NK cells contribute to the skin graft rejection promoted by CD4+ T cells activated through the indirect allorecognition pathway.

Rejection of solid organ allografts is promoted by T cells. Recipient T cells can directly recognize intact allo-MHC molecules on donor cells and can also indirectly recognize processed donor-derived allo-peptides presented by recipient antigen-presenting cells in the context of self-MHC molecules. Although CD4(+) T cells primed through the indirect allorecognition pathway alone are sufficient to promote acute allograft rejection, it is unknown how they can mediate graft destruction without cognate recognition of donor cells. In this study, we analyzed the indirect effector mechanism of skin allograft rejection using a mouse model in which SCID recipients bearing MHC class II-deficient skin allografts were adoptively transferred with CD4(+) T cells. Histologically, entire graft necrosis was preceded by mononuclear cell infiltration in the graft epithelia with epithelial cell apoptosis, indicating cell-mediated cytotoxicity against donor cells as an effector mechanism. Beside CD4(+) T cells and macrophages, NK cells infiltrated in the rejecting grafts. Depletion of NK cells as well as blocking of the activating NK receptor NKG2D allowed prolonged survival of the grafts. Expression of NKG2D ligands was up-regulated in the rejecting grafts. These results suggest that NK cells activated through NKG2D contribute to the skin allograft rejection promoted by indirectly primed CD4(+) T cells.

Ito A ，Shimura H ，Nitahara A ，Tomiyama K ，Ito M ，Kanekura T ，Okumura K ，Yagita H ，Kawai K ... -  《-》

The indirect pathway of allorecognition. The occurrence of self-restricted T cell recognition of allo-MHC peptides early in acute renal allograft rejection and its inhibition by conventional immunosuppression.

There is evidence that T cells can "directly" recognize intact allo-MHC molecules on the surface of allogeneic stimulator or target cells, and/or "indirectly" recognize processed allo-MHC peptides presented by self antigen-presenting cells (APCs). We and others have recently demonstrated that in vivo-primed rat CD4+ T cells recognize and proliferate to specific polymorphic amino acid sequences when presented as MHC allopeptides by self APCs. Studies on the mechanisms of indirect T cell recognition of alloantigen are now reported. First, we studied the immunogenicity of 4 synthetic polymorphic class II MHC allopeptides representing full-length sequences of the hypervariable domains of RT1.Du beta (DR or I-E-like) in several responder strains: LEW (RT1(l)), ACI (RT1a), BUF (RT1b), BN (RT1n), and control syngeneic WF (RT1u) strains. Immunogenicity of the individual 25mer allopeptides varied in the different responder strains, indicating that self-restricted T cell recognition of allo-MHC peptides is determined not only by polymorphisms, but also by the responder MHC genotype. Self-restricted CD4+ T cell recognition of processed allo-MHC peptides has been shown to occur during acute skin and cardiac allograft rejection, and there is evidence that this pathway may play an important role in initiating and amplifying the immune response to allografts. T cells from LEW animals primed in vivo by WF (RT1u) vascularized renal allografts were capable of proliferating to the RT1.Du beta peptides as early as 3 days postengraftment, when presented by self APCs. We then tested the effects of various immunosuppressive drugs on self-restricted primed T cell proliferative response to an immunogenic MHC allopeptide in vitro. Methylprednisolone, cyclosporine, and FK506 inhibited the proliferative response of RT1.Du beta 2-primed LEW T cells in a dose-dependent fashion. In addition, a single injection of cyclosporine (25 mg/kg i.m.) to LEW recipients of WF renal allografts on the day of transplantation completely abolished the proliferative response of in vivo-primed T cells to RT1.Du beta 2, indicating the susceptibility of the indirect pathway of allorecognition to conventional immunosuppressive drugs.

Gallon L ，Watschinger B ，Murphy B ，Akalin E ，Sayegh MH ，Carpenter CB ... -  《TRANSPLANTATION》

Adoptively transferred CD4+ lymphocytes from CD8 -/- mice are sufficient to mediate the rejection of MHC class II or class I disparate skin grafts.

