Cancer-initiating cells from colorectal cancer patients escape from T cell-mediated immunosurveillance in vitro through membrane-bound IL-4

Crosstalk between colorectal CSCs and immune cells in tumorigenesis, and strategies for targeting colorectal CSCs

Cancer immunotherapy has emerged as a promising strategy in the treatment of colorectal cancer, and relapse after tumor immunotherapy has attracted increasing attention. Cancer stem cells (CSCs), a small subset of tumor cells with self-renewal and differentiation capacities, are resistant to traditional therapies such as radiotherapy and chemotherapy. Recently, CSCs have been proven to be the cells driving tumor relapse after immunotherapy. However, the mutual interactions between CSCs and cancer niche immune cells are largely uncharacterized. In this review, we focus on colorectal CSCs, CSC-immune cell interactions and CSC-based immunotherapy. Colorectal CSCs are characterized by robust expression of surface markers such as CD44, CD133 and Lgr5; hyperactivation of stemness-related signaling pathways, such as the Wnt/β-catenin, Hippo/Yap1, Jak/Stat and Notch pathways; and disordered epigenetic modifications, including DNA methylation, histone modification, chromatin remodeling, and noncoding RNA action. Moreover, colorectal CSCs express abnormal levels of immune-related genes such as MHC and immune checkpoint molecules and mutually interact with cancer niche cells in multiple tumorigenesis-related processes, including tumor initiation, maintenance, metastasis and drug resistance. To date, many therapies targeting CSCs have been evaluated, including monoclonal antibodies, antibody‒drug conjugates, bispecific antibodies, tumor vaccines adoptive cell therapy, and small molecule inhibitors. With the development of CSC-/niche-targeting technology, as well as the integration of multidisciplinary studies, novel therapies that eliminate CSCs and reverse their immunosuppressive microenvironment are expected to be developed for the treatment of solid tumors, including colorectal cancer.

Tumor Microenvironment: A Niche for Cancer Stem Cell Immunotherapy

Tumorigenic Cancer Stem Cells (CSCs), often called tumor-initiating cells (TICs), represent a unique subset of cells within the tumor milieu. They stand apart from the bulk of tumor cells due to their exceptional self-renewal, metastatic, and differentiation capabilities. Despite significant progress in classifying CSCs, these cells remain notably resilient to conventional radiotherapy and chemotherapy, contributing to cancer recurrence. In this review, our objective is to explore novel avenues of research that delve into the distinctive characteristics of CSCs within their surrounding tumor microenvironment (TME). We will start with an overview of the defining features of CSCs and then delve into their intricate interactions with cells from the lymphoid lineage, namely T cells, B cells, and natural killer (NK) cells. Furthermore, we will discuss their dynamic interplay with myeloid lineage cells, including macrophages, neutrophils, and myeloid-derived suppressor cells (MDSCs). Moreover, we will illuminate the crosstalk between CSCs and cells of mesenchymal origin, specifically fibroblasts, adipocytes, and endothelial cells. Subsequently, we will underscore the pivotal role of CSCs within the context of the tumor-associated extracellular matrix (ECM). Finally, we will highlight pre-clinical and clinical studies that target CSCs within the intricate landscape of the TME, including CAR-T therapy, oncolytic viruses, and CSC-vaccines, with the ultimate goal of uncovering novel avenues for CSC-based cancer immunotherapy.

Immunobiology of cancer stem cells and their immunoevasion mechanisms

Cancer stem cells (CSCs) defined as a small fraction of cells within malignancies have been isolated from tumors with different histological origins with stem related characteristics such as self-replicating potential, tumorigenesis, and therapy resistance. The dynamic communication between CSCs and tumor microenvironment particularly immune cells orchestrates their fate and plasticity as well as the patient outcome. According to recent evidence, it has been reported that they harness different immunological pathways to escape immunosurveillance and express aberrantly immunomodulatory agents or decreased levels of factors forming antigen presenting machinery (APM), subsequently followed by impaired antigen presentation and suppressed immune detection. As effective therapies are expected to be able to eradicate CSCs, mechanistic understanding of such interactions can provide insights into causes of therapy failure particularly in immunotherapy. Also, it can contribute to enhance the practical interventions against CSCs and their immunomodulatory features resulting in CSCs eradication and improving patient clinical outcome. The aim of this review is to explain the present knowledge regarding the immunobiology of CSCs and the immunoevasion mechanisms they use.

Fusobacterium & Co. at the Stem of Cancer: Microbe-Cancer Stem Cell Interactions in Colorectal Carcinogenesis

Adult stem cells lie at the crossroads of tissue repair, inflammation, and malignancy. Intestinal microbiota and microbe-host interactions are pivotal to maintaining gut homeostasis and response to injury, and participate in colorectal carcinogenesis. Yet, limited knowledge is available on whether and how bacteria directly crosstalk with intestinal stem cells (ISC), particularly cancerous stem-like cells (CR-CSC), as engines for colorectal cancer initiation, maintenance, and metastatic dissemination. Among several bacterial species alleged to initiate or promote colorectal cancer (CRC), the pathobiont Fusobacterium Nucleatum has recently drawn significant attention for its epidemiologic association and mechanistic linkage with the disease. We will therefore focus on current evidence for an F. nucleatum-CRCSC axis in tumor development, highlighting the commonalities and differences between F. nucleatum-associated colorectal carcinogenesis and gastric cancer driven by Helicobacter Pylori. We will explore the diverse facets of the bacteria-CSC interaction, analyzing the signals and pathways whereby bacteria either confer "stemness" properties to tumor cells or primarily target stem-like elements within the heterogeneous tumor cell populations. We will also discuss the extent to which CR-CSC cells are competent for innate immune responses and participate in establishing a tumor-promoting microenvironment. Finally, by capitalizing on the expanding knowledge of how the microbiota and ISC crosstalk in intestinal homeostasis and response to injury, we will speculate on the possibility that CRC arises as an aberrant repair response promoted by pathogenic bacteria upon direct stimulation of intestinal stem cells.

Treatment with decitabine induces the expression of stemness markers, PD-L1 and NY-ESO-1 in colorectal cancer: potential for combined chemoimmunotherapy

BACKGROUND

The mechanism of tumor immune escape and progression in colorectal cancer (CRC) is widely investigated in-vitro to help understand and identify agents that might play a crucial role in response to treatment and improve the overall survival of CRC patients. Several mechanisms of immune escape and tumor progression, including expression of stemness markers, inactivation of immunoregulatory genes by methylation, and epigenetic silencing, have been reported in CRC, indicating the potential of demethylating agents as anti-cancer drugs. Of these, a chemotherapeutic demethylating agent, Decitabine (DAC), has been reported to induce a dual effect on both DNA demethylation and histone changes leading to an increased expression of target biomarkers, thus making it an attractive anti-tumorigenic drug.

METHODS

We compared the effect of DAC in primary 1076 Col and metastatic 1872 Col cell lines isolated and generated from patients' tumor tissues. Both cell lines were treated with DAC, and the expression of the NY-ESO-1 cancer-testis antigen, the PD-L1 immunoinhibitory marker, and the CD44, Nanog, KLF-4, CD133, MSI-1 stemness markers were analyzed using different molecular and immunological assays.

