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 alloantigens in vivo have not been established. Two alloantigen presentation pathways may be used: (1) allogeneic class I and class II MHC molecules may be recognized directly by host CD8+ and CD4+ cells, respectively, or (2) allogeneic MHC molecules may be processed as foreign peptide and presented by host antigen-presenting cells to CD4+ cells 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 activation in vivo. Limiting dilution analysis was used to quantify the localization of interleukin-2-producing helper T lymphocytes (HTL) following implantation of sponge allografts. Allografts either were disparate at both class I and class 11, or were derived from beta2-microglobulin knockout (beta2M-/-) mice, which express class II but are deficient in class I. Two measures of in vivo HTL function were monitored: (1) the accumulation of HTL within the allograft (a process that is dependent 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 the allograft, and cytolytic alloantibodies were present in the serum. When mice were implanted with beta2M-/- sponge allografts, HTL failed to infiltrate these class I-deficient allografts, and alloantibodies were not detectable in the sera of recipients of beta2M-/- sponge allografts. This in vivo requirement for class I expression was not reflected by traditional in vitro measures of HTL function; cells obtained from lymphoid tissues mounted a mixed lymphocyte response and produced interleukin-2 when stimulated with beta2M-/- splenocytes in vitro. One possible interpretation of these data is that in vivo HTL functions are dependent upon the presence of class I-reactive CD8+ T cells. However, HTL readily infiltrated grafts expressing both class I and class II when recipients depleted of CD8+ T cells, and alloantibodies were produced. These observations support the idea that indirect presentation of allogeneic class I molecules plays a critical role in regulating CD4+ HTL functions associated with allograft rejection in vivo.