## 5 Biomarkers of Response and/or Resistance to Immune Checkpoint Inhibitors
### Markers of Response and/or Resistance in Tumor Genome  
#### 5.1.1 Tumor Mutational Burden  
The immune response against tumor cells is mainly elicited by tumor-specific neoantigen peptides, produced as a result of somatic mutations in cancer genomes and recognized as non-self by the immune system. Mutations in driver genes or passenger mutations, as well as other genomic and post-transcriptomic alterations, can induce the formation of neoantigens [74; 75]. The number of neoantigens has been positively correlated with tumor mutational burden (TMB) [76], and the overall mutational load is an objective but indirect measure of tumor immunogenicity, considered a predictor of tumor-specific T cell immunity [74]. Therefore, it is reasonable to hypothesize that TMB may affect the odds of generating immunogenic peptides and influence ICIs response [16]. Indeed, a high TMB has been associated with tumor immune infiltration, response to ICIs and survival in several solid tumor types in which TMBs of \(>\)10 mutations per megabase (mut/Mb) are common, such as melanoma and non-small-cell lung cancer (NSCLC) [77]. This association \begin{table}
\begin{tabular}{l l l} \hline \hline
**Response** & **Refs** & **Resistance** & **Ref** \\ \hline \hline _Biomarkers of response/resistance in tumor genome_ & & & \\ \hline Activation of IFN signaling pathways & [60; 61] & Copy number alterations (CNA)—loss & [34; 62; 63] \\
4-genes signature & & of heterozygosity of HLA alleles & \\
11-genes signature & & CTNNB1 mutations & [64] \\  & & Activation of WNT/\(\beta\)-catenin pathway & [64; 65] \\ \hline \hline _Biomarkers of response/resistance in tumor tissue_ & & & \\ \hline \hline \(\uparrow\) T cell infiltration & [34; 66] & \(\uparrow\) cancer-associated fibroblast & [34] \\ M1 macrophage polarization & [8; 33; 34] & \(\uparrow\) stromal activation & [33; 34] \\ \(\uparrow\) cytotoxicity & [34] & M2 macrophage polarization & [33; 34] \\ \(INF_{\gamma}\), \(PRF1\), \(GZMB\), \(TBX21\), \(KLRK1\) & & \(\uparrow\) CD8+ PD1+ T cells & [67] \\ \(\uparrow\) Immune cell homing & [34; 61] & \(CCL5\), \(CCL3\), \(GZMK\), \(GZMA\), \(CXCR6\) & \\ \(CXCL9\), \(CXCL10\) & [34] & \(\uparrow\) CD4 Treg cells & [34] \\ Antigen presentation—dendrtic cells & [35; 40; 41; 61] & \(\uparrow\) A42 PD-1\({}^{+}\) tumor-infiltrating T cells & [68] \\ PD-L1 expression in cancer cells and/or immune cells & & & \\ \hline \hline _Biomarkers of response/resistance in blood_ & & & \\ \hline \(\uparrow\) Baseline CD137 & [66] & Antidrug antibodies (ADAs) & [3; 70] \\ \(\downarrow\) AFP during treatment & [69] & & \\ \hline _Biomarkers of response/resistance associated to the host_ & & & \\ \hline Gut microbiota diversity & [71] & Etiology—\(NASH\) & [60; 67; 72] \\ \hline _Clinical markers of immunotherapy activity_ & & & \\ \hline Development of immune-related adverse events & [73] & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Biomarkers of response to immune checkpoint inhibitors in advanced HCC. has been confirmed in several other tumor types [78], and a meta-analysis including the data of >1000 ICI-treated patients demonstrated that TMB was the strongest predictor of response to therapy [79]. Therefore, the association between mutational burden or neoantigen load and patient survival after immunotherapy is definitely established for a large share of cancer types [77]. By contrast, the utility of TMB as a genomic biomarker is limited in HCC [80; 81], due to a low mutational burden (<3 mut/Mb) and a lack of association between high TMB and greater immune infiltration [33]. The presence of molecular alterations impairing the antigen presenting system such as loss of heterozygosity or hypermethylation of HLA, that hinder the recognition of cancer cells by the immune system, might explain these findings [62; 82; 83]. Indeed, in patients with HCC responding to ICIs, MHC II molecules are upregulated [84].