Introduction
Although most new therapeutic approaches to sepsis have focused on blocking the early hyper-inflammatory phase, recent studies have highlighted the profound immunosuppressive state that occurs after the initial stage of the disorder [
1‐
4]. Numerous interacting mechanisms of immunosuppression occur in sepsis, including increased T regulatory cells, increased myeloid derived suppressor cells, apoptotic depletion of immune effector cells, and a shift from a TH1 to an anergic or TH2 immune phenotype [
5‐
8]. Another recently recognized mechanism of immunosuppression in sepsis is T cell exhaustion [
3]. T cell exhaustion was first described in states of chronic viral infection with persistent high levels of antigen exposure [
9‐
11]. It is typified by the presence of T cells which have lost effector function, that is, they fail to proliferate, produce cytokines or induce cytotoxic cell death in targeted cells [
10]. Exhausted T cells also have an increased tendency to undergo apoptosis because of changes in the ratio of pro- and anti-apoptotic Bcl-2 family members. One of the contributing factors for development of T cell exhaustion is signaling by the negative co-stimulatory molecule PD-1 (CD279), a member of the B7-CD28 super family, following interaction with its ligands PD-L1 (CD274) and PD-L2 (CD273) [
9,
11‐
13]. Following T cell activation, PD-1 is promptly induced and subsequently expressed on the surface of CD4 and CD8 T cells whereupon it interacts with PD-L1 and PD-L2. PD-L1 is broadly expressed on both hematopoietic and non-hematopoietic cells and its expression is significantly up-regulated during states of inflammation such as sepsis [
11].
Although much of the focus and excitement of anti-PD-1 antibody therapy has been in the field of oncology, in which it has been demonstrated to be highly effective in inducing remissions in patients with a variety of malignancies [
14,
15], anti-PD-1 has also shown significant success in infectious disease. Multiple independent investigators have reported that blockade of the PD-1:PD-L1 pathway restores T cell effector function, increases IFN-γ production, prevents apoptosis and improves survival in various pathologic models of sepsis [
16‐
20]. The present study compared and contrasted the ability of anti-PD-1 and anti-PD-L1 antibodies to decrease apoptosis and improve effector function in leukocytes from patients with sepsis. Another goal of the study was to determine if a correlation existed between lymphocyte apoptosis and putative mediators of apoptosis including lymphocyte PD-1 and PD-L1 expression and monocyte PD-L1 expression to gain insight into possible mechanisms for apoptotic cell death and the lymphocytopenia that typically accompany sepsis.
Discussion
The present results show that blockade of either PD-1 or its ligand PD-L1 reverses two pathophysiologic hallmarks of sepsis. Anti-PD-1 and anti-PD-L1 antibodies markedly decreased sepsis-induce lymphocyte apoptosis and restored the ability of immune effector cells to produce cytokines that are essential for host immunity. These
in vitro findings in patient leukocytes strengthen the concept that blockade of the PD-1:PD-L1 pathway offers a promising new approach in the treatment of sepsis [
17,
28]. Although most previous therapeutic trials in sepsis have focused on blockade of the initial hyper-inflammatory phase, there is increased recognition that if patients survive this initial stage of the disorder, they progress to an immunosuppressive state [
4,
28‐
32]. New treatment protocols have resulted in the fact that the majority of deaths in sepsis now occur after the first four days of sepsis (the hyper-inflammatory phase) and during the immunosuppressive phase [
33]. Furthermore, microbiologic studies of patients dying of sepsis showed that over 50% of the infecting organisms were classified as opportunistic pathogens (opportunistic bacteria and fungi), a finding which is highly compatible with impaired immunity [
33]. In this setting, use of immuno-adjuvant agents including anti-PD-1 or anti-PD-L1 antibodies is a logical approach to restore host immunity and potentially improve survival.
