A. Le Moine

Endotoxin-Induced Myeloid-Derived Suppressor Cells Inhibit Alloimmune Responses via Heme Oxygenase-1

Inflammation and cancer are associated with impairment of T‐cell responses by a heterogeneous population of myeloid‐derived suppressor cells (MDSCs) coexpressing CD11b and GR‐1 antigens. MDSCs have been recently implicated in costimulation blockade‐induced transplantation tolerance in rats, which was under the control of inducible NO synthase (iNOS). Herein, we describe CD11b+GR‐1+MDSC‐compatible cells appearing after repetitive injections of lipopolysaccharide (LPS) using a unique mechanism of suppression. These cells suppressed T‐cell proliferation and Th1 and Th2 cytokine production in both mixed lymphocyte reaction and polyclonal stimulation assays. Transfer of CD11b+ cells from LPS‐treated mice in untreated recipients significantly prolonged skin allograft survival. They produced large amounts of IL‐10 and expressed heme oxygenase‐1 (HO‐1), a stress‐responsive enzyme endowed with immunoregulatory and cytoprotective properties not previously associated with MDSC activity. HO‐1 inhibition by the specific inhibitor, SnPP, completely abolished T‐cell suppression and IL‐10 production. In contrast, neither iNOS nor arginase 1 inhibition did affect suppression. Importantly, HO‐1 inhibition before CD11b+ cell transfer prevented the delay of allograft rejection revealing a new MDSC‐associated suppressor mechanism relevant for transplantation.

Atypical Acute Rejection After Hand Transplantation

Skin rejection after hand transplantation is characterized by a maculopapular erythematous rash that may be diffuse, patchy or focal, and distributed over forearms and dorsum of the hands. This ‘classical’ pattern of rejection usually spares the skin of the palm and does not affect the nails. Herein, we report the experience on four cases presenting with an ‘atypical’ pattern of rejection that is novel in involving the palmar skin and the nails. All patients were young and exposed to repetitive and persistent mechanical stress of the palm. Characteristic features of rejection included a desquamative rash associated with dry skin, red papules, scaling and lichenification localized to the palm. Skin lesions were associated with nail dystrophy, degeneration, deformation or loss. Histology of the skin and nail bed revealed a lymphocytic infiltrate with predominance of T cells (CD3+, CD4+ and CD8+), with small numbers of B cells (CD20+ and CD79a+) and a low number of Forkhead transcription factor 3 (FOXP3)‐positive cells in one patient. The lesions persisted over weeks to months, responded poorly to steroid treatment and were managed with antithymocyte globulin (ATG; Thymoglobulin, Genzyme, Cambridge, MA), alemtuzumab and/or intensified maintenance immunosuppression.

CTLA4-Ig restores rejection of MHC class-II mismatched allografts by disabling IL-2-expanded regulatory T cells

Allograft acceptance and tolerance can be achieved by different approaches including inhibition of effector T cell responses through CD28‐dependent costimulatory blockade and induction of peripheral regulatory T cells (Tregs). The observation that Tregs rely upon CD28‐dependent signals for development and peripheral expansion, raises the intriguing possibility of a counterproductive consequence of CTLA4‐Ig administration on tolerance induction. We have investigated the possible negative effect of CTLA4‐Ig on Treg‐mediated tolerance induction using a mouse model of single MHC class II‐mismatched skin grafts in which long‐term acceptance was achieved by short‐term administration of IL‐2/anti‐IL‐2 complex. CTLA4‐Ig treatment was found to abolish Treg‐dependent acceptance in this model, restoring skin allograft rejection and Th1 alloreactivity. CTLA4‐Ig inhibited IL‐2‐driven Treg expansion, and prevented in particular the occurrence of ICOS+ Tregs endowed with potent suppressive capacities. Restoring CD28 signaling was sufficient to counteract the deleterious effect of CTLA4‐Ig on Treg expansion and functionality, in keeping with the hypothesis that costimulatory blockade inhibits Treg expansion and function by limiting the delivery of essential CD28‐dependent signals. Inhibition of regulatory T cell function should therefore be taken into account when designing tolerance protocols based on costimulatory blockade.

