A modified live PRRSV vaccine and the pathogenic parent strain induce regulatory T cells in pigs naturally infected with Mycoplasma hyopneumoniae

The lack of heterologous protection by porcine reproductive and respiratory syndrome virus (PRRSV) vaccines is currently a major problem in the field. Heterologous protection by PRRS vaccines depends on the ability of the vaccine to induce an interferon gamma (IFN-γ) response. One mechanism by which the virus evades the immune system is by activating regulatory T cells (Tregs), resulting in induction of interleukin 10 rather than IFN-γ. Our hypothesis that current PRRS vaccines do not differ from pathogenic strains in the ability to induce Tregs was tested by inoculating three groups of pigs with two pathogenic viruses and an attenuated vaccine strain and evaluating the number of Tregs in peripheral blood mononuclear cells. Before inoculation, the pigs, although vaccinated became infected naturally with Mycoplasma hyopneumoniae before shipment to our research facility. Our results show that the PRRSV vaccine strain and parent strain are equally able to induce Tregs in pigs naturally infected with M. hyopneumoniae. Pigs in the vaccine and PRRSV groups had higher lung lesion scores than pigs in the control groups. The results suggest that the exacerbation M. hyopneumoniae respiratory disease may be due to the ability of PRRSV vaccination and viral infection to induce regulatory T cells.