Circulating BLT1⁺ T cells are CD27⁺ preterminally differentiated memory T cells that express CD38 and HLA-DR. (A) BLT1⁺ T-cell coexpression of differentiation markers. Approximately 92% of circulating BLT1⁺CD4⁺ T cells and 86% of BLT1⁺ CD8⁺ T cells coexpress CD45RO. The majority of BLT1⁺ CD4⁺ and CD8⁺ T cells are CD27⁺ (90% of CD4 and 89% of CD8) and CD28⁺ (97% CD4 and 76% CD8), comparable with levels of CD27 and CD28 expressed by bulk T cells. Representative data are shown gated on FSC^hi and CD3⁺ cells; these data are representative of 7 (CD45RO and CD27) and 4 (CD28) different experiments. Numbers represent percentages of gated population. (B) Coexpression of activation markers on total BLT1⁺ CD3⁺ CD4⁺ and CD3⁺ CD8⁺ T cells. Cells were simultaneously gated on the total lymphocyte gate, CD3⁺ CD4 or CD8 gate, and the BLT1⁺ gate, to define the frequency of BLT1⁺ CD4⁺ or CD8⁺ cells that coexpress the activation marker of interest. These data are representative of 4 to 10 separate donors. (C) BLT1⁺ T cells are significantly enriched for CD38 (n = 10) and HLA-DR (n = 6) compared with bulk. Error bars represent SEM.