Il4i1, also known as Interleukin-4-induced gene 1, is an L-amino acid oxidase enzyme. It is mainly secreted by inflammatory antigen-presenting cells, like macrophages in T helper type 1 granulomas and tumors. Il4i1 is involved in the regulation of B-and T-cell adaptive immune responses, and plays a role in tryptophan metabolism. The tryptophan catabolites generated by Il4i1 can activate the aryl hydrocarbon receptor (AHR), which is related to tumor malignancy and immune suppression [1,2,4].

In cancer research, Il4i1 has been shown to promote tumor progression. In a mouse model of chronic lymphocytic leukemia (CLL), Il4i1 enhanced the disease progression, associated with reduced survival in glioma patients [1]. In glioma, it was significantly expressed in M2-like tumor-associated macrophages (TAMs), promoting glioma cell invasion and migration, and inducing M2-like macrophage polarization [3]. Also, in the tumor microenvironment, Il4i1 and IDO1, another tryptophan-catabolic enzyme, have overlapping expression patterns in myeloid cells, and both can suppress oxidative cell death ferroptosis [2].

In conclusion, Il4i1 is a key enzyme in regulating immune responses and tryptophan metabolism. Its role in promoting tumor progression, as revealed by in vivo studies such as those in mouse models of CLL and glioma, makes it a potential target for cancer therapy. Understanding Il4i1 function can provide new strategies for cancer treatment, especially considering its involvement in immunosuppressive mechanisms in the tumor microenvironment [1,2,3].