B cell superstimulatory influenza virus (H2-subtype) induces B cell proliferation by a PKC-activating, Ca(2+)-independent mechanism.

The influenza virus hemagglutinin glycoprotein (HA) induces a vigorous B cell proliferation and Ig-synthesis by an unknown activation mechanism, which is susceptible to the inhibitory effects of anti-Ig and anti-class II mAbs. To gain further insight into the activation mode of this T cell-independent, B cell "superstimulatory" virus, we analyzed the sensitivity of H2-subtype virus-mediated B cell activation to the inhibitory effects of various signal transduction-blocking agents and compared it to the well characterized anti-mu-mediated and the LPS-employed pathway. Cyclic-AMP agonists (cAMP-analogues, pentoxifylline, cholera toxin, and forskolin) blocked HA-mediated activation of B cells only at concentrations at least 50-fold higher than required for blocking of anti-mu-induced activation. However, HA-treatment failed to induce an increase in intracellular cAMP levels in responding B cells. The B cell response to HA was highly resistant to calcineurin-inhibitory cyclosporin-A treatment and did not result in a measurable Ca2+ influx. Similarly, HA failed to induce an increase in tyrosine phosphorylations, including phosphorylation of phospholipase C gamma 2. HA-activated B cells showed an increase in membrane-associated protein kinase C activity, and depletion of protein kinase C by pretreatment of B cells with phorbol esters inhibited a subsequent activation by HA. Collectively, our results provide a new example of B cell stimulation by multivalent type-2 Ags, which seems to be mediated by a phosphatidylinositol- and Ca(2+)-independent signaling pathway.