Pathways of helper CD4 T cell allorecognition in generating alloantibody and CD8 T cell alloimmunity

Background. The relative contributions of the "direct" and "indirect" pathways of CD4 T cell allorecognition in providing help for generating effective humoral and CD8 T cell alloimmunity remain unclear. Here, the generation of alloantibody and cytotoxic CD8 T cell responses to a vascularized allograft were examined in a murine adoptive-transfer model in which help could only be provided by transferred CD4 T cells recognizing alloantigen exclusively through the direct pathway. Methods. Rejection kinetics and the development of alloantibody and cytotoxic CD8 T cell responses to MHC-mismatched H-2(d) heart grafts were compared when CD4 T cell help was present (wild-type H-2(b) recipients), or absent (CD4 T cell deficient, MHC class II-/- H-2(b) recipients [B6CII(-/-)]), or available only through the direct pathway (B6CII(-/-) mice reconstituted with wild-type CD4 T cells). Results. BALB/c allografts were rejected by B6 mice rapidly (median survival time [MST] 7 days) with strong CD8 T cell effector and alloantibody responses, but were rejected by B6CII(-/-) mice more slowly (MST 23 days), with markedly reduced CD8 T cell responses and no detectable alloantibody. CD4 T cell reconstitution of B6CII(-/-) recipients accelerated heart graft rejection to near that of wild-type recipients (MST 13 days), with complete restoration of cytotoxic CD8 T cell responses but without detectable IgM or IgG alloantibody. Conclusions. Different pathways of helper T cell allorecognition are responsible for generating humoral and CD8 T cell alloimmunity. CD4 T cell help provided exclusively through the direct pathway generates strong cytotoxic CD8 T cell responses that effect rapid heart graft rejection.