RESULTS

DAC treatment significantly upregulated stemness markers in both primary 1076 Col and meta-static 1872 Col cell lines, although a lower effect occurred on the latter: CD44 (7.85 fold; ***p = 0.0001 vs. (4.19 fold; *p = 0.0120), Nanog (4.1 fold; ***p < 0.0001 vs.1.69 fold; ***p = 0.0008), KLF-4 (4.33 fold; ***p < 0.0001 vs.2.48 fold; ***p = 0.0005), CD133 (16.77 fold; ***p = 0.0003 vs.6.36 fold; *p = 0.0166), and MSI-1 (2.33 fold; ***p = 0.0003 vs.2.3 fold; ***p = 0.0004), respectively. Interestingly, in the metastatic 1872 Col cells treated with DAC, the expression of both PD-L1 and NY-ESO-1 was increased tenfold (*p = 0.0128) and fivefold (***p < 0.0001), respectively.

CONCLUSIONS

We conclude that the upregulation of both stemness and immune checkpoint markers by DAC treatment on CRC cells might represent a mechanism of immune evasion. In addition, induction of NY-ESO-1 may represent an immuno-therapeutic option in metastatic CRC patients. Finally, the combination of DAC and anti-PD-1/anti-PD-L1 antibodies treatment should represent a potential therapeutic intervention for this group of patients.

Cancer Stem Cell Relationship with Pro-Tumoral Inflammatory Microenvironment

Inflammatory processes and cancer stem cells (CSCs) are increasingly recognized as factors in the development of tumors. Emerging evidence indicates that CSCs are associated with cancer properties such as metastasis, treatment resistance, and disease recurrence. However, the precise interaction between CSCs and the immune microenvironment remains unexplored. Although evasion of the immune system by CSCs has been extensively studied, new research demonstrates that CSCs can also control and even profit from the immune response. This review provides an overview of the reciprocal interplay between CSCs and tumor-infiltrating immune cells, collecting pertinent data about how CSCs stimulate leukocyte reprogramming, resulting in pro-tumor immune cells that promote metastasis, chemoresistance, tumorigenicity, and even a rise in the number of CSCs. Tumor-associated macrophages, neutrophils, Th17 and regulatory T cells, mesenchymal stem cells, and cancer-associated fibroblasts, as well as the signaling pathways involved in these pro-tumor activities, are among the immune cells studied. Although cytotoxic leukocytes have the potential to eliminate CSCs, immune evasion mechanisms in CSCs and their clinical implications are also known. We intended to compile experimental findings that provide direct evidence of interactions between CSCs and the immune system and CSCs and the inflammatory milieu. In addition, we aimed to summarize key concepts in order to comprehend the cross-talk between CSCs and the tumor microenvironment as a crucial process for the effective design of anti-CSC therapies.

Ecological niches for colorectal cancer stem cell survival and thrival

To date, colorectal cancer is still ranking top three cancer types severely threatening lives. According to cancer stem cell hypothesis, malignant colorectal lumps are cultivated by a set of abnormal epithelial cells with stem cell-like characteristics. These vicious stem cells are derived from intestinal epithelial stem cells or transformed by terminally differentiated epithelial cells when they accumulate an array of transforming genomic alterations. Colorectal cancer stem cells, whatever cell-of-origin, give rise to all morphologically and functionally heterogenous tumor daughter cells, conferring them with overwhelming resilience to intrinsic and extrinsic stresses. On the other hand, colorectal cancer stem cells and their daughter cells continuously participate in constructing ecological niches for their survival and thrival by communicating with adjacent stromal cells and circulating immune guardians. In this review, we first provide an overview of the normal cell-of-origin populations contributing to colorectal cancer stem cell reservoirs and the niche architecture which cancer stem cells depend on at early stage. Then we survey recent advances on how these aberrant niches are fostered by cancer stem cells and their neighbors. We also discuss recent research on how niche microenvironment affects colorectal cancer stem cell behaviors such as plasticity, metabolism, escape of immune surveillance as well as resistance to clinical therapies, therefore endowing them with competitive advantages compared to their normal partners. In the end, we explore therapeutic strategies available to target malignant stem cells.

Current understanding of cancer stem cells: Immune evasion and targeted immunotherapy in gastrointestinal malignancies

As a relatively rare population of cancer cells existing in the tumor microenvironment, cancer stem cells (CSCs) possess properties of immune privilege to evade the attack of immune system, regulated by the microenvironment of CSCs, the so-called CSCs niche. The bidirectional interaction of CSCs with tumor microenvironment (TME) components favors an immunosuppressive shelter for CSCs' survival and maintenance. Gastrointestinal cancer stem cells (GCSCs) are broadly regarded to be intimately involved in tumor initiation, progression, metastasis and recurrence, with elevated tumor resistance to conventional therapies, which pose a major hindrance to the clinical efficacy for treated patients with gastrointestinal malignancies. Thus, a multitude of efforts have been made to combat and eradicate GCSCs within the tumor mass. Among diverse methods of targeting CSCs in gastrointestinal malignancies, immunotherapy represents a promising strategy. And the better understanding of GCSCs immunomodulation and immunoresistance mechanisms is beneficial to guide and design novel GCSCs-specific immunotherapies with enhanced immune response and clinical efficacy. In this review, we have gathered available and updated information to present an overview of the immunoevasion features harbored by cancer stem cells, and we focus on the description of immune escape strategies utilized by CSCs and microenvironmental regulations underlying CSCs immuno-suppression in the context of gastrointestinal malignancies. Importantly, this review offers deep insights into recent advances of CSC-targeting immunotherapeutic approaches in gastrointestinal cancers.

Inhibition of DCLK1 kinase reverses epithelial-mesenchymal transition and restores T-cell activity in pancreatic ductal adenocarcinoma

Immunotherapy has recently become a promising cancer therapy with extensive applications of immune checkpoint inhibitors (ICIs). However, pancreatic ductal adenocarcinoma (PDAC) appears to be unresponsive to immunotherapy due to the immunosuppressive microenvironment. Recent studies showed that cancer stem cell marker DCLK1 promoted the initiation and development of PDAC. Nevertheless, the mechanism driving this process remains unclear. Here, by performing gain-of-function investigations in PDAC cell lines, we demonstrate that both DCLK1 long (DCLK1-iso1, DCLK1-AS) and short (DCLK1-iso4, DCLK1-BL) isoforms can efficiently activate EMT leading to tumor migration and invasion. Consistent with experiments in vitro, bioinformatic analysis demonstrates that DCLK1 may act as a driver of EMT activation in PDAC. Further analysis showed that EMT was associated with an immunosuppressive microenvironment, which includes more immunosuppressive cells and chemokines, and patients with a higher EMT score were less sensitive to immune checkpoint inhibitors according to the TIDE (Tumor Immune Dysfunction and Exclusion) algorithm. Multiplexed immunofluorescence results demonstrated the close correlation between DCLK1, EMT and immunosuppression in PDAC patients. The findings were further confirmed in vivo reflected by decreased CD4+, CD8+ T cells and increased M2 macrophages as well as E-cad loss in DCLK1-overexpressing subcutaneous tumors. Importantly, the highly-specific DCLK1 inhibitor (DCLK1-IN-1) was able to effectively block EMT process and restore T-cell activity. Altogether, our data demonstrate that DCLK1 is strongly associated with tumor immune escape in PDAC and inhibiting DCLK1 kinase activity may be a promising therapeutic modality.

