Exploiting scavenger receptors in cancer immunotherapy: Lessons from CD5 and SR-B1

Macrophage scavenger receptors: Tumor support and tumor inhibition

Tumor-associated macrophages (TAMs) are a heterogeneous population of myeloid cells that constitute up to 50% of the cell mass of human tumors. TAMs interact with the components of the tumor microenvironment (TME) by using scavenger receptors (SRs), a large superfamily of multifunctional receptors that recognize, internalize and transport to the endosomal/lysosomal pathway apoptotic cells, cytokines, matrix molecules, lipid modified lipoproteins and other unwanted-self ligands. In our review, we summarized state-of-the art for the role of macrophage scavenger receptors in tumor development and their significance as cancer biomarkers. In this review we focused on functional activity of TAM-expressing SRs in animal models and in patients, and summarized the data for different human cancer types about the prognostic significance of TAM-expressed SRs. We discussed the role of SRs in the regulation of cancer cell biology, cell-cell and cell-matrix interaction in TME, immune status in TME, angiogenesis, and intratumoral metabolism. Targeting of tumor-promoting SRs can be a promising therapeutic approach in anti-cancer therapy. In our review we provide evidence for both tumor supporting and tumor inhibiting functions of scavenger receptors expressed on TAMs. We focused on the key differences in the prognostic and functional roles of SRs that are specific for cancer types. We highlighted perspectives for inhibition of tumor-promoting SRs in anti-cancer therapy.

The New General Biological Property of Stem-like Tumor Cells (Part II: Surface Molecules, Which Belongs to Distinctive Groups with Particular Functions, Form a Unique Pattern Characteristic of a Certain Type of Tumor Stem-like Cells)

An ability of poorly differentiated cells of different genesis, including tumor stem-like cells (TSCs), to internalize extracellular double-stranded DNA (dsDNA) fragments was revealed in our studies. Using the models of Krebs-2 murine ascites carcinoma and EBV-induced human B-cell lymphoma culture, we demonstrated that dsDNA internalization into the cell consists of several mechanistically distinct phases. The primary contact with cell membrane factors is determined by electrostatic interactions. Firm contacts with cell envelope proteins are then formed, followed by internalization into the cell of the complex formed between the factor and the dsDNA probe bound to it. The key binding sites were found to be the heparin-binding domains, which are constituents of various cell surface proteins of TSCs-either the C1q domain, the collagen-binding domain, or domains of positively charged amino acids. These results imply that the interaction between extracellular dsDNA fragments and the cell, as well as their internalization, took place with the involvement of glycocalyx components (proteoglycans/glycoproteins (PGs/GPs) and glycosylphosphatidylinositol-anchored proteins (GPI-APs)) and the system of scavenger receptors (SRs), which are characteristic of TSCs and form functional clusters of cell surface proteins in TSCs. The key provisions of the concept characterizing the principle of organization of the "group-specific" cell surface factors of TSCs of various geneses were formulated. These factors belong to three protein clusters: GPs/PGs, GIP-APs, and SRs. For TSCs of different tumors, these clusters were found to be represented by different members with homotypic functions corresponding to the general function of the cluster to which they belong.

Cholesterol supports bovine granulosa cell inflammatory responses to lipopolysaccharide

During bacterial infections of the bovine uterus or mammary gland, ovarian granulosa cells mount inflammatory responses to lipopolysaccharide (LPS). In vitro, LPS stimulates granulosa cell secretion of the cytokines IL-1α and IL-1β, and the chemokine IL-8. These LPS-stimulated inflammatory responses depend on culturing granulosa cells with serum, but the mechanism is unclear. Here we tested the hypothesis that cholesterol supports inflammatory responses to LPS in bovine granulosa cells. We used granulosa cells isolated from 4-8 mm and > 8.5 mm diameter ovarian follicles and manipulated the availability of cholesterol. We found that serum or follicular fluid containing cholesterol increased LPS-stimulated secretion of IL-1α and IL-1β from granulosa cells. Conversely, depleting cholesterol using methyl-β-cyclodextrin diminished LPS-stimulated secretion of IL-1α, IL-1β and IL-8 from granulosa cells cultured in serum. Follicular fluid contained more high-density lipoprotein cholesterol than low-density lipoprotein cholesterol, and granulosa cells expressed the receptor for high-density lipoprotein, scavenger receptor class B member 1 (SCARB1). Furthermore, culturing granulosa cells with high-density lipoprotein cholesterol, but not low-density lipoprotein or very low-density lipoprotein cholesterol, increased LPS-stimulated inflammation in granulosa cells. Cholesterol biosynthesis also played a role in granulosa cell inflammation because RNA interference of mevalonate pathway enzymes inhibited LPS-stimulated inflammation. Finally, treatment with follicle-stimulating hormone, but not luteinizing hormone, increased LPS-stimulated granulosa cell inflammation, and follicle-stimulating hormone increased SCARB1 protein. However, changes in inflammation were not associated with changes in oestradiol or progesterone secretion. Taken together these findings imply that cholesterol supports inflammatory responses to LPS in granulosa cells.

