In vivo isotype regulation of humoral responses to dextran B1355 in BALB/c mice. Double-class IgG3/IgM producers as a function of age.

This report shows that, in 8- to 10-month-old BALB/c mice immunized intraperitoneally with dextran B1355, approximately 75% of IgG3 anti-alpha (1----3) polyglucan (anti-dex) plaque-forming cells (PFC) detected in the spleen were identified as double-Ig class producers secreting simultaneously IgG3 and IgM antibodies with the same specificity for the dex epitope. Under the same conditions of immunization, however, IgA anti-dex PFC were mostly single-class secretors. IgA PFC developed in the spleen in highest numbers (equal to IgM), but in Peyer's patches IgA PFC were sevenfold more numerous than IgM. Furthermore, spleen IgG3 anti-dex PFC responses were low compared with spleen IgA and IgM anti-dex PFC responses and appeared only late in ontogeny. The possibility is discussed whether a TH dependence of the IgA anti-dex response and a TH-independent generation of the IgG3 response are responsible for the different pattern of isotype expression.