A Novel Candidate Vaccine for Cytauxzoonosis Inferred from Comparative Apicomplexan Genomics

Abstract

Cytauxzoonosis is an emerging infectious disease of domestic cats (Felis catus) caused by the apicomplexan protozoan parasite Cytauxzoon felis. The growing epidemic, with its high morbidity and mortality points to the need for a protective vaccine against cytauxzoonosis. Unfortunately, the causative agent has yet to be cultured continuously in vitro, rendering traditional vaccine development approaches beyond reach. Here we report the use of comparative genomics to computationally and experimentally interpret the C. felis genome to identify a novel candidate vaccine antigen for cytauxzoonosis. As a starting point we sequenced, assembled, and annotated the C. felis genome and the proteins it encodes. Whole genome alignment revealed considerable conserved synteny with other apicomplexans. In particular, alignments with the bovine parasite Theileria parva revealed that a C. felis gene, cf76, is syntenic to p67 (the leading vaccine candidate for bovine theileriosis), despite a lack of significant sequence similarity. Recombinant subdomains of cf76 were challenged with survivor-cat antiserum and found to be highly seroreactive. Comparison of eleven geographically diverse samples from the south-central and southeastern USA demonstrated 91–100% amino acid sequence identity across cf76, including a high level of conservation in an immunogenic 226 amino acid (24 kDa) carboxyl terminal domain. Using in situ hybridization, transcription of cf76 was documented in the schizogenous stage of parasite replication, the life stage that is believed to be the most important for development of a protective immune response. Collectively, these data point to identification of the first potential vaccine candidate antigen for cytauxzoonosis. Further, our bioinformatic approach emphasizes the use of comparative genomics as an accelerated path to developing vaccines against experimentally intractable pathogens.

Mitochondrial Genome Sequences and Structures Aid in the Resolution of Piroplasmida phylogeny

“…parva had been considered the closest relatives of C . felis , and have been focused on as model species in comparative studies investigating novel therapeutic and vaccination strategies to combat cytauxzoonosis [97,98]. However, the genomic and phenotypic observations made in this studies and others [95] suggest that non-transforming Theileria species and Cytauxzoon felis may be better model species for one another, and should be taken into consideration in future comparative studies.…”

Two Tales of Cytauxzoon felis Infections in Domestic Cats

“…They often carry intraerythrocytic merozoites, commonly known as piroplasms, after recovering from infection. Upon ingestion by a suitable vector, i.e., Amblyomma americanum, the lone star tick, or Dermacentor variabilis, the American dog tick, gametocytes are released into the tick's gut after digestion of erythrocytes and develop into ray bodies, also known as gametes (52). A male gamete fertilizes a female one, resulting in the formation of a diploid zygote.…”

“…A male gamete fertilizes a female one, resulting in the formation of a diploid zygote. Zygotes mature into motile haploid kinetes that invade the salivary gland, in which sporozoites form as a result of sporogony (52). These sporozoites are infective to a feline host, such as a bobcat or a domestic cat.…”

“…Therefore, these hard tick vectors serve as the definitive (final) hosts ( Fig. 3) (52). Only nymphs and adults are infective to felids, since there is no evidence of transovarial transmission.…”

“…They undergo asexual reproduction by schizogony, resulting in formation of mature schizonts. The latter release numerous merozoites upon rupture of the infected macrophages (52). Subsequently, merozoites infect eryth- European strain from domestic and wild animals.…”