Functional interaction between B cell subpopulations defined by CD23 expression

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Abstract
Using the CD23 monoclonal antibody (mAb) MHM6 and sheep anti-mouse Ig bound to magnetic beads we have obtained highly purified populations of MHM6+ and MHM6− tonsil B cells. We have found that the increased expression of MHM6 reactivity seen on B cells after activation results from up-regulation of antigen on cells already weakly positive and not from expression of new antigen on the previously negative population. The strong proliferative responses of MHM6+ cells seen in the presence of anti-IgM (αμ) and interleukin 4 (IL4) or the CDw40 mAb G28-5, and with Staphylococcus aureus Cowan I (SAC), and to a lesser extent with the phorbol ester 12-0-tetradecanoylphorbol 13-acetate (TPA), resemble that seen among unfractionated B cells. In contrast, the MHM6− population cultured alone responds only weakly to αμ+ G28-5 or SAC and exhibits virtually no response to αμ + IL4 or TPA With all these mitogenic stimuli, tritiated thymidine uptake by the MHM6− population is augmented three- to sixfold by the addition of mitomycin C (MC)-treated MHM6+ cells. Pretreatment of cells with anti-leukocyte functional antigen 1 mAb has little effect on the subsequent proliferation of the MHM6− population but shows cell contact to be critical for the proliferation of MHM6′ cells. Such pretreatment has revealed that the functional interaction observed between MHM6+ and MHM6− cells is dependent on both cell contact and the presence of an MHM6+ cell-derived soluble component We have found that addition of soluble CD23, purified from Epstein-Barr virus-transformed lymphoblastoid cell line supernatant, increases the proliferative response of MHM6− tonsil B cells to mitogenic stimuli in the presence of inactivated MHM6+ cells but has no effect on proliferation when MHM6+ cells are absent By way of contrast to normal B lymphocytes, we have examined functional responses of prolymphocytic leukemia (PLL) B cells. Although these cells, when freshly isolated, show comparable levels of CD23 expression to normal B cells, this expression is not increased upon activation. In addition, in contrast to normal B cells, the PLL MHM6− population cultured alone shows a strong proliferative response to various mitogenic stimuli, comparable to that of MHM6+ or unfractionated cells, and this response is not augmented by the addition of MC-treated MHM6+ cells. Thus, a novel functional interaction is described between normal, but not leukemic, B cell populations defined by their expression of CD23. This interaction involves two components, one mediated through cell contact and the other by the presence of soluble factor(s) derived from the MHM6+ cells with neither component alone being effective.