Alexander Kroemer

The Innate NK Cells, Allograft Rejection, and a Key Role for IL-15

Transplant rejection is mediated primarily by adaptive immune cells such as T cells and B cells. The T and B cells are also responsible for the specificity and memory of the rejection response. However, destruction of allografts involves many other cell types including cells in the innate immune system. As the innate immune cells do not express germline-encoded cell surface receptors that directly recognize foreign Ags, these cells are thought to be recruited by T cells to participate in the rejection response. In this study, we examined the alloreactivity of the innate NK cells in Rag−/− mice using a stringent skin transplant model and found that NK cells at a resting state readily reject allogeneic cells, but not the skin allografts. We also found that IL-15, when preconjugated to its high affinity IL-15Rα-chain, is remarkably potent in stimulating NK cells in vivo, and NK cells stimulated by IL-15 express an activated phenotype and are surprisingly potent in mediating acute skin allograft rejection in the absence of any adaptive immune cells. Furthermore, NK cell-mediated graft rejection does not show features of memory responses. Our data demonstrate that NK cells are potent alloreactive cells when fully activated and differentiated under certain conditions. This finding may have important clinical implications in models of transplantation and autoimmunity.

Deletion of CD39 on natural killer cells attenuates hepatic ischemia/reperfusion injury in mice

Natural killer (NK) cells play crucial roles in innate immunity and express CD39 (Ecto‐nucleoside triphosphate diphosphohydrolase 1 [E‐NTPD1]), a rate‐limiting ectonucleotidase in the phosphohydrolysis of extracellular nucleotides to adenosine. We have studied the effects of CD39 gene deletion on NK cells in dictating outcomes after partial hepatic ischemia/reperfusion injury (IRI). We show in mice that gene deletion of CD39 is associated with marked decreases in phosphohydrolysis of adenosine triphosphate (ATP) and adenosine diphosphate to adenosine monophosphate on NK cells, thereby modulating the type‐2 purinergic (P2) receptors demonstrated on these cells. We note that CD39‐null mice are protected from acute vascular injury after single‐lobe warm IRI, and, relative to control wild‐type mice, display significantly less elevation of aminotransferases with less pronounced histopathological changes associated with IRI. Selective adoptive transfers of immune cells into Rag2/common gamma null mice (deficient in T cells, B cells, and NK/NKT cells) suggest that it is CD39 deletion on NK cells that provides end‐organ protection, which is comparable to that seen in the absence of interferon gamma. Indeed, NK effector mechanisms such as interferon gamma secretion are inhibited by P2 receptor activation in vitro. Specifically, ATPγS (a nonhydrolyzable ATP analog) inhibits secretion of interferon gamma by NK cells in response to interleukin‐12 and interleukin‐18, providing a mechanistic link between CD39 deletion and altered cytokine secretion. Conclusion: We propose that CD39 deficiency and changes in P2 receptor activation abrogate secretion of interferon gamma by NK cells in response to inflammatory mediators, thereby limiting tissue damage mediated by these innate immune cells during IRI. (HEPATOLOGY 2010.)

OX40/OX40L Costimulation Affects Induction of Foxp3+ Regulatory T Cells in Part by Expanding Memory T Cells In Vivo

OX40 is a member of the TNFR superfamily and has potent T cell costimulatory activities. OX40 also inhibits the induction of Foxp3+ regulatory T cells (Tregs) from T effector cells, but the precise mechanism of such inhibition remains unknown. In the present study, we found that CD4+ T effector cells from OX40 ligand-transgenic (OX40Ltg) mice are highly resistant to TGF-β mediated induction of Foxp3+ Tregs, whereas wild-type B6 and OX40 knockout CD4+ T effector cells can be readily converted to Foxp3+ T cells. We also found that CD4+ T effector cells from OX40Ltg mice are heterogeneous and contain a large population of CD44highCD62L− memory T cells. Analysis of purified OX40Ltg naive and memory CD4+ T effector cells showed that memory CD4+ T cells not only resist the induction of Foxp3+ T cells but also actively suppress the conversion of naive CD4+ T effector cells to Foxp3+ Tregs. This suppression is mediated by the production of IFN-γ by memory T cells but not by cell-cell contact and also involves the induction of T-bet. Importantly, memory CD4+ T cells have a broad impact on the induction of Foxp3+ Tregs regardless of their origins and Ag specificities. Our data suggest that one of the mechanisms by which OX40 inhibits the induction of Foxp3+ Tregs is by inducing memory T cells in vivo. This finding may have important clinical implications in tolerance induction to transplanted tissues.

