The success of cancer immunotherapy relies on the knowledge of the tumor microenvironment and the immune evasion mechanisms in which the tumor, stroma, and infiltrating immune cells function in a complex network. The potential barriers that profoundly challenge the overall clinical outcome of promising therapies need to be fully identified and counteracted. Although cancer immunotherapy has increasingly been applied, we are far from understanding how to utilize different strategies in the best way and how to combine therapeutic options to optimize clinical benefit. This review intends to give a contemporary and detailed overview of the different roles of immune cells, exosomes, and molecules acting in the tumor microenvironment and how they relate to immune activation and escape. Further, current and novel immunotherapeutic options will be discussed.

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The Tumor Microenvironment: A Milieu Hindering and Obstructing Antitumor Immune Responses

Matrix metalloproteinases (MMPs) are a family of zinc-dependent extracellular matrix (ECM) remodeling endopeptidases that have the capacity to degrade almost every component of the ECM. The degradation of the ECM is of great importance, since it is related to embryonic development and angiogenesis. It is also involved in cell repair and the remodeling of tissues. When the expression of MMPs is altered, it can generate the abnormal degradation of the ECM. This is the initial cause of the development of chronic degenerative diseases and vascular complications generated by diabetes. In addition, this process has an association with neurodegeneration and cancer progression. Within the ECM, the tissue inhibitors of MMPs (TIMPs) inhibit the proteolytic activity of MMPs. TIMPs are important regulators of ECM turnover, tissue remodeling, and cellular behavior. Therefore, TIMPs (similar to MMPs) modulate angiogenesis, cell proliferation, and apoptosis. An interruption in the balance between MMPs and TIMPs has been implicated in the pathophysiology and progression of several diseases. This review focuses on the participation of both MMPs (e.g., MMP-2 and MMP-9) and TIMPs (e.g., TIMP-1 and TIMP-3) in physiological processes and on how their abnormal regulation is associated with human diseases. The inclusion of current strategies and mechanisms of MMP inhibition in the development of new therapies targeting MMPs was also considered.

https://www.mdpi.com/1422-0067/21/24/9739/pdf

The Roles of Matrix Metalloproteinases and Their Inhibitors in Human Diseases.

Mesenchymal stromal cells (MSCs) are self-renewing, culture-expandable adult stem cells that have been isolated from a variety of tissues, and possess multipotent differentiation capacity, immunomodulatory properties, and are relatively non-immunogenic. Due to this unique set of characteristics, these cells have attracted great interest in the field of regenerative medicine and have been shown to possess pronounced therapeutic potential in many different pathologies. MSCs' mode of action involves a strong paracrine component resulting from the high levels of bioactive molecules they secrete in response to the local microenvironment. For this reason, MSCs' secretome is currently being explored in several clinical contexts, either using MSC-conditioned media (CM) or purified MSC-derived extracellular vesicles (EVs) to modulate tissue response to a wide array of injuries. Rather than being a constant mixture of molecular factors, MSCs' secretome is known to be dependent on the diverse stimuli present in the microenvironment that MSCs encounter. As such, the composition of the MSCs' secretome can be modulated by preconditioning the MSCs during in vitro culture. This manuscript reviews the existent literature on how preconditioning of MSCs affects the therapeutic potential of their secretome, focusing on MSCs' immunomodulatory and regenerative features, thereby providing new insights for the therapeutic use of MSCs' secretome.

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Mesenchymal Stromal Cell Secretome: Influencing Therapeutic Potential by Cellular Pre-conditioning.

