Porcine circovirus type 2 (PCV2) is the necessary agent for the occurrence of post-weaning multisystemic wasting syndrome (PMWS) in pigs. It has been suggested that PMWS-affected pigs are immunosuppressed and, therefore, more prone to develop co-infections. In this study, we elucidated that PCV2 downregulates in vitro the immune cell functions during recall antigen responses. We showed that PCV2, but not the non-pathogenic porcine circovirus type 1, induces interleukin (IL)-10 secretion by monocytic cells. Notably, PCV2-induced IL-10 led to effective repression of IL-12 in blood peripheral mononuclear cells (PBMCs). Besides alpha and gamma interferon synthesis by PBMCs from pseudorabies virus (PRV)-immunized animals, activated in vitro PRV also was repressed by subsequent infection by PCV2. The ability of PCV2 to hamper the development of immune responses may contribute to the Th1 suppressed responses, immune suppression and co-infections. © 2008 SGM.