Downregulation of MHC class I molecules by human
cytomegalovirus-encoded US2 and US11
Barel, M.T.
Citation
Barel, M. T. (2005, October 27). Downregulation of MHC class I molecules by human
cytomegalovirus-encoded US2 and US11. Retrieved from https://hdl.handle.net/1887/4294
Version:
Corrected Publisher’s Version
License:
Licence agreement concerning inclusion of doctoral thesis in the
Institutional Repository of the University of Leiden
CHAPTER 1
Introduction
D e fe ns e m e ch a nis m s a g a ins t v irus e s
In healthy individuals, the immune system is able to
limit the damag e in many viral infec tions. W hen a viral
p artic le manag es to invade the body by c rossing
p hysic al barriers suc h as the sk in and muc us
membranes, it bec omes subjec t to detec tion and
elimination by several immune effec tor c ells. A s
viruses c annot rep lic ate on their ow n, they must infec t
c ells in order to ex p loit the host c ell’s mac hinery for
the p roduc tion of p rog eny and to enable further virus
sp read.
H ost c ell entry c an be bloc k ed by antibody-p roduc ing
B c ells, w hic h sp ec ific ally rec og niz e foreig n struc tures
on the viral p artic le. T he antibodies c annot only
steric ally hinder viral entry, but they c an also induc e
elimination of the viral p artic les via different p athw ays.
T hese inc lude c omp lement ac tivation,
antibody-dep endent c ell-mediated c ytotox ic ity and p hag
o-c ytosis
1 ,2. T w o subtyp es of B c ells arise after
ac tivation of B c ells, antibody p roduc ing p lasma c ells
and memory B c ells. T he latter subtyp e is c ap able of
mounting an immediate hig h-affinity resp onse up on
renew ed c ontac t w ith the antig en.
O nc e virus p artic les are inside host c ells, T c ells c ome
into p lay. T hey c an rec og niz e foreig n antig en
frag ments disp layed in the c ontex t of major
histoc omp atibility c omp lex (M H C ) molec ules at the
c ell surfac e
3 ,4. T here are tw o c lasses of M H C
molec ules, w hic h differ in struc ture, ex p ression
p attern and sourc e of antig enic p ep tides.
M H C c lass I molec ules are found on all nuc leated
c ells and mainly p resent p ep tides derived from
endog enously synthesiz ed p roteins. C irc ulating C D 8
+c ytotox ic T c ells (C T L s) insp ec t antig enic frag ments
disp layed by M H C c lass I molec ules. In c ase of
infec tion, C T L s rec og niz ing foreig n antig ens c an,
up on ac tivation, k ill suc h c ells.
E x p ression of M H C c lass II molec ules is restric ted to
antig en-p resenting c ells, i.e. B c ells, mac rop hag es
and dendritic c ells. C lass II molec ules disp lay p ep tide
frag ments, derived from an ex og enous sourc e of
p roteins (e.g . viral c ap sid p roteins), for insp ec tion by
C D 4
+T c ells. U p on rec og nition of foreig n p ep tides,
these T c ells p rovide help to C D 8
+T c ells to elic it a
c ytotox ic resp onse. In addition, C D 4
+T help er c ells
are req uired for the maintenanc e and func tionality of
memory C D 8
+T c ells
5.
T he p resentation of ex og enous p ep tides is not
c omp letely restric ted to M H C c lass II molec ules, as
p ep tides derived from internaliz ed p roteins c an also
enter the M H C c lass I p athw ay
6 ,7. T he ability to c
ross-p resent seems to be limited to a sross-p ec ific subset of
antig en p resenting c ells, mainly C D 8
+dendritic c ells
8.
In addition, endog enous p roteins c an g ain ac c es to
the M H C c lass II p athw ay via autop hag y
9.
B esides an interac tion betw een M H C molec ules and
their sp ec ific rec ep tors, a lig ation betw een ac c essory
molec ules on the antig en p resenting c ell and c
ounter-rec ep tors on the T c ell (c o-stimulation) is req uired for
T c ell ac tivation.
N atural k iller (N K ) c ells are also c ap able of sensing
virally infec ted c ells, but c an only do so by virtue of
altered ex p ression levels of c ertain native surfac e
p roteins, inc luding M H C c lass I molec ules. T he
ac tivation of N K c ells, and the subseq uent release of
c ytotox ins, is c ontrolled by a set of N K c ell rec ep tors.
D ep ending on the typ e of rec ep tor, inhibitory or
ac tivating sig nals are ig nited up on binding lig ands on
the p otential targ et c ell. If inhibitory rec ep tor sig naling
fails to q uenc h the effec t of eng ag ement of ac tivating
rec ep tors, the N K c ell w ill be trig g ered to k ill its targ et
c ell.
T his thesis w ill further foc us on M H C c lass I
molec ules and their imp lic ation as lig ands for T and
N K c ell rec ep tors in the c ontex t of human c ytomeg
alo-virus infec tion.
H um a n cy tom e g a lov irus
H uman c ytomeg alovirus (H C M V ) is found in human
p op ulations w orldw ide and c an infec t 6 0 – 90 %
of
individuals, dep ending on the p op ulation studied.
H C M V is a member of the Herpesviridae, w hic h are
k now n to establish a life-long relationship w ith their
hosts. H C M V infec tion is usually asymp tomatic , but
c an c ause disease in an op p ortunistic manner. In
immunoc omp romised individuals, suc h as A ID S
p atients and org an transp lant rec ip ients, unc ontrolled
H C M V reac tivation c an c ause severe illness or death
1 0 ,1 1
. A dditionally, H C M V is a notorious risk fac tor for
c ong enital birth defec ts. In most c ases, suc h c linic al
manifestations are found in mothers w ho underg o
p rimary rather than rec urrent infec tions
1 2.
In vivo, HCMV can infect endothelial cells, epithelial
cells, monocytes/macrophages, smooth muscle cells,
stromal cells, neuronal cells, neutrophils, fibroblasts,
and hepatocytes
13,14. In vitro, all tested vertebrate cell
types were susceptible to HCMV infection
15. Common
HCMV-associated illnesses include retinitis (which
may cause blindness), hepatitis, encephalitis,
pneumonia, and bowel disease
16.
F ollowing primary infection, HCMV persists in a latent
state in cells of the myeloid lineage, with intermittent
viral reactivation and shedding from mucosal surfaces,
and containment by the host immune response
17.
Another potential cell type latently or persistently
infected by HCMV are vascular endothelial cells
18.
Interestingly, HCMV infection has different effects on
the functioning and survival of these various types of
host cells. In macrovascular aortic endothelial cells
HCMV infection is not lytic and results in the
accumulation of significant amounts of extracellular,
but not intracellular virus. In addition, the cell cycle is
not inhibited by HCMV and cells continuously release
infectious virus. This is in contrast with the rapid, lytic
infection of brain microvascular endothelial cells and
human foreskin cells
18. It also differs from HCMV
infection in monocyte-derived macrophages, which
results in non-lytic accumulation of intracellular virus
and lack of extracellular virus
19.
At present, the viral and host mechanisms of HCMV
latency and reactivation are unclear, but the virus may
be reactivated by cellular differentiation, and hormonal
or cytokine (TNF D) stimulation.
HCMV particles can be found in all body fluids and
can be transmitted by multiple means, including
aerosol droplets, sexual contact, nursing,
trans-plantation and blood transfusion
20.
HCMV has a large double stranded DNA genome
(t230 kb), encoding approximately 192 proteins
21.
The size of the genome varies among HCMV strains.
Compared to low-passage clinical HCMV isolates, the
laboratory-adapted AD169 and Towne strains lack 22
and 19 viral genes, respectively
22.
In infected cells, viral replication occurs according to a
cascade of three consecutive phases, called
immediate early (IE), early (E), and late (L). The IE
gene products play an important role in regulating the
expression of viral E and L genes, as well as of
cellular genes. E and L viral gene products include
viral functions associated with viral DNA replication
and virus packaging. The viral genome also encodes
several proteins involved in immune evasion
strategies, dealing with various immune effector
mechanisms of the host. These include interference
with MHC class I and II antigen presentation to CD8
+and CD4
+T cells, NK cell activity, cytokine and
chemokine signalling, and apoptosis
23-37.
