Development of canine PD-1/PD-L1 specific monoclonal antibodies and amplification of canine T cell function

Validation of a PD-1/CD28 chimeric switch receptor to augment CAR-T function in dogs with spontaneous B cell lymphoma

Chimeric antigen receptor (CAR) T cell therapy has achieved unprecedented clinical outcomes in patients with relapsed/refractory B cell leukemias; however, response rates in patients with large B cell lymphoma (LBCL) are less impressive. Expression of PD-1 on activated T cells and PD-L1 on malignant, stromal, and immune cells within the tumor microenvironment (TME) contribute to CAR-T exhaustion, hypofunction, and treatment failures. Here, a comparative approach is taken to develop a chimeric switch receptor (CSR) with potential to augment CAR-T persistence, function, and clinical efficacy in immune competent, pet dogs with spontaneous B cell lymphoma (BCL). We show that similar to human CAR-T cells, expression of a PD-1/CD28 CSR in canine CAR-T cells results in enhanced function against PD-L1+ targets and preserves central memory phenotype. We also demonstrate that these effects depend upon active CSR signaling. This work paves the way for in vivo studies in canine BCL patients to inform human trial design.

A review of CD4+ T cell differentiation and diversity in dogs

CD4+ T cells are an integral component of the adaptive immune response, carrying out many functions to combat a diverse range of pathogenic challenges. These cells exhibit remarkable plasticity, differentiating into specialized subsets such as T helper type 1 (TH1), TH2, TH9, TH17, TH22, regulatory T cells (Tregs), and follicular T helper (TFH) cells. Each subset is capable of addressing a distinct immunological need ranging from pathogen eradication to regulation of immune homeostasis. As the immune response subsides, CD4+ T cells rest down into long-lived memory phenotypes-including central memory (TCM), effector memory (TEM), resident memory (TRM), and terminally differentiated effector memory cells (TEMRA) that are localized to facilitate a swift and potent response upon antigen re-encounter. This capacity for long-term immunological memory and rapid reactivation upon secondary exposure highlights the role CD4+ T cells play in sustaining both adaptive defense mechanisms and maintenance. Decades of mouse, human, and to a lesser extent, pig T cell research has provided the framework for understanding the role of CD4+ T cells in immune responses, but these model systems do not always mimic each other. Although our understanding of pig immunology is not as extensive as mouse or human research, we have gained valuable insight by studying this model. More akin to pigs, our understanding of CD4+ T cells in dogs is much less complete. This disparity exists in part because canine immunologists depend on paradigms from mouse and human studies to characterize CD4+ T cells in dogs, with a fraction of available lineage-defining antibody markers. Despite this, every major CD4+ T cell subset has been described to some extent in dogs. These subsets have been studied in various contexts, including in vitro stimulation, homeostatic conditions, and across a range of disease states. Canine CD4+ T cells have been categorized according to lineage-defining characteristics, trafficking patterns, and what cytokines they produce upon stimulation. This review addresses our current understanding of canine CD4+ T cells from a comparative perspective by highlighting both the similarities and differences from mouse, human, and pig CD4+ T cell biology. We also discuss knowledge gaps in our current understanding of CD4+ T cells in dogs that could provide direction for future studies in the field.

A Review on Canine and Human Soft Tissue Sarcomas: New Insights on Prognosis Factors and Treatment Measures

Soft tissue sarcomas (STSs) represent a diverse group of tumors arising from mesenchymal cells, affecting both humans and animals, including dogs. Although STSs represent a class of rare tumors, especially in humans, they pose significant clinical challenges due to their potential for local recurrence and distant metastasis. Dogs, as a model for human STSs, offer several advantages, including exposure to similar environmental risk factors, genetic diversity among breeds, and the spontaneous development of tumors. Furthermore, canine tumors closely mimic the heterogeneity and complexity of human tumors, making them valuable for research into disease progression and treatment effectiveness. Current treatment approaches for STSs in both dogs and humans primarily involve surgery, radiation therapy, and chemotherapy, with treatment decisions based on tumor characteristics and patient factors. However, the development of novel therapeutic strategies is essential, given the high failure rate of new drugs in clinical trials. To better design new tailored treatments, comprehension of the tumor microenvironment (TME) is fundamental, since it plays a crucial role in STS initiation and progression by modulating tumor behavior, promoting angiogenesis, and suppressing immune responses. Notably, TME features include cancer-associated fibroblasts (CAFs), extracellular matrix (ECM) alterations, and tumor-associated macrophages (TAMs) that, depending on their polarization state, can affect immune responses and thus the patient's prognosis. In this review, new therapeutical approaches based on immunotherapy will be deeply explored as potential treatment options for both dogs and humans with STSs. In conclusion, this review provides an overview of the current understanding of STSs in dogs and humans, emphasizing the importance of the TME and potential treatment strategies.