Research into the mechanistic basis of immunosuppression in sepsis has determined that multiple overlapping etiologies exist including increased T regulatory and myeloid derived suppressor cells and apoptotic depletion of T and B cells [
5‐
8]. A relatively newly recognized etiology of immunosuppression in sepsis is T cell exhaustion. T cell exhaustion was first reported in animal models of chronic viral infection and was thought to be due to persistent exposure to high levels of antigen [
9‐
11]. Patients with sepsis often have a protracted course with primary and secondary infections, a scenario that likely includes persistent high circulating antigens thereby facilitating development of T cell exhaustion [
3,
33,
34]. A recent postmortem study of spleens and lungs obtained from patients dying of sepsis demonstrated findings highly consistent with T cell exhaustion [
3,
10]. These findings included severely depressed splenocyte cytokine production, decreased T cell IL-7 receptor (CD127) expression, and increased PD-1 and PD-L1 expression on T cells and macrophages, respectively. These postmortem studies also demonstrated that PD-L1 was highly expressed on tissue parenchymal cells, that is, on splenic endothelial and bronchial epithelial cells, thereby providing opportunity for PD-1 activation [
3]. Guignant and colleagues documented a correlation between PD-1 expression on circulating immune cells of septic patients and decreased T cell proliferative capacity, increased nosocomial infections and mortality [
35]. Zhang
et al. reported that anti-PD-1 was increased on monocytes from septic patients and that anti-PD-1 antibody decreased T cell apoptosis and improved immune effector function [
36]. A recent important study by Singh
et al. showed that
in vitro blockade of PD-1 improved T cell IFN-γ production and decreased apoptosis in patients with active infections due to
M. tuberculosis[
37]. A second major finding of these investigators was that when patients with active tuberculosis were treated with effective medication to eradicate
M. tuberculosis, the number of PD-1-expressing T cells decreased and inversely correlated with IFN-γ T-cell response against
M. tuberculosis. We believe that this work has major implications for the broader field of sepsis because of the similarities of active tuberculosis with protracted sepsis.
In addition to data that T cell exhaustion exists in patients with chronic viral infections and sepsis, there is evidence from animal studies that treatment with anti-PD-1 and anti-PD-L1 antibodies can reverse T cell dysfunction, increase pathogen clearance and improve survival. Four different investigative teams reported that blockade of the PD-1 pathway prevents apoptotic cell death, restores host immunity and decreases mortality in clinically-relevant models of bacterial and fungal sepsis [
16‐
20]. Given that T cell exhaustion is postulated to occur after chronic antigen exposure, it is somewhat surprising that anti-PD-1 and anti-PD-L1 antibodies were effective in particular animal models of sepsis even though the antibodies were administered relatively quickly after sepsis began, that is, often within the first 24 to 48 h after sepsis onset. These findings suggest either that other unidentified PD-1 mediated immunosuppressive mechanisms arise quickly after sepsis or that the term “exhaustion” should be more narrowly restricted. Some investigators prefer the term immune “reprioritization” rather than immune “exhaustion” in this setting. Despite this controversy, the present results showing that anti-PD-1 and anti-PD-L1 antibodies restore cytokine production and prevent apoptosis in immune cells from patients with sepsis are highly consistent with these animal studies and underscore their potential efficacy in clinical sepsis. The effect of anti-PD-1 and anti-PD-L1 to improve IFN-γ production by T cells may be particularly beneficial in sepsis given its ability to improve monocyte function, which is impaired in sepsis [
4,
38,
39]. A clinical trial of IFN- γ in sepsis is currently underway and is being targeted to those patients whose circulating monocytes have low HLA-DR expression, (see clinicaltrials.gov Trial number NCT01649921).
An important factor in the potential clinical utility of anti-PD-1 or anti-PD-L1 antibodies in sepsis is identifying which patients would be optimal candidates for blocking therapy. Anti-PD-1 antibody has been highly successful in a subset of patients with various types of malignancies [
14,
15]. In general, those patients whose tumors expressed PD-L1 on immunohistochemical analysis have responded to therapy with anti-PD-1 antibody. As PD-1 and PD-L1 can also be early activation markers, it is inadvisable to use these markers alone to diagnose an immunosuppressive state. Currently, patients with sepsis whose monocytes have decreased HLA-DR expression and/or patients whose LPS-stimulated whole blood response shows decreased TNF-α production are considered good candidates for immuno-stimulatory therapy [
4]. Increased CD8 T cell PD-1 expression in conjunction with these two criteria might identify patients who are good candidates for anti-PD-1 antibody in sepsis. Recent studies, as well as work from our own investigations, have shown that patients with sepsis who have a persistently low absolute lymphocyte count have a greatly increased risk of dying of sepsis [4, 6 and unpublished data]. We postulate that these patients would be ideal candidates for anti-PD-1 antibody. The positive correlation between PD-1 expression on CD4 T cells and apoptosis (Figure
9C), as well as the potent anti-apoptotic effect of anti-PD-1 suggests that anti-PD-1 would be highly advantageous in this setting by acting to increase lymphocyte numbers and function.
