Feedback Enhancement of Antibody Responses via Complement and Fc Receptors

Sammanfattning: IgG, IgM and IgE in complex with antigen have the capacity to regulate specific immune responses. In this investigation, the role of Fc receptors for IgG (Fc?RI, Fc?RII and Fc?RIII) and complement receptors 1 and 2 (CR1/2) for antibody-mediated enhancement of antibody responses are investigated.IgM is known to efficiently activate complement and thereby enhance specific antibody responses but it is not known if this involves binding to CR1/2. Using CR1/2 deficient mice, immunized with sheep erythrocytes alone or together with specific IgM, we present evidence that IgM-mediated enhancement is completely dependent on CR1/2 expression, whereas IgG or IgE in complex with bovine serum albumin (BSA) induce strong antibody responses in CR1/2-deficient mice. Enhancement by IgE is mediated via the low affinity receptor for IgE (Fc?RII, CD23). However, the receptors which are involved in IgG-mediated enhancement are not known. We find that ?-chain-deficient mice (lacking Fc?RI and Fc?RIII) have impaired antibody responses to IgG/BSA complexes. In contrast, Fc?RIII deficient mice have normal responses, suggesting that Fc?RI mediates the effect. Furthermore, IgG/BSA complexes induce up to 189-fold stronger antibody responses in Fc?RIIB-deficient mice than in wild-type mice. The threshold dose of IgG/BSA required was lower, the response was sustained for longer and initiated earlier in Fc?RIIB-deficient than in wild-type animals. The findings suggest that Fc?RIIB acts as a "safety-valve" preventing excessive antibody production during an immune response. We show for the first time that IgG3/BSA complexes can mediate enhancement of specific antibody responses. Their effect does not involve known Fc? receptors.