One Health: Therapies Targeting Genetic Variants in Human and Canine Histiocytic and Dendritic Cell Sarcomas

The precise cause of HS/DCS is still unknown. The relatively low incidence in humans urges for an animal model with a high incidence to accelerate knowledge about genetics and optimal treatment of HS/DCS. Namely, until now, the therapies targeting genetic variants are still more experimental and sparsely used, while consensus is missing. In addition, the literature about variants and possible mutation-targeted therapies in humans and dogs consists mainly of case reports scattered throughout the literature. Therefore, an overview is provided of all currently known genetic variants in humans and dogs with HS/DCS and its subtypes, their possible mutation-targeted therapies, their efficacy, and a contemplation about the future. Several genetic variants have already been discovered in HS/DCS, of which many are shared between canine and human HS/DCS, but unique variants exist as well. Unfortunately, none of these already found variants seem to be specifically causal for HS/DCS, and the puzzle of its landscape of genetic variation is far from complete. The use of mutation-targeted therapies, including MAPK-/MEK-inhibitors and the future use of PTPN11-, CDK4/6- and PD-1-inhibitors, seems to be promising for these specific variants, but clearly, clinical trials are needed to determine optimal inhibitors and standardised protocols for all variants. It can be concluded that molecular analysis for variants and subsequent mutation-targeted therapy are an essential addition to cancer diagnostics and therapy. A joint effort of humans and dogs in research is urgently needed and will undoubtedly increase knowledge and survival of this devastating disease in dogs and humans.

Comparative characterization of two monoclonal antibodies targeting canine PD-1

Monoclonal antibodies targeting immune checkpoints have revolutionized oncology. Yet, the effectiveness of these treatments varies significantly among patients, and they are associated with unexpected adverse events, including hyperprogression. The murine research model used in drug development fails to recapitulate both the functional human immune system and the population heterogeneity. Hence, a novel model is urgently needed to study the consequences of immune checkpoint blockade. Dogs appear to be uniquely suited for this role. Approximately 1 in 4 companion dogs dies from cancer, yet no antibodies are commercially available for use in veterinary oncology. Here we characterize two novel antibodies that bind canine PD-1 with sub-nanomolar affinity as measured by SPR. Both antibodies block the clinically crucial PD-1/PD-L1 interaction in a competitive ELISA assay. Additionally, the antibodies were tested with a broad range of assays including Western Blot, ELISA, flow cytometry, immunofluorescence and immunohistochemistry. The antibodies appear to bind two distinct epitopes as predicted by molecular modeling and peptide phage display. Our study provides new tools for canine oncology research and a potential veterinary therapeutic.

PD-L1 mRNA and protein expression in canine mammary carcinomas: Correlation with histopathological grade and molecular markers

Programmed death ligand 1 (PD-L1) is an immune checkpoint molecule that plays a crucial role in regulating antitumor immune responses. Canine mammary carcinomas (CMCs) are common tumors of dogs. Despite extensive studies on the heterogeneity of CMCs, there is still a lack of effective precision therapies for the treatment of CMCs. In this study, we aimed to investigate the correlation between PD-L1 mRNA and protein expression in CMCs and explore its association with histopathological grade and molecular markers, including the estrogen receptor, epidermal growth factor receptor 2, and cytokeratin 5/6 (CK5/6). Formalin-fixed paraffin-embedded samples were evaluated for PD-L1 mRNA expression using RNA in situ hybridization and PD-L1 protein expression using immunohistochemistry. We observed no substantial correlation between PD-L1 mRNA and protein expression in CMCs; however, PD-L1 mRNA levels were significantly higher in grade 3 than in grade 1 tumors (P = .001). In addition, we observed a positive correlation between PD-L1 protein expression and CK5/6 expression in CMCs (P = .032). These findings suggest that PD-L1 expression in CMCs is heterogeneous and may be regulated post-transcriptionally. Further studies are needed to explore the prognostic and therapeutic implications of PD-L1 expression in different molecular subtypes of CMCs and their potential as predictive biomarkers for immunotherapy.

