Role of B cells in the expression of the PPD response of human lymphocytes in vitro.

Human blood lymphocytes were tested for their proliferative response to purified protein derivative (PPD) in vitro. Purified T cells showed a lower response than the orginial purified lymphocyte suspension. Purified B cells were unresponsive to PPD. Mixtures of preparations of T and B cells were stimulated to a greater extent by this antigen than anticipated by adding the responses of the two cell types tested separately. This cell synergy could largely be explained by soluble factors released by PPD-stimulated T cells, which were found to be mitogenic for B cells.