Immunoglobulin-binding factors.

Abstract

It has been known for more than a decade that lymphocytes [1] and also granulocytes [2] and macrophages [3] release molecules capable of binding immunoglobulin (Ig) isotypes. First IgG-binding factors (IgG-BF) [1], then IgE-BF [4], IgA-BF [5] and more recently IgD-BF [6], were reported. Soon after the discovery of IBF, three of their major properties were characterized: (i) they are produced by cells, mostly T lymphocytes, bearing Fc receptors (FcR) for the same Ig isotype; thus, IgG-BF are produced by Fcq/R positive cells, lgE-BF, IgA-BF and IgD-BF are released by FceR, FcaR and Fc6Rpositive cells, respectively; (ii) their production increases upon interaction of Ig with FcR, IgG inducing IgG-BF release [7], IgE and IgA inducing the production of IgE-BF and IgA-BF, respectively [5, 8]; when lymphocytes are incubated in serum-free buffer, the appearance of IBF in the supernatant parallels the shedding of membrane FcR [9]; (iii) IBF have been found to regulate Ig production in vitro, in a strictly isotype-specific manner for IgE-BF and IgA-BF, and in a broader way for IgG-BF (which regulates IgG and lgM responses). Moreover, some IBF have been reported to exert dual effects: IgE-BF produced by T cells either potentiated or suppressed IgE production, depending on their glycosylation [10]. Both suppressive and potentiating IgA-BF [5, 11] and suppressive IgG-BF [7, 12] have also been described. Taken together, these obervations led, soon after the description of IBF, to the proposal that these molecules were in fact shed FcR [13] (the term solu-