Anti-PD-L1 peptide improves survival in sepsis

BACKGROUND

Sepsis remains a leading cause of death in most intensive care units. Many deaths in sepsis are due to nosocomial infections in patients who have entered the immunosuppressive phase of the disorder. One cause of immunosuppression in sepsis is T-cell exhaustion mediated by programmed cell death-1 (PD-1) interaction with its ligand (PD-L1). Studies demonstrated that blocking the interaction of PD-1 with PD-L1 with knockout mice or inhibitory antibodies reversed T-cell dysfunction and improved sepsis survival. This study assessed the efficacy of a novel short-acting peptide (compound 8) that inhibits PD-1:PD-L1 signaling in a clinically relevant second-hit fungal sepsis model.

METHODS

Mice underwent cecal ligation and puncture to induce peritonitis. Three days later, mice received intravenous injection of Candida albicans. Forty-eight hours after Candida infection, mice were treated with compound 8 or inactive peptide. The effect of Candida infection on expression of coinhibitory molecules, PD-1, and PD-L1 were quantitated by flow cytometry on CD4+ cells, CD8+ cells, natural killer (NK) cells, and natural killer T-cells (NKT). The effect of compound 8 on survival was also examined.

RESULTS

Four days after fungal infection, PD-1 and PD-L1 expressions were markedly increased on CD4+, NK, and NKT cells in septic versus sham-operated mice (%PD-1 on CD4+, 11.9% versus 2.8%; and %PD-L1 on NKT, 14.8% versus 0.5%). Compared with control, compound 8 caused a 2-fold increase in survival from 30% to 60%, P < 0.05.

CONCLUSIONS

Compound 8 significantly improved survival in a clinically relevant immunosuppressive model of sepsis. These results support immunoadjuvant therapy targeting T-cell exhaustion in this lethal disease.

Inhibition of intestinal epithelial apoptosis improves survival in a murine model of radiation combined injury

World conditions place large populations at risk from ionizing radiation (IR) from detonation of dirty bombs or nuclear devices. In a subgroup of patients, ionizing radiation exposure would be followed by a secondary infection. The effects of radiation combined injury are potentially more lethal than either insult in isolation. The purpose of this study was to determine mechanisms of mortality and possible therapeutic targets in radiation combined injury. Mice were exposed to IR with 2.5 Gray (Gy) followed four days later by intratracheal methicillin-resistant Staphylococcus aureus (MRSA). While either IR or MRSA alone yielded 100% survival, animals with radiation combined injury had 53% survival (p = 0.01). Compared to IR or MRSA alone, mice with radiation combined injury had increased gut apoptosis, local and systemic bacterial burden, decreased splenic CD4 T cells, CD8 T cells, B cells, NK cells, and dendritic cells, and increased BAL and systemic IL-6 and G-CSF. In contrast, radiation combined injury did not alter lymphocyte apoptosis, pulmonary injury, or intestinal proliferation compared to IR or MRSA alone. In light of the synergistic increase in gut apoptosis following radiation combined injury, transgenic mice that overexpress Bcl-2 in their intestine and wild type mice were subjected to IR followed by MRSA. Bcl-2 mice had decreased gut apoptosis and improved survival compared to WT mice (92% vs. 42%; p<0.01). These data demonstrate that radiation combined injury results in significantly higher mortality than could be predicted based upon either IR or MRSA infection alone, and that preventing gut apoptosis may be a potential therapeutic target.

Blockade of the negative co-stimulatory molecules PD-1 and CTLA-4 improves survival in primary and secondary fungal sepsis

Introduction

Fungal sepsis is an increasingly common problem in intensive care unit patients.Mortality from fungal sepsis remains high despite antimicrobial therapy that is highly active against most fungal pathogens, a finding consistent with defective host immunity that is present in many patients with disseminated fungemia.One recently recognized immunologic defect that occurs in patients with sepsis is T cell "exhaustion" due to increased expression of programmed cell death -1 (PD-1).This study tested the ability of anti-PD-1 and anti-programmed cell death ligand -1 (anti-PD-L1) antagonistic antibodies to improve survival and reverse sepsis-induced immunosuppression in two mouse models of fungal sepsis.

Methods

Fungal sepsis was induced in mice using two different models of infection, that is, primary fungal sepsis and secondary fungal sepsis occurring after sub-lethal cecal ligation and puncture (CLP).Anti-PD-1 and anti-PD-L1 were administered 24 to 48 h after fungal infection and effects on survival, interferon gamma production, and MHC II expression were examined.

