PRF1 and chronic obstructive pulmonary disease: There was a higher percentage of CD28nullCD8+ T cell and NKT-cells expressing perforin and granzyme b in COPD patients compared with control subjects (eg., 45 ± 13 (14 ± 12) p = .037; and 33 ± 13 (12 ± 8) p = .025 for the percentage of CD28null CD8+ T cells expressing granzyme b and perforin (median ± sd) from COPD patients (controls).