CLL B cells offer T-cell costimulatory type help to target, nonmalignant B cells. (A) CLL B cells induce normal, target B cells to produce antibodies in the presence of PWM. A total of 5 × 10⁴ autologous T cells (AutoT_xr) from the normal target B-cell donor, CLL B cells (CLLB_xr), or T cells derived from the CLL patient (CLLT_xr) were irradiated (2,000 rad) and placed in culture with 5 × 10⁴ target, purified B cells from the normal donor, in the absence or presence of PWM. Control circumstances included 5 × 10⁴ target, unirradiated B cells alone, with and without PWM, and 5 × 10⁴ irradiated CLL B cells, in the absence of target B cells. IgG production was assayed by ELISA from supernatants obtained after 7 days of coculture. (B) CLL B-cell–induced, PWM-dependent IgG production by normal B cells is inhibited by antibody to CD40L. The experiment shown is typical of a series in which 8 × 10⁴irradiated CLL B cells were exposed to MoAbs (final concentration, 5 μg/mL) before coculture with 4 × 10⁴ normal, unirradiated B cells and assayed for IgG production. In this case, CLL B cells from case no. 13 induced greater than 400 ng/mL of IgG in the supernatant at 7 days, in an effect that was reduced in the presence of antibodies including antibody to CD40L.