Injury-induced suppression of effector T cell immunity requires CD1d-positive APCs and CD1d-restricted NKT cells

CD1d deficiency limits tolerogenic properties of peritoneal macrophages

Invariant natural killer T (iNKT) cells are involved in various autoimmune diseases. Although iNKT cells are arthritogenic, transforming growth factor beta (TGFβ)-treated tolerogenic peritoneal macrophages (Tol-pMφ) from wild-type (WT) mice are more tolerogenic than those from CD1d knock-out iNKT cell-deficient mice in a collagen-induced arthritis (CIA) model. The underlying mechanism by which pMφ can act as tolerogenic antigen presenting cells (APCs) is currently unclear. To determine cellular mechanisms underlying CD1d-dependent tolerogenicity of pMφ, in vitro and in vivo characteristics of pMφ were investigated. Unlike dendritic cells or splenic Mφ, pMφ from CD1d^+/− mice showed lower expression levels of costimulatory molecule CD86 and produced lower amounts of inflammatory cytokines upon lipopolysaccharide (LPS) stimulation compared to pMφ from CD1d-deficient mice. In a CIA model of CD1d-deficient mice, adoptively transferred pMφ from WT mice reduced the severity of arthritis. However, pMφ from CD1d-deficient mice were unable to reduce the severity of arthritis. Hence, the tolerogenicity of pMφ is a cell-intrinsic property that is probably confer-red by iNKT cells during pMφ development rather than by interactions of pMφ with iNKT cells during antigen presentation to cognate T cells.

Innate T cells in the intensive care unit

Rapid onset of acute inflammation is a hallmark of critical illnesses that bring patients to the intensive care unit (ICU). In critical illness, innate T cells rapidly reach full activation and drive a robust acute inflammatory response. As "cellular adjuvants," innate T cells worsen inflammation and mortality in several common critical illnesses including sepsis, ischemia-reperfusion injury, stroke, and exacerbations of respiratory disease. Interestingly, innate T cell subsets can also promote a protective and anti-inflammatory response in sepsis, ischemia-reperfusion injury, and asthma. Therapies that target innate T cells have been validated in several models of critical illness. Here, we review the role of natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells and γδ T cells in clinical and experimental critical illness.

Contribution of programmed cell death receptor (PD)-1 to Kupffer cell dysfunction in murine polymicrobial sepsis

Recent studies suggest that coinhibitory receptors appear to be important in contributing sepsis-induced immunosuppression. Our laboratory reported that mice deficient in programmed cell death receptor (PD)-1 have increased bacterial clearance and improved survival in experimental sepsis induced by cecal ligation and puncture (CLP). In response to infection, the liver clears the blood of bacteria and produces cytokines. Kupffer cells, the resident macrophages in the liver, are strategically situated to perform the above functions. However, it is not known if PD-1 expression on Kupffer cells is altered by septic stimuli, let alone if PD-1 ligation contributes to the altered microbial handling seen. Here we report that PD-1 is significantly upregulated on Kupffer cells during sepsis. PD-1-deficient septic mouse Kupffer cells displayed markedly enhanced phagocytosis and restoration of the expression of major histocompatibility complex II and CD86, but reduced CD80 expression compared with septic wild-type (WT) mouse Kupffer cells. In response to ex vivo LPS stimulation, the cytokine productive capacity of Kupffer cells derived from PD-1-/- CLP mice exhibited a marked, albeit partial, restoration of the release of IL-6, IL-12, IL-1β, monocyte chemoattractant protein-1, and IL-10 compared with septic WT mouse Kupffer cells. In addition, PD-1 gene deficiency decreased LPS-induced apoptosis of septic Kupffer cells, as indicated by decreased levels of cleaved caspase-3 and reduced terminal deoxynucleotidyl transferase dUTP nick end-labeling-positive cells. Exploring the signal pathways involved, we found that, after ex vivo LPS stimulation, septic PD-1-/- mouse Kupffer cells exhibited an increased Akt phosphorylation and a reduced p38 phosphorylation compared with septic WT mouse Kupffer cells. Together, these results indicate that PD-1 appears to play an important role in regulating the development of Kupffer cell dysfunction seen in sepsis.

