Regulatory T cells (T-reg) provide a balance to immune T cell activation thereby protecting the body from pathogen-induced immunopathology. Several persistent viruses induce Treg that subvert protective immune mechanisms and promote viral persistence. Epstein-Barr virus (EBV) generally infects children subclinically and persists thereafter, but primary infection in early adulthood may cause immunopathological damage manifest as infectious mononucleosis. In this study the role of T-reg was investigated in acute infectious mononucleosis and healthy EBV seropositive donors. The proportion of CD4(+)CD25(high) T cells in blood from infectious mononucleosis patients was significantly lower than in seropositive donors (P=0.05). Using the FOXP3 marker for T-reg the same frequency and extra-follicular distribution of T-reg was noted in infectious mononucleosis and control tonsils. Regulatory cytokines, interleukin (IL)-10 and transforming growth factor (TGF)-beta, were significantly raised in infectious mononucleosis compared to seropositive donor plasma (P=0.0001, P=0.0004 respectively) although levels of IL-10 peaked earlier in infectious mononucleosis than TGF-beta. Previous studies identified EBV latent membrane protein (LMP)-1-induced T-reg activity [Marshall et al. (2003): J Immunol 170:6183-6189; Marshall et al. (2007): Brit J Haematol 139:81-89], and in this study a significant reduction in interferon-gamma production was found from infectious mononucleosis but not seropositive donor lymphocytes after stimulation with a recall antigen when LMP-1 peptide PRG was added (P=0.03). It is possible that T-reg are important in controlling primary EBV infection to a subclinical level in most cases and that infectious mononucleosis represents a failure of this protective mechanism. J. Med. Virol 81:870-877, 2009. (C) 2009 Wiley-Liss, Inc.