Systemic Interleukins' Profile in Early and Advanced Colorectal Cancer

Tumor microenvironment (TME) is characterized by mutual interactions of the tumor, stromal and immune cells. Early and advanced colorectal tumors differ in structure and present altered serum cytokine levels. Mutual crosstalk among TME infiltrating cells may shift the balance into immune suppressive or pro-inflammatory, antitumor response this way influencing patients' prognosis. Cancer-related inflammation affects all the body and this way, the systemic level of cytokines could reflect TME processes. Despite numerous studies, it is still not known how systemic cytokines levels change during colorectal cancer (CRC) tumor development. Better understanding tumor microenvironment processes could help in planning therapeutic interventions and more accurate patient prognosis. To contribute to the comprehension of these processes within TME, we reviewed cytokines levels from clinical trials in early and advanced colorectal cancer. Presented data were analyzed in the context of experimental studies and studies analyzing tumor infiltration with immune cells. The review summarizes clinical data of cytokines secreted by tumor microenvironment cells: lymphocytes T helper 1 (Th1), lymphocytes T helper 2 (Th2), lymphocytes T helper 17 (Th17), regulatory T cells (Treg cells), regulatory T cells (Breg cells), M1/M2 macrophages, N1/N2 neutrophils, myeloid-derived suppressor cells (MDSC), dendritic cells (DC), innate lymphoid cells (ILC) natural killer (NK) cells and tumor cells.

Differential pre-malignant programs and microenvironment chart distinct paths to malignancy in human colorectal polyps

Colorectal cancers (CRCs) arise from precursor polyps whose cellular origins, molecular heterogeneity, and immunogenic potential may reveal diagnostic and therapeutic insights when analyzed at high resolution. We present a single-cell transcriptomic and imaging atlas of the two most common human colorectal polyps, conventional adenomas and serrated polyps, and their resulting CRC counterparts. Integrative analysis of 128 datasets from 62 participants reveals adenomas arise from WNT-driven expansion of stem cells, while serrated polyps derive from differentiated cells through gastric metaplasia. Metaplasia-associated damage is coupled to a cytotoxic immune microenvironment preceding hypermutation, driven partly by antigen-presentation differences associated with tumor cell-differentiation status. Microsatellite unstable CRCs contain distinct non-metaplastic regions where tumor cells acquire stem cell properties and cytotoxic immune cells are depleted. Our multi-omic atlas provides insights into malignant progression of colorectal polyps and their microenvironment, serving as a framework for precision surveillance and prevention of CRC.

A single-cell resolution atlas of human colorectal polyps maps out distinct paths for pre-cancer to cancer transformation, accompanied by differential immune microenvironment features.

Basophils as a potential therapeutic target in cancer

Basophils, which are considered as redundant relatives of mast cells and the rarest granulocytes in peripheral circulation, have been neglected by researchers in the past decades. Previous studies have revealed their vital roles in allergic diseases and parasitic infections. Intriguingly, recent studies even reported that basophils might be associated with cancer development, as activated basophils synthesize and release a variety of cytokines and chemokines in response to cancers. However, it is still subject to debate whether basophils function as tumor-protecting or tumor-promoting components; the answer may depend on the tumor biology and the microenvironment. Herein, we reviewed the role of basophils in cancers, and highlighted some potential and promising therapeutic strategies.

Long Non-Coding RNAs (lncRNAs) in Response and Resistance to Cancer Immunosurveillance and Immunotherapy

Long non-coding RNAs (lncRNAs) are critical regulatory elements in cellular functions in states of both normalcy and disease, including cancer. LncRNAs can influence not only tumorigenesis but also cancer features such as metastasis, angiogenesis and resistance to chemo-and immune-mediated apoptotic signals. Several lncRNAs have been demonstrated to control directly or indirectly the number, type and activities of distinct immune cell populations of adaptive and innate immunities within and without the tumor microenvironment. The disruption of lncRNA expression in both cancer and immune cells may reflect alterations in tumor responses to cancer immunosurveillance and immunotherapy, thus providing new insights into lncRNA biomarker-based prognostic and therapeutic cancer assessment. Here we present an overview on lncRNAs’ functions and underlying molecular mechanisms related to cancer immunity and conventional immunotherapy, with the expectation that any elucidations may lead to a better understanding and management of cancer immune escape and response to current and future immunotherapeutics.

Modulation of Immune Components on Stem Cell and Dormancy in Cancer

Cancer stem cells (CSCs) refer to a certain subpopulation within the tumor entity that is characterized by restricted cellular proliferation and multipotent differentiation potency. The existence of CSCs has been proven to contribute to the heterogeneity of malignancies, accounting for intensified tumorigenesis, treatment resistance, and metastatic spread. Dormancy was proposed as a reversible state of cancer cells that are temporarily arrested in the cell cycle, possessing several hallmarks that facilitate their survival within a devastating niche. This transient period is evoked to enter an actively proliferating state by multiple regulatory alterations, and one of the most significant and complex factors comes from local and systemic inflammatory reactions and immune components. Although CSCs and dormant cancer cells share several similarities, the clear relationship between these two concepts remains unclear. Thus, the detailed mechanism of immune cells interacting with CSCs and dormant cancer cells also warrants elucidation for prevention of cancer relapse and metastasis. In this review, we summarize recent findings and prospective studies on CSCs and cancer dormancy to conclude the relationship between these two concepts. Furthermore, we aim to outline the mechanism of immune components in interfering with CSCs and dormant cancer cells to provide a theoretical basis for the prevention of relapse and metastasis.

CAR-T therapy: Prospects in targeting cancer stem cells

Cancer stem cells (CSCs), a group of tumour cells with stem cell characteristics, have the ability of self-renewal, multi-lineage differentiation and tumour formation. Since CSCs are resistant to conventional radiotherapy and chemotherapy, their existence may be one of the root causes of cancer treatment failure and tumour progression. The elimination of CSCs may be effective for eventual tumour eradication. Because of the good therapeutic effects without major histocompatibility complex (MHC) restriction and the unique characteristics of CSCs, chimeric antigen receptor T-cell (CAR-T) therapy is expected to be an important method to eliminate CSCs. In this review, we have discussed the feasibility of CSCs-targeted CAR-T therapy for cancer treatment, summarized current research and clinical trials of targeting CSCs with CAR-T cells and forecasted the challenges and future direction from the perspectives of toxicity, persistence and potency, trafficking, infiltration, immunosuppressive tumour microenvironment, and tumour heterogeneity.

Modulating cell differentiation in cancer models

Cancer has been traditionally viewed as a disease characterised by excessive and uncontrolled proliferation, leading to the development of cytotoxic therapies against highly proliferating malignant cells. However, tumours frequently relapse due to the presence of slow-cycling cancer stem cells eluding chemo and radiotherapy. Since these malignant stem cells are largely undifferentiated, inducing their lineage commitment has been proposed as a potential intervention strategy to deplete tumours from their most resistant components. Pro-differentiation approaches have thus far yielded clinical success in the reversion of acute promyelocytic leukaemia (APL), and new developments are fast widening their therapeutic applicability to solid carcinomas. Recent advances in cancer differentiation discussed here highlight the potential and outstanding challenges of differentiation-based approaches.