Role of cellular events in the pathophysiology of sepsis

INTRODUCTION

Sepsis is a dysregulated host immune response due to an uncontrolled infection. It is a leading cause of mortality in adult intensive care units globally. When the host immune response induced against a local infection fails to contain it locally, it progresses to sepsis, severe sepsis, septic shock and death.

METHOD

Literature survey was performed on the roles of different innate and adaptive immune cells in the development and progression of sepsis. Additionally, the effects of septic changes on reprogramming of different immune cells were also summarized to prepare the manuscript.

FINDINGS

Scientific evidences to date suggest that the loss of balance between inflammatory and anti-inflammatory responses results in reprogramming of immune cell activities that lead to irreversible tissue damaging events and multi-organ failure during sepsis. Many surface receptors expressed on immune cells at various stages of sepsis have been suggested as biomarkers for sepsis diagnosis. Various immunomodulatory therapeutics, which could improve the functions of immune cells during sepsis, were shown to restore immunological homeostasis and improve survival in animal models of sepsis.

CONCLUSION

In-depth and comprehensive knowledge on the immune cell activities and their correlation with severity of sepsis will help clinicians and scientists to design effective immunomodulatory therapeutics for treating sepsis.

Prevention of NKT cell activation accelerates cutaneous wound closure and alters local inflammatory signals

We previously reported that in the absence of NKT cells, wound closure was accelerated in a murine excisional punch wound model. Here, we explored whether purposefully inhibiting NKT cell activation had similar effects on wound closure and the dermal inflammatory response to injury. We found that prevention of NKT cell activation accelerated wound closure in a dose-responsive manner. If anti-CD1d was administered before wounding, NKT cell infiltration into cutaneous wounds was diminished without quantitative changes in cellular infiltrates. Furthermore, prevention of NKT cell activation transiently enhanced the local production of a subset of chemokines, including MIP-2, MCP-1, MIP-1α, and MIP-1β, and altered the relative expression of CD69 and CXCR2 on the surface of both circulating and wound NKT cells. Taken together, these findings suggest that wounding activates NKT cells via CD1d presentation of glycolipid antigen and help further define a role for NKT cells in the regulation of wound inflammation and closure. Many soluble factors have been targeted as potential wound healing therapies, but their clinical success has been limited. Given our findings, the NKT cell may be an attractive target for wound healing therapies.

Human NKT cells direct the differentiation of myeloid APCs that regulate T cell responses via expression of programmed cell death ligands

NKT cells are innate lymphocytes that can recognize self or foreign lipids presented by CD1d molecules. NKT cells have been shown to inhibit the development of autoimmunity in murine model systems, however, the pathways by which they foster immune tolerance remain poorly understood. Here we show that autoreactive human NKT cells stimulate monocytes to differentiate into myeloid APCs that have a regulatory phenotype characterized by poor conjugate formation with T cells. The NKT cell instructed myeloid APCs show elevated expression of the inhibitory ligand PD-L2, and blocking PD-L1 and PD-L2 during interactions of the APCs with T cells results in improved cluster formation and significantly increased T cell proliferative responses. The elevated expression of PD-L molecules on NKT-instructed APCs appears to result from exposure to extracellular ATP that is produced during NKT-monocyte interactions, and blocking purinergic signaling during monocyte differentiation results in APCs that form clusters with T cells and stimulate their proliferation. Finally, we show that human monocytes and NKT cells that are injected into immunodeficient mice co-localize together in spleen and liver, and after 3 days in vivo in the presence of NKT cells a fraction of the myeloid cells have upregulated markers associated with differentiation into professional APCs. These results suggest that autoreactive human NKT cells may promote tolerance by inducing the differentiation of regulatory myeloid APCs that limit T cell proliferation through expression of PD-L molecules.

