Immunoregulation of the anti-bovine serum albumin response by polyclonal and monoclonal antibodies

Regulation of antibody responses via antibodies, complement, and Fc receptors

Antibodies can completely suppress or enhance the antibody response to their specific antigen by several hundredfold. Immunoglobulin M (IgM) enhances antibody responses via the complement system, and complement activation by IgM probably starts the chain of events leading to antibody responses to suboptimal antigen doses. IgG can enhance primary antibody responses in the absence of the complement system and seems to be dependent on Fc receptors for IgG (FcgammaRs). IgE enhances antibody responses via the low-affinity receptor for IgE (FcepsilonRII/CD23). The precise effector mechanisms that cause enhancement are not known, but direct B-cell signaling, antigen presentation, and increased follicular localization are all possibilities. IgG, IgE, and IgM may also suppress antibody responses when used in certain immunization regimes, and it seems reasonable that an important mechanism behind suppression is the masking of antigenic epitopes by antibodies. In addition, FcgammaRIIB, which contains a cytoplasmic inhibitory motif, acts as a negative regulator of antibody responses. This receptor, however, may prevent the antibody responses from exceeding a certain level rather than causing complete suppression.

Immune response to diphtheria toxin and to different CNBr fragments: evidence for different B and T cell reactivities

The antigenic structure of the diphtheria toxin has been studied in man. B and T cell responses to diphtheria toxoid and to different fragments of the toxin molecule were analyzed in 4 individuals one month after booster immunization. Studies on the B cell response showed that: part of the response was directed against assembled topographic sites; 80% of the response was directed against determinants present on fragment A; the few determinants present on the CNBr peptides of B cross-react with determinants present on A; and reduction of the second disulfide bridge of fragment B diminishes the response. In contrast to the antibody response, most of the T cell reactivity was directed against the B fragment or CNBr peptides from this fragment. Analysis of the fine specificity of T lymphocyte clones revealed that some CNBr fragments share common T cell determinants. These studies indicate that T and B cell determinants are differently distributed on the molecule and that large cross-reactivities that are not explained by the analysis of the amino acid sequence could be found at the B and T cell level.

Enhancement of IgG anti-carrier responses by IgG2 anti-hapten antibodies in mice

Isologous IgG2a and IgG2b anti-hapten antibodies injected along with trinitrophenylated or fluoresceinated keyhole limpet hemocyanin (KLH) were found to considerably stimulate primary IgG responses to the carrier protein in mice. No such stimulation was observed with IgG1, IgM and IgA anti-hapten antibodies. Depending on the antigen antibody combination, the amplification obtained after a single injection of IgG2-complexed haptenated KLH ranged from 20- to 1000-fold. By comparison, secondary responses were only marginally enhanced (approximately 3-fold) when IgG2-complexed rather than free antigen was used to boost irradiated recipients previously reconstituted with primed spleen cells. The IgG2-mediated enhancement was effective over a wide range of antigen/antibody ratios, did not require the use of high affinity antibodies and occurred whatever the route of injection. The stimulation was specific for the complexed antigen and developed to the same extent whether complexes were formed in vitro or in vivo. A comparable stimulation of the anti-carrier response was obtained with several other haptenated proteins but with formaldehyde-treated diphtheria and tetanus toxoids inhibition rather than stimulation was observed.