Results

Anti-PD-1 and anti-PD-L1 antibodies were highly effective at improving survival in primary and secondary fungal sepsis.Both antibodies reversed sepsis-induced suppression of interferon gamma and increased expression of MHC II on antigen presenting cells.Blockade of cytotoxic T-lymphocyte antigen-4 (CTLA-4), a second negative co-stimulatory molecule that is up-regulated in sepsis and acts like PD-1 to suppress T cell function, also improved survival in fungal sepsis.

Conclusions

Immuno-adjuvant therapy with anti-PD-1, anti-PD-L1 and anti-CTLA-4 antibodies reverse sepsis-induced immunosuppression and improve survival in fungal sepsis.The present results are consistent with previous studies showing that blockade of PD-1 and CTLA-4 improves survival in bacterial sepsis.Thus, immuno-adjuvant therapy represents a novel approach to sepsis and may have broad applicability in the disorder.Given the relative safety of anti-PD-1 antibody in cancer clinical trials to date, therapy with anti-PD-1 in patients with life-threatening sepsis who have demonstrable immunosuppression should be strongly considered.

Dendritic cells produce CXCL13 and participate in the development of murine small intestine lymphoid tissues

In the adult intestine, luminal microbiota induce cryptopatches to transform into isolated lymphoid follicles (ILFs), which subsequently act as sites for the generation of IgA responses. The events leading to this conversion are incompletely understood. Dendritic cells (DCs) are components of cryptopatches (CPs) and ILFs and were therefore evaluated in this process. We observed that the adult murine intestine contains clusters of DCs restricted to the CP/ILF continuum. A numerical and cell associative hierarchy in the adult intestine and a chronologic hierarchy in the neonatal intestine demonstrated that these clusters form after the coalescence of CD90+ cells to form CPs and before the influx of B220+ B lymphocytes to form ILFs. Cluster formation was dependent on lymphotoxin and the lymphotoxin beta receptor and independent of lymphocytes. The ILF DC population was distinguished from that of the lamina propria by the absence of CD4+CD11c+ cells and an increased proportion of CD11c+B220+ cells. The formation of clusters was not limited by DC numbers but was induced by luminal microbiota. Moreover, in the absence of the chemokine CXCL13, CP transformation into ILF was arrested. Furthermore, ILF DCs express CXCL13, and depletion of DCs resulted in regression of ILFs and disorganization of CPs. These results reveal DC participation in ILF transformation and maintenance and suggest that in part this may be due to CXCL13 production by these cells.

IL-7 promotes T cell viability, trafficking, and functionality and improves survival in sepsis

Sepsis is a highly lethal disorder characterized by widespread apoptosis-induced depletion of immune cells and the development of a profound immunosuppressive state. IL-7 is a potent antiapoptotic cytokine that enhances immune effector cell function and is essential for lymphocyte survival. In this study, recombinant human IL-7 (rhIL-7) efficacy and potential mechanisms of action were tested in a murine peritonitis model. Studies at two independent laboratories showed that rhIL-7 markedly improved host survival, blocked apoptosis of CD4 and CD8 T cells, restored IFN-gamma production, and improved immune effector cell recruitment to the infected site. Importantly, rhIL-7 also prevented a hallmark of sepsis (i.e., the loss of delayed-type hypersensitivity), which is an IFN-gamma- and T cell-dependent response. Mechanistically, rhIL-7 significantly increased the expression of the leukocyte adhesion markers LFA-1 and VLA-4, consistent with its ability to improve leukocyte function and trafficking to the infectious focus. rhIL-7 also increased the expression of CD8. The potent antiapoptotic effect of rhIL-7 was due to increased Bcl-2, as well as to a dramatic decrease in sepsis-induced PUMA, a heretofore unreported effect of IL-7. If additional animal studies support its efficacy in sepsis and if current clinical trials continue to confirm its safety in diverse settings, rhIL-7 should be strongly considered for clinical trials in sepsis.

Sepsis-induced human lymphocyte apoptosis and cytokine production in "humanized" mice

Sepsis is the leading cause of death in critically ill patients in the United States with over 210,000 deaths annually. One stumbling block to an effective therapy of sepsis has been the lack of a clinically relevant animal model. There are important distinctions in the mouse versus human immune system regarding the host response to invading pathogens. These differences may explain the disappointing results in many sepsis clinical trials despite the clear efficacy of these agents in mouse models of sepsis. The purpose of the present study was to develop a "humanized" mouse model of sepsis and to determine if the model recapitulated the major findings of lymphocyte apoptosis and cytokine response that exist in patients with sepsis. Two-day-old NOD-scid IL2rgamma(null) mice received an adoptive transfer of hCD34(+) hematopoietic cord blood stem cells. These mice acquired a functional human innate and adaptive immune system, as evidenced by the development of all lineages of human immune cells as well as by mounting a DTH response. Eight weeks post-transfer, mice were made septic using the highly clinical relevant CLP model of sepsis, and sepsis induced marked elevations in human pro- and anti-inflammatory cytokines as well as a dramatic increase in human T and B cell apoptosis. Collectively, these results show that the humanized mouse model recapitulates many of the classic findings in patients with sepsis. Therefore, it represents an advanced, clinically relevant model for mechanistic studies of sepsis and testing of novel therapies.

