Figure 2. Expression of ICOS in resting and activated memory T cell subsets.

PBMCs from 11 healthy, young (n=6) and older (n=5) rhesus monkeys were analyzed for ICOS expression by flow cytometry. (A) Memory subsets were defined using CD28 and CD95 expression (left) to derive naïve T cells (CD28⁺CD95^lo; T_N); central memory T cells (CD28⁺CD95^hi; T_CM); and effector memory T cells (CD28⁻CD95^hi; T_EM). Memory subsets were re-defined by CD28 and ICOS expression and overlaid (right). Resting T_CM cells (blue overlay) express ICOS, while T_N cells (green) and T_EM cells (red) do not express ICOS at rest. Representative data from a young monkey is shown. (B, C) PBMCs were stimulated for 24 hours with PMA/ionomycin. CD4⁺ (B) and CD8⁺ (C) CD28⁺ and CD28⁻ subsets were analyzed for ICOS expression. Data is expressed as % ICOS⁺ T cells in CD4⁺CD28⁺, CD4⁺CD28⁻, CD8⁺CD28⁺ or CD8⁺CD28⁻ subsets. Activated CD8⁺CD28⁻ T cells expressed significantly less ICOS than CD8⁺CD28⁺ T cells (* p=0.0001).