Changes in Receptor Isotype and T/B Ratio in an Antigen-Binding Cell Population after in vitro Immunization

Abstract

Abstract T and B antigen-binding cells (T ABC, B ABC) specific for sheep erythrocytes (SRC) have been generated in a primary in vitro Mishell-Dutton-type culture system. On days 1 to 3 of the in vitro response, ABC were approximately 60% T cells and 40% B cells, although on days 4 to 6 there were more B ABC than T ABC. T ABC increased significantly in the first day of culture, but no increase in B ABC was seen until day 3. Absolute numbers of both T and B ABC were maximal on day 4, representing a 10-fold increase in T ABC and about a 4-fold increase in B ABC. The increase in T and B ABC was antigen-driven. In unstimulated cultures, T and B ABC maintained nonimmune levels until day 4. However, T and B non-ABC decreased synchronously, whether antigen-stimulated or not. The isotypes of ABC receptors were examined by inhibiting antigen binding with heavy chain-specific antisera. All ABC, including T ABC, were inhibitable by rabbit anti-mouse immunoglobulin (RxMIg) containing antibodies to γ, µ, κ, and λ. The proportion of ABC inhibitable by anti-µ did not change from the day 0 level in antigen-stimulated or unstimulated cultures. There was an antigen-driven gain of surface IgG (sIgG) by ABC between day 2 and day 3 in stimulated cultures not seen in unstimulated cultures. Loss of surface IgD (sIgD) occurred to the same extent and at the same rate from ABC in stimulated and unstimulated cultures, providing no conclusive evidence that this loss was an antigen-driven differentiation event, but demonstrating that the loss of sIgD could be dissociated from the expression of sIgG. On day 3, when sIgG first appeared, it was expressed mainly or exclusively on surface IgM (sIgM)-bearing cells, the majority of which still expressed sIgD.