Lactococcus lactis subsp. cremoris C60 is a probiotic strain of lactic acid bacteria (LAB) which induces various immune modifications in myeloid lineage cells. These modifications subsequently reg-ulate T cell function, resulting in enhanced immunity both locally and systemically. Here, we report that C60 suppresses tumor growth by enhancing macrophage function via metabolic al-terations, thereby increasing adenosine triphosphate (ATP) production in a murine melanoma model. Intragastric (i.g.) administration of C60 significantly reduced tumor volume compared to saline administration in mice. The anti-tumor function of intratumor (IT) macrophage was up-regulated in mice administered with C60, as evidenced by an increased inflammatory phenotype (M1) rather than an anti-inflammatory/reparative (M2) phenotype, along with enhanced anti-gen-presenting ability, resulting in increased tumor antigen-specific CD8+ T cells. Through this functional modification, we identified that C60 establishes a glycolysis-dominant metabolism, rather than fatty acid oxidation (FAO), in IT macrophages, leading to increased intracellular ATP levels. To address the question of why orally supplemented C60 exhibits functions in distal places, we found a possibility that bacterial cell wall components, which could be distributed throughout the body from the gut, may induce stimulatory signals in peripheral macrophages via Toll-like receptors (TLRs) signaling activation. Thus, C60 strengthens macrophage anti-tumor immunity by promoting a predominant metabolic shift towards glycolysis upon TLR-mediated stimulation, thereby increasing substantial energy production.

Programmed death ligand 1 (PD-L1) is a co-inhibitory molecule expressed on the surface of various cell types, known for its suppressive effect on T cells through interaction with PD-1. Neutrophils also express PD-L1, and its expression is elevated in specific situations; however, the immunobiological role of PD-L1+ neutrophil has not been fully characterized. Here, we report that PD-L1-expressing neutrophils increased in methicillin-resistant Staphylococcus aureus (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. An 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 increased plasma alanine transaminase (ALT) level 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 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.