Polyclonal Treg cells enhance the activity of a mucosal adjuvant

The efficacy of vaccines can be greatly improved by adjuvants that enhance and modify the magnitude and the duration of the immune response. Several approaches to design rational adjuvants are based on the suppression of regulatory T-cell (Treg) function. Here, we evaluated whether removal or addition of Treg at the time of vaccination with tetanus toxoid and the mucosal adjuvant cholera toxin (CT), would affect immune responses. We found that depletion/inactivation of CD4(+)CD25(+) Treg, either by treatment of BALB/c mice with anti-CD25 monoclonal antibodies or by adoptive transfer of CD4(+)CD25(-) T lymphocytes depleted of CD4(+)CD25(+) Treg into nu/nu mice, impaired antibody production after mucosal immunization in the presence of CT. Conversely, transfer of polyclonal, but not Ag-specific, CD4(+)CD25(+)Foxp3(+) Treg to normal BALB/c mice enhanced CT-induced antibody responses. An increased titer of both immunoglobulin IgG1 and IgG2a antibody subclasses was found, however, the ratio between IgG1/IgG2a with or without polyclonal Treg was comparable, suggesting that polyclonal Treg influence the magnitude, but not the quality of the immune response. Recipients of polyclonal Treg that had been immunized with CT had an increased number of Ag-specific CD4(+) T cells with an activated phenotype (CD44(hi)) in the draining lymph nodes. This accumulation of Ag-specific CD4(+) T lymphocytes could favour the germinal centre formation and may promote T-dependent B-cell responses. Overall, our study indicates that Foxp3(+) Treg can not only function as suppressor cells but also as helper T cells, depending on the type of immune response being evaluated and the microenvironment in which the response is generated.