Although the effects of lymphopenia in adoptive hosts could not be excluded, our results showed that the most important factor affecting expansion of Treg cell numbers during FV infection was, in fact, the concomitant response to FV of a conventional effector CD4+ T cell population, which could be providing an essential Treg cell growth factor, such as IL-2 (Almeida et al., 2006) or inducing elevated expression of Treg cell-stimulating self-peptide-MHC II complexes. This evidence concerns the gene CD4 and lymphopenia.