Background Bactericidal/Permeability-Increasing Protein (BPI) is an antibacterial and antiinflammatory protein stored in polymorphonuclear cells (PMN). BPI has been detected in synovial fluid (SF) of patients with reactive arthritis, rheumatoid arthritis (RA), psoriatic arthritis (PsA) and osteoarthritis (OA).

Objectives The aims of this study were to evaluate the presence of BPI in SF of patients with crystal-induced arthritis and to compare its levels with those of other arthropathies. Furthermore, we tested whether BPI exerts antiinflammatory effects in in vitro models of sterile inflammation.

Methods SF was collected from the knees of 50 untreated patients: 9 with gout, 9 with calcium pyrophosphate (CPP) crystal-induced arthritis (CPP-IA), 8 with OA, 8 with OA and CPP crystals in SF (OA+CPP), 8 with RA and 8 with PsA. SF was examined under optical light microscopy. White cell count (WBC) and the PMN percentage were determined in SF according to standard procedures. SF BPI levels were assayed by ELISA.

As regard the study in vitro, the THP-1 cell line was primed for 3h with phorbole myristate acetate (300 ng/ml), reincubated overnight, and treated with sterile synthetic monosodium urate (MSU) (0.5 mg/ml) or CPP (0.1 mg/ml) crystals for 24h in presence or absence of BPI (1 or 5 μg/ml). Cell supernatants were tested by ELISA for IL-1β and IL-8 production.

Results The results of SF analysis and BPI levels are reported in the table.

BPI levels were similar and the lowest in SFs from OA and OA+CPP. Although the BPI concentrations in gout and CPP-IA SFs were higher than in OA and OA+CPP, the differences did not reach statistical significance. RA SF showed the highest BPI levels (p<0.001 vs gout, CPP-IA, OA, OA+CPP, PsA). In the total group of patients, we found a correlation between BPI and WBC count (r=0.474; p=0.004) and PMN (r=0.354; p=0.037).

Exposure of THP-1 cells to MSU or CPP crystals increased the production of IL-1β (MSU: 655.5±28.28 pg/ml; CPP: 496.5±9.19 pg/ml) and IL-8 (MSU: 4538.5±37.48 pg/ml; CPP: 4424.7±9.55 pg/ml). The treatment with BPI decreased the release of both cytokines induced by crystals in a dose dependent manner. Indeed, in the presence of BPI 1 or 5 μg/ml, the IL-1β secretion was reduced by 19% and 40%, respectively. IL-8 crystal-induced production was inhibited in the presence of BPI 1 μg/ml (MSU: 20%; CPP: 27%), and abolished when THP-1 cells were treated with BPI 5 μg/ml.

Conclusions This study is the first to define the presence of BPI in SF of crystal-induced arthritis. Our results demonstrate that although SFs from gout and CPP-IA contained the WBC and PMN highest levels, BPI was present at significantly lower concentrations than in RA and PsA. This could be explained by the lack of PMN activators able to induce the release of BPI in crystal-induced arthritis.

The use of BPI in our in vitro models of crystal-induced inflammation leads to a significant inhibition of the proinflammatory cytokine release, suggesting an antiinflammatory activity of this protein also in sterile inflammation.

Disclosure of Interest None declared