CD16/32-specific biotinylated 2.4G2 single-chain Fv complexed with avidin-FITC enhances FITC-specific humoral immune response in vivo in a CD16-dependent manner

Fc gamma receptors (Fc gamma Rs) play an essential role in the regulation of immune response due to their ability to bind immune complexes. Activating Fc gamma Rs may facilitate antigen presentation and dendritic-cell maturation, while in the late phase of the immune response, the inhibitory Fc gamma RIIb may down-regulate B-cell activation upon cross-linking with activating receptors. In this study, we investigated the in vivo role of Fc gamma Rs on the modulation of humoral immune response. In order to get well-defined immune complexes that can bind to both the activating and the inhibitory Fc gamma Rs, we designed a mono-biotinylated single-chain fragment variable construct from the rat anti-mouse CD16/32 clone 2.4G2, linked to avidin-FITC, and tested its effect on the FITC-hapten-specific T-independent type 2 (TI-2) and T-dependent (TD) immune response. When injected intravenously in mice, the complex bound to a small portion of B220+, CD11b(high) and CD11c(high) cells and was localized in the spleen on marginal zone macrophages 15 min after treatment. When applied as a booster following primary immunization with TI-2 (FITC-dextran) or TD (FITC-keyhole limpet haemocyanin) antigens, the complex elevated the number of hapten-specific IgM/IgG-producing B cells. This effect was diminished in CD16KO mice, suggesting that the activating-type Fc gamma RIII might be a key mediator of this mechanism.