The cholinergic anti-inflammatory pathway delays TLR-induced skin allograft rejection in mice: cholinergic pathway modulates alloreactivity.

Activation of innate immunity through Toll-like receptors (TLR) can abrogate transplantation tolerance by revealing hidden T cell alloreactivity. Separately, the cholinergic anti-inflammatory pathway has the capacity to dampen macrophage activation and cytokine release during endotoxemia and ischemia reperfusion injury. However, the relevance of the α7 nicotinic acetylcholine receptor (α7nAChR)-dependent anti-inflammatory pathway in the process of allograft rejection or maintenance of tolerance remains unknown. The aim of our study is to investigate whether the cholinergic pathway could impact T cell alloreactivity and transplant outcome in mice. For this purpose, we performed minor-mismatched skin allografts using donor/recipient combinations genetically deficient for the α7nAChR. Minor-mismatched skin grafts were not rejected unless the mice were housed in an environment with endogenous pathogen exposure or the graft was treated with direct application of imiquimod (a TLR7 ligand). The α7nAChR-deficient recipient mice showed accelerated rejection compared to wild type recipient mice under these conditions of TLR activation. The accelerated rejection was associated with enhanced IL-17 and IFN-γ production by alloreactive T cells. An α7nAChR-deficiency in the donor tissue facilitated allograft rejection but not in recipient mice. In addition, adoptive T cell transfer experiments in skin-grafted lymphopenic animals revealed a direct regulatory role for the α7nAChR on T cells. Taken together, our data demonstrate that the cholinergic pathway regulates alloreactivity and transplantation tolerance at multiple levels. One implication suggested by our work is that, in an organ transplant setting, deliberate α7nAChR stimulation of brain dead donors might be a valuable approach for preventing donor tissue inflammation prior to transplant.

Critical role of regulatory T cells in Th17-mediated minor antigen-disparate rejection.

Th17-mediated immune responses have been recently identified as novel pathogenic mechanisms in a variety of conditions; however, their importance in allograft rejection processes is still debated. In this paper, we searched for MHC or minor Ag disparate models of skin graft rejection in which Th17 immune responses might be involved. We found that T cell-derived IL-17 is critical for spontaneous rejection of minor but not major Ag-mismatched skin grafts. IL-17 neutralization was associated with a lack of neutrophil infiltration and neutrophil depletion delayed rejection, suggesting neutrophils as an effector mechanism downstream of Th17 cells. Regulatory T cells (Tregs) appeared to be involved in Th17 reactivity. We found that in vivo Treg depletion prevented IL-17 production by recipient T cells. An adoptive cotransfer of Tregs with naive monospecific antidonor T cells in lymphopenic hosts biased the immune response toward Th17. Finally, we observed that IL-6 was central for balancing Tregs and Th17 cells as demonstrated by the prevention of Th17 differentiation, the enhanced Treg/Th17 ratio, and a net impact of rejection blockade in the absence of IL-6. In conclusion, the ability of Tregs to promote the Th17/neutrophil-mediated pathway of rejection that we have described should be considered as a potential drawback of Treg-based cell therapy.

IL-17 production elicited by allo-major histocompatibility complex class II recognition depends on CD25posCD4pos T cells.

BACKGROUND: Interleukin (IL)-17 is involved in autoimmune inflammatory disorders and naturally occurring CD25pos regulatory T cells were shown to promote IL-17 synthesis. Because IL-17 is overproduced in certain types of allograft rejection, it is important to characterize the cells responsible for IL-17 synthesis and to define how IL-17 is regulated during alloimmune responses. METHODS: Splenic CD4pos T cells were isolated from C57BL/6 mice and fractionated according to CD25 expression. T cells were stimulated by major histocompatibility complex class II-mismatched bone marrow-derived dendritic cells from bm12 mice, either immature or made mature by exposure to lipopolysaccharide. To track T cell populations, CD25negCD4pos and CD25posCD4pos were isolated from Thy1.1 and congenic Thy1.2 mice, respectively. Cell proliferation was quantified by CFSE dilution. IL-17-producing cells and FOXP3pos cells were enumerated by intracytoplasmic staining and cytokine levels in culture supernatants were measured by ELISA. RESULTS: Addition of CD25posCD4pos T cells to CD25negCD4pos T cells inhibited IL-2, interferon-[gamma], and IL-13 production but promoted IL-17 synthesis on stimulation by allogenic immature DC. In this setting, IL-17 originated from CD25intCD4posFOXP3neg memory T cells, which depend on IL-2 to produce IL-17. Alloreactive CD25negCD4pos T cells were also induced to produce IL-17 when stimulated by mature DC in the presence of CD25highCD4posFOXP3pos T cells. CONCLUSIONS: We conclude that (1) the cellular source of IL-17 during an antiallo major histocompatibility complex class II response depends on the maturation status of allogenic DC, (2) whereas suppressing Th1 and Th2 cytokine synthesis, naturally occurring regulatory T cells, allow IL-17 production by alloreactive CD4pos T cells.

