The dual ability of B cells to stimulate and inhibit T-cell proliferation. Cocultures of CFSE-labeled, activated T cells with resting or activated B cells were established at different T:B cell ratios. (A) On the left, proliferation of T cells alone. In the middle, freshly harvested resting B cells promoted proliferation of activated T cells in a representative coculture. On the right, activated B cells inhibited T-cell proliferation in a representative coculture. (B) Results for 3 independent coculture experiments with T cells and resting B cells and 3 independent coculture experiments with T cells and activated B cells. All cocultures were established with autologous T and B cells. (C) 5′-AMP production by resting or activated B cells in the presence of eATP. 25 000 B cells were incubated in 200 μL PBS in wells of 96-well plates in the presence of 20 μL ATP for various time periods. 5′-AMP production was measured by mass spectrometry. Control wells contained ATP alone. Representative data from 1 of 3 experiments performed with cells of different normal donors.