Immunogenicity of antigen complexed with antibody. I. Role of different isotypes.

IgM-, IgG1-, IgG2a-, IgG2b- and IgE-type anti-ovalbumin antibodies were isolated from rat immune sera and the complexes formed by antibodies of a defined isotype and the antigen were compared for antibody avidity and interaction with homologous complement. IgG2a-containing complexes consumed total complement with the greatest efficiency. IgG2a-, IgG2b- and IgM-containing complexes displayed similar activities in activation of the alternative pathway while IgG1 was less efficient. Reduction and alkylation of IgG2a antibodies abolished the capacity of the complex to activate the alternative pathway, but not their total complement consumption. The complement-dependent solubilization was greatest when complexes contained IgG1- or IgM-type antibodies and lowest with IgG2a-containing complexes. Inbred Long Evans rats immunized by complexes containing IgG1 or IgG2b antibodies displayed a markedly lower delayed-type hypersensitivity (DTH) compared with those immunized by antigen alone. Immunization with IgG2a- or IgE-containing complexes resulted in a slightly decreased DTH, while IgM-containing complexes induced DTH like ovalbumin alone. IgG2a-containing complexes elicited an antibody response markedly higher than that found in animals immunized by antigen alone. The same effect was found when complexes contained reduced-alkylated IgG2a. Immunization with complexes containing the other isotypes involved in the study induced an antibody response similar to the antigen in phosphate-buffered saline.