Lysine-Dependent Binding of OspE to the C-terminus of Factor H Mediates Complement Resistance in Borrelia burgdorferi

Serum resistance of Borrelia burgdorferi strains belonging to the B. afzelii and B. burgdorferi sensu stricto genospecies is dependent on binding of complement inhibitor factor H. We recently reported that factor H binding by B. burgdorferi is due to inducible expression of several approximately 20 kDa plasmid‐encoded, surface‐exposed lipoproteins related to OspE (e.g. ErpA, ErpP and P21). In addition, a second class of factor H‐binding proteins of approximately 27–35 kDa has been described. The OspE‐related lipoproteins are dramatically induced by B. burgdorferi during transmission from its tick vector into the mammalian host. The induction of OspE‐related lipoproteins during mammalian infection may play a key a role in the borrelial evasion of the host's immune system. The goal of the present study was to define the factor H‐binding regions of OspE‐related proteins using mutagenesis, peptide mapping and surface plasmon resonance analysis (Biacore). The combined studies revealed that the C‐terminal regions of both human and mouse factor H (SCRs 18–20) specifically bind to OspE‐related lipoproteins. We also found FHR‐1, whose C‐terminal SCRs 3–5 are homologous to SCRs 18–20 of factor H, to bind to OspE. Peptide mapping revealed five putative regions (designated I‐V) in OspE that could directly interact with factor H. Deleting the C‐terminal 15 amino acid residues from region V of P21 abolished its ability to bind factor H. At the same time, however, synthetic peptides corresponding to the C‐termini of OspE, P21 and ErpP did not inhibit factor H binding to OspE. Thus, the C‐terminal‐binding region V appears to be necessary but not sufficient for factor H binding. When a more specific mutation strategy was employed, where single amino acid residues in peptides spanning over the factor H‐binding regions were mutated to alanines, we observed that lysines in the factor H‐binding regions of OspE were required for factor H binding. The combined data have revealed that key lysine residues in OspE‐related lipoproteins and ionic interactions are crucial for factor H interactions. Furthermore, binding of OspE to the C‐termini of both mouse and human factor H suggests that Borrelia spirochetes utilize analogous complement resistance mechanisms in both rodents and man. In Borrelia garinii strains, which in in vitro analyses have been found to be sensitive to complement killing, differences in the OspE sequences as well as in the expression of factor H‐binding proteins may account for their susceptibility to serum lysis.