PDL1 immunohistochemistry in canine neoplasms: Validation of commercial antibodies, standardization of evaluation, and scoring systems

Immuno-oncology research has brought to light the paradoxical role of immune cells in the induction and elimination of cancer. Programmed cell death protein 1 (PD1), expressed by tumor-infiltrating lymphocytes, and programmed cell death ligand 1 (PDL1), expressed by tumor cells, are immune checkpoint proteins that regulate the antitumor adaptive immune response. This study aimed to validate commercially available PDL1 antibodies in canine tissue and then, applying standardized methods and scoring systems used in human pathology, evaluate PDL1 immunopositivity in different types of canine tumors. To demonstrate cross-reactivity, a monoclonal antibody (22C3) and polyclonal antibody (cod. A1645) were tested by western blot. Cross-reactivity in canine tissue cell extracts was observed for both antibodies; however, the polyclonal antibody (cod. A1645) demonstrated higher signal specificity. Canine tumor histotypes were selected based on the human counterparts known to express PDL1. Immunohistochemistry was performed on 168 tumors with the polyclonal anti-PDL1 antibody. Only membranous labeling was considered positive. PDL1 labeling was detected both in neoplastic and infiltrating immune cells. The following tumors were immunopositive: melanomas (17 of 17; 100%), renal cell carcinomas (4 of 17; 24%), squamous cell carcinomas (3 of 17; 18%), lymphomas (2 of 14; 14%), urothelial carcinomas (2 of 18; 11%), pulmonary carcinomas (2 of 20; 10%), and mammary carcinomas (1 of 31; 3%). Gastric (0 of 10; 0%) and intestinal carcinomas (0 of 24; 0%) were negative. The findings of this study suggest that PDL1 is expressed in some canine tumors, with high prevalence in melanomas.

Precision Medicine in Veterinary Science

Precision medicine focuses on the clinical management of the individual patient, not on population-based findings. Successes from human precision medicine inform veterinary oncology. Early evidence of success for canines shows how precision medicine can be integrated into practice. Decreasing genomic profiling costs will allow increased utilization and subsequent improvement of knowledge base from which to make better informed decisions. Utility of precision medicine in canine oncology will only increase for improved cancer characterization, enhanced therapy selection, and overall more successful management of canine cancer. As such, practitioners are called to interpret and leverage precision medicine reports for their patients.

Establishment of Anti-Dog Programmed Cell Death Ligand 1 Monoclonal Antibodies for Immunohistochemistry

Immune checkpoint blockade therapy has shown successful clinical outcomes in multiple human cancers. In dogs, several types of tumors resemble human tumors in many respects. Therefore, several groups have developed the anti-dog programmed cell death ligand 1 (dPD-L1) monoclonal antibodies (mAbs) and showed efficacy in several canine tumors. To examine the abundance of dPD-L1 in canine tumors, anti-dPD-L1 diagnostic mAbs for immunohistochemistry are required. In this study, we immunized the peptide in the dPD-L1 intracellular domain, and established anti-dPD-L1 mAbs, L1Mab-352 (mouse IgG1, kappa), and L1Mab-354 (mouse IgG1, kappa). In enzyme-linked immunosorbent assay, L1Mab-352 and L1Mab-354 showed high-binding affinity to the dPD-L1 peptide, and the dissociation constants (KD) were determined as 6.9 × 10-10 M and 7.2 × 10-10 M, respectively. Furthermore, L1Mab-352 and L1Mab-354 were applicable for the detection of dPD-L1 in immunohistochemical analysis in paraffin-embedded dPD-L1-overexpressed cells. These results indicated that L1Mab-352 and L1Mab-354 are useful for detecting dPD-L1 in immunohistochemical analysis.

