Immune Suppression in CLL Is Mediated By the L-Amino Acid Oxidase IL4I1, a Reason for the Treatment Failure of IDO1 Inhibitors

A better understanding of cancer-induced immune suppression has led to the development of novel immunotherapy approaches, some of which are considered as breakthrough in cancer treatment. One of the promising targets that was followed in clinical trials is indoleamine-2,3-dioxygenase 1 (IDO1) which is a tryptophan (Trp)-catabolizing enzyme that produces metabolites such as kynurenine (Kyn) which promote immune evasion by suppressing T cell activity, and therefore tumor progression. Enhanced expression of IDO1 was described for patients with B cell lymphoma, including chronic lymphocytic leukemia (CLL), and we observed a strong upregulation of IDO1 and a significantly increased Kyn to Trp serum ratio in the Eµ-TCL1 mouse model of CLL. To evaluate the therapeutic potential of this pathway in CLL, we treated Eµ-TCL1 mice with the IDO1 inhibitors 1-Methyl-D-tryptophan or Epacadostat. Both drugs led to initial treatment responses, but failed to improve tumor development over time which is in line with disappointing results of a first phase III clinical trial combining Epacadostat with immune checkpoint blockade for patients with melanoma.

In search for an explanation for this failure, we investigated genes that are induced upon resistance to immune checkpoint blockade and identified the phenylalanine-catabolizing enzyme interleukin-4-induced 1 (IL4I1). IL4I1 is strongly expressed in cancer-associated myeloid cells as well as tumor cells of different entities, including CLL and other B cell lymphoma, and was attributed immunosuppressive functions. We thus explored its role in cancer immune escape in bone marrow-chimeric mice that lack IL4I1 in the hematopoietic tumor microenvironment. Adoptive transfer of Eµ-TCL1 leukemia in these mice showed that lack of IL4I1 in hematopoietic cells resulted in a dramatically reduced tumor development compared to respective control mice (Figure 1A). In-depth analysis of the immune compartment of these mice by flow cytometry and transcriptome analyses revealed that IL4I1 deficiency was associated with lower expression of immune checkpoint molecules including PD-1 on CD8 T cells. Moreover, CD8⁺ effector T cells of IL4I1-deficient mice were highly enriched in a transcriptional signature of genes that was downregulated upon T-cell exhaustion (Figure 1B). We further observed less suppressive regulatory T cells, and enhanced antigen presentation capacity of dendritic cells in the IL4I1-deficient chimeric mice developing CLL. Altogether, these results provide evidence for a central role for IL4I1 in CLL-associated immune suppression and suggest IL4I1 as attractive novel target for immunotherapy of cancer, including tumors that do not respond to IDO1 inhibitors.

Figure 1: CLL development and CD8 T cell phenotype in mice with IL4I1-deficient microenvironment

A: Bone marrow (BM)-chimeric mice were generated by i.v. transplantation of 5 x 10⁶ BM cells of wildtype (WT) or Il4i1^-/- mice in lethally irradiated C57BL/6 mice. After reconstitution of the hematopoietic system, 10 weeks after BM transfer, mice were i.v. transplanted with 1 x 10⁷ malignant B cells of Eµ-TCL1 mice and CLL development was monitored in peripheral blood over time by quantifying absolute number of CD5⁺ CD19⁺ CLL cells by flow cytometry. B: 5 weeks after CLL cell transfer, mice were sacrificed and splenic CD8⁺ effector T cells were sorted for transcriptome analysis by microarrays. Gene set enrichment analysis of ranked-list of genes (fold change) was performed using published signatures which showed an enrichment of genes that are downregulated in exhausted versus memory CD8⁺ T cells in Il4i1^-/- in comparison to WT mice.