Background: In type 1 diabetes (T1D), a prototypic autoimmune disease, effector T cells destroy beta cells. Normally, CD4(+)CD25(+high), or natural regulatory T cells (Tregs), counter this assault. In autoimmunity, the failure to suppress CD4(+)CD25(low) T cells is important for disease development. However, both Treg dysfunction and hyperactive responder T-cell proliferation contribute to disease.
Methods/principal findings: We investigated human CD4(+)CD25(low) T cells and compared them to CD4(+)CD25(-) T cells in otherwise equivalent in vitro proliferative conditions. We then asked whether these differences in suppression are exacerbated in T1D. In both single and co-culture with Tregs, the CD4(+)CD25(low) T cells divided more rapidly than CD4(+)CD25(-) T cells, which manifests as increased proliferation/reduced suppression. Time-course experiments showed that this difference could be explained by higher IL-2 production from CD4+CD25(low) compared to CD4+CD25- T cells. There was also a significant increase in CD4+CD25(low) T-cell proliferation compared to CD4+CD25- T cells during suppression assays from RO T1D and at-risk subjects (n = 28, p = 0.015 and p = 0.024 respectively).
Conclusions/significance: The in vitro dual suppression assays proposed here could highlight the impaired sensitivity of certain responder T cells to the suppressive effect of Tregs in human autoimmune diseases.