In the absence of direct alloreactivity, a low indirect alloresponse is likely to result in the lack of differentiation of functional CTL, a phenomenon we previously documented in the case of corneal allotransplants [48]. effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8+cytotoxic T cells response and markedly delayed the rejection of skin allografts while it inducedde novochronic rejection of heart transplants. This shows that Th2 cells activatedviaindirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants. Keywords:Chronic graft rejection, Heart transplantation, Skin graft, T-cell allorecognition, Th1/Th2 cells == Introduction == Transplantation of allogeneic tissues triggers a vigorous inflammatory immune response that is initiatedviathe recognition of donor Ag by the recipient CD4+T lymphocytes. Such T-cell allorecognition is mediatedviatwo distinct pathways: the direct pathway in which T cells are stimulated by intact allogeneic MHC II molecules displayed on donor APC [1,2] and the indirect pathway in which T cells interact with processed alloantigen peptides presented by self-MHC II molecules at the surface of recipient Phenylbutazone (Butazolidin, Butatron) APC [35]. In direct alloreactivity, donor bone marrow-derived passenger leukocytes present a multitude of peptides in an MHC class II context therefore eliciting a polyclonal CD4+T-cell response in the hosts Phenylbutazone (Butazolidin, Butatron) lymphoid organs. Similarly, it is traditionally accepted that this reaction represents the traveling push behind early acute graft rejection. On the other hand, the indirect T-cell alloresponse is definitely oligoclonal in that it is mediated by a selected set of alloreactive T-cell clones realizing a few dominating peptides offered by self-MHC [6,7]. Despite its low rate of recurrence, the indirect alloresponse is sufficient to ensure acute rejection of pores and skin allotransplants [8,9]. However, it is unclear whether, in the absence of a direct CD4+T-cell alloresponse, indirect alloreactivity can mediate on its own acute rejection of less immunogenic vascularized solid organ transplants, including kidneys Phenylbutazone (Butazolidin, Butatron) and hearts. This presumably depends upon the strength of the indirect response and its ability to promote the differentiation of practical alloreactive CD8+cytotoxic T cells (CTL). Early acute allograft rejection of solid organ transplants can be prevented with immunosuppressive medicines such as calcineurin TIE1 inhibitors. However, a large portion of heart, lung and kidney transplants ultimately succumb to chronic rejection, a slow process involving perivascular swelling, fibrosis and arteriosclerosis associated with intimal thickening and subsequent luminal occlusion of graft vessels [1012]. It is believed the CD4+T-cell direct alloresponse gradually fades aside after transplantation as donor passenger leukocytes vanish. In contrast, the indirect alloresponse is definitely perpetuatedviaspreading to formerly cryptic determinants on alloantigens and graft tissue-specific Ag [1318]. Also, indirect alloreactivity is definitely thought to play a critical part in the production of alloantibodies [1921] that are known mediators of the chronic rejection process [2226]. Therefore, there is strong circumstantial evidence suggesting that indirect rather than direct type of alloreactivity initiates chronic rejection of allografts. In support of this view, some correlation between the presence of indirect alloreactivity and chronic rejection has been reported [2731]. Additionally, a study by Madsens group demonstrates immunization having a donor MHC peptide can accelerate the onset of chronic allograft vasculopathy in heart-transplanted miniature swines [32]. However, there is still no formal demonstration the indirect alloresponse by CD4+T cells is necessary and/or adequate to inducede novochronic rejection of organ allotransplants. Here, we investigated the contribution of alloreactivity by Phenylbutazone (Butazolidin, Butatron) Th1 and Th2 cells to the acute and chronic allograft rejection of pores and skin and heart transplants in one MHC class I-disparate donor/recipient (BALB/c to BALB/c-dm2 (dm2)) mouse combination. With this model, pores and skin allografts are acutely declined within 10 days post-transplantation while heart transplants are approved permanently without indications of chronic rejection. We observed that induction of a pro-inflammatory Th1 response to a dominating MHC class I allopeptide (Ld6180) accelerated acute rejection of pores and skin but it did not provoke the rejection of heart allotransplants. In contrast, while immune deviation of the alloreactivity toward a Th2 response markedly delayed acute rejection of pores and skin allografts by Th1 cells, it induced chronic rejection of cardiac transplants. This demonstrates Th2 cells can exert dual effects on acute and chronic forms of allograft rejection. The implications of these findings for the design of selective immune therapies in allotransplantation are discussed. == Results == == Immunogenicity of.