[Construction of recombinant canine adenovirus type 2 expressing Canine coronavirus spike glycoprotein and its immunogenicity]

In order to construct a recombinant Canine adenovirus type 2 (CAV-2) expressing the spike glycoprotein of Canine coronavirus (CCV), the S1 gene fragment of CCV strain DXMV, encoding major antigenic region A, B, C and D of S protein, was amplified by RT-PCR and cloned into pVAX1 vector. The complete S1 expression cassette was subcloned into the shuttle vector pVAXE3, then further cloned into the backbone vector pPoly2-CAV2 containing complete genome of CAV-2. To gain the recombinant Canine adenovirus, the recombinant plasmid pCAV-2-CCV-S1 was linearized by Cla I/Asc I to release recombinant genome, and then transfected into MDCK cell. The recombinant virus CAV-2-S1 was gained through 4 passages in MDCK, which showed classical CPE of CAV-2. The expressed S1 protein of CCV, which was identified by RT-PCR and Western blot, can be specifically recognized by polyclonal antibody against CCV. The immunization in dogs indicated that the recombinant CAV-2 could effectively induce the specific antibodies against CCV and CAV.

Detection and differentiation of CAV-1 and CAV-2 by polymerase chain reaction

Canine adenovirus type 1 (CAV-1) and type 2 (CAV-2) can be categorized in the laboratory by haemagglutination and neutralization tests, but they are difficult to differentiate from each other in specimens, especially when infection occurs in the digestive tract. The object of this study was to develop a simple method of detecting and differentiating them. One pair of common primers was designed and synthesized according to the sequences of the E3 and flanking regions and a polymerase chain reaction (PCR) assay was established using these two primers to amplify the virus-specific DNA fragment from clinical specimens as well as from cell cultures. After elecctrophoresis, under the same amplification conditions, 508 bp and 1030 bp PCR products were observed for CAV-1 and CAV-2, respectively. These were further shown to be adenovirus specific by dot hybridization and sequencing. As only one pair of primers was involved in the PCR procedure, it was faster and easier to perform than any of the other assays used for detecting canine adenovirus, making it applicable in the rapid confirmation of diagnosis and differentiation of the two types of canine adenoviruses.

Sequence analysis of the canine adenovirus 2 fiber-encoding gene

The gene coding for the fiber protein (Fip) of canine adenovirus type 2 (CAV-2) has been cloned and sequenced. The putative protein has 80% sequence similarity with the CAV-1 Fip. The observed differences may contribute to the known differences in cell tropism and virulence of the two types of CAV.

Comparison of the immunological properties of two canine adenoviruses

By means of immunological methods, including neutralization (NT), neutralization-enhancement (NE), hemagglutination-inhibition (HI), as well as immunodiffusion experiments, the two known canine adenoviruses, infectious canine hepatitis (ICH) virus and infectious canine laryngotracheitis (ICL) virus, have been shown to be two distinct serotypes. The relationship between the two viruses was shown to be predominantly one-sided as ICH hexon antisera could neutralize ICL virus infectivity to a much higher extent than the reciprocal cross-reaction. The one-way cross-reaction was also found to involve the fiber components as demonstrated in HI tests with fiber-specific antisera and virions and soluble components as antigens. In contrast, NE tests with fiber-specific antisera were found to be type-specific. A HI test using virion antisera and soluble complete hemagglutinin as antigen was found to be a rapid and specific method for distinguishing between the two virus types. The specificity of this HI test was interpreted to be based on the presence of type-specific antibodies against the vertex capsomere (penton base). Immuno-diffusion precipitin analyses substantiated the data obtained in the preceding experiments, as spur formation in two directions was found when fiber antisera was tested against purified fiber. Precipitin patterns obtained with purified hexon and hexon antisera indicated a unique determinant associated with the ICL hexon, presumably the type specificity detected in the hexon antisera NT tests. On the basis of data obtained in this and previous studies on the relationship of the two canine adenoviruses, it is proposed that ICL virus be officially recognized as a distinct canine adenovirus serotype.