IL-2 EXPANDS AND MAINTAINS IGM PLASMABLASTS FROM A CD5+ SUBSET CONTAINED WITHIN THE GERMINAL CENTER CELL-ENRICHED (SURFACE IGD-/CD39- BUOYANT) FRACTION OF HUMAN TONSIL

IL-2 was found to promote the rapid growth of a minority population contained within the germinal centre (GC) cell-enriched (CD39- and/or IgD- buoyant) fraction of human tonsillar B lymphocytes. The cells emerging in response to IL-2 had a high mitotic index and morphologically resembled plasmablasts. Cultures could be maintained in the absence of feeder cells for up to 3 weeks in IL-2 and were characterized by large amounts of IgM in their supernatants: approximately 40% of the cells contained readily detectable cytoplasmic IgM by day 10 of culture. Negligible quantities of IgG and IgA were found. The target population for IL-2-driven expansion and IgM secretion was smIg+/CD38+ and was subject to suppression by anti-IgM antibody. While only 8% of cells within the GC cell-enriched fraction were CD5+ (compared with 15% of high density resting B cells), their removal led to an 83% reduction in the amount of IgM produced in response to IL-2. IL-2 selectively expanded this minor CD5+ subset such that by day 6 of culture they comprised 57% of all viable cells. Cultures established with IL-2 showed increasing expression of cytoplasmic Bcl-2 and withdrawal of growth factor resulted in cell death via apoptosis. We discuss these results in relation to CD5+ B cells and their potential role in antibody responses to TD antigens.