Antigen-induced changes in the proportion of antigen-binding cells expressing the IgM, IgG, and IgD receptors

Changes in the isotype of the antigen receptor after immunization were assessed by using antigen-binding cells (ABC) from mouse spleen specific for sheep erythrocytes (SRC). Rabbit antisera specific for mouse heavy chains μ, γ, and δ, as well as polyspecific anti-Ig were used to inhibit SRC binding to the receptors of nonimmmune ABC and immune ABC 5 days after immunization with SRC. Nonimmune ABC were more heterogeneous than immune ABC in their sensitivity to inhibition by polyspecific anti-Ig and anti-μ in that they included a greater proportion of cells inhibited by low concentrations of these antisera. The maximum percent of cells inhibitable by anti-μ (60 to 80%) did not change after immunization. Where IgG was not detectable on nonimmune ABC, after immunization 40 to 50% of the ABC expressed IgG. The porportion of ABC with IgD receptors fell from about 45% in nonimmune mice to around 25% in 5-day immune mice. From the inhibition produced by two or three antiheavy chain sera present individually or simultaneously, the proportion of cells bearing various combinations of surface Ig isotypes were estimated. Thus, nonimmune and immune ABC-bearing IgD also appeared to bear IgM although a few immune ABC expressed only IgM. In immune ABC, IgG-bearing ABC overlaped extensively with the IgM-bearing population but not with the IgD-bearing population. ABC bearing IgD alone were not detected, but immune animals probably contained μ⁺δ⁺γ⁺ ABC. These results suggest that antigen contact with specific ABC triggers changes of receptor isotypes during the expansion of the ABC population including a partial loss of IgD and the appearance of IgG.