**1. Introduction**

According to the GLOBOCAN database, colorectal cancer (CRC) represents the second most frequent cancer type diagnosed in women and the third in men. Globally, the highest incidence rates of CRC are seen in New Zeeland, Australia, North America, and Europe [1]. In contrast, the lowest incidence is found in South-Centre Asia and Africa. The existing discrepancies among geographic regions are mainly attributed to lower screening rates in undeveloped countries, socioeconomic status, lifestyle, and dietary disparities [2]. Age is considered a risk factor for CRC. However, recent epidemiologic studies reported an increased incidence in people under 50 years old due to lifestyle changes and genetic implications [3].

Despite the sustained efforts focused on developing new treatment options for CRC, metastatic CRC (mCRC) patients still have a very poor prognosis [4]. For advanced and metastatic CRC treatment, the breakthrough was the addition of oxaliplatin and irinotecan to the original 5-fluorouracil (5-FU) regimen. The combination almost doubled the survival rates and has been the standard of care for more than 20 years. The addition of targeted agents, such as bevacizumab (anti-VEGF), panitumumab, and cetuximab (anti-EGFR), further increased the efficacy of the treatment [5]. In recent times, treatment strategies focused on altering the immune system, like immune checkpoint inhibitors (ICIs), have made their way into oncology practice after showing promising results in solid tumors like melanoma and lung cancer. These approaches have been demonstrated to be less effective in CRC patients [6]. However, a better understanding of the tumor immune contexture and CRCs' molecular subtypes demonstrated that a specific subset of patients having a hypermutated phenotype might benefit from ICIs [7]. Mainly, these tumors are distinguished by a robust immune activation and high microsatellite instability (MSI-H) due to dysfunctions of the mismatch repair (MMR) genes-dMMR. By contrast, in tumors with low microsatellite instability (MSI-L) and proficient mismatch repair (pMMR) function, ICIs are ineffective [8]. To date, many novel combinatorial approaches have been researched in order to overcome the relative resistance seen in CRCs.

This chapter aims to overview the immune landscape and immunotherapeutic strategies in CRC.

### **2. Immune landscape of colorectal cancer**

The pathogenesis of CRC is a very complex multistep event linked to the accumulations of both the epigenetic and genetic alterations [9]. Other exogenous factors, including lifestyle, diet, and microbiota, contribute to this process [10]. Moreover, another essential aspect correlated with CRC development is the host immune dysfunction, primarily relying on escape mechanisms and immune evasion, which create a favorable environment for tumor growth [11]. The immune system can distinguish tumor antigens after their presentation via major histocompatibility complex (MHC) proteins present on antigen-presenting cells adenomatous polyposis coli to T cell receptors (TCR) found on the surface of T cells. The interaction between MHC proteins and TCR is insufficient for T cell activation. These pathways are further modulated by co-inhibitory and co-stimulatory signals, which tumor cells exploit to evade recognition and destruction [12, 13]. Among the co-stimulatory molecules that positively influence T cell activation and expansion after interaction with their ligands, we mention CD80 and CD86, found on cancer cells or APC. Other co-stimulatory molecules recently described include 4-1BB, GITR, and X40 [14].

On the other hand, co-inhibitory molecules, including cytotoxic T lymphocyte antigen 4 (CTLA4), programmed cell death protein-1 (PD-1), LAG-3, and TIM-3, antagonize the effects mentioned above upon interaction with their ligands. These signaling pathways prevent excessive immune responses and autoimmune phenomena [15]. Tumor cells often hijack these mechanisms, overexpress co-inhibitory molecules, which promote the activation of immunosuppressive regulatory T cells (Treg) instead of effector T cells (Teff), and, therefore, evade immune surveillance [16].

ICIs using anti-PD1, anti-PD-L1 (programmed cell death protein-ligand 1), and anti-CTLA4 molecules have been successfully used in various cancer types to *Immunotherapy for Colorectal Cancer in the Era of Precision Medicine DOI: http://dx.doi.org/10.5772/intechopen.105377*

promote an effective antitumor immune response and overcome immune evasion mechanisms (**Figure 1**).

It was initially assumed that CRC is not an immunogenic cancer type, and therefore, immunotherapy would not be successful in this setting. Further studies identified a subset of patients harboring MSI-H/dMMR phenotype that could benefit from these therapeutic strategies [17]. Mutations in MMR genes are associated with microsatellite instability (MSI) and, therefore, a high tumor mutational burden. Consequently, these tumors contain an increased number of neoantigen, which will be recognized as foreign and will generate a robust immune response by the host. Moreover, MSI-H/dMMR tumors are characterized by the upregulation of immune checkpoints (PD-1 and PD-L1), which further enhances immune evasion [18].

### **2.1 Colorectal cancer molecular subtypes**

Furthermore, CRC has been classified into four consensus molecular subtypes (CMS) to correlate the tumor phenotype with the clinical behavior and guide treatment. CMS1 (MSI immune subtype, 14%) tumors are frequently located in the proximal colon and are characterized by an increased immune infiltration in the tumor microenvironment (TME) (particularly CD8+, CD4+, and NK). In addition, these tumors have a high BRAFV600E mutation rate, are hypermethylated, and are associated with an impaired MMR system [19]. Owing to their particular phenotype, the immune-activated CMS1 subgroup has a clinical benefit from treatment with ICIs.

The CMS2 subtype (canonical, 37%) result from the canonical adenoma-tocarcinoma sequence. This cell phenotype is typically characterized by loss of tumor suppressor gene adenomatosis polyposis coli, followed by Kirsten rat sarcoma virus (KRAS) mutation and TP53 loss [20]. Moreover, these tumors present with low levels of hypermethylation and microsatellite stability (MSS). The CMS2 subtype is also characterized by the activation of WNT and MYC pathways, high expression of oncogenes epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), and a significant risk of distant relapse. However, CMS2 tumors have the highest 5-year overall survival (OS), at 77% among all the subtypes [21].

**Figure 1.** *Mechanism of anti-PD-1 antibodies.*

CMS3 tumors (metabolic, 13%) have a chromosomal instability (CIN) genomic phenotype but with fewer copy number alterations. 30% of these tumors have microsatellite instability and an intermediate gene hypermethylation level. Moreover, CMS3 tumors are enriched with Kirsten rat sarcoma virus (KRAS) mutations [19, 20].

CMS4 (mesenchymal, 23%) has a phenotype distinguished by the activation of pathways associated with epidermal-mesenchymal transition (EMT) and by the overexpression of proteins involved in complement signaling and extracellular matrix remodeling [22]. The tumor microenvironment of CMS4 tumors is pro-inflammatory, with high levels of Treg, T helper, and myeloid derivated suppressor cells. CMS4 tumors are often diagnosed in advanced stages, have a poor prognosis, and show no benefit from adjuvant chemotherapy. Regarding the metastatic setting, CMS4 tumors are resistant to anti-EGFR, independently of KRAS status [23].

In a recent translational study of over 1700 tumor samples, 55% of them had ≥2 CMS subgroups, suggesting that intratumoral heterogenicity is a common finding [24]. However, intratumoral heterogenicity was associated with worse OS and reduced disease-free survival (DFS) [25].
