Predicting anti-PD-1 responders in malignant melanoma from the frequency of S100A9+ monocytes in the blood.

Our results suggest that monocytic cell populations can critically determine the outcome of PD-1 blockade, particularly the subset expressing S100A9, which should be further explored as a possible predictive biomarker. Detailed knowledge of the biological role of S100A9+ monocytes is of high translational relevance.