Proof-of-concept study of the caninized anti-canine programmed death 1 antibody in dogs with advanced non-oral malignant melanoma solid tumors

BACKGROUND

The anti-programmed death 1 (PD-1) antibody has led to durable clinical responses in a wide variety of human tumors. We have previously developed the caninized anti-canine PD-1 antibody (ca-4F12-E6) and evaluated its therapeutic properties in dogs with advance-staged oral malignant melanoma (OMM), however, their therapeutic effects on other types of canine tumors remain unclear.

OBJECTIVE

The present clinical study was carried out to evaluate the safety profile and clinical efficacy of ca-4F12-E6 in dogs with advanced solid tumors except for OMM.

METHODS

Thirty-eight dogs with non-OMM solid tumors were enrolled prospectively and treated with ca-4F12-E6 at 3 mg/kg every 2 weeks of each 10-week treatment cycle. Adverse events (AEs) and treatment efficacy were graded based on the criteria established by the Veterinary Cooperative Oncology Group.

RESULTS

One dog was withdrawn, and thirty-seven dogs were evaluated for the safety and efficacy of ca-4F12-E6. Treatment-related AEs of any grade occurred in 13 out of 37 cases (35.1%). Two dogs with sterile nodular panniculitis and one with myasthenia gravis and hypothyroidism were suspected of immune-related AEs. In 30 out of 37 dogs that had target tumor lesions, the overall response and clinical benefit rates were 6.9% and 27.6%, respectively. The median progression-free survival and overall survival time were 70 days and 215 days, respectively.

CONCLUSIONS

The present study demonstrated that ca-4F12-E6 was well-tolerated in non-OMM dogs, with a small number of cases showing objective responses. This provides evidence supporting large-scale clinical trials of anti-PD-1 antibody therapy in dogs.

Inhibitory checkpoint molecule mRNA expression in canine soft tissue sarcoma

Canine soft tissue sarcomas (STS) are common neoplasms and considered immune deserts. Tumour infiltrating lymphocytes are sparse in STS and, when present, tend to organize around blood vessels or at the periphery of the neoplasm. This pattern is associated with an immunosuppressive tumour microenvironment linked to overexpression of molecules of the PD-axis. PD-1, PD-L1 and PD-L2 expression correlates with malignancy and poor prognosis in other neoplasms in humans and dogs, but little is known about their role in canine STS, their relationship to tumour grade, and how different therapies affect expression. The objective of this study was to evaluate the expression of checkpoint molecules across STS tumour grades and after tumour ablation treatment. Gene expression analysis was performed by reverse-transcriptase real-time quantitative PCR in soft tissue sarcomas that underwent histotripsy and from histologic specimens of STS from the Virginia Tech Animal Laboratory Services archives. The expression of PD-1, PD-L1 and PD-L2 was detected in untreated STS tissue representing grades 1, 2, and 3. Numerically decreased expression of all markers was observed in tissue sampled from the treatment interface relative to untreated areas of the tumour. The relatively lower expression of these checkpoint molecules at the periphery of the treated area may be related to liquefactive necrosis induced by the histotripsy treatment, and would potentially allow TILs to infiltrate the tumour. Relative increases of these checkpoint molecules in tumours of a higher grade and alongside immune cell infiltration are consistent with previous reports that associate their expression with malignancy.

Development and characterization of mouse anti-canine PD-L1 monoclonal antibodies and their expression in canine tumors by immunohistochemistry in vitro

