Insights from a novel three-dimensional in vitro model of lyme arthritis -Standardized analysis of cellular and molecular interactions between Borrelia burgdorferi and synovial explants and fibroblasts

Objective. To develop a novel 3-dimensional (3-D) in vitro model of Lyme ar thritis to use in the study of the interactions between Borrelia burgdorfer i (Bb) and human synovial host cells with respect to phagocytosis and poten tial persistence of Bb as well as the induction of proinflammatory cytokine s and chemokines. Methods. Two distinct culture systems, consisting of synovial membrane expl ants or interactive synovial cells embedded in 3-D fibrin matrices, were ch osen. Both systems were artificially infected with Bb, and the interactions between Bb and synovial tissue/cells were studied by histology, immunohist ochemistry, and electron microscopy. Functional analyses included the induc tion/secretion of cytokines by Bb in the model system. Results. Both culture systems proved to be stable and reproducible. The hos t cells and spirochetes showed high levels of viability and maintained thei r physiologic shape for >3 weeks, Bb invaded the synovial tissue and the ar tificial matrix in a time-dependent manner. Host cells were activated by Bb , as indicated by the induction of interleukin-1 beta and tumor necrosis fa ctor alpha. Electron microscopic analysis revealed Bb intracellularly withi n macrophages as well as synovial fibroblasts, suggesting that not only pro fessional phagocytes, but also resident synovial cells are capable of phago cytosing Bb. Most interestingly, the uptake of the spirochetes appeared to cause severe damage of the synovial fibroblasts, since the majority of thes e cells displayed ultrastructural features of disintegration. Conclusion. A novel 3-D in vitro model has been established that allows the study of distinct aspects of Lyme arthritis under conditions that resemble the pathologic condition in humans. This reproducible, standardized model supplements animal studies and conventional 2-D cultures. The disintegratio n of synovial fibroblasts containing Bb or Bb fragments challenges the conc ept of an intracellular persistence of Bb and may instead reflect a mechani sm that contributes to the inflammatory processes characteristic of Lyme ar thritis.