Deregulation of HLA-I in cancer and its central importance for immunotherapy

It is now well accepted that many tumors undergo a process of clonal selection which means that tumor antigens arising at various stages of tumor progression are likely to be represented in just a subset of tumor cells. This process is thought to be driven by constant immunosurveillance which applies selective pressure by eliminating tumor cells expressing antigens that are recognized by T cells. It is becoming increasingly clear that the same selective pressure may also select for tumor cells that evade immune detection by acquiring deficiencies in their human leucocyte antigen (HLA) presentation pathways, allowing important tumor antigens to persist within cells undetected by the immune system. Deficiencies in antigen presentation pathway can arise by a variety of mechanisms, including genetic and epigenetic changes, and functional antigen presentation is a hard phenomenon to assess using our standard analytical techniques. Nevertheless, it is likely to have profound clinical significance and could well define whether an individual patient will respond to a particular type of therapy or not. In this review we consider the mechanisms by which HLA function may be lost in clinical disease, we assess the implications for current immunotherapy approaches using checkpoint inhibitors and examine the prognostic impact of HLA loss demonstrated in clinical trials so far. Finally, we propose strategies that might be explored for possible patient stratification.

Cancer stem cell-immune cell crosstalk in tumour progression

Cellular heterogeneity and an immunosuppressive tumour microenvironment are independent yet synergistic drivers of tumour progression and underlie therapeutic resistance. Recent studies have highlighted the complex interaction between these cell-intrinsic and cell-extrinsic mechanisms. The reciprocal communication between cancer stem cells (CSCs) and infiltrating immune cell populations in the tumour microenvironment is a paradigm for these interactions. In this Perspective, we discuss the signalling programmes that simultaneously induce CSCs and reprogramme the immune response to facilitate tumour immune evasion, metastasis and recurrence. We further highlight biological factors that can impact the nature of CSC-immune cell communication. Finally, we discuss targeting opportunities for simultaneous regulation of the CSC niche and immunosurveillance.

Immune deserts in head and neck squamous cell carcinoma: A review of challenges and opportunities for modulating the tumor immune microenvironment

Immunotherapy revolutionized cancer treatment but has yet to elicit durable responses in the majority of patients with head and neck squamous cell carcinoma (HNSCC). HNSCC is generally characterized by a high tumor mutational burden, which has translated to a large neoantigen load that could prime the immune system to recognize and eliminate malignant cells. Studies are increasingly showing, however, that HNSCC is an "immune desert" tumor that can hijack multiple parts of the tumor immunity cycle in order to evade immune recognition and suppress immune system activation. Herein we will review how HNSCC tumors modulate their architecture, cellular composition, and cytokine milieu to maximize immunosuppression; as well as relevant therapeutic opportunities and emerging issues facing the field of HNSCC immuno-oncology.

Natural Products as a Promising Therapeutic Strategy to Target Cancer Stem Cells

Cancer is still a deadly disease, and its treatment desperately needs to be managed in a very sophisticated way through fast-developing novel strategies. Most of the cancer cases eventually develop into recurrencies, for which cancer stem cells (CSCs) are thought to be responsible. They are considered as a subpopulation of all cancer cells of tumor tissue with aberrant regulation of self-renewal, unbalanced proliferation, and cell death properties. Moreover, CSCs show a serious degree of resistance to chemotherapy or radiotherapy and immune surveillance as well. Therefore, new classes of drugs are rushing into the market each year, which makes the cost of therapy increase dramatically. Natural products are also becoming a new research area as a diverse chemical library to suppress CSCs. Some of the products even show promise in this regard. So, the near future could witness the introduction of natural products as a source of new chemotherapy modalities, which may result in the development of novel anticancer drugs. They could also be a reasonably-priced alternative to highly expensive current treatments. Nowadays, considering the effects of natural compounds on targeting surface markers, signaling pathways, apoptosis, and escape from immunosurveillance have been a highly intriguing area in preclinical and clinical research. In this review, we present scientific advances regarding their potential use in the inhibition of CSCs and the mechanisms by which they kill the CSCs.

Long non-coding RNAs in cancer stem cells

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Long non coding RNAs are involved in the regulation of multiple cellular processes.

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Cancer stemness and escape from immunological anti-cancer mechanisms are important mechanisms of resistance to anti-cancer agents and are pivotal in controlling cancer development and metastases.

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Long non coding RNAs have deep effects on the immune-modulation and on the control of cancer stem cells.

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Several pathways involved in immunological escape and cancer stemness are modulated by long non coding RNAs.

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Targeting long non coding RNAs is a potential new strategy to control tumor development and metastases.

In recent years, it has been evidenced that the human transcriptome includes several types of non-coding RNAs (ncRNAs) that are mainly involved in the regulation of different cellular processes. Among ncRNAs, long-non-coding RNAs (lncRNAs) are defined as longer than 200 nucleotides and have been shown to be involved in several physiological and pathological events, including immune system regulation and cancer. Cancer stem cells (CSCs) are defined as a population of cancer cells that possess characteristics, such as resistance to standard treatments, cancer initiation, ability to undergo epithelial-to-mesenchymal transition, and the ability to invade, spread, and generate metastases. The cancer microenvironment, together with genetic and epigenetic factors, is fundamental for CSC maintenance and tumor growth and progression. Unsurprisingly, lncRNAs have been involved in both CSC biology and cancer progression, prognosis and recurrence. Here we review the most recent literature on IncRNAs involvement in CSC biology and function.

Clinical Implications of Colorectal Cancer Stem Cells in the Age of Single-Cell Omics and Targeted Therapies

The cancer stem cell (CSC) concept emerged from the recognition of inherent tumor heterogeneity and suggests that within a given tumor, in analogy to normal tissues, there exists a cellular hierarchy composed of a minority of more primitive cells with enhanced longevity (ie, CSCs) that give rise to shorter-lived, more differentiated cells (ie, cancer bulk populations), which on their own are not capable of tumor perpetuation. CSCs can be responsible for cancer therapeutic resistance to conventional, targeted, and immunotherapeutic treatment modalities, and for cancer progression through CSC-intrinsic molecular mechanisms. The existence of CSCs in colorectal cancer (CRC) was first established through demonstration of enhanced clonogenicity and tumor-forming capacity of this cell subset in human-to-mouse tumor xenotransplantation experiments and subsequently confirmed through lineage-tracing studies in mice. Surface markers for CRC CSC identification and their prospective isolation are now established. Therefore, the application of single-cell omics technologies to CSC characterization, including whole-genome sequencing, RNA sequencing, and epigenetic analyses, opens unprecedented opportunities to discover novel targetable molecular pathways and hence to develop novel strategies for CRC eradication. We review recent advances in this field and discuss the potential implications of next-generation CSC analyses for currently approved and experimental targeted CRC therapies.

Cancer Stem Cells Are Possible Key Players in Regulating Anti-Tumor Immune Responses: The Role of Immunomodulating Molecules and MicroRNAs

Simple Summary

This review provides a critical overview of the state of the art of the characterization of the immunological profile of a rare component of the tumors, denominated cancer stem cells (CSCs) or cancer initiating cells (CICs). These cells are endowed with the ability to form and propagate tumors and resistance to therapies, including the most innovative approaches. These investigations contribute to understanding the mechanisms regulating the interaction of CSCs/CICs with the immune system and identifying novel therapeutic approaches to render these cells visible and susceptible to immune responses.

