Soluble CD14 is a novel monocyte-derived survival factor for chronic lymphocytic leukemia cells, which is induced by CLL cells in vitro and present at abnormally high levels in vivo.

Accumulation of leukemic cells in patients with chronic lymphocytic leukemia (CLL) is due to prolonged cell survival rather than increased proliferation. Survival of CLL cells depends on microenvironmental factors. Even though long-lived in vivo, CLL cells rapidly die by spontaneous apoptosis in vitro unless cocultured with stromal cells or their conditioned medium. In the present study, we show that survival of CLL cells is maintained in high cell density cultures, where the main prosurvival activity is delivered by monocytes. Cytokine array and enzyme-linked immunosorbent assay studies revealed increased expression of soluble CD14 by monocytes in the presence of CLL cells. The addition of recombinant soluble CD14 to primary CLL cells resulted in significantly increased cell survival rates, which were associated with higher activity nuclear factor κB. Quantification of serum levels of soluble CD14 revealed abnormally high levels of this protein in CLL patients, indicating a potential role of soluble CD14 in vivo. In summary, the presented data show that monocytes help in the survival of CLL cells by secreting soluble CD14, which induces nuclear factor κB activation in these cells, and that CLL cells actively shape their microenvironment by inducing CD14 secretion in accessory monocytes.