Macrotrabecular hepatocellular carcinoma: Unique immunovascular characteristics

HCC is characterized by the histological diversities closely linked to the clinicopathological and biological features. The current World Health Organization classification of HCC defines histological variants, including scirrhous, steatohepatitic, clear, chromophobe, macrotrabecular-massive, lymphocyte-rich, and neutrophil-rich, in addition to the conventional HCC. Among them, macrotrabecular-massive-HCC is characterized by the proliferation of thick tumor trabeculae wrapped by endothelial cells (macrotrabecular [MT] pattern), and associated with elevated VEGF-A and angiopoietin-2 expression and TP53 mutation.1 Although not treated as a distinct histopathological category, vessels encapsulating tumor clusters (VETC) is another histopathological entity characterized by tumor clusters wrapped by endothelial cells, which is associated with elevated expression of angiopoietin-2 and FGF2 and more frequently observed in HCCs with Wnt/β-catenin pathway activation including CTNNB1 mutation.2 The vascular structure in MT and VETC patterns is quite characteristic since the histological polarity between tumor nest and vasculature is inverted in these variants; in usual HCC, sinusoidal vessels form tube-like structures and are surrounded by tumor cells, while in MT and VETC patterns, tumor clusters/trabecular are wrapped by endothelial cells. Since tumor trabeculae and clusters wrapped by endothelial cells may easily bulge into venous blood flow and establish epithelial-mesenchymal transition–independent metastasis, MT and VETC patterns are associated with increased risk of recurrence after surgical resection and worse prognosis.2 Experimental studies have shown that increased expression of angiogenic factors, VEGF-A and angiopoietin-2, can directly induce this dynamic change in tumor vasculature to form MT and VETC patterns (Figure 1).2,3 In addition, tumor angiogenesis is affected by interaction with other stromal components, including immune cells and fibrotic stroma. For example, IFN-γ from activated T cells and CXCL9/10/11 from activated macrophages are potent inhibitors of abnormal tumor angiogenesis, which also contributes to the process of “vessel normalization,” while angiogenic factors, including VEGF-A and FGF2 strongly suppress antitumor immunity by inhibiting the function of dendritic cells and macrophages.4 Accordingly, the development of MT and VETC patterns in HCC is negatively associated with lymphocytic infiltration and IFN-γ–related cytokine/chemokine expression.5,6 This immunovascular interaction is also the target of current combined immunotherapy against advanced HCC (eg, atezolizumab plus bevacizumab [atezo+bev]) and important to predict the therapeutic effect.7 In the current issue of Hepatology, Taniai et al8 have established the murine tumor model of macrotrabecular-massive-HCC and revealed the pivotal roles of 2 additional angiogenic factors, angiopoietin-like protein 2 (ANGPTL2) and IL-11, in the formation of MT pattern (Figure 1). Angiopoietin-like protein family (ANGPTL1-7) consists of secretory glycoproteins structurally similar to angiopoietins but lacks the ability to bind angiopoietin receptors Tie1 and Tie2.9 Among ANGPTL1-7, expression of ANGPTL2 is upregulated in various types of human cancers and reported to increase tumor cell motility and tumor angiogenesis in a different manner from angiopoietins. IL-11 is a member of IL-6 cytokine family whose receptors share the signal-transducing receptor β-subunit, gp130 (IL-6Rβ), and expected to support tumor angiogenesis and inhibit Th1-mediated antitumor immune response like IL-6.10 In their work, Taniai and colleagues have first showed that introduction of both TP53 mutation and MYC overexpression transforms 3T3 tumor into aggressive phenotype histologically characterized by the formation of MT pattern and decreased lymphocytic infiltration (3T3PM tumor). When compared to 3T3 tumor, 3T3PM tumor showed increased expression of ANGPTL2 and IL-11, and the deletion of ANGPTL2 or IL11 gene in 3T3PM tumor was sufficient to inhibit the formation of MT pattern. Interestingly, deletion of ANGPTL2 or IL11 gene in 3T3PM tumor also induced infiltration of T cells, which synergized with anti-PD-1 antibody treatment. In addition, inhibition of angiogenic signaling by tyrosine kinase inhibitor, lenvatinib, also inhibited the formation of the MT pattern and increased the T-cell infiltration in 3T3PM tumor, which was also synergized with an anti-PD-1 antibody treatment. The roles of ANGPTL2 and IL-11 in tumor angiogenesis revealed by Taniai and colleagues may be applicable to other tumor types and clearly important to understand the immunovascular interaction in the microenvironment of human tumors. The work by Taniai and colleagues also poses several important questions in the biology of HCC. First, although the angiogenic factors (VEGF-A, angiopoietin-2, ANGPTL2, and IL-11) and underlying molecular mechanisms (TP53 mutation and MYC overexpression) important in MT pattern formation have been revealed, the driving growth factors/cytokines required for VETC pattern is still largely unknown except for angiopoietin-2. Several studies have shown the correlation among VETC pattern formation, Wnt/β-catenin pathway activation, including CTNNB1 mutation, and FGF2 expression6; however, it is unclear if Wnt/β-catenin pathway activation and FGF2 expression can directly induce VETC pattern formation and future studies will be required. Second, the mechanism of why MT and VETC pattern is associated with lower T-cell infiltration needs to be further elucidated. This may partly be attributed to the suppression of dendritic cells and macrophages by VEGF-A and probably by FGF2 and IL-11, and to the immaturity in the tumor vasculature4; however, the detailed phenotypes of myeloid and endothelial cells and the relationship with other immunosuppressive mechanism in MT and VETC patterns is still largely unclear and may require further research. The recent advancement in the understanding of MT and VETC patterns and the immunovascular microenvironment in HCC will contribute to the more detailed and accurate pathological and radiological assessment of these unique patterns of HCC. As one example, recent studies have shown that heterogeneous enhancement pattern in the arterial phase of contrast-enhanced CT and Gd-EOB-DTPA (EOB)-MRI is associated with MT and VETC patterns in HCC. Histologically, this is due to the nearly complete lack of arterial tumor vessels in MT and VETC regions,11 probably because, in the dilated vascular channels of MT and VETC patterns, tumor vessels may not be selected for developing into arterial tumor vessels. Microenvironmentally, the presence of MT and VETC pattern strongly indicates immunosuppressive microenvironment that may be resistant in immunotherapy.6,8 In the current study, Taniai and colleagues also presented unresectable HCC cases in which image scoring system based on contrast-enhanced CT indicated the presence of MT/VETC, and these tumors responded to atezo+bev therapy or lenvatinib monotherapy followed by atezo+bev therapy. These cases support the importance of combined inhibition of tumor angiogenesis and immune checkpoint molecules. In addition to vascular structure, regulatory T cells and some populations of immunosuppressive macrophages have been revealed as prognostic indicators after atezo+bev treatment;7,12 therefore, further development of methods to noninvasively evaluate tumor microenvironment will be required. This kind of integrative assessment of HCC among pathologists, radiologists, and clinicians will provide further important rationale in making a decision to choose therapeutic regimens from multiple candidates.FIGURE 1: Development of MT and VETC patterns in HCC.