The macrocyclic peptide rhesus theta defensin 1 activates interferon and antiviral pathways in human monocytes

Rhesus theta defensin (RTD)-1, a cyclic antimicrobial peptide, regulates gene expression and immune signaling pathways in cell culture and animal models of immune-mediated diseases. In lipopolysaccharide-stimulated cells, RTD-1 inhibition of proinflammatory cytokine secretion and gene expression is mediated through inhibition of the NF-κB and MAP kinase signaling pathways. To gain insights into RTD-1 regulation of naïve cells, we performed RNA sequencing (RNA-seq) to determine the effect of the peptide on global gene expression in human monocytes and THP-1 monocytes. In both cell types, analysis of differentially expressed genes revealed stimulation of interferon and antiviral gene expression pathways. RTD-1 induced Y701 phosphorylation of STAT1 and activated the ISRE reporter in a JAK-dependent manner. Stimulation of the ISRE reporter by RTD-1 was interferon-α/β receptor dependent but was independent of its NF-κB inhibitory activity in lipopolysaccharide-stimulated cells. RTD-1 inhibited infection of vesicular stomatitis virus pseudotyped with G glycoprotein or SARS-CoV-2 spike protein in THP-1 and Vero E6 cells, respectively. RTD-1 also inhibited infection of Calu-3 2B4 cells by SARS-CoV-2 virus, demonstrating antiviral activity of RTD-1 in diverse cell types. These results demonstrate that RTD-1 stimulates interferon and antiviral pathways, potentially priming cells for resistance to viral infection.