gms | German Medical Science

Objectives: Previously, we found increased osteoclastogenesis to be a main driver for the compromised bone structure in PJI. Inhibition of osteoclast differentiation and function is a potential salvage therapy to restore bone homeostasis and subsequently reduce the risk of prosthesis failure. In this study, we investigate targeting PD-1 as a promising strategy to improve immunocompetence while suppressing excessive osteoclast generation and function.

Methods: We collected peripheral blood from patients undergoing total knee arthroplasty due to primary osteoarthritis (NC) or PJI-dependent implant removal surgery. Soluble PD-L1 levels were tested using ELISA. Monocytes were isolated for flow cytometry, cell differentiation, and bone resorption pit assay. Osteoclastogenesis was induced by MCSF and RANKL with additional stimulation: (i) none, (ii) PD-L1, (iii) PD-1 inhibitor, (iv) PD-L1 + PD-1 inhibitor. RNA was isolated from osteoclasts for real-time PCR. Cell medium was acquired to test activity of resorptive enzymes associated with osteoclast activity.

Results and conclusion: Monocyte cell count (0.86±0.35 v 0.21±0.03/nl, p=.02) and PD-1 expression on monocytes (92.50±6.83 v 35.34±15.43%, p<.01) was elevated in PJI. After differentiation, in PJI, total osteoclast surface area was larger (47.40±3.19% v 26.75±3.49%, p<.01) and increased significantly further (+64.04±1.72%, p<.01) after PD-L1 stimulation. Addition of PD-1 inhibitor reduced osteoclast surface area in PJI (36.51±3.95 v 64.04±1.72%, p<.01). Expression of osteoclast differentiation marker Nfatc1 and Ctsk were higher in the PJI group (Nfatc1: 1.09±0.20 v 0.45±0.06, p<.01; Ctsk: 2.27±1.00 v 0.73±0.23, p=.02), but did not differ in Mmp9 and Acp5 expression. In PJI, expression of all four genes were upregulated after PD-L1 stimulation, while PD-1 inhibitor treatment reversed this effect. PD-L1 stimulation led to increased osteoclast bone resorption compared to without stimulation (7.65±3.15 v 2.49±0.67%, p=.05), while addition of PD-1 inhibitor had no impact in the NC group. In PJI, osteoclast surface resorption increased after PD-L1 stimulation (20.69±6.54 v 8.06±1.54, p=.01). Osteoclast resorptive activity was increased after PD-1 stimulation (TRAP: x1.54, p<.01; CTSK: x1.27, p<.01). Conversely, osteoclast function was reversed after treatment with PD-1 inhibitor (surface resorption area: 4.23±0.55 v 8.06±1.54, p=.03; TRAP: x0.62, p=0.02; CTSK: x0.16, p<.01).

Elevated numbers of monocytic osteoclast-progenitor cells and increased osteoclastic differentiation and function may contribute to decreased bone volume and impact prosthesis osseointegration in patients with PJI. Our results show PD-L1 can stimulate osteoclast generation and function and that this can be reversed by PD-1 inhibitor. This effect is particularly prominent in PJI. Targeting PD-1 can be a potential therapy to inhibit monocyte-mediated osteoclastogenesis while retaining anti-microbial activity of macrophages to limit the impact of PJI on the bone stock.