Veterinary Microbiology 2008, Vol. 132: 74–86Previously, it was shown that modulation of the immune system enhances porcine circovirus type 2 (PCV2) replication in pigs. In the present study, the effect of the mitogen concanavalin A (ConA) on PCV2 replication was investigated. Since ConA induces T-lymphocyte activation and initiates the production of interferon-gamma (IFN-g), a cytokine that enhances PCV2 replication in porcine epithelial and monocytic cell lines in vitro, it was examined if the effects observed with ConA were mediated by IFN-g. In an in vitro study, ConA but not IFN-g enhanced PCV2 replication in peripheral blood mononuclear cells (PBMC). Up to 2.08% and 0.96% of PBMC were antigen positive for PCV2 strains 1121 and Stoon-1010, respectively, and a low virus production was observed. PCV2-infected PBMC were identified as CD4+ (40%), CD8+ (54%) and IgM+ (11%). In a subsequent in vivo study, caesarean-derived colostrum-deprived piglets were injected with ConA or IFN-g 12 h before inoculation and every 3 days for 9 days after inoculation with strain 1121. PCV2 was isolated from inguinal lymph node biopsies from 10 days post-inoculation (dpi) in ConA-treated pigs and from 15 dpi in non-treated and IFN-g-treated pigs. ConA increased PCV2 replication levels, but disease was not observed. Half of the ConA-treated and IFN-g-treated pigs showed a delayed humoral immune response, but this delay did not result in increased PCV2 replication in these pigs. These experiments demonstrated that ConA enhances PCV2 replication in PBMC in vitro and in lymphoid tissues in vivo. # 2008 Elsevier B.V. All rights reserved.