[Chronic MRSA periprosthetic joint infection induces formation of mature tertiary lymphoid structures and osteolysis in mice]

To investigate the histomorphological and functional characteristics of tertiary lymphoid structures (TLS) in a mouse model of chronic knee periprosthetic joint infection (PJI) with methicillin-resistant Staphylococcus aureus (MRSA) and explore their association with local osteolysis.

C57BL/6J mice receiving prosthesis implantation in the knee joint were randomized into control group and PJI group for injection of sterile PBS and of MRSA USA300 (1×10⁴ CFU) into the knee joint cavity, respectively. Peripheral blood leukocyte counts, erythrocyte sedimentation rates, and C-reactive protein levels were monitored regularly. Sixty days after the injections, periprosthetic tissues were harvested for HE staining to measure the cross-sectional area of mature TLS. Multiplex immunofluorescence staining was used to analyze spatial segregation of T and B cells (CD3/B220) and the network of follicular dendritic cells (FDCs) and detect Ki67 and PNAd expressions for assessing TLS functionality. The number of osteoclast per unit of bone surface (N.Oc/BS) in the periprosthetic region was determined using histomorphometric analysis with TRAP staining.

At 21 days post-surgery, the mice in PJI group showed significant elevation of all the peripheral blood inflammatory markers. Numerous mature TLS were observed in the periprosthetic tissues in PJI group with an average cross-sectional area significantly greater than that in the control group. The TLS in PJI group exhibited a typical spatial organization, characterized by a B-cell-centered core surrounded by T cells with an FDC network; abundant Ki67-positive proliferating cells were observed within the TLS, and PNAd-positive high endothelial venules were visible at the periphery and within the parenchyma. The N.Oc/BS was significantly greater in PJI group than in the control group (28.75±4.13 vs 1.63±0.74 mm^-1, P<0.0001).

Chronic MRSA infection induces formation of mature TLS with well-defined structural organization and functional activity in the periprosthetic region of mice, accompanied by significant bone loss.