A regulatory CD4⁺ T cell subset in the BB rat model of autoimmune diabetes expresses neither CD25 nor Foxp3

Biobreeding (BB) rats model type 1 autoimmune diabetes (T1D). BB diabetes-prone (BBDP) rats develop T1D spontaneously. BB diabetes-resistant (BBDR) rats develop T1D after immunological perturbations that include regulatory T cell (Treg) depletion plus administration of low doses of a TLR ligand, polyinosinic-polycytidylic acid. Using both models, we analyzed CD4 ⁺CD25⁺ and CD4⁺CD45RC^- candidate rat Treg populations. In BBDR and control Wistar Furth rats, CD25⁺ T cells comprised 5-8% of CD4⁺ T cells. In vitro, rat CD4 ⁺CD25⁺ T cells were hyporesponsive and suppressed T cell proliferation in the absence of TGF-β and IL-10, suggesting that they are natural Tregs. In contrast, CD4⁺CD45RC^- T cells proliferated in vitro in response to mitogen and were not suppressive. Adoptive transfer of purified CD4⁺CD25⁺ BBDR T cells to prediabetic BBDP rats prevented diabetes in 80% of recipients. Surprisingly, CD4 ⁺CD45RC^-CD25^- T cells were equally protective. Quantitative studies in an adoptive cotransfer model confirmed the protective capability of both cell populations, but the latter was less potent on a per cell basis. The disease-suppressing CD4⁺CD45RC^-CD25 ^- population expressed PD-1 but not Foxp3, which was confined to CD4⁺CD25⁺ cells. We conclude that CD4⁺CD25 ⁺ cells in the BBDR rat act in vitro and in vivo as natural Tregs. In addition, another population that is CD4⁺CD45RC^-CD25 ^- also participates in the regulation of autoimmune diabetes.