Porcine circovirus associated disease (PCVAD) is currently one of the most economically important diseases facing the global swine industry. Porcine circovirus type 2 (PCV2) is the primary and essential causative agent of PCVAD, but development of clinical disease typically requires co-infection with other swine pathogens such as porcine reproductive and respiratory syndrome virus (PRRSV). The specific mechanisms of co-infection that lead to clinical disease are not fully understood, but immune modulation by the co-infecting viruses is thought to play a critical role. The ability of dendritic cells (DC) infected with PRRSV, PCV2 or both to induce regulatory T cells (Tregs) was evaluated in vitro. DCs infected with PCV2 significantly increased CD4+CD25+FoxP3+ Tregs (p<0.05) and DCs co-infected with PRRSV and PCV2 induced significantly higher numbers of Tregs than with PCV2 alone (p<0.05). This Treg induction was found to be dependent on TGF-β and not IL-10. Further investigation of the in vivo swine immune response to acute co-infection with PCV2 and PRRSV failed to detect activation of Tregs in peripheral blood mononuclear cells (PBMCs) or bronchoalveolar lavage samples. The Treg response to in vitro and in vivo PRRSV challenge in pigs persistently infected with PCV2 or vaccinated against PCV2 was evaluated. There was no significant difference in Tregs in PBMCs among chronically PCV2-infected, vaccinated PCV2 challenged or negative control pigs. However, following in vitro infection of monocyte-derived dendritic cells with PCV2, PRRSV, or both viruses, co-cultured lymphocytes from chronically infected and PCV2 vaccinated pigs had significantly (p<0.05) decreased Treg expression in the virus infected groups compared to the negative controls. In separate experiments, pigs vaccinated against PCV2 and subsequently challenged with an attenuated PRRSV strain and its pathogenic parental strain developed increased CD4+CD25+FoxP3+ Tregs (p<0.05) in PBMC samples compared to uninfected controls, and this correlated with increased suppressor activity and IL-10 expression. The findings from these studies indicate that the interaction of PCV2 and PRRSV in swine modulates the host immune response mediated in part through the activity of Tregs. However, the extent to which Tregs orchestrate a dysregulated immune response in the pathogenesis of PCVAD in vivo remains to be determined.