Immune escape is the hallmark of carcinogenesis. This widely known mechanism is the overexpression of immune checkpoint ligands, such as programmed cell death protein 1 and programmed death-ligand 1 (PD-1/PD-L1), leading to T cell anergy. Therefore, cancer immunotherapy with specific binding to these receptors has been developed to treat human cancers. Due to the lack of cross-reactivity of these antibodies in dogs, a specific canine PD-1/PD-L1 antibody is required. The aim of this study is to develop mouse anti-canine PD-L1 (cPD-L1) monoclonal antibodies and characterize their in vitro properties. Six mice were immunized with recombinant cPD-L1 with a fusion of human Fc tag. The hybridoma clones that successfully generated anti-cPD-L1 antibodies and had neutralizing activity were selected for monoclonal antibody production. Antibody properties were tested by immunosorbent assay, surface plasmon resonance, and immunohistochemistry. Four hybridomas were effectively bound and blocked to recombinant cPD-L1 and cPD-1-His-protein, respectively. Candidate mouse monoclonal antibodies worked efficiently on formalin-fixed paraffin-embedded tissues of canine cancers, including cutaneous T-cell lymphomas, mammary carcinomas, soft tissue sarcomas, squamous cell carcinomas, and malignant melanomas. However, functional assays of these anti-cPD-L1 antibodies need further investigation to prove their abilities as therapeutic drugs in dogs as well as their applications as prognostic markers.

Expansion of platform physiologically-based pharmacokinetic model for monoclonal antibodies towards different preclinical species: cats, sheep, and dogs

Monoclonal antibodies (mAbs) are becoming an important therapeutic option in veterinary medicine, and understanding the pharmacokinetic (PK) of mAbs in higher-order animal species is also important for human drug development. To better understand the PK of mAbs in these animals, here we have expanded a platform physiological-based pharmacokinetic (PBPK) model to characterize the disposition of mAbs in three different preclinical species: cats, sheep, and dogs. We obtained PK data for mAbs and physiological parameters for the three different species from the literature. We were able to describe the PK of mAbs following intravenous (IV) or subcutaneous administration in cats, IV administration in sheep, and IV administration dogs reasonably well by fixing the physiological parameters and just estimating the parameters related to the binding of mAbs to the neonatal Fc receptor. The platform PBPK model presented here provides a quantitative tool to predict the plasma PK of mAbs in dogs, cats, and sheep. The model can also predict mAb PK in different tissues where the site of action might be located. As such, the mAb PBPK model presented here can facilitate the discovery, development, and preclinical-to-clinical translation of mAbs for veterinary and human medicine. The model can also be modified in the future to account for more detailed compartments for certain organs, different pathophysiology in the animals, and target-mediated drug disposition.

Potential Promises and Perils of Human Biological Treatments for Immunotherapy in Veterinary Oncology

The emergence of immunotherapy for the treatment of human cancers has heralded a new era in oncology, one that is making its way into the veterinary clinic. As the immune system of many animal species commonly seen by veterinarians is similar to humans, there is great hope for the translation of human therapies into veterinary oncology. The simplest approach for veterinarians would be to adopt existing reagents that have been developed for human medicine, due to the potential of reduced cost and the time it takes to develop a new drug. However, this strategy may not always prove to be effective and safe with regard to certain drug platforms. Here, we review current therapeutic strategies that could exploit human reagents in veterinary medicine and also those therapies which may prove detrimental when human-specific biological molecules are used in veterinary oncology. In keeping with a One Health framework, we also discuss the potential use of single-domain antibodies (sdAbs) derived from camelid species (also known as Nanobodies™) for therapies targeting multiple veterinary animal patients without the need for species-specific reformulation. Such reagents would not only benefit the health of our veterinary species but could also guide human medicine by studying the effects of outbred animals that develop spontaneous tumors, a more relevant model of human diseases compared to traditional laboratory rodent models.

Development of an Anti-canine PD-L1 Antibody and Caninized PD-L1 Mouse Model as Translational Research Tools for the Study of Immunotherapy in Humans

Immune checkpoint blockade therapy, one of the most promising cancer immunotherapies, has shown remarkable clinical impact in multiple cancer types. Despite the recent success of immune checkpoint blockade therapy, however, the response rates in patients with cancer are limited (∼20%-40%). To improve the success of immune checkpoint blockade therapy, relevant preclinical animal models are essential for the development and testing of multiple combination approaches and strategies. Companion dogs naturally develop several types of cancer that in many respects resemble clinical cancer in human patients. Therefore, the canine studies of immuno-oncology drugs can generate knowledge that informs and prioritizes new immuno-oncology therapy in humans. The challenge has been, however, that immunotherapeutic antibodies targeting canine immune checkpoint molecules such as canine PD-L1 (cPD-L1) have not been commercially available. Here, we developed a new cPD-L1 antibody as an immuno-oncology drug and characterized its functional and biological properties in multiple assays. We also evaluated the therapeutic efficacy of cPD-L1 antibodies in our unique caninized PD-L1 mice. Together, these in vitro and in vivo data, which include an initial safety profile in laboratory dogs, support development of this cPD-L1 antibody as an immune checkpoint inhibitor for studies in dogs with naturally occurring cancer for translational research. Our new therapeutic antibody and caninized PD-L1 mouse model will be essential translational research tools in raising the success rate of immunotherapy in both dogs and humans.

