Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy

Abstract

Sepsis - which is a severe life-threatening infection with organ dysfunction - initiates a complex interplay of host pro-inflammatory and anti-inflammatory processes. Sepsis can be considered a race to the death between the pathogens and the host immune system, and it is the proper balance between the often competing pro- and anti-inflammatory pathways that determines the fate of the individual. Although the field of sepsis research has witnessed the failure of many highly touted clinical trials, a better understanding of the pathophysiological basis of the disorder and the mechanisms responsible for the associated pro- and anti-inflammatory responses provides a novel approach for treating this highly lethal condition. Biomarker-guided immunotherapy that is administered to patients at the proper immune phase of sepsis is potentially a major advance in the treatment of sepsis and in the field of infectious disease.

Figure 1. Competing theories of the host immune response in sepsis

(Theory #1): Recent studies show that activation of both pro- and anti-inflammatory immune responses occurs promptly after sepsis onset. Cells of the innate immune system including monocytes and neutrophils release large amounts of pro-inflammatory cytokines that drive inflammation (blue line – days 1–3). The intensity of the initial inflammatory response varies in individual patients, depending on multiple factors, including pathogen load and virulence, patient co-morbidities and host genetic factors. Early deaths in sepsis (top red line – day 3) are typically due to a hyper-inflammatory “cytokine storm” response with fever, refractory shock, acidosis and hyper-catabolism. An example of this scenario would be a young patient dying of toxic shock syndrome or meningococcemia. Most patients have restoration of innate and adaptive immunity and survive the infection (recovery – day 6). If sepsis persists, failure of critical elements of both innate and adaptive immune system occurs such that patients enter a profound immunosuppressive state (blue and red lines –after day 6). Deaths are due to an inability of the patient to clear infections and development of secondary infections. (Theory #2): A competing theory of sepsis agrees that there is an early activation of innate immunity and suppression of adaptive immunity. This theory holds that deaths in sepsis are due to persistent activation of the innate immunity with resultant intractable inflammation and organ injury. According to this theory, late deaths in sepsis are due to persistent underlying innate immune-driven inflammation.

Figure 2. Immunohistochemical staining for CD4⁺ T cells, CD8⁺ T cells, and HLA-DR in spleens from septic or trauma patients

Spleens from patients with sepsis or patients requiring splenectomy for traumatic injury were obtained and underwent immunostaining for CD4⁺ or CD8⁺ T cells or HLA-DR. Sepsis induces apoptotic death of splenocytes including CD4⁺ and CD8⁺ T cells leading to profound depletion of these critical immune effector cells. Note the loss in periarteriolar CD4⁺ and CD8⁺ T cells (stained a brown color) in spleens from septic versus trauma patients (200X). The loss in CD4⁺ T cells in patients with sepsis is frequently as severe as occurs in patients with AIDS. The number of HLA-DR⁺ cells (predominantly B cells) and the levels of HLA-DR expression (as determined by the intensity of the staining) are both decreased in septic versus trauma patients. In contrast to the spleen from trauma patients, the spleen from septic patients also demonstrated an increase in the expression of HLA-DR by capillary endothelial cells.

Figure 3. Impact of sepsis on innate and adaptive immunity

a)Sepsis has diverse and profound effects on all cellular elements comprising the innate immune system. Sepsis rapidly triggers extensive apoptosis in dendritic cells, monocytes and immature macrophages, natural killer (NK) cells, and myeloid derived suppressor cells (MDSCs). Conversely, sepsis delays neutrophil apoptosis, a result thought to be secondary to the mechanisms of neutrophil activation. After initial mobilization and activation of neutrophils, subsequent neutrophils that are released from bone marrow have lower bactericidal functions and decreased cytokine production. Recent data show that a subset of neutrophils release large amounts of the immunosuppressive cytokine interleukin-10 (IL-10). Decreased HLA-DR expression on antigen presenting cells including monocyte/macrophages and dendritic cells is a hallmark of sepsis, which may impair the optimal presentation of microbial antigens to T cells. b) Sepsis causes massive loss of CD4⁺ and CD8⁺ T cells as well as B cells. T regulatory (Treg) cells are more resistant to sepsis-induced apoptosis and consequently, there is an increased percentage of TReg cells in the circulation relative to the other lymphocyte subsets. This contributes on a more immunosuppressive phenotype. Surviving CD4⁺ and CD8⁺ T cells have either a shift from a pro-inflammatory Th1 cell to an anti-inflammatory Th2 cell phenotype or develop an “exhaustive” phenotype characterized by increased programmed cell death-1 expression and reduced cytokine secretion. CD4⁺ T cells have decreased expression of CD28 and reduced T cell receptor (TCR) diversity, which both likely contributing to the impaired anti-microbial response to invading pathogens.

Figure 4. Immunotherapy of sepsis

There are several immunotherapeutic agents that have shown promise in reversing the immunosuppressive phase of sepsis including recombinant interleukin-7 (IL-7), programmed cell death 1 (PD1)- or PDL1-specific antibodies, recombinant interferon-γ (IFNγ) and recombinant granulocyte/macrophage colony-stimulating factor (GM-CSF). GM-CSF and IFNγ act primarily on monocytes and macrophages to increase HLA-DR expression and induce activation. IL-7 and PD1-specific antibodies have the advantage of targeting CD4⁺ and CD8⁺ T cells to restore the function of adaptive immune system. By reengaging CD4⁺ T cells, both recombinant IL-7 and PD1-specific antibodies will have effects not only on adaptive immune cells but indirectly on monocytes and macrophages. PD1—and PDL1-specific antibodies will prevent and/or reverse T cell exhaustion. Thus, the net effects of these antibodies will be to prevent decreased interferon-γ (IFNγ) production, T cell apoptosis and decreased CD8⁺ T cell cytotoxicity. IL-7 will block T cell apoptosis, induce T cell proliferation, increase IFNγ production by T cells, increase T cell receptor (TCR) diversity, and increase T cell trafficking by increasing the expression of cell integrins such as lymphocyte function-associated antigen 1 (LFA1). The application of immunotherapeutic agents will depend on the use of various biomarkers or tests of immune function to document that patients have entered the immunosuppressive phase of sepsis.