Overcoming the limitations of cytokines to improve cancer therapy

Cytokines are pleiotropic soluble proteins used by immune cells to orchestrate a coordinated response against pathogens and malignancies. In cancer immunotherapy, cytokine-based drugs can be developed potentiating pro-inflammatory cytokines or blocking immunosuppressive cytokines. However, the complexity of the mechanisms of action of cytokines requires the use of biotechnological strategies to minimize systemic toxicity, while potentiating the antitumor response. Sequence mutagenesis, fusion proteins and gene therapy strategies are employed to enhance the half-life in circulation, target the desired bioactivity to the tumor microenvironment, and to optimize the therapeutic window of cytokines. In this review, we provide an overview of the different strategies currently being pursued in pre-clinical and clinical studies to make the most of cytokines for cancer immunotherapy.

Scavenger Receptors: Novel Roles in the Pathogenesis of Liver Inflammation and Cancer

The scavenger receptor superfamily represents a highly diverse collection of evolutionarily-conserved receptors which are known to play key roles in host homeostasis, the most prominent of which is the clearance of unwanted endogenous macromolecules, such as oxidized low-density lipoproteins, from the systemic circulation. Members of this family have also been well characterized in their binding and internalization of a vast range of exogenous antigens and, consequently, are generally considered to be pattern recognition receptors, thus contributing to innate immunity. Several studies have implicated scavenger receptors in the pathophysiology of several inflammatory diseases, such as Alzheimer's and atherosclerosis. Hepatic resident cellular populations express a diverse complement of scavenger receptors in keeping with the liver's homeostatic functions, but there is gathering interest in the contribution of these receptors to hepatic inflammation and its complications. Here, we review the expression of scavenger receptors in the liver, their functionality in liver homeostasis, and their role in inflammatory liver disease and cancer.

Cholesterol-Ester Transfer Protein Alters M1 and M2 Macrophage Polarization and Worsens Experimental Elastase-Induced Pulmonary Emphysema

Cholesterol-ester transfer protein (CETP) plays a role in atherosclerosis, the inflammatory response to endotoxemia and in experimental and human sepsis. Functional alterations in lipoprotein (LP) metabolism and immune cell populations, including macrophages, occur during sepsis and may be related to comorbidities such as chronic obstructive pulmonary disease (COPD). Macrophages are significantly associated with pulmonary emphysema, and depending on the microenvironment, might exhibit an M1 or M2 phenotype. Macrophages derived from the peritoneum and bone marrow reveal CETP that contributes to its plasma concentration. Here, we evaluated the role of CETP in macrophage polarization and elastase-induced pulmonary emphysema (ELA) in human CETP-expressing transgenic (huCETP) (line 5203, C57BL6/J background) male mice and compared it to their wild type littermates. We showed that bone marrow-derived macrophages from huCETP mice reduce polarization toward the M1 phenotype, but with increased IL-10. Compared to WT, huCETP mice exposed to elastase showed worsened lung function with an increased mean linear intercept (Lm), reflecting airspace enlargement resulting from parenchymal destruction with increased expression of arginase-1 and IL-10, which are M2 markers. The cytokine profile revealed increased IL-6 in plasma and TNF, and IL-10 in bronchoalveolar lavage (BAL), corroborating with the lung immunohistochemistry in the huCETP-ELA group compared to WT-ELA. Elastase treatment in the huCETP group increased VLDL-C and reduced HDL-C. Elastase-induced pulmonary emphysema in huCETP mice promotes lung M2-like phenotype with a deleterious effect in experimental COPD, corroborating the in vitro result in which CETP promoted M2 macrophage polarization. Our results suggest that CETP is associated with inflammatory response and influences the role of macrophages in COPD.

