Human CD4⁺ T cells regulate peripheral immune responses in rheumatoid arthritis via insulin-like growth factor–like family member 2

Human CD4⁺ T cells play a central role in the pathogenesis of autoimmune diseases, but their immunoregulatory mechanisms driving pathogenesis remain to be elucidated. We show that human T peripheral helper cells (T_PH cells) regulate peripheral immune responses via insulin-like growth factor–like family member 2 (IGFL2), an inflammatory factor found exclusively in primates. Single-cell RNA sequencing of seropositive rheumatoid arthritis (RA) synovium showed that IGFL2 is specifically expressed by CD4⁺ T cells, predominantly T_PH cells. IGFL2 promotes transforming growth factor–β–induced CXCL13 production in CD4⁺ T cells, activates nuclear factor κB signaling, and induces monocyte gene signatures like those of pathogenic macrophages. CRISPR-Cas9 knockout of IGFL2 in synovial T_PH cells suppressed this gene signature in cocultured monocytes. Blood IGFL2 protein levels correlated with RA disease severity and could be used as a potential biomarker. These findings highlight the involvement of IGFL2 in RA pathogenesis, emphasizing how human T_PH cells regulate local immune responses via IGFL2.