CX3CR1 and chronic obstructive pulmonary disease: The increased expression of CX3CR1, FGG, EGR1, VCAM1 and PTGS2 is positively associated with COPD, and the underexpression of PPBP, FPR1, FPR2, S100A12, ARG1, CD163, ORM1, S100A8 and S100A9 is negatively associated with COPD.