Inhibition of B cell proliferation with anti‐CD19 monoclonal antibodies: anti‐CD19 antibodies do not interfere with early signaling events triggered by anti‐IgM or interleukin 4

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Abstract
The 95‐kDa antigen recognized by the anti‐CD19 panel of monoclonal antibodies is found on the surface of most cells of the B cell lineage. Anti‐CD19 antibodies inhibit B cell proliferation in response to anti‐Ig plus interleukin 4 (IL4), but enhance the response to mitogenic concentrations of either phorbol 12‐myristate 13‐acetate (PMA) or Epstein‐Barr virus. This dichotomy in the effect of anti‐CD19 antibodies suggested that the inhibitory action may be directed at the transmembrane signaling pathways utilized by anti‐IgM and IL4. To investigate this hypothesis, an attempt was made to determine the mechanism of signal transduction utilized by the CD19 antigen, and elucidate its effect on transmembrane signaling invoked by anti‐immunoglobulin and IL4. Binding of anti‐CD19 antibody to B cells did not promote activation of either the phosphoinositide or cAMP signaling pathways. In addition, anti‐CD19 antibody did not inhibit phosphatidylinositol bisphosphate (PIP2) hydrolysis induced by anti‐IgM or IL4, nor did it interfere with cAMP induction by IL4. We also found that anti‐CD19 antibody inhibited PMA plus calcium ionophore‐induced B cell proliferation. This evidence indicates that anti‐CD19 mAb interrupts the signaling cascade at a point distal to receptor‐mediated breakdown of PIP2 and/or activation of adenyl cyclase. This conclusion was fully consistent with experiments in which anti‐CD19 antibody was shown to inhibit DNA but not RNA synthesis, and the observation that anti‐CD19 antibody must be present between 6 h and 20 h after the initiation of the culture suggesting that anti‐CD19 mAb exerts its inhibitory effect in late G0 or G1, after the initial signaling events.