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RIIB Signaling in Resting and Activated B Cells1
*
Department of Immunology, National Jewish Medical and Research Center, Denver, CO 80206;
Department of Immunology, University of Colorado Health Science Center, Denver, CO 80206; and
Terry Fox Laboratory, British Columbia Cancer Agency, Vancouver, British Columbia, Canada
Fc
RIIB functions as an inhibitory receptor to dampen B cell Ag
receptor signals and immune responses. Accumulating evidence indicates
that ex vivo B cells require the inositol 5-phosphatase, Src homology
domain 2-containing inositol 5-phosphatase (SHIP), for
FcRIIB-mediated inhibitory signaling. However, we report here that
LPS-activated primary B cells do not require SHIP and thus differ from
resting B cells. SHIP-deficient B cell blasts display efficient
FcRIIB-dependent inhibition of calcium mobilization as well as Akt
and extracellular signal-related protein kinase phosphorylation.
Surprisingly, FcRIIB-dependent degradation of phosphatidylinositol
3,4,5-trisphosphate and conversion into phosphatidylinositol
3,4-bisphosphate occur in SHIP-deficient B cell blasts, demonstrating
the function of an additional inositol 5-phosphatase. Further analysis
reveals that while resting cells express only SHIP, B cell blasts also
express the recently described inositol 5-phosphatase, SHIP-2. Finally,
data suggest that both SHIP-2 and SHIP can mediate downstream biologic
consequences of FcRIIB signaling, including inhibition of the
proliferative response.
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