1. Field of the Invention
This invention relates generally to compositions comprising Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) antigens with improved immunogenicity and methods for their use. The compositions and methods described herein result in improved immunogenic responses of pigs to PRRSV antigens, thus providing for improved protection of pigs to PRRSV infection.
2. Related Art
PRRSV is an economically important pathogen that affects pigs. Infection of sows and gilts with PRRSV can result in reproductive failure. PRRSV also causes respiratory disease in pigs of all ages. It is possible to vaccinate the pigs to protect them from infection with PRRSV. However, the current commercially available vaccines (most of which are live attenuated vaccines) are somewhat ineffective and therefore should be improved. The complete immunologic mechanisms of protection against PRRSV are not clear; however, it has been clearly shown that PRRSV-neutralizing antibodies are central to this protection. Unfortunately, the PRRSV itself (in its wild-type form) or the current live vaccines derived from it have poor ability to induce virus-specific neutralizing antibodies in a timely manner and at effective (i.e., protective) levels.
U.S. Pat. No. 6,500,662, “Infectious cDNA clone of North American porcine reproductive and respiratory syndrome (PRRS) virus and uses thereof” (by Calvert et al., Dec. 31, 2002) describes the development of an infectious North American PRRSV cDNA clone and its use as a vaccine. However, U.S. Pat. No. 6,500,662 does not disclose PRRSV vaccines that comprise hypoglycosylated PRRSV antigens.
In another study, sequences of the GP5 protein (or ORF5 protein) from various North American PRRSV strains were compared to one another and to the representative European PRRSV isolate known as the Lelystad strain, revealing that the N-linked glycosylation sites at Asparagine 44 (N44) and Asparagine 51 (N51) of the GP5 consensus sequence were conserved in all of the PRRSV isolates examined (Pirzadeh et al., Can. J. Vet Res., 1998, 62:170-177). However, the N-glycosylation site located at Asparagine 31 (N31) of the GP5 consensus sequence was absent in certain North American PRRSV isolates and absent in the European PRRSV Lelystad strain isolate. Recombinant GST-GP5 fusion proteins from four (4) North American PRRSV strains and the Leylstad strain were produced in E. coli as insoluble inclusion bodies, renatured, and used as immunogens in rabbits. Such recombinant proteins produced in E. coli retain but do not glycosylate their native N-glycosylation sites.
Inoculation of pigs with a DNA vaccine comprising a CMV promoter fusion to the GP5 gene of the IAF-Klop North American PRRSV isolate has also been shown to provide protection in immunized animals against PRRSV challenge (Pirzadeh and Dea, 1998, J. General Virology, 79, 989-999). This particular GP5 gene isolate encodes a GP5 protein containing the N31, N44 and N51 Asparagine residues that are presumably glycosylated when expressed in pigs immunized with the DNA vaccine. Vaccination of pigs with E. coli produced GST-GP5, which retain but do not glycosylate the native N-glycosylation sites of the GP5 gene of the IAF-Klop strain, did not protect the lungs of virus-challenged pigs.
European PRRSV infectious clones containing mutations that result in expression of hypoglycosylated PRRSV proteins have also been described (Wissink et al., 2004, J. Gen. Virol. 85:3715-23). This particular reference reports that PRRSV containing mutations in the Asparagine Residue 53 (N53) of the PRRSV Lelystad strain GP(5) protein that prevent N-linked glycosylation of that site are infectious and can produce infectious PRRSV Lelystad strain virus particles. In contrast, PRRSV containing mutations in the N46 of the PRRSV Lelystad strain GP(5) protein that prevent N-linked glycosylation of that site are not infectious and do not produce infectious PRRSV Lelystad strain virus particles. Wissink et al. speculate that N-glycan sites in the European PRRSV GP2a protein, and, by analogy, the N53 site of the GP5 protein, could act at many different levels in the natural host, including receptor interactions or immune shielding.
In viruses other than PRRSV, glycan residues have been implicated in a variety of roles. The N-linked glycosylation, in general, is important for correct folding, targeting, and biological activity of proteins (Helenius, A. and M. Aebi., Annu. Rev. Biochem. 73:1019-1049, 2004; Williams, D. B. and Glycoconj J., 12:iii-iv, 1995; Zhang, et al., Glycobiology 14:1229-46, 2004). In many enveloped viruses, the envelope proteins are modified by addition of sugar moieties and the N-linked glycosylation of envelope protein plays diverse functions of viral glycoproteins such as receptor binding, membrane fusion, penetration into cells, and virus budding (Braakman, I. and E. van Anken, Traffic 1:533-9, 2000; Doms et al. Virology 193:545-62, 1993). Recent studies have demonstrated the role of N-linked glycosylation of Hantaan virus glycoprotein in protein folding and intracellular trafficking (Shi, X. and R. M. Elliott, J. Virol. 78:5414-22, 2004) as well as in biological activity and antigenicity of influenza virus hemagglutinin (HA) protein (Abe, Y., et al., J. Virol. 78:9605-11, 2004). Furthermore, it has become evident that glycosylation of viral envelope proteins is a major mechanism for viral immune evasion and persistence used by several different enveloped viruses to escape, block or minimize the virus-neutralizing antibody response. Examples of this effect have been reported for SIV (Reitter, J. N. et al., Nat. Med. 4:679-84, 1998) and HIV-1 (Wei, X. et al., Nature 422:307-12, 2003), HBV (Lee, J. et al. Biochem. Biophys. Res. Commun. 303:427-32, 2003), influenza (Skehel, J. J. et al., Proc. Natl. Acad. Sci. USA 81:1779-83, 1984) and the arterivirus LDV (Chen, Z. et al. Virology 266:88-98, 2000).