Modeling NK-Cell Lymphoma in Mice Reveals Cell-of-Origin, Microenvironment, and Therapeutic Targets

Extranodal NK/T-cell lymphoma (ENKTCL) is an Epstein-Barr virus (EBV)-related neoplasm preferentially involving the upper aerodigestive tract. However, what mechanisms underlie such site predilection and which NK-cell subpopulation is responsible for ENKTCL remain unclear. As current treatment shows limited efficacy, exploring the pathogenic mechanisms and developing novel therapeutic strategies are required in ENKTCL. However, an important limitation of the field is the lack of a genetically engineered mouse model (GEMM) recapitulating genetic events in human ENKTCL. To address this, we developed a mouse model by NK-cell-specific Trp53 disruption (Trp53^-/-), which died earlier than wild-type mice, with a median survival of 417 days. Macroscopic evaluation at death demonstrated the enlargement of the salivary glands, in addition to hepatosplenomegaly and occasional abdominal mass. Histological and flow cytometric analyses of the tumors revealed atypical lymphoid infiltration with a population of Lin^-CD122⁺NK1.1^- cells, resembling immature NK cells. Thus, Trp53^-/- mice developed NK-cell lymphomas involving the salivary glands and the hematopoietic system.

Salivary gland NK cells are tissue-resident NK cells that are distinct from conventional NK cells. We investigated how Trp53 deficiency has different effects on NK cells across these tissues in the pre-tumor state. Remarkably, Trp53 knockout caused an increase of Lin^-CD122⁺NK1.1^- cells and extensive gene expression changes of NK cells exclusively in the salivary glands. Thus, we evaluated characteristic phenotypes of tissue-resident NK cells and found that not only murine NK-cell tumors but also human ENKTCL samples expressed tissue-resident markers such as CD69 and CD49a, suggesting tissue-resident NK cells as their cell-of-origin. To investigate genetic profiles of Trp53^-/- tumors, we performed whole-exome sequencing, which revealed recurrent Myc amplifications. In addition, RNA sequencing (RNA-seq) showed the significant enrichment of MYC target gene signatures in these tumors.

To investigate the role of EBV-encoded latent membrane protein 1 (LMP1) in NK-cell lymphomagenesis, we introduced the expression of LMP1 into our mouse model (Trp53^-/-LMP1⁺). LMP1 expression significantly accelerated NK-cell lymphomagenesis. Single-cell RNA-seq of the tumors showed LMP1-mediated propagation of myeloid cells, including monocytes and conventional dendritic cells (DC), through interferon-γ signaling. Cell-cell interaction analysis revealed that LMP1-induced myeloid cell expansion promoted tumor cell proliferation through upregulated CXCL16-CXCR6 signaling. We validated the presence of CD11c⁺ DC in the tumor microenvironment and CXCR6 and CXCR3 protein expressions in tumor cells by immunohistochemical analysis of human ENKTCL samples. Importantly, inhibition of CXCL16-CXCR6 signaling was effective against NK-cell tumors in vivo.

Using single-cell RNA-seq and flow cytometric analyses, we also found the significant expansion of KLRG1-expressing cells within the tumors. These cells harbored the capacity to repopulate tumors in secondary recipients. We validated the upregulation of KLRG1 expression in human ENKTCL samples through RNA-seq data and immunohistochemical analysis. We then examined whether targeting tumor cells with anti-KLRG1 antibody is effective against murine NK-cell tumors and found that anti-KLRG1 antibody strikingly prolonged the survival of mice transplanted with Trp53^-/- tumors. As the effectiveness of anti-KLRG1 antibody was modest against Trp53^-/-LMP1⁺ tumors, we further explored another therapeutic target and identified further upregulation of MYC target gene signature in Trp53^-/-LMP1⁺ tumors. Therefore, we treated mice bearing Trp53^-/-LMP1⁺ tumors with silvestrol, an eIF4A inhibitor that blocks Myc translation. While silvestrol alone slightly improved the survival, combining it with an anti-KLRG1 antibody further extended the survival.

In conclusion, using GEMM recapitulating the human disease, we have delineated the cell-of-origin and microenvironmental changes and exploited novel therapeutic targets, including CXCL16, KLRG1, and MYC for ENKTCL. These observations prove that our mouse models will serve as valuable tools for elucidating the pathogenic mechanisms and refining diagnostic and therapeutic strategies in ENKTCL.