Cancer immunotherapy is based on the idea that the host immune system can be stimulated to eliminate malignant cells. The discovery of immune checkpoints and immune checkpoint inhibitors (ICI) was a breakthrough in the history of medical oncology. Immunotherapy for colorectal cancer (CRC) began 40 years ago, with the development of levamisole for surgically treated localized CRC. This antihelmintic was found to display immunomodulatory effects and was evaluated in adjuvant setting for CRC patients in the 1980s [1
]. Positive results reported by Mortel and colleagues with levamisole plus 5-fluorouracil paved the way for a decade of clinical trials testing various combinations of these two compounds, until the emergence of leucovorin plus 5-fluorouracil regimens [3
]. Adding levamisole to this new standard failed to enhance outcomes, and this immunomodulatory agent disappeared from the therapeutic arsenal [6
In the era of immune checkpoint blockade, CRC holds a singular position, with a minority of tumors being highly sensitive to ICI (i.e., CRC with microsatellite instability (MSI) and/or mismatch-repair deficiency (dMMR)), but a vast majority of cold CRC refractory to these compounds [8
]. MSI is a molecular phenotype related to a deficient DNA MMR system, resulting from MMR gene germline mutations (i.e., Lynch syndrome) or from an epigenetic silencing of the MMR system (i.e., sporadic cancer), the latter being frequently associated with the BRAFV600E
mutation. Compared to MSS/pMMR (microsatellite stable/proficient MMR) CRC, MSI/dMMR tumors are also characterized by: poor differentiation, mucinous component, proximal location, female and older age (but also young patients for Lynch-related cases), distant lymph node metastases, and peritoneal carcinomatosis [9
]. MSI/dMMR has long been used in adjuvant setting as a positive prognostic parameter and as a pre-screening test for Lynch syndrome. It is also used for therapeutic management since fluoropyrimidines alone are not indicated in the adjuvant setting for patients with stage II dMMR CRC, given their favorable survival and the lack of impact of chemotherapy in this situation [10
]. In contrast, adjuvant therapy with a fluoropyrimidine alone is the standard of care for MSS/pMMR stage II CC harboring high-risk features. For stage III diseases, the association of a fluoropyrimidine and oxaliplatin for 3–6 months is recommended, whatever the MSI/dMMR status [12
MMR deficiency is not, per se, a direct transforming event. Most genetic alterations found in dMMR tumors are somatic events that occur as a result of MSI. The MSI-driven oncogenic pathway leads to a high tumor mutational burden, with highly immunogenic neoantigens arising from frameshift mutations. As a consequence, MSI tumors are highly infiltrated by cytotoxic T lymphocytes [15
]. Two signals are required to initiate an adaptive immune response by T cells: MHC-antigen peptide recognition by the T cell receptor and co-stimulation via an array of receptors interacting with cognate ligands on antigen-presenting cells. It has been shown that MSI tumors are likely to persist in their hostile microenvironment because of immunoescape and the dramatic overexpression of immune checkpoints [15
]. Based on these findings, ICI have been developed in MSI/dMMR tumors.
While outcomes with ICI-treated CRC patients were initially disappointing in basket trials, results from phase II biomarker-guided, non-randomized trials were, in contrast, highly impressive for MSI/dMMR metastatic CRC [19
]. Compared to standard of care chemotherapy ± bevacizumab or cetuximab, pembrolizumab as first-line therapy for patients with MSI/dMMR metastatic CRC is associated with a clinically meaningful and statistically significant improvement in progression-free survival, and should consequently become the new standard of care for these patients [24
MSI/dMMR has now become a major predictor for the efficacy of ICI. Given its frequency in localized colon cancer (CC) (10–15% compared to 5% of metastatic CRC), the development of ICI in adjuvant setting may concern a sizable group of patients [25
]. Moreover, results of the NICHE trial that showed impressive results with preoperative nivolumab plus ipilimumab for early stage MSI/dMMR CC but also for MSS/pMMR CC, has generated considerable attention for the implementation of neoadjuvant immunotherapy in CC [27
In this review, we will focus on the key issues at stake in the development of ICI for patients with resected CC, with a particular interest for the MSI/dMMR population. Finally, we will highlight potential strategies to expand the use of ICI in localized CRC beyond the MSI/dMMR phenotype.
4. Immunotherapy for Localized CC Beyond MSI/dMMR
4.1. The Other Lesson From the NICHE Study: Neoadjuvant Immunotherapy for MSS/pMMR Tumors
The NICHE study provided hypothesis-generating data for patients with MSS/pMMR tumors [27
]. Four of 15 evaluable pMMR CC patients (27%) treated with nivolumab plus ipilimumab had a pathological response, with two complete pathological responses and one case harboring 1% of residual viable tumor. Four other patients had some evidence of a pathological response. Celecoxib did not seem to improve sensitivity to ICI treatment. Interestingly, responses in pMMR tumors were observed despite low TMB and low number of insertions-deletions (indels). This response rate among pMMR tumors is striking, even more so since the clinical activity of ICI for MSS/pMMR metastatic CRCs has been clearly disappointing [19
]. These results confirm data observed in other cancer types, that early-stage cancers may be more responsive to ICI, especially as neoadjuvant treatment [73
In this small cohort, the only biomarker found to predict response among pMMR tumors was the presence of T cells with co-expression of CD8 and PD1. Other potential factors such as CD3+ CD8+ FOXP3+ T cell infiltration, TMB, interferon-gamma score, tertiary lymphoid structures, TMB or the consensus molecular subtype (CMS) classification did not significantly differ between pMMR responders and non-responders. Nevertheless, the NICHE study clearly shows there is a window of opportunity for immune checkpoint inhibitors as neoadjuvant treatment for patients with early-stage MSS/pMMR CC.