OX40 Controls Functionally Different T Cell Subsets and Their Resistance to Depletion Therapy

T cell depletion is a widely used approach in clinical transplantation. However, not all T cells are equally sensitive to depletion therapies and a significant fraction of T cells persists even after aggressive treatment. The functional attributes of such T cells and the mechanisms responsible for their resistance to depletion are poorly studied. In the present study, we showed that CD4+ T cells that are resistant to polyclonal anti-lymphocyte serum (ALS) mediated depletion exhibit phenotypic features of memory cells and uniformly express OX40 on the cell surface. Studies using the foxp3gfp knockin mice revealed that the remaining CD4+OX40+ cells consist of Foxp3+ Tregs and Foxp3− T effector/memory cells. The ALS-resistant CD4+OX40+ cells failed to mediate skin allograft rejection upon adoptive transferring into congenic Rag−/− mice, but removal of Foxp3+ Tregs from the OX40+ cells resulted in prompt skin allograft rejection. Importantly, OX40 is critical to survival of both Foxp3+ Tregs and T effector/memory cells. However, OX40 exhibits opposing effects on the functional status of Foxp3+ Tregs and T effector/memory cells, as stimulation of OX40 on T effector cells induced amplified cell proliferation but stimulation of OX40 on the Foxp3+ Tregs impaired their suppressor functions. Our study demonstrates that OX40 is a critical molecule in regulating survival and functions of depletion-resistant T cells; and these findings may have important clinical implications.

The innate natural killer cells in transplant rejection and tolerance induction

PURPOSE OF REVIEW The roles of adaptive immune cells in transplant models have been extensively studied, but very little is known about the role of innate immune cells in the allograft response, especially in tolerance induction. In this review, we summarized the latest developments in the study of the role of natural killer cells in mediating graft rejection and tolerance induction. RECENT FINDINGS Natural killer cells are potent cytolytic cells; they also induce tissue inflammation by producing powerful proinflammatory cytokines. Thus, natural killer cells can act as effector cells in transplant rejection. Recent studies, however, have demonstrated additional roles for natural killer cells in the induction of transplant tolerance. We found that natural killer cells control survival of graft-derived donor cells and killing of donor dendritic cells by host natural killer cells inhibits direct priming of alloreactive T cells. Natural killer cells are also shown directly to suppress the activation of T cells. In other models, natural killer cells are found to regulate the induction of regulatory T cells. These new findings may have important clinical implications in tolerance induction. SUMMARY Natural killer cells are involved in both graft rejection and tolerance induction; such opposing effects may be mediated by differences in the activation status of natural killer cells. We believe that natural killer cells can be therapeutically modified for the induction of transplant tolerance.

A Role for Neuronal Alpha-Synuclein in Gastrointestinal Immunity

Background: Alpha-synuclein (αS) is a nerve cell protein associated with Parkinson disease (PD). Accumulation of αS within the enteric nervous system (ENS) and its traffic from the gut to the brain are implicated in the pathogenesis and progression of PD. αS has no known function in humans and the reason for its accumulation within the ENS is unknown. Several recent studies conducted in rodents have linked αS to immune cell activation in the central nervous system. We hypothesized that αS in the ENS might play a role in the innate immune defenses of the human gastrointestinal (GI) tract. Methods: We immunostained endoscopic biopsies for αS from children with documented gastric and duodenal inflammation and intestinal allograft recipients who contracted norovirus. To determine whether αS exhibited immune-modulatory activity, we examined whether human αS induced leukocyte migration and dendritic cell maturation. Findings: We showed that the expression of αS in the enteric neurites of the upper GI tract of pediatric patients positively correlated with the degree of acute and chronic inflammation in the intestinal wall. In intestinal allograft subjects who were closely monitored for infection, expression of αS was induced during norovirus infection. We also demonstrated that both monomeric and oligomeric αS have potent chemoattractant activity, causing the migration of neutrophils and monocytes dependent on the presence of the integrin subunit, CD11b, and that both forms of αS stimulate dendritic cell maturation. Interpretation: These findings strongly suggest that αS is expressed within the human ENS to direct intestinal inflammation and implicates common GI infections in the pathogenesis of PD.

KLRG1+ NK Cells Protect T-bet–Deficient Mice from Pulmonary Metastatic Colorectal Carcinoma

We studied the developmental and functional mechanisms behind NK cell–mediated antitumor responses against metastatic colorectal carcinoma (CRC) in mice. In particular, we focused on investigating the significance of T-box transcription factors and the immunotherapeutic relevance of IL-15 in the development and function of tumor-reactive NK cells. Pulmonary CRC metastases were experimentally seeded via an adoptive i.v. transfer of luciferase-expressing CT26 CRC cells that form viewable masses via an in vivo imaging device; genetically deficient mice were used to dissect the antitumor effects of developmentally different NK cell subsets. IL-15 precomplexed to IL-15 receptor-α was used in immunotherapy experiments. We found that mice deficient for the T-box transcription factor T-bet lack terminally differentiated antitumor CD27lowKLRG1+ NK cells, leading to a terminal course of rapid-onset pulmonary CRC metastases. The importance of this NK cell subset for effective antitumor immunity was shown by adoptively transferring purified CD27lowKLRG1+ NK cells into T-bet–deficient mice and, thereby, restoring immunity against lung metastasis formation. Importantly, immunity to metastasis formation could also be restored in T-bet–deficient recipients by treating mice with IL-15 precomplexed to IL-15 receptor-α, which induced the development of eomesodermin+KLRG1+ NK cells from existing NK cell populations. Thus, contingent upon their T-bet–dependent development and activation status, NK cells can control metastatic CRC in mice, which is highly relevant for the development of immunotherapeutic approaches in the clinic.

