Immunotolerance during bacterial pneumonia and sepsis - Chapter 1: General introduction and outline of this thesis

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Immunotolerance during bacterial pneumonia and sepsis

Hoogerwerf, J.J.

Publication date

2010

Link to publication

Citation for published version (APA):

Hoogerwerf, J. J. (2010). Immunotolerance during bacterial pneumonia and sepsis.

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General Introduction

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Introduction

Infectious diseases are a major cause of morbidity and mortality worldwide. Massive

use of antibiotics promotes pathogen resistance and as a consequence, the incidence

of drug-resistant bacteria is increasing (WHO; The world health report 2000, Health

Systems: improving performance). Therefore, it is of the utmost importance to expand

our comprehension of host responses against invading pathogens in order to develop

new treatment strategies. This thesis focuses on the immune response against

bacteria during (nosocomial) pneumonia and sepsis.

Bacterial pneumonia

Bacterial pneumonia is one of the most common infectious diseases and the most

frequent source of sepsis

. Depending on the circumstances in which the patient

acquires pneumonia, community-acquired pneumonia can be distinguished from

hospital-acquired (nosocomial) pneumonia occurring in patients with pre-existing

conditions. The most frequent causative pathogen in community-acquired pneumonia

is Streptococcus pneumoniae

, whereas Pseudomonas aeruginosa and Klebsiella

pneumoniae are prominent bacteria causing nosocomial pneumonia

.

The host response against bacterial pneumonia

The airways are in direct contact with the outside environment and therefore

continuously exposed to respiratory pathogens. The first line of defense in the upper

respiratory tract is formed by physical mechanisms like coughing and sneezing. When

respiratory pathogens overcome these structural defenses and enter the alveolar

space, the innate immune response is primarily responsible for the elimination of

these pathogens. Upon recognition of invading pathogens, innate immune cells like

respiratory epithelial cells and resident alveolar macrophages will then orchestrate an

innate immune response leading to the secretion of cytokines, chemokines and

antimicrobial peptides

(Figure 1.1). Moreover, alveolar macrophages are able to bind

and phagocytose pathogens and subsequently kill them intracellularly. The secreted

cytokines and chemokines mediate recruitment and activation of neutrophils from the

circulation to the site of inflammation in the lung. Recruited neutrophils effectively

phagocytose and eliminate pathogens

(Figure 1.1). Besides the elimination of

pathogens, alveolar macrophages are able to phagocytose apoptotic neutrophils and

thereby contribute to the resolution of pneumonia

. Furthermore, the innate immune

response is thought to orchestrate the adaptive immune response that primarily

consists of T- and B-cell responses that provide specific memory of infection

.

Introduction

⏐

Figure 1.1 Normal (left side) and inflamed (right side) alveolus (adapted from 9

).

Recognition of pathogens by Toll-like receptors

In the alveolar space, innate immune cells distinguish potential pathogens from self,

using receptors that recognize highly conserved motifs (pathogen-associated

molecular patterns; PAMPs) on pathogens that are not found in higher eukaryotes.

The receptors recognizing these PAMPs have been termed “pattern recognition

receptors” or PRRs. Among other receptor families, Toll-like receptors (TLRs) occupy a

central position as PRRs in the initiation of cellular innate immune responses

. TLRs

are distinguished from other PRRs by their ability to recognize, but moreover, to

discriminate between different classes of pathogens. Presently, thirteen TLRs are

described, of which TLR2 and TLR4 are of great importance in bacterial pneumonia.

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TLR4 recognizes lipopolysaccharide (LPS), part of the outer membrane of

gram-negative bacteria

, whereas TLR2 recognizes lipoteichoic acid (LTA), a major

constituent of gram-positive bacteria

(Figure 1.2). Although many investigations

have been published on the effects of LPS in humans

, the human response to LTA

in vivo has never been studied. Knowledge of the effects of LTA in humans is

important considering the prominent place of gram-positive pathogens in both

community-acquired and nosocomial infections.

Figure 1.2 Overview of PAMPs as part of the membrane of gram-positive and gram-negative bacteria (adapted from 20

)

Coagulation and fibrinolysis

The acute inflammatory response is frequently accompanied by activation of

coagulation and inhibition of fibrinolysis in the bronchoalveolar space during

pneumonia

. These hemostatic effects can be considered host-protective in

containing inflammation to the site of infection

. However, procoagulant activity can

also be disadvantageous by modulating inflammatory activity, leading to excessive

activation of inflammation in the alveolar compartment during pneumonia

. LPS have

been demonstrated to reproduce the hemostatic alterations of pneumonia in the

lungs of healthy humans when administered in the airways by bronchial

instillation

. In contrast, knowledge of the hemostatic balance in inflammation

caused by gram-positive pathogens is limited.

Sepsis

Sepsis is one of the leading causes of death in the Western world and its mortality

rate remains unacceptably high between 20-40%

. Sepsis is a heterogenous clinical

Introduction

⏐

syndrome broadly defined as the systemic host response to an infection. Although any

bacterial infection can progress and cause systemic inflammation, respiratory tract

infections are the most common source for sepsis

. Furthermore, patients with

sepsis are prone to develop nosocomial infections, in particular pneumonia, which has

a large impact on outcome.