It is evident that these mechanisms are not exploited
in a way that is life-threatening to the
immuno-competent host. Several studies support important
roles for B, T and NK cells in protection against HCMV
disease
38-44. However, these immune evasion
strategies may prevent complete eradication of the
virus and contribute to the ability of HCMV to cause
lifelong persistent infections.
Before discussing the immune escape mechanisms
exploited by HCMV to interfere with MHC class I
surface expression, an introduction on MHC class I
complex formation and the different functions of the
various MHC class I locus products will be given.
MHC class I molecules
for peptides binding different alleles of MHC class I
Figure 1. Features of MHC class I molecules. A) S ch em atic diagram of an MHC class I m olecu le sh ow ing th e ex ternal dom ains, th e transm em brane segm ent and th e cy toplasm ic region. T h e peptide binding cleft is form ed by th e m em brane distal 1 and 2 dom ains of class I. B ) R epresentation of th e h u m an class I HL A -A 2 m olecu le determ ined by x -ray cry stallograph ic analy sis. T h e -strands are depicted as th ick arrow s and th e -h elices as h elical ribbons. D isu lfide bonds are represented as tw o interconnected sph eres. C) A plot of th e v ariability in th e am ino acid seq u ence of allelic class I m olecu les in h u m ans dem onstrates th at th e v ariable residu es are clu stered in th e 1 and 2 dom ains. D ) R epresentation of th e 1 and 2 dom ains as v iew ed from th e top of a class I m olecu le, sh ow ing th e cleft consisting of a base of anti-parallel beta strands and sides of -h elices. E ) E x am ples of anch or residu es in nonam eric peptides elu ted from tw o class I MHC m olecu les. A nch or residu es (grey ) tend to be h y droph obic am ino acids and interact w ith th e class I MHC m olecu le. (Adopted from J. Kuby, Immunology 4th ed.,
for peptides binding different alleles of MHC class I
molecules. Altogether, this ensures the display of a
wide variety of peptide antigens for CD8
+T cell
inspection.
MHC class I assembly and interactions with
comp onents of MHC class I antigen p resentation
p athway
The folding and assembly into mature trimeric
complexes involves a series of events and requires
the action of several accessory molecules (see Figure
2). As HCMV-encoded proteins intervene at different
stages of this process, the MHC class I antigen
presentation pathway will be discussed in more detail.
MHC class I heavy chains encode a signal peptide,
which directs insertion into the ER during translation.
Once in the ER, the signal sequence is cleaved off by
a signal peptidase, while oligosaccharyl transferase
equips the HC with an N-linked oligosaccharide.
These free HCs are soon found in association with the
general ER chaperones BiP
45and membrane bound
calnexin, the latter of which has lectin-like activity
46,47.
BiP transiently binds to many newly synthesized
proteins. Misfolded proteins and unassembled
subunits are bound by BiP in a prolonged fashion.
48,49
. Binding of calnexin is regulated by glucose
trimming of nascent N-linked oligosaccharides
50.
Calnexin generally binds proteins with
mono-glucosylated (G lc1Man9-7G lnNAc2) oligosaccharides
51
. Calnexin and BiP predominantly associate with free
MHC class I heavy chains and the assembly with E2m
abolishes the interaction of the heavy chain with these
chaperones
45,52,53. Before binding the light chain,
heavy chains also interact with ERp57, a member of
the protein disulfide isomerase (P DI) family, who are
involved in disulfide bond oxidation, reduction and
isomerization reactions
54-56. Mature MHC class I
molecules harbor three intra-molecular disulfide
bridges, the formation of which is likely to be mainly
assisted by ERp57. After binding E2m, MHC class I
molecules are found in association with another,
soluble ER chaperone with lectin-like activity,
calreticulin
57,58. Like calnexin, calreticulin binds to
proteins
with
G lc1Man9-7G lnNAc2
N-linked
oligosaccharides
59,60. MHC class I molecules than
become associated with the peptide-loading complex,
which besides calreticulin includes ERp57, tapasin,
and the transporter associated with antigen
processing (TAP ). Tapasin mediates the interaction of
class I with TAP
57,58,60. The structure of MHC class
I-peptide complexes suggests that oxidation of the
cysteines in the HC D2-domain is a prerequisite for
stable peptide binding, a process likely facilitated by
ERp57
61. The peptides are generated from
endogenous proteins by cytosolic proteasomes (large
protease complexes, discussed in more detail in a
later section) and may be further trimmed by
aminopeptidases before and after translocation into
the ER via TAP
62,63. In the ER, peptides are loaded
onto HC-E2m heterodimers. These trimeric
HC-E2m-peptide complexes then dissociate from the loading
complex and are released into the secretory pathway
64
.
MHC class I variants
In humans, a single type of light chain (B2m) is found
that is complexed with one of multiple types of heavy
chains, HLA-A, B, C, E, G , of which up to two different
allelic forms can be expressed. They are encoded by
separate clusters within the MHC region of the human
genome, located on chromosome 6. This region also
encodes other proteins involved in antigen
presentation (e.g. MHC class II, TAP 1, TAP 2, LMP
genes). The light chain is encoded by a gene located
on another site, on chromosome 15. The various
types of heavy chains differ from each other in several
ways, as will be discussed below.
Polymorphism
One outstanding difference among class I haplotypes,
is the degree of polymorphism that can be found in
the world population. To date, 372 different alleles
have been reported for HLA-A, and 661 for HLA-B.
HLA-C shows somewhat less variation with 190
different alleles. P olymorphism of HLA-E and HLA-G
is very limited, with 5 (HLA-E) and 15 (HLA-G )
different alleles described to date (IMG T/HLA
database,
65.
T issu e distrib u tion / su rfac e ex pression
Tissue distribution and cell surface levels differ for the
various locus products. Most host cells express
HLA-A, B, C and E alleles, whereas HLA-G expression is
restricted to the thymus and certain placental tissues,
e.g. trophoblast cells. The trophoblast cells at the
maternal-fetal interface lack surface expression of
HLA-A and -B alleles
66,67. Outside this
immune-privileged site, the surface levels of HLA-A, -B, and -C
alleles may vary between cell types
68,69. Several
aspects that influence MHC class I (surface)
expression levels will be discussed below.
Figure 2. M H C c la s s I a s s em b ly . A ) Model for folding and complex formation of MHC class I molecules and interactions with components of the class I antigen presentation pathway (see text for further description). Calnexin (Cnx), calreticulin (Crt), tapasin (T pn). T he N -link ed gly can is mark ed “N ”. B ) N -link ed gly cans are added to proteins in the E R as "core oligosaccharides" that hav e the structure shown. T hese are b ound to the poly peptide chain through an N -gly cosidic b ond with the side chain of an asparagine that is part of the A sn-X -S er/T hr consensus seq uence. T he three glucoses are remov ed b y glucosidase I and II, and terminal mannoses b y one or more different E R mannosidases. A ssociation of gly copeptides with Cnx/Crt is dependent upon the presence of a single terminal glucose residue b ound to the G lcN A c2Man9 gly can precursor.
(Adapted from ref.142
attrib uted to differences in their promoter regions.
69-7 1
. T he cy tok ines T N F -D, IF N -J and IF N -E can
increase av erage MHC class I surface lev els up to 1 0
-fold
6 9. T he HL A -G promoter region differs from other
MHC class I genes, as some of the conserv ed
regulatory b oxes hav e b een deleted or altered,
including the IF N regulatory seq uence
7 0 ,7 2. In
addition, the cy tok ine IL 1 0 has b een reported to
selectiv ely upregulate HL A -G expression
7 3. Cell ty
pe-specific factors may contrib ute to differential locus
expression as well
6 8 ,6 9.
MHC class I complex formation and stability
T h e le v e ls o f th e d iffe re n t M H C c la s s I m o le c u le s th a t
a re fo u n d a t th e c e ll s u rfa c e a re d e te rm in e d b y
tra n s c rip t le v e ls , a s w e ll a s b y th e s u c c e s s o f s ta b le
c o m p le x fo rm a tio n , a n d tu rn o v e r ra te s . G e n e ra lly ,
s u rfa c e le v e ls o f H L A -C lo c u s p ro d u c ts a re ra th e r lo w ,
a b o u t 1 0 % o f th e a v e ra g e le v e l o f H L A -A a n d -B
m o le c u le s
7 4 -7 6. S e v e ra l e x p la n a tio n s h a v e b e e n
p ro p o s e d , in c lu d in g lo w e r tra n s c rip t le v e ls , lo w e r
a s s e m b ly ra te s fo r H L A -C h e a v y c h a in s w ith E2m ,
a n d a n in e ffic ie n t s u p p ly o f h ig h a ffin ity H L A C
-s p e c ific p e p tid e -s .