Autoreactive natural killer T cells: promoting immune protection and immune tolerance through varied interactions with myeloid antigen-presenting cells

Natural killer T (NKT) cells are innate T lymphocytes that are restricted by CD1d antigen-presenting molecules and recognize lipids and glycolipids as antigens. NKT cells have attracted attention for their potent immunoregulatory effects. Like other types of regulatory lymphocytes, a high proportion of NKT cells appear to be autoreactive to self antigens. Thus, as myeloid antigen-presenting cells (APCs) such as monocytes, dendritic cells (DCs) and myeloid-derived suppressor cells (MDSCs) constitutively express CD1d, NKT cells are able to interact with these APCs not only during times of immune activation but also in immunologically quiescent periods. The interactions of NKT cells with myeloid APCs can have either pro-inflammatory or tolerizing outcomes, and a central question is how the ensuing response is determined. Here we bring together published results from a variety of model systems to highlight three critical factors that influence the outcome of the NKT-APC interaction: (i) the strength of the antigenic signal delivered to the NKT cell, as determined by antigen abundance and/or T-cell receptor (TCR) affinity; (ii) the presence or absence of cytokines that costimulate NKT cells [e.g. interleukin (IL)-12, IL-18 and interferon (IFN)-alpha]; (iii) APC intrinsic factors such as differentiation state (e.g. monocyte versus DC) and Toll-like receptor (TLR) stimulation. Together with recent findings that demonstrate new links between NKT cell activation and endogenous lipid metabolism, these results outline a picture in which the functions of NKT cells are closely attuned to the existing biological context. Thus, NKT cells may actively promote tolerance until a critical level of danger signals arises, at which point they switch to activating pro-inflammatory immune responses.

A novel role for NKT cells in cutaneous wound repair

Here, we report the novel observation that natural killer T (NKT) cells contribute to the cutaneous wound repair process. Using an excisional wound model in wild-type versus NKT cell-deficient mice, this report shows that when NKT cells are absent, initial wound closure is markedly accelerated. We report here for the first time that NKT cells are a significant constituent of early wound inflammation and that they regulate the local production of a key subset of neutrophil and monocyte/macrophage chemokines, as well as TGF-β1 content and collagen deposition. Together, our findings support the concept that NKT cells regulate the early inflammatory and fibroproliferative phases of nonpathologic healing wounds, positioning the NKT cell as an attractive potential therapeutic target for modulation of impaired wound healing.

ERK and not p38 pathway is required for IL-12 restoration of T cell IL-2 and IFN-gamma in a rodent model of alcohol intoxication and burn injury

Previous studies from our laboratory have shown that acute alcohol/ethanol (EtOH) intoxication combined with burn injury suppresses T cell IL-2 and IFN-gamma production by inhibiting p38 and ERK activation. Because IL-12 plays a major role in Th1 differentiation and IFN-gamma production, we examined whether diminished IL-2 and IFN-gamma production after EtOH plus burn injury resulted from a decrease in IL-12. Furthermore, we investigated whether IL-12 utilizes the p38/ERK pathway to modulate T cell IL-2 and IFN-gamma production after EtOH and burn injury. Male rats ( approximately 250 g) were gavaged with 5 ml of 20% EtOH 4 h before approximately 12.5% total body surface area burn or sham injury. Rats were sacrificed on day 1 after injury, and mesenteric lymph node T cells were isolated. T cells were stimulated with anti-CD3 in the absence or presence of rIL-12 (10 ng/ml) for 5 min and lysed. Lysates were analyzed for p38/ERK protein and phosphorylation levels using specific Abs and Western blot. In some experiments, T cells were cultured for 48 h with or without the inhibitors of p38 (10 microM SB203580/SB202190) or ERK (50 microM PD98059) to delineate the role of p38 and ERK in IL-12-mediated restoration of IL-2 and IFN-gamma. Our findings indicate that IL-12 normalizes both p38 and ERK activation in T cells, but the results obtained using p38 and ERK inhibitors indicate that the restoration of ERK plays a predominant role in IL-12-mediated restoration of T cell IL-2 and IFN-gamma production after EtOH and burn injury.

Invariant natural killer T cells regulate breast cancer response to radiation and CTLA-4 blockade

PURPOSE

Immunoregulatory and suppressive mechanisms represent major obstacles to the success of immunotherapy in cancer patients. We have shown that the combination of radiotherapy to the primary tumor and CTL-associated protein 4 (CTLA-4) blockade induces antitumor immunity, inhibiting metastases and extending the survival of mice bearing the poorly immunogenic and highly metastatic 4T1 mammary carcinoma. Similarly to patients with metastatic cancer, however, mice were seldom cured. Here we tested the hypothesis that invariant natural killer T (iNKT) cells, a subset with unique regulatory functions, can regulate the response to radiotherapy and CTLA-4 blockade.

