ANTIGEN‐INITIATED B LYMPHOCYTE DIFFERENTIATION

Summary The humoral immune response to tile hapten NIP (4‐hydroxy‐3‐iodo‐5‐nitrophenylacetic acid) coupled to POL (polymerised bacterial flagellin) has been investigated by an adoptive transfer system. The primary adoptive immune response to this antigen involved exclusively IgM antibody‐forming cells (AFC), whereas the response of animals primed to the NIP determinant invoked predominantly IgG AFC with variable amounts of IgM AFC. Failure to observe a primary IgG response was partially due to a low level of IgG AFC‐progenitors and partially due to the absence of the appropriate collaborating T cells. Both the primary and secondary IgM responses were obtained in the absence of T (thymus‐derived) lymphocytes as shown by adoptive transfer of anti‐ θ treated spleen cells. The secondary IgG response by the same test was, however, T cell dependent, both for the generation and expression of memory IgG AFC‐progenitors. The NIP‐POL system is suggested for the identification and comparison of B cell AFC‐progenitors responsible for “virgin” and “memory” IgM and IgG responses. IgM AFC‐progenitors in a primed or unprimed cell population can be directly assayed by transfer of cells to irradiated recipients, though the question of T cell suppression must be considered. NIP IgG AFC‐progenitors can be directly assayed by transfer of cells to recipient animals pre‐primed to the carrier protein by injecting low doses of monomeric flagellin 4 days before irradiation. Such “helper” recipients were able to restore an IgG response which was reduced by anti‐ θ treatment of the transferred cells.