7 1 ,7 7 ,7 8. V a ria tio n b e tw e e n M H C
c la s s I m o le c u le s in b in d in g c o m p o n e n ts o f th e
p e p tid e lo a d in g m a c h in e ry , ta p a s in /T A P , m a y a ls o
c o n trib u te to d iffe re n c e s in M H C c la s s I s u rfa c e le v e ls .
E x p e rim e n ts w ith w ild ty p e a n d m u ta n t H L A -A 2
m o le c u le s in d ic a te d th a t re s id u e s 1 3 2-1 3 4 in th e D2
d o m a in a re im p o rta n t d e te rm in a n ts fo r a s s o c ia tio n
w ith T A P . H L A -A 2 T 1 3 4 K /S 1 3 2C m u ta n ts d o n o t
a s s o c ia te w ith ta p a s in /T A P a n d a re re le a s e d in to th e
s e c re to ry p a th w a y m u c h fa s te r (a s in d ic a te d b y
re a c h in g E n d o H re s is ta n c e ) th a n th e ir w ild ty p e
c o u n te rp a rts
7 9. T h e re is n o s e q u e n c e v a ria tio n in th is
p a rtic u la r re g io n . R e s id u e s 1 1 4 a n d 1 1 6 , w h ic h a re
p o ly m o rp h ic , w e re s h o w n to b e re s p o n s ib le fo r
d iffe re n c e s in p e p tid e -lo a d in g c o m p le x in te ra c tio n s
8 0-8 4
. C la s s I m o le c u le s th a t a s s o c ia te in e ffic ie n tly w ith
lower peptide ligand binding affinities and render the
heterotrim eric M H C c lass I c om plex es form ed less
stable. O nc e at the c ell su rfac e, ty rosine or di-leu c ine
m otifs present in m ost M H C c lass I m olec u les c an
fu nc tion as endoc y tosis signals. H L A -G m isses
potential endoc y tosis signals and its half-life has been
shown to be alm ost twic e that of H L A -A 2 m olec u les
8 5.
Peptide binding / repertoire
H L A -A /B and H L A -C /G /E differ in their ligand-binding
c apabilities: H L A -A /B m olec u les are prom isc u ou s,
whereas the other c lass I m olec u les bind a lim ited
nu m ber of ligands. H L A -C loc u s produ c ts, for
instanc e, hav e a preferenc e for peptides that are
poorly transported into the E R , and that req u ire a 1 0
-fold higher peptide c onc entration for release into the
sec retory pathway
7 8 ,8 6 -8 8. H L A -G binds m any
self-peptides with a defined m otif and its peptide binding
div ersity is estim ated to be abou t fiv e fold lower than
that of H L A -A
8 9 ,9 0. T he m ost c om m on H L A -E ligands
are nonam ers deriv ed from signal seq u enc es of other
H L A c lass I m olec u les
9 1 ,9 2.
S pec ia l fea tu res / a lterna tiv e s plic e v a ria nts
D u e to a prem atu re stop c odon in ex on 6 , H L A -G has
a relativ ely short c y toplasm ic tail (6 residu es)
c om pared to other H L A c lass I m olec u les (2 9 -3 3
residu es)
9 3. C onseq u ently , H L A -G not only lac k s
potential endoc y tosis signals present in other c lass I
m olec u les, bu t it also gains an E R retriev al/retential
signal with the dily sine residu es positioned at 4 and
-5 from the C -term inu s. T his m otif c an m ediate
rec y c ling of assem bled H L A -G m olec u les between the
E R and the c is -G olgi
9 4 ,9 5. H igh affinity peptide binding
seem s to be req u ired to end the rec y c ling and allow
egress to the c ell su rfac e
8 5.
P rim ary H L A -G m R N A transc ripts are differentially
splic ed, giv ing rise to m u ltiple isoform s
9 6. O nly the fu ll
length isoform (H L A -G 1 ) is ex pressed at the c ell
su rfac e
9 7. In addition to this m em brane-bou nd
isoform , a solu ble (sec reted) isoform was detec ted in
plac ental tissu es (sH L A G 1 /H L A G 5 ). S olu ble H L A
-G 1 is translated from a transc ript with a retained intron
4 , whic h introdu c es a prem atu re stopc odon after the
D1 -3 dom ain enc oding seq u enc e. T his isoform c an
assoc iate with E2 m
9 8 ,9 9.
L iga nds for T a nd N K c ell rec eptors
M H C c lass I m olec u les c an be ligands for c y totox ic T
c ell rec eptors, as well as for N K c ell rec eptors.
T hrou gh rec ognition of M H C -peptide c om plex es,
C D 8
+T c ells c an k ill infec ted target c ells. H L A -A and
B m olec u les are the restric tion elem ents in the
m ajority of C T L responses, althou gh there are also
som e ex am ples of H L A -C , -G and -E m olec u les
presenting antigenic peptides to C T L s
1 0 0 -1 0 3. T his
m ay v ery well ex plain the high degree of
poly m orphism that is partic u larly desc ribed for H L A -A
and – B alleles.
U nlik e for C T L s, the distinc tion between self and
non-self antigens in the c ontex t of M H C c lass I m olec u les
is not the m ajor restric tion elem ent for triggering of N K
c ell ly sis. T his trigger is determ ined by the presenc e
or absenc e of N K c ell rec eptor ligands on the su rfac e
of the target c ell. T he total inpu t of sev eral ty pes of
ligands, inc lu ding M H C I m olec u les, and their
engagem ent with both inhibitory and ac tiv ating N K
rec eptors c ontrols N K c ell c y toly sis
1 0 4 ,1 0 5. M any M H C
c lass I m olec u les c an c ontribu te to the regu lation of
N K c ell fu nc tioning. O ther N K c ell rec eptor ligands
inc lu de sev eral m olec u les that are distantly related to
M H C c lass I m olec u les, e.g. M IC A /B , U L 1 6 binding
protein (U L B P ) 1 /2 /3 , and C D 1 5 5 . A n ov erv iew of
c u rrently k nown N K c ell rec eptors and ligands is
presented in T able 1
1 0 6 ,1 0 7.
A partic u lar N K c ell c lone c an ex press u p to nine
different rec eptors, whic h together determ ine the
ac tiv ation statu s of the c ell. T hese rec eptors c an be
display ed on ov erlapping su bsets within the total N K
c ell popu lation, and the repertoire of ex pressed
rec eptors is heterogeneou s in different indiv idu als
1 0 8 ,1 0 9
. S om e of these rec eptors are also fou nd on
other im m u ne effec tor c ell ty pes. V ariation in su rfac e
ex pression of only one ty pe of M H C c lass I m olec u le
c an already m ak e a differenc e for c ell su rv iv al.
Table 1. Natural killer cell receptors and their ligands N am e of rec eptor T y pe of signal L igand(s) K IR 2 D L 1 (p5 8 .1 ) inhibitory H L A -CL y s8 0
K IR 2 D L 2 (p5 8 .2 ) inhibitory H L A -C A sn8 0
K IR 2 D L 4 (p4 9 ) inhibitory H L A -G K IR 3 D L 1 (p7 0 ) inhibitory H L A -B w4 K IR 3 D L 2 (p1 4 0 ) inhibitory H L A -A 3 , A 1 1 L IR 1 /IL T 2 inhibitory H L A -A ,-B ,-C ,-E ,-F ,-G L IR 2 /IL T 4 inhibitory H L A -A ,-B ,-C ,-E ,-F ,-G C D 9 4 /N K G 2 A inhibitory H L A -E K IR 2 D S 1 (p5 0 .1 ) ac tiv ating H L A -C L y s8 0
K IR 2 D S 2 (p5 0 .2 ) ac tiv ating H L A -C A sn8 0
K IR 2 D S 4 (p5 0 .3 ) ac tiv ating u nk nown C D 9 4 /N K G 2 C ac tiv ating H L A -E P 4 0 /L A IR 1 inhibitory u nk nown p7 5 /A IR M 1 inhibitory u nk nown N K p3 0 (1 C 7 /N K -A 1 ) ac tiv ating u nk nown N K p4 4 ac tiv ating u nk nown N K p4 6 ac tiv ating u nk nown N K p8 0 ac tiv ating u nk nown N K G 2 D ac tiv ating M IC A /B , U L B P 1 -3 2 B 4 ac tiv ating C D 4 8
Surface expression of HLA-E was shown to be
sufficient to either inhibit N K cells expressing
C D 9 4 /N K G 2 A or to enhance k illing by cells
expressing C D 9 4 /N K G 2 C
1 1 0. Lik ewise, HLA-G
expression has been shown to protect HLA class I
d eficient targ ets from N K cell m ed iated ly sis, throug h
eng ag ing inhibitory N K cell receptors
1 1 1 ,1 1 2. T he
selectiv e expression of only HLA-C , -E, and – G alleles
on cells at the fetal m aternal interface m ay help to
protect them from m aternal cy totoxic T cell and N K
cell attack .
