MYC Oncogene Abrogates Natural Killer (NK) Cell-Mediated Immune Surveillance of B- and T- Lymphoid Malignancies By Suppressing STAT1/2-Type I IFN Signaling

Background: Many high-risk B- and T- lymphoid malignancies including Acute Lymphoblastic Leukemia (ALL) and lymphomas exhibit hyperactivation of the MYC oncogene and MYC-associated pathways. Experimentally, direct targeting of MYC in mouse models of MYC^High lymphoid cancers sustains tumor regression. However, the requirement of MYC in normal lymphocyte physiology has impeded the development of MYC inhibitors. Hence, the development of targeted therapies against MYC^High lymphoid cancers requires the identification of cell-intrinsic and cell-extrinsic (immune microenvironment) processes uniquely regulated by 'oncogenic' MYC (MYC^High B/T-lymphoblasts) but not by 'normal' MYC (MYC^Low B/T-lymphocytes).

Approach: We employed an inducible transgenic mouse model of MYC-driven T-ALL (SRα-tTA/Tet-O-hMYC mice; Felsher and Bishop, Molecular Cell, 1999) to study leukemia-intrinsic, and leukemia-extrinsic immune surveillance mechanisms upon MYC activation (MYC^High/ON, overt T-ALL), and MYC inactivation (MYC^Low/OFF, regressed T-ALL). Inducible regulation of the human MYC (hMYC) transgene specifically in T-lymphoblasts enables us to elucidate how T-ALL-intrinsic MYC impacts normal immune cells during leukemogenesis in vivo. Using mass cytometry (CyTOF), and CIBERSORT to profile the immune microenvironment of MYC^High/ON and MYC^Low/OFF T-ALLs in SRα-tTA/Tet-O-hMYC mice, we identified specific anti- and pro-tumorigenic immune subsets that can be modulated to develop targeted immunotherapies against MYC-driven lymphoid cancers.

Results: By conducting CyTOF-based immune profiling of lymphoid organs in healthy mice, and mice bearing MYC^ON or MYC^OFF T-ALL, we demonstrated a significant reduction in numbers of Natural Killer (NK) cells, and an increase in the absolute counts of neutrophils and dendritic cells (DCs) in MYC^ON mice, in comparison to healthy controls and MYC^OFF mice. The reduction in NK cell numbers in MYC^ON mice led us to hypothesize that the NK subset may play an anti-tumorigenic role in MYC-driven T-ALLs. Since anti-tumor immune subsets can be developed as therapies against MYC-driven lymphoid cancers, we decided to focus on how MYC impacts NK cell-mediated immune surveillance.

We demonstrated that mature CD3^-NKp46⁺ Natural Killer (NK) cells are specifically 'excluded' from the T-ALL microenvironment, in a MYC-dependent fashion. Residual NK cells in MYC^ON T-ALL-bearing mice exhibited suppression of the NK cell maturation/cytotoxicity marker, NKp46. Concordant with the suppression of NKp46 on NK cells in MYC^ON mice, we observed a blockade in early NK cell development from the NK precursor (NKP) to the immature NK (iNK) stage which is marked by the expression of NKp46. Next, we showed that adoptive transfer of mature CD3^- NKp46⁺ syngeneic NK cells alone is sufficient to delay the initiation of MYC^ON T-ALL, and the recurrence of MYC^OFF T-ALL.

Further investigation into the molecular mechanism behind blockade of NK cell maturation in MYC-driven B/T-lymphoid cancers revealed that cancer-intrinsic MYC transcriptionally represses STAT1/2-Type I IFN signaling required for early NK cell maturation from NKP to iNK stage. We observed that treating T-ALL-bearing SRα-tTA/Tet-O-hMYC mice (MYC^ON)with Type I IFN improves survival by rescuing NK cell maturation. We showed that that low expression of both STAT1 and STAT2 in patients with MYC^High B- and T-lymphoid neoplasms correlates significantly with the absence of activated NK cells, and predicts unfavorable clinical outcomes. Of note, aggressive MYC^High B/T-lymphoid cancers are often treated with Type I IFNs, but the molecular mechanisms underlying the anti-cancer properties of Type I IFNs are not completely understood. We demonstrate for the first time that MYC-mediated suppression of NK surveillance may in part be responsible for the sensitivity of B/T-lymphoid cancers to Type I IFN therapy.

Conclusion: We conclude that subversion of NK cell-mediated immune surveillance is critical for MYC-induced leukemogenesis. Our studies thus provide a rationale for developing targeted NK cell-based therapies as alternatives to direct MYC inhibition for treating refractory MYC^High B- and T- lymphoid malignancies.