Yersinia lipopolysaccharide is modified by human monocytes

Reactive arthritis is usually self-limiting polyarthritis, which develops after certain gastrointestinal or urogenital tract infections, mostly in susceptible HLA B27-positive individuals. In the pathogenesis of this arthritis, it is probably important that structures of the causative bacteria are found in the affected joints. The structure found in the synovial fluid phagocytes of the patients with reactive arthritis after Yersinia, Salmonella, and Shigella infections has always been lipopolysaccharide (LPS) of the causative bacteria. It has been in a highly processed form but still immunoreactive. To follow the degradation process of LPS, we fed peripheral blood monocytes of healthy blood donors with heat-killed Yersinia enterocolitica O:3 bacteria in vitro and monitored the fate of LPS by immunofluorescence and immunoblotting methods. Heat-killed bacteria were used since Y. enterocolitica O:3 bacteria are able to live inside monocytes in vitro and dividing intracellular bacteria would have made it impossible to monitor the degradation process of LPS with these methods. Both the core region and the O-polysaccharide chain of LPS persisted in cytoplasmic vacuoles and on plasma membrane of monocytes through the 7-day follow-up time. Migration properties of processed LPS in sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggested structural modifications of LPS. We also demonstrated that core epitopes appearing on the surface of Yersinia-fed monocytes on day 4 of incubation were processed intracellularly, suggesting that LPS-containing phagocytes are a constant source of membrane-active LPS in their microenvironment as well as in the joints of arthritic patients.