T cell and NK cell escape mechanisms of HCMV
Sev eral im m une escape m echanism s exploited by
hum an cy tom eg alov irus seem to focus on prev ention
of d etection and elim ination by cy totoxic T cells and
N K cells.
HCMV and escape from cytotoxic T cell killing
D uring latent infections, HC M V v iral g ene expression
is lim ited which helps to av oid im m une surv eillance by
cy totoxic T cells. D uring activ e infection, HC M V
exploits another m echanism to av oid d isplay of v iral
antig ens, that is by d own-reg ulating M HC m olecules.
C ell surface expression of M HC class I m olecules is
affected by the concerted action of a set of proteins
encod ed within the uniq ue short (U S) reg ion of the
HC M V g enom e that are expressed along the d ifferent
stag es of v iral infection. U S3 is the first to be
sy nthesiz ed and prev ents traffick ing of newly
sy nthesiz ed M HC class I m olecules, by entrapping
them in the ER
3 0 ,1 1 3. N ext, U S2 and U S1 1 com e into
play and ind uce proteoly tic d eg rad ation of M HC class
I m olecules. Im m ed iately after their sy nthesis and
translocation into the ER , class I heav y chains are
transported into the cy tosol where they are d epriv ed
of their N -link ed g ly can and subseq uently d eg rad ed
by proteasom es
3 7 ,1 1 4. At early and late tim es post
infection, U S6 prev ents peptid e load ing of M HC class
I m olecules by block ing the T ransporter associated
with Antig en P rocessing (T AP )
2 9 ,3 3 ,1 1 5. Another U
S-encod ed g ene, U S1 0 , has been reported to d elay
m aturation of M HC class I m olecules
2 5.
HCMV and N K cell escape
A com plete red uction of M HC class I expression could
hav e serious conseq uences for the surv iv al of the
v irus, as cells lack ing M HC class I surface m olecules
are m ore susceptible to N K -cell attack
2 4. Sev eral
proteins encod ed within the U niq ue Long (U L) reg ion
of the HC M V g enom e (U L1 6 , U L1 8 , U L4 0 ) appear to
protect infected cells ag ainst N K -cell ly sis. T hey either
block expression of lig and s that activ ate N K cells
(U L1 6 ) or allow expression of lig and s that can inhibit
N K -cell trig g ering (U L1 8 , U L4 0 )
3 4.
U L1 6 , expressed at an early stag e of infection,
interferes with M IC B / U LB P 1 , 2 -specific trig g ering of
activ ating N K cell receptors (N K G 2 D ) by d
own-reg ulating their lig and s
1 1 6. At the sam e tim e, U L4 0
can prom ote surface expression of HLA-E by
prov id ing it with T AP -ind epend ent peptid es, thereby
supply ing a lig and for the inhibitory C D 9 4 /N K G 2 A
receptor, which is found on m ost N K cells
1 0 2. At a late
stag e of infection, HC M V encod es a v iral M HC class I
hom olog ue, U L1 8 , which serv es as d ecoy for M HC
class I at the cell surface and can bind the LIR 1
inhibitory N K cell receptor
1 1 7. R ecently , a new
early /late HC M V -encod ed g ene prod uct has been
id entified , U L1 4 1 , which prev ents surface expression
of C D 1 5 5 , a lig and for the activ ating N K cell receptor
C D 2 2 6
3 6. It is im portant to note that this U L1 4 1 g ene
is present in v arious clinical HC M V strains, whereas it
is lost in laboratory AD 1 6 9 and T owne strains
2 1 ,2 2 ,3 6.
Alternativ e way s to preserv e inhibitory sig nals to N K
cells could includ e a m ore selectiv e d own-reg ulation
of M HC class I surface expression. If the specificity of
the U S proteins were such that those M HC class I
m olecules that present v iral antig ens (m ostly HLA-A
and -B alleles) are affected pred om inantly , HC M V
could escape both T cell and N K cell k illing .
S electiv ity and degree of MHC class I dow n-regu lation
b y HCMV
T he success of im m une escape by HC M V from both T
cell and N K cell attack throug h m od ulation of M HC
class I surface expression is lik ely to be influenced by
the efficiency as well as by the specificity of the
d ifferent v iral ev asion proteins.
T he d eg ree of d own-m od ulation that can be
accom plished lik ely d epend s on the balance of targ et
M HC class I and U S protein lev els. As m entioned
before, M HC class I surface lev els can be upreg ulated
by cy tok ines. T he expression lev els of the U S proteins
v ary d uring the course of infection and m ay also
d epend on v iral load
1 1 8. A hig her am ount of v iral
particles can lead to a m ore sev ere red uction in M HC
class I surface expression
1 1 9. As m entioned in
Several laboratories have studied allelic differences
between MHC class I molecules with respect to
sensitivity to US2, US3, US6, and US11
29,33,67,102,113,115,119-126
.
Our studies were focused on the specificity of MHC
class I down-regulation by US2 and US11. In addition,
we aimed to characteriz e the precise regions in MHC
class I alleles that determine sensitivity or resistance
to US2 and US11. The approach for US2 was based
on previous results and on crystal structure data from
a soluble HLA-A2/E2m/US2 complex (see Figure 3)
published by Gewurz et al.
123.
Dislocation and degradation of MHC class I
m olecules
In the ER, US2 and US11 bind newly synthesiz ed
MHC class I heavy chains and target them to the
cytosol for subsequent degradation by proteasomes
31,37
. Retro-transport is a mechanism commonly used
for the disposal of improperly folded and unassembled
ER lumenal proteins, including MHC class I heavy
chains. The exact requirements for dislocation of
these membrane-anchored proteins from the ER into
the cytosol are still relatively unknown. However, there
are indications that certain ER chaperones may be
involved.
ER quality control and ER-associated degradation of
incom pletely folded or assem bled M H C class I
m olecules
W hen properly folded heterotrimeric HC-E2m-peptide
complexes are formed, the MHC class I complex is
released of all auxiliary molecules as it follows the
secretory pathway. But until this point, several quality
control mechanism are active to prevent surface
expression of malfolded or unassembled class I heavy
chains. For instance in the absence of one of the
components of the complex, e.g. in case of a defect in
E2m expression, no MHC class I molecules can be
detected at the cell surface
127-130. If the supply of
peptides is hampered by TAP inhibition, or loading is
obstructed by an impaired interaction of TAP and
HC’s in the absence of tapasin, then the surface
expression of class I molecules is also severely
reduced
57,131.
