Abstract
Phenotypic markers that denote different developmental stages of thymocytes are important for understanding T cell development in the thymus. Here, we show that CD1b is a critical discriminator of thymocyte maturation stage in cynomolgus monkeys. CD1b was expressed by immature thymocytes prior to β-selection, and its expression decreased as cells became fully mature in the thymus. MHC-I expression was lowest at the CD3(lo)CD1b(+) immature double-positive (DP) stage, while the ratio of CD1d:MHC-I expression was significantly higher at this stage than at other developmental stages. PLZF was expressed by < 0.2% of thymocytes; most PLZF(+) thymocytes were CD3(-/lo)CD1b(+) immature DP thymocytes with the potential to produce IL-4. EOMES(+) thymocytes, which accounted for > 2% of total thymocytes, were mostly CD3(+)CD1b(-) mature thymocytes and predominantly of the CD8 single-positive (SP) lineage. An unconventional CD8(+) T cell subset expressing the NKG2AC(+)CXCR3(+) innate-like T cell marker was identified within the EOMES(+) CD8 SP lineage; these cells exhibited a memory phenotype. Taken together, these findings show that CD1b is a valuable discriminatory marker of thymocyte development. The data presented herein can be used to characterize the features of PLZF- and EOMES-associated unconventional T cells in the thymus.