Identification of CLL-relevant microenvironmental factors as potential targets for novel therapies

Survival and proliferation of CLL cells is regulated by extrinsic factors provided by the microenvironment. In the peripheral blood, CLL cells are cell cycle arrested and show prolonged survival, whereas in the bone marrow and lymph nodes, proliferation of CLL cells is induced by the local niche. To analyze the role of microenvironmental factors in the pathogenesis of CLL, we have established coculture models that either maintain long-term survival of primary CLL cells or induce their proliferation and therefore mimic different microenvironmental niches. Using these cocultures, we have performed microarray-based expression profiling and cytokine quantification by antibody arrays. Integrative analysis of these data as well as functional studies revealed the importance of an inflammatory microenvironment in CLL and identified soluble CD14 as a novel survival factor for CLL cells (see Seiffert et al., 2010 and Schulz et al., 2011). We further quantified a selection of cytokines in a large cohort of clinically well-defined CLL serum samples (CLL8 cohort of the German CLL study group) and thereby identified several novel factors with potential relevance in CLL. Currently these protein data are evaluated for their potential use as diagnostic or prognostic markers for CLL, as well as for their ability to predict treatment response.
In search of new therapeutic options for patients with CLL, the immunomodulatory drug lenalidomide turned out to have significant clinical activity in CLL. But its mechanism of action is complex and not well understood. As our data showed that myeloid cells constitute an important part of the microenvironment in CLL, we investigated whether lenalidomide interferes with the supportive cell-cell interactions in primary co-cultures of monocytes and CLL cells. Our results showed that lenalidomide significantly reduced the viability of CLL cells in these co-cultures. Among the effects induced by the treatment, we observed changes in the immunophenotype and the cytokine secretion of the myeloid cells. As a consequence of that, not only the expression of cytoskeleton- and migration-related genes was altered in CLL cells but also the migratory potential of CLL cells was impaired, which likely affects the circulation and homing of CLL cells in vivo (see Schulz et al., 2013). The observed effects might therefore explain partly the mode of action of lenalidomide in CLL.
In addition to in vitro studies using cocultures of selected primary cells that are part of the CLL microenvironment, we investigate alterations within hematopoietic tissues in the Eµ-TCL1 mouse line (see CLL mouse models).

Schematic diagram of the interactions between CLL cells and monocytes

In primary cocultures CLL cells impact on the secretome of monocytes (e.g. increase in CCL2 and CD14 release) and thereby contribute to the creation of a pro-tumorigenic microenvironment. Monocyte-derived CD14 directly increases CLL cell viability in the culture, whereas CCL2 acts as a chemotactic factor and is known to induce differentiation of monocytes to tumor-associated macrophages (Seiffert et al., 2010; Schulz et al., 2011). Our data further showed that the immunomodulatory drug lenalidomide impacts on the secretome and differentiation of Nurse-like cells and thereby reduces the viability of CLL cells in these cocultures (Schulz et al., 2013).

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