IL-2 expands and maintains IgM plasmablasts from a CD5+ subset contained within the germinal centre cell-enriched (surface IgD−/CD39− buoyant) fraction of human tonsil

Abstract

IL-2 was found to promote the rapid growth of a minority population contained within the germinal centre (GC) cell-enriched (CD39⁻ and/or lgD⁻ buoyant) fraction of human tonslllar B lymphocytes. The cells emerging in response to IL-2 had a high mitotic index and morphologically resembled plasmablasta. 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: ∼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-drlven expansion and IgM secretion was 8mlg⁺/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 apoptosls. We discuss these results in relation to CD5⁺ B cells and their potential role in antibody responses to TD antigens.