Prominin-1 (CD133) and Metastatic Melanoma: Current Knowledge and Therapeutic Perspectives

Innovative approaches to specifically target the melanoma subpopulation responsible for local invasion and metastatic dissemination are needed. Prominin-1 (CD133) expression has been observed in many melanoma cell lines, as well as in primary and metastatic melanomas from patients. Although its function(s) in melanoma is presently unknown, prominin-1 may represent a molecular target, due to its association with melanoma stem cells and with the metastatic phenotype.

Synergistic cytotoxicity of ex vivo expanded natural killer cells in combination with monoclonal antibody drugs against cancer cells

The adoptive transfer of highly cytotoxic natural killer (NK) cells is an emerging tool for cancer immunotherapy. Antibody-dependent cellular cytotoxicity (ADCC) has recently been identified as one of the critical factors for the clinical efficacy of anticancer antibodies, in which NK cells are the major effectors of ADCC. NK cells were expanded from PBMC by a feeder-cell-free expansion method. NK cell expansion efficiency was evaluated within a period of 21 days. The kinetics of NK cell expansion and the expression of activating and inhibitory receptors on NK cells were monitored. NK cells producing IFN-γ and TNF-α were detected by intracellular cytokine staining. The cytotoxicity of expanded NK cells against various cancer cells was compared with that of freshly isolated NK cells. The ADCC functions of expanded NK cells in combination with rituximab against CD20+ lymphoma cell lines were evaluated. Our method efficiently expanded NK cells ex vivo, which showed a much higher activity to induce the expression of activating receptors and to produce IFN-γ and TNF-α as well as cytotoxicity against various cancer cell lines including CD133+ primary cancer cells than freshly isolated NK cells. We observed a synergistic cytotoxicity of our expanded NK cells against CD20+ B lymphoma cell lines as well as higher IFN-γ and TNF-α production when combined with rituximab. Our results suggest that the adoptive transfer of a large number of ex vivo expanded NK cells, particularly in combination with monoclonal antibody drugs, is a useful tool for cancer immunotherapy.

CD133+ melanoma subpopulations contribute to perivascular niche morphogenesis and tumorigenicity through vasculogenic mimicry

Tumor cell subpopulations that express cancer stem cell markers such as CD133 (prominin1) or ABCB5 are thought to be crucial for tumor initiation and heterogeneity, but their biological significance in melanoma has been controversial. Here, we report that CD133(+) and ABCB5(+) subpopulations are colocalized in melanomas in perivascular niches that contain CD144 (VE-cadherin)(+) melanoma cells forming vessel-like channels, a phenomenon termed vasculogenic mimicry (VM). RNAi-mediated attenuation of CD133 established its critical function in morphogenesis of these perivascular niches as well as in melanoma tumorigenicity. Niche-associated genes CD144 and ABCB5 were downregulated in tumors derived from CD133 knockdown (KD) melanoma cells compared with controls. CD133KD cells also lacked the ability to form CD144(+) VM-like channels in a manner that was associated with a depletion of the ABCB5(+) cell subpopulation. Finally, CD133 KD cells exhibited poorer tumor growth in vivo. Taken together, our findings corroborate models in which CD133(+)/ABCB5(+) melanoma cells reside in a complex anastomosing microvascular niche that encompasses CD144(+) VM channels as well as authentic endothelial cell-lined blood vessels. Further, they indicate that CD133(+) cells act as stem-like cells, which drive tumor growth by promoting VM and the morphogenesis of a specialized perivascular niche in melanoma.

Expression of CD133 and other putative stem cell markers in uveal melanoma

'Cancer stem cells' (CSCs) are tumor cells with stem cell properties hypothesized to be responsible for tumorigenesis, metastatis, and resistance to treatment, and have been identified in different tumors including cutaneous melanoma, using stem cell markers such as CD133. This study explored expression of CD133 and other putative stem cell markers in uveal melanoma. Eight uveal melanoma cell lines were subjected to flow-cytometric (fluorescence-activated cell sorting) analysis of CD133 and other stem cell markers. Eight paraffin-embedded tumors were analyzed by immunohistochemistry for CD133, Pax6, Musashi, nestin, Sox2, ABCB5, and CD68 expressions. Ocular, uveal melanoma, and hematopoietic stem cell distributions of C-terminal and N-terminal CD133 mRNA splice variants were compared by reverse-transcription PCR. Fluorescence-activated cell sorting analysis revealed a population of CD133-positive/nestin-positive cells in cell lines Mel270, OMM 2.3, and OMM2.5. All cell lines studied were positive for nestin, CXCR-4, CD44, and c-kit. Immunohistochemistry identified cells positive for CD133, Pax6, Musashi, nestin, Sox2, ABCB5, and CD68 predominantly at the invading tumor front. C-terminal primers interacting with CD133 splice variant s2 detected a novel variant lacking exon 27. Differential expression of CD133 splice variants was found in iris, ciliary body, retina, and retinal pigment epithelium/choroid as well as in uveal melanoma cell lines. mRNA for nestin, Sox2, and Musashi was present in all studied cell lines. Uveal melanoma such as cutaneous melanoma may therefore contain CSCs. Further experiments are needed to isolate stem cell marker-positive cells, to evaluate their functional properties and to explore therapeutical approaches to these putative CSCs in uveal melanoma.