Significance

Our cPD-L1 antibody and unique caninized mouse model will be critical research tools to improve the efficacy of immune checkpoint blockade therapy in both dogs and humans. Furthermore, these tools will open new perspectives for immunotherapy applications in cancer as well as other autoimmune diseases that could benefit a diverse and broader patient population.

Development of Monoclonal Antibodies Targeting Canine PD-L1 and PD-1 and Their Clinical Relevance in Canine Apocrine Gland Anal Sac Adenocarcinoma

Canine apocrine gland anal sac adenocarcinoma (AGASACA) is an aggressive canine tumor originating from the anal sac glands. Surgical resection, with or without adjuvant chemotherapy, represents the standard of care for this tumor, but the outcome is generally poor, particularly for tumors diagnosed at an advanced stage. For this reason, novel treatment options are warranted, and a few recent reports have suggested the activation of the immune checkpoint axis in canine AGASACA. In our study, we developed canine-specific monoclonal antibodies targeting PD-1 and PD-L1. A total of 41 AGASACAs with complete clinical and follow-up information were then analyzed by immunohistochemistry for the expression of the two checkpoint molecules (PD-L1 and PD-1) and the presence of tumor-infiltrating lymphocytes (CD3 and CD20), which were evaluated within the tumor bulk (intratumor) and in the surrounding stroma (peritumor). Seventeen AGASACAs (42%) expressed PD-L1 in a range between 5% and 95%. The intratumor lymphocytes were predominantly CD3+ T-cells and were positively correlated with the number of PD-1+ intratumor lymphocytes (ρ = 0.36; p = 0.02). The peritumor lymphocytes were a mixture of CD3+ and CD20+ cells with variable PD-1 expression (range 0-50%). PD-L1 expression negatively affected survival only in the subgroup of dogs treated with surgery alone (n = 14; 576 vs. 235 days). The presence of a heterogeneous lymphocytic infiltrate and the expression of PD-1 and PD-L1 molecules support the relevance of the immune microenvironment in canine AGASACAs and the potential value of immune checkpoints as promising therapeutic targets.

The PD-1/PD-L1 Pathway: A Perspective on Comparative Immuno-Oncology

Simple Summary

The programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) pathway inhibits the function of activated immune cells. This mediates immune tolerance and prevents immune-mediated tissue destruction. The malfunction of this pathway is involved in the pathogenesis of chronic infections, autoimmunity, and cancer. The PD-1/PD-L1 pathway is an excellent example of the research benefits of comparative pathology and attests to the importance of the “one health one medicine” concept. Pioneering research was mainly focused on the examination of cells and tissues of human and mouse origin. It mainly revealed that PD-L1-positive tumor cells can paralyze PD-1-bearing immune cells, which prevents immunological destruction of cancer cells. This led to a major breakthrough in cancer treatment, i.e., the use of antibodies that block the interaction of these molecules and restore anti-cancer immune defense (immune checkpoint therapy). Further studies provided more detailed information on the tissue-specific context and fine-tuning of this pathway. The most recent research has extended the investigations to the examination of several animal species with the aim of improving disease diagnostics and treatment for certain animal diseases, in particular cancer, which is a major cause of disease and death in companion animals.

