Mechanism of complement resistance of pathogenic Borrelia burgdorferi isolates

Borrelia burgdorferi, the causative agent of Lyme disease, differ in their susceptibility to normal human serum and are consequently classified as complement-resistant, complement-sensitive and intermediate complement-sensitive. Most isolates belonging to the genospecies B. afzelii are complement-resistant, while particularly B. garinii isolates were rapidly killed by complement. In general, isolates of the genospecies B. burgdorferi sensu stricto (s.s.) are intermediate complement-sensitive. Independent of the genospecies, all Borreliae were capable to activate the classical and/or the alternative pathway. Deposition of the activation products C3, C6, and TCC is much stronger by B. burgdorferi s.s. and B. garinii isolates than by B. afzelii isolates. The mechanism(s) on how Borreliae evade complement-mediated bacteriolysis has recently been described by showing that complement-resistant B. afzelii isolates but not the complement-sensitive B. garinii isolates absorb human complement regulators FHL-1/reconectin and factor H. Surface-attached FHL-1/reconectin maintains its complement regulatory activity and supports factor I-mediated C3b cleavage to iC3b. In complement-resistant Borreliae, two outer surface proteins, the 27.5 kDa (CRASP-1, complement regulator-acquiring surface protein 1) and the 20/21 kDa (CRASP-2), are responsible for the surface attachment of the two complement regulators. CRASP-1, which is present in complement-resistant Borreliae, binds preferentially FHL-1/reconectin while CRASP-2, which is restrictively expressed, binds preferentially factor H. Thus, complement-resistant Borreliae bind human complement regulators and control complement activation on their surface and prevent the formation of toxic activation products.