CD4 and infection: In addition, we found the decline of peripheral CD4 + Trm (cluster 5) and CD4 + Tm (cluster 8, ANXA1 +) in COPD patient and evidence has shown that CD4 + Trm (cluster 5) could respond rapidly to pathogen re-exposure and facilitate CD8 + T cell recruitment [38], suggesting its importance in protecting the host against infection.