AbstractRecent studies highlight the importance of baseline functional immunity for immune checkpoint blockade therapies. High‐dimensional systemic immune profiling is performed in a cohort of non‐small‐cell lung cancer patients undergoing PD‐L1/PD‐1 blockade immunotherapy. Responders show high baseline myeloid phenotypic diversity in peripheral blood. To quantify it, we define a diversity index as a potential biomarker of response. This parameter correlates with elevated activated monocytic cells and decreased granulocytic phenotypes. High‐throughput profiling of soluble factors in plasma identifies fractalkine (FKN), a chemokine involved in immune chemotaxis and adhesion, as a biomarker of response to immunotherapy that also correlates with myeloid cell diversity in human patients and murine models. Secreted FKN inhibits lung adenocarcinoma growth in vivo through a prominent contribution of systemic effector NK cells and increased tumor immune infiltration. FKN sensitizes murine lung cancer models refractory to anti‐PD‐1 treatment to immune checkpoint blockade immunotherapy. Importantly, recombinant FKN and tumor‐expressed FKN are efficacious in delaying tumor growth in vivo locally and systemically, indicating a potential therapeutic use of FKN in combination with immunotherapy.