PDCD1 and melanoma: Enterococcus faecium, expressed NlpC/p60 peptidoglycan hydrolase SagA, can release peptidoglycan fragments, produce Nucleotide-binding oligomerization domain 2 (NOD2) active polypeptides, activate innate immunity, induce a microenvironment conducive to anti-PD-1 immunotherapy, enhance the effectiveness of melanoma immunotherapy (86).