Chikungunya Virus Infection of Monocytes Generates a Persistent Macrophage Reservoir That Promotes IFN-π/IL-27-STAT1- and NF-κB-Driven Chronic Arthritis

Chikungunya virus (CHIKV) infection causes acute febrile illness that frequently progresses to chronic arthralgia. Although monocytes are CHIKV targets, whether viral exposure drives their differentiation into macrophages to sustain persistent inflammation remains incompletely understood. To address this, we integrated transcriptomic profiling of pediatric CHIKV patients during acute and convalescent phases, in vitro differentiation of primary human monocytes into macrophages under CHIKV infection, and single-cell RNA-sequencing (scRNA-seq) of joint-associated macrophages from an immunocompetent mouse model of chronic CHIKV arthritis at 28 days post-infection. In acute patients, we observed a marked upregulation of monocyte/macrophage markers alongside robust, IFN-dependent JAK-STAT signaling. In vitro, CHIKV-infected monocytes differentiated into persistently infected macrophages (MDM-CHIKV) that closely aligned with an M1-polarized transcriptional profile. These cells exhibited STAT1/NF-{kappa}B-dependent inflammation, ISG induction, and enhanced antigen presentation capacity. Remarkably, MDM-CHIKV uniquely produced IFN-{pi}/IL-27, revealing an alternative non-canonical axis where JAK-STAT activation persists despite the absence of classical type I IFNs. Functionally, MDM-CHIKV displayed an enhanced respiratory burst upon immune complex uptake, pointing to Fc{gamma}R-mediated pathology, as well as complement activation and T-cell stimulatory capacity. However, upon TLR4 challenge, these cells retained partial regulatory responses via IL-10 and TGF-{beta}; this hybrid immunophenotype offers a potential mechanistic explanation for relapsing arthralgia. Additionally, scRNA-seq of murine joints confirmed that persistent CHIKV replication was restricted to a distinct macrophage subset that resembles MDM-CHIKV, establishing these cells as the primary in vivo viral reservoir. Mirroring our human findings, joint-associated murine macrophage subsets exhibited elevated IFN-{pi}/IL-27 expression alongside robust NF-{kappa}B/STAT1-dependent inflammatory signatures (Tnf, Il1{beta}, Il6, Ccl2, Ccl5, Cxcl9, Cxcl10, Cxcl1-3, and/or Ptgs2), underscoring their critical role in sustaining chronic arthritis. Collectively, our findings demonstrate that CHIKV actively drives monocyte-to-macrophage differentiation into a persistently infected, M1-skewed hybrid phenotype that serves as a viral reservoir and fuels pro-arthritic inflammation. Consequently, macrophage reprogramming, the non-canonical IFN-{pi}/IL-27-STAT1 axis, Fc{gamma}R-mediated phagocytosis, and complement activation emerge as central pathogenic nodes and promising therapeutic targets for mitigating chronic post-CHIKV arthropathy.