Structural Basis for Recombinatorial Permissiveness in the Generation of Anaplasma marginale Msp2 Antigenic Variants

Abstract

Sequential expression of outer membrane protein antigenic variants is an evolutionarily convergent mechanism used by bacterial pathogens to escape host immune clearance and establish persistent infection. Variants must be sufficiently structurally distinct to escape existing immune effectors yet retain the core structural elements required for localization and function within the outer membrane. We examined this balance using Anaplasma marginale, which generates antigenic variants in the outer membrane protein Msp2 using gene conversion. The overwhelming majority of Msp2 variants expressed during long-term persistent infection are mosaics, derived by recombination of oligonucleotide segments from multiple alleles to form unique hypervariable regions (HVR). As a result, the mosaics are not under long-term selective pressure to encode a functional protein; consequently, we hypothesized that the Msp2 HVR is structurally permissive for mosaic expression. Using an integrated approach of predictive modeling with determination of the native Msp2 protein structure and function, we demonstrate that structured elements, most notably, ␤-sheets, are significantly concentrated in the highly conserved N-and C-terminal domains. In contrast, the HVR is overwhelmingly a random coil, with the structured ␣-helices and ␤-sheets being confined to the genomically defined structural tethers that separate the antigenically variable microdomains. This structure is supported by the surface exposure of the HVR microdomains and the slow diffusion-type porin function in native Msp2. Importantly, the predominance of the random coil provides plasticity for the formation of functional HVR mosaics and realization of the full potential of segmental gene conversion to dramatically expand the variant repertoire. Highly antigenically variable microbes use a diversity of mechanisms to generate molecules that allow escape from host immune effectors and result in long-term persistence. Bacterial pathogens across multiple genera, including Anaplasma, Borrelia, Neisseria, and Treponema, generate outer membrane protein variants by gene conversion, resulting in novel expressed surface antigens (1-3). The potential repertoire of antigenic variants is determined by the number of donor alleles in the pathogen genome combined with the occurrence of segmental recombinatorial events that can generate unique expression site mosaics (1-3). Notably, segmental gene conversion results in expressed variants not represented by any one allele but generated in situ by recombination of donated oligonucleotides derived from different alleles. This generation of unique mosaics can tremendously expand the potential antigenic repertoire. The number of alleles encoding unique Msp2 variants in Anaplasma marginale is Ͻ10, and segmental recombination using oligonucleotide segments derived from these alleles allows the generation of Ͼ1,000 potential variants (4-6). Similarly, segmental gene conversion in Treponema pallidum exponentially increases the potential number of TpK varia...

Novel variants of the newly emerged Anaplasma capra from Korean water deer (Hydropotes inermis argyropus) in South Korea

“…Consistent with our results, sequence variation at the studied gene markers has been reported previously among isolates from ticks, sheep and goats [12,[15][16][17][18][20][21][22][23][24][25]. Similarly, genetic variation is common in other Anaplasma species [40][41][42][43][44][45]. Although genetic diversity is reportedly related to infectivity, virulence, pathogenicity, niche preference, immune evasion, and/or host adaptability [46][47][48][49], this has not been established in A. capra and further studies are needed to evaluate these relationships.…”

Segmental Variation in a Duplicated msp2 Pseudogene Generates Anaplasma marginale Antigenic Variants

“…The complete loss of antibody binding to microdomain 1 in the locus K variant across all biological replicates is consistent with the importance of this HVR domain in immune evasion (15). Underlying this mechanism is the random coil structure of the HVR, allowing sufficient plasticity in the surface-exposed microdomains while maintaining overall Msp2 structural integrity as a membrane protein (16). Msp2 is encoded by an orthologous gene family present in multiple species of the genus Anaplasma.…”

Immunization with Anaplasma centrale Msp2 HVRs Is Less Effective than the Live A. centrale Vaccine against Anaplasmosis

“…Interestingly, the most common variant detected was V1, which had the lowest similarity of any of the variants that were expressed during challenge when comparing across the HVR when compared with the A. centrale HVRs (Supplemental Table S1). This variant has been detected in previous A. marginale St. Maries strain infections and has been examined to show that it retains similar porin functionality as Msp2 molecules that are derived from full-length pseudogene HVRs [25]. The pseudogenes that are retained in the genome are subject to selection, whereas the transient recombined expression site mosaics that arise during the course of infection are not; therefore, it is notable that variant V1 is known to retain functionality.…”