The Role of microRNAs in Metabolic Syndrome-Related Oxidative Stress

Oxidative stress (OxS) is the cause and the consequence of metabolic syndrome (MetS), the incidence and economic burden of which is increasing each year. OxS triggers the dysregulation of signaling pathways associated with metabolism and epigenetics, including microRNAs, which are biomarkers of metabolic disorders. In this review, we aimed to summarize the current knowledge regarding the interplay between microRNAs and OxS in MetS and its components. We searched PubMed and Google Scholar to summarize the most relevant studies. Collected data suggested that different sources of OxS (e.g., hyperglycemia, insulin resistance (IR), hyperlipidemia, obesity, proinflammatory cytokines) change the expression of numerous microRNAs in organs involved in the regulation of glucose and lipid metabolism and endothelium. Dysregulated microRNAs either directly or indirectly affect the expression and/or activity of molecules of antioxidative signaling pathways (SIRT1, FOXOs, Keap1/Nrf2) along with effector enzymes (e.g., GPx-1, SOD1/2, HO-1), ROS producers (e.g., NOX4/5), as well as genes of numerous signaling pathways connected with inflammation, insulin sensitivity, and lipid metabolism, thus promoting the progression of metabolic imbalance. MicroRNAs appear to be important epigenetic modifiers in managing the delicate redox balance, mediating either pro- or antioxidant biological impacts. Summarizing, microRNAs may be promising therapeutic targets in ameliorating the repercussions of OxS in MetS.

Reconfiguring Nature's Cholesterol Accepting Lipoproteins as Nanoparticle Platforms for Transport and Delivery of Therapeutic and Imaging Agents

Apolipoproteins are critical structural and functional components of lipoproteins, which are large supramolecular assemblies composed predominantly of lipids and proteins, and other biomolecules such as nucleic acids. A signature feature of apolipoproteins is the preponderance of amphipathic α-helical motifs that dictate their ability to make extensive non-covalent inter- or intra-molecular helix-helix interactions in lipid-free states or helix-lipid interactions with hydrophobic biomolecules in lipid-associated states. This review focuses on the latter ability of apolipoproteins, which has been capitalized on to reconstitute synthetic nanoscale binary/ternary lipoprotein complexes composed of apolipoproteins/peptides and lipids that mimic native high-density lipoproteins (HDLs) with the goal to transport drugs. It traces the historical development of our understanding of these nanostructures and how the cholesterol accepting property of HDL has been reconfigured to develop them as drug-loading platforms. The review provides the structural perspective of these platforms with different types of apolipoproteins and an overview of their synthesis. It also examines the cargo that have been loaded into the core for therapeutic and imaging purposes. Finally, it lays out the merits and challenges associated with apolipoprotein-based nanostructures with a future perspective calling for a need to develop "zip-code"-based delivery for therapeutic and diagnostic applications.

Cellular Stress and General Pathological Processes

From the viewpoint of the general pathology, most of the human diseases are associated with a limited number of pathogenic processes such as inflammation, tumor growth, thrombosis, necrosis, fibrosis, atrophy, pathological hypertrophy, dysplasia and metaplasia. The phenomenon of chronic low-grade inflammation could be attributed to non-classical forms of inflammation, which include many neurodegenerative processes, pathological variants of insulin resistance, atherosclerosis, and other manifestations of the endothelial dysfunction. Individual and universal manifestations of cellular stress could be considered as a basic element of all these pathologies, which has both physiological and pathophysiological significance. The review examines the causes, main phenomena, developmental directions and outcomes of cellular stress using a phylogenetically conservative set of genes and their activation pathways, as well as tissue stress and its role in inflammatory and para-inflammatory processes. The main ways towards the realization of cellular stress and its functional blocks were outlined. The main stages of tissue stress and the classification of its typical manifestations, as well as its participation in the development of the classical and non-classical variants of the inflammatory process, were also described. The mechanisms of cellular and tissue stress are structured into the complex systems, which include networks that enable the exchange of information with multidirectional signaling pathways which together make these systems internally contradictory, and the result of their effects is often unpredictable. However, the possible solutions require new theoretical and methodological approaches, one of which includes the transition to integral criteria, which plausibly reflect the holistic image of these processes.

