Table I.

Expression of PD-1 by T cells from young and aged mice^a.

	CD4⁺			CD8⁺
	Young	Aged	p^b	Young	Aged	p^b
Spleen	10.4 ± 1.4	46.5 ± 7.2	< 0.01	7.6 ± 0.8	49.7 ± 6.4	< 0.01
LN	7.7 ± 1.1	23.6 ± 5.2	0.01	4.6 ± 0.7	12.8 ± 1.8	< 0.01
Lungs	6.6 ± 1.1	27.3 ± 7.5	0.04	ND^c	ND^c
PP	19.2 ± 4.0	29.7 ± 6.5	0.26	19.6 ± 6.9	18.2 ± 4.2	0.86

	CD4⁺CD44⁺			CD8⁺CD44⁺
	Young	Aged	p^b	Young	Aged	p^b
Spleen	28.6 ± 4.7	45.7 ± 6.7	0.11	26.3 ± 2.5	35.1 ± 6.8	0.45
LN	24.7 ± 3.4	33.3 ± 7.4	0.29	12.9 ± 1.4	18.7 ± 3.2	0.10
Lungs	19.9 ± 4.8	28.6 ± 8.1	0.42	ND^c	ND^c
PP	33.8 ± 7.6	34.2 ± 7.8	0.97	40.7 ± 18.2	19.5 ± 5.0	0.21

	T_reg
	Young	Aged	p^b
Spleen	20.5 ± 4.1	46.7 ± 7.6	0.03
LN	17.5 ± 3.6	28.8 ± 6.5	0.14
Lungs	15.9 ± 6.1	25.1 ± 5.7	0.30
PP	28.9 ± 7.2	26.4 ± 6.8	0.81

Cells were obtained from young mice (≤2-month-old, N≥ 6) and aged mice (≥16-month-old; N≥ 6) and stained for the surface markers CD4, CD8, CD44 and PD-1, followed by staining for the intracellular marker FoxP3. T_reg were defined as CD4⁺FoxP3⁺ T cells. Values represent the mean (±SEM) percentage of PD-1 positive cells in the different subsets of T cells.

p values (t-test) compare the percentage of PD-1⁺ cells in young and aged mice.

ND: not determined