Developmentally regulated expression of the PD-1 protein on the surface of double-negative (CD4(-)CD8(-)) thymocytes

PD-1, a member of the Ig superfamily, was previously isolated from an apoptosis-induced T cell hybridoma 2B4.11 by subtractive hybridization, Expression of the PD-1 mRNA is restricted to thymus in adult mice, Using an anti-PD-1 mAb (J43), we examined expression of the PD-1 protein during differentiation of thymocytes in normal adult, fetal and RAG-2(-/-) mice with or without anti-CD3 mAb stimulation, While PD-1 was expressed only on 3-5% of total normal thymocytes, similar to 34% of the CD4(-)CD8(-) double-negative (DN) fraction are PD-1(+) cells with two distinct expression levels (low and high), PD-1(high) thymocytes belonged to TCR gamma delta lineage cells, In the UN compartment of the TCR alpha beta lineage, PD-1 expression started at the low level from the CD44(+)CD25(+) stage and the majority of thymocytes expressed PD-1 at the CD44(-)CD25(-) stage in which thymocytes express TCR beta chains, The anti-CD3 epsilon antibody administration augmented the PD-1 expression as well as the differentiation of the CD44(-)CD25(+) DN cells into the CD44(-)CD25(-) UN stage, not only in normal mice but also in RAG-2-deficient mice, The fraction of the PD-1(low) cells in the CD4(+)CD8(+) double-positive (DP) compartment was very small (<5%) but increased by stimulation with the anti-CD3 antibody, although the total number of Up cells was drastically reduced, The results show that PD-1 expression is specifically induced at the stages preceding clonal selection.