It was shown as long as half a century ago that bone marrow is a source of not only hematopoietic stem cells, but also stem cells of mesenchymal tissues. Then the term "mesenchymal stem cells" (MSCs) was coined in the early 1990s, and more than a decade later, the criteria for defining MSCs have been released by the International Society for Cellular Therapy. The easy derivation from a variety of fetal and adult tissues and undemanding cell culture conditions made MSCs an attractive research object. It was followed by the avalanche of reports from preclinical studies on potentially therapeutic properties of MSCs, such as immunomodulation, trophic support and capability for a spontaneous differentiation into connective tissue cells, and differentiation into the majority of cell types upon specific inductive conditions. Although ontogenesis, niche, and heterogeneity of MSCs are still under investigation, there is a rapid boost of attempts at clinical applications of MSCs, especially for a flood of civilization-driven conditions in so quickly aging societies, not only in the developed countries, but also in the populous developing world. The fields of regenerative medicine and oncology are particularly extensively addressed by MSC applications, in part due to the paucity of traditional therapeutic options for these highly demanding and costly conditions. There are currently almost 1,000 clinical trials registered worldwide at ClinicalTrials.gov, and it seems that we are starting to witness the snowball effect with MSCs becoming a powerful global industry; however, the spectacular effects of MSCs in the clinic still need to be shown. Stem Cells 2019;37:855-864.

https://stemcellsjournals.onlinelibrary.wiley.com/doi/epdf/10.1002/stem.3016

Concise Review: Mesenchymal Stem Cells: From Roots to Boost.

Mesenchymal stem/stromal cells (MSCs) are commonly used as a source of cellular therapy due to their strong immunosuppressive and regenerative effects. One of the key mechanisms of MSC efficacy appears to derive from their paracrine activity. Recently, it has been shown that the secretion of different factors through extracellular vesicles known as exosomes, orchestrate the principle mechanisms of action of MSCs after infusion. The use of MSC-derived exosomes may provide considerable advantages over their counterpart live cells, potentially reducing undesirable side effects including infusional toxicities. In this review, we examine clinical trials of MSC-derived exosomes currently in progress for gene delivery, regenerative medicine, and immunomodulation. In addition, we summarize the limitations and clinical potential of this cell-free therapeutic strategy.

Mesenchymal stem cell-derived exosomes for clinical use.

Earlier research primarily attributed the effects of mesenchymal stem cell (MSC) therapies to their capacity for local engrafting and differentiating into multiple tissue types. However, recent studies have revealed that implanted cells do not survive for long, and that the benefits of MSC therapy could be due to the vast array of bioactive factors they produce, which play an important role in the regulation of key biologic processes. Secretome derivatives, such as conditioned media or exosomes, may present considerable advantages over cells for manufacturing, storage, handling, product shelf life and their potential as a ready-to-go biologic product. Nevertheless, regulatory requirements for manufacturing and quality control will be necessary to establish the safety and efficacy profile of these products. Among MSCs, human uterine cervical stem cells (hUCESCs) may be a good candidate for obtaining secretome-derived products. hUCESCs are obtained by Pap cervical smear, which is a less invasive and painful method than those used for obtaining other MSCs (for example, from bone marrow or adipose tissue). Moreover, due to easy isolation and a high proliferative rate, it is possible to obtain large amounts of hUCESCs or secretome-derived products for research and clinical use.

https://www.mdpi.com/1422-0067/18/9/1852/pdf

Mesenchymal Stem Cell Secretome: Toward Cell-Free Therapeutic Strategies in Regenerative Medicine

It has been reported that stromal cell features may affect the clinical outcome of breast cancer patients. Cancer associated fibroblasts (CAFs) represent one of the most abundant cell types within the breast cancer stroma. Here, we aimed to explore the influence of CAFs on breast cancer gene expression, as well as on invasion and angiogenesis. qRT-PCR was used to evaluate the expression of several cancer progression related genes (S100A4, TGFβ, FGF2, FGF7, PDGFA, PDGFB, VEGFA, IL-6, IL-8, uPA, MMP2, MMP9, MMP11 and TIMP1) in the human breast cancer-derived cell lines MCF-7 and MDA-MB-231, before and after co-culture with CAFs. Stromal mononuclear inflammatory cell (MIC) MMP11 expression was used to stratify primary tumors. In addition, we assessed the in vitro effects of CAFs on both MDA-MB-231 breast cancer cell invasion and endothelial cell (HUVEC) tube formation. We found that the expression levels of most of the genes tested were significantly increased in both breast cancer-derived cell lines after co-culture with CAFs from either MMP11+ or MMP11- MIC tumors. IL-6 and IL-8 showed an increased expression in both cancer-derived cell lines after co-culture with CAFs from MMP11+ MIC tumors. We also found that the invasive and angiogenic capacities of, respectively, MDA-MB-231 and HUVEC cells were increased after co-culture with CAFs, especially those from MMP11+ MIC tumors. Our data indicate that tumor-derived CAFs can induce up-regulation of genes involved in breast cancer progression. Our data additionally indicate that CAFs, especially those derived from MMP11+ MIC tumors, can promote breast cancer cell invasion and angiogenesis.