Prognostic Value of CD1B in Localised Prostate Cancer

Cluster of differentiation (CD) antigens are cell surface markers used to differentiate haematopoietic cell types. These antigens are present in various malignancies and are reportedly linked to patient prognosis; however, they have not been implemented as prostate cancer progression markers. Here, we aimed to assess the impact of genetic variation in haematopoietic cell CD markers on clinical outcomes in patients with prostate cancer. An association study of 458 patients with prostate cancer was conducted to identify single-nucleotide polymorphisms in 11 candidate CD marker genes associated with biochemical recurrence (BCR) after radical prostatectomy. Identified predictors were further evaluated in an additional cohort of 185 patients. Joint population analyses showed that CD1B rs3181082 is associated with BCR (adjusted hazard ratio 1.42, 95% confidence interval 1.09-1.85, p = 0.010). In addition, rs3181082 overlapped with predicted transcriptional regulatory elements and affected CD1B expression. Furthermore, low CD1B expression correlated with poorer BCR-free survival. Our results indicated that CD1B rs3181082 confers prostate cancer progression and may help improve clinical prognostic stratification.

New Aspects Towards a Molecular Understanding of the Allicin Immunostimulatory Mechanism via Colec12, MARCO, and SCARB1 Receptors

The allicin pleiotropic effects, which include anti-inflammatory, anti-oxidant, anti-tumoral, and antibacterial actions, were well demonstrated and correlated with various molecular pathways. The immunostimulatory mechanism of allicin has not been elucidated; however, there is a possible cytokine stimulation from immunoglobulin release caused by allicin. In this study, when Wistar female rats and CD19+ lymphocytes were treated with three different doses of allicin, immunoglobulins, glutathione, and oxidative stress markers were assayed. Molecular docking was performed between S-allylmercaptoglutathione (GSSA)—a circulating form of allicin in in vivo systems formed by the allicin interaction with glutathione (GSH)—and scavenger receptors class A and B from macrophages, as well as CD19+ B lymphocytes. Our data demonstrated a humoral immunostimulatory effect of allicin in rats and direct stimulation of B lymphocytes by S-allyl-mercapto-glutathione, both correlated with decreased catalase (CAT) activity. The molecular docking revealed that S-allyl-mercapto-glutathione interacting with Colec12, MARCO (class A), and SCARB1 (class B) scavenger receptors in in vitro tests demonstrates a direct stimulation of immunoglobulin secretion by GSSA in CD19+ B lymphocytes. These data collectively indicate that GSSA stimulates immunoglobulin secretion by binding on scavenger receptors class B type 1 (SCARB1) from CD19+ B lymphocytes.

Immunohistochemical Labeling of Low-Density Lipoprotein Receptor and Scavenger Receptor Class B Type 1 Are Increased in Canine Lymphoma

Altered lipid metabolism is a well-documented hallmark of neoplastic transformation and impacts disease progression. Two major lipoprotein receptors, the low-density lipoprotein receptor (LDL-R) and scavenger receptor class B, type 1 (SR-BI) are overexpressed in a number of cancer types in people. These receptors serve to deliver cholesterol to the tumor cells and have been used to target drug therapies. In this study, we performed a retrospective analysis of LDL-R and SR-B1 expression in canine lymphoma using archived formalin-fixed tissue samples. Cases were immunophenotyped and classified according to World Health Organization (WHO) standards prior to immunostaining for the LDL_R and SR-B1. A total of 45 cases were evaluated; 21 high grade B (HGB), 11 low grade B (LGB), 7 high grade T (HGT), and 6 low grade T (LGT) lymphomas. One sided Wilcoxon rank sum tests were used to compare staining intensity between neoplastic and hyperplastic lymphoid tissue. The relationships between histological score and tumor grade and score and stage at presentation were assessed using non-parametric Kruskal-Wallis tests. Neoplastic lymphoid tissue expressed higher levels of both receptors compared to reactive lymph nodes. Median LDL-R score was 85.0 (interquartile range = 101.7), Median SR-B1 score was 209.0 (interquartile range 105.2). No relationship between LDL-R or SR-B1 staining score and tumor grade or phenotype was found. Serum cholesterol concentration was compared between dogs with high and low grade tumors using a two sample T-test, and correlations between cholesterol concentration and histological score, and between the score for the two receptors were determined using a Spearman correlation. The high expression level of these lipoprotein receptors on most of the tumors could underlie the lack of relationship between score and tumor grade. The overexpression of LDL-R and SR-B1 in canine lymphoma holds therapeutic potential particularly in dogs that overexpress one or both of these receptors, and this warrants further investigation.

