Evolution of the vls Antigenic Variability Locus of the Lyme Disease Pathogen and Development of Recombinant Monoclonal Antibodies Targeting Conserved VlsE Epitopes

Meta-analysis of the Vmp-like sequences of Lyme disease Borrelia: evidence for the evolution of an elaborate antigenic variation system

VMP-like sequence (vls) antigenic variation systems are present in every Lyme disease Borrelia strain with complete genome sequences. The linear plasmid-encoded vls system consists of a single expression site (vlsE) and contiguous array(s) of silent cassettes that have ~90% identity with the central cassette region of the cognate vlsE gene; antigenic variation occurs through random, segmental, and unidirectional recombination of vls silent cassette sequences into the vlsE expression site. Automated annotation programs do not accurately recognize vls silent cassette sequences, so these regions are not correctly annotated in most genomic sequences. In this study, the vls sequences were re-analyzed in the genomic sequences of 31 available Lyme disease Borrelia and one relapsing fever Borrelia organisms, and this information was utilized to systematically compare the vls systems in different species and strains. In general, the results confirm the conservation of the overall architecture of the vls system, such as the head-to-head arrangement of vlsE and a contiguous series of vlsS silent cassette sequences and presence of inverted repeat sequences between the two regions. However, the data also provide evidence for the divergence of the vls silent cassette arrays through point mutations, short indels, duplication events, and rearrangements. The probable occurrence of convergent evolution toward a vls system-like locus is exemplified by Borrelia turcica, a variable large protein (Vlp) expressing organism that is a member of the relapsing fever Borrelia group.

Outsmarting Pathogens with Antibody Engineering

There is growing interest in identifying antibodies that protect against infectious diseases, especially for high-risk individuals and pathogens for which no vaccine is yet available. However, pathogens that manifest as opportunistic or latent infections express complex arrays of virulence-associated proteins and are adept at avoiding immune responses. Some pathogens have developed strategies to selectively destroy antibodies, whereas others create decoy epitopes that trick the host immune system into generating antibodies that are at best nonprotective and at worst enhance pathogenesis. Antibody engineering strategies can thwart these efforts by accessing conserved neutralizing epitopes, generating Fc domains that resist capture or degradation and even accessing pathogens hidden inside cells. Design of pathogen-resistant antibodies can enhance protection and guide development of vaccine immunogens against these complex pathogens. Here, we discuss general strategies for design of antibodies resistant to specific pathogen defense mechanisms.

Novelty Search Promotes Antigenic Diversity in Microbial Pathogens

Driven by host–pathogen coevolution, cell surface antigens are often the fastest evolving parts of a microbial pathogen. The persistent evolutionary impetus for novel antigen variants suggests the utility of novelty-seeking algorithms in predicting antigen diversification in microbial pathogens. In contrast to traditional genetic algorithms maximizing variant fitness, novelty-seeking algorithms optimize variant novelty. Here, we designed and implemented three evolutionary algorithms (fitness-seeking, novelty-seeking, and hybrid) and evaluated their performances in 10 simulated and 2 empirically derived antigen fitness landscapes. The hybrid walks combining fitness- and novelty-seeking strategies overcame the limitations of each algorithm alone, and consistently reached global fitness peaks. Thus, hybrid walks provide a model for microbial pathogens escaping host immunity without compromising variant fitness. Biological processes facilitating novelty-seeking evolution in natural pathogen populations include hypermutability, recombination, wide dispersal, and immune-compromised hosts. The high efficiency of the hybrid algorithm improves the evolutionary predictability of novel antigen variants. We propose the design of escape-proof vaccines based on high-fitness variants covering a majority of the basins of attraction on the fitness landscape representing all potential variants of a microbial antigen.