Differential lymphopenia-induced homeostatic proliferation for CD4+ and CD8+ T cells following septic injury

Sepsis is a severe, life-threatening infection and a leading cause of death in hospitals. A hallmark of sepsis is the profound apoptosis-induced depletion of lymphocytes generating a lymphopenic environment. As lymphopenia can induce nonantigen-driven homeostatic proliferation (HP), we examined this process during sepsis. CD4(+) and CD8(+) T cells, which were depleted within 24 h of sepsis induction, remained at significantly reduced levels until Day 21 when normal numbers were detected. When HP was examined, naïve CD8(+) T cells proliferated between Day 7 and Day 21 post-cecal ligation and puncture, developing into memory cells with relatively few cells expressing an activation phenotype. Conversely, naïve CD4(+) T cells did not undergo HP, but proportionally higher numbers expressed activation markers. Adoptive transfer studies revealed that T cells from mice that had recovered from sepsis were not protective when transferred to naïve mice undergoing sepsis. In addition, the TCR repertoire was not skewed toward any specific Vbeta type but resembled the repertoire found in normal mice, suggesting that T cells were not primed to antigens resulting from the infection. Interestingly, depletion of endogenous CD8(+) but not CD4(+) T cells restored the ability of naive CD4(+) T cells to undergo HP, increasing the number of CD4(+) T cells with memory but not activation markers. We conclude that homeostatic control in the postseptic environment permits recovery of the T cell repertoire to normal levels without generating antigen-specific memory or aberrant T cell specificities. Restoration of homeostatic control mechanisms might be a rational therapy for this disorder.

Alpha4beta7/MAdCAM-1 interactions play an essential role in transitioning cryptopatches into isolated lymphoid follicles and a nonessential role in cryptopatch formation

The alpha(4) integrins alpha(4)beta(7) and alpha(4)beta(1), and their ligands mucosal vascular addressin cell adhesion molecule 1 (MAdCAM-1) and VCAM-1, have diverse functions, including roles in the formation of secondary lymphoid tissues at early time points during the colonization and clustering of the fetal lymphoid tissue inducer (LTi) cells and at later time points during the recruitment of lymphocytes. In this study, we evaluated the role of alpha(4) integrins in the development of a recently appreciated class of intestinal lymphoid tissues, isolated lymphoid follicles (ILFs). We observed that diverse ILF cellular populations express alpha(4)beta(7) and alpha(4)beta(1), including the LTi-like cells and lymphocytes, while ILF stromal cells and vessels within ILFs express VCAM-1 and MAdCAM-1, respectively. Evaluation of adult and neonatal beta(7)(-/-) mice and adult and neonatal mice given blocking Abs to alpha(4)beta(7), MAdCAM-1, or VCAM-1 did not identify a role for alpha(4) integrins in cryptopatch (CP) development; however, these studies demonstrated that alpha(4)beta(7) and MAdCAM-1 are required for the transitioning of CP into lymphoid tissues containing lymphocytes or ILFs. Competitive bone marrow transfers demonstrated that beta(7)(-/-) LTi-like cells had a reduced but not significantly impaired ability to localize to CP. Bone marrow transfers and adoptive transfers of B lymphocytes revealed that beta(7) expression by B lymphocytes was essential for their entry into the developing ILFs. These findings demonstrate an essential role for alpha(4)beta(7)/MAdCAM-1 in ILF development corresponding to the influx of beta(7)-expressing lymphocytes and a nonessential role for beta(7)-localizing LTi-like cells to the small intestine.

