Immunoglobulin M receptors on memory cells of immunoglobulin G antibody‐forming cell clones

Abstract

The memory cells of two antibody-forming cell clones had receptors of the IgM class, even though the clones had been producing IgG₁ or IgG_2a anti-2,4-dinitrophenyl antibodies for 9–15 months previously (on exposure to antigen). Thus a phenotypic switch in heavy chain constant region evidently occurred after re-exposure of these memory cells to antigen. To show that, we first removed the clonal cells' surface immunoglobulins by “capping” and “stripping”, with class- or subclass-specific antisera. Then, to assay their remaining receptor activity, the cells were incubated with antigen in vitro, washed and transferred (together with carrier-primed cells) to irradiated recipients, and their antibody responses to this in vitro boost were assayed by isoelectric focusing. Pretreatment with anti-μ serum, as well as with anti-Fab(k), prevented the responses of the IgG₁ and IgG_2a clones to an in vitro boost, while anti-γ₁, and anti-γ_2a antisera had no effect. An anti-serum to the putative mouse IgD also had no effect. The anti-μ serum failed to react with the IgG₁ and IgG_2a clonal serum antibodies in the test tube. Some other contaminating clones were suppressed completely only by the anti-Fab serum. This result strongly suggests that switching in class commitment may occur during the differentiation of memory cells to antibody producers, and may therefore be antigen-dependent. It also implies that some apparently naïve cells with surface IgM may., in reality, be B memory cells.