Breslow thickness and Clark level are prognostic factors for human cutaneous melanomas. Breslow thickness is measured with an ocular micrometer from the top of the granular layer of the epidermis to the deepest invasive cell across the broad base of the tumor, while Clark level is based on the anatomical level of invasion through the layers of the dermis. Because of the anatomical differences between humans and dogs, we evaluated the tumor thickness and a modified Clark level in 77 canine primary cutaneous melanocytic tumors. Tumor thickness (using both a traditional and a more convenient system) and modified Clark level were measured and associated with histological diagnosis and clinical outcome. Tumor thickness was a prognostic factor, being greater in animals with shorter overall survival and disease-free time. Cutoffs of 0.95 cm and 0.75 cm defined a higher hazard for an unfavorable outcome and to develop recurrence/metastasis, respectively. Because of an excellent agreement between the 2 methods, it was concluded that tumor thickness could be measured with a ruler when an ocular micrometer is not available. Modified Clark level was not found to be relevant for prognosis. However, we suggest that both tumor thickness and a modified Clark level can be valid additional parameters when histological diagnosis is uncertain. Further studies, including a wider sample population, would be worthwhile to confirm the prognostic significance of these 2 parameters.

The study of the immune response in several types of tumors has been rapidly increasing in recent years with the dual aim of understanding the relationship between neoplastic and immune cells as well as identifying targets for cancer immunotherapy. Despite being considered one of the most immunogenic tumor types, melanoma can progress in the presence of abundant lymphocytic infiltration, therefore suggesting that the immune response is not able to efficiently control tumor growth. The purpose of this study was to investigate whether the density, distribution and grade of tumor-infiltrating lymphocytes (TILs) in 97 canine melanocytic tumors is associated with histologic indicators of malignancy and can be considered a prognostic factor in the dog. As a further step in the characterization of the immune response in melanocytic tumors, an immunohistochemical investigation was performed to evaluate the two main populations of TILs, T-lymphocytes (CD3 +) and B-lymphocytes (CD20 +). The results of our study show that TILs are present in a large proportion of canine melanocytic tumors, especially in oral melanomas, and that, differently from humans, the infiltrate is usually mild. The quantity of CD20 + TILs was significantly associated with some histologic prognostic factors, such as the mitotic count, the cellular pleomorphism and the percentage of pigmented cells. Remarkably, a high infiltration of CD20 + TILs was associated with tumor-related death, presence of metastasis/recurrence, shorter overall and disease-free survival, increased hazard of death and of developing recurrence/metastasis, hence representing a potential new negative prognostic factor in canine melanocytic tumors.

Human melanoma is one of the deadliest forms of cancer, with poor prognosis and high resistance to chemotherapy and radiotherapy. The discovery of immunosuppressive mechanisms in the human melanoma microenvironment led to the use of new prognostic markers and to the development of immunotherapies targeting immune checkpoint molecules. Immunoescape mechanisms in canine melanoma have not yet been investigated, and no such immunotherapy has been tested. The aim of this study was to provide preliminary data on the expression of transcription factor forkhead box protein P3 (FoxP3) and indoleamine 2,3-dioxygenase (IDO) in primary canine melanocytic tumors and to investigate their prognostic role. Formalin-fixed, paraffin-embedded samples from 74 canine melanocytic tumors (26 oral melanomas, 23 cutaneous melanomas, and 25 cutaneous melanocytomas) were retrospectively evaluated by immunohistochemistry to explore the expression of FoxP3 and IDO. An increased risk of death due to melanoma was associated with a higher number of FoxP3+ cells per high-power field (FoxP3+/HPF), a higher percentage of CD3+ cells that were also FoxP3+ infiltrating and surrounding the tumor (%FoxP3), and a higher number of IDO+ cells/HPF (IDO+/HPF). A prognostic value for FoxP3 and IDO is suggested by our study, with optimal cutoffs of 14.7 FoxP3+ cells/HPF, 6.1 IDO+ cells/HPF, and 12.5% FoxP3+ cells. Both markers were also associated with tumor type. Multivariable analysis identified IDO+/HPF ( P < .001) as an independent prognostic marker. Even though stratification by diagnosis caused a loss of significance, results from the present study suggest a prognostic role for IDO and FoxP3, possibly related to the establishment of an immunosuppressive microenvironment.

Interactions between tumor cells and tumor microenvironment are considered critical in carcinogenesis, tumor invasion and metastasis. To examine transcriptome changes and to explore the relationship with tumor microenvironment in canine cutaneous melanocytoma and melanoma, we extracted RNA from formalin-fixed, paraffin-embedded (FFPE) specimens and analyzed them by means of RNA-seq for transcriptional analysis. Melanocytoma and melanoma samples were compared to detect differential gene expressions and significant enriched pathways were explored to reveal functional relations between differentially expressed genes. The study demonstrated a differential expression of 60 genes in melanomas compared to melanocytomas. The differentially expressed genes cluster in the extracellular matrix-receptor interaction, protein digestion and absorption, focal adhesion and PI3K-Akt (phosphoinositide 3-kinase/protein kinase B) signaling pathways. Genes encoding for several collagen proteins were more commonly differentially expressed. Results of the RNA-seq were validated by qRT-PCR and protein expression of some target molecules was investigated by means of immunohistochemistry. We hypothesize that the developing melanoma actively promotes collagen metabolism and extracellular matrix remodeling as well as enhancing cell proliferation and survival contributing to disease progression and metastasis. In this study, we also detected unidentified genes in human melanoma expression studies and uncover new candidate drug targets for further testing in canine melanoma.

