Abstract:

Introduction: Borrelia burgdorferi (Bb) infection can persist in protected sites like articular joints, presenting as migratory oligoarthritis characteristic of Lyme Arthritis (LA). LA is characterized by intermittent synovitis and joint swelling, but why acute infection is relapsing-remitting and not persistent remains unknown. We hypothesize that the immune response is unique to the joint space, particularly the response of Resident Synovial Macrophages (RSMs), and may be important in understanding the clinical course of acute LA and beyond. This study aimed to investigate the polarization and functional profiles of these joint lining macrophages in response to acute Bb exposure in ex vivo human synovial tissue from patients undergoing total joint replacement. Methods: Synovial biopsies from seven patients were exposed to three inoculate doses of Bb (250k, 500k, 1000k) in our Joint Space Analysis System (JSAS). Tissue samples and supernatant were collected at 24, 48, and 72 hours for histology, immunofluorescence (IF), RNAscope, and ELISA. Results: We found dose- and time-dependent changes in synovial intimal lining (IntL) thickness and cellularity. IntL integrity significantly decreased with 1000k Bb at Day 2 (p = 0.021) and remained decreased with 1000k Bb at Day 3 (p = 0.005). IF revealed sustained expression of the M2 marker CD206 in joint-lining macrophages despite infection. RNAscope demonstrated a spatially limited co-localization of Bb within the tissue and alteration of tight junction ZO-1 subcellular localization within the IntL. ELISA indicated reduced Osteoprotegerin at Day 3, suggesting potential for altered bone remodeling activity. IL-10 secretion increased over time and Bb dose. Discussion: These findings demonstrate that Bb interaction with joint-lining macrophages induces rapid, structural, and functional alterations, with loss of IntL integrity and macrophage dysregulation. Compromise of the synovial barrier may facilitate immune cell infiltration, contributing to disease pathogenesis, in the setting of increased markers of synovitis and increased regulatory cytokines.