MYC choreographs the crosstalk between tumor cells and macrophages to drive immune evasion in cancer

The MYC oncogene is one of the most commonly activated oncogenes in human cancer [1]. Hepatocellular carcinoma (HCC), an aggressive malignancy with prognosis, shows MYC activation in upto 70% of patients [1]. The coordinated effects of MYC on a multitude of cellular processes results in enforcement of the hallmarks of cancer. While the role of MYC in regulating cancer cell autonomous functions like cell growth, proliferation and metabolic dysfunction are well characterized [2--4], the mechanisms by which MYC modulates the host immune response are not fully understood. Macrophages are immune cells part of the mononuclear phagocyte innate immune system which play a crucial role in tumor progression and metastasis [5,6]. They are one of the largest immune populations infiltrating tumors and have a strong prognostic and therapeutic role [7]. This dissertation focuses on the mechanistic and therapeutic role of the MYC oncogene in regulating the crosstalk between liver cancer cells and macrophages. Chapter 1 of this dissertation provides a broad overview of the role of the MYC oncogene in enabling the cancer cells to evade host immune surveillance. I describe the major MYC genetic aberrations and alternate mechanisms by which the MYC-related pathways are activated in a majority of cancers. I summarize recent studies which demonstrate how MYC signaling enables tumor cells to dysregulate their microenvironment and reprogram the host immune response. I discuss how MYC pathways not only dictate cancer cell pathophysiology but also suppress the host immune response against that cancer by promoting exclusion of surveilling lymphocytes, regulating the expression of immune checkpoints and reprogramming innate immune cells. I also outline major efforts currently underway to target the MYC pathway to effectively treat cancer. Currently, there are very limited therapeutic options for the management of liver cancer [8]. While MYC is a prominent therapeutic target in HCC, it is generally considered undruggable. In chapter 2 I present data which establishes the therapeutic role of MYC in liver cancer. Antisense oligonucleotides (ASOs) were used to target and reduce the expression of MYC to impede tumor progression and phenotypically elicits oncogene addiction in the transgenic mouse models of MYC-driven primary hepatocellular carcinoma (HCC). I show that the ASOs effectively depleted MYC mRNA and protein expression without significant toxicity. Treatment with MYC ASO in vivo, but not with a control ASO, decreased proliferation, induced apoptosis, increased senescence, and remodeled the tumor microenvironment by recruitment of CD4+ T cells. I further demonstrate that MYC ASO inhibits the growth of human liver cancer xenografts in vivo. Thus, our results illustrate the feasibility of therapeutically targeting MYC in liver cancer. Metastasis is a major cause of mortality in cancer. In chapter 3 I demonstrate how MYC cooperates with the transcription factor Twist1 to promote metastasis in liver cancer. We generated an autochthonous transgenic mouse model whereby conditional expression of MYC and Twist1 enables hepatocellular carcinoma (HCC) to metastasize in > 90% of mice. We show that MYC and Twist1 cooperate to elicit a transcriptional program associated with the expression of a cytokinome that leads to the recruitment and polarization of tumor-associated macrophages (TAM). Systemic treatment with CCL2 and IL13 induced MYC-HCCs to metastasize; whereas, blockade of CCL2 and IL13 abrogated MYC/Twist1-HCC metastasis. Further, I show that MYC and TWIST1 in 33 human cancers (n=9502) predicts poor survival, CCL2/IL13 expression and TAM infiltration. Finally, I conducted a prospective clinical study to confirm that plasma levels of CCL2 and IL13 predicted invasive liver tumors. We conclude that MYC and TWIST1 cooperate to elicit crosstalk between cancer cells and macrophages to drive metastasis. The role of MYC and macrophages in determining response to immune checkpoint therapy is not fully appreciated. In chapter 4, I show MYC oncogene induces a Th2-like immune profile, with reduced CD8 T cell infiltration, activates immune checkpoint expression, and predicts responsiveness to immune checkpoint inhibition in human HCC. I confirm experimentally that MYC-driven tumors suppress pro-inflammatory antigen-presenting macrophages with increased CD40 and MHCII expression, which in turn impedes T cell response. Further, I demonstrate that the MYC-driven suppression of macrophages can be reversed by combined but not individual blockades of PDL1 and CTLA4. Moreover, depletion of macrophages abrogated the anti-neoplastic effects of PDL1 and CTLA4 blockade. Hence, I establish that MYC is a predictor of responsiveness to immunotherapy through suppression of pro-inflammatory macrophages, and this immune evasion can be overcome by combined PDL1 and CTLA4 blockade. MYC inactivation leads to reversible tumor dormancy in liver cancer [9]. The immunogenic mechanisms governing MYC-induced tumor dormancy are illustrated in chapter 5. Using transgenic mouse models of MYC driven liver cancer and human HCC therapy-resistant dormancy, I establish that the TGFβ pathway is activated in quiescent cancer stem cell-like dormant tumor cells. Single cell RNA sequencing and multiplex immunofluorescence demonstrate that immunosuppressive PDL1 expressing macrophages engulf the dormant tumor cells in the perivascular niche and secrete TGFβ1 thus enabling their survival. Lastly, I confirm that inhibition of the both TGFβ and PDL1 pathways is required to eliminate dormant tumor cells and prevent cancer recurrence. I show the translational relevance of these findings using human samples therapy-resistant HCC. Thus, I have identified a novel, therapeutically vulnerable, mechanism governing macrophage-mediated dormancy in MYC-driven cancers. My thesis work deciphers the complex interplay between cancer cells and macrophages in liver cancer and establishes the critical role of the MYC oncogene in orchestrating this crosstalk. The molecular insights gained from this work have strong therapeutic implications for preventing cancer metastasis and recurrence, the two most important drivers of cancer mortality

Advisor: Felsher Dean (Dean Walton)
Oro Anthony
Wang Kevin K. W.