STLV-1 infection induces a constitutive activation of the simian immune system. (A) Comparison of the CD3⁺ cell number per cubed millimeter of blood between noninfected (5 animals) and STLV-1–infected (15 animals) monkeys before initiation of the treatment. Cells were stained with CD3 antibodies. The percentages obtained were then reported to the total lymphocyte number determined by clinical automated lymphocyte count. (B) Comparison of T-CD4⁺/T-CD8⁺ ratio between noninfected and infected monkeys before treatment. Cells were stained with CD4, CD8, and CD3 antibodies. (C) Comparison of CD8⁺ subpopulations count per cubed millimeter of blood determined before treatment. Cells were stained for CD3, CD8, CD95, and CD28. *Significant difference (P < .05; Mann-Whitney test).