Type 3 Innate Lymphoid Cells Promote Acute Myeloid Leukemia Immune Evasion and Expansion

Acute Myeloid Leukemia (AML) is an aggressive myeloid malignancy originated in the bone marrow with poor overall survival rates of less than 30%. The immune system has been shown to play an important role in detecting and eliminating leukemic cells. Recent studies have described a population of immune cells termed Innate lymphoid cells (ILCs). ILCs are known to regulate both innate and adaptive immunity in an antigen-independent manner and are believed to be among the first immune cells to interact with malignantly transformed cells. Natural killer (NK) cells are a subset of ILCs and are known to inhibit AML progression, however the impact of other ILCs is not yet fully understood. Type 3 ILCs (ILC3s) have key roles in maintaining homeostasis in mucosal immunity through their ability to secrete cytokines including IL-17, IL-22, TNFα and GM-CSF (pro-myeloid). An expansion of ILC3s has recently been observed in solid tumors, however the overall functional impact of ILC3s in the setting of AML remains poorly understood and warrants further investigation.

We first utilized an MLL-partial tandem duplication and FLT3-internal tandem duplication Flt3-ITD double knock-in mutation murine model of AML that recapitulates two clinically relevant mutations in human AML. Using this AML model, we discovered an expansion of ILC3s in the bone marrow of leukemic mice compared to wild-type control mice (2.364% +/- 0.40% AML vs 0.045% +/- 0.40% WT, n=5, p<0.0001). We also observed an increase in ILC3s in the bone marrow of untreated AML patients relative to age matched donor controls (7.013% +/- 2.6% vs. 2.328% +/- 2.6%). Our published data have previously shown AML blasts secrete aryl hydrocarbon receptor (AHR) ligands, which can bind and activate the transcription factor AHR in ILC precursors and mature ILCs. AHR is an important transcription factor in ILC3 development. Therefore, we hypothesized that the ILC3 increase was due at least in part to AHR-mediated increased ILC3 differentiation. To address this hypothesis, we isolated the ILC precursor (ILCP) from normal human donors and co-cultured with AML cells. By regulating AHR activity in this system, we showed that ILC3s are expanded in AML in part due to AML-mediated AHR activation (28.02% +/- 3.89% without AML, 60% +/- 5.5% with AML and 27% +/- 5% AML+AHR inhibitor, n=15, p<0.001).

Next, we sought to determine the direct impact of ILC3s on AML proliferation and differentiation that could contribute to AML relapse. We utilized a colony forming assay (CFU) assay in the presence or absence of ILC3s. Bone marrow from primary untreated AML patients was cultured alone or with ILC3s (Lin-CD94-NKp44+). AML samples co-cultured with ILC3s had significantly increased colony formation relative to AML alone (69.4 CFU AML+ILC3 vs 42.7 CFU AML alone, n=8, p<0.05). Post-culture, we observed significant increases in the transcript of GM-CSF and TNFα in ILC3s co-cultured with AML marrow as compared to ILC3s alone. Furthermore, in our immunocompetent murine AML model, we observed significantly higher production of GM-CSF and TNFα by ILC3s in the bone marrow of leukemic mice relative to nonleukemic controls (GM-CSF: 11.08% +/- 1.6 AML vs 1.128% +/- 1.6 WT, n=10, p<0.0001; TNFα: 5.456% +/- 1.3 AML vs 1.307% +/- 1.3 WT, n=10, p<0.05), suggesting a potential mechanism of ILC3-mediated increased colony formation.

Lastly, to determine if ILC3 expansion impacted NK cell immune surveillance, we co-cultured normal donor human ILC3s with AML cells, separated by transwell, for 48 hours before subjecting the AML cells to an NK cell cytotoxicity assay. We observed significantly reduced NK cell killing of AML cells when co-cultured with ILC3s compared to controls, suggesting ILC3s protect AML cells from NK cell-mediated killing (24.3% killing +/- 3.81% AML alone vs 14.57% +/- 1.35% AML with ILC3, n=18, p<0.01). Collectively, these data support a model in which AML-mediated AHR activation promotes ILC3 expansion to both support AML proliferation and protect AML blasts from immune surveillance. Future studies will focus in exploring how to target these interactions to design effective therapeutic approaches and improve patient clinical outcomes.