Melanoma cells inhibit natural killer cell function by modulating the expression of activating receptors and cytolytic activity

Natural killer (NK) cells play a key role in tumor immune surveillance. However, adoptive immunotherapy protocols using NK cells have shown limited clinical efficacy to date, possibly due to tumor escape mechanisms that inhibit NK cell function. In this study, we analyzed the effect of coculturing melanoma cells and NK cells on their phenotype and function. We found that melanoma cells inhibited the expression of major NK receptors that trigger their immune function, including NKp30, NKp44, and NKG2D, with consequent impairment of NK cell-mediated cytolytic activity against various melanoma cell lines. This inhibitory effect was primarily mediated by indoleamine 2,3-dioxygenase (IDO) and prostaglandin E2 (PGE2). Together, our findings suggest that immunosuppressive barriers erected by tumors greatly hamper the antitumor activity of human NK cells, thereby favoring tumor outgrowth and progression.

Autologous CTL response against cancer stem-like cells/cancer-initiating cells of bone malignant fibrous histiocytoma

Malignant fibrous histiocytoma (MFH) of the bone is an aggressive tumor with high rates of local recurrence and metastasis. The development of novel therapeutic approaches is critical to improve the prognosis of patients with MFH. We reported previously that the side population (SP) cells of the MFH2003 bone MFH cell line have the characteristics of cancer stem-like cells (CSC)/cancer-initiating cells. In the present study, to establish immunotherapy targeting CSC, we analyzed cell surface immune molecules on SP cells of the MHF2003 cell line, as well as autologous CTL responses against these SP cells in the tumor microenvironment and peripheral circulating lymphocytes, using autologous tumor-infiltrating lymphocytes and autologous CTL clones derived from peripheral blood, respectively. We found that the SP cells expressed human leukocyte antigen (HLA) Class I molecules on the cell surface. The autologous tumor-infiltrating lymphocyte line TIL2003 recognized both the SP and main population cells of the MFH2003 cell line. Next, we induced the CTL clone Tc4C-6 by mixed lymphocyte tumor cell culture using autologous peripheral blood mononuclear cells and freshly isolated SP cells, followed by a limiting dilution procedure. The Tc4C-6 clone showed specific cytotoxicity against the SP cells. Moreover, the cytotoxicity against SP cells was blocked by the anti-HLA Class I antibody W6/32. In conclusion, the findings of the present study support the idea that CSC of bone MFH are recognized by autologous CTL in the tumor microenvironment and peripheral circulating lymphocytes. Thus, CTL-based immunotherapy could target CSC of bone sarcoma to help prevent tumor recurrence.

Immunotherapy targeting colon cancer stem cells

In the last 10 years, cancer stem cells have interested the scientific community because this small tumorigenic population is also associated with tumor progression in human patients and specific targeting of cancer stem cells could be a strategy to eradicate cancers currently resistant to conventional therapy. Clinical studies have recently demonstrated that adding immune therapy to chemotherapy has survival benefits in comparison with chemotherapy alone that can sensitize tumors to immune cell-mediated killing (e.g., increasing sensitivity of tumor cells to subsequent cytotoxicity by T cells via upregulation of death receptors DR5 and Fas). However, loss of MHC molecules is often observed in cancer cells, rendering tumor cells resistant to CD8 T-cell-mediated cytotoxicity. For this reason, we review the role of other T-cell subsets, such as γδ T and NK cells that are able to efficiently recognize and kill tumor cells and that could be used in passive or active immunotherapy in cancer stem cell eradication.

Human melanoma-initiating cells express neural crest nerve growth factor receptor CD271

The question whether tumorigenic cancer stem cells exist in human melanomas has arisen recently1. Here we show that in melanomas, tumor stem cells (MTSC) can be isolated prospectively as a highly enriched CD271+ MTSC population using a process that maximizes viable cell transplantation1,6. In this study the tumors sampled were taken from a broad spectrum of sites and stages. High viability FACS isolated cells resuspended in a matrigel vehicle were implanted into T, B, and NK deficient Rag2−/− γc−/− mice (RG) mice. The CD271+ subset of cells was the tumor initiating population in 9/10 melanomas tested. Transplantation of isolated melanoma cells into engrafted human skin or bone in RG mice resulted in melanoma from CD271+ but not CD271− cells. We also showed that tumors transplanted by CD271+ patient cells were capable of metastasis in-vivo. Importantly, CD271+ melanoma cells lacked expression of TYR, MART and MAGE in 86%, 69% and 68% of melanoma patients respectively suggesting why T cell therapies directed at these antigens usually result in only temporary tumor shrinkage.

