Rapamycin treatment ameliorates HLA-B27-mediated gut inflammation and alters the microbiome in experimental spondyloarthritis

<h4>Objective</h4>To determine whether rapamycin affects HLA-B27-mediated gut inflammation in experimental spondyloarthritis (SpA).<h4>Methods</h4>HLA-B27/human β₂-microglobulin transgenic (B27-Tg) rats with gut inflammation were treated with rapamycin (1.5 mg/kg intraperitoneally, 3 times a week) or vehicle for 5 weeks. Healthy age-matched wild type (WT) rats were treated in parallel. Gut inflammation was evaluated via stool scoring and histological assessment. Transcriptome and microbiome analyses were performed on colon tissue and cecal luminal contents, respectively. Bulk immune cells were isolated from the colonic lamina propria of B27-Tg and WT animals, treated with rapamycin <i>ex vivo</i>, and pro-inflammatory cytokine expression was measured using qPCR.<h4>Results</h4>Rapamycin treatment reduced stool and colon histological scores in B27-Tg rats compared to vehicle-treated B27-Tg controls. Transcriptome analysis revealed that rapamycin reduced expression of key pro-inflammatory cytokines like <i>Il17a, Il17f</i>, <i>Tnf</i>, <i>Il1a</i>, <i>IL1b</i>, and <i>Il22</i> in B27-Tg colon tissue compared to vehicle-treated B27-Tg controls. <i>Ex vivo</i> treatment of bulk immune cells isolated from B27-Tg rat colon with rapamycin reduced expression of <i>Il17a, Il17f</i>, <i>Ifng</i>, and <i>Il22</i> compared to vehicle-treated cells. Rapamycin treatment decreased the abundance of cecum microbiota associated with inflammation in B27-Tg rats. Rapamycin also altered the gut microbiome in WT rats, without associated changes in the tissue transcriptome.<h4>Conclusion</h4>Our study demonstrates that rapamycin treatment substantially reduces HLA-B27-mediated gut inflammation in experimental SpA. Results from this pre-clinical model suggest further evaluation of rapamycin as a therapeutic strategy in HLA-B27 associated diseases is warranted.