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In three successive experiments, the immune functions of pigs persistently infected with the porcine reproductive and respiratory syndrome virus (PRRSV) have been evaluated. Non-specific immune responses were analyzed over a period of 12 weeks post-infection (PI). In addition, the capacity of PRRSV-infected pigs to develop an efficient immune response against pseudorabies virus (PRV) glycoproteins and to resist to a subsequent virulent challenge was investigated. Our results demonstrate that PRRSV produced minor effects on the immune system of pigs. The skin delayed type hypersensitivity (DTH) in response to phytohemagglutinine injection was slightly diminished one week after challenge, but was restored thereafter. However, three weeks after the infection, the total white blood cell count, and the number of CD2+, CD8+ and IgM+ cells were enhanced. The increase in numbers of CD8+ cells persisted for three consecutive weeks. Serum immunoglobulins in infected pigs also increased by week 3 PI and up to 8 weeks PI. These results show that PRRSV may have stimulating effects on the pig immune system during the phase of long-lasting infection. After immunization with PRV glycoproteins, the production of anti-PRV antibodies and skin DTH response against PRV glycoproteins were not affected. On the contrary, following a virulent PRV challenge, PRRSV-infected pigs developed a better secondary antibody response and their resistance to the infection was as effective as in control pigs. Taken together, our data do not support a systemic immunosuppressive effect of PRRSV, during the persistent phase of infection. Other mechanisms may therefore apply to explain the emergence of secondary infections in endemically infected herds.

Virus-like particles (VLPs) produced by recombinant expression of the major viral structural proteins could be an attractive method for severe acute respiratory syndrome (SARS) control. In this study, using the baculovirus system, we generated recombinant viruses that expressed S, E, M and N structural proteins of SARS-CoV either individually or simultaneously. The expression level, size and authenticity of each recombinant SARS-CoV protein were determined. In addition, immunofluorescence and FACS analysis confirmed the cell surface expression of the S protein. Co-infections of insect cells with two recombinant viruses demonstrated that M and E could assemble readily to form smooth surfaced VLPs. On the other hand, simultaneous high level expression of S, E and M by a single recombinant virus allowed the very efficient assembly and release of VLPs. These data demonstrate that the VLPs are morphological mimics of virion particles. The high level expression of VLPs with correct S protein conformation by a single recombinant baculovirus offers a potential candidate vaccine for SARS.