Antibody density on rat red cells determines the rate of activation of the complement component Cl

Abstract

It is a common observation that there is variability in the rate of activation of C1, the first component of complement, when bound to immune complexes. The cause of this variation has been investigated with experiments designed to assess separately the effect of antibody, antigen and C1 density. Using 125I-labeled C1 and a rat monoclonal antibody specific for the class I antigen, it has been found that the rate of activation is primarily dependent on antibody density on the cell surface and not on antigen or C1 density. This finding supports the suggestion that direct contact between the C1r2C1s2 subcomponent of C1 and antibody may be required for potentiation of C1 activation.

Monoclonal IgG Antibodies Influence the Migration Patterns of Lymphocytes in vivo

“…Furthermore, in marked contrast to our previous findings in decomplemented animals where both R3/13 and R2/15S targeted erythrocytes almost exclusively to the splenic Fc receptor–dependent clearance system [32, 38, 44], the predominant site of accumulation of the lymph node cells was the liver in CVF–treated rats preinjected with these MoAb (table2). These effects could be attributed to a much higher level of the RT1A a antigen expression on lymph node cells, thereby allowing more efficient complement activation by the target cell–bound antibody [25]. This interpretation is in agreement with the in vitro studies which have shown that complement activation by antigen–bound antibody is highly dependent on the target epitope density [21, 23].…”

“…Some animal studies not only suggest the importance of antibody specificity and the ability of some MoAb to activate the complement system [8, 16], but also show correlation between antigen density on the target cells and efficient therapeutic effects of MoAb serotherapy [17]. Although the properties of different IgG isotypes to mediate Fc–region–dependent effector functions have been examined in several in vitro experimental systems using human or chimeric MoAb–containing human Fc regions [18, 19, 20, 21, 22, 23], and rat antibodies [24, 25, 26, 27, 28, 29], the effector mechanisms responsible for an effective immunotherapy in vivo still remain unclear. It is now apparent from the literature that lymphocyte depletion may not be necessary to observe beneficial effects of MoAb therapy for the treatment of autoimmune diseases both in humans [3]and animal models [30].…”

Complement Is Activated by IgG Hexamers Assembled at the Cell Surface

“…Potent complement activation by monoclonal antibodies is restricted to certain antigens and epitopes (12, 19, 22), presumably because antigen size, density, and fluidity may affect activation (18, 22–26) and because IgG orientation resulting from epitope geometry imposes additional structural constraints (12, 19, 22, 25, 27). Polyclonal antibodies appear to be less sensitive than monoclonal antibodies to such constraints (24, 28, 29), potentially because binding of antibodies to a variety of antigens or epitopes facilitates clustering of Fc segments, thereby allowing efficient Fc-Fc assembly. Monovalent binding of IgG molecules in the platform is consistent with earlier observations (30) and could be envisaged to provide more degrees of freedom for the Fc segments, allowing their optimal positioning for C1q recruitment.…”

Complement in therapy and disease

“…The epitope recognized by the antibody is a critical factor in determining its ability to engage the complement system. Indeed antigen size, density, geometry, fluidity, as well as IgG orientation imposed by epitope positioning, have a strong impact (Bindon et al, 1988;Cragg et al, 2003;de Weers et al, 2011;Hughes-Jones et al, 1985;Parce et al, 1983;Pawluczkowycz et al, 2009;Teeling et al, 2004Teeling et al, , 2006Xia et al, 1993b). These observations can be explained by structural prerequisites affecting Fc movement and Fc:Fc positioning required for efficient hexamer formation (Diebolder et al, 2014).…”