Peripheral blood CD4(+)CD8(+) lymphocytes in cynomolgus monkeys are of resting memory T lineage

In this study, we analyzed peripheral blood CD4(+)CD8(+) double-positive (DP) lymphocytes in adult cynomolgus monkeys (Macaca fascicularis). Forty of 55 monkeys had >5% of the peripheral blood DP subpopulation (9.3 +/- 5.9%; mean +/- SD) in peripheral blood lymphocytes (PBL) in contrast to a low percentage of peripheral blood DP cells in humans and mice. In a cross-sectional study, the peripheral blood DP cells were found to increase in proportion with age. To clarify whether peripheral blood DP lymphocytes were immature precursors released from thymus without prior differentiation, the expressions of CD8 chains and CD1b on peripheral blood DP lymphocytes were compared with those on thymocytes. The peripheral blood DP lymphocytes were CD8 alpha(+)beta(-) and CD1b(-), while thymic DP lymphocytes were CD8 alpha(+)beta(+) and CD1b(+), suggesting that the peripheral blood DP cells are extrathymic T lymphocytes. Furthermore, the peripheral blood DP lymphocytes exhibited a resting memory T cell phenotype with CD2(hi)CD3(+)CD28(-)CD29(hi)CD49d(hi)CD69(-)CD80(lo). Taken together, adult cynomolgus monkeys possess a unique peripheral blood DP T cell subpopulation which expresses a resting memory T cell phenotype. In addition, similar phenotypic properties of DP lymphocytes were distributed in the spleen and lymph nodes, although the proportion was less in the spleen and much less in lymph nodes than in PBL.