In E2m and TAP-deficient cell lines, incompletely
folded and assembled MHC class I molecules are
removed from the ER and released into the cytosol,
where they are degraded by proteasomes
132. The
exact mechanism by which molecules are dislocated
remains elusive, but there are indications that ER
F igure 3 . Cry stal structure of a soluble U S 2 /HL A -A 2 / 2 m /Tax com plex . Crystal structure of a soluble US2/HLA-A2/ 2m/Tax complex, as determined by B. Gewurz et al. Data for this image were derived from the PUBMED Protein Data Base (reference 1IM3) and visualiz ed as solid ribbon using W ebViewerLite. HLA-A2 (light grey), 2m (darker grey), US2 (darkest grey) and Human T lymphotropic virus type I Tax peptide (LLFGY PVY V, black). Amino acid residues in HLA-A2 that make direct contact with US2 are depicted in dark grey and are marked with their corresponding position numbers in the class I heavy chain.
chaperones such as BiP and calnexin may play a role
in this process. BiP retains many misfolded proteins in
the ER, including unassembled HC’s
45,133,134. Studies
involving Kar2p (the yeast homologue of BiP) and
glycoprotein CPY * , have linked the ATPase activity of
BiP with release of malfolded proteins into the cytosol
135
. The release from BiP and retro-transport of
unassembled Ig L chain, a soluble nonglycosylated
protein, has been found to be tightly coupled with
proteasome activity
136. It is unclear whether these
effects of BiP on dislocation are restricted to
malfolded soluble ER proteins. Besides this, BiP has
been implicated to play a role in the unfolded protein
response (UPR), as sensor of accumulating misfolded
proteins. UPR directs the upregulation of a number of
stress-related proteins (likely involved in disposal
processes), including BiP
137,138. Like BiP, Calnexin
retains incompletely assembled MHC class I HC’s in
the ER by using its cytoplasmic tail
52,139. Surface
expression of MHC class I heavy chains in a
E2m-deficient cell line could be restored by introducing
calnexin with a truncated tail
52.
As mentioned before, trimming of the N-linked
precursor oligosaccharide Glc3Man9GlcNAc2 in the
ER yields Glc1Man9-7GlnNAc2, which enables
and quality control process is then initiated that
consists of cycles of deglucosylation by glucosidase II,
release from the chaperone, reglucosylation by the
folding
sensor
UDP-Glc:glycoprotein
glucosyl-transferase, and re-association with calnexin and
calreticulin
140. Glycoproteins that achieve proper
folding are no longer recognized by glucosyltranferase
and leave this cycle. Differential mannose trimming by
ER-mannosidases I and II has been proposed to
signal the degradation of terminally misfolded
glycoproteins
141-143. Inhibitors that prevent mannose
trimming were found to inhibit degradation of many
defective glycoproteins
141,143-147. Loss of the mannose
residue that is the acceptor for glucosyltranferase
leads to release from the reglucosylation cycles
involved in the rescue of improperly folded
glycoproteins and association to the putative mannose
lectin EDEM
148-150. The glycoproteins are then
dis-located to the cytosol, deglycosylated and degraded
by the proteasome
151.
Ubiquitin and proteasomal degradation
The turnover of many cellular proteins is regulated by
a process, which involves initial earmarking via the
ubiquitin system and subsequent degradation by
proteasomes (see Figure 4). This earmarking involves
the linkage of ubiquitin, a highly conserved protein of
~ 8.5kDa, to the substrate protein through an
isopep-tide bond between the C-terminal glycine (Gly76) of
ubiquitin and the H-NH
2group of a lysine residue in
the target protein. Protein ubiquitination involves a
cascade reaction with subsequent activities of three
different types of enzymes: ubiquitin-activating
enzyme (E1), ubiquitin-conjugating enzyme (E2) and
ubiquitin-ligase (E3)
152,153. Polyubiquitin chains are
formed by the attachment of additional ubiquitin
moieties via Gly76, to Lys48 of the next ubiquitin
moiety. Only poly-ubiquinated proteins are recognized
as substrates for proteasomal degradation. A fourth
type of enzyme (E4) has been postulated to function
as multi-protein chain assembly factor
154.
Ubiquitin-mediated proteolysis is an important
mechanism for the regulation of many cellular
processes, as it mediates the turnover of cell cycle
regulators, tumor suppressors and growth modulators,
transcriptional activators and inhibitors, cell surface
receptors, in addition to misfolded proteins. This
requires a substrate specific mode of action for the
E1-3 enzymes. E1 enzymes are highly conserved and
so far only a single ubiquitin-activating enzyme (Uba1)
has been found in yeast and humans. The
ubiquitin-conjugating (ubc) enzymes are part of a homologous
family of proteins. To date, 13 different E2 enzymes
have been described in yeast, and at least 18 in
humans. Likely, E3 enzymes contribute most to the
biological specificity of the ubiquitin system, with
hundreds, if not thousands of often structurally
unrelated enzymes. These include Skp/Cullin/F-box
(SCF), SOCS-box, Anaphase Promoting Complex
(APC), HECT family, RING finger domain, U-box,
VBC and Parkin-like E3 ligases.
Proteasomes, found both in the nucleus and cytosol,
are large multi-subunit complexes, made up of a 20S
core particle and one or two regulatory “cap”
complexes (19S or 11S/PA28)
155. The 20S
barrel-shaped core particle consists of four stacked rings:
two identical outer D-rings, and two identical inner
E-rings. Each of these rings contain seven different
subunits. The catalytic activity is provided by 3
subunits (called X , Y and Z ) in the E-rings. The 19S
cap is composed of 17 subunits, 9 of which are part of
a base connected to the D-ring, and 8 of which are
found in a structure referred to as the lid. This cap
recognizes poly-ubiquitinated proteins and is believed
to mediate entry into the narrow pore of the 20S core
particle by unfolding the substrate and widening the
entrance via the D-ring. Immuno-modulatory cytokines
such as IFN-J can alter the composition of the
proteasome. IFN-J promotes expression of the
regulatory PA28/11S complex, which reportedly
enhances proteasome activity
156,157. In addition, IFN-J
induces the incorporation of LMP2, LMP7 and
MECL-1 subunits into the E-rings of the 20S core particle,
instead of the standard X , Y, and Z subunits
158-160.
Proteasomes harboring IFN-J subunits are therefore
referred to as immunoproteasomes.
Immuno-proteasomes may enhance the specificity of the
proteasome and promote the generation of peptides
presented by MHC class I molecules.
D islocation of ER-lumenal proteins and involvement of
cytosolic factors
It is still largely unknown how dislocation of
ER-lumenal substrates is accomplished. Proteasomes
may play an active role in the extraction of
ubiquitinated substrates. In mammalian cells ~ 10% of
the proteasomes are located at the cytosolic face of
the ER membrane, and proteasomal activity appear
to be required for the extraction of several
trans-membrane proteins
161-163.
final membrane detachment of partially dislocated
substrates
1 6 4 ,1 6 5. It is a h ex am eric rin g c o m p lex , th at
sen ses bo th p o ly ubiq uitin ated as w ell as n o n
-ubiq uitin ated p ro tein s
1 6 5. It lik ely ac ts in c o n c ert w ith
o th er (E R -m em bran e an c h o red ) p ro tein s ex ertin g a
m o re substrate-sp ec ific ro le.
In stud ies o f U S 1 1 -m ed iated d eg rad atio n o f M H C
c lass I h eav y c h ain s, D erlin 1 (th e m am m alian
eq uiv alen t o f y east D er1 ), w as id en tified as an
ad d itio n al c o m p o n en t in v o lv ed in d islo c atio n
1 6 6 -1 6 8.
Scope of this thesis
T h e stud ies d esc ribed in th is th esis fo c us o n
strateg ies ex p lo ited by h um an c y to m eg alo v irus to
esc ap e im m un e d etec tio n by its h o st th ro ug h
m o d ulatio n o f surfac e ex p ressio n o f M H C c lass I
m o lec ules by U S 2 an d U S 1 1 (rev iew ed in C h ap ter 1 ).
U S 2 an d U S 1 1 c an bo th targ et n ew ly sy n th esiz ed
M H C c lass I m o lec ules fo r p ro teaso m al d eg rad atio n .
T h is c an p ro h ibit th e d isp lay o f v iral an tig en s o n th e
surfac e o f in fec ted c ells an d frustrate in sp ec tio n by
c y to to x ic T c ells. H o w ev er, th e absen c e o f surfac e
c lass I m o lec ules c o uld also alert N K c ells an d trig g er
c y to ly sis by th ese im m un e effec to r c ells.
If the specificity of the viral US2 and US11 proteins
w as su ch that those M H C class I m olecu les
presenting viral antig ens (m ostly H L A -A and -B
alleles) are preferentially affected, these US proteins
cou ld contrib u te to an escape of H C M V -infected cells
from b oth T cell and N K cell k illing . In this thesis, w e
investig ated w hether US2 and US11 indeed display
selectivity in their dow n-reg u lation of M H C class I
locu s produ cts (C hapters 2-4 ).
A lthou g h at first sig ht the m ode of action of US2 and
US11 seem s very sim ilar, it is u nlik ely that H C M V
encodes tw o different proteins w ith identical fu nction.
