CD28 and chronic obstructive pulmonary disease: Having shown a loss of CD28 and up-regulation of CD137 on CD28null T cells in patients with COPD compared with healthy controls [5] we then aimed to determine the effect of blocking antibody to CD137 on granzyme B expression and IFNγ and TNFα production by NK cells and CD28nullCD8- and CD28nullCD8+ T cells and NKT-like cells.