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Human B cells isolated from peripheral blood were activated and induced to proliferate by either Epstein–Barr virus (EBV) or the T cell-derived mitogens CD40 ligand (CD40L) plus interleukin (IL)-4. Although both populations initially proliferated as B-blasts, significant differences were revealed over a longer period. EBV infection resulted in continuously proliferating lymphoblastoid cell lines (LCLs), whereas most of the CD40L/IL-4-stimulated B cells had a finite proliferative lifespan of 3–4 weeks. Cell cycle analysis, trypan blue staining and Western blot analysis for cleavage of poly(ADP-ribose) polymerase (PARP) all demonstrated that the decrease in proliferation in CD40L/IL-4-stimulated B cells is not due to cell death. Instead, these cells arrest, accumulate in G0/G1 and undergo alterations in cell surface marker expression, cellular morphology and immunoglobulin production, all consistent with plasmacytoid differentiation. In contrast, B cells infected with EBV continued to proliferate and retained a blast-like phenotype. Differences in both cytokine production and the expression of cell cycle regulators were identified between the two B-cell populations, which might contribute to the differentiation of the CD40L/IL-4-stimulated B cells and suggest potential mechanisms by which EBV may overcome this. The study has also identified a window of opportunity during which a comparison of isogenic populations of EBV- and mitogen-driven B blasts can be made.
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