CC chemokine receptor 6 expression by B lymphocytes is essential for the development of isolated lymphoid follicles

Isolated lymphoid follicles (ILFs) are organized lymphoid structures that facilitate the efficient interaction of antigen, antigen-presenting cells, and lymphocytes to generate controlled adaptive immune responses within the intestine. Because CC chemokine receptor 6 (CCR6) deficiency affects the generation of mucosal immune responses, we evaluated a potential role for CCR6 in the development of ILFs. We observed that CCR6 and its ligand CCL20 are highly expressed within ILFs and that B lymphocytes are the largest CCR6-expressing population within ILFs. ILF development was profoundly arrested in the absence of CCR6. Concordant with a block in ILF development at a stage corresponding to the influx of B lymphocytes, we observed that CCR6-deficient mice had a diminished population of intestinal B lymphocytes. Bone marrow reconstitution studies demonstrated that ILF development is dependent on CCR6-sufficient B lymphocytes, and adoptive transfers demonstrated that CCR6(-/-) B lymphocytes were inefficient at localizing to intestinal lymphoid structures. Paralleling these findings, we observed that CCR6-deficient mice had a reduced proportion of Peyer's patch B lymphocytes and an associated re-duction in the number and size of Peyer's patch follicular domes. These observations define an essential role for CCR6 expression by B lymphocytes in localizing to intestinal lymphoid structures and in ILF development.

Murine isolated lymphoid follicles contain follicular B lymphocytes with a mucosal phenotype

Isolated lymphoid follicles (ILFs) are organized intestinal lymphoid structures whose formation can be induced by luminal stimuli. ILFs have been demonstrated to act as inductive sites for the generation of immune responses directed toward luminal stimuli; however, the phenotype of the immune response initiated within ILFs has largely been uninvestigated. To gain a better understanding of the immune responses initiated within ILFs, we examined phenotypic and functional aspects of the largest cellular component of the murine ILF lymphocyte population, B lymphocytes. We observed that murine ILF B lymphocytes are composed of a relatively homogenous population of follicular B-2 B lymphocytes. Consistent with their proximity to multiple stimuli, ILF B lymphocytes displayed a more activated phenotype compared with their counterparts in the spleen and Peyer's patch (PP). ILF B lymphocytes also expressed higher levels of immunomodulatory B7 and CD28 family members B7X and programmed death-1 compared with their counterparts in the spleen and PP. ILF B lymphocytes preferentially differentiate into IgA-producing plasma cells and produce more IL-4 and IL-10 and less interferon-gamma compared with their counterparts in the spleen. Immunoglobulin repertoire analysis from individual ILFs demonstrated that ILFs contain a polyclonal population of B lymphocytes. These findings indicate that murine ILFs contain a polyclonal population of follicular B-2 B lymphocytes with a phenotype similar to PP B lymphocytes and that, in unchallenged animals, ILFs promote immune responses with a homeostatic phenotype.

Adaptive immune responses are dispensable for isolated lymphoid follicle formation: antigen-naive, lymphotoxin-sufficient B lymphocytes drive the formation of mature isolated lymphoid follicles

Isolated lymphoid follicles (ILFs) are recently appreciated members of the mucosal immune system. The architecture, composition, and inducible nature of these structures indicates that these structures are tertiary lymphoid structures. The process leading to the formation of tertiary lymphoid structures, lymphoid neogenesis, has been observed in a number of inflammatory and autoimmune conditions. Given this association, there is considerable interest in identifying the factors promoting lymphoid neogenesis, and understanding the steps in this process. Using murine ILF formation as a model, we have examined the roles of different cellular sources of lymphotoxin (LT) and the adaptive immune response in lymphoid neogenesis. In this study, we report that, although other cellular sources of LT may supplant B lymphocytes in the formation of immature ILFs (loosely organized clusters of B lymphocytes), LT-sufficient B lymphocytes are required for the progression of immature ILFs to mature ILFs (organized lymphoid aggregates with a follicle-associated epithelium). ILF formation occurs in the absence of T lymphocytes and Ag-specific B lymphocyte responses, and ILF B lymphocytes express elevated levels of LT in the absence of antigenic stimulation. Consistent with a role for chemokines inducing LT expression in Ag-naive B lymphocytes, and a chemokine-driven positive-feedback loop driving mature ILF formation, mature ILFs express elevated levels of B lymphocyte chemoattractant in the absence of Ag-specific B lymphocyte stimulation. These observations indicate that ILFs contain Ag-naive lymphocytes, and suggest that events occurring within ILFs shape subsequent immune responses mediated by these lymphocytes.

