To identify novel protein therapeutics we have utilized an integrated screening system in which a collection of full-length human cDNA clones, encoding virtually all secreted proteins and receptors, was assembled. Soluble secreted proteins were expressed in a high-throughput format using human cells as the expression host and tested in high-throughput cell-based assays. Through this approach, a novel cytokine, FPT025, was identified in a human monocyte proliferation assay. FPT025 has no apparent sequence homology to known cytokines or any other genes and is expressed in human spleen, skin, brain, and other tissues. The purified recombinant FPT025 protein stimulated human primary monocyte proliferation and/or survival. FPT025 protein specifically bound to human primary monocytes and activated ERK1/2 phosphorylation in primary monocytes as well as in a human monocytic cell line, THP-1. FPT025 promoted formation of the myeloid lineage colonies, CFU-M, in a human bone marrow colony formation assay and enhanced proliferation of cells with myeloid cell surface markers from human monocytes. To identify the receptor of FPT025, a collection of extracellular domains (ECD) of transmembrane proteins was screened for their ability to block FPT025 activation of monocyte proliferation. In this screen, we found that FPT025 binds to the M-CSF receptor (M-CSFR) with high affinity. Furthermore, we showed that the binding of FPT025 to M-CSFR was specific and could be competed by M-CSF. The soluble ECD of M-CSFR inhibited both the binding of FPT025 to M-CSFR and the activity of FPT025 in monocyte proliferation. Therefore, FPT025 functions as a novel ligand of the M-CSF receptor and participates in the regulation of myeloid lineage differentiation, proliferation, and survival.

Disclosures: Five Prime Therapeutics, Inc.; Five Prime Therapeutics, Inc.; Five Prime Therapeutics, Inc.

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