SPNS2 and chronic obstructive pulmonary disease: Further studies including primary cell co-culture systems are now designed to explore the direct biological effects of differential expression of Spns2 by alveolar macrophages and bronchial epithelial cells and how cigarette smoke- or COPD disease-induced changes of other molecules or factors might affect Spns2 expression, S1P release and related macrophage phagocytic dysfunction.