EXPERIMENTAL DESIGN

The growth of 4T1 primary tumors and lung metastases was compared in wild-type and iNKT cell-deficient (iNKT-/-) mice. Treatment was started on day 13 when the primary tumors were palpable. Mice received radiotherapy to the primary tumor in two doses of 12 Gy in combination or not with 9H10 monoclonal antibody against CTLA-4. Response to treatment was assessed by measuring primary tumor growth delay/regression, survival, and number of lung metastases.

RESULTS

The response to radiotherapy plus 9H10 was markedly enhanced in the absence of iNKT cells, with 50% of iNKT-/- versus 0% of wild-type mice showing complete tumor regression, long-term survival, and resistance to a challenge with 4T1 cells. Administration of the iNKT cell activator alpha-galactosylceramide did not enhance the response of wild-type mice to radiotherapy plus 9H10. Tumor-infiltrating iNKT cells were markedly reduced in wild-type mice treated with radiotherapy plus 9H10.

CONCLUSIONS

iNKT cells play a major role in regulating the response to treatment with local radiotherapy and CTLA-4 blockade.

Regulation of secondary antigen-specific CD8(+) T-cell responses by natural killer T cells

The physiologic function of natural killer T (NKT) cells in adaptive immunity remains largely unknown because most studies have used NKT cell agonists. In the present study, the role of NKT cells during the secondary effector phase was investigated separately from the primary immunization phase via adoptive transfer of differentiated effector T cells into naive recipients. We found that secondary antitumor CD8(+) T-cell responses were optimal when NKT cells were present. Tumor-specific CD8(+) effector T cells responded less strongly to tumor cell challenge in NKT cell-deficient recipients than in recipients with intact NKT cells. NKT cell-mediated enhancement of the secondary antitumor CD8(+) T-cell response was concurrent with increased number and activity of tumor-specific CD8(+) T cells. These findings provide the first demonstration of a direct role for NKT cells in the regulation of antigen-specific secondary T-cell responses without the use of exogenous NKT cell agonists such as alpha-galactosylceramide (alpha-GalCer). Furthermore, forced activation of NKT cells with alpha-GalCer during the secondary immune response in suboptimally immunized animals enhanced otherwise poor tumor rejection responses. Taken together, our findings strongly emphasize the importance of NKT cells in secondary CD8(+) T-cell immune responses.

The compensatory anti-inflammatory response syndrome (CARS) in critically ill patients

Like the systemic inflammatory response syndrome (SIRS), the compensatory anti-inflammatory response syndrome (CARS) is a complex pattern of immunologic responses to severe infection or injury. The difference is that while SIRS is a proinflammatory response tasked with killing infectious organisms through activation of the immune system, CARS is a global deactivation of the immune system tasked with restoring homeostasis. Much research now suggests that the timing and relative magnitude of this response have a profound impact on patient outcomes.

The role of hepatic invariant NKT cells in systemic/local inflammation and mortality during polymicrobial septic shock

NKT cells have been described as innate regulatory cells because of their rapid response to conserved glycolipids presented on CD1d via their invariant TCR. However, little is known about the contribution of the hepatic NKT cell to the development of a local and/or systemic immune response to acute septic challenge (cecal ligation and puncture (CLP)). We found not only that mice deficient in invariant NKT cells (Jalpha18(-/-)) had a marked attenuation in CLP-induced mortality, but also exhibited an oblation of the systemic inflammatory response (with little effect on splenic/peritoneal immune responsiveness). Flow cytometric data indicated that following CLP, there was a marked decline in the percentage of CD3(+)alpha-galactosylceramide CD1d tetramer(+) cells in the mouse C57BL/6J and BALB/c liver nonparenchymal cell population. This was associated with the marked activation of these cells (increased expression of CD69 and CD25) as well as a rise in the frequency of NKT cells positive for both Th1 and Th2 intracellular cytokines. In this respect, when mice were pretreated in vivo with anti-CD1d-blocking Ab, we observed not only that this inhibited the systemic rise of IL-6 and IL-10 levels in septic mice and improved overall septic survival, but that the CLP-induced changes in liver macrophage IL-6 and IL-10 expressions were inversely effected by this treatment. Together, these findings suggest that the activation of hepatic invariant NKT cells plays a critical role in regulating the innate immune/systemic inflammatory response and survival in a model of acute septic shock.

