Objective: To develop a novel 3-dimensional (3-D) in vitro model of Lyme arthritis to use in the study of the interactions between Borrelia burgdorferi (Bb) and human synovial host cells with respect to phagocytosis and potential persistence of Bb as well as the induction of proinflammatory cytokines and chemokines.
Methods: Two distinct culture systems, consisting of synovial membrane explants or interactive synovial cells embedded in 3-D fibrin matrices, were chosen. Both systems were artificially infected with Bb, and the interactions between Bb and synovial tissue/cells were studied by histology, immunohistochemistry, and electron microscopy. Functional analyses included the induction/secretion of cytokines by Bb in the model system.
Results: Both culture systems proved to be stable and reproducible. The host cells and spirochetes showed high levels of viability and maintained their physiologic shape for >3 weeks. Bb invaded the synovial tissue and the artifical matrix in a time-dependent manner. Host cells were activated by Bb, as indicated by the induction of interleukin-1beta and tumor necrosis factor alpha. Electron microscopic analysis revealed Bb intracellularly within macrophages as well as synovial fibroblasts, suggesting that not only professional phagocytes, but also resident synovial cells are capable of phagocytosing Bb. Most interestingly, the uptake of the spirochetes appeared to cause severe damage of the synovial fibroblasts, since the majority of these 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 disintegration of synovial fibroblasts containing Bb or Bb fragments challenges the concept of an intracellular persistence of Bb and may instead reflect a mechanism that contributes to the inflammatory processes characteristic of Lyme arthritis.