Interleukin-6 dimers produced by endothelial cells inhibit apoptosis of B-chronic lymphocytic leukemia cells.

Tumoral lymphocytes from patients with B-chronic lymphocytic leukemia (B-CLL) are long-lived cells in vivo, but they die rapidly by apoptosis in vitro. Here, it is reported that endothelial cells (ECs) inhibit the apoptosis of B-CLL cells, as determined by 4 different flow cytometric methods, and that this antiapoptotic effect is mediated mainly by soluble factor(s), as can be deduced from the following findings. First, EC-conditioned medium (ECCM) inhibited the apoptotic rate in B-CLL to approximately 50% of control. Second, the antiapoptotic effect mediated by EC/B-CLL cell contact was more apparent than real; using a fluorescence-based phagocytosis assay, it was demonstrated that this effect was due to the phagocytic capacity of ECs, which internalized apoptotic cells. Third, the protective effect of ECCM was associated neither with proliferation nor differentiation signals. Fourth, the survival factor was a dimeric form of IL-6 because anti-IL-6 antibodies completely neutralized the antiapoptotic effect mediated not only by the crude ECCM but also by the 45- to 55-kd active fractions obtained after gel filtration, which contained high levels of IL-6. These IL-6 dimers (IL-6(D)) were noncovalently associated. Sixth, human recombinant IL-6(D) (hrIL-6(D)) inhibited B-CLL apoptosis, whereas hrIL-6 monomers (hrIL-6(M)) did not. Binding and functional competition experiments showed not only that monomers and dimers had similar affinity for the IL-6R, but also that hrIL-6(M) inhibited the antiapoptotic activity of hrIL-6(D). These data suggest that IL-6(D) derived from ECs promote the survival of B-CLL cells.