Background <p>Chemoimmunotherapy has become the standard first-line treatment for advanced non-small cell lung cancer (NSCLC). Deciphering the T-cell subset responsible for chemoimmunotherapy and easily tested conveniently is critical in predicting the treatment outcomes.</p> Methods <p>Based on peripheral blood collected from patients enrolled from a phase 2 clinical study (ClinicalTrials.gov NCT04836728), we performed multi-color flow cytometry and unsupervised analysis to explore correlations with therapeutic outcomes. We integrated single-cell RNA and T-cell receptor (TCR) sequencing in 36 samples, including peripheral blood, tumors and non-tumor tissues, from 8 NSCLC patients to interpret the correlation, which was further verified using blood samples, orthotopic and subcutaneous lung cancer mouse model.</p> Results <p>The baseline CD28⁻KLRG1⁺CD57⁺ and on-treatment CD28⁻KLRG1⁺ CD8⁺ T cells in peripheral blood were independent factors which indicated improved treatment outcomes in advanced NSCLC patients receiving first-line chemoimmunotherapy. While being in a late-differentiated T-cell status, these cells were clonally expanded and reinvigorated during chemoimmunotherapy, serving as a peripheral T-cell pool for supplying potential tumor-reactive T cells in tumors, and reversely differentiating into less-differentiated subsets. The zinc-metallothionein pathway regulated the CD28⁻KLRG1⁺ CD8⁺ T-cell subset. Zinc supplementation combined with chemoimmunotherapy improved both local and systemic antitumor immune responses in mouse model.</p> Conclusions <p>Circulating CD28⁻KLRG1⁺ CD8⁺ T cells are valuable and convenient biomarkers for first-line chemoimmunotherapy in advanced NSCLC and provide insight into how late-differentiated or senescent T cells engage in the antitumor immunity when immunotherapy is added to conventional therapies.</p>