The Journal of Immunology, Vol 144, Issue 11 4147-4155, Copyright © 1990 by American Association of Immunologists
Antigen-induced Ca2+ signaling and desensitization in B cells
AH Lazarus, GB Mills and TL Delovitch
Banting and Best Department of Medical Research, University of Toronto, Canada.
Cross-linking of B cell surface Ig (sIg) by anti-Ig results in
transmembrane signaling. However, the capacity of a thymus-dependent (TD)
Ag to mediate B cell signal transduction has been less well documented.
Therefore, we examined Ag-induced intracellular free calcium concentration
[( Ca2+]) in B cells by using TD Ag that would be expected to either
cross-link or not cross-link sIgM and/or induce the coupling of sIgM to
FcR. Stimulation of mouse TA3 hybridoma B cell transfectants that express
the SP6 anti-TNP specific sIgM with either TNP-OVA or anti-IgM antibodies
resulted in a maximal fourfold increase in [Ca2+]i. The net increase in
[Ca2+]i in response to TNP-OVA was dependent upon both the Ag dose and the
TNP:OVA molar ratio. Because occupancy of several cell-surface receptor
types leads to a loss of response to subsequent stimulation by ligand
(homologous desensitization), we examined the ability of Ag to induce
homologous desensitization of sIgM in these B cells. TNP1-OVA at all
concentrations tested (up to 500 micrograms/ml) did not lead to any change
in [Ca2+]i or desensitization. Cross-linking of TNP1-OVA (10 micrograms/ml)
with F(ab')2 of anti-OVA antibody induced both a rise in [Ca2+]i and
homologous desensitization of sIg, suggesting that cross- linking of sIgM
by Ag is sufficient to induce both these processes. TNP6-OVA at a
concentration of 10 micrograms/ml induced changes in [Ca2+]i and partially
desensitized TNP-specific B cells to stimulation by anti-IgM.
Interestingly, a high dose (180 micrograms/ml) of TNP6-OVA stimulated
minimal changes in [Ca2+]i yet did not lead to desensitization. However,
cross-linking of TNP6-OVA at this high dose with F(ab')2 of rabbit anti-OVA
elevated [Ca2+]i and elicited partial desensitization. Complete
desensitization of sIgM by Ag was achieved when intact (Fc-containing)
anti-OVA antibody was used, suggesting that the FcR can play a role in
desensitization. Ag- and antibody-mediated desensitization was not caused
by steric hindrance of sIg. Thus, we have observed two forms of Ag-induced
desensitization of sIgM, both of which involve sIg cross-linking and one of
which is mediated by the physiologic coupling of sIg to FcR.
