This study was designed to assess the parameters influencing the magnitude and type of immune responses generated to plasmids encoding the hemagglutinin/neuraminidase (HN) and fusion (F) proteins of bovine parainfluenzavirus type 3 (BPIV3). Mice immunized with plasmids expressing HN or F under control of the Rous sarcoma virus long terminal repeat promoter were primed, but they did not develop measurable immune responses. In contrast, strong humoral and cellular immune responses were induced with constructs containing the human cytomegalovirus immediate-early promoter and intron A. After immunization with both HN- and F-encoding plasmids, enhanced responses were observed. Analysis of in vitro protein synthesis confirmed that the presence of the intron is crucial for the expression of the BPIV3 HN gene. Plasmid encoding HN induced significantly higher serum antibody titers by intradermal injection than by intramuscular delivery, whereas antigen- specific T cell proliferation was stronger in intramuscularly injected mice. Both the isotype ratios and the cytokine profiles indicated a Th1-type response after intramuscular immunization and a mixed to Th2-type response in intradermally immunized mice. A plasmid encoding a truncated, secreted form of HN induced a Th2-type immune response, regardless of the route of delivery. In cotton rats, HN- and F-encoding plasmids conferred protection from BPIV3 challenge.
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