Synergistic T-T cell interaction present in alloreactivity: determination of 'MLR helper' T cell subsets.

CD4 and CD8 T cell subsets involved in primary allo-MLR responses were re-evaluated in the present study. The slope analysis of B6 allo-MLR responses to MHC class I-disparate bm1 and class II-disparate bm12 revealed the presence of a T-T cell interaction between CD4 and CD8 T cell subsets. The allo-MLR response of B6 CD4 T cells to bm12 was completely blocked by anti-L3T4, indicating that the function of CD4 T cells is required for allo-MLR responses to bm12. It was further demonstrated that the function of CD8 T cells in class II-disparate allo-MLR is to augment the proliferative response of CD4 T cells. On the other hand, CD8 T cells primarily responded to class I-disparate allo-MLR, and CD4 T cells function to augment the proliferative response of CD8 T cells. It was further indicated that CD4 and CD8 T cells recognize both class I and class II MHC. The CD4 T cell recognition of allo-class I and CD8 T cell recognition of allo-class II induces insufficient signals for proliferation of these T cells. Thus, the class I-reactive CD4 and class II-reactive CD8 T cell subsets function as helper cells ('MLR helper' T cells) for proliferating T cells in MLR responses.