Humoral immune response in a marsupial Monodelphis domestica: anti-isotypic and anti-idiotypic responses detected by species-specific monoclonal anti-immunoglobulin reagents.

We examined the humoral immune response of the laboratory opossum (Monodelphis domestica) as a model marsupial species. To evaluate antibody responses, IgM and IgG preparations were purified from the sera of naïve Monodelphis. These two immunoglobulin (Ig) preparations were used to generate specific murine monoclonal antibodies for use in ELISA-based serology. Individual Monodelphis were then immunized with a multideterminant protein antigen, a murine monoclonal antibody (Mab) IgG preparation designated Pab 405. In contrast with the primary IgM response of eutherian mammals, the primary response of Monodelphis to mouse IgG involved both IgG and IgM. The specificity of this anti-mouse IgG response appeared isotypic in nature, specifically, the immune sera recognized Ig determinants common to both Pab 405 and a control IgG Mab. Further, to evaluate the antibody responses to mouse IgG, immune sera were adsorbed against a control IgG Mab to remove the anti-isotypic reactivity. The adsorbed Monodelphis sera recognized idiotope specificities expressed on Pab 405. Based on an inhibition ELISA, the anti-idiotype (anti-Id) response recognized an idiotope on Pab 405 associated with its antigen combining site. These results demonstrate that Monodelphis respond to a multideterminant protein antigen such as murine IgG, similarly but not identically to eutherian mammals, and can serve as a useful marsupial model for additional comparative immunological studies.