SHIP Down-Regulates FcεR1-Induced Degranulation at Supraoptimal IgE or Antigen Levels

Abstract

Cross-linking of the IgE-loaded high-affinity IgE receptor (FcεR1) by multivalent Ags results in mast cell activation and subsequent release of multiple proinflammatory mediators. The dose-response curve for FcεR1-mediated degranulation is bell-shaped, regardless of whether the IgE or the Ag concentration is varied. Although overall calcium influx follows this bell-shaped curve, intracellular calcium release continues to increase at supraoptimal IgE or Ag concentrations. As well, overall calcium mobilization adopts more transient kinetics when stimulations are conducted with supraoptimal instead of optimal Ag concentrations. Moreover, certain early signaling events continue to increase whereas degranulation drops under supraoptimal conditions. We identified SHIP, possibly in association with the FcεR1 β-chain, as a critical negative regulator acting within the inhibitory (supraoptimal) region of the dose-response curve that shifts the kinetics of calcium mobilization from a sustained to a transient response. Consistent with this, we found that degranulation of SHIP-deficient murine bone marrow-derived mast cells was not significantly reduced at supraoptimal Ag levels. A potential mediator of SHIP action, Bruton’s tyrosine kinase, did not seem to play a role within the supraoptimal suppression of degranulation. Interestingly, SHIP was found to colocalize with the actin cytoskeleton (which has been shown previously to mediate the inhibition of degranulation at supraoptimal Ag doses). These results suggest that SHIP, together with other negative regulators, restrains bone marrow-derived mast cell activation at supraoptimal IgE or Ag concentrations in concert with the actin cytoskeleton.

Granzyme B is expressed in mouse mast cells in vivo and in vitro and causes delayed cell death independent of perforin

“…Mast cells differentiated in vitro from bone marrow of C57BL/6 mouse strain (B6), 16 termed bone marrow-derived mast cells (BMMC), consisted of greater than 95% CD117 þ ST2 þ cells with only a minority (between 0.6 to 1%), if any, also staining for Gr-1 (data not shown), Mac-1, NK1.1, B220, CD4 or CD8 (Figure 2a). Further analysis by EM verified that the majority of CD117 þ ST2 þ cells are of mast cell origin (Figure 2b).…”

Negative Signaling in Fc Receptor Complexes

“…Supporting this interpretation, the inducible tyrosyl-phosporylation of SHIP1 dose-dependently increased with the concentration of antigen, even after supra-optimal concentrations were reached. Most importantly, inhibition of secretion induced by an excess of antigen in mast cells derived from wt mice was abrogated in mast cells derived from SHIP1-deficient mice (Gimborn et al, 2005). These data altogether indicate that SHIP1, possibly recruited by FcRβ when heavily phosphorylated as a result of supra-optimal receptor aggregation, is the effector of autonomous negative regulation of FcεRI signaling that dampens mast cell activation in excess of ligand.…”

“…Indeed, drugs such as latrunculin, which prevent actin polymerization, enhance mast cell degranulation (Frigeri and Apgar, 1999). Interestingly, inhibition of degranulation observed in excess of antigen was markedly reduced in cells treated with latrunculin B, and actin could coprecipitate with SHIP1 in BMMC (Gimborn et al, 2005).…”

“…Thus, following FcεRI engagement, FcRβ and FcRγ, as well as Syk, undergo rapid c-Cbl-dependent E3 ligase-mediated ubiquitination (Gimborn et al, 2005). Lyn also associates with c-Cbl and is ubiquitinated and degraded in IgE-activated mast cells (Kyo et al, 2003)).…”

Regulation of allergy by Fc receptors

“…This was thought to result from decreased receptor aggregation, due to a competition of antigen for IgE. Intracellular signals do not decrease, however, but increase, as antigen concentration increases, including the phopshorylation of the inositol 5-phosphatase SHIP1, and inhibition observed in antigen excess was abrogated in SHIP1 -/-mast cells [22]. SHIP1 therefore determines both the threshold, as proposed earlier, and the upper limit of aggregation that triggers degranulation.…”