The proprotein convertase subtilisin/kexins (PCSKs) regulate biological actions by cleaving immature substrate proteins. The archetype PCSK, FURIN, promotes the pathogenicity of viruses by proteolytically processing viral proteins. FURIN has also important regulatory functions in both innate and adaptive immune responses but its role in the CD8 + CTLs remains enigmatic. We used a T-cell-specific FURIN deletion in vivo to demonstrate that FURIN promotes host response against the CTL-dependent lymphocytic choriomeningitis virus by virtue of restricting viral burden and augmenting interferon gamma (IFNG) production. We also characterized Furin KO CD8 + T cells ex vivo, including after their activation with FURIN regulating cytokines IL12 or TGFB1. Furin KO CD8 + T cells show an inherently activated phenotype characterized by the upregulation of effector genes and increased frequencies of CD44 + , TNF + , and IFNG + cells. In the activated CTLs, FURIN regulates the productions of IL2, TNF, and GZMB and the genes associated with the TGFBRsignaling pathway. FURIN also controls the expression of Eomes, Foxo1, and Bcl6 and the levels of ITGAE and CD62L, which implies a role in the development of CTL memory. Collectively, our data suggest that the T-cell expressed FURIN is important for host responses in viral infections, CTL homeostasis/activation, and memory development.

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