Adoptive transfer of CD4(+)Foxp3(+) regulatory T cells to C57BL/6J mice during acute infection with Toxoplasma gondii down modulates the exacerbated Th1 immune response.

Infection of C57BL/6J mice with the parasite Toxoplasma gondii triggers a powerful Th1 immune response that is detrimental to the host. During acute infection, a reduction in CD4(+)Foxp3(+) regulatory T cells (Treg) has been reported. We studied the role of Treg during T. gondii infection by adoptive transfer of cells purified from transgenic Foxp3(EGFP) mice to infected wild type animals. We found a less severe weight loss, a significant delayed mortality in infected Treg-transferred mice, and reduced pathology of the small intestine that were associated with lower IFN-γ and TNF-α levels. Nevertheless, higher cyst number and parasite load in brain were observed in these mice. Treg-transferred infected mice showed reduced levels of both IFN-γ and TNF-α in sera. A reduced number of CD4(+) T cells producing IFN-γ was detected in these mice, while IL-2 producing CD4(+) T cells were restored to levels nearly similar to uninfected mice. CD25 and CD69 expression of CD4(+) T cells were also down modulated. Our data show that the low Treg cell number are insufficient to modulate the activation of CD4(+) T cells and the production of high levels of IFN-γ. Thus, a delicate balance between an optimal immune response and its modulation by Treg cells must exist.