W e evalu ated if US2 and US11 cou ld act
com plem entary to each other. T hey cou ld do so in
variou s w ays, e.g . b y targ eting different M H C class I
su b sets for deg radation, b y u sing different pathw ays
for deg radation of class I m olecu les, or b y acting at
different stag es in the folding and assem b ly process
of M H C class I m olecu les. T herefore, w e evalu ated
several of these aspects.
F irst of all, w e ex plored the stru ctu ral req u irem ents of
M H C class I m olecu les to b ecom e targ ets for US2
(C hapters 2 and 4 ) or US11 (C hapters 3 and 4 ) u sing
variou s chim eric and m u tant H L A class I m olecu les.
Ub iq u itin serves as an earm ark to targ et proteins for
proteasom al deg radation and it cou ld also provide a
handle for pu lling E R protein into the cytosol.
T herefore, w e also assessed the role of the u b iq u itin
system in US11-m ediated dislocation of M H C class I
m olecu les (C hapter 5 ).
F inally, w e evalu ated w hether US2 and US11 can act
in early stag es of M H C class I folding and assem b ly,
b y targ eting u nassem b led H C s for deg radation
(C hapter 6 ).
References
1. K u b y, J . Immunology, 3rd ed. W .H . F reem an& C o., N ew Y ork . 2. P arren, P . W . and D . R . B u rton. 20 0 1. T he antiviral activity of
antib odies in vitro and in vivo. A dv .Immunol. 7 7 :19 5 -26 2. 3 . Z ink ernag el, R . M . and P . C . D oherty. 19 7 4 . Im m u nolog ical
su rveillance ag ainst altered self com ponents b y sensitised T lym phocytes in lym phocytic choriom ening itis. N a ture 25 1:5 4 7 -5 4 8 .
4 . M cM ichael, A . J ., A . T ing , H . J . Z w eerink , and B . A . A sk onas. 19 7 7 . H L A restriction of cell-m ediated lysis of influ enz a viru s-infected hu m an cells. N a ture 27 0 :5 24 -5 26 .
5 . B evan, M . J . 20 0 4 . H elping the C D 8 (+ ) T -cell response. N a t.R ev .Immunol. 4 :5 9 5 -6 0 2.
6 . A ck erm an, A . L . and P . C ressw ell. 20 0 4 . C ellu lar m echanism s g overning cross-presentation of ex og enou s antig ens. N a t.Immunol. 5 :6 7 8 -6 8 4 .
7 . G rom m e, M . and J . N eefjes. 20 0 2. A ntig en deg radation or presentation b y M H C class I m olecu les via classical and non-classical pathw ays. M ol.Immunol. 3 9 :18 1-20 2.
8 . H eath, W . R ., G . T . B elz , G . M . B ehrens, C . M . Sm ith, S. P . F orehan, I. A . P arish, G . M . D avey, N . S. W ilson, F . R . C arb one, and J . A . V illadang os. 20 0 4 . C ross-presentation, dendritic cell su b sets, and the g eneration of im m u nity to cellu lar antig ens. Immunol.R ev . 19 9 :9 -26 .
9 . P alu dan, C ., D . Schm id, M . L andthaler, M . V ock erodt, D . K u b e, T . T u schl, and C . M u nz . 20 0 5 . E ndog enou s M H C class II processing of a viral nu clear antig en after au tophag y. S c ienc e 3 0 7 :5 9 3 -5 9 6 .
10 . L ju ng m an, P . 19 9 6 . C ytom eg aloviru s infections in transplant patients. S c a nd.J .Infec t.D is .S up p l 10 0 :5 9 -6 3 .
11. R am say, M . E ., E . M iller, and C . S. P eck ham . 19 9 1. O u tcom e of confirm ed sym ptom atic cong enital cytom eg aloviru s infection. A rc h .D is .C h ild 6 6 :10 6 8 -10 6 9 .
12. A dler, S. P . 19 9 2. C ytom eg aloviru s and preg nancy. C urr.O p in.O b s tet.G ynec ol. 4 :6 7 0 -6 7 5 .
13 . Ib anez , C . E ., R . Schrier, P . G haz al, C . W iley, and J . A . N elson. 19 9 1. H u m an cytom eg aloviru s produ ctively infects prim ary differentiated m acrophag es. J .V irol. 6 5 :6 5 8 1-6 5 8 8 . 14 . Sinz g er, C ., M . K ahl, K . L aib , K . K ling el, P . R ieg er, B . P lachter,
and G . J ahn. 20 0 0 . T ropism of hu m an cytom eg aloviru s for endothelial cells is determ ined b y a post-entry step dependent on efficient translocation to the nu cleu s. J .G en.V irol. 8 1:3 0 21-3 0 21-3 5 .
15 . N ow lin, D . M ., N . R . C ooper, and T . C om pton. 19 9 1. E x pression of a hu m an cytom eg aloviru s receptor correlates w ith infectib ility of cells. J .V irol. 6 5 :3 114 -3 121.
16 . P ass, R . F . 20 0 1. C ytom eg aloviru s. In F ields V irology, V ol. 4 . D . M . K nipe and P . M . H ow ley, eds. L ippincott-R aven, pp. 26 7 5 -27 0 5 .
17 . B orysiew icz , L . K ., S. G raham , J . K . H ick ling , P . D . M ason, and J . G . Sissons. 19 8 8 . H u m an cytom eg aloviru s-specific cytotox ic T cells: their precu rsor freq u ency and stag e specificity. E ur.J .Immunol. 18 :26 9 -27 5 .
18 . F ish, K . N ., C . Soderb erg -N au cler, L . K . M ills, S. Steng lein, and J . A . N elson. 19 9 8 . H u m an cytom eg aloviru s persistently infects aortic endothelial cells. J .V irol. 7 2:5 6 6 1-5 6 6 8 . 19 . F ish, K . N ., A . S. D epto, A . V . M oses, W . B ritt, and J . A .
N elson. 19 9 5 . G row th k inetics of hu m an cytom eg aloviru s are altered in m onocytederived m acrophag es. J .V irol. 6 9 :3 7 3 7 -3 7 4 -3 .
20 . R ob ain, M ., N . C arre, E . D u ssaix , D . Salm on-C eron, and L . M eyer. 19 9 8 . Incidence and sex u al risk factors of cytom eg aloviru s seroconversion in H IV -infected su b jects. T he SE R O C O Stu dy G rou p. S ex T ra ns m.D is . 25 :4 7 6 -4 8 0 . 21. M u rphy, E ., D . Y u , J . G rim w ood, J . Schm u tz , M . D ick son, M .
A . J arvis, G . H ahn, J . A . N elson, R . M . M yers, and T . E . Shenk . 20 0 3 . C oding potential of lab oratory and clinical strains of hu m an cytom eg aloviru s. P roc .N a tl.A c a d.S c i.U .S .A 10 0 :14 9 7 6 -14 9 8 1.
22. C ha, T . A ., E . T om , G . W . K em b le, G . M . D u k e, E . S. M ocarsk i, and R . R . Spaete. 19 9 6 . H u m an cytom eg aloviru s clinical isolates carry at least 19 g enes not fou nd in lab oratory strains. J .V irol. 7 0 :7 8 -8 3 .
23 . B odag hi, B ., T . R . J ones, D . Z ipeto, C . V ita, L . Su n, L . L au rent, F . A renz ana-Seisdedos, J . L . V ireliz ier, and S. M ichelson. 19 9 8 . C hem ok ine seq u estration b y viral chem oreceptors as a novel viral escape strateg y: w ithdraw al of chem ok ines from the environm ent of cytom eg aloviru s-infected cells. J .E x p .M ed. 18 8 :8 5 5 -8 6 6 .
25. Furman, M. H., N. Dey, D. Tortorella, and H. L. Ploegh. 2002. The human cytomegalovirus US10 gene product delays trafficking of major histocompatibility complex class I molecules. J.Virol. 76:11753-11756.
26. Goldmacher, V. S., L. M. Bartle, A. Skaletskaya, C. A. Dionne, N. L. Kedersha, C. A. Vater, J. W. Han, R. J. Lutz, S. Watanabe, E. D. Cahir McFarland, E. D. Kieff, E. S. Mocarski, and T. Chittenden. 1999. A cytomegalovirus-encoded mitochondria-localized inhibitor of apoptosis structurally unrelated to Bcl-2. Proc.Natl.Acad.Sci.U.S.A 96:12536-12541. 27. Hegde, N. R., R. A. Tomazin, T. W. Wisner, C. Dunn, J. M. Boname, D. M. Lewinsohn, and D. C. Johnson. 2002. Inhibition of HLA-DR assembly, transport, and loading by human cytomegalovirus glycoprotein US3: a novel mechanism for evading major histocompatibility complex class II antigen presentation. J.Virol. 76:10929-10941.
