Abstract Introduction: While cancer-associated fibroblasts (CAFs) are known to promote and sustain the growth of tumors, the underlying mechanisms remain incompletely understood. Previous work in our lab identified a novel mechanism of communication in which CAFs secrete cardiotrophin-like cytokine factor 1 (CLCF1), a cytokine that binds ciliary neurotrophic factor receptor (CNTFR) on tumor cells and promotes neoplastic growth. CNTFR is a component of the tripartite receptor complex formed by CNTFR-gp130-LIFR and is capable of activating several oncogenic signaling cascades, including Jak-STAT. Here we demonstrate that targeting CNTFR represents a potentially novel strategy for treating non-small cell lung cancer (NSCLC). Results: Gene expression analyses reveal that increased CNTFR and CLCF1 levels correlate with poor patient outcomes and that CLCF1 expression is consistently higher in CAFs when compared to normal lung fibroblasts (NLFs) from the same patient. Elevated CNTFR expression was also seen in NSCLC cell lines by western blot, immunohistochemistry, and flow cytometry. Addition of recombinant CLCF1 increased the proliferation of NSCLC cell lines in culture (A549, H23, and H358), while CNTFR knockdown decreased cell proliferation and colony formation in vitro and tumor growth in A549, H23, and H2009 xenografts. CNTFR knockdown also decreased phosphorylation of STAT3 and ERK and decreased expression of several dedifferentiation markers. Therefore, CLCF1-CNTFR signaling may promote tumor growth by activating the Jak-STAT and Ras-Raf-MEK-ERK pathways, and by promoting the dedifferentiation of normal epithelial tissue. Finally, we tested a novel “ligand trap” to bind soluble CLCF1 thereby abrogating the CLCF1-CNTFR signaling axis. This ligand trap was designed using directed protein engineering to increase binding compared to the native CNTFR. In vivo treatment of cell lines and patient-derived xenografts (PDXs) led to decreased tumor growth. Currently we are testing this novel biologic in an autochthonous mouse tumor model. These results identify a pro-oncogenic role for CLCF1-CNTFR signaling in lung cancer and a potentially novel approach to therapy using a ligand decoy. Perspectives: We identify for the first time a critical role for CLCF1-CNTFR signaling in NSCLC progression. We validate this role in both a cell culture model and in PDXs, and we also demonstrate novel links between CLCF1-CNTFR and two well established oncogenic pathways. Our findings may shed light on oncogenic mechanisms in other cancer contexts and facilitate the development of novel therapeutic strategies. Citation Format: Cesar Marquez, Jun Kim, Amato Giaccia, Jennifer Cochran, Alejandro Sweet-Cordero. Targeting the CLCF1-CNTFR signaling axis using directed evolution for lung cancer therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3918.