Lipoproteome screening of the Lyme disease agent identifies inhibitors of antibody-mediated complement killing

Abstract

Significance Spirochetal pathogens encode an abundance of lipoproteins that can provide a critical interface with the host environment. Borrelia burgdorferi , the model species for spirochetal biology, must survive an enzootic life cycle defined by fluctuations between vector (tick) and vertebrate host. While B. burgdorferi expresses over 80 surface lipoproteins—many of which likely contribute to host survival—the B. burgdorferi lipoproteome is poorly characterized. Here, we generated a platform to rapidly identify targets of B. burgdorferi surface lipoproteins and identified two paralogs that confer resistance to antibody-initiated complement killing that may promote survival in immunocompetent hosts. This work expands our understanding of complement evasion mechanisms and points toward a discovery approach for identifying host–pathogen interactions central to spirochete pathogenesis.

Outer surface lipoproteins from the Lyme disease spirochete exploit the molecular switch mechanism of the complement protease C1s

“…Of particular note is that each Elp protein has high affinity for purified C1r (31). Interestingly, however, and different from BBK32, binding of C1r by Elp proteins did not prevent cleavage of its natural substrate (i.e., zymogen C1s), nor of small synthetic C1r peptidic substrates (31). This finding is consistent with the inhibitory mechanism proposed here, as C4 exosites are clearly absent in C1r.…”

“…Unlike BBK32, binding of activated C1r did not directly block its cleavage of zymogen C1s. Instead, we found that ElpB and ElpQ block the CP by binding directly to C1s and inhibiting cleavage of its two natural substrates, C2 and C4 (31). Along with B. burgdorferi BBK32, which binds and inhibits both zymogen and active forms of C1r (28), ElpB and ElpQ may protect Lyme disease spirochetes from antibody-mediated complement killing during host infection.…”

“…We have previously shown that residues 19-378 of ElpB and residues 19-343 of ElpQ, which constitute the mature proteins lacking the secretion and lipidation sequences, prevent the deposition of C4b in a serum-based ELISA assay that utilizes IgM as a specific activator of the classical pathway (31). To determine if this inhibitory activity could be isolated to a specific region of the Elp proteins, we designed, produced, and purified a set of twelve truncation mutants of ElpB and ElpQ.…”

“…Using a surface lipoproteome library (32), we recently identified a new player in the B. burgdorferi complement evasion system in the form of two paralogous outer surface lipoproteins of the OspE/F-related protein family, termed ElpB and ElpQ (31) (also termed ErpB and ErpQ, respectively (33–35)). Genes encoding Elps are located on 32-kilobase circular plasmids (i.e.…”

“…cp32 genetic elements) and are expressed during the mammalian phase of infection, suggesting a potential role in host interaction, which is further supported by the identification of laminin as a binding partner for ErpX/ElpX (35–39). We showed that ElpB and ElpQ bound with high affinity to the CP proteases C1r and C1s and were selective for the activated forms of each protease (31). Unlike BBK32, binding of activated C1r did not directly block its cleavage of zymogen C1s.…”