It is interesting to note that critically-ill non-septic patients had increased expression of PD-1 on CD4 and CD8 T cells (see Figure
1) compared to results in healthy volunteers (unpublished data). In addition to sepsis, trauma and major surgery are known to lead to a state of immunosuppression [
5,
28] and it is possible that PD-1:PD-L1 may be contributing to impaired host immunity in this setting as well. Conceivably, critically-ill non-septic patients who have persistent elevation of lymphocyte PD-1 expression and who are at high risk of infection might be candidates for therapy with anti-PD-1 antibodies to boost their immunity and prevent or ameliorate these infections.
A surprising finding was the potent effect of anti-PD-1 and anti-PD-L1 antibodies to increase production of IFN-γ in NKT cells from septic patients (Figure
6). Sepsis severely suppressed IFN-γ by NKT cells (Figure
5) and both anti-PD-1 and anti-PD-L1 increased the percent of IFN-γ positive T cells by approximately 50% in septic patients (Figure
6). Although the data on the role of NKT cells in sepsis are conflicting, recent studies indicate that NKT cells bridge the gap between innate and adaptive immunity and play an important role in response to particular classes of pathogens, including
Streptococcus pneumonia, a very common cause of community acquired pneumonia [
40]. NKT cells have also recently been shown to play an important role in regulating peritoneal macrophage phagocytic function in a murine sepsis model [
41]. Therefore, these findings, showing a potent effect of anti-PD-1 and anti-PD-L1 in patient PBMCs, are highly relevant.
Anti-PD-1 and anti-PD-L1 antibodies have had extraordinary success in cancer trials and are considered to represent a major breakthrough in the field [
42]. Anti-PD-1 antibody induced remission in approximately 20 to 25% of patients with a diversity of tumors, including malignant melanoma, renal cell cancer and non-small cell lung cancer. A remarkable feature of anti-PD-1 and anti-PD-1 therapy is the fact that some patients have durable cancer remissions that last for many months in the absence of continued therapy [
43]. Cancer and sepsis share many of the same immunosuppressive mechanisms, including increased T regulatory cells, increased myeloid derived suppressor cells, and T cell exhaustion [
4‐
8,
44]. This commonality in immune pathology in cancer and sepsis could be due to the fact that both cancer and sepsis may evolve into states of chronic low grade inflammation and persistent antigen exposure. Therefore, immunotherapy that is effective in reversing immune dysfunction in cancer might have similar effects in sepsis. This finding could explain why anti-PD-1 and anti-PD-L1 are effective in these two seemingly disparate disorders. Both anti-PD-1 and anti-PD-L1 antibodies have been well tolerated in clinical trials to date [
14,
15,
45]. Although serious autoimmune reactions can occur in patients treated with anti-PD-1 or anti-PD-L1 antibodies, these reactions are uncommon. Patients with sepsis typically may not require as prolonged a therapy with anti-PD-1/anti-PD-L1 as patients with cancer. Therefore, severe autoimmune reactions will likely be less of a problem in patients with sepsis.
Competing interests
Dr. Hotchkiss has received research laboratory funding from MedImmune, Bristol Meyers Squibb, Pfizer, Agennix, Aurigene and the National Institutes of Health grants GM055194 and GM044118. Catherine Svabek is an Associate Scientist at MedImmune. Drs. Robbins, Ulbrandt, Suzich, Blair, Patera, and Krishnan are also employees of MedImmune.
Authors’ contributions
KC helped design the studies, performed flow cytometry and analyzed data. CV and BS enrolled patients and entered data. CS, PR, NU, AP and JS helped analyze data. DR and SW entered and helped analyze data. JG helped design the studies and write the manuscript. RH, SK, JG, JS, PR, AP and WB helped design the studies, analyze data, and write the manuscript. All authors read and approved the final manuscript.