CXCL9 and chronic obstructive pulmonary disease: ,32 Although we have no clear explanation for why circulating levels of CXCL9 and CXCL10 were lower in the transplant COPD cohort compared to control samples, which contrasts with other studies,32,33 the lack of an increase may result from the absence of viral or bacterial infections in the transplant COPD cohort at the time of transplantation.