The immune response results from the interactions of lymphocytes of many specificities and functions. One of these, the helper T cell, is required for the optimal expression of the functions of other subsets of T cells as well as B lymphocytes. The experiments are designed to further our understanding of helper T-cell function and the mechanisms that control their interactions with B lymphocytes. First, we are evaluating the role of interleukin-2 in the responses of B lymphocytes. We have identified a novel Thy-1+ cell that arises in cultures containing interleukin-2, whose function may be to induce resting B cells to enter cell cycle. We are continuing studies directed at understanding this population. Second, we are evaluating the function of surface immunoglobulin and class II molecules in the triggering of B lymphocytes. Using a B-cell lymphoma that expressed I-A snd I-E molecules but utilizes only I-E molecules as triggering receptors, we are evaluating the relative mobilities of these molecules in the membrane, their numbers, and the dynamics of these molecules in the membrane of unactivated and activated cells. Third, we are investigating the selective use of the I-E molecule as a triggering receptor and its relationship with the expression of certain immunoglobulin V regions during the immune response (i.e., immune response gene control of B-cell idiotypes). Together, these studies will provide information necessary to understand the regulation of interactions of functionally distinct subpopulations of lymphocytes, the basis of specificity in the immune response, and the selection of the B-lymphocyte repertoire. The results of these experiments will also provide insights into the nature of aberrant lymphocyte responses and growth. (LB)