Immune responses in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV)

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.