Recent studies revealed that CD4+ cells initiate allograft rejection through direct recognition of allogeneic MHC class II Ags and indirect recognition of MHC peptides processed by self APCs. Both pathways were shown to help CD8+ cells that eventually lysed allogeneic MHC class I-presenting targets. There was little evidence, however, that CD4+ cells are sufficient for graft rejection. We studied skin graft rejection by CD8-deficient (CD8 -/-) mice. We showed that BALB/cJ(H-2d) CD8 -/- mice could reject allogeneic skin from C57BL/6J(H-2b) mice deficient in MHC class I or in MHC class II Ags. To understand the role of CD4+ cells in this process, we isolated them from CD8 -/- mice and transferred them to BALB/cJ nude mice that had been grafted with allogeneic skin (H-2b) from animals deficient in MHC class I or MHC class II. Nude mice injected with CD4+ cells rejected MHC class II and, albeit more slowly, MHC class I disparate skins. We showed in vitro evidence that CD4+ cells were not cytotoxic toward MHC class I or MHC class II disparate targets and that they recognized MHC class I allogeneic targets through indirect recognition. CD4+ cells produced Th1 cytokines, but not IL-4, following stimulation with allogeneic cells. Furthermore, intragraft TNF-alpha was elevated in skin grafted onto nude mice reconstituted with CD4+ cells compared with nonreconstituted mice. This suggests that MHC class II- or MHC class I-guided CD4+ cells alone are sufficient to induce rejection by the generation of cytokine-induced lesions.

Dalloul AH ，Chmouzis E ，Ngo K ，Fung-Leung WP ... -  《JOURNAL OF IMMUNOLOGY》

The replacement of graft endothelium by recipient-type cells conditions allograft rejection mediated by indirect pathway CD4+ T cells.

Whereas the participation of alloreactive T cells sensitized by indirect allorecognition in graft rejection is well documented, the nature of recipient antigen presenting cells recognized by indirect pathway CD4+ T cells within the graft has yet to be identified. The purpose of this study was to determine the role played by graft endothelium replacement in the immune recognition of cardiac allografts rejected by indirect pathway CD4+ T cells. Transgenic RAG2-/- mice expressing I-Ab-restricted male antigen H-Y-specific TcR were studied for their capacity to reject H-2k male cardiac allografts. Chronic vascular rejection in this model was due to the indirect recognition of H-Y antigen shed from H-2k male allograft and presented by the recipient's own I-Ab APC to transgenic T cells. Immunohistochemical analysis of rejected grafts revealed the presence of numerous microvascular endothelial cells (EC) that expressed recipient's I-Ab MHC class II molecules. This observation suggested that graft endothelium replacement by I-Ab-positive cells of recipient origin could stimulate the rejection of male H-2k graft by I-Ab-restricted H-Y-specific T cells. To investigate further this possibility, hearts from H-2b-into-H-2k irradiation bone marrow (BM) chimera were transplanted in transgenic recipients. A direct correlation was observed between the presence of I-Ab-positive EC within myocardial microvessels and the induction of acute rejection of chimeric H-2k male cardiac allografts transplanted in transgenic recipients. We conclude that graft endothelium replacement by recipient-type cells is required for the rejection of cardiac allograft mediated by indirect pathway alloreactive CD4+ T cells.

Kapessidou Y ，Habran C ，Buonocore S ，Flamand V ，Barvais L ，Goldman M ，Braun MY ... -  《TRANSPLANTATION》

Langerhans cells, orthotopic corneal allografts, and direct and indirect pathways of T-cell allorecognition.

To determine after orthotopic corneal allografting the role of Langerhans cells in activation of T cells via the direct and indirect pathways of allorecognition and the relationship between these pathways and the rapidity of graft rejection. Corneas from eyes of normal mice and from eyes after superficial cauterization were grafted to eyes of major histocompatibility complex (MHC) and/or minor histocompatibility (H)-disparate recipient mice. The grafts were analyzed through time for content of class 1 MHC- bearing Langerhans cells and for rejection or acceptance. Graft recipients were evaluated for acquisition of delayed hypersensitivity (DH) and cytotoxic T cells (Tc) directed at donor MHC and minor H alloantigens. Langerhans cells migrated more rapidly into epithelium of cauterized grafts than normal grafts. Unlike normal grafts, the vast majority of cauterized allografts were rejected within 2 weeks. Normal grafts induced neither DH nor Tc directed at donor MHC antigens, whereas cauterized grafts induced both DH and Tc specific for donor MHC. All grafts induced DH directed at donor minor H antigens, but only rejected grafts correlated with acquisition of Tc directed at donor minor H antigens. CONCLUSIONS. The rapidity of orthotopic corneal allograft rejection correlated with density of Langerhans cells within epithelium and with acquisition of donor-specific DH and Tc. Although recipient-derived Langerhans cells promoted minor H-specific, self-MHC-restricted T cells (indirect pathway) and subacute graft rejection, donor-derived Langerhans cells promoted early, acute rejection in conjunction with allogeneic MHC-specific Tc (direct pathway).

Sano Y ，Ksander BR ，Streilein JW 《INVESTIGATIVE OPHTHALMOLOGY & VISUAL SCIENCE》