New soluble CSF-1R-dimeric mutein with enhanced trapping of both CSF-1 and IL-34 reduces suppressive tumor-associated macrophages in pleural mesothelioma.

Background: Colony stimulating factor-1 receptor (CSF-1R) and its ligands CSF-1 and interleukin (IL)-34 have tumorigenic effects through both induction of suppressive macrophages, and survival/proliferation of tumor cells. In addition, the IL-34 tumorigenic effect can also be mediated by its other receptors, protein-tyrosine phosphatase zeta, Syndecan-1 (CD138) and triggering receptor expressed on myeloid cells 2. Small tyrosine kinase inhibitors are used to block CSF-1R signaling but lack specificity. Neutralizing anti-CSF-1 and/or IL-34 antibodies have been proposed, but their effects are limited. Thus, there is a need for a more specific and yet integrative approach.

Methods: A human mutated form of the extracellular portion of CSF-1R was in silico modelized to trap both IL-34 and CSF-1 with higher affinity than the wild-type CSF-1R by replacing the methionine residue at position 149 with a Lysine (_M149K). The extracellular portion of the mutated CSF-1R M149K was dimerized using the immunoglobulin Fc sequence of a silenced human IgG1 (sCSF-1R_M149K-Fc). Signaling through CSF-1R, survival of monocytes and differentiation of suppressive macrophages were analyzed using pleural mesothelioma patient's samples and mesothelioma/macrophage spheroids in vitro and in vivo in the presence of sCSF-1R_M149K-Fc or sCSF-1R-Fc wild type control (sCSF-1R_WT-Fc).

Results: We defined that the D1 to D5 domains of the extracellular portion of CSF-1R were required for efficient binding to IL-34 and CSF-1. The mutein sCSF-1R_M149K-Fc trapped with higher affinity than sCSF-1R_WT-Fc both CSF-1 and IL-34 added in culture and naturally produced in mesothelioma pleural effusions. sCSF-1R_M149K-Fc inhibited CSF-1R signaling, survival and differentiation of human suppressive macrophage in vitro and in vivo induced by pleural mesothelioma cells. Neutralization of IL-34 and CSF-1 by sCSF-1R_M149K-Fc also resulted in higher killing of pleural mesothelioma cells by a tumor-specific CD8⁺ T cell clone in mesothelioma/macrophage spheroids.

Conclusions: sCSF-1R_M149K-Fc efficiently traps both CSF-1 and IL-34 and inhibits CSF-1R signaling, monocyte survival and suppressive macrophage differentiation induced by pleural mesothelioma cells producing CSF-1 and IL-34, as well as restores cytotoxic T-cell responses. sCSF-1R_M149K-Fc has therapeutic potential vs other therapies under development targeting single components of this complex cytokine pathway involved in cancer.