Despite promising immunotherapy strategies in human melanoma, there are few studies on the immune environment of canine melanocytic tumors. In humans, the activation of immunosuppressive cell subpopulations, such as regulatory T cells (Tregs) that express forkhead box protein P3 (FoxP3), the engagement of immunosuppressive surface receptors like cytotoxic T lymphocyte antigen (CTLA-4), and the secretion of molecules inhibiting lymphocyte activation, such as indoleamine-pyrrole 2,3-dioxygenase (IDO), are recognized as immunoescape mechanisms that allow tumor growth and progression. The aim of our study was to investigate the expression of these immunosuppression markers in canine melanocytic tumors and to postulate their possible role in melanoma biology and progression. Fifty-five formalin-fixed, paraffin-embedded canine melanocytic tumors (25 oral melanomas; 20 cutaneous melanomas; 10 cutaneous melanocytomas) were selected to investigate the expression of FoxP3, CTLA-4, and IDO by immunohistochemistry and RT-qPCR (real-time quantitative polymerase chain reaction). All of the tested markers showed high gene and protein expression in oral melanomas and were differently expressed in cutaneous melanomas when compared to their benign counterpart. IDO expression was associated with an increased hazard of death both in univariable and multivariable analyses ( P < .05). FoxP3 protein expression >6.9 cells/HPF (high-power field) was an independent predictor of death ( P < .05). CTLA-4 gene and protein expressions were associated with a worse prognosis, but only in the univariable analysis ( P < .05). FoxP3, CTLA-4, and IDO likely play a role in canine melanoma immunoescape. Their expression, if supported by future studies, could represent a prognostic tool in canine melanoma and pave the way to future immunotherapeutic approaches in dogs.

One of the next challenges in cancer immunotherapy is the resistance of tumors to T-cell-based treatments through loss of MHC class I. Here, we show that under these circumstances, the Toll-like receptor (TLR)-7/8 ligand imiquimod, but not the TLR3 ligand poly I:C or TLR9 ligand CpG, mediated an effective antitumor response. The rejection of these immune-escaped cancers was mediated by NK cells and CD4 T cells, whereas activated CD8 T cells were dispensable. Application of the innate immune stimulator at a distant site activated NK cells and thereby elicited tumor-specific T-cell responses in tumor-bearing mice. Mechanistically, imiquimod activated NK cells to kill tumor cells, resulting in release of tumor antigens and induction of tumor-specific CD4 T cells. These T helper cells provoked a strong induction of CXCL9 and CXCL10 in the tumor environment. Simultaneously, imiquimod induced the expression of the cognate chemokine receptor CXCR3 on peripheral lymphocytes. This ignited intratumoral CD4 T-cell infiltration and accumulation, which was critical for tumor rejection; CXCR3 blocking antibodies mitigated the clinical response. In the effector phase, NK cell recruitment to tumors and their activation depended on CD4 T cells. Together, we have uncovered a potent immune axis of tumor-specific CD4 T cells and NK cells that eliminates escaped MHC-I tumors. .

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Uncontrolled proliferation is a key feature of tumor progression and malignancy. This suggests that cell-cycle related factors could be exploited as cancer biomarkers and that pathways specifically involved in the cell cycle, such as the Rb-E2F pathway, could be targeted as an effective anti-tumor therapy. We investigated 34 formalin-fixed paraffin-embedded (FFPE) tissue samples of canine cutaneous melanocytoma, cutaneous melanoma, and oral melanoma. Corresponding clinical follow-up data were used to determine the prognostic value of the mRNA expression levels of several cell cycle regulated E2F target genes (E2F1, DHFR, CDC6, ATAD2, MCM2, H2AFZ, GINS2, and survivin/BIRC5). Moreover, using four canine melanoma cell lines, we explored the possibility of blocking the Rb-E2F pathway by using a CDK4/6 inhibitor (Palbociclib) as a potential anti-cancer therapy. We investigated the expression levels of the same E2F target gene transcripts before and after treatment to determine the potential utility of these molecules as predictive markers. The E2F target gene H2AFZ was expressed in 91.43% of the primary tumors and H2AFZ expression was significantly higher in cases with unfavorable clinical outcome. Among the other tested genes, survivin/BIRC5 showed as well-promising results as a prognostic marker in canine melanoma. Three of the four tested melanoma cell lines were sensitive to the CDK4/6 inhibitor. The resistant cell line displayed higher expression levels of H2AFZ before treatment compared to the CDK4/6 inhibitor-sensitive cell lines. The present results suggest that CDK4/6 inhibitors could potentially be used as a new anti-cancer treatment for canine melanoma and that H2AFZ could serve as a prognostic and predictive marker for patient selection.