Chronic lymphocytic leukemia (CLL) cells are highly dependent on their microenvironment. External stimuli provided by bone marrow stromal cells or non-malignant leukocytes are required for their survival and proliferation. Interestingly, peripheral blood-derived monocytes differentiate in the presence of CLL cells to so-called Nurse-like cells (NLCs), which are round or fibroblast-shaped adherent cells that were shown to promote survival of CLL cells in vitro and to exist in lymph nodes of CLL patients. In search of new therapeutic options for patients with CLL, the immunomodulatory drug lenalidomide turned out to have significant clinical activity in CLL. Lenalidomide does not induce apoptosis in CLL cells directly, but is rather believed to act via the microenvironment. Several studies described that it alters cytokine levels and the activation status of the cells. Further, a CLL-specific T-cell defect was shown to be repaired by lenalidomide, which might represent a major activity of this drug in CLL. However, its mechanism of action seems to be complex and is not well understood.
As monocytes as well as NLCs are very effective in maintaining survival of CLL cells, we aimed to investigate whether lenalidomide interferes with these supportive cell-cell interactions. To do this, we established primary co-cultures of monocytes and CLL cells in the presence or absence of lenalidomide and observed a significantly decreased viability of CLL cells after 14 days of treatment, suggesting an impact of this drug on the survival support of NLCs. Therefore, we analyzed the immunophenotype of NLCs by flow cytometry, as well as the secretion of cytokines in the co-cultures by ELISA and antibody-coupled bead arrays. Among the effects induced by lenalidomide, we observed reduced cell surface expression of the MHC II protein HLA-DR on NLCs as well as lower levels of the chemokine CCL2, but higher levels of IL-10 in the culture supernatant, indicating an altered inflammatory milieu in the co-cultures. The enhanced IL-10 levels resulted in an increase in STAT1 phosphorylation in CLL cells as measured by Western blot analysis. As a consequence, enhanced expression of the adhesion molecule ICAM-1 (CD54) and an altered expression of cytoskeletal genes (e.g. RHOC and CORO1B) were observed in CLL cells after lenalidomide treatment. Chemotaxis assays using transwell culture dishes and SDF1-α as chemoattractant revealed an impaired migratory potential of lenalidomide-treated CLL cells, which was not due to reduced expression of the SDF1-α receptor CXCR4. In summary, our data show that lenalidomide reduces the survival support of NLCs for CLL cells in vitro, suggesting that this drug effects the myeloid microenvironment in CLL in vivo. Furthermore, lenalidomide impairs the migratory potential of CLL cells which may affect circulation and homing of CLL cells in vivo.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.