Endotoxin hyperresponsiveness upon CD4+ T cell reconstitution in lymphopenic mice: control by natural regulatory T cells.

Natural CD4(+)CD25(+) regulatory T cells (nTreg) have been shown to control graft-versus-host disease after hematopoietic stem cell transplantation (HSCT). Herein, we considered the possibility that the beneficial action of nTreg upon immune reconstitution in lymphopenic hosts involves dampening of the inflammatory response induced by bacterial products. We first observed that transfer of syngeneic CD4(+)CD25(-) T cells in RAG-deficient mice dramatically enhanced release of inflammatory cytokines and associated pathology upon endotoxin injection. Interferon (IFN)-gamma produced by T cells undergoing homeostatic proliferation was shown to be involved in the endotoxin hyperresponsiveness induced by CD4(+) T cell reconstitution. Co-transfer of CD4(+)CD25(+) nTreg with CD4(+)CD25(-) T cells inhibited the expansion of IFN-gamma-producing T cells and reduced endotoxin responses in RAG(-/-) mice. We conclude that (1) CD4(+) T cell reconstitution sensitizes lymphopenic hosts to endotoxin-induced pathology and (2) nTreg prevent this process by limiting the emergence of IFN-gamma-producing cells.

Nicotine protects kidney from renal ischemia/reperfusion injury through the cholinergic anti-inflammatory pathway.

Kidney ischemia/reperfusion injury (I/R) is characterized by renal dysfunction and tubular damages resulting from an early activation of innate immunity. Recently, nicotine administration has been shown to be a powerful inhibitor of a variety of innate immune responses, including LPS-induced toxaemia. This cholinergic anti-inflammatory pathway acts via the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). Herein, we tested the potential protective effect of nicotine administration in a mouse model of renal I/R injury induced by bilateral clamping of kidney arteries. Renal function, tubular damages and inflammatory response were compared between control animals and mice receiving nicotine at the time of ischemia. Nicotine pretreatment protected mice from renal dysfunction in a dose-dependent manner and through the alpha7nAChR, as attested by the absence of protection in alpha7nAChR-deficient mice. Additionally, nicotine significantly reduced tubular damages, prevented neutrophil infiltration and decreased productions of the CXC-chemokine KC, TNF-alpha and the proinflammatory high-mobility group box 1 protein. Reduced tubular damage in nicotine pre-treated mice was associated with a decrease in tubular cell apoptosis and proliferative response as attested by the reduction of caspase-3 and Ki67 positive cells, respectively. All together, these data highlight that nicotine exerts a protective anti-inflammatory effect during kidney I/R through the cholinergic alpha7nAChR pathway. In addition, this could provide an opportunity to overcome the effect of surgical cholinergic denervation during kidney transplantation.

Donor T-cell development in host thymus after heterotopic limb transplantation in mice.

We developed a mouse model of heterotopic limb transplantation in which we took advantage of Thy1.1 and Thy1.2 congenic strains to track and characterize donor T cells, to determine the role of recipient's thymus in mixed T-cell chimerism induction as well as transplant immunocompetence. The vascularized Thy1.1 limb graft composed of femur, muscle, and skin (VBT) survived long-term in more than 87.5% of Thy1.2 recipients. Percentages of donor-type Thy1.1 T cells increased from day 30 to 90 in thymus and spleen of recipients. Most peripheral donor T cells displayed a naïve phenotype and a few others were regulatory T cells. Thymectomy prevented peripheral T-cell chimerism. Congenic VBT in immunodeficient RAG mice restored their ability to reject skin allografts. These observations suggest that donor T cells differentiated in host thymus might contribute to maintenance of mixed chimerism after transplantation of tissue composite grafts that include vascularized bone.