Cyclosporine A Drives a Th17- and Th2-Mediated Posttransplant Obliterative Airway Disease

Calcineurin‐inhibitor refractory bronchiolitis obliterans (BO) represents the leading cause of late graft failure after lung transplantation. T helper (Th)2 and Th17 lymphocytes have been associated with BO development. Taking advantage of a fully allogeneic trachea transplantation model in mice, we addressed the pathogenicity of Th cells in obliterative airway disease (OAD) occurring in cyclosporine A (CsA)‐treated recipients. We found that CsA prevented CD8+ T cell infiltration into the graft and downregulated the Th1 response but affected neither Th2 nor Th17 responses in vivo. In secondary mixed lymphocyte cultures, CsA dramatically decreased donor‐specific IFN‐γ production, enhanced IL‐17 production and did not affect IL‐13. As CD4+ depletion efficiently prevented OAD in CsA‐treated recipients, we further explored the role of Th2 and Th17 immunity in vivo. Although IL‐4 and IL‐17 deficient untreated mice developed an OAD comparable to wild‐type recipients, a single cytokine deficiency afforded significant protection in CsA‐treated recipients. In conclusion, CsA treatment unbalances T helper alloreactivity and favors Th2 and Th17 as coexisting pathways mediating chronic rejection of heterotopic tracheal allografts.

Interleukin-9 promotes eosinophilic rejection of mouse heart allografts.

Background. Eosinophils participate in allograft rejection when donor-reactive helper T lymphocytes are T-helper type 2 (Th2)-biased. Whereas the involvement of interleukin (IL)-4 and IL-5 in these forms of rejection is well established, the role of IL-9, another Th2-type cytokine promoting eosinophilia, has not been determined. Methods. We first used real-time polymerase chain reaction to quantify IL-9 mRNA in rejected allografts in a mouse model of fully mismatched heart transplantation in which recipients were devoid of CD8 T cells and developed a Th2 alloimmune response. We then compared allograft survival in wild-type versus IL-9–deficient mice depleted of CD8 T cells. Finally, we compared the fate of major histocompatibility complex class II-mismatched cardiac transplants from wild-type versus IL-9 transgenic donors to determine the influence of IL-9 overexpression within the graft. Results. The Th2 alloimmune response in CD8-deficient mice was associated with the accumulation of IL-9 mRNA in the rejected graft. In IL-9–deficient recipients depleted of CD8 T cells, eosinophil infiltration of heart allografts did not develop, but rejection still occurred. In the major histocompatibility complex class II disparate model, heart allografts from IL-9 transgenic donors were acutely rejected, whereas grafts from wild-type donors did not develop rejection. Acute rejection of IL-9 transgenic hearts was associated with massive eosinophil infiltration and prevented by neutralization of either IL-4 or IL-5. Conclusion. IL-9 is critically involved in heart transplant eosinophilia in conjunction with IL-4 and IL-5.

CD8+ T-Cell depletion and rapamycin synergize with combined coreceptor/stimulation blockade to induce robust limb allograft tolerance in mice.

The growing development of composite tissue allografts (CTA) highlights the need for tolerance induction protocols. Herein, we developed a mouse model of heterotopic limb allograft in a stringent strain combination in which potentially tolerogenic strategies were tested taking advantage of donor stem cells in the grafted limb. BALB/c allografts were transplanted into C57BL/6 mice treated with anti‐CD154 mAb, nondepleting anti‐CD4 combined to either depleting or nondepleting anti‐CD8 mAbs. Some groups received additional rapamycin. Both depleting and nondepleting mAb combinations without rapamycin only delayed limb allograft rejection, whereas the addition of rapamycin induced long‐term allograft survival in both combinations. Nevertheless, robust donor‐specific tolerance, defined by the acceptance of a fresh donor‐type skin allograft and simultaneous rejection of third‐party grafts, required initial CD8+ T‐cell depletion. Mixed donor‐recipient chimerism was observed in lymphoid organs and recipient bone marrow of tolerant but not rejecting animals. Tolerance specificity was confirmed by the inability to produce IL‐2, IFN‐γ and TNF‐α in MLC with donor antigen while significant alloreactivity persisted against third‐ party alloantigens. Collectively, these results show that robust CTA tolerance and mixed donor‐recipient chimerism can be achieved in response to the synergizing combination of rapamycin, transient CD8+ T‐cell depletion and costimulation/coreceptor blockade.

