NCAM1 and chronic obstructive pulmonary disease: Having established that lung CD56+ cells (which include both NK cells and CD56+ T cells) are able to kill autologous lung target cells, we then separated the subjects into smokers with normal pulmonary function (n = 6), mild COPD (n = 12) and severe COPD (n = 10) and analyzed % cytotoxicity (Fig. 5A).