Abstract

The programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) pathway mainly attracted attention in immuno-oncology, leading to the development of immune checkpoint therapy. It has, however, much broader importance for tissue physiology and pathology. It mediates basic processes of immune tolerance and tissue homeostasis. In addition, it is involved in the pathogenesis of chronic infectious diseases, autoimmunity, and cancer. It is also an important paradigm for comparative pathology as well as the “one health one medicine” concept. The aim of this review is to provide an overview of novel research into the diverse facets of the PD-1/PD-L1 pathway and to give insights into its fine-tuning homeostatic role in a tissue-specific context. This review details early translational research from the discovery phase based on mice as animal models for understanding pathophysiological aspects in human tissues to more recent research extending the investigations to several animal species. The latter has the twofold goal of comparing this pathway between humans and different animal species and translating diagnostic tools and treatment options established for the use in human beings to animals and vice versa.

Using the canine microbiome to bridge translation of cancer immunotherapy from pre-clinical murine models to human clinical trials

The microbiome has clearly been established as a cutting-edge field in tumor immunology and immunotherapy. Growing evidence supports the role of the microbiome in immune surveillance, self-tolerance, and response to immune checkpoint inhibitors such as anti PD-L1 and CTLA-4 blockade (1-6). Moreover, recent studies including those using fecal microbial transplantation (FMT) have demonstrated that response to checkpoint immunotherapies may be conferred or eliminated through gut microbiome modulation (7, 8). Consequently, studies evaluating microbiota-host immune and metabolic interactions remain an area of high impact research. While observations in murine models have highlighted the importance of the microbiome in response to therapy, we lack sufficient understanding of the exact mechanisms underlying these interactions. Furthermore, mouse and human gut microbiome composition may be too dissimilar for discovery of all relevant gut microbial biomarkers. Multiple cancers in dogs, including lymphoma, high grade gliomas, melanomas and osteosarcoma (OSA) closely resemble their human analogues, particularly in regard to metastasis, disease recurrence and response to treatment. Importantly, dogs with these spontaneous cancers also have intact immune systems, suggesting that microbiome analyses in these subjects may provide high yield information, especially in the setting of novel immunotherapy regimens which are currently expanding rapidly in canine comparative oncology (9, 10). Additionally, as onco-microbiotic therapies are developed to modify gut microbiomes for maximal responsiveness, large animal models with intact immune systems will be useful for trialing interventions and monitoring adverse events. Together, pre-clinical mechanistic studies and large animal trials can help fully unlock the potential of the microbiome as a diagnostic and therapeutic target in cancer.

Inhaled recombinant human IL-15 in dogs with naturally occurring pulmonary metastases from osteosarcoma or melanoma: a phase 1 study of clinical activity and correlates of response

PURPOSE

Although recombinant human interleukin-15 (rhIL-15) has generated much excitement as an immunotherapeutic agent for cancer, activity in human clinical trials has been modest to date, in part due to the risks of toxicity with significant dose escalation. Since pulmonary metastases are a major site of distant failure in human and dog cancers, we sought to investigate inhaled rhIL-15 in dogs with naturally occurring lung metastases from osteosarcoma (OSA) or melanoma. We hypothesized a favorable benefit/risk profile given the concentrated delivery to the lungs with decreased systemic exposure.

EXPERIMENTAL DESIGN

We performed a phase I trial of inhaled rhIL-15 in dogs with gross pulmonary metastases using a traditional 3+3 cohort design. A starting dose of 10 µg twice daily × 14 days was used based on human, non-human primate, and murine studies. Safety, dose-limiting toxicities (DLT), and maximum tolerated dose (MTD) were the primary objectives, while response rates, progression-free and overall survival (OS), and pharmacokinetic and immune correlative analyses were secondary.

RESULTS

From October 2018 to December 2020, we enrolled 21 dogs with 18 dogs reaching the 28-day response assessment to be evaluable. At dose level 5 (70 μg), we observed two DLTs, thereby establishing 50 µg twice daily × 14 days as the MTD and recommended phase 2 dose. Among 18 evaluable dogs, we observed one complete response >1 year, one partial response with resolution of multiple target lesions, and five stable disease for an overall clinical benefit rate of 39%. Plasma rhIL-15 quantitation revealed detectable and sustained rhIL-15 concentrations between 1-hour and 6 hour postnebulization. Decreased pretreatment lymphocyte counts were significantly associated with clinical benefit. Cytotoxicity assays of banked peripheral blood mononuclear cells