Complement activation is required for IgM-mediated enhancement of the antibody response.

Abstract

The ability of IgM antibodies to specifically enhance the thymus-dependent humoral immune response to particulate antigens is well documented. We have used two approaches to test whether complement factors play a role in this process. First, mice were depleted of C3 by treatment with cobra venom factor (CVF) and then immunized with SRBC with or without IgM-anti-SRBC. CVF treatment severely impaired the capacity of IgM to induce an enhanced anti-SRBC response. Moreover, it was shown that IgM can potentiate the response in C5-deficient AKR mice, thus demonstrating that the complement factors acting before C5 are the crucial ones. A second test compared the enhancing properties of two monoclonal IgM-anti-TNP antibodies where, because of a point mutation in the mu chain constant region, one of the antibodies is impaired in its capacity to activate complement. We show that the mutant antibody lacks the enhancing properties of the wild-type IgM. Activation of C3 by IgM antibodies as well as localization of antigen in the spleen seem to be necessary steps in the IgM-mediated enhancement of antibody responses. Our data offer an explanation to the immunosuppression described in CVF-treated animals as well as the low humoral immune responses in certain hereditary complement deficiencies. It is suggested that IgM indeed has an important physiological function in enhancing antibody responses to foreign substances.

Requirement for complement in antibody responses is not explained by the classic pathway activator IgM

“…To test whether C activation by natural IgM explains the requirement for classic pathway activation in primary Ab responses, we generated knock-in (Cμ13) mice using homologous recombination in ES cells. The point mutation introduced is identical to the one previously reported to abrogate the ability of IgM to initiate C-dependent lysis (42) and to bind C1 (43) as well as to enhance Ab responses (34). A codon change resulting in substitution of proline for serine at amino acid position 436 in the third constant domain of the μ-heavy chain was introduced.…”

“…Based on previous observations, one possibility seemed to be that natural IgM could activate C in naive animals (i.e., in a primary response) because (i) one of the most potent activators of C1q is IgM; (ii) mice lacking secretory IgM have lower Ab responses than WT mice and can be rescued by transfer of IgM from normal mouse serum (36,37); and (iii) specific IgM is known to feedback-enhance Ab responses, and this process is Cdependent (34,35). To test the hypothesis, the Cμ13 knock-in mouse strain with a point mutation in the gene encoding the CH3 domain of the μ-heavy chain, making its IgM unable to activate C, was generated.…”

“…First, Ag-specific IgM, passively administered to mice or humans together with Ag can enhance the Ab response to the Ag via Ab feedback regulation (31)(32)(33)(34)(35). This enhancement depends on the ability of IgM to activate C (34,35). Second, mice lacking secretory IgM have lower Ab responses than WT mice (36)(37)(38), and the response can be restored by transfer of IgM from normal mouse serum (36,38).…”

“…It seemed plausible that the enhancing effect of natural IgM on Ab responses (36)(37)(38) would depend on its ability to activate C, and thereby explain why classic pathway activation plays a crucial role in primary Ab responses. Early primary Ag-specific IgM could then further enhance the Ab response via the feedback-enhancing effect (32)(33)(34)(35).…”

“…Two lines of research support this view. First, Ag-specific IgM, passively administered to mice or humans together with Ag can enhance the Ab response to the Ag via Ab feedback regulation (31)(32)(33)(34)(35). This enhancement depends on the ability of IgM to activate C (34,35).…”