Background: Joint pain and inflammation are common early features of a wide range of systemic diseases. These include autoimmune disorders such as systemic lupus erythematosus (SLE), as well as infections originating outside the musculoskeletal system, which can manifest as reactive arthritis. We recently demonstrated that highly permeable fenestrated capillaries are specifically located at the periphery of the synovium, allowing circulating stimuli to enter the joint. This region is tightly surrounded by macrophages and nociceptors, forming blood–joint barrier (BJB) [1]. However, the detailed ultrastructure of the BJB under physiological and pathological conditions remains poorly understood.

Objectives: This study sought to generate a three-dimensional molecular and ultrastructural atlas of the BJB using whole-mount imaging and volume electron microscopy (vEM) of the synovium under both healthy and arthritic conditions.

Methods: For in vivo studies, arthritis was induced in C57BL/6J mice using immune complex injection and the collagen antibody–induced arthritis model. Whole-mount imaging of the synovium was performed using a TCS SP8 3X gated STED confocal inverted microscope. We developed a vEM protocol that enables reconstruction of serial ultrathin sections, providing nanometer-scale resolution of endothelial cell morphology, including fenestrations and transcytotic vesicles.

Results: Under physiological conditions, PV1 ⁺ fenestrated capillaries were specifically localised at the lining–sublining interface in the peripheral synovium where it attaches to bone. Following immune challenges from the circulation, extensive sprouting emerged from PV1 ⁺ capillaries, leading to tip cell formation and displacement of surrounding macrophages. vEM analysis revealed marked alterations in endothelial fenestration ultrastructure and in interactions with adjacent cell types, resulting in disruption of the BJB.

Conclusions: Our findings highlight the central role of PV1 ⁺ fenestrated capillaries in the synovium in linking systemic immune conditions to joint pathology. Structural remodeling of endothelial cells and surrounding macrophages contributes to disruption of the blood–joint barrier, providing mechanistic insight into inflammation-associated joint disease.

REFERENCES: [1] Hasegawa T, Lee C.Y.C., Hotchen A.J. et al. Macrophages and nociceptor neurons form a sentinel unit around fenestrated capillaries to defend the synovium from circulating immune challenge. Nature Immunol, 2024, 25:2270–2283.

Acknowledgments: NIL.

Disclosure of Interests: None declared.