Exogenous IGF-1 enhances thymic function in PSGL-1KO mice without a disproportionate increase in ETPs. PSGL-1KO mice and age-matched wild-type controls were given a 2-week course of IGF-1, after which thymocyte subsets, ETPs, and peripheral (combined spleen and lymph node) LSK were enumerated. (A) Changes in thymocyte number and thymic TREC number in PSGL-1KO mice treated with IGF-1. (B) ETP numbers in wild-type mice compared with diluent- and IGF-1–treated PSGL-1KO mice. (C,D) changes in frequencies of ETPs (C) and peripheral LSK (D) in diluent- and IGF-1–treated wild-type and PSGL-1KO mice. (E) LSK/ETP ratios in diluent- and IGF-1–treated wild-type and PSGL-1KO mice. The normalized IGF-1/diluent ratios for LSK/ETP ratios in B6 and PSGL-1KO mice are indicated. n = 6 to 11 mice per group combined from 3 independent experiments (*P < .05). Error bars represent SEM.