Human melanoma-initiating cells express neural crest nerve growth factor receptor CD271

The question of whether tumorigenic cancer stem cells exist in human melanomas has arisen in the last few years. Here we show that in melanomas, tumour stem cells (MTSCs, for melanoma tumour stem cells) can be isolated prospectively as a highly enriched CD271(+) MTSC population using a process that maximizes viable cell transplantation. The tumours sampled in this study were taken from a broad spectrum of sites and stages. High-viability cells isolated by fluorescence-activated cell sorting and re-suspended in a matrigel vehicle were implanted into T-, B- and natural-killer-deficient Rag2(-/-)gammac(-/-) mice. The CD271(+) subset of cells was the tumour-initiating population in 90% (nine out of ten) of melanomas tested. Transplantation of isolated CD271(+) melanoma cells into engrafted human skin or bone in Rag2(-/-)gammac(-/-) mice resulted in melanoma; however, melanoma did not develop after transplantation of isolated CD271(-) cells. We also show that in mice, tumours derived from transplanted human CD271(+) melanoma cells were capable of metastatsis in vivo. CD271(+) melanoma cells lacked expression of TYR, MART1 and MAGE in 86%, 69% and 68% of melanoma patients, respectively, which helps to explain why T-cell therapies directed at these antigens usually result in only temporary tumour shrinkage.

Emerging strategies for the identification and targeting of cancer stem cells

The hypothesis of cancer stem cells (CSCs) is receiving increasing interest and has become the subject of considerable debate among cancer researchers. Recent rapid progress in CSC research has encountered increasing difficulties and challenges. Understanding the biologic characteristic of CSCs is crucial to start with better identification and diagnosis based on CSC markers and eventually targeting to CSCs will undoubtedly result in improved prevention and treatment of many types of CSCs. We discuss here some of the approaching strategies that include establishing special methods of identifying CSCs and targeting therapies of CSCs.

High log-scale expansion of functional human natural killer cells from umbilical cord blood CD34-positive cells for adoptive cancer immunotherapy

Immunotherapy based on natural killer (NK) cell infusions is a potential adjuvant treatment for many cancers. Such therapeutic application in humans requires large numbers of functional NK cells that have been selected and expanded using clinical grade protocols. We established an extremely efficient cytokine-based culture system for ex vivo expansion of NK cells from hematopoietic stem and progenitor cells from umbilical cord blood (UCB). Systematic refinement of this two-step system using a novel clinical grade medium resulted in a therapeutically applicable cell culture protocol. CD56⁺CD3⁻ NK cell products could be routinely generated from freshly selected CD34⁺ UCB cells with a mean expansion of >15,000 fold and a nearly 100% purity. Moreover, our protocol has the capacity to produce more than 3-log NK cell expansion from frozen CD34⁺ UCB cells. These ex vivo-generated cell products contain NK cell subsets differentially expressing NKG2A and killer immunoglobulin-like receptors. Furthermore, UCB-derived CD56⁺ NK cells generated by our protocol uniformly express high levels of activating NKG2D and natural cytotoxicity receptors. Functional analysis showed that these ex vivo-generated NK cells efficiently target myeloid leukemia and melanoma tumor cell lines, and mediate cytolysis of primary leukemia cells at low NK-target ratios. Our culture system exemplifies a major breakthrough in producing pure NK cell products from limited numbers of CD34⁺ cells for cancer immunotherapy.

Susceptibility of human melanoma cells to autologous natural killer (NK) cell killing: HLA-related effector mechanisms and role of unlicensed NK cells

Background

Despite Natural Killer (NK) cells were originally defined as effectors of spontaneous cytotoxicity against tumors, extremely limited information is so far available in humans on their capability of killing cancer cells in an autologous setting.

Methodology/Principal Findings

We have established a series of primary melanoma cell lines from surgically resected specimens and here showed that human melanoma cells were highly susceptible to lysis by activated autologous NK cells. A variety of NK cell activating receptors were involved in killing: particularly, DNAM-1 and NKp46 were the most frequently involved. Since self HLA class I molecules normally play a protective role from NK cell-mediated attack, we analyzed HLA class I expression on melanomas in comparison to autologous lymphocytes. We found that melanoma cells presented specific allelic losses in 50% of the patients analyzed. In addition, CD107a degranulation assays applied to NK cells expressing a single inhibitory receptor, revealed that, even when expressed, specific HLA class I molecules are present on melanoma cell surface in amount often insufficient to inhibit NK cell cytotoxicity. Remarkably, upon activation, also the so called “unlicensed” NK cells, i.e. NK cells not expressing inhibitory receptor specific for self HLA class I molecules, acquired the capability of efficiently killing autologous melanoma cells, thus additionally contributing to the lysis by a mechanism independent of HLA class I expression on melanoma cells.

Conclusions/Significance

We have investigated in details the mechanisms controlling the recognition and lysis of melanoma cells by autologous NK cells. In these autologous settings, we demonstrated an efficient in vitro killing upon NK cell activation by mechanisms that may be related or not to abnormalities of HLA class I expression on melanoma cells. These findings should be taken into account in the design of novel immunotherapy approaches against melanoma.