**2. What is an Immunoscore?**

*Multidisciplinary Approach for Colorectal Cancer*

instability pathway (CIN) in left-sided colon cancer.

mixed features and accounts for 13% of the cancers [5, 6].

There are four stages of colon cancer considering their size, number of lymph nodes, and distant metastasis (TNM). Stage 1 comprehends the T1 and T2 tumors (extension to submucosa and muscularis propria), and the treatment is only chirurgical. Stage 2 englobes the T3 and T4 tumors (subserosa, invasion of visceral

anti-inflammatory drugs [4].

There are several risk factors for CRC: inherited predisposition (the involvement of at least one first-degree relative doubles the risk, and the risk is even higher if the affected case was prior to the age of 60), obesity, total caloric intake, red meat, sedentary lifestyle and physical inactivity, alcohol consumption, and prolonged cigarette smoking. A low incidence of CRC is associated with high-fiber diet (it dilutes fecal carcinogens, decreases colon transit time, and generates a favorable luminal environment), fruits and vegetables, aspirin, and nonsteroidal

An anatomic shift is being observed once the incidence of right-sided or proximal cancer is rising. This shift is due to increased longevity, a response to luminal carcinogens and genetic defects like defects in mismatch repair genes with resulting microsatellite instability (MSI) in proximal colon cancers and chromosomal

Since 2015, there are five consensus molecular subtypes (CMS) of CRC [5]. The

CMS1 is the MSI-immune and accounts for 14% of the cancers. This subtype is characterized by proximal colon locations, high BRAF V600E mutation rate, hypermethylation of CpG islands which causes loss of tumor suppressor function, an association with an impaired DNA mismatch repair (MMR) system, an infiltration of immunogenic lymphocytes in the microenvironment, and MSI. MSI cancers are also considered "hypermutated" with approximately 47 mutations per 106 bases, compared to microsatellite stable (MSS or CMS2) tumors which average 2.8/106 bases. The clinical implications of this subtype are that early stage MSI tumors (most CMS1 cancers) have better prognosis than MSS cancers. Stage II cancers with MSI have a low recurrence rate and thus are generally not considered for adjuvant chemotherapy. Patients with stage III MSI tumors do not benefit from fluorouracil monotherapy but are responsive to combination fluorouracil, leucovorin, and oxaliplatin (FOLFOX) adjuvant chemotherapy. CMS1 tumors have a favorable outcome when detected before disease dissemination. In part, the good prognosis may be linked to the presence of specific T-cell populations: CD8+ cytotoxic T lymphocytes, CD4+ activated type 1 T helper cells (Th1), and natural killer cells. However, CMS1 tumors were associated with worse survival after relapse [5, 6]. The CMS2 is the canonical subtype and accounts for 37% of the cancers. This subtype is characterized by a low mutation rate. Five-year overall survival for all stages of CMS2 is the highest, and it has the highest survival rate after relapse. Additionally, CMS2 cancers were more commonly left-sided lesions (59%) [5, 6]. The CMS3 is the metabolic subtype and accounts for 13% of the cancers. This subtype is characterized by RAS mutations (68% of the cancers) which predict poor response to epidermal growth factor receptor (EGFR) monoclonal antibodies (e.g., cetuximab) [5, 6]. The CMS4 is the mesenchymal subtype and accounts for 23% of the cancers. This subtype is characterized by very high pro-inflammatory microenvironment. Additionally, they exhibit extremely low levels of hypermutation and are MSS status. CMS4 cancers, often diagnosed at advanced stages, have a poor prognosis with the worst 5-year overall survival (62%) and relapse-free survival (60%) of any molecular subtype. Although standard adjuvant therapy (FOLFOX) for stage III is recommended, CMS4 cancers show no benefit from systemic adjuvant treatments. For metastatic disease, CMS4 cancers are resistant to anti-EGFR therapy, independent of RAS mutation status. Anti-angiogenesis therapies such as bevacizumab are standard additions for stage IV disease [5, 6]. Finally, the last subtype is the

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The clinical outcome and the tumor progression are now considered the result of a balance between the invasiveness of the tumor and the immune response of the patient against the tumor. The immune system has the ability to control and shape cancer through a mechanism called immunoediting, which include elimination, equilibrium, and escape [7].

It was already shown that the strength of the in situ adaptive immune reaction is strongly correlated with time to recurrence and overall survival of CRC [8]. This in situ immune cell infiltration in cancer, called high density of tumor-infiltrating lymphocytes (TIL), is associated with a favorable prognostic effect [8]. Once cancer becomes clinically detectable, the adaptive immune response plays a critical role in preventing tumor recurrence, metastization, and clinical outcome. A protective response is maintained by the ability of memory T cells to recall previously encountered antigens [9]. Concerning to regulatory T cells (Tregs), Sinicrope et al. showed an association between a low CD3+/FoxP3+ cell ratio and shorter survival [10], but Salama et al. showed the opposite, a high Treg density in the tumor was associated with improved survival [11]. Regarding TH17 and TH1 immune response, TH17 is associated with poor prognosis [12], and TH1 is associated with prolonged diseasefree survival [13].

The consensus Immunoscore is a scoring system that outlines the density of CD3+ and CD8+ T-cell effectors existent in the tumor and its invasive margin. The pre-existing intra-tumoral immunity could be enhanced and activated by immunotherapy. Immunoscore could be a good prognostic marker, by identifying patients at high risk of tumor recurrence and stratifying patients who could benefit from adjuvant therapies [14]. This score is based in the numeration of two lymphocyte populations (CD3/CD45RO, CD3/CD8 or CD8/CD45RO), in density (Cells/mm2) and the location (in the core of the tumor or in the invasive margin) [15]. The score ranges from Immunoscore 0 (I0) when low densities of both cell types are found in both regions to Immunoscore 4 (I4) when high densities are found in both regions.
