CD14+ blood mononuclear cells co-cultured with chronic lymphocytic leukemia (CLL) B cells can differentiate into large, round, adherent cells that can attract and support survival of CLL cells in vitro. These cells, called nurselike cells (NLC), have distinctive high-level expression of CD68, a property also shared by lymphoma-associated macrophages (LAM), which are found in the secondary lymphoid tissues of patients with follicular lymphoma and appear more prevalent in lymph nodes of patients with therapy-resistant disease. We found that NLC express significantly higher levels of B-cell activating factor belonging to the tumor necrosis factor family (BAFF) than common macrophages, which also can differentiate from CD14+ blood mononuclear cells in vitro. BAFF can induce activation of NF-kB in CLL cells and enhance leukemia-cell survival. To investigate the influence of CLL cells on the differentiation of NLC in vitro, we cultured purified CLL cells for 24h in to generate conditioned media. When CD14+ blood mononuclear cells of healthy donors were cultured for 7 days in the conditioned media, the cells assumed the morphology of NLC and acquired high-level expression of BAFF. In contrast, CD14+ blood mononuclear cells cultured in non-conditioned media did not acquire such changes. Serial dilutions of condition media into non-conditioned media revealed a dose-response relationship between the relative-concentration of the condition media and the capacity of the media to induce NLC-changes in the CD14+ blood mononuclear cells. Although the CLL cells from each patient examined (n=6) could generate conditioned media that was effective in inducing NLC-changes in CD14+ blood mononuclear cells, initial experiments suggest that conditioned media derived from CLL cells with features associated with more aggressive disease (e.g. expression of unmutated IgVH and ZAP-70) showed a higher BAFF-inducing activity compared to condition media derived from CLL cells that expressed mutated IgVH and lacked expression of ZAP-70. Condition media was generated under identical culture condition in all cases. Although IL-10 and TNF-a have been shown to increase BAFF expression in other systems, we did not observe increase in BAFF protein expression on monocytes cultured with increasing amounts with either of these cytokine alone. These studies reveal that CLL cells elaborate a factor(s) that can induce CD14+ blood mononuclear cells to assume properties of NLC, which apparently are similar to the LAM also identified in the tissues of patients with follicular lymphoma. Preliminary biochemical characterization suggests that these factor(s) are heat- and nuclease resistant. The results show a previously unrecognized symbiosis between CLL cells and their microenvironment. Because the survival of neoplastic B cells might depend upon NLC and other such cells in the microenvironment of lymphoid tissues, identification of this factor(s), and development of agents that can block its capacity to induce maturation of CD14+ blood mononuclear cells into NLC, could provide a novel target for therapy of CLL and other indolent B-cell lymphomas.

Author notes

Disclosure: No relevant conflicts of interest to declare.