## 5 Biomarkers of Response and/or Resistance to Immune Checkpoint Inhibitors
### Markers of Response and/or Resistance in Tumor Tissue  
#### 5.2.1 Immunogenomic Classification in the Prediction of Response to ICIs  
In recent years, some studies attempted to create a classification of HCC according to tumor immune features [3]. A study performed an immunogenic analysis based on the clustering of immune-related gene-expression signatures in several cancers, identifying six immune subtypes. HCCs were predominantly classifiable in the \"Inflammatory\" and the \"Lymphocyte depleted\" subclasses [93]. The first study specifically investigating HCC samples was performed by [PERSON] et al. in 2017 [33] and analyzed the gene expression pattern of inflammatory cells infiltrating TME. It identified an \"Immune\" class, accounting for about 25% of HCCs, characterized by markers of a significant inflammatory response; in particular, a high level of immune infiltration, high expression of immune checkpoints, and enrichment in the signatures involved in responsiveness to immunotherapy in other solid tumor types characterize this HCC group. In this \"immune\" class, two subtypes were identified according to distinct TME characteristics: (1) an \"Immune Active\" subclass, with high tumor infiltrating lymphocytes (TILs), increased CD8\({}^{+}\) T cell and M1 macrophages infiltration, activated IFN signaling, and overexpression of adaptative immune response related genes; tumors pertaining to this subclass demonstrate favorable prognosis and better response to ICIs; and (2) an \"Immune Exhausted\" subclass, characterized by an activated stroma, increased infiltration of M2 macrophages and exhausted T cells, and high levels of TGF8 signaling [33].  
Some refinements have been recently introduced in this classification after the identification of an \"Inflamed\" class of HCC [34]. This class, beyond including both the previously described \"Immune\" subclasses (\"immune active\" and \"immune exhausted\"), is expanded by the addition of the newly defined \"Immune-like\" subclass, which has features very similar to the \"Immune active\" subclass (high activation of interferon signaling, cytolytic activity, expression of immune-effector cytokines, and a more diverse T cell repertoire), but it is associated with the presence of _CTNNB1_ mutations and increased WNT-\(\beta\)catenin signaling. [PERSON] et al. [84] recently showed that HCCs belonging to the \"Inflammatory class\" are frequent among patients having clinical benefits from anti-PD-1/PD-L1 the apy. The proportion of inflamed HCCs is approximately 35%, a percentage higher than that reported in several malignancies (intrahepatic or extrahepatic cholangiocarcinoma, 11% [94; 95]; small-cell lung cancer, 17% [96]; melanoma, 23% [97]), but lower than in others (squamous head and neck cancer, 40% [98]). The remaining 65% of \"non-inflamed\" HCCs are characterized by a low level of immune infiltrate, downregulation of immune checkpoint molecules, and low activation of interferon signaling. These tumors can be divided in two subclasses according to their immune escape mechanisms: (1) an \"Intermediate\" class, with a high level of chromosomal instability and, consequently, frequent deletions of immune-related genes (involved in IFN signaling or antigen presentation), and frequent _TP53_ mutations; and (2) an \"Excluded\" class characterized by _CTNNB1_ mutations, with increased WNT-\(\beta\)catenin signaling and _PTK2_ overexpression due to gene amplification and promoter hypomethyllation. The hallmark of the \"Excluded\" subclass is the presence of characteristics of immune desertification [34]. A schematic representation of HCC immune classes is shown in Table 3.