Integration of activatory and inhibitory signals in human B-cells (original) (raw)

Fc7 receptors type lib1 (FcTRIIbl) inhibit B-cell activation when co-ligated with B-cell antigen receptors BCR) by immune complexes. In murine B-cells the inhibition is mediated by the interaction of the phosphorylated immunoreceptor tyrosine-based inhibitory motif (P-ITIM) of FcTRIIbl with the SH2 domain containing protein tyrosine phosphatase, SHPI. To clarify the mechanism of FcTRIIb mediated inhibition of human B-cells we have studied the association of signaling molecules with human FcTRIIbl after co-ligating with BCR. FcTRIIbl were affinity purified from the Burkitt lymphoma cell line, BL41. Several tyrosine phosphorylated proteins were co-isolated with FcTRIIbl at 145, 110, and 50-60 kDa, which were not present in FcTRIIbl free immune complexes. Among these molecules we have identified the p52 Shc adaptor protein. Furthermore. we have shown that the insolubilised synthetic peptide corresponding P-ITIM bound Shc, Lyn and the p75 and p110 unidentified tyrosine phosphorylated proteins. Here we describe that the cell membrane associated Shc is partially dephosphorylated in BCR-Fc,'RIlbl co-ligated samples, suggesting that its function in regulating p21ras monomeric G protein is impaired. Indeed. we have detected a lower p21ras activity in BCR-FcyRIIbl co-crosslinked samples. These data indicate that co-ligation of BCR and Fc;'Rllbl interrupts signal transduction between protein tyrosine kinase activation and p21ras mediated activation pathway. Since in contrast to the mouse B-cells both Fc),RIIbl and FcTRIIb2 are expressed in human B-cells, we have investigated the inhibitory function of the two receptors in Fc),RIIb negative Burkitt lymphoma cell line ST486 transfected with FcyRIIbl and Fc;.'RIIb2. respectively. Both FcTRIIbl and Fc?,RIIb2 inhibited the rise of intracellular Ca 2 + induced by the crosslinking of BCR. The rate of the inhibition depended on the ratio of the co-crosslinked receptors (BCR-FcyRIIbl) to the crosslinked BCR (BCR-BCR). Co-crosslinking of the two receptors inhibited not only the capacitive Ca 2+ entry but rather the total Ca 2+ response in both FcTRIlbl and FcTRIlb2 transfected human B-cells. CD19 represents the signal transduction unit of complement receptor. CR2 ICD21). and is responsible for the complement activating IgM-immune complex induced enhancement of B-cell activation. Co-crosslinking of CD19 and BCR was shown to enhance B-cell activation due to the recruitment of further signaling molecules Io the activator complex by the phosphorylated tyrosine residues of CDI9. Here we show a novel finding that co-ligation of CDI9 with Fc;,Rllbl inhibits the CD19-induced upregulation of Ca 2+ response. The results indicate that IgG plus complement containing immune complexes may inhibit B-cell activation in vivo, due to the FcyRIIbl-mediated interruption of signal transduction via both BCR and CD19.