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,
Departments of
*
Medicine and
Microbiology, University of Alabama, Birmingham, AL 35294; and the
Department of Veterans Affairs, Birmingham, AL 35233
Splenic marginal zone (MZ) and follicular mantle (FO) B cells
differ in their responses to stimuli in vitro and in vivo. We have
previously shown that MZ cells exhibit greater calcium responses after
ligation of membrane IgM (mIgM). We have now investigated the molecular
mechanism underlying the difference in calcium responses following
ligation of mIgM and studied the response to total B cell receptor
ligation in these two subsets. We compared key cellular proteins
involved in calcium signaling in MZ and FO cells. Tyrosine
phosphorylation and activity of phospholipase C-
2 and Syk protein
tyrosine kinase were significantly higher in MZ cells than in FO cells
after mIgM engagement, providing a likely explanation for our previous
findings. Tyrosine phosphorylation of CD22 and expression of Src
homology 2-containing inositol phosphatase and Src homology
2-containing protein tyrosine phosphatase-1 were also higher in the MZ
cells. Expression and tyrosine phosphorylation of Btk, BLNK, Vav, or
phosphatidylinositol 3-kinase were equivalent. In contrast, stimulation
with anti-
induced equivalent increases in calcium and
activation of Syk in the two subsets. These signals were also
equivalent in cells from IgM transgenic, JH knockout mice,
which have equivalent levels of IgM in both subsets. With total spleen
B cells, Btk was maximally phosphorylated at a lower concentration of
anti- than Syk. Thus, calcium signaling in the subsets of mature
B cells reflects the amount of Ig ligated more than the isotype or the
subset and this correlates with the relative tyrosine phosphorylation
of Syk.
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