Borrelia burgdorferi BmpA Is a Laminin-Binding Protein

The Borrelia burgdorferi BmpA outer surface protein plays a significant role in mammalian infection by the Lyme disease spirochete and is an important antigen for the serodiagnosis of human infection. B. burgdorferi adheres to host extracellular matrix components, including laminin. The results of our studies indicate that BmpA and its three paralogous proteins, BmpB, BmpC, and BmpD, all bind to mammalian laminin. BmpA did not bind mammalian type I or type IV collagens or fibronectin. BmpA-directed antibodies significantly inhibited the adherence of live B. burgdorferi to laminin. The laminin-binding domain of BmpA was mapped to the carboxy-terminal 80 amino acids. Solubilized collagen inhibited BmpA-laminin binding, suggesting interactions through the collagen-binding domains of laminin. These results, together with previous data, indicate that BmpA and its paralogs are targets for the development of preventative and curative therapies for Lyme disease.Early during the course of Lyme disease, humans frequently produce antibodies directed against a Borrelia burgdorferi antigen originally described as “P39” (66). Antibodies recognizing P39 are considered to be specific and diagnostic for Lyme disease spirochete infection (5, 18, 30, 62, 64). The antigenic protein was subsequently identified as BmpA (Borrelia membrane protein A) (65). The bmpA gene is located on the main borrelial chromosome, adjacent to three paralogous genes named bmpB, bmpC, and bmpD, which together form a complex operon (3, 4, 28, 32, 55, 56, 65). These other Bmp proteins are also often antigenic in infected humans (14). In addition to the serological data described above, examination of B. burgdorferi within skin and joint tissues confirmed the production of BmpA protein during mammalian infection (21, 49). BmpA is located in the borrelial outer membrane (46), where it is exposed to the external environment and can be a target of bactericidal antibodies (49, 63; F. Cabello, personal communication). BmpA and its paralogs have been implicated as playing roles in some symptoms of Lyme disease (49, 72). B. burgdorferi mutants in which bmpA or bmpB is specifically deleted are unable to persist in mouse joint tissues (49), indicating an important role for these proteins in the maintenance of mammalian infection. Despite the extensive research conducted on these important antigens, functions for the Bmp proteins had not been determined previously.B. burgdorferi is an extracellular organism, frequently found associated with its hosts'' connective tissues (6-9, 16, 17, 24, 26, 31, 36, 39, 48). In the laboratory, B. burgdorferi shows affinity for various host extracellular matrix (ECM) components, such as type I collagen, fibronectin, and decorin (16, 33, 34, 50, 74). We recently determined that B. burgdorferi also adheres to mammalian laminin, an important component of many mammalian ECMs (13). Ligand affinity blot analyses of a B. burgdorferi cell fraction enriched for outer membrane components revealed that the type strain, B31, can produce several distinct laminin-binding proteins, one of which we previously identified as being the surface-exposed outer membrane lipoprotein ErpX (11, 13, 69). We now present data indicating that BmpA and its paralogs are also laminin-binding proteins.