28. Hegde, N. R. and D. C. Johnson. 2003. Human cytomegalovirus US2 causes similar effects on both major histocompatibility complex class I and II proteins in epithelial and glial cells. J.Virol. 77:9287-9294.
29. Hengel, H., J. O. Koopmann, T. Flohr, W. Muranyi, E. Goulmy, G. J. Hammerling, U. H. Koszinowski, and F. Momburg. 1997. A viral ER-resident glycoprotein inactivates the MHC-encoded peptide transporter. Immunity. 6:623-632.
30. Jones, T. R., E. J. Wiertz, L. Sun, K. N. Fish, J. A. Nelson, and H. L. Ploegh. 1996. Human cytomegalovirus US3 impairs transport and maturation of major histocompatibility complex class I heavy chains. Proc.Natl.Acad.Sci.U.S.A 93:11327-11333.
31. Jones, T. R. and L. Sun. 1997. Human cytomegalovirus US2 destabilizes major histocompatibility complex class I heavy chains. J.Virol. 71:2970-2979.
32. Kotenko, S. V., S. Saccani, L. S. Izotova, O. V. Mirochnitchenko, and S. Pestka. 2000. Human cytomegalovirus harbors its own unique IL-10 homolog (cmvIL-10). Proc.Natl.Acad.Sci.U.S.A 97:1695-1700. 33. Lehner, P. J., J. T. Karttunen, G. W. Wilkinson, and P.
Cresswell. 1997. The human cytomegalovirus US6 glycoprotein inhibits transporter associated with antigen processing-dependent peptide translocation. Proc.Natl.Acad.Sci.U.S.A 94:6904-6909.
34. Orange, J. S., M. S. Fassett, L. A. Koopman, J. E. Boyson, and J. L. Strominger. 2002. Viral evasion of natural killer cells. Nat.Immunol. 3:1006-1012.
35. Skaletskaya, A., L. M. Bartle, T. Chittenden, A. L. McCormick, E. S. Mocarski, and V. S. Goldmacher. 2001. A cytomegalovirus-encoded inhibitor of apoptosis that suppresses caspase-8 activation. Proc.Natl.Acad.Sci.U.S.A 98:7829-7834.
36. Tomasec, P., E. C. Wang, A. J. Davison, B. Vojtesek, M. Armstrong, C. Griffin, B. P. McSharry, R. J. Morris, S. Llewellyn-Lacey, C. Rickards, A. Nomoto, C. Sinzger, and G. W. Wilkinson. 2005. Downregulation of natural killer cell-activating ligand CD155 by human cytomegalovirus UL141. Nat.Immunol. 6:181-188.
37. Wiertz, E. J., T. R. Jones, L. Sun, M. Bogyo, H. J. Geuze, and H. L. Ploegh. 1996. The human cytomegalovirus US11 gene product dislocates MHC class I heavy chains from the endoplasmic reticulum to the cytosol. Cell 84:769-779. 38. Biron, C. A., K. S. Byron, and J. L. Sullivan. 1989. Severe
herpesvirus infections in an adolescent without natural killer cells. N.Engl.J.Med. 320:1731-1735.
39. Biron, C. A. 1997. Activation and function of natural killer cell responses during viral infections. Curr.Opin.Immunol. 9:24-34. 40. Kern, F., T. Bunde, N. Faulhaber, F. Kiecker, E. Khatamzas, I.
M. Rudawski, A. Pruss, J. W. Gratama, R. Volkmer-Engert, R. Ewert, P. Reinke, H. D. Volk, and L. J. Picker. 2002. Cytomegalovirus (CMV) phosphoprotein 65 makes a large
contribution to shaping the T cell repertoire in CMV-exposed individuals. J.Infect.Dis. 185:1709-1716.
41. Landini, M. P., M. X . Guan, G. Jahn, W. Lindenmaier, M. Mach, A. Ripalti, A. Necker, T. Lazzarotto, and B. Plachter. 1990. Large-scale screening of human sera with cytomegalovirus recombinant antigens. J.Clin.Microbiol. 28:1375-1379.
42. Nastke, M. D., L. Herrgen, S. Walter, D. Wernet, H. G. Rammensee, and S. Stevanovic. 2005. Major contribution of codominant CD8 and CD4 T cell epitopes to the human cytomegalovirus-specific T cell repertoire. Cell Mol.L ife Sci. 62:77-86.
43. Walter, E. A., P. D. Greenberg, M. J. Gilbert, R. J. Finch, K. S. Watanabe, E. D. Thomas, and S. R. Riddell. 1995. Reconstitution of cellular immunity against cytomegalovirus in recipients of allogeneic bone marrow by transfer of T-cell clones from the donor. N.Engl.J.Med. 333:1038-1044. 44. Wills, M. R., A. J. Carmichael, K. Mynard, X . Jin, M. P.
Weekes, B. Plachter, and J. G. Sissons. 1996. The human cytotoxic T-lymphocyte (CTL) response to cytomegalovirus is dominated by structural protein pp65: frequency, specificity, and T-cell receptor usage of pp65-specific CTL. J.Virol. 70:7569-7579.
45. Nossner, E. and P. Parham. 1995. Species-specific differences in chaperone interaction of human and mouse major histocompatibility complex class I molecules. J.Exp.Med. 181:327-337.
46. Degen, E. and D. B. Williams. 1991. Participation of a novel 88-kD protein in the biogenesis of murine class I histocompatibility molecules. J.Cell B iol. 112:1099-1115. 47. Galvin, K., S. Krishna, F. Ponchel, M. Frohlich, D. E.
Cummings, R. Carlson, J. R. Wands, K. J. Isselbacher, S. Pillai, and M. Ozturk. 1992. The major histocompatibility complex class I antigen-binding protein p88 is the product of the calnexin gene. Proc.Natl.Acad.Sci.U.S.A 89:8452-8456. 48. Leitzgen, K. and I. G. Haas. 1998. Protein maturation in the
ER. In Chemtracts, B iochemistry and Molecular B iology., pp. 423-445.
49. Gething, M. J., S. Blond-Elguindi, K. Mori, and J. F. Sambrook. 1994. Structure, function and regulation of the ER chaperone BiP. In The biology of heat shock proteins and molecular chaperones. Cold Spring Harbor Press, New York, pp. 115-135.
50. Hammond, C., I. Braakman, and A. Helenius. 1994. Role of N-linked oligosaccharide recognition, glucose trimming, and calnexin in glycoprotein folding and quality control. Proc.Natl.Acad.Sci.U.S.A 91:913-917.
51. Ellgaard, L. and A. Helenius. 2003. Q uality control in the endoplasmic reticulum. Nat.Rev.Mol.Cell B iol. 4:181-191. 52. Rajagopalan, S. and M. B. Brenner. 1994. Calnexin retains
unassembled major histocompatibility complex class I free heavy chains in the endoplasmic reticulum. J.Exp.Med. 180:407-412.
53. Sugita, M. and M. B. Brenner. 1994. An unstable beta 2-microglobulin: major histocompatibility complex class I heavy chain intermediate dissociates from calnexin and then is stabilized by binding peptide. J.Exp.Med. 180:2163-2171. 54. Walker, K. W. and H. F. Gilbert. 1997. Scanning and escape
during protein-disulfide isomerase-assisted protein folding. J.B iol.Chem. 272:8845-8848.
55. Ellgaard, L. and A. Helenius. 2001. ER quality control: towards an understanding at the molecular level. Curr.Opin.Cell B iol. 13:431-437.
56. High, S., F. J. Lecomte, S. J. Russell, B. M. Abell, and J. D. Oliver. 2000. Glycoprotein folding in the endoplasmic reticulum: a tale of three chaperones? FEB S L ett. 476:38-41. 57. Sadasivan, B., P. J. Lehner, B. Ortmann, T. Spies, and P.
glycoprotein, tapasin, in the interaction of MHC class I molecules with TAP. Immunity. 5:103-114.
