Adoptive immunotherapy after HLA mismatched stem cell
transplantation
Oosten, L.E.M.
Citation
Oosten, L. E. M. (2007, November 21). Adoptive immunotherapy after HLA
mismatched stem cell transplantation. Retrieved from
https://hdl.handle.net/1887/12446
Version: Corrected Publisher’s Version
License: Licence agreement concerning inclusion of doctoral
thesis in the Institutional Repository of the University
of Leiden
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123
Summary
Summary
125 Allogeneic stem cell transplantation (SCT) is a well established treatment for a
variety of hematological malignancies. After SCT, donor-derived T cells present in the stem cell graft elicit immune responses that mediate both beneficial graft-versus- leukemia effects (GvL) and detrimental graft-versus-host disease (GvHD). The best stem cell donor in terms of minimal GvHD is a human leukocyte antigen (HLA) -identical sibling, but for many patients such donors are not available. SCT across HLA mismatches requires T cell depletion to minimize GvHD. However, because T cell depletion also reduces GvL, the risk of leukemia relapse after SCT is significantly increased. Leukemia relapse after HLA-matched SCT can be treated with donor lymphocyte infusions (DLI). Unfortunately, DLI therapy is often accompanied by GvHD as the donor T cells have not been selected for preferential reactivity against the patient’s malignant cells. The use of DLI after HLA-mismatched SCT is associated with severe GvHD-related morbidity and mortality.
An attractive strategy to treat relapsed leukemia after HLA-mismatched SCT comprises adoptive immunotherapy with ex vivo generated donor T cells that display reactivity limited to the patient’s residual leukemic or hematopoietic cells. This may be achieved by employing the patient’s mismatched HLA molecules as antigen-presenting molecules for hematopoietic system-specifi c antigens to generate non-self- (allo) HLA-restricted T cells from the stem cell donor. The minor histocompatibility antigens (mHags) HA-1 and HA-2 are suitable target antigens as they display hematopoietic system-restricted tissue distribution and relevant expression on leukemic cells and their progenitors.
In theory, alloHLA-restricted mHag-specifi c cytotoxic T cells should lyse the patient’s hematopoietic cells, including leukemic cells, but not non-hematopoietic cells that do not express the relevant mHag. Likewise, graft-derived donor hematopoietic cells, which lack the appropriate HLA restriction molecule, will not be affected. Current protocols for the generation of alloHLA-restricted cytotoxic T cells are complex and associated with a high risk of adventitious expansion of broad alloHLA-specifi c T cells that are potentially harmful to the patient. In this thesis, we explored the alloHLA-reactive T cell repertoire and investigated several new strategies to generate HA-1- and HA-2-specifi c alloHLA-A2- restricted T cells.
In chapter 2, we investigated whether artifi cial antigen-presenting constructs (aAPCs) could replace professional antigen-presenting cells (APCs) for the generation of mHag-specifi c T cells. To that end, we coated cell-sized latex beads with recombinant HLA-A2/mHag complexes and the costimulatory molecules CD80 and CD54. These aAPCs effectively stimulated mHag-specifi c T cell clones in a ligand-density dependent manner and could be used to enrich polyclonal HA-1-specifi c T cell lines without affecting T cell phenotype or cytolytic activity. In addition, these aAPCs did not stimulate broad alloHLA-A2-reactive T cells (data incorporated in chapter 4). Thus, HLA-A2/mHag complex-coated aAPCs can be exploited in vitro to promote selective outgrowth of mHag-specifi c alloHLA-restricted T cells present in a pool of alloHLA-reactive T cells.
Adoptive Immunotherapy after HLA-mismatched Stem Cell Transplantation
126
In chapter 3, we examined whether APCs transduced with a transporter associated with antigen processing (TAP) -inhibiting protein could serve as antigen-specifi c stimulators for mHag-specifi c T cells. Inhibition of TAP signifi cantly reduces endogenous antigen presentation but does not impair exogenous peptide loading. APCs transduced with the herpesvirus-derived proteins US6, ICP47, or UL49.5, that are known to inhibit TAP, exhibited a stable decrease in cell surface HLA class I expression and were protected from lysis by mHag-specifi c T cells. Exogenous addition of mHag peptide fully restored target cell recognition. UL49.5 showed the most pronounced inhibitory effect, reducing cell- surface HLA class I expression and mHag-specifi c lysis by 99%. UL49.5 also signifi cantly diminished alloHLA-reactivities mediated by alloHLA-specifi c T cells. However, reduction of alloHLA-specifi c lysis was not complete. Therefore, inhibition of TAP alone does not reduce an APC’s alloHLA-antigenicity to the extent that adventitious generation of broad alloHLA-reactive T cells would be prevented.
In chapter 4, we aimed to determine whether alloHLA-restricted T cells generated after repeated stimulation with alloHLA-expressing APCs are predominantly peptide-specifi c or peptide-independent. Hereto, we generated alloHLA-A2-reactive T cell populations from HLA-A2neg donors. Peptide-specifi c alloHLA-A2 recognition was determined using a panel of HLA-A2 tetramers representing peptides that are derived from ubiquitously expressed self-proteins. Each alloHLA-A2-reactive T cell population contained distinct T cell subsets binding a single specifi c tetramer. Using the aAPC described in chapter 2, we observed that none of these T cell populations could be stimulated by aAPCs coated with HLA-A2 complexes representing irrelevant peptides, whereas aAPCs coated with a relevant HLA-A2/peptide complex provided adequate stimulation. Thus, the majority of alloHLA- A2-reactive T cell lines, generated using a conventional stimulation protocol, displayed a peptide-specifi c and not a peptide-independent recognition pattern.
In chapter 5, we describe the results of two multimer-based strategies to generate alloHLA- restricted mHag-specifi c T cells. Both methods introduced recombinant HLA-A2/mHag complexes as the sole allogeneic target antigens. In the one strategy, we explored the feasibility of generating mHag-specifi c T cells from HLA-A2neg donors using autologous HLA-A2neg dendritic cells coated with monomeric HLA-A2/mHag complexes. In the other strategy, alloHLA-A2/HA-2-specifi c T cells were directly isolated from the peripheral blood of HLA-A2neg parous females with HLA-A2pos offspring using tetrameric HLA-A2/HA-2 complexes. Both methods yielded T cells with selective tetramer-staining profi les. However, these T cells were either of low avidity or displayed selective, but not solely mHag-specifi c, cytolytic activity. Apparently, alloHLA-restricted T cells tend to recognize HLA/peptide complexes in a peptide-dependent but not peptide-specifi c manner.
In chapter 6, we discuss these fi ndings and conclude that alloHLA-recognition by T cells is inherently crossreactive. A degree of crossreactivity is required to ensure effective pathogen surveillance. AlloHLA-reactivity seems to be the (unwanted) by-product of a crossreactive T cell pool originally selected for reactivity to self-HLA molecules. Consequently, T cell crossreactivity is far more prominent in the HLA-mismatched setting than in the HLA-
Summary
127 matched setting. Unfortunately, as we show in chapter 5, currently available technologies
can not readily distinguish between crossreactive and antigen-specifi c alloHLA-restricted T cells. We therefore conclude that generation of alloHLA-restricted leukemia- or hematopoietic system antigen-specifi c T cells is as yet not feasible in a clinical setting.
