CD28 and chronic obstructive pulmonary disease: The inhibitory effect of anti-CD137 blocking antibody (10 μg/mL) (a137), 10-6 M methylprednisolone (MP) alone and in combination on granzyme B, IFNγ and TNFα expression by CD28null(28-) and CD28 + (28+) CD8+ NKT-like cells and NK cells following stimulation of PBMC from COPD patients (mean ± SEM, mean of 8 experiments) compared with control antibody