Immunomodulatory effects of soluble CD5 on experimental tumor models

Modulation of antitumor immune responses by targeting immune checkpoint regulators has been proven successful in the treatment of many different tumors. Recent evidence shows that the lymphocyte receptor CD5 –a negative regulator of TCR-mediated signaling- may play a role in the anti-tumor immune response. To explore such an issue, we developed transgenic C57BL/6 mice expressing a soluble form of human CD5 (shCD5EμTg), putatively blocking CD5-mediated interactions (“decoy receptor” effect). Homozygous shCD5EμTg mice showed reduced growth rates of tumor cells of melanoma (B16-F0) and thymoma (EG7-OVA) origin. Concomitantly, increased CD4⁺ and CD8⁺ T cell numbers, as well as reduced proportion of CD4⁺CD25⁺FoxP3⁺ (T_reg) cells were observed in tumor draining lymph nodes (TdLN). TdLN cell suspensions from tumor-bearing shCD5EμTg mice showed increased both tumor specific and non-specific cytolitic activity. Moreover, subcutaneous peritumoral (p.t.) injection of recombinant shCD5 to wild-type (WT) mice slowed B16-F0 tumor growth, and reproduced the above mentioned TdLN cellular changes. Interestingly, lower intratumoral IL-6 levels –an inhibitor of Natural Killer (NK) cell cytotoxity- were observed in both transgenic and rshCD5-treated WT mice and the anti-tumor effect was abrogated by mAb-induced NK cell depletion. Taken together, the results further illustrate the putative regulatory role of CD5-mediated interactions in anti-tumor immune responses, which would be at least in part fostered by NK cells.

Enhancement of antibody-dependent cellular cytotoxicity of cetuximab by a chimeric protein encompassing interleukin-15

Enhancement of antibody-dependent cellular cytotoxicity (ADCC) may potentiate the antitumor efficacy of tumor-targeted monoclonal antibodies. Increasing the numbers and antitumor activity of NK cells is a promising strategy to maximize the ADCC of standard-of-care tumor-targeted antibodies. For this purpose, we have preclinically tested a recombinant chimeric protein encompassing the sushi domain of the IL15Rα, IL-15, and apolipoprotein A-I (Sushi-IL15-Apo) as produced in CHO cells. The size-exclusion purified monomeric fraction of this chimeric protein was stable and retained the IL-15 and the sushi domain bioactivity as measured by CTLL-2 and Mo-7e cell proliferation and STAT5 phosphorylation in freshly isolated human NK and CD8⁺ T cells. On cell cultures, Sushi-IL15-Apo increases NK cell proliferation and survival as well as spontaneous and antibody-mediated cytotoxicity. Scavenger receptor class B type I (SR-B1) is the receptor for ApoA-I and is expressed on the surface of tumor cells. SR-B1 can adsorb the chimeric protein on tumor cells and can transpresent IL-15 to NK and CD8⁺ T cells. A transient NK-humanized murine model was developed to test the increase of ADCC attained by the chimeric protein in vivo. The EGFR⁺ human colon cancer cell line HT-29 was intraperitoneally inoculated in immune-deficient Rag2^-/-γc^-/- mice that were reconstituted with freshly isolated PBMCs and treated with the anti-EGFR mAb cetuximab. The combination of the Sushi-IL15-Apo protein and cetuximab reduced the number of remaining tumor cells in the peritoneal cavity and delayed tumor engraftment in the peritoneum. Furthermore, Sushi-IL15-Apo increased the anti-tumor effect of a murine anti-EGFR mAb in Rag1^-/- mice bearing subcutaneous MC38 colon cancer transfected to express EGFR. Thus, Sushi-IL15-Apo is a potent tool to increase the number and the activation of NK cells to promote the ADCC activity of antibodies targeting tumor antigens.