SPHK1 and chronic obstructive pulmonary disease: Unlike S1PR2, there were no correlations between S1PR5 and smoking status, or with SPHK1 or SPHK2 or S1PR2. Currently, the role of S1PR5 in macrophages is unclear, although our findings of a strong negative correlation between expression of S1PR5 and phagocytosis of apoptotic cells suggest that it may play an important role in this function and contribute to the defective macrophage function in COPD.