Complement activation is not required for IgG‐mediated suppression of the antibody response

Abstract

Feedback suppression of the antibody response by IgG is known to be dependent on intact Fc regions. However, it is not clear which of the Fc-mediated effector functions is required. In the present report we have studied whether ability or inability of the IgG antibodies to activate the complement system was of consequence for their immunosuppressive effect. First, a monoclonal IgG1-anti-2,4,6-trinitrophenyl (TNP) antibody, unable to activate complement via the classical or alternate pathway, was shown to be able to inhibit more than 90% of the in vivo sheep erythrocyte-specific antibody response in mice when TNP coupled to sheep erythrocytes was used as antigen. Second, we investigated the immunosuppressive ability of a non-complement-activating mutant IgG2a-anti-TNP monoclonal antibody. The mutant differs from the wild type by a single amino acid substitution in the CH2 domain leading to inability to fix complement factor C1q. However, the mutant has the same affinity for antigen and the same Fc receptor-binding capacity as the wild type antibody. It is demonstrated that the mutant was as efficient as the wild type antibody in inhibiting an in vitro antibody response to TNP-coupled sheep erythrocytes. These findings confirm the non-determinant specificity and Fc dependence of IgG-mediated suppression, and show that the Fc-mediated effector mechanism is independent of complement activation. The results instead suggest binding to Fc receptors as a necessary step in feedback immunosuppression and favor inactivation of B cells by cross-linking of Fc and antigen receptors on their surface rather than elimination of antigen by complement-dependent phagocytosis as the effector mechanism.

FcγRIIB in IgG-Mediated Suppression of Antibody Responses: Different Impact In Vivo and In Vitro

“…Therefore, it is interesting that IgGmediated suppression in vitro, unlike suppression in vivo, to a large extent is dependent on Fc␥RIIB. This agrees with previous in vitro studies in the same system showing that aglycosylated monoclonal IgG (that did not activate C nor bind to Fc␥Rs) lost its ability to suppress (22), whereas monoclonal point-mutated IgG (that bound to Fc␥Rs but did not activate C) retained its suppressive ability (30). The present finding also agrees with the observation that IgG-mediated suppression in vitro was counteracted by a mAb that blocked Fc␥RIIB and Fc␥RIII (31).…”

“…Fc␥R-mediated mechanisms seem unlikely because suppression of SRBC responses takes place in mice lacking not only Fc␥RIIB, but also Fc␥RI plus IIB plus III (9), and because both IgM and IgE can suppress (9,11,33). Complementmediated effects also appear unlikely because IgG, unable to activate C, is still able to suppress (30). A thorough discussion about possible explanations for the apparently paradoxical findings in the mouse vs the human system has recently been published (35).…”

Efficient IgG-mediated suppression of primary antibody responses in Fcγ receptor-deficient mice

“…Because no involvement of Fc␥Rs in IgG-mediated suppression could be demonstrated, and because the other major Fc-mediated function of IgG, complement activation, is not required for suppression (41), the ability of F(abЈ) 2 fragments (1999) to suppress was reinvestigated. F(abЈ) 2 fragments were prepared from purified monoclonal TNP-specific IgG2a (clone 7B4).…”

Studies on the Mechanism by Which Antigen‐Specific IgG Suppresses Primary Antibody Responses: Evidence for Epitope Masking and Decreased Localization of Antigen in the Spleen

“…IgG molecules binding in close proximity to each other are more likely to bind C1q and to activate the classical complement pathway than IgG molecules binding far apart. However, as the ability of IgG to suppress the Ab response is not dependent on its ability to activate complement [19] this possibility is highly unlikely. It could also be claimed that SRBC-NIP high are for some reason cleared more efficiently than SRBC-NIP low and that immunization with the former would therefore cause lower Ab responses owing to preferential elimination of the high density SRBC-NIP.…”

“…Moreover, the finding that IgG-mediated suppression is normal in mice lacking not only FccRIIB, but in addition FccRI, FccRIII and FccRIV as well as FcRn [17], suggests an altogether FcR-independent mechanism. Critical involvement of the complement system was excluded by the finding that monoclonal IgG antibodies unable to activate complement were still efficient suppressors [19].…”