Prevention of injury-induced suppression of T-cell immunity by the CD1d/NKT cell-specific ligand alpha-galactosylceramide

Infection, sepsis, and multiple organ failure continue to be significant factors leading to morbidity and mortality after severe injury. Studies by our laboratory and others have identified injury-induced defects in both innate and adaptive components of host defense. We previously reported that CD1d-restricted natural killer T (NKT) cells actively suppress effector T-cell immunity after burn injury via production of excess IL-4 and failure to produce IFN-gamma. alpha-Galactosylceramide (alpha-GalCer) is a synthetic NKT cell-specific ligand presented exclusively to invariant NKT cells and is known to improve immunity against tumors and infection by promoting IFN-gamma production. Here, we confirmed the role of Valpha14-Jalpha281 invariant NKT cells in mouse model of burn injury-induced suppression of T-cell immunity and further asked whether alpha-GalCer can improve immunity after injury via similar mechanisms. We observed that systemic treatment with alpha-GalCer prevented the injury-induced suppression of Ag-specific T-cell responsiveness both in vitro and in vivo and restored the ability of splenic lymphocytes to produce both IL-2 and IFN-gamma. Moreover, burn injury was associated with diminished expression of major histocompatibility complex II and CD40 on antigen presenting cells that were both restored by alpha-GalCer treatment to levels seen in sham-treated mice. Collectively, these data suggest that, via manipulation of the NKT cell population, we may be able to maintain T-cell function and improve host defense after burn injury.

Inhibition of protein tyrosine phosphatases prevents mesenteric lymph node T-cell suppression following alcohol intoxication and burn injury

Previously, we have shown that acute alcohol (EtOH) intoxication before burn injury potentiates the suppression of mesenteric lymph node T-cell effector responses. Moreover, the suppression in T-cell was accompanied with a decrease in p-38 and extracellular-signal-regulated kinase (ERK) activation. This study examined the role of protein tyrosine phosphatases (PTP) in suppressed T-cell p-38, ERK, and cytokine production after EtOH intoxication and burn injury. A blood EtOH level of approximately 100 mg/dl in male rats (approximately 250 g) was achieved by gavaging animals with 5 ml of 20% EtOH suspension 4 hours before burn or sham injury (approximately 12.5% or 25% total body surface area [TBSA]). One day after injury, rats were killed and mesenteric lymph node T-cell cytokine (IL-2/IFN-gamma) production, p-38, and ERK activation were measured. As compared with shams, there was a significant decrease in T-cell cytokine production after 25% and not 12.5% TBSA burn injury. However, T-cell IL-2/IFN-gamma levels were significantly decreased in rats receiving a combined insult of EtOH and burn injury regardless of the percentage of burn area. Furthermore, we found a significant decrease in p-38 and ERK-1/2 phosphorylation in T-cells of rats receiving a combined insult of EtOH and 12.5% TBSA burn compared with shams. Treatment of cells with PTP inhibitor pervanadate (10 muM) prevented T-cell p-38/ERK suppression. The suppression in IL-2/IFN-gamma production was also attenuated in T-cells cultured in the presence of pervanadate. These findings suggest that an increase in PTP activity may contribute to T-cell suppression after EtOH intoxication and burn injury.

Immunoparalysis after multiple trauma

The immunological sequelae following multiple trauma constitute an ongoing challenge in critical care management. The overall immune response to multiple trauma is a multilevel complex interdependently involving neurohormonal, cellular and haemodynamic factors. Immunoparalysis is characterised by a reduced capacity to present antigens via downregulated HLA-DR and an unbalanced monocyte-T cell interaction. Trauma-induced death of functionally conducive immune cells in the early recovery phase is significant in the emergence of posttraumatic multiple organ dysfunction or failure. Novel findings may contribute to more appropriate immunomonitoring and improved treatment. We must consider the preservation and support of immune function as the ultimate therapeutic goal, which may override the current strategy of simply antagonising excessive pro- or anti-inflammatory immune responses of the severely injured person. This review focuses on the injury-induced conduct of key immune effector cells and associated effects promoting immunoparalysis after multiple trauma.

