The high metabolic needs of T lymphocytes in response to activation make them particularly vulnerable to modifications of their biochemical milieu. Immunosuppressive enzymes produced in the tumor microenvironment modify nutrient availability by catabolizing essential or semi-essential amino acids and producing toxic catabolites, thus participating in the local sabotage of the antitumor immune response. L-amino-acid oxidases are FAD-bound enzymes found throughout evolution, from bacteria to mammals, and are often endowed with anti-infectious properties. IL4I1 is a secreted L-phenylalanine oxidase mainly produced by inflammatory antigen-presenting cells—in particular, macrophages present in T helper type 1 granulomas and in various types of tumors. In the last decade, it has been shown that IL4I1 is involved in the fine control of B- and T-cell adaptive immune responses. Preclinical models have revealed its role in cancer immune evasion. Recent clinical data highlight IL4I1 as a new potential prognostic marker in human melanoma. As a secreted enzyme, IL4I1 may represent an easily targetable molecule for cancer immunotherapy.

MΦ and DC are key elements in the control of tissue homeostasis and response to insult. In this work, we demonstrate that MΦ and DC are the major producers of the phenylalanine catabolizing enzyme IL‐4‐induced gene 1 (IL4I1) under inflammatory conditions. IL4I1 was first described in B cells, which indeed can produce IL4I1 in vitro, although at much lower levels. In vivo, IL4I1 is highly expressed by MΦ and DC of Th1 granulomas (sarcoidosis, tuberculosis) but poorly detected in Th2 granulomas (schistosomiasis). In vitro, expression of the enzyme is induced in mononuclear phagocytes by various pro‐inflammatory stimuli through the activation of the transcription factors NF‐κB and/or STAT1. B cells also express IL4I1 in response to NF‐κB‐activating stimuli such as CD40L; however, in contrast to myeloid cells, B cells are insensitive to IFN‐γ but respond to stimulation of the IL‐4/STAT6 axis. As we show that the expression of IL4I1 by a monocytic cell line inhibits T‐cell proliferation and production of IFN‐γ and inflammatory cytokines, we propose that IL4I1 participates in the downregulation of Th1 inflammation in vivo.

The L‐phenylalanine oxidase IL4I1 inhibits T‐cell proliferation in vitro through H₂O₂ production, and is highly expressed in tumor‐associated macrophages. IL4I1 is also detected by immunohistochemistry in neoplastic cells from several B‐cell lymphomas and some non‐lymphoid tumors. To evaluate IL4I1's effect on tumor growth, we developed a mouse melanoma model constitutively coexpressing IL4I1 and the GP33 epitope. After GP33 vaccination, tumors developed more frequently in mice injected with IL4I1‐expressing cells in comparison with mice receiving control cells. Tumor escape was preceded by a rapid diminution of IFN‐γ‐producing cytotoxic antitumor CD8⁺ T cells. Moreover, tumor incidence was already increased when only 20% of the injected cells expressed IL4I1. The minimal IL4I1 activities leading to tumor escape were close to those detected in human melanoma and mesothelioma. Thus, we demonstrate the immunosuppressive functions of IL4I1 in vivo and suggest that IL4I1 facilitates human tumor growth by inhibiting the CD8⁺ antitumor T‐cell response.

L-amino acid oxidases (LAAO) are flavoproteins that catalyze the oxidative deamination of L-amino acids to a keto-acid along with the production of H₂O₂ and ammonia. Interleukin 4 induced gene 1 (IL4I1) is a secreted LAAO expressed by macrophages and dendritic cells stimulated by microbial derived products or interferons, which is endowed with immunoregulatory properties. It is the first LAAO described in mammalian innate immune cells. In this work, we show that this enzyme blocks the in vitro and in vivo growth of Gram negative and Gram positive bacteria. This antibiotic effect is primarily mediated by H₂O₂ production but is amplified by basification of the medium due to the accumulation of ammonia. The depletion of phenylalanine (the primary amino acid catabolized by IL4I1) may also participate in the in vivo inhibition of staphylococci growth. Thus, IL4I1 plays a distinct role compared to other antibacterial enzymes produced by mononuclear phagocytes.