CD8A and melanoma: Greater upregulation of Ki67 in CD4<sup>+</sup> central memory cells significantly differentiated responders and non-responders after one cycle of treatment (p=0.0086, area under the curve (AUC)=0.74, 95% CI 0.59 to 0.88), while higher on-treatment TIM-3 frequency within CD8<sup>+</sup> T cells differentiated patients who experienced severe toxicity (p=0.0086, AUC=0.74, 95% CI 0.59 to 0.88).<h4>Conclusions</h4>We here show that response and toxicity to cICB in advanced melanoma are driven by distinct immune features evident after only one cycle of treatment.