Canine Distemper Virus DNA Vaccination Induces Humoral and Cellular Immunity and Protects against a Lethal Intracerebral Challenge

We have studied the immune responses to the two glycoproteins of the Morbillivirus canine distemper virus (CDV) after DNA vaccination of BALB/c mice. The plasmids coding for both CDV hemagglutinin (H) and fusion protein (F) induce high levels of antibodies which persist for more than 6 months. Intramuscular inoculation of the CDV DNA induces a predominantly immunoglobulin G2a (IgG2a) response (Th1 response), whereas gene gun immunization with CDV H evokes exclusively an IgG1 response (Th2 response). In contrast, the CDV F gene elicited a mixed, IgG1 and IgG2a response. Mice vaccinated (by gene gun) with either the CDV H or F DNA showed a class I-restricted cytotoxic lymphocyte response. Immunized mice challenged intracerebrally with a lethal dose of a neurovirulent strain of CDV were protected. However, approximately 30% of the mice vaccinated with the CDV F DNA became obese in the first 2 months following the challenge. This was not correlated with the serum antibody levels.Inoculation of plasmid DNA into muscle and the subsequent expression of the encoded protein have opened up new approaches in vaccination and gene therapy (for an extensive review see reference 25). Although initial studies used intramuscular (i.m.) inoculation to deliver the DNA, other routes have been shown to be equally or more efficient in inducing immune responses, which may be related to the types of antigen-presenting cells (APCs) which are involved (9). Recent observations suggest that after i.m. inoculation, muscle cells probably act as a reservoir for the foreign antigen, while the bone marrow cells seem to act as the APCs (8, 12, 26, 27). For DNA delivery to the skin, the APCs have not yet been identified but could well include cells of dendritic origin (21). Although both intradermal and i.m. DNA inoculations induce a strong Th1 response, inoculation of DNA precipitated onto gold beads and delivered by means of a gene gun favors a Th2 response (7). Whether this is due to the targeting of different APCs has not been determined.We have been studying the possibility of using DNA vaccination to protect against canine distemper virus (CDV). CDV is a member of the genus Morbillivirus, in the Paramyxoviridae family, and although this virus primarily infects dogs, the disease has also been described in several animal species both in nature and in captivity (10, 18, 22). The currently available live attenuated vaccine efficiently protects dogs once maternal antibodies have disappeared, but it is not sufficiently attenuated for certain other animal species in which a fatal infection may ensue (6). This has led to a problem in protecting members of rare animal species living in captivity.In the present study, we have expressed the two CDV glycoproteins, the attachment protein (hemagglutinin [H]) and the fusion protein (F), from plasmids driven by a cytomegalovirus (CMV) promoter. We show that i.m. and intradermal inoculation of the CDV H-encoding plasmid induces a Th1 response, whereas gene gun inoculation of the same plasmid induces a Th2-type response. In contrast, the CDV F gene administered with the gene gun elicited a mixed Th response. Mice immunized with either of the plasmids were protected against a lethal intracerebral (i.c.) infection.