Recombinant IL-38 Alleviates Temporomandibular Joint Synovial Inflammation via IL-1R1-NF-κB-IL1β Pathway

Introduction and aims: Interleukin (IL)-38 has been identified as an anti-inflammatory cytokine; however, its specific role in temporomandibular joint (TMJ) synovial inflammation and the identity of its functional receptors remain elusive. This study aimed to investigate the therapeutic efficacy of IL-38 in TMJ synovial inflammation and to elucidate its potential receptor mechanisms. Methods: A rat model of temporomandibular joint osteoarthritis (TMJOA) was established via bilateral TMJ injection of mono-iodoacetate (MIA). Fifteen Sprague–Dawley (SD) rats were randomly allocated into three groups: sham (PBS), model (MIA), and treatment (MIA +human recombinant IL-38, hIL-38). Protein BLAST analysis was utilized to predict IL-38 function and receptor affinity. Synovial histopathology was assessed using HE staining. Quantitative real-time PCR (qPCR) was employed to quantify inflammatory gene expression, while immunohistochemistry (IHC) was utilized to detect inflammatory proteins and IL-1 receptor type 1 (IL-1R1) expression. Protein–protein interaction (PPI) network and protein-protein docking analyses were performed to predict the interaction between IL-38 and IL-1R1. Results: IL-38 exhibited 43% identity and 55% positivity with interleukin-1 receptor antagonist (IL-1Ra), its precursor, and its isoforms X1 and X2. HE staining demonstrated that hIL-38 significantly reduced synovial inflammatory cell infiltration. IHC staining revealed that hIL-38 inhibited macrophage infiltration (CD68⁺) and suppressed the expression of iNOS and COX-2, thereby attenuating synovial inflammation. Docking and PPI analyses corroborated a direct interaction between IL-38 and IL-1R1. Furthermore, IHC staining indicated that IL-38 down-regulated the protein levels of IL-1R1, NF-κB p65, and IL-1β. Conclusions: IL-38 functions as a novel anti-inflammatory cytokine that alleviates TMJ synovial inflammation, potentially by antagonizing IL-1R1 and blocking the NF-κB signalling cascade. Clinical Relevance: IL-38 may offer a novel therapeutic strategy for alleviating synovial inflammation in TMJOA.