Isolated lymphoid follicle formation is inducible and dependent upon lymphotoxin-sufficient B lymphocytes, lymphotoxin beta receptor, and TNF receptor I function

The gastrointestinal mucosa contains a complex network of lymphoid compartments that have evolved to efficiently protect the host from invading pathogens. Recently, an additional lymphoid structure resembling Peyer's patches (PP) in composition and architecture has been identified in the murine small intestine, the isolated lymphoid follicle (ILF). In this study we examine the nature and factors required for ILF formation. We observed a spectrum of structures fitting the previous descriptions of ILFs, ranging from clusters of B220(+) cells (which we have termed immature ILFs) to well-organized lymphoid nodules (which we have termed mature ILFs). Here we demonstrate that that similar to PP formation, ILF formation requires lymphotoxin (LT)- and LT beta receptor-dependent events. However unlike PP formation, the LT- and LT beta receptor-dependent events required for ILF formation can occur in adulthood and require LT-sufficient B lymphocytes. We demonstrate that mature ILF formation occurs in response to lumenal stimuli, including normal bacterial flora, and requires TNF receptor I function. These findings suggest that ILFs are organized intestinal lymphoid structures whose formation can be induced and whose mass can be expanded in response to mucosal challenges.

Postgestational lymphotoxin/lymphotoxin beta receptor interactions are essential for the presence of intestinal B lymphocytes

Lymphotoxin (LT), a cytokine belonging to the TNF family, has established roles in the formation of secondary lymphoid structures and in the compartmentalization of T and B lymphocyte areas of the spleen. In this study, we examine the role of LT in directing the composition of intestinal lymphocytes. We report that mice deficient in LT have a normal composition of intestinal lamina propria (LP) T lymphocytes, and an absence of intestinal LP B lymphocytes. We further refine this observation to demonstrate that the interaction of LT with the LTbetaR is essential for the presence LP B lymphocytes. The LT/LTbetaR-dependent events relevant for the presence of LP B lymphocytes occur after birth, do not require the presence of Peyer's patches, lymph nodes, or the spleen; and therefore, are distinct and independent from the previously identified roles of LT/LTbetaR. The LT-dependent signal relevant for the presence of LP B lymphocytes is optimally supplied by a LT-sufficient B lymphocyte, and requires a LTbetaR-sufficient radio-resistant, non-bone marrow-derived cell. Based upon the severity of the deficit of LP B lymphocytes we observed, these novel LT/LTbetaR-dependent events are of primary importance in directing the entry and residence of LP B lymphocytes.

Chloroquine-induced neuronal cell death is p53 and Bcl-2 family-dependent but caspase-independent

Chloroquine is a lysosomotropic agent that causes marked changes in intracellular protein processing and trafficking and extensive autophagic vacuole formation. Chloroquine may be cytotoxic and has been used as a model of lysosomal-dependent cell death. Recent studies indicate that autophagic cell death may involve Bcl-2 family members and share some features with caspase-dependent apoptotic death. To determine the molecular pathway of chloroquine-induced neuronal cell death, we examined the effects of chloroquine on primary telencephalic neuronal cultures derived from mice with targeted gene disruptions in p53, and various caspase and bcl-2 family members. In wild-type neurons, chloroquine produced concentration- and time-dependent accumulation of autophagosomes, caspase-3 activation, and cell death. Cell death was inhibited by 3-methyladenine, an inhibitor of autophagic vacuole formation, but not by Boc-Asp-FMK (BAF), a broad caspase inhibitor. Targeted gene disruptions of p53 and bax inhibited and bcl-x potentiated chloroquine-induced neuron death. Caspase-9- and caspase-3-deficient neurons were not protected from chloroquine cytotoxicity. These studies indicate that chloroquine activates a regulated cell death pathway that partially overlaps with the apoptotic cascade.

Spontaneous and continuous cyclooxygenase-2-dependent prostaglandin E2 production by stromal cells in the murine small intestine lamina propria: directing the tone of the intestinal immune response

The mechanisms allowing the gastrointestinal immune system to avoid an inappropriate inflammatory response to nonpathogenic luminal Ags are poorly understood. We have previously described a role for cyclooxygenase (COX)-2-dependent arachidonic acid metabolites produced by the murine small intestine lamina propria in controlling the immune response to a dietary Ag. To better understand the role of COX-2-dependent arachidonic acid metabolites produced by the lamina propria, we examined the pattern of expression and the cellular source of COX-2 and COX-2-dependent PGE(2). We now demonstrate that non-bone marrow-derived lamina propria stromal cells have basal COX-2 expression and that COX-2-dependent PGE(2) production by these cells is spontaneous and continuous. The other mucosal and nonmucosal lymphoid compartments examined do not share this phenotype. In contrast to the majority of descriptions of COX-2 expression, COX-2 expression by lamina propria stromal cells is not dependent upon exogenous stimuli, including adhesion, LPS signaling via Toll-like receptor 4, or the proinflammatory cytokines TNF-alpha, IFN-gamma, and IL-1 beta. These findings, in conjunction with the known immunomodulatory capacities of PGs, suggest that COX-2 expression by the small intestine lamina propria is a basal state contributing to the hyporesponsiveness of the intestinal immune response.