a, Scheme used for longitudinal interventional study of control and fT reg knockout mice which indicates when particular assays were conducted and whose results are described in b–g, in which rosiglitazone (Rosi) was introduced in diet after firmly establishing obesity with a HFD alone for 12 weeks (n = 8 mice per group). b, Cohort weights during course of study. Black arrow indicates introduction of rosiglitazone to the diet. c, Homeostatic model assessment of IR (HOMA-IR). d, e, Glucose tolerance test (d) and glucose excursions of glucose tolerance test (e) described as area under curve (AUC). f, g, Insulin tolerance test (f) and bar-graph quantitation of relative serum glucose decrease during insulin tolerance test (g) described as area above curve (AAC). h, Scheme used for parallel prophylactic study of control and fT reg knockout mice, the results of which are described in i–l, in which mice were placed on a HFD or HFD with rosiglitazone for 12 weeks (n = 8 mice per group). i, Cohort weights at end of study. j, HOMA-IR. k, l, Glucose and insulin tolerance tests of control (k) or fT reg knockout (l) mice fed HFD or HFD with rosiglitazone. m, Scheme used to determine temporal relationship of TZD-induced fT reg expansion and TZD-induced insulin-sensitization in wild-type mice, the results of which are described in n–q, where mice were fed HFD or HFD with rosiglitazone for up to 11 weeks (n = 10 mice per group, 5 mice of each group were euthanized at 5 weeks after diet introduction and remaining 5 mice were euthanized at 11 weeks). n, HOMA-IR at 4 weeks. o, p, Glucose (o) and insulin (p) tolerance tests at 5 weeks. q, Relative fT reg cell enrichment of mice fed HFD with rosiglitazone versus mice fed HFD alone at 5 and at 11 weeks. Data are mean ± s.e.m. *P < 0.05, **P < 0.01, ***P < 0.001, Student’s t-test.