Plasmodium falciparum: natural and experimental transmission-blocking immunity

Fc-dependent regulation of humoral immune responses was investigated by immunization of BALB/c mice with immune complexes. These complexes were composed of DNP- and PC-conjugated KLH or Ficoll, and monoclonal T15 idiotype-positive anti-PC antibodies of different isotypes but indistinguishable V-region properties. Since the response to DNP was analyzed, effects due to masking of antigenic determinants by anti-PC antibodies are excluded. The responses to free and complexed antigens showed significant differences in the proportions of DNP-specific IgM, IgG1, IgG2 and IgG3 antibodies. Complexes containing the T-dependent carrier KLH elicited serum antibodies to DNP with significantly decreased IgM levels, irrespective of the isotype in the complex. Under these conditions, however, DNP-specific IgG classes were augmented to various extents, depending on the isotype in the complex. In the case of the T-independent carrier Ficoll, only complexes with IgM or IgG3 suppressed the IgM response. Moreover, immunization with complexes composed of IgM or IgG2a led to a significant decrease of DNP-specific IgG1. In contrast to changes induced by antibodies bound to the T-dependent antigen, immunization with T-independent complexes did not enhance the production of any of the immunoglobulin isotypes.