SPHK1 and chronic obstructive pulmonary disease: For 13 COPD subjects, a strong negative correlation was found between SPHK1, S1PR3 and S1PR5 mRNA expression levels and the ability of alveolar macrophages to phagocytose apoptotic cells (SPHK1 r = -0.59, p<0.05; S1PR3 r = -0.70, p<0.05; S1PR5 r = -0.87 p<0.05; SPHK1 data presented in Fig 5B.