Involvement of the CD1d–Natural Killer T Cell Pathway in Neointima Formation After Vascular Injury

Recent studies have established that the immune system plays an important role in the development of atherosclerosis. However, its role in regulating the arterial response to mechanical injury is less well studied. Arterial injury is associated with local accumulation of antibodies, and mice lacking functional T and B cells exhibit increased neointima formation, indicating that adaptive immune responses to neoantigens in the damaged tissue modulate the vascular repair process. To study the role of lipid antigen presentation in the arterial response to injury, we analyzed neointima formation in mice deficient in the lipid antigen-presenting molecule CD1d using a carotid collar model. As compared with control mice, neointima formation was reduced by >60% (P<0.01) in CD1d-/- mice. Moreover, carotid injury of wild-type C57BL/6 mice was associated with expansion of CD1d-restricted natural killer T cells in the spleen and accumulation of natural killer T cells in the periadventitial space of injured arteries. The results suggest that presentation of lipid antigens through the CD1d-natural killer T cell pathway modulates vascular repair responses.

Regulatory T cell populations in sepsis and trauma

Sepsis syndrome remains the leading cause of mortality in intensive care units. It is now believed that along with the body's hyperinflammatory response designated to eliminate the underlying pathogen, mechanisms are initiated to control this initial response, which can become deleterious and result in immune dysfunctions and death. A similar state of immune suppression has been described after numerous forms of severe trauma/injury. Although the evidence for immune dysfunctions after sepsis has grown, much remains to be understood about mechanisms underpinning its development and how it acts to increase the morbid state of the critically ill patient. In this context, although the majority of clinical and basic science conducted so far has focused on the roles of myeloid cell populations, the contribution of T lymphocytes and in particular, of regulatory T cells has been somewhat ignored. The studies presented here support the concept that regulatory T lymphocytes (CD4+CD25+ regulatory, gammadelta, and NK T cells) play a role in the control of immune responses and are affected by injury and sepsis. This may be related to their capacity to interact with components of the innate and adaptive immune responses and to their ability to be activated nonspecifically by bacterial products and/or cytokines and to regulate through direct cell-cell and/or soluble mediators. It is our hope that a better understanding of the mechanism through which those rare lymphocyte subsets exert such a profound effect on the immune response may help in improving our ability not only to diagnose but also to treat the critically ill individual.

The biology of NKT cells

Recognized more than a decade ago, NKT cells differentiate from mainstream thymic precursors through instructive signals emanating during TCR engagement by CD1d-expressing cortical thymocytes. Their semi-invariant alphabeta TCRs recognize isoglobotrihexosylceramide, a mammalian glycosphingolipid, as well as microbial alpha-glycuronylceramides found in the cell wall of Gram-negative, lipopolysaccharide-negative bacteria. This dual recognition of self and microbial ligands underlies innate-like antimicrobial functions mediated by CD40L induction and massive Th1 and Th2 cytokine and chemokine release. Through reciprocal activation of NKT cells and dendritic cells, synthetic NKT ligands constitute promising new vaccine adjuvants. NKT cells also regulate a range of immunopathological conditions, but the mechanisms and the ligands involved remain unknown. NKT cell biology has emerged as a new field of research at the frontier between innate and adaptive immunity, providing a powerful model to study fundamental aspects of the cell and structural biology of glycolipid trafficking, processing, and recognition.

Innate Lymphocyte Subsets and Their Immunoregulatory Roles in Burn Injury and Sepsis

The vast majority of clinical and basic science research on the immune consequences of burn injury and sepsis conducted during the last three decades has focused mainly on the roles of macrophages, neutrophils and, to a lesser extent, conventional T lymphocytes. During recent years, however, it has become increasingly clear that minor subsets of innate immune cells, innate regulatory lymphocytes in particular, are central to processes involved in both protective immunity and immunopathology. Recent reports by our laboratory and others have just begun to shed light on the critical roles of innate lymphocyte subsets, including natural killer T cells, natural killer cells, gamma-delta T cells, and naturally occurring CD4+CD25+ regulatory T cells during the immune response to burn injury and sepsis. Given their emerging importance and documented upstream regulatory capacities over macrophage, dendritic cell, and T lymphocyte functions, innate regulatory lymphocytes represent attractive new targets for therapeutic intervention for the overall immune paralysis that occurs with injury and sepsis. Here, we provide an overview of the current state of knowledge of these particular cell subsets in the immune response to burn injury and sepsis.