Identification of CD8+CD25+FOXP3+ regulatory T cells in Type 1 Diabetes: assembling the puzzle in the disease etiopathogenesis

Insulin-dependent diabetes mellitus (Type 1 diabetes, T1D) is an autoimmune disease where autoreactive T lymphocytes destroy pancreatic beta cells. Common human autoimmune diseases show quantitative or qualitative T regulatory (Treg) defects. We previously reported a defect in CD4+ Tregs cell proliferation and reduced CD4+ Tregs PD-1 expression in diabetic patients (Perri 2015). Further, another regulatory subset, CD8+ Tregs, endowed with immunosuppressive functions, has recently raised interest. These cells, evaluated as CD8+CD25+FOXP3+ cells, are represented as a much lower percentage (below 1%) of population in human PBMC in respect to CD4+ Tregs (8%). Despite their reduced abundancy, CD8+ Tregs require serious consideration in the regulation of immune responses due to their effective suppressive activity.

Different CD8+ T cell populations, their proliferation capacity and expression in PD-1 molecule were evaluated by flow-cytometer analysis in a cohort of 31 T1D patients, 18 newly-diagnosed and 13 long-term, compared to 20 healthy donors. The various subsets were studied under basal conditions and after 3 and 5 days of PMA/ionomycin stimulation.

Under basal conditions, CD8+ Tregs and CD8+ Teffs were seemingly represented among study groups while there was evidence of diminished expression of PD-1 in the patients eff cells. After 3 days of PMA/ionomycin stimuli patients CD8+ Tregs showed decreased percentage in respect to control group. PD-1+CD8+ Tregs were represented as a much lower percentage in diabetic patients, in respect to control subjects. Importantly, patients CD8+ Tregs and CD8+ Teffs presented a significant proliferation defect in respect to the control group. In conclusion, CD8+ Tregs could represent a novel target of immunotherapy in T1D patients.