Targeted transcriptomic analysis of synovial tissues from horses with septic arthritis treated with immune-activated mesenchymal stromal cells reveals induction of T-cell response pathways

Abstract

Abstract OBJECTIVE To investigate mechanistically the reported beneficial effects of immune-activated mesenchymal stromal cell (MSC) therapy to treat equine septic arthritis, leveraging Nanostring technology. ANIMALS 8 Quarter Horses with induced tibiotarsal Staphylococcus aureus septic arthritis treated IA with either Toll-like receptor-3 agonist polyinosinic:polycytidylic acid–activated MSCs + vancomycin antimicrobials (TLR-MSC-VAN; n = 4) or antimicrobials (VAN; 4). METHODS Synovial tissues were collected and fixed in neutral-buffered 10% formalin, and formalin-fixed paraffin-embedded synovial and osteochondral tissues were sequenced using a custom-designed 200-gene equine Nanostring nCounter immune panel to directly quantify expression of key immune and cartilage-related genes. Immunohistochemistry to detect CD3+ T cells was performed on synovial tissues to further quantify T-cell infiltration in TLR-MSC-VAN– versus VAN-treated joints. RESULTS Comparison of synovial transcriptomes between groups revealed moderate changes in differential gene expression, with upregulated expression of 9 genes and downregulated expression of 17 genes with fold change ≥ 2 or ≤ −2 and a significant false discovery rate–adjusted P value of ≤ .05. The most upregulated genes in TLR-MSC-VAN–treated horses included those related to T-lymphocyte recruitment and function, while pathways related to innate immune activation and inflammation were significantly downregulated. Immunohistochemistry and quantitation of CD3+ T-cell infiltrates revealed a numerically greater infiltrate in synovial tissues of TLR-MSC-VAN–treated horses, which did not reach statistical significance in this small sample set (P = .20). CLINICAL RELEVANCE Targeted transcriptomic analyses using an equine Nanostring immune and cartilage health panel provided new mechanistic insights into how innate and adaptive immune cells within synovial tissues respond to TLR-activated MSC treatment when used to treat septic arthritis.