58. Solheim, J. C., M. R. Harris, C. S. Kindle, and T. H. Hansen. 1997. Prominence of beta 2-microglobulin, class I heavy chain conformation, and tapasin in the interactions of class I heavy chain with calreticulin and the transporter associated with antigen processing. J.Immunol. 158:2236-2241.
59. Peterson, J. R., A. Ora, P. N. Van, and A. Helenius. 1995. Transient, lectin-like association of calreticulin with folding intermediates of cellular and viral glycoproteins. Mol.Biol.Cell 6:1173-1184.
60. van Leeuwen, J. E. and K. P. Kearse. 1996. Deglucosylation of N-linked glycans is an important step in the dissociation of calreticulin-class I-TAP complexes. Proc.Natl.Acad.Sci.U.S.A 93:13997-14001.
61. Dick, T. B. 2004. Assembly of MHC class I peptide complexes from the perspective of disulfide bond formation. Cell Mol.Life Sci. 61:547-556.
62. Serwold, T., F. Gonzalez, J. Kim, R. Jacob, and N. Shastri. 2002. ERAAP customizes peptides for MHC class I molecules in the endoplasmic reticulum. Nature 419:480-483. 63. York, I. A., S. C. Chang, T. Saric, J. A. Keys, J. M. Favreau, A.
L. Goldberg, and K. L. Rock. 2002. The ER aminopeptidase ERAP1 enhances or limits antigen presentation by trimming epitopes to 8-9 residues. Nat.Immunol. 3:1177-1184. 64. Harris, M. R., Y. Y. Yu, C. S. Kindle, T. H. Hansen, and J. C.
Solheim. 1998. Calreticulin and calnexin interact with different protein and glycan determinants during the assembly of MHC class I. J.Immunol. 160:5404-5409.
65. Robinson, J., M. J. Waller, P. Parham, N. de Groot, R. Bontrop, L. J. Kennedy, P. Stoehr, and S. G. Marsh. 2003. IMGT/HLA and IMGT/MHC: sequence databases for the study of the major histocompatibility complex. Nucleic Acids Res. 31:311-314.
66. Crisa, L., M. T. McMaster, J. K. Ishii, S. J. Fisher, and D. R. Salomon. 1997. Identification of a thymic epithelial cell subset sharing expression of the class Ib HLA-G molecule with fetal trophoblasts. J.Exp.Med. 186:289-298.
67. Schust, D. J., D. Tortorella, and H. L. Ploegh. 1999. HLA-G and HLA-C at the feto-maternal interface: lessons learned from pathogenic viruses. Semin.Cancer Biol. 9:37-46. 68. Johnson, D. R. 2000. Differential expression of human major
histocompatibility class I loci: HLA-A, -B, and -C. H um.Immunol. 61:389-396.
69. Johnson, D. R. 2003. Locus-specific constitutive and cytokine-induced HLA class I gene expression. J.Immunol. 170:1894-1902.
70. Gobin, S. J., M. van Zutphen, A. M. Woltman, and P. J. van den Elsen. 1999. Transactivation of classical and nonclassical HLA class I genes through the IFN-stimulated response element. J.Immunol. 163:1428-1434.
71. Tibensky, D. and T. L. Delovitch. 1990. Promoter region of HLA-C genes: regulatory elements common to and different from those of HLA-A and HLA-B genes. Immunogenetics 32:210-213.
72. Gobin, S. J. and P. J. van den Elsen. 2000. Transcriptional regulation of the MHC class Ib genes HLA-E, HLA-F, and HLA-G. H um.Immunol. 61:1102-1107.
73. Moreau, P., F. Adrian-Cabestre, C. Menier, V. Guiard, L. Gourand, J. Dausset, E. D. Carosella, and P. Paul. 1999. IL-10 selectively induces HLA-G expression in human trophoblasts and monocytes. Int.Immunol. 11:803-811.
74. Le Bouteiller, P. 1994. HLA class I chromosomal region, genes, and products: facts and questions. Crit Rev.Immunol. 14:89-129.
75. McCutcheon, J. A., J. Gumperz, K. D. Smith, C. T. Lutz, and P. Parham. 1995. Low HLA-C expression at cell surfaces
correlates with increased turnover of heavy chain mRNA. J.Exp.Med. 181:2085-2095.
76. Snary, D., C. J. Barnstable, W. F. Bodmer, and M. J. Crumpton. 1977. Molecular structure of human histocompatibility antigens: the HLA-C series. Eur.J.Immunol. 7:580-585.
77. Neefjes, J. J. and H. L. Ploegh. 1988. Allele and locus-specific differences in cell surface expression and the association of HLA class I heavy chain with beta 2-microglobulin: differential effects of inhibition of glycosylation on class I subunit association. Eur.J.Immunol. 18:801-810.
78. Neisig, A., J. Roelse, A. J. Sijts, F. Ossendorp, M. C. Feltkamp, W. M. Kast, C. J. Melief, and J. J. Neefjes. 1995. Major differences in transporter associated with antigen presentation (TAP)-dependent translocation of MHC class I-presentable peptides and the effect of flanking sequences. J.Immunol. 154:1273-1279.
79. Lewis, J. W., A. Neisig, J. Neefjes, and T. Elliott. 1996. Point mutations in the alpha 2 domain of HLA-A2.1 define a functionally relevant interaction with TAP. Curr.Biol. 6:873-883. 80. Neisig, A., R. Wubbolts, X. Zang, C. Melief, and J. Neefjes. 1996. Allele-specific differences in the interaction of MHC class I molecules with transporters associated with antigen processing. J.Immunol. 156:3196-3206.
81. Park, B., S. Lee, E. Kim, and K. Ahn. 2003. A single polymorphic residue within the peptide-binding cleft of MHC class I molecules determines spectrum of tapasin dependence. J.Immunol. 170:961-968.
82. Peh, C. A., S. R. Burrows, M. Barnden, R. Khanna, P. Cresswell, D. J. Moss, and J. McCluskey. 1998. HLA-B27-restricted antigen presentation in the absence of tapasin reveals polymorphism in mechanisms of HLA class I peptide loading. Immunity. 8:531-542.
83. Turnquist, H. R., H. J. Thomas, K. R. Prilliman, C. T. Lutz, W. H. Hildebrand, and J. C. Solheim. 2000. HLA-B polymorphism affects interactions with multiple endoplasmic reticulum proteins. Eur.J.Immunol. 30:3021-3028.
84. Turnquist, H. R., E. L. Schenk, M. M. McIlhaney, H. D. Hickman, W. H. Hildebrand, and J. C. Solheim. 2002. Disparate binding of chaperone proteins by HLA-A subtypes. Immunogenetics 53:830-834.
85. Park, B., S. Lee, E. Kim, S. Chang, M. Jin, and K. Ahn. 2001. The truncated cytoplasmic tail of HLA-G serves a quality-control function in post-ER compartments. Immunity. 15:213-224.
86. Falk, K., O. Rotzschke, B. Grahovac, D. Schendel, S. Stevanovic, V. Gnau, G. Jung, J. L. Strominger, and H. G. Rammensee. 1993. Allele-specific peptide ligand motifs of HLA-C molecules. Proc.Natl.Acad.Sci.U.S.A 90:12005-12009. 87. Neefjes, J., E. Gottfried, J. Roelse, M. Gromme, R. Obst, G. J.
Hammerling, and F. Momburg. 1995. Analysis of the fine specificity of rat, mouse and human TAP peptide transporters. Eur.J.Immunol. 25:1133-1136.
88. Sidney, J., M. F. del Guercio, S. Southwood, V. H. Engelhard, E. Appella, H. G. Rammensee, K. Falk, O. Rotzschke, M. Takiguchi, R. T. Kubo, and . 1995. Several HLA alleles share overlapping peptide specificities. J.Immunol. 154:247-259. 89. Diehl, M., C. Munz, W. Keilholz, S. Stevanovic, N. Holmes, Y.
W. Loke, and H. G. Rammensee. 1996. Nonclassical HLA-G molecules are classical peptide presenters. Curr.Biol. 6:305-314.
90. Lee, N., A. R. Malacko, A. Ishitani, M. C. Chen, J. Bajorath, H. Marquardt, and D. E. Geraghty. 1995. The membrane-bound and soluble forms of HLA-G bind identical sets of endogenous peptides but differ with respect to TAP association. Immunity. 3:591-600.
