The Toll-Like Receptor-Like Molecule CD180 and Soluble CD14 Transmit Survival Signals in B-Cell Chronic Lymphocytic Leukemia Cells Presumably by Acting As Co-Receptors,

Abstract 3883

Survival and proliferation of B-cell chronic lymphocytic leukemia (CLL) cells strongly depend on external factors. When removed from their natural microenvironment CLL cells rapidly undergo spontaneous apoptosis in vitro unless cocultured with stromal cells or non-malignant leukocytes. Recently, we could show that monocytes effectively support long-term survival of CLL cells in vitro. Our results from cytokine antibody arrays and extensive transcriptome analyses of primary CLL cell cocultures suggested a functional role of several soluble factors as well as signaling pathways of innate immunity, like Toll-like receptor-, TREM1- and NRF2-mediated signaling. The most interesting soluble factors are currently quantified in serum samples of the german CLL8 study cohort of CLL patients by cytometric bead arrays. So far, our data show that two of these candidates, CCL2 and soluble CD14, are significantly increased in the serum of CLL patients. In vitro studies using recombinant soluble CD14 demonstrated that CLL cell survival was significantly increased in the presence of this factor. CD14 which is expressed in particular by monocytes and macrophages is an important mediator of innate immunity. Along with TLR-4, CD14 acts as a co-receptor for the detection of bacterial lipopolysaccharide (LPS). Alternatively, LPS can also bind to the toll-like receptor-like molecule CD180, which shows strong homology to TLR-4, but does not harbor an intracellular signaling domain. Since TLR-4 is not expressed in CLL cells, we investigated the potential role of CD180 in CD14-mediated cell survival. Flow cytometry analysis revealed an upregulation of CD180 surface expression in CLL cells under survival-inducing culture conditions. Stimulation of CD180 with a cross-linking antibody resulted in activation of CLL cells measured by increased cell size and upregulation of the activation marker CD86, and significantly increased survival rates of CLL cells. Both CD14- and CD180-mediated survival signals lead to an increase in NF-κB activity and up-regulation of its target gene BCL-2. Depletion of CD180 surface expression in CLL cells abolished the pro-survival effect of soluble CD14, suggesting that this factor mediates its signals via binding to CD180. In summary, our data demonstrate that both, soluble CD14 and the toll-like receptor-like molecule CD180 transmit pro-survival signals in CLL cells, most likely by acting as co-receptors. Currently, we characterize the intracellular signaling machinery which is involved in these processes.