Antigen-initiated B lymphocyte differentiation. XVIII. Pre-progenitor B cells that give primary adoptive responses are s-IgM+IgD-Ia+.

Abstract

Abstract A subset of B lymphocytes termed “pre-progenitors,” responsible for certain adoptive immune responses, has previously been defined on the basis of particular physical parameters and biologic characteristics. These cells show a nonspecific macrophage-mediated proliferative response to antigenic stimulation of the animal. This leads to amplification of B cell numbers and differentiation into the more numerous “direct progenitor” B cells, which then respond specifically to antigen, finally producing AFC. Cell surface antigenic markers on such primary “pre-progenitor” B cells have now been determined, in order to integrate this information with other models of B cell development. Unprimed adult mouse spleen cells were treated with cytotoxic antisera and complement, or were labeled with fluorescent antisera and separated on a fluorescence-activated cell sorter. The isolated fractions were then assayed in an adoptive system, with a sequential nonspecific then specific antigen stimulation protocol, and a readout of hapten-specific IgM AFC in the spleen 8 days post-transfer. Most progenitors were surface s-lg+, and in particular s-lgM+, but around 20% of the activity in the assay was due to sIg− cells, presumably pre-B cells. Cytotoxic monoclonal anti-lgD and complement did not eliminate any “pre-progenitor” activity from C57 mouse spleen and enriched the response from the remaining B cells. Sorting experiments with CBA mice also showed that most activity was in the s-lgD− fraction, though a little remained with the slgD+ cells. Treatment with cytotoxic anti-la antisera eliminated activity, as did treatment with monoclonal cytotoxic B2A2 antibody, a reagent that kills most B cells but spares stem cells. These results define primary “pre-progenitors” as an intermediate B cell category, predominantly slgM+lgD−la+B2A2+, distinct from “pre-B” cells and from the majority of mature B cells. In contrast to other studies on primary adoptive responses, the defined assay system used has focused attention on the s-lgD−lgM+ cells, a subset that may have a crucial role in B lymphocyte differentiation.