Abstract

Cancer cells endowed with stemness properties and representing a rare population of cells within malignant lesions have been isolated from tumors with different histological origins. These cells, denominated as cancer stem cells (CSCs) or cancer initiating cells (CICs), are responsible for tumor initiation, progression and resistance to therapies, including immunotherapy. The dynamic crosstalk of CSCs/CICs with the tumor microenvironment orchestrates their fate and plasticity as well as their immunogenicity. CSCs/CICs, as observed in multiple studies, display either the aberrant expression of immunomodulatory molecules or suboptimal levels of molecules involved in antigen processing and presentation, leading to immune evasion. MicroRNAs (miRNAs) that can regulate either stemness properties or their immunological profile, with in some cases dual functions, can provide insights into these mechanisms and possible interventions to develop novel therapeutic strategies targeting CSCs/CICs and reverting their immunogenicity. In this review, we provide an overview of the immunoregulatory features of CSCs/CICs including miRNA profiles involved in the regulation of the interplay between stemness and immunological properties.

Cellular Immunotherapy Targeting Cancer Stem Cells: Preclinical Evidence and Clinical Perspective

The term “cancer stem cells” (CSCs) commonly refers to a subset of tumor cells endowed with stemness features, potentially involved in chemo-resistance and disease relapses. CSCs may present peculiar immunogenic features influencing their homeostasis within the tumor microenvironment. The susceptibility of CSCs to recognition and targeting by the immune system is a relevant issue and matter of investigation, especially considering the multiple emerging immunotherapy strategies. Adoptive cellular immunotherapies, especially those strategies encompassing the genetic redirection with chimeric antigen receptors (CAR), hold relevant promise in several tumor settings and might in theory provide opportunities for selective elimination of CSC subsets. Initial dedicated preclinical studies are supporting the potential targeting of CSCs by cellular immunotherapies, indirect evidence from clinical studies may be derived and new studies are ongoing. Here we review the main issues related to the putative immunogenicity of CSCs, focusing on and highlighting the existing evidence and opportunities for cellular immunotherapy approaches with T and non-T antitumor lymphocytes.

Targeting Oncoimmune Drivers of Cancer Metastasis

Residual metastasis is a major cause of cancer-associated death. Recent advances in understanding the molecular basis of the epithelial-mesenchymal transition (EMT) and the related cancer stem cells (CSCs) have revealed the landscapes of cancer metastasis and are promising contributions to clinical treatments. However, this rarely leads to practical advances in the management of cancer in clinical settings, and thus cancer metastasis is still a threat to patients. The reason for this may be the heterogeneity and complexity caused by the evolutional transformation of tumor cells through interactions with the host environment, which is composed of numerous components, including stromal cells, vascular cells, and immune cells. The reciprocal evolution further raises the possibility of successful tumor escape, resulting in a fatal prognosis for patients. To disrupt the vicious spiral of tumor-immunity aggravation, it is important to understand the entire metastatic process and the practical implementations. Here, we provide an overview of the molecular and cellular links between tumors' biological properties and host immunity, mainly focusing on EMT and CSCs, and we also highlight therapeutic agents targeting the oncoimmune determinants driving cancer metastasis toward better practical use in the treatment of cancer patients.

Possible Roles of Interleukin-4 and -13 and Their Receptors in Gastric and Colon Cancer

Interleukin (IL)-4 and -13 are structurally and functionally related cytokines sharing common receptor subunits. They regulate immune responses and, moreover, are involved in the pathogenesis of a variety of human neoplasms. Three different receptors have been described for IL-4, but only IL-4 receptor type II (IL-4Rα/IL-13Rα1) is expressed in solid tumors. While IL-13 can also bind to three different receptors, IL-13 receptor type I (IL-4Rα/IL-13Rα1/IL-13Rα2) and type II (IL-4Rα/IL-13Rα1) are expressed in solid tumors. After receptor binding, IL-4 and IL-13 can mediate tumor cell proliferation, survival, and metastasis in gastric or colon cancer. This review summarizes the results about the role of IL-4/IL-13 and their receptors in gastric and colon cancer.

Expression of IL4Rα and IL13Rα1 are associated with poor prognosis of soft-tissue sarcoma of the extremities, superficial trunk, and retroperitoneum

Background

IL4Rα and IL13Rα1 are constituents of the type II IL4 receptor. Recently, IL4Rα and IL13Rα1 were reported to have roles in cancer progression and suggested as potential prognostic markers. However, studies on IL4Rα and IL13Rα1 in soft-tissue sarcomas have been limited.

Methods

This study investigated the immunohistochemical expression of IL4Rα and IL13Rα1 in 89 soft-tissue sarcomas of the extremities, superficial trunk, and retroperitoneum. Immunohistochemical staining for IL4Rα and IL13Rα1 were scored according to a combination of staining intensity and staining area in tissue microarray samples. Positivity for the immunohistochemical expression of IL4Rα and IL13Rα1 were determined using receiver operating curve analysis. Statistical analysis was performed using regression analysis and a chi-square test.

Results

In human soft-tissue sarcomas, immunohistochemical expression of IL4Rα was significantly associated with IL13Rα1 expression. Nuclear and cytoplasmic expression of IL4Rα and IL13Rα1 were significantly associated with shorter survival of soft-tissue sarcoma patients in univariate analysis. Multivariate analysis indicated that nuclear expression of IL4Rα and IL13Rα1 were independent indicators of shorter overall survival (IL4Rα; p = 0.002, IL13Rα1; p = 0.016) and relapse-free survival (IL4Rα; p = 0.022, IL13Rα1; p < 0.001) of soft-tissue sarcoma patients. Moreover, the co-expression pattern of nuclear IL4Rα and IL13Rα1 was an independent indicator of shorter survival of soft-tissue sarcoma patients (overall survival; overall p < 0.001, relapse-free survival; overall p < 0.001).

Conclusions

This study suggests IL4Rα and IL13Rα1 are associated with the progression of soft-tissue sarcoma, and the expression of IL4Rα and IL13Rα1 might be novel prognostic indicators of soft-tissue sarcoma patients.

Circulating Interleukin-4 Is Associated with a Systemic T Cell Response against Tumor-Associated Antigens in Treatment-Naïve Patients with Resectable Non-Small-Cell Lung Cancer

Simple Summary

Cytokines can increase the activity of T cells specific for tumor-associated antigens and thereby promote tumor-specific immune responses. In this study, cytokine profiles and T cell responses against 14 tumor-associated antigens were investigated in 36 treatment-naïve patients with resectable non-small cell lung cancer. Based on these results, preoperative serum interleukin-4 levels can play a role in predicting T cell responses specific for tumor-associated antigens and recurrence-free survival regardless of tumor stage. This is clinically relevant as patients with high preoperative serum interleukin-4 levels could be at high risk of postoperative tumor recurrence and, therefore, should be considered for adjuvant or neoadjuvant treatment. From this perspective, preoperative serum interleukin-4 levels may become a useful option to assess the risk of postoperative tumor recurrence in non-small-cell lung cancer.

