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CD19 signal transduction in normal human B cells: linkage to downstream pathways requires phosphatidylinositol 3‐kinase, protein kinase C and Ca2+

CD19 is required for normal antibody responses in mice. We have shown that CD19 enhances the activation of extracellular signal‐regulated kinase (ERK) 2 by membrane (m) IgM but otherwise little is known of CD19 signaling in primary human cells. We now ask which pathways link CD19 with ERK2 in human tonsillar B cells. In analyses of signaling intermediates, the phosphatidylinositol 3‐kinase (PI3K) inhibitor wortmannin partially suppressed the release of Ca2+ induced by coligation of CD19 and mIgM but the selective PKC inhibitor bisindolylmaleimide I (BIM‐I) did not. The Ca2+ chelator 1,2‐bis(2‐aminophenoxy)ethane‐N,N,N',N'‐tetraacetic acid, BIM‐I and wortmannin each had only a small effect on ERK2 activation induced by surface IgM alone but blocked the enhancement of that activation by CD19/mIgM coligation. To analyze the mitogen‐activated protein kinase (MAPK) cascade, we measured activation of Raf, MAPK‐ or ERK kinase (MEK) 1 and ERK2. CD19 consistently enhanced activation of ERK2 and MEK1. However, synergistic activation of Raf was variably observed. In subpopulation analyses, synergistic activation of Raf1 was consistently observed in the IgDlow but not in the IgDhigh cells. Thus, in normal human B cells, PI3K is upstream of the Ca2+ response while PI3K, Ca2+ release and protein kinase C are all required for ERK2 activation, and CD19 enhances the MAPK cascade at multiple levels, depending on the state of differentiation.