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Anaplasma marginale subsp. centrale is a naturally attenuated subtype that has been used as a vaccine for a century. We sequenced the genome of this organism and compared it to those of virulent senso stricto A. marginale strains. The comparison markedly narrows the number of outer membrane protein candidates for development of a safer inactivated vaccine and provides insight into the diversity among strains of senso lato A. marginale.
Sir Arnold Theiler described Anaplasma marginale as the “cause of a specific tick-borne disease of cattle” in 1908 (14), providing the first identification of a rickettsial pathogen. Two years later, Theiler isolated a less virulent organism, which he designated A. marginale subtype centrale (15). This naturally attenuated strain has been used as a live vaccine to prevent severe disease due to A. marginale senso stricto strains for 100 years. Understanding the genetic similarities and differences between the vaccine strain and wild-type A. marginale strains will provide clues as to how the vaccine provides protection. To that end, we have sequenced the A. marginale subsp. centrale vaccine strain using a whole-genome shotgun sequencing strategy.
Genomic DNA, obtained from Kimron Veterinary institute, was fragmented by hydroshearing and ligated into pSmartLCKan (Lucigen). A total of 10,752 paired-end sequence reads (~6.5× coverage) were generated. Assembly with Phrap (www.phrap.org) resulted in 148 contigs. Closure was achieved by applying the genome walking method across gap-spanning subclones and genomic DNA amplicons. For polymorphic loci, the most frequently observed subclone sequence was selected.
Coding sequences (CDSs) in the single, circular, 1,206,806-bp chromosome were predicted using Glimmer2 and Glimmer3 (4, 5, 12). Annotation was as described previously for A. marginale senso stricto genomes (2, 3). There are 925 predicted CDSs, 19 pseudogenes, 37 tRNA genes, and a single set of rRNA genes in the genome. A. marginale subsp. centrale contains 10 putative genes not found in the closed-core genomes of senso stricto strains (3). Similarly, 18 genes found in senso stricto strains are absent from A. marginale subsp. centrale. This divergence is consistent with the subspecies nomenclature (15), but the findings do not resolve whether these genetic differences warrant classification of the vaccine strain as a distinct species within the genus Anaplasma (6).
The ability of live A. marginale subsp. centrale to protect against a diversity of A. marginale strains indicates that epitopes critical for protective immunity are broadly conserved (11). As immunity against A. marginale can be induced by immunization with purified outer membrane protein (OMP) complexes (8-10, 13), identification of OMPs conserved between A. marginale subsp. centrale and senso stricto A. marginale may narrow the vaccine candidate list. A. marginale OMPs cluster predominately into two protein superfamilies, major surface protein 1 (Msp1) and Pfam01617/Msp2 (2). Members of the Msp1 superfamily from senso stricto strains (1, 2) are not well conserved (e.g., Msp1a, Msp1b-1, Msp1b-2, and Mlp2 to Mlp4; 13 to 48% amino acid identity) or are nonexistent (e.g., the products of Msp1b partial genes 1 to 3) in A. marginale subsp. centrale, suggesting that immunity induced by the live vaccine strain is unlikely to be associated with the Msp1 superfamily.
Comparative analysis of the Pfam01617/Msp2 superfamily (2, 8) reveals both conservation and diversity. OpAG1 to OpAG3 and Msp4 are generally well conserved, while the family comprising Omp1 to Omp15 found in senso stricto strains (2, 3, 8) is reduced in A. marginale subsp. centrale: genes for the closely related proteins Omp7 to Omp9 are collapsed into a single CDS, and genes for homologs of Omp2, Omp3, Omp6, and Omp15 are missing. The OMP complex capable of inducing protective immunity contains 11 proteins (7, 8). By excluding those without homologs in the vaccine strain and the highly variable Msp2 and Msp3, the number of candidates is narrowed to six: four Msp2 superfamily members (Msp4, Omp1, Omp7, and OpAG2) and two non-superfamily members (AM779/ACIS557 and AM854/ACIS486). The degree of identity among these candidates from the vaccine strain and senso stricto A. marginale strains ranges from 63% (for OpAG2 proteins) to 88% (for Msp4 homologs). While the next steps in vaccine development will require strain analysis for epitope conservation in these candidates and immunization trials to test in vivo efficacy, progress will be accelerated using the minimal candidate list defined by the comparative genomics approach.
The genome sequence and annotation were deposited in GenBank with accession number CP001759.
This research was supported by BARD grant no. US-3315-02C, NIH grant no. R01 AI44005, USDA ARS-CRIS grant no. 5348-32000-027-03S, and the Wellcome Trust grant no. GR075800M.
Published ahead of print on 23 October 2009.