Abstract

Spontaneous T cell responses to tumor-associated antigens (TAs) in the peripheral blood of patients with non-small-cell lung cancer (NSCLC) may be relevant for postoperative survival. However, the conditions underlying these T cell responses remain unclear. We quantified the levels of 27 cytokines in the peripheral blood and tumor tissues from treatment-naïve patients with NSCLC (n = 36) and analyzed associations between local and systemic cytokine profiles and both TA-specific T cell responses and clinical parameters. We defined T cell responders as patients with circulating T cells that were reactive to TAs and T cell nonresponders as patients without detectable TA-specific T cells. TA-specific T cell responses were correlated with serum cytokine levels, particularly the levels of interleukin(IL)-4 and granulocyte colony-stimulating factor (G-CSF), but poorly correlated with the cytokine levels in tumor tissues. Nonresponders showed significantly higher serum IL-4 levels than responders (p = 0.03); the predicted probability of being a responder was higher for individuals with low serum IL-4 levels. In multivariable Cox regression analyses, in addition to IL-4 (hazard ratio (HR) 2.8 (95% confidence interval (CI): 0.78–9.9); p = 0.116), the age-adjusted IL-8 level (HR 3.9 (95% CI: 1.05–14.5); p = 0.042) predicted tumor recurrence. However, this study included data for many cytokines without adjustment for multiple testing; thus, the observed differences in IL-4 or IL-8 levels might be incidental findings. Therefore, additional studies are necessary to confirm these results.

Group 2 Innate Lymphoid Cells: A Double-Edged Sword in Cancer?

Group 2 Innate Lymphoid Cells (ILC2s) belong to the family of helper ILCs which provide host defense against infectious agents, participate in inflammatory responses and mediate lymphoid organogenesis and tissue repair, mainly at the skin and mucosal level. Based on their transcriptional, phenotypic and functional profile, ILC2s mirror the features of the adaptive CD4+ Th2 cell subset, both contributing to the so-called type 2 immune response. Similar to other ILCs, ILC2s are rapidly activated by signals deriving from tissue and/or other tissue-resident immune cells. The biologic activity of ILCs needs to be tightly regulated in order to prevent them from contributing to severe inflammation and damage in several organs. Indeed, ILC2s display both enhancing and regulatory roles in several pathophysiological conditions, including tumors. In this review, we summarize the actual knowledge about ILC2s ability to induce or impair a protective immune response, their pro- or antitumor activity in murine models, human (children and adults) pathologies and the potential strategies to improve cancer immunotherapy by exploiting the features of ILC2s.

Immune Adaptation of Colorectal Cancer Stem Cells and Their Interaction With the Tumor Microenvironment

Metastasis is the primary cause of death in colorectal cancer (CRC) patients. Emerging evidence has shown that CRC stem cells (CRCSCs) play a significant role in metastatic dissemination and tumor recurrence. However, strategies for targeting CRCSCs are limited because CRCSCs are resistant to therapeutic interventions and because the tumor microenvironment (TME) provides a supportive niche. Moreover, growing evidence highlights the critical role of CRCSCs in immune adaptation and modulation of the TME. CRCSCs escape immune surveillance by avoiding recognition by the innate immune system and shaping the TME through exosomes, cytokines, and chemokines to generate an immunosuppressive niche that facilitates cancer progression. In this review, we summarize studies investigating the immunomodulatory properties of CRCSCs and their underlying mechanisms in order to improve the efficacy of treatment strategies against advanced CRC.

Cancer stem cell secretome in the tumor microenvironment: a key point for an effective personalized cancer treatment

Cancer stem cells (CSCs) represent a tumor subpopulation responsible for tumor metastasis and resistance to chemo- and radiotherapy, ultimately leading to tumor relapse. As a consequence, the detection and eradication of this cell subpopulation represent a current challenge in oncology medicine. CSC phenotype is dependent on the tumor microenvironment (TME), which involves stem and differentiated tumor cells, as well as different cell types, such as mesenchymal stem cells, endothelial cells, fibroblasts and cells of the immune system, in addition to the extracellular matrix (ECM), different in composition to the ECM in healthy tissues. CSCs regulate multiple cancer hallmarks through the interaction with cells and ECM in their environment by secreting extracellular vesicles including exosomes, and soluble factors such as interleukins, cytokines, growth factors and other metabolites to the TME. Through these factors, CSCs generate and activate their own tumor niche by recruiting stromal cells and modulate angiogenesis, metastasis, resistance to antitumor treatments and their own maintenance by the secretion of different factors such as IL-6, VEGF and TGF-ß. Due to the strong influence of the CSC secretome on disease development, the new antitumor therapies focus on targeting these communication networks to eradicate the tumor and prevent metastasis, tumor relapse and drug resistance. This review summarizes for the first time the main components of the CSC secretome and how they mediate different tumor processes. Lastly, the relevance of the CSC secretome in the development of more precise and personalized antitumor therapies is discussed.

Signal transducer and activator of transcription 6 as a target in colon cancer therapy

Signal transducer and activator of transcription 6 (STAT6) is a member of the STAT family of proteins that serve key roles in the initiation of tumorigenesis and malignant transformation. STAT6 is highly expressed in several types of cancer, including breast, pancreatic, prostate and colorectal cancer. STAT6 transduces signals in response to the binding of interleukin (IL)-4 and IL-13 to their receptors and regulates the expression of genes involved in the immune response, cell survival, tumor proliferation and metastasis. Patients with colorectal cancer exhibit high STAT6 activity in the colonic epithelium, and STAT6 expression is associated with lower survival rates, lymph node metastasis, changes in the epithelial barrier function and alterations in the inflammatory response. A number of studies investigating experimental models and cancer cell lines have revealed that STAT6 is associated with tumor growth and development, as well as with increased invasion and metastasis, suggesting that STAT6 inhibition may serve as a novel therapeutic strategy in colon cancer. The present review summarizes the evidence with regard to the implications of STAT6 in cancer biology and the direct and indirect effects on colon tumor transformation. Furthermore, the current treatment strategies targeting the IL-4/IL-13/STAT6 axis in colon cancer are discussed.

Influence of Innate Immunity on Cancer Cell Stemness

Even if cancer stem cells (CSCs) represent only a small proportion of the tumor mass, they significantly account for tumor maintenance, resistance to therapies, relapse and metastatic spread, due to their increased capacity of self-renewal, multipotency, tumorigenicity and quiescence. Emerging evidence suggests that the immune contexture within the tumor microenvironment (TME) determines both the response to therapy and the clinical outcome. In this context, CSCs acquire immune evasion skills by editing immune cell functions and sculpting the immunosuppressive landscape of TME. Reciprocally, infiltrating immune cells influence CSCs self-renewal, tumorigenicity and metastasis. In this review, we summarize the immunomodulatory properties of CSCs, as well as the impact of innate immune cells on cancer cells stemness in the different phases of cancer immunoediting process and neoplastic progression.

Targeting Cancer Stem Cells by Genetically Engineered Chimeric Antigen Receptor T Cells

The term cancer stem cell (CSC) starts 25 years ago with the evidence that CSC is a subpopulation of tumor cells that have renewal ability and can differentiate into several distinct linages. Therefore, CSCs play crucial role in the initiation and the maintenance of cancer. Moreover, it has been proposed throughout several studies that CSCs are behind the failure of the conventional chemo-/radiotherapy as well as cancer recurrence due to their ability to resist the therapy and their ability to re-regenerate. Thus, the need for targeted therapy to eliminate CSCs is crucial; for that reason, chimeric antigen receptor (CAR) T cells has currently been in use with high rate of success in leukemia and, to some degree, in patients with solid tumors. This review outlines the most common CSC populations and their common markers, in particular CD133, CD90, EpCAM, CD44, ALDH, and EGFRVIII, the interaction between CSCs and the immune system, CAR T cell genetic engineering and signaling, CAR T cells in targeting CSCs, and the barriers in using CAR T cells as immunotherapy to treat solid cancers.