Immunotolerance in sepsis

Until recently, the high mortality rate of sepsis was thought to be the result of an

uncontrolled hyperinflammatory response of the host to an infection. However,

failure of clinical trials with anti-inflammatory strategies in sepsis patients and the

development of animal models more closely imitating clinical sepsis have led to the

reconsideration of the pathogenesis of sepsis. Sepsis is currently considered a

misbalance between hyperinflammatory responses and immunotolerance (Figure

1.3).

Figure 1.3 Misbalance of hyperinflammation and immunotolerance in the host response during sepsis (adapted from 20

).

Hyperinflammation is designed to eliminate invading pathogens, but is at the same

time responsible for tissue damage. In contrast, immunotolerance is believed to

dampen excessive inflammation and subsequent tissue damage, but may contribute

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to the susceptibility of septic patients to nosocomial infections

. Clear evidence

of immunotolerance in sepsis comes from studies showing hyporesponsiveness of

immunocompetent cells upon recurrent exposures to microbial agents or products

(often referred to as tolerance to LPS)

.

Various mechanisms are thought to contribute to immunotolerance, among which

anti-inflammatory cytokines such as interleukin (IL)-10 and transforming

growth-factor (TGF)-β. Likewise, deregulated apoptosis of lymphocytes, dendritic cells,

monocytes/macrophages and granulocytes, has been implicated to play a role in

immunotolerance

(Figure 1.3).

Alongside upregulation of anti-inflammatory mediators and deregulated apoptosis of

immune cells, inhibitors of TLRs such as MyD88 short, A20, interleukin-1

receptor-associated kinase (IRAK)-M and ST2 are thought to play a role in the immunotolerance

in septic patients

(Figure 1.3 and 1.4).

ST2

The receptor ST2 emerges as a

transmembrane variant (ST2L) and a

soluble secreted variant (sST2).

Originally described as a Th2 marker

,

several other cell-types also express

ST2 including mast cells

, eosinophils

and macrophages

. ST2L is linked to

important Th2 effector functions

,

but concomitantly, ST2L has been

shown to play an important negative

regulatory function in TLR signaling

(Figure 1.4). Therefore, ST2 is thought

to play a role in the

immuno-suppression in septic patients. Soluble

ST2 probably acts as a decoy receptor

by binding IL-33 (ligand of ST2L),

thereby inhibiting signaling by ST2L

.

Figure 1.4 Overview of extra- and intracellular Toll-like receptor regulators54

Introduction

⏐

IRAK-M

IRAK-M is an intracellular proximal inhibitor of TLR signaling expressed by epithelial

cells and macrophages in the lung. IRAK-M inhibits the IRAK-1/IRAK-4 complex and

thereby mitigates intracellular responses elicited by all MyD88 dependent receptors

.

Considering its central position in the regulation of TLR signaling and its expression in

the two most prominent resident cells in the bronchoalveolar space, IRAK-M likely

plays an important role in the host response to bacterial infection. Importantly, in

septic mice, enhanced IRAK-M expression in pulmonary macrophages resulted in a

strongly impaired host defense response during secondary (i.e. following sepsis)

Pseudomonas pneumonia, suggesting that IRAK-M contributes to immunotolerance

.

Outline of this thesis

The general aim of this thesis is to enhance our knowledge of the host response to

bacterial pneumonia and sepsis and to increase our insight into the underlying

mechanisms of immunotolerance as a feature of patients with sepsis. In the first part

we used a model of 1) lung inflammation: bronchial instillation of LTA or LPS in the

human lung in healthy volunteers, in order to mimick the pulmonary response during

gram-positive or gram-negative pneumonia respectively; and 2) lung infection:

K. pneumoniae pneumonia in mice. Chapter 2 describes the inflammatory host

response to LTA versus known LPS-induced responses in the human bronchoalveolar

space. In chapter 3 the effects of LTA on hemostasis in the human lung was described

and compared with the known hemostatic effects to LPS. In chapter 4 we investigated

the effect of in vivo LPS bronchial instillation on the responsiveness of alveolar

macrophages to further stimulation with bacterial products. Chapter 5 reports on the

role of TLR-inhibitor IRAK-M in the host response during gram-negative pneumonia. In

the next part the influence of apoptosis and TLR-inhibitor ST2 was investigated in

sepsis. Chapter 6 describes the gene expression profiles of apoptosis regulators in

purified leukocyte subsets in human sepsis. The extent of soluble ST2 (a decoy

receptor for TLR inhibitor ST2) release during human sepsis was investigated in

Chapter 7. Last, the role of ST2 in modulating host defense in the lung during sepsis

was investigated using a murine model of cecal ligation and puncture (CLP)-induced

sepsis followed by secondary challenge with intranasal P. aeruginosa (chapter 8).

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