Immune Complexes That Bind to ELISA Plates Not Coated with Antigen in Mice Infected with Lactate Dehydrogenase-Elevating Virus: Relationship to IgG 2a - and IgG 2b -Specific Polyclonal Activation of B Cells

We have further investigated the nature of IgG-containing complexes of 150–300 kD that rapidly appear in the circulation of mice of various strains after infection with lactate dehydrogenase-elevating virus (LDV) and are recognized and quantitated by their binding in the presence of 0.05% Tween 20 to certain enzyme-linked immunosorbent assay (ELISA) plates with high protein affinity that have not been coated with protein antigen (5). These binding complexes have been found to contain primarily IgG2a or, in some mice, IgG_2b. Their isotype specificity and time course of formation correlated with those of the polyclonal production of immunoglobulins in these mice, as measured by increases in total IgG_2a or IgG_2b in the circulation. In contrast, anti-LDV antibodies exhibited much broader isotype specificities in all mouse strains investigated. Depletion of BALB/c mice of CD4⁺T cells or lack of T cells in nude Swiss mice only partly reduced the polyclonal activation of B cells and the formation of ELISA plate-binding complexes, whereas anti-LDV antibody formation was completely blocked. Only a small proportion of the total IgG_2a or IgG_2b formed as a result of the LDV-induced polyclonal activation of B cells was recovered in plate-binding complexes, which sedimented in sucrose density gradients between 150 and 300 kD. Diverse monoclonal antibodies of different IgG isotopes did not bind to the plates at concentrations at which LDV-induced immune complexes exhibited binding activity. We suggest that the LDV-induced immune complexes do not contain anti-LDV antibodies, but are complexes of auto-antibodies and self-antigen(s). However, additional features must be responsible for the high affinity of these complexes for ELISA plates since various immune complexes formed in vitro failed to bind to the plates, and binding activity of the immune complexes formed in LDV-infected mice could not be regenerated in vitro once the complexes had been dissociated by a low pH treatment.