IL4I1 and neoplasm: A unique class of unconventional mucosal associated invariant (MAIT) cells with a profile of the marker genes KLRB1 (CD161), NCR3, RORA, and SLC4A10 and the inhibitory NK cell receptors KLRG1, KLRB1, and IL4I1 (a tumor-derived tryptophan catabolic enzyme that promotes tumor invasion) exists in the intestinal mucosal tissues.