CD40 LIGAND-INDEPENDENT B-CELL ACTIVATION REVEALED BY CD40 LIGAND-DEFICIENT T-CELL CLONES - EVIDENCE FOR DISTINCT ACTIVATION REQUIREMENTS FOR ANTIBODY-FORMATION AND B-CELL PROLIFERATION

We report the capacity of CD40 ligand (CD40L)-negative T cell clones t o activate human B cells. CD40L-negative T cells induce a level of B c ell proliferation 10-20% of that seen with normal T cells. The signal provided by the negative clones is synergistic with that derived from a CD40L transfectant, and restores B cell proliferation to normal leve ls, showing that CD40L-negative T cell clones are not inherently inhib itory for B cells. Although their capacity to induce proliferation was much reduced, CD40L-negative T cell clones were still strong inducers of B cell differentiation to plasma cells. This differentiation to pl asma cells was inhibited by a CD40L transfectant. The data are discuss ed with regard to the normal in vivo mechanism for maintaining B cell memory and memory antibody responses to T-dependent antigens.