Neutrophils Expressing Programmed Death-Ligand 1 Play an Indispensable Role in Effective Bacterial Elimination and Resolving Inflammation in Methicillin-Resistant Infection.

Programmed death ligand 1 (PD-L1) is a co-inhibitory molecule expressed on the surface of various cell types and known for its suppressive effect on T cells through its interaction with PD-1. Neutrophils also express PD-L1, and its expression is elevated in specific situations; however, the immunobiological role of PD-L1 neutrophils has not been fully characterized. Here, we report that PD-L1-expressing neutrophils increased in methicillin-resistant (MRSA) infection are highly functional in bacterial elimination and supporting inflammatory resolution. The frequency of PD-L1 neutrophils was dramatically increased in MRSA-infected mice, and this population exhibited enhanced activity in bacterial elimination compared to PD-L1- neutrophils. The administration of PD-L1 monoclonal antibody did not impair PD-L1 neutrophil function, suggesting that PD-L1 expression itself does not influence neutrophil activity. However, PD-1/PD-L1 blockade significantly delayed liver inflammation resolution in MRSA-infected mice, as indicated by their increased plasma alanine transaminase (ALT) levels and frequencies of inflammatory leukocytes in the liver, implying that neutrophil PD-L1 suppresses the inflammatory response of these cells during the acute phase of MRSA infection. Our results reveal that elevated PD-L1 expression can be a marker for the enhanced anti-bacterial function of neutrophils. Moreover, PD-L1 neutrophils are an indispensable population attenuating inflammatory leukocyte activities, assisting in a smooth transition into the resolution phase in MRSA infection.