Prolonged phenotypic, functional, and molecular change in group I Burkitt lymphoma cells on short-term exposure to CD40 ligand

Abstract

Group I Burkitt lymphoma (BL) cell lines (L3055, Elijah, Louckes, BL2, and BL29) retaining the original biopsy phenotype were found to undergo prolonged phenotypic, functional, and molecular change on short-term exposure to soluble recombinant CD40L trimer. Sensitivity to, extent of, and duration of change appeared to reflect passage number in that the earliest passaged lines, L3055 and BL29, were generally the most susceptible. Culture of group I BL lines with CD40L resulted in significant growth arrest (without apoptosis) that, for L3055 cells, was sustained for 7 to 9 days after 72 hours of exposure. This was accompanied by the formation of large homotypic aggregates together with gross changes in individual cell morphology and a concomitant upregulation of CD54 (ICAM-1). Three of the five group I BL lines exhibited rapid downregulation of the hallmark CD77 surface antigen, which, for L3055 cells, was maintained for at least 12 days after 72 hours of incubation with CD40L. With the exception of BL2, all group I BL lines were induced to express CD40 homodimers on CD40-stimulation, whereas only monomers were detected in unstimulated cells. Experiments using CD40-transfected Rat-1 fibroblasts showed that the ability to signal for dimer formation required Thr234 of CD40. For L3055 and BL29 cells, an initial 72 hours of exposure to CD40L resulted in the maintenance of homodimers for up to 14 and 10 days, respectively. There was a close correlation between the induction and duration of CD40 homodimers and the appearance of Bcl-2. For L3055 cells, which are sensitive to apoptosis-induction on BCR-engagement, exposure to CD40L for 72 hours was found to provide considerable protection from anti-IgM, which was still significant to 20 days. The implications of such sustained effects on relatively short-term exposure of tumor B cells to CD40L are discussed.