Evidence implicating L3T4 in class II MHC antigen reactivity; monoclonal antibody GK1.5 (anti-L3T4a) blocks class II MHC antigen-specific proliferation, release of lymphokines, and binding by cloned murine helper T lymphocyte lines.

Monoclonal antibody GK1.5 recognizes a determinant, designated L3T4a, on the murine T cell surface molecule L3T4. The expression of L3T4a by functional murine T cell clones appears to correlate primarily with class II MHC antigen reactivity rather than with functional phenotype. In previous studies, antigen-specific cytolysis by a cloned class II MHC antigen(I-Ak)-reactive CTL line was found to be blocked entirely by monoclonal antibody (mAb) GK1.5, at a step before the lethal hit. In the present studies, we demonstrate that mAb GK1.5 profoundly blocks antigen-specific proliferation and release of lymphokines by cloned murine class II MHC antigen-reactive helper T lymphocyte (HTL) lines. Analysis of cloned T cell hybridomas, however, suggests that there exists clonal heterogeneity in the degree of inhibition of class II MHC antigen-specific function by mAb GK 1.5. Finally, we present evidence that mAb GK1.5 blocks class II MHC antigen-specific function by blocking class II MHC antigen-specific binding. The data presented here lend considerable support to the concept both that L3T4 and the human Leu-3/T4 molecules are similar and that L3T4 plays a role in class II MHC antigen-reactivity by murine T cells.