Spatial and functional characterization of ER stress proteins links fibroblast plasticity and proliferative processes in human osteoarthritic synovitis.

A strong crosstalk exists between endoplasmic reticulum (ER) stress and synovitis. Beyond their canonical role in protein folding, ER stress chaperones may promote inflammation, cell survival, and fibroblast activation under pathological conditions. This study aimed at localizing and quantifying 11 ER stress proteins (BiP, HYOU1, MANF, PDIA4, GANAB, HSP90B1, TXNDC5, DNAJB11, LMAN1, ERP29, CALR) in human inflamed synovial membranes and at investigating their expression in fibroblast-like synoviocytes (FLS) under ER stress, pro-inflammatory, or pro-fibrotic stimuli. By immunohistochemistry, on a first cohort of formalin-fixed paraffin-embedded (FFPE) biopsies obtained from patients with osteoarthritis (OA), chronic pyrophosphate arthropathy (CPPA), and rheumatoid arthritis (RA), these ER chaperones were primarily localized to the lining in low-grade inflammation (Tak <4) and expanded to the sublining under high inflammatory conditions (Tak ≥4), with a widespread distribution in RA. Imaging mass cytometry, applied to a second cohort of FFPE tissue samples collected from patients diagnosed with OA and RA, revealed the co-expression of ER stress proteins with CD55⁺ FLS in the lining and their progressive infiltration into the sublining along with CD34⁺CD31- FLS during inflammation. These observations were confirmed by immunofluorescence on a larger cohort of OA patients. As inflammation progresses, there is a loss of co-expression with CD55 in the lining, accompanied by a gradual shift towards co-expression with CD34 in the sublining. In vitro, ER stress proteins, particularly BiP, HYOU1, MANF, PDIA4, HSP90B1, LMAN1, CALR, and DNAJB11 are overexpressed in human OA FLS following ER stress, pro-inflammatory or pro-fibrotic stimulation, with BiP, PDIA4, HSP90B1, ERP29, and CALR also being secreted. PDIA4 emerged as a central player: its depletion significantly impaired FLS proliferation and migration, highlighting a direct role in driving synovitis. This study provides the first spatial and functional characterization of ER chaperones in human arthritic synovium, linking ER stress to fibroblast plasticity, inflammation, and fibrosis.