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Colorectal cancer (CRC) is one of the most common cancers worldwide. The patient’s prognosis largely depends on the tumor stage at diagnosis. The pathological TNM Classification of Malignant Tumors (pTNM) staging of surgically resected cancers represents the main prognostic factor and guidance for decision-making in CRC patients. However, this approach alone is insufficient as a prognostic predictor because clinical outcomes in patients at the same histological tumor stage can still differ. Recently, significant progress in the treatment of CRC has been made due to improvements in both chemotherapy and surgical management. Immunotherapy-based approaches are one of the most rapidly developing areas of tumor therapy. This review summarizes the current knowledge about the tumor microenvironment (TME), immune response and its interactions with CRC development, immunotherapy and prognosis.
Uncontrollable proliferation and metastasis of cancer cells within major organs, such as the liver and lung, constitute the leading causes of death in CRC patients (
We searched for keywords “immunotherapy colorectal cancer”, “tumor infiltrating lymphocytes colorectal cancer” and “PD-L1 colorectal cancer” in the database PubMed, which gave us a total of 794 studies. After discarding duplicates, we obtained 320 studies that were scanned. We excluded 240 studies unrelated to the topic, thus acquiring 65 studies that were used as a basis for this review. Further 44 papers were identified by cross-referencing.
Three major pathways of CRC tumorigenesis have been thoroughly described: chromosomal instability (CIN), microsatellite instability (MSI), and the CpG island methylator phenotype (CIMP) pathway (
The third pathway, MSI, is a result of DNA mismatch repair deficiency (dMMR) caused by the inactivation of genes encoding proteins responsible for repairing DNA errors, mainly occurring during replication. The prognosis of tumors developing through MSI pathway is better than that of other pathways (
The immune system plays an important role in cancer control and treatment response and the presence and quantity of key immune cell subtypes within the TME of CRC are known to possess prognostic potential (
CTLA-4 is considered the “leader” of immune checkpoints as it stops potentially autoreactive T cells as soon as the initial stage of naïve T-cell activation, typically in lymph nodes (
A core concept in cancer immunotherapy is that tumor cells, which would normally be recognized by T cells, have developed ways to evade the host immune system by taking advantage of peripheral tolerance and hiding from immune recognition (
PD-1 and PD-L1 are crucial checkpoints of the immune system response. They provide negative feedback that inhibits T helper 1 (Th1) cytotoxic immune responses, causing T-cell exhaustion or tolerance (
Recent reports indicated that while PD-L1 expression on CRC tumor cells was commonly associated with poor outcome or was not prognostic, lymphocytic expression of PD-L1 and PD-1 in CRC was associated with good clinical outcome, particularly in certain colon locations (
Inhibition of the PD-1/PD-L1 signaling is a feasible strategy for normalizing the TME and up to now, this approach has been used in the treatment of various cancers, such as melanoma, non-small cell lung cancer, gastric cancer, renal cancer, liver cancer, urothelial cancer, lymphoma, and all microsatellite instability-high (MSI-H) cancers (
Clinicopathologically, PD-L1 positivity, as well as PD-1 density, tend to associate with proximal tumor location and poor differentiation (
According to recent resources, the expression of PD-L1 in immune cells is significantly higher in MSI CRCs when compared to non-MSI tumors (
Chronic T cell activation, which may occur in persistent infections or cancer following repeated exposures to antigens, is also associated with an increased expression of inhibitory receptors such as PD-1 and CTLA-4, which leads to T cell dysfunction compromising productive T cell responses (
In addition to PD-1/PD-L1 signaling inhibition, immunotherapy relying on the combination of multiple checkpoint targets is another viable possibility for (not only) CRC treatment. Fiegle et al. showed that the dual CTLA-4 and PD-L1 blockade exerts synergistic inhibitory effects on the growth and metastasis of CRC in orthotopic mice by increasing the number of CD8+ and CD4+ T cells. This increase is associated with a Th1 response mediated by CTLA-4 inhibition and by inducing a higher number of M1 macrophages, which can mostly be ascribed to PD-L1 blockade (
During T cell activation, PD-1 is expressed on the surface of T cells as well as B cells and NK cells (
Also, intratumoral heterogeneity needs to be considered, in particular the differences between the tumor center and tumor margin. Kwak et al. demonstrated that the higher total number of CD3+ and CD8+ T cells in the immune infiltrate are predictive of survival. This is true for both the invasive margin and tumor center; the presence of these T cells in both these parts of the tumor further improves the prognosis (
Some studies found that a high total lymphocyte score predicted cancer-related survival more accurately than any of the individual lymphocyte scores analyzed separately (
There are, of course, also other immune cells with specialized functions. NK cells are involved in the lysis of tumor cells that lose MHC class I expression (
B cells can modulate immune responses through various mechanisms, including inhibition of T cell responses (
The innate immune components of the microenvironment represent the first line of defense, providing rapid response to foreign factors as well as anti-tumor activities (
Dendritic cells (DCs) are professional antigen-presenting cells and their interrupted development allows tumor cells to evade immune recognition. To avoid immune surveillance, cancer cells may suppress DCs through multiple mechanisms, including the secretion of immunosuppressive TGF-β or IL-10 (
Mast cells (MCs) are multifunctional cells whose role is to participate in allergic and infectious events. In cancers, their presence has been described mainly at the tumor margins and at peri-vascular regions. It was associated with increased blood vessel density in the TME (
Myeloid-derived suppressor cells (MDSC) represent a heterogeneous population of granulocytes and monocytes that rapidly expand during infection, inflammation, and cancer (
It follows that most of the immune cells may actually have a dual activity—anti- and pro-tumor, depending on the signals received from the TME. These signals may support anti-tumorigenic functions or modulate immune cells into a protumorigenic phenotype. This knowledge reflects the potential and importance of the role of TME and the potential of treatment approaches targeting TME for the treatment of CRC. As an example, we can mention therapies blocking CTLA-4, PD-1/PD-L1 pathways, those limiting monocyte infiltration, reprogramming polarization of TAMs, blocking secretion of immunosuppressive TGF-β or IL-10 by cancer cells, or inhibiting the activity of MDSCs.
Various mechanisms have been proposed to explain how the TME contributes to tumor progression, tumor invasion, and metastasis. For instance, the following mechanisms were proposed: 1) impacting the proliferation and survival of cancer cells; 2) increasing their stem-like properties and favoring epithelial-to-mesenchymal transition (EMT); (
Cancer-associated fibroblasts (CAFs), non-neoplastic cells present in the tumor, are a major contributor to EMT. CAFs produce desmoplastic reaction (DR), commonly observed in invasive CRC. Desmoplasia is the rearrangement of organized, anisotropic extracellular matrix fibers in a pathological microenvironment associated with metastasis or poor patient survival (
Evaluation of the amount and type of TIL seems to be a suitable complement to the standard TNM classification, especially when deciding on adjuvant therapy. The immunoscore (IS) methodology quantifies and detects various types of immune cells in tumor tissue, and, besides, determines the density of their infiltration and localization in the center and invasive margin. It offers ratings from 0 (low immune system cell infiltration in both areas) to 4 (high immune system cell infiltration in both areas). There are two advantages to this examination: 1) IS appears to be a strong prognostic factor for disease-free survival and overall survival and 2) it has biological significance (adaptive and also an innate immune response to the presence of tumor cells). It can, therefore, be also used as a tool for the management of therapy, including immunotherapy (
Landmark studies indicating the value of different cell populations in predicting CRC prognosis.
Cell population | Author | Cancer location | TNM stage | Sample size | Main results |
---|---|---|---|---|---|
CAF | Akishima-Fukasawa et al. ( |
I–III | 110 | PGP9.5 expression is an independent prognostic factor for overall and recurrence-free survival. | |
CAF | Glentis et al. ( |
CRC and adenoma | Not defined | 40 | CAFs actively assist cancer cells to breach the basement membrane. |
CAF | Ren et al. ( |
CRC | Not defined | not defined | CAFs promote the stemness and chemoresistance of CRC by transferring exosomal H19. |
CAF | Zhang et al. ( |
CRC | Not defined | not defined | Colorectal-cancer-CAFs-derived HGF induced up-regulation of CD44 which mediated adhesion of CRC cells to endothelial cells, and subsequently resulted in enhancement of metastasis of CRC. |
CAF | Miyazaki et al. ( |
CRC and breast cancer | Not defined | not defined | The direct interaction with CAFs, as well as environmental cytokines, contributes to the collective invasion of cancers. |
CAF | Unterleuthner et al. ( |
CRC | Not defined | not defined | WNT2 has a pivotal role in sustaining an activated CAF phenotype, which is associated with the maintenance of a pro-angiogenic secretome and contributes to elevated tumor angiogenesis in CRC. |
DC | Bauer et al. ( |
CRC (MSI-H and MSS) | Not defined | 69 | Impaired DC maturation may contribute to local immune evasion in CRC. |
DC | Gulubova et al. ( |
CRC | I - IV | 145 | The infiltration of colon cancer with DCs is related to tumor progression and patient prognosis, suggesting a central role of DCs in controlling local tumor immunity. |
DC | Hu et al. ( |
CRC | not defined | 19 | Treatment with anti-PD-L1 may promote the maturation of Dcs and enhance the functionality of DC1 subtype. |
DC | Miller et al. ( |
CRC | III | 221 | PD-L1-expressing DC in the tumor microenvironment are associated with improved survival in stage III colon cancer and likely reflect an immunologically “hot” tumor microenvironment. |
B cells | Berntsson et al. ( |
CRC | I–IV | 557 | Dense infiltration of CD20+ B-cells is an independent predictor of a favourable clinical outcome. |
B cells | Edin et al. ( |
CRC | I–IV | 316 | There is a positive prognostic role of tumour-infiltrating CD20+ B lymphocytes in CRC patients. |
B cells | Mullins et al. ( |
CRC and rectal cancer | III–IV | 25 | Tumor-infiltrating B cells can contribute to tumor control in a dula role of sole antigen-presentation and additionally anti-tumoral Ig-production. |
B cells | Toor et al. ( |
CRC | I–IV | 50 | Decreased levels of B cells and selective IC-expressing CD8+ TILs are associated with tumor progression. MSI-H tumors could show favorable prognosis/improved response to cancer immunotherapy. |
T cells | Li et al. ( |
CRC | I–IV | 356 | Higher expressions of PD-1 and PD-L1 correlates with a better prognosis in CRC patients. |
T cells | Berntsson et al. ( |
CRC | I–IV | 557 | A high density of cytotoxic T cells is an independent prognostic factor in right-sided tumours and regulatory T cells predict longer survival only in patients with rectal tumours. |
T cells | Digiacomo et al. ( |
BRAF-mCRC | IV | 59 | A simultaneous evaluation of MSI, CD8 T-cell content, and neuroendocrine markers could allow for the identification of subsets of BRAF-mCRC with a different prognosis and potential eligibility for specific treatments. |
T cells | Glaire et al. ( |
CRC | II–III | 1804 | The prognostic value of intratumoral CD8+ cell infiltration in stage II/III CRC varies across tumour and nodal risk strata. |
T cells | Fiegle et al. ( |
Mouse model of CRC | 25 | Dual CTLA- and PD-L1 blockade exert synergistic inhibitory effects on growth and metastasis of the orthotopic CT26 colon tumors by increasing CD8+ and CD4+ T cells. | |
T cells | Kuwahara et al. ( |
CRC | I–IV | 342 | Intratumoral CD4+ T-cell density and combined CD4+ and FOXP3+ T-cell densities were stronger prognostic factors than other clinicopathological features. |
T cells | Craig et al. ( |
CRC | II–IV | 1724 | Immune cold patients by assessment of CD3, CD4 and CD8 IHC are linked with difficult-to-treat, poor prognosis hypoxic biology, which may be potentially amenable to targeted therapy or monitoring for disease progression. |
T cells | Noh et al. ( |
CRC | I–IV | 489 | Tumours with PD-L1-positive tumour cells and high-CD8 TIL is associated with the best prognosis, and show stronger CD8/PD-L1/Pd-1 signalling interaction compared to the other types. |
T cells | Hartman et al. ( |
CRC | I–IV | 259 | The prognostic value of MMR protein deficiency is most likely attributed to increased tumor-associated CD8-positive T cells and that automated quantitative CD8 T-cell analysis is a better biomarker of patient prognosis. |
T cells | Fuchs et al. ( |
CRC | I–IV | 1034 | ITWG systém for assessing TILs is a powerful predictor of all-cause survival in CRC independent of many prognostic factors and superior to the assessment of intraepithelial lymphocytes using a traditional system. |
T cells | Lalos et al. ( |
CRC | I–IV | 613 | The combination of high CD8+ T-cell density and expression of SDF-1 represents an independent, favorable, prognostic condition in CRC, mostly in patients with stage III disease. |
T cells | Al-Badran et al. ( |
CRC | I–III | 773 | Individual and combined high expression of TIM-3, LAG-3, and PD-1 on stromal immune cells are associated with better colorectal cancer prognosis. |
The predictive potential of IS has been reported in several studies. For example, Morris et al. reported longer overall survival in patients with TILs who received adjuvant chemotherapy based on 5-fluorouracil (
The analyses of the effects of the immune infiltrate in specific subgroups of patients and their clinical outcome led to the development of a new classification system based on the consensus between different classification systems proposed by various research groups. Thus, four “consensus molecular subtypes” (CMSs) of CRC were defined (
The biological and prognostic associations of CMS in the context of metastatic tumor heterogeneity is slightly more complicated. Eide et al. performed exploratory analyses of a dataset of 317 primary tumor samples and 295 liver metastasis samples. Almost 90% of metastases belonged to CMS2 or CMS4 subtypes, which is in line with the aforementioned poorer prognosis of these two subtypes (
However, it must be considered that even within CMS groups, there is high inter- and intratumor heterogeneity, which limits universal application of the CMS classification in the clinical practice as a predictive prognostic marker. In view of this, further subclassification based, for example, on detailed gene expression profiling methods like single-cell RNA sequencing and analyses might refine this classification system and improve its clinical usability.
This review shows us that despite the large amount of available information and relatively detailed knowledge of the immune mechanisms that take place inside the tumors and in their vicinity, many factors and variables remain unclear, which ultimately determine success of proposed therapy. The immune contexture of each CRC is highly relevant for the design and allocation of therapeutic approaches that depend on anti-tumoral immune responses and include conventional therapeutic options (chemo- and radiotherapy) as well as immunotherapy. PD-L1+ positive tumor cells and CD8+ TIL are, therefore, key prognostic biomarkers for locally advanced CRC patients treated with neoadjuvant chemoradiotherapy. Upregulation of immune response with immunotherapy has been successful in a subset of patients with dMMR/MSI-H CRC patients. Choosing the right combination of drugs and the right therapeutic strategy for each individual patient including those with metastasized CRC is the key to successful therapy. These combinations and individualization will be crucial aspects of a future clinical trial also at our department. Future directions in the study of immunotherapy in CRC will include identifying improved biomarkers in patients with pMMR/MSS, and identifying novel immunotherapies with improved efficacy, such as specific treatments targeting innate immune cells supporting tumor growth.
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
MW and JS did the literature research and selected relevant studies. MW summarized data from these studies and wrote the manuscript. MR made the major revision of the text. All authors contributed to manuscript revision, read, and approved the submitted version.
This work was supported by MH CZ–DRO-FNOs/2016 and NU21-03-00372.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.