Modulation of immune response by combined targeting of complement receptors and low-affinity Fcγ receptors

Immune complexes (ICs) induce effective pathogen-specific innate and humoral immune response via immunecomplex-binding receptors, such as murine complement receptor type 1 and 2 (mCR1/2) and murine low-affinity Fc receptors for IgG (mFcγRII and III). The exact function of mCR1/2 in cooperation with mFcγRII/III in modulation of humoral immunity has not yet been adequately clarified. The aim of this study was to target these receptors by specific single-chain fragments of antibody (scFv), either individually or in combination, thus modelling the action of IC. For targeting, we used scFv derived from the well-characterized 7g6 and 2.4g2 monoclonal antibodies recognizing mCR1/2 and mFcγRII/III, respectively. These scFvs were monobiotinylated and conjugated to streptavidin or streptavidin-coated microspheres. Such complexes were investigated with respect to target receptor recognition and in vivo localization. Antibody response against the constructs was measured by ELISA and ELISPOT. Results show that targeting streptavidin complexes to mFcγRII/III induces stronger IgG1 response than targeting to mCR1/2 yet both strategies enhance the antibody response compared to the control group immunized with non-targeted peptide–streptavidin complexes. Moreover, the immunogenicity of coupled antigens increased using microspheres as carrier, instead of using soluble streptavidin. In summery, our in vivo experiments reveal that mFcγRII/III is more potent a target than CR1/2 and show that combined targeting of CR1/2 and FcγRII/III receptors does not result in cumulative enhancement of the antigen specific immune response. In addition, microparticle-mediated enhancement of immunization can be further improved by FcγRII/III targeting.