"IL-40: A novel immunoregulatory axis at the crossroads of inflammation, tissue remodeling, and cancer immunity".

Interleukin-40 (IL-40), encoded by the C17orf99 gene, is a recently identified B cell-associated cytokine that bridges innate and adaptive immune responses. Since its discovery in 2017, IL-40 has emerged as a pleiotropic immunoregulatory factor with broad implications in human diseases. Elevated IL-40 levels are consistently reported across systemic and organ-specific autoimmune disorders, including rheumatoid arthritis, systemic lupus erythematosus, Sjögren's syndrome, systemic sclerosis, and autoimmune thyroid diseases. In these conditions, IL-40 correlates with disease activity, inflammatory burden, and adverse outcomes. Mechanistically, beyond B-cell differentiation and IgA production, IL-40 induces neutrophil extracellular trap (NET) formation (NETosis), triggering extracellular release of peptidylarginine deiminase 4. PAD4 catalyzes citrullination of intracellular proteins, converting arginine to citrulline residues, generating neoepitopes that break immune tolerance and drive anti-citrullinated protein antibody (ACPA) production-a hallmark of rheumatoid arthritis and related autoimmunities. Beyond autoimmunity, dysregulated IL-40 is observed in viral infections (COVID-19, Epstein-Barr virus), sepsis, and pneumonia, suggesting roles in host defense, systemic inflammation, and immunopathology. IL-40 deficiency impairs IgA homeostasis and alters gut microbiota, implicating it in mucosal immunity and microbiome crosstalk. In oncology, aberrant IL-40 expression is reported in solid tumors (breast cancer, hepatocellular carcinoma) and hematologic malignancies (acute/chronic myeloid leukemias, Hodgkin lymphoma), indicating context-dependent functions in tumor biology. Elevated IL-40 in obesity, metabolic syndrome, type 2 diabetes, and allergic diseases further implicates this cytokine in low-grade chronic inflammation and hypersensitivity. Overall, IL-40 represents a promising immunoregulatory cytokine with diagnostic and therapeutic potential across immune-mediated diseases and cancer. However, current evidence remains largely observational and animal-based, while mechanistic and clinical investigations are scarce. Future research should prioritize identifying IL-40 receptor(s) and downstream signaling pathways through well-designed cellular and molecular studies, alongside large-scale multicenter trials, to fully elucidate its biological functions and clinical utility.