Cancer-associated fibroblasts affect breast cancer cell gene expression, invasion and angiogenesis.

Although the mechanisms underlying the genesis and progression of breast cancer are better understood than ever, it is still the most frequent malignant tumor in women and one of the leading causes of cancer death. Therefore, we need to establish new approaches that lead us to better understand the prognosis of this heterogeneous systemic disease and to propose new therapeutic strategies. Cancer is not only a malignant transformation of the epithelial cells merely based on their autonomous or acquired proliferative capacity. Today, data support the concept of cancer as an ecosystem based on a cellular sociology, with diverse components and complex interactions between them. Among the different cell types that make up the stroma, which have a relevant role in the dynamics of tumor/stromal cell interactions, the main ones are cancer associated fibroblasts, endothelial cells, immune cells and mesenchymal stromal cells. Several factors expressed by the stroma of breast carcinomas are associated with the development of metastasis, such as matrix metalloproteases, their tissular inhibitors or some of their regulators like integrins, cytokines or toll-like receptors. Based on the expression of these factors, two types of breast cancer stroma can be proposed with significantly different influence on the prognosis of patients. In addition, there is evidence about the existence of bi-directional signals between cancer cells and tumor stroma cells with prognostic implications, suggesting new therapeutic strategies in breast cancer.

https://www.mdpi.com/2072-6694/11/5/664/pdf

Breast Cancer Tumor Stroma: Cellular Components, Phenotypic Heterogeneity, Intercellular Communication, Prognostic Implications and Therapeutic Opportunities.

Prognostic Influence of Tumor Stroma on Breast Cancer Subtypes

The aim of this work was to evaluate the expression and clinical relevance of some cytokines in breast carcinomas. An immunohistochemical study using tissue arrays and specific antibodies against interleukin 1β (IL-1β), IL-6, IL-10, IL-17, interferon β (IFNβ) and nuclear factor kappa B (NFκB) was performed in 108 breast carcinomas. Most studied cytokines were mainly expressed by cancer cells but also by stromal cells as cancer-associated fibroblasts (CAFs) or mononuclear inflammatory cells (MICs). Global expression (score) of IL-1β and IL-17 was positively associated with histological grade; human epidermal growth factor receptor 2-positive tumors showed a higher global expression of IFNβ but a lower global expression of NFκB; and node-negative tumors showed a higher global expression of IL-6. High score of IL-6 was significantly associated with both longer relapse free-survival (RFS) and overall survival (OS). Moreover, the expression of IL-1β by each stromal cells (CAFs and MICs) was significantly associated with both longer RFS and OS, whereas the expression of IL-10 by these cells was significantly associated with both shorter RFS and OS. However, the combination of IL-1β, IL-6 and IL-10 expression by MICs reached an important association with prognosis and improved our previously reported prognostic signification based on the matrix metalloprotease 11 status by MICs. The combination of IL-1β, IL-6 and IL-10 expression by MICs was significant and independently associated with distant RFS in a multivariate analysis. Therefore, the combination of the expression of IL-1β, IL-6 and IL-10 may serve as promising biomarkers of MICs with prognostic significance, contributing to a better characterization of breast carcinomas microenvironment.

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Sandra Cid

Papers

Mesenchymal Stem Cell Secretome: Toward Cell-Free Therapeutic Strategies in Regenerative Medicine

Cancer-associated fibroblasts affect breast cancer cell gene expression, invasion and angiogenesis.

Breast Cancer Tumor Stroma: Cellular Components, Phenotypic Heterogeneity, Intercellular Communication, Prognostic Implications and Therapeutic Opportunities.

Prognostic Influence of Tumor Stroma on Breast Cancer Subtypes

Prognostic significance of inflammatory factors expression by stroma from breast carcinomas