Interaction of Borrelia burgdorferi with peripheral blood fibrocytes, antigen-presenting cells with the potential for connective tissue targeting

Background: Borrelia Burgdorferi has a predilection for collagenous tissue and can interact with fibronectin and cellular collagens. While the molecul ar mechanisms of how B. burgdorferi targets connective tissues and causes a rthritis are not understood, the spirochetes can bind to a number of differ ent cell types, including fibroblasts. A novel circulating fibroblast-like cell called the peripheral blood fibrocyte has recently been described. Fib rocytes express collagen types I and III as well as fibronectin. Besides pl aying a role in wound healing, fibrocytes have the potential to target to c onnective tissue and the functional capacity to recruit, activate, and pres ent antigen to CD4(+) cells. Materials and Methods: Rhesus monkey fibrocytes were isolated and character ized by now cytometry. B. burgdorferi were incubated with human or monkey f ibrocyte cultures in vitro and the cellular interactions analyzed by light and electron microscopy. The two strains of B. burgdorferi studied included JD1, which is highly pathogenic for monkeys, and M297, which lacks the cel l surface OspA and OspB proteins. Results: In this study, we demonstrate that B. burgdorferi binds to both hu man and monkey (rhesus) fibrocytes in vitro. This process does not require OspA or OspB. In addition, the spirochetes are not phagocytosed but are tak en into deep recesses of the cell membrane, a process that may protect them from the immune system. Conclusions: This interaction between B. burgdorferi and peripheral blood f ibrocytes provides a potential explanation for the targeting of spirochetes to joint connective tissue and may contribute to the inflammatory process in Lyme arthritis.