Small, resting B cells can be induced to proliferate by direct signals from activated helper T cells.

We have investigated the role of T lymphocytes in the activation of small, resting B cells. Homogeneous populations of small human B cells were purified with a fluorescence-activated cell sorter by positive and negative immunofluorescent staining techniques. When these small B cells were cultured with mitogen-activated Leu-3+ T cells, significant B cell proliferation was observed. This T cell-dependent B cell proliferative response did not require cross-linkage of B cell surface immunoglobulin molecules or B cell exposure to the mitogens employed for T cell activation. Culture supernatants from activated helper T cells could neither induce small B cell proliferation nor augment the B cell response observed with limiting numbers of T cells. These data indicate that activated helper T cells can provide all the signals required for the activation and proliferation of small, resting B cells and suggest that direct T-B cell contact is a critical element for this T-B cell interaction.