ANTIGEN-DRIVEN DIFFERENTIATION OF NAIVE IG-TRANSGENIC B-CELLS IN-VITRO

We have established a culture system in which naive B cells bearing a transgenic, chicken OVA (cOVA)-specific tg differentiate to plasma cel ls in vitro after interaction with cOVA plus cOVA-specific helper T ce lls. B cell-enriched populations from Ig-transgenic mice, but not from nontransgenic mice, proliferated after presenting nanomolar concentra tions of cross-linked cOVA to DO11.70 (cOVA plus IA(d)-specific) T cel ls. After 6 to 9 days of culture with Ag and specific T cells, the B c ells acquired a plasma cell phenotype and secreted the transgene-deriv ed Ig at high levels. Engagement of B cell surface tg was not essentia l for primary B cell differentiation. Differentiating B cells enlarged , clustered, and acquired two plasma cell markers, Syndecan and CD43. B cell CD45 isoform expression changed: the B220 isoform was lost in a T cell-dependent manner, whereas the CD45 RB isoform was gained in a T-independent manner. Although unstimulated B cells survived less than 72 h in vitro, those in Ag-stimulated cultures showed reduced early d eath, a surge of proliferation at 3 to 5 days, and increased death lat e in the culture. Using a large population of naive B cells of defined antigenic specificity permits us to study a primary immune response t o an Ag, rather than to less physiologic polyclonal stimuli. Because a ll steps of differentiation occurred in vitro, they are easily accessi ble for study. This coculture system provides an opportunity to observ e Ag-specific T cell-B cell collaboration.