Transposable elements (TEs) are mobile genomic sequences that encompass roughly 50% of the human genome. Class 1 TEs, or "retrotransposons," mobilize through the production of an RNA intermediate that is then reverse transcribed to form complementary DNA (cDNA) molecules capable of genomic reinsertion. While TEs are traditionally silenced to maintain genomic integrity, the recognition of immunostimulatory cues, such as those provided by microorganisms, drastically alters host transcription to induce the differential expression of TEs. Emerging evidence demonstrates that the inducible production of TE cDNA is not an inert phenomenon but instead has been coopted by host immunity to facilitate cross talk between host and constituents of the microbiota by agonizing intrinsic antiviral receptors. Here, we demonstrate that immunostimulation of toll-like receptor 4 (TLR4) with lipopolysaccharide (LPS) and TLR5 with bacterial flagella (FLA) alters the expression of retrotransposons, such as human endogenous retroviruses (HERVs) and long interspersed nuclear elements (LINEs). Next, we demonstrate that reverse transcriptase inhibitor (RTi) delivery ameliorates the acute production of the proinflammatory cytokine "tumor necrosis factor alpha" (TNF-α) in response to FLA in a monocytic cell line (THP-1). Collectively, our findings demonstrate that TLR5-mediated cross talk between the host and microbiota is partially dependent on the reverse transcription (RT) of retrotransposons. IMPORTANCE The microbiota is a potent reservoir of immunostimulatory and immunosuppressive motifs that fundamentally shape host immunity. Despite broad associations between microbial composition and host immunity, the mechanisms underlying host microbiota-induced immunoregulation remain poorly defined. Here, we demonstrate a novel mechanism by which motifs overabundant during dysbiotic conditions influence host immunity through the upregulation of endogenous RT to produce motifs that agonize antiviral receptors.
