Antigen molecules are recognized by the immune system after internal processing by antigen-presenting cells, generally mononuclear phagocytic cells such as macrophages and B lymphocytes. In order to present a proteinaceous antigen, the antigen-presenting cell first internalizes the antigen which is then broken down into small peptidic fragments by enzymes contained in vesicles in the cytoplasm of the antigen-presenting cells. After fragmentation, the peptides are linked to cellular major histocompatibility complex (MHC) molecules and presented on the presenting cell's surface to the immune system. Peptides presented in this way are recognized by the T-cell receptor which engages T-lymphocytes into the immune response against this antigen. This antigen presentation also stimulates the B lymphocytes for specific antibody production.
Complexes of antibody and antigen have been used to stimulate an immune response against the antigen. Antigen uptake through antigen-antibody complexes bound to Fc receptors for IgG (Fc.gamma.R) increases the efficiency of antigen presentation and thereby antigen-specific T-cell activation by human and mouse macrophages, (Celis et al (1984) Science 224:297-299; Chang (1985) Immunol. Today 6:245-259; Manca et al. (1988) Immunol. 140:2893-2898; Schalke et al. (1985) J. Immunol. 134:3643-3648; and Snider et al (1987) J. Immunol. 139:1609-1616). The binding of these complexes to Fc.gamma.R is mediated by the Fc region of the antibody. This binding is susceptible to inhibition by physiological concentrations of IgG.