4.2. Selecting Patients According to the Immune Microenvironment
The analysis of the microenvironment might be valuable for the selection of patients who might benefit from immunotherapeutic strategies. Currently, the only data concerning immunotherapy for early-stage CC originate from the NICHE study that detected a significant correlation between pathological response to neoadjuvant ICI and the CD8+ PD1+ T cell infiltrate in pMMR tumors.
Different methods of immune microenvironment evaluation have been developed for localized CC [17
]. They provide important prognostic information: patients with low infiltration of CD3+ and CD8+ T cells exhibiting a higher risk of relapse, whatever the MSI/dMMR status [74
]. The analysis of the Immunoscore®
in the IDEA France trial confirmed its strong prognostic value in stage III CC, independently of the MSI/dMMR status [77
]. The three-year disease-free survival was 66.80% (95%CI 62.23–70.94) and 77.14% (95%CI 73.50–80.35) for patients with low or intermediate to high immunoscore, respectively. Moreover, a predictive value of the Immunoscore®
for disease-free survival benefit with a longer duration of FOLFOX (six months vs. three months) was detected for patients with an intermediate to high score. On the opposite, a lack of benefit from six-month FOLFOX was observed in patients with low immunoscore®
These results show that a higher risk of relapse or death does not necessarily translate in a higher efficacy of adjuvant treatment. They highlight the Immunoscore® as a potential useful tool to guide immuno(chemo)therapeutic strategies for early-stage CC, in both MSI/dMMR and MSS/pMMR populations. Nevertheless, except for the data about the CD8+ PD1+ T cell infiltrate on the 15 pMMR tumors from the NICHE study, there is currently no published data that may support the assumption that the evaluation of tumor-associated inflammatory microenvironment is predictive for the efficacy of ICI for patients with MSS/pMMR CC.
4.3. Targeting Tumors with High Tumor Mutation Load: Polymerase-Mutated CCs
Emerging literature shows that high TMB is predictive for the efficacy of ICI. In the context of CRC, the vast majority of hypermutated tumors are MSI/dMMR. In the TCGA CRC cohort (n
= 276), 16% of samples (n
= 44) exhibited a hypermutated phenotype (defined as a TMB greater than 12 mutations per 106
bases). Among these, 37 were MSI. All MSS hypermutated tumors (n
= 7) harbored exonucleasic proofreading domain POLE
-mutations (DNA epsilon polymerase) [78
]. Similarly, in a study by Stadler and colleagues, 31 out of 224 CRC samples were hypermutated, of which 3 were MSS and displayed a deleterious POLE
-mutation CRC usually exhibits a MSS/pMMR phenotype, but are “ultra-mutated”, with a higher TMB than MSI/dMMR cancers [78
]. These tumors are highly sensitive to immune checkpoint tumors [82
]. They arise from the left colon and rectum, in younger males. Though, polymerase-mutated CRCs are infrequent (<1%) and associated with favorable outcomes, with rare metastatic relapse [86
]. Therefore, the added value of ICI compared to surgery alone and/or surgery followed by conventional adjuvant chemotherapy might be hard to prove. It is noteworthy that patients with CRC harboring polymerase epsilon mutation are eligible for the POLEM trial [table].
All in all, the overlap between hypermutated CC and MSI/dMMR or POLE
-mutated tumors seem complete. Given the technical pitfalls related to the assessment of TMB, the added value of this biomarker to MSI/dMMR and POLE
mutation testing is uncertain. Still, since not all POLE
mutations seem to be driving hypermutagenesis, with a hypermutated phenotype being restricted to specific hotspots, the evaluation of TMB might provide useful “functional” information [81
Immune checkpoint blockade represents a major therapeutic breakthrough for cancer patients. The development of ICI in adjuvant situation, alone or in combination standard chemotherapy, for early CC faces several challenges.
For the MSI/dMMR population, the favorable prognosis associated with MSI/dMMR status in stage II and III CC, with few DFS events (relapse or death), raises questions about the feasibility of such studies, with the necessity of including a large number of patients to detect a small therapeutic effect. The good prognosis of early stage MSI/dMMR CC highlights the question of the benefit–risk balance of adjuvant therapies. Because of the high efficacy of ICI in MSI/dMMR CRC, one question concerns what should the experimental arm of a study be for this population of patients: ICI alone or combined with oxaliplatin-based chemotherapy? To conduct a study of adjuvant ICI therapy, with a reasonable number of patients and a chance to improve DFS, it is probably more feasible to focus on patients with T4 or N2 MSI/dMMR CC, for whom the expected magnitude of effect is high. Selecting patients with persistent circulating tumor DNA after surgery and a high risk of relapse might be a seductive strategy for which clinical studies are ongoing. Concerning MSI/dMMR rectal cancers, neoadjuvant strategies with ICI and/or chemoradiotherapy need to be evaluated in clinical trials.
For MSS tumors, new predictive biomarkers (tumor infiltrating lymphocytes, TMB or others) are required. In this context, the consideration of the immune microenvironment seems to be the next step to take, at least as a prognostic parameter, eventually as a therapy-guiding biomarker if its predictive value for ICI efficacy is demonstrated. Finally, one should keep in mind that designing biomarker-guided clinical trials is fraught with specific challenges that have to be addressed, especially in the context of immune checkpoint blockade.