ALLOGENEIC CLASS-I MHC REQUIREMENT FOR ALLOANTIGEN-REACTIVE HELPER T-LYMPHOCYTE RESPONSES IN-VIVO - EVIDENCE FOR INDIRECT PRESENTATION OF ALLOANTIGEN

The mechanisms by which host T cells recognize transplant-associated a lloantigens in vivo have not been established, Two alloantigen present ation pathways may be used: (I) allogeneic class I and class II MHC mo lecules may be recognized directly by host CD8(+) and CD4(+) cells, re spectively, or (2) allogeneic MHC molecules may be processed as foreig n peptide and presented by hose antigen-presenting cells to CD4(+) cel ls in the context of self class II proteins, In this study, the sponge matrix allograft model was used to examine the relative contributions of these alloantigen presentation pathways to CD4(+) T-cell activatio n in vivo, Limiting dilution analysis was used to quantify the localiz ation of interleukin-2-producing helper T lymphocytes (HTL) following implantation of sponge allografts, Allografts either were disparate at both class I and class II, or were derived from beta 2-micro-globulin knockout (beta 2M-/-) mice, which express class II but are deficient in class I. Two measures of in vivo HTL function were monitored: (1) t he accumulation of HTL within the allograft: (a process that is depend ent upon antigen-driven cytokine production), and (2) the development of cytolytic. alloantibodies, After implantation of sponge allografts expressing both class I and class II, HTL were readily detectable in t he allograft., and cytolytic alloantibodies were present in the? serum , When mice were implanted with beta 2M-/- sponge allografts, HTL fail ed to infiltrate these class I-deficient allografts, and alloantibodie s were not detectable in. the sera of recipients of beta 2M-/- sponge allografts. This in vivo requirement for class I expression was not re flected by traditional in vitro measures of HTL function; cells obtain ed from lymphoid tissues mounted a mixed lymphocyte response and produ ced interleukin-2 when stimulated with beta 2M-/- splenocytes in vitro , One possible interpretation of these data is that in vivo HTL functi ons are dependent upon the presence of class I-reactive CD8(+) T cells , However, HTL readily infiltrated grafts expressing both class I and class Il when recipients were depleted of CD8(+) T cells, and alloanti bodies were produced. These observations support the idea that indirec t presentation of allogeneic class I molecules plays a critical role i n regulating CD4(+) HTL functions associated with allograft rejection in vivo.