Intracellular nicotinamide adenine dinucleotide promotes TNF-induced necroptosis in a sirtuin-dependent manner.

Cellular necrosis has long been regarded as an incidental and uncontrolled form of cell death. However, a regulated form of cell death termed necroptosis has been identified recently. Necroptosis can be induced by extracellular cytokines, pathogens and several pharmacological compounds, which share the property of triggering the formation of a RIPK3-containing molecular complex supporting cell death. Of interest, most ligands known to induce necroptosis (including notably TNF and FASL) can also promote apoptosis, and the mechanisms regulating the decision of cells to commit to one form of cell death or the other are still poorly defined. We demonstrate herein that intracellular nicotinamide adenine dinucleotide (NAD+) has an important role in supporting cell progression to necroptosis. Using a panel of pharmacological and genetic approaches, we show that intracellular NAD+ promotes necroptosis of the L929 cell line in response to TNF. Use of a pan-sirtuin inhibitor and shRNA-mediated protein knockdown led us to uncover a role for the NAD+-dependent family of sirtuins, and in particular for SIRT2 and SIRT5, in the regulation of the necroptotic cell death program. Thus, and in contrast to a generally held view, intracellular NAD+ does not represent a universal pro-survival factor, but rather acts as a key metabolite regulating the choice of cell demise in response to both intrinsic and extrinsic factors.

A brief focus on memory cells in transplantation.

T and B memory cells are critical for host defences against pathogens. However, converging lines of evidence indicate that alloreactive memory T cells can play a detrimental role in the transplantation setting. This emergence of memory cells seems to be facilitated by several induction therapies and immunosuppressive agents, which lead to T cell loss. Herein, we briefly review some clinical and experimental observations from the literature, highlighting the immunological risk associated with enhanced T-cell memory responses.

Impact of Interleukin-2–expanded Regulatory T Cells in Various Allogeneic Combinations on Mouse Skin Graft Survival

The impact of in vivo regulatory T cells (Treg) expansion using short-term injections of interleukin-2 (IL-2) coupled to a specific anti-IL-2 antibody was examined in various allogeneic combinations of murine skin transplantations. In a model of a single major histocompatibility complex (MHC) class II disparity, the IL-2-expanded Tregs infiltrated the transplanted skin, inhibited Th1 alloreactivity, and prevented acute graft rejection. However, in the presence of increased load of CD4-recognized alloantigens, exogenous IL-2 only moderately prolonged graft survival as attested by CD8 T cell-depletion in full minor plus major mismatched recipients treated with IL-2. If direct CD8 alloreactivity remained intact, the IL-2/anti-IL-2-mediated Tregs expansion failed to delay allograft rejection. This observation was confirmed by the inability of expanded Tregs to delay rejection of multiple minor disparate (MHC matched) skin allografts. Altogether, these results warn that cross-reactive CD8(+) T cells represent an important hurdle to Treg-based tolerance induction.

Autophagy for tolerance induction in transplantation.

Autophagy is a homeostatic and ubiquitous process by which cells recycle injured organelles and proteins through intracellular formation of vesicles (autophagosomes), which undergo secondary fusion with lysosomes for degradation. In steady states, this recycling process facilitates cell longevity, particularly in slow-dividing cells such as podocytes where autophagy plays a role in preventing glomeruloscerosis (1). During metabolic stress, autophagosomes provide nutrients through degradation of their cargo. In these two settings, autophagy ensures cell survival and protects against ischemia-reperfusion injury. In other circumstances, such as chemotherapy administration, autophagy is critically involved in the induced programmed cell death that differs from classical apoptosis (2). Thus, autophagy can promote either cellular survival or death depending on the context.