Inhibition of mTORC2 component RICTOR impairs tumor growth in pancreatic cancer models

Mammalian Target of Rapamycin complex 2 (mTORC2) and its regulatory component Rapamycin-insensitive companion of mTOR (RICTOR) are increasingly recognized as important players in human cancer development and progression. However, the role of RICTOR in human pancreatic ductal adenocarcinoma (PDAC) is unclear so far. Here, we sought to analyze the effects of RICTOR inhibition in human pancreatic cancer cell lines in vitro and in vivo. Furthermore, RICTOR expression was determined in human PDAC samples. Results demonstrate that depletion of RICTOR with siRNA (transient knock-down) or shRNA (stable knock-down) has an inhibitory effect on tumor growth in vitro. Moreover, RICTOR inhibition led to impaired phosphorylation/activity of AGC kinases (AKT, SGK1). Interestingly, hypoxia-induced expression of hypoxia-induced factor-1α (HIF-1α) was diminished and secretion of vascular-endothelial growth factor-A (VEGF-A) was impaired upon targeting RICTOR. Stable RICTOR knock-down led to significant inhibition of tumor growth in subcutaneous and orthotopic tumor models which was accompanied by significant reduction of tumor cell proliferation. Finally, immunohistochemical analyses of 85 human PDAC samples revealed significantly poorer survival in patients with higher RICTOR expression. In conclusion, these findings provide first evidence for mTORC2/RICTOR as an attractive novel target for treatment of human PDAC.

CD27low natural killer cells prolong allograft survival in mice by controlling alloreactive CD8+ T cells in a T-bet-dependent manner.

Background Natural killer (NK) cells play a dichotomous role in alloimmune responses because they are known to promote both allograft survival and rejection. The aim of this study was to investigate the role of functionally distinct NK cell subsets in alloimmunity with the hypothesis that this dichotomy is explained by the functional heterogeneity of distinct NK cell subsets. Methods Because T-bet controls the maturation of NK cells from CD27high to terminally differentiated CD27low NK cells, we used Rag−/−T-bet−/− mice that lack mature CD27low NK cells to study the distinct roles of CD27low versus CD27high NK cells in a model of T cell–mediated skin transplant rejection under costimulatory blockade conditions. Results We found that T cell–reconstituted Rag1−/− recipients (possessing CD27low NK cells) show significantly prolonged allograft survival on costimulatory blockade when compared to Rag1−/−T-bet−/− mice (lacking CD27low NK cells), indicating that CD27low but not CD27high NK cells enhance allograft survival. Critically, Rag1−/−T-bet−/− recipients showed strikingly increased alloreactive memory CD8+ T cell responses, as indicated by increased CD8+ T cell proliferation and interferon-&ggr; production. Therefore, we speculated that CD27low NK cells directly regulate alloreactive CD8+ T cell responses under costimulatory blockade conditions. To test this, we adoptively transferred CD27low NK cells into Rag1−/−T-bet−/− skin transplant recipients and found that the CD27low NK cells restore better allograft survival by inhibiting the proliferation of alloreactive interferon-&ggr;+CD8+ T cells. Conclusions In summary, mature CD27low NK cells promote allograft survival under costimulatory blockade conditions by regulating alloreactive memory CD8+ T-cell responses.

The microbiome and its implications in intestinal transplantation.

Purpose of reviewThis article summarizes the complex interplay between the microbiota and host immune responses, and its impact on intestinal transplantation and allograft rejection. Recent findingsRecent findings highlight the importance of Paneth cells as crucial producers of antimicrobial peptides that control the intestinal host-microbial interface as well as the essential role of NOD2 as a master regulator of antimicrobial host defenses. Moreover, complex interactions between innate and adaptive immune responses have been shown to critically shape host antimicrobial Th17 responses, which may be key for the pathogenesis of inflammatory bowel diseases and intestinal allograft rejection. SummaryA growing body of evidence indicates that crosstalk between the microbiome and innate and adaptive host immunity determines alloimmune responses and outcomes in intestinal transplantation. Elaboration of this emerging field might lead to novel mechanistic insight into these complex interactions and allow for new therapeutic approaches.