GPR18 and viral infectious disease: Compared to cluster 4, cluster 2 cells showed higher expression of SLAMF7, ZEB2, GZMB, GPR18, CD72, PDCD1 (PD1) and CRTAM that are known to be expressed in terminally differentiated effector cells in viral infections and various cancers [3, 73–76] (Figures 2b, 4c and 5b,d).