High Expression of the Sda Synthase B4GALNT2 Associates with Good Prognosis and Attenuates Stemness in Colon Cancer

BACKGROUND

The carbohydrate antigen Sd^a and its biosynthetic enzyme B4GALNT2 are highly expressed in normal colonic mucosa but are down-regulated to a variable degree in colon cancer tissues. Here, we investigated the clinical and biological importance of B4GALNT2 in colon cancer.

METHODS

Correlations of B4GALNT2 mRNA with clinical data were obtained from The Cancer Genome Atlas (TCGA) database; the phenotypic and transcriptomic changes induced by B4GALNT2 were studied in LS174T cells transfected with B4GALNT2 cDNA.

RESULTS

TCGA data indicate that patients with high B4GALNT2 expression in cancer tissues display longer survival than non-expressers. In LS174T cells, expression of B4GALNT2 did not affect the ability to heal a scratch wound or to form colonies in standard growth conditions but markedly reduced the growth in soft agar, the tridimensional (3D) growth as spheroids, and the number of cancer stem cells, indicating a specific effect of B4GALNT2 on the growth in poor adherence and stemness. On the transcriptome, B4GALNT2 induced the down-regulation of the stemness-associated gene SOX2 and modulated gene expression towards an attenuation of the cancer phenotype.

CONCLUSIONS

The level of B4GALNT2 can be proposed as a marker to identify higher- and lower-risk colorectal cancer patients.

Impact of sialyltransferase ST6GAL1 overexpression on different colon cancer cell types

Cancer-associated glycan structures can be both tumor markers and engines of disease progression. The structure Siaα2,6Galβ1,4GlcNAc (Sia6LacNAc), synthesized by sialyltransferase ST6GAL1, is a cancer-associated glycan. Although ST6GAL1/Sia6LacNAc are often overexpressed in colorectal cancer (CRC), their biological and clinical significance remains unclear. To get insights into the clinical relevance of ST6GAL1 expression in CRC, we interrogated The Cancer Genome Atlas with mRNA expression data of hundreds of clinically characterized CRC and normal samples. We found an association of low ST6GAL1 expression with microsatellite instability (MSI), BRAF mutations and mucinous phenotype but not with stage, response to therapy and survival. To investigate the impact of ST6GAL1 expression in experimental systems, we analyzed the transcriptome and the phenotype of the CRC cell lines SW948 and SW48 after retroviral transduction with ST6GAL1 cDNA. The two cell lines display the two main pathways of CRC transformation: chromosomal instability and MSI, respectively. Constitutive ST6GAL1 expression induced much deeper transcriptomic changes in SW948 than in SW48 and affected different genes in the two cell lines. ST6GAL1 expression affected differentially the tyrosine phosphorylation induced by hepatocyte growth factor, the ability to grow in soft agar, to heal a scratch wound and to invade Matrigel in the two cell lines. These results indicate that the altered expression of a cancer-associated glycosyltransferase impacts the gene expression profile, as well as the phenotype, although in a cancer subtype-specific manner.

The Cancer Stem Cell in Hepatocellular Carcinoma

The recognition of intra-tumoral cellular heterogeneity has given way to the concept of the cancer stem cell (CSC). According to this concept, CSCs are able to self-renew and differentiate into all of the cancer cell lineages present within the tumor, placing the CSC at the top of a hierarchical tree. The observation that these cells—in contrast to bulk tumor cells—are able to exclusively initiate new tumors, initiate metastatic spread and resist chemotherapy implies that CSCs are solely responsible for tumor recurrence and should be therapeutically targeted. Toward this end, dissecting and understanding the biology of CSCs should translate into new clinical therapeutic approaches. In this article, we review the CSC concept in cancer, with a special focus on hepatocellular carcinoma.

Cancer Stem Cells: Devil or Savior-Looking behind the Scenes of Immunotherapy Failure

Although the introduction of immunotherapy has tremendously improved the prognosis of patients with metastatic cancers of different histological origins, some tumors fail to respond or develop resistance. Broadening the clinical efficacy of currently available immunotherapy strategies requires an improved understanding of the biological mechanisms underlying cancer immune escape. Globally, tumor cells evade immune attack using two main strategies: avoiding recognition by immune cells and instigating an immunosuppressive tumor microenvironment. Emerging data suggest that the clinical efficacy of chemotherapy or molecularly targeted therapy is related to the ability of these therapies to target cancer stem cells (CSCs). However, little is known about the role of CSCs in mediating tumor resistance to immunotherapy. Due to their immunomodulating features and plasticity, CSCs can be especially proficient at evading immune surveillance, thus potentially representing the most prominent malignant cell component implicated in primary or acquired resistance to immunotherapy. The identification of immunomodulatory properties of CSCs that include mechanisms that regulate their interactions with immune cells, such as bidirectional release of particular cytokines/chemokines, fusion of CSCs with fusogenic stromal cells, and cell-to-cell communication exerted by extracellular vesicles, may significantly improve the efficacy of current immunotherapy strategies. The purpose of this review is to discuss the current scientific evidence linking CSC biological, immunological, and epigenetic features to tumor resistance to immunotherapy.

Bidirectional Crosstalk Between Cancer Stem Cells and Immune Cell Subsets

Cancer stem cells (CSCs), also known as tumor-initiating cells, are characterized by an increased capacity for self-renewal, multipotency, and tumor initiation. While CSCs represent only a small proportion of the tumor mass, they significantly account for metastatic dissemination and tumor recurrence, thus making them attractive targets for therapy. Due to their ability to sustain in dormancy, chemo- and radiotherapy often fail to eliminate cancer cells with stemness properties. Recent advances in the understanding of the tumor microenvironment (TME) illustrated the importance of the immune contexture, determining the response to therapy and clinical outcome of patients. In this context, CSCs exhibit special properties to escape the recognition by innate and adaptive immunity and shape the TME into an immunosuppressive, pro-tumorigenic landscape. As CSCs sculpt the immune contexture, the phenotype and functional properties of the tumor-infiltrating immune cells in turn influence the differentiation and phenotype of tumor cells. In this review, we summarize recent studies investigating main immunomodulatory properties of CSCs and their underlying molecular mechanisms as well as the impact of immune cells on cancer cells with stemness properties. A deeper understanding of this bidirectional crosstalk shaping the immunological landscape and determining therapeutic responses will facilitate the improvement of current treatment modalities and the design of innovative strategies to precisely target CSCs.

Aldehyde dehydrogenase-positive melanoma stem cells in tumorigenesis, drug resistance and anti-neoplastic immunotherapy

Cancer stem cells (CSCs), a rare subset of cancer cells, are well known for their self-renewing capacity. CSCs play a critical role in therapeutic failure and are responsible for poor prognosis in leukemia and various solid tumors. However, it is still unclear how CSCs initiate carcinogenesis and evade the immune response. In humans, the melanoma initiating cells (MICs) are recognized as the CSCs in melanomas, and were verified to possess CSC potentials. The enzymatic system, aldehyde dehydrogenase (ALDH) is considered to be a specific marker for CSCs in several tumors. The expression of ALDH in MICs may be closely correlated with phenotypic heterogeneity, melanoma-genesis, metastasis, and drug resistance. The ALDH⁺ CSCs/MICs not only serve as an indicator for therapeutic efficacy, but have also become a target for the treat of melanoma. In this review, we initially introduce the multiple capacities of MICs in melanoma. Then, we summarize in vivo and in vitro studies that illustrate the relationship between ALDH and MICs. Furthermore, understanding of chemotherapy resistance in melanoma relies on ALDH⁺ MICs. Finally, we review studies that focus on melanoma immunotherapies, rendering ALDH a potential marker to evaluate the efficacy of anti-neoplastic therapies or an adjuvant anti-melanoma target.

Tumor propagating cells: drivers of tumor plasticity, heterogeneity, and recurrence

Tumorigenesis is associated with the development of a highly variable pattern of cellular diversity, consequence of genetic and epigenetic diversification, followed by clonal selection and expansion. This process is shaped by the microenvironment and leads to intratumoral heterogeneity, which is characterized by differences between cancer cells in terms of gene expression, phenotypic markers, growth dynamics, and resistance to treatment. Another relevant aspect in intratumor heterogeneity is cell plasticity-the ability of a cell to switch to new identities. In this review, we focus on the mechanisms that regulate cancer cell plasticity within a tumor, and explore the concept of tumor propagating cells, or TPCs, a cancer cell able to propagate/phenocopy the parental tumor and recapitulate tumor heterogeneity. We discuss the influence of the microenvironment and driver mutations on TPCs formation and function, the existence of phenotypically distinct TPC clones within a tumor, the evolution of TPCs with disease progression, and their implications for therapy.

The Cross Talk between Cancer Stem Cells/Cancer Initiating Cells and Tumor Microenvironment: The Missing Piece of the Puzzle for the Efficient Targeting of these Cells with Immunotherapy

Cancer Stem Cells/Cancer Initiating Cells (CSCs/CICs) is a rare sub-population within a tumor that is responsible for tumor formation, progression and resistance to therapies. The interaction between CSCs/CICs and tumor microenvironment (TME) can sustain “stemness” properties and promote their survival and plasticity. This cross-talk is also pivotal in regulating and modulating CSC/CIC properties. This review will provide an overview of the mechanisms underlying the mutual interaction between CSCs/CICs and TME. Particular focus will be dedicated to the immunological profile of CSCs/CICs and its role in orchestrating cancer immunosurveillance. Moreover, the available immunotherapy strategies that can target CSCs/CICs and of their possible implementation will be discussed. Overall, the dissection of the mechanisms regulating the CSC/CIC-TME interaction is warranted to understand the plasticity and immunoregulatory properties of stem-like tumor cells and to achieve complete eradications of tumors through the optimization of immunotherapy.

XIAOPI Formula Inhibits Breast Cancer Stem Cells via Suppressing Tumor-Associated Macrophages/C-X-C Motif Chemokine Ligand 1 Pathway

Macrophages are the most abundant stromal cells associated with the host immune system in multiple malignancies including breast cancer. With proven clinical efficacy and no noticeable adverse effects, XIAOPI formula (XPS) has been approved for breast hyperplasia treatment by the State Food and Drug Administration of China (SFDA) in 2018. The existing knowledge about the anti-breast cancer activities and mechanisms of XPS has been very limited. The present study aimed to investigate whether XPS could exert an anti-breast cancer effect by regulating tumor-associated macrophages (TAMs) in tumor microenvironment. Herein, breast cancer cells and TAMs were co-cultured using the transwell co-culture system to simulate the coexistence of them. XPS could significantly inhibit the proliferation, colony formation, breast cancer stem cells (CSCs) subpopulation, mammosphere formation abilities as well as stemness-related genes expression in both human and mouse breast cancer cells in the co-culture system. Additionally, XPS could suppress M2 phenotype polarization as well as C-X-C motif chemokine ligand 1 (CXCL1) expression and secretion of TAMs. Notably, further mechanistic explorations verified TAMs/CXCL1 as the critical target of XPS in inhibiting breast CSCs self-renewal in the co-culture system as the exogenous CXCL1 administration could abrogate the inhibitory effect of XPS on breast CSCs self-renewal. More importantly, XPS significantly inhibited mammary tumor growth, breast CSCs subpopulation, and TAMs/CXCL1 activity in mouse 4T1-Luc xenografts in vivo without any detectable side effects. Taken together, this study not only uncovers the immunomodulatory mechanism of XPS in treating breast cancer but also sheds novel insights into TAMs/CXCL1 as a potential molecular target for breast CSCs elimination.

Cell stemness, epithelial-to-mesenchymal transition, and immunoevasion: Intertwined aspects in cancer metastasis

Recent advances in tumor immunology, fostered by dramatic outcomes with cancer immunotherapy, have opened new scenarios in cancer metastasis. The cancer stemness/mesenchymal phenotype and an excess of immune suppressive signals are emerging as Intertwined aspects of human tumors. This review examines recent studies that explored the mechanistic links between cancer cell stemness and immunoevasion, and the evidence points to these key events in cancer metastasis as two sides of the same coin. This review also covers the mechanisms involved in tumor expression of programmed cell death ligand 1 (PD-L1), a major factor exploited by human neoplasias to suppress immune control. We highlight the convergence of mesenchymal traits and PD-L1 expression and examine the functions of this immune inhibitory molecule, which confers cancer cell resistance and aggressiveness.

Cancer stem cell immunology and immunotherapy: Harnessing the immune system against cancer's source

Despite recent advances in diagnosis and therapy having improved cancer outcome, many patients still do not respond to treatments, resulting in the progression or relapse of the disease, eventually impairing survival expectations. The limited efficacy of therapy is often attributable to its inability to affect cancer stem cells (CSCs), a small population of cells resistant to current radio- and chemo-therapies. CSCs are characterized by self-renewal and tumor-initiating capabilities, and function as a reservoir for the local and distant recurrence of the disease. Therefore, new therapeutic approaches able to effectively target and deplete CSCs are urgently needed. Immunotherapy is facing a renewed interest for its potential in cancer treatment, and the possibility of harnessing the immune system to target CSCs is being addressed by a new exciting research field. In this chapter, we discuss the cancer stem cell model and illustrate CSC biological and molecular properties, critically addressing theoretical and practical issues linked with their definition and study. We then review the existing literature regarding the immunological properties of CSCs and the complex interplay occurring between CSCs and immune cells. Finally, we present up-to-date studies on CSC immunotargeting and its potential future perspective. In conclusion, understanding the interplay between CSC biology and tumor immunology will provide a deeper understanding of the mechanisms that regulate CSC immunological properties. This will contribute to the design of new CSC-directed immunotherapeutic strategies with the potential of strongly improving cancer outcomes.

Recent Advances in Cancer Stem Cell-Targeted Immunotherapy

Cancer stem cells (CSCs) are one of the reasons for the relapse of cancer cells and metastasis. They have drug resistance against most chemotherapeutic agents. CSCs are also responsible for tumor cell heterogeneity and cause minimal residual disease. In order to achieve complete regression of tumors, CSCs have to be targeted. Recent advances in immunotherapies have shown promising outcomes in curing cancer, which are also applicable to target CSCs. CSCs express immune markers and exhibit specific immune characteristics in various cancers, which can be used in immunotherapies to target CSCs in the tumor microenvironment. Recently, various strategies have been used to target CSCs. Adaptive T-cells, dendritic cell (DC)-based vaccines, oncolytic viruses, immune checkpoint inhibitors, and combination therapies are now being used to target CSCs. Here, we discuss the feasibility of these immunological approaches and the recent trends in immunotherapies to target CSCs.