title: Antigen-induced Ca2+ signaling and desensitization in B cells.
abstract: 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.
authors.affiliation: Banting and Best Department of Medical Research, University of Toronto, Canada.
authors.forename: A H
authors.initials: AH
authors.lastname: Lazarus
authors.forename: G B
authors.initials: GB
authors.lastname: Mills
authors.forename: T L
authors.initials: TL
authors.lastname: Delovitch
mesh_terms: D000818:Animals; D000938:Antigen-Presenting Cells / Q000276:immunology; D001402:B-Lymphocytes / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D002460:Cell Line; D002999:Clone Cells; D007075:Immunoglobulin M / Q000502:physiology; D066298:In Vitro Techniques; D051379:Mice; D010047:Ovalbumin / Q000276:immunology; D011940:Receptor Aggregation; D011947:Receptors, Antigen, B-Cell / Q000502:physiology*; D015398:Signal Transduction; D014301:Trinitrobenzenes / Q000276:immunology
chemical_list: D007075:Immunoglobulin M; D011947:Receptors, Antigen, B-Cell; D014301:Trinitrobenzenes; D010047:Ovalbumin; D002118:Calcium