In summary, our findings demonstrate that circulating T-cell CX3CR1 reflects dynamic change of CD8+ TILs in the tumor microenvironment early after initiation of ICI therapy, and that an increase in the frequency of the CX3CR1+ subset in PB CD8+ T cells correlates with response to anti-PD-1 therapy early on-treatment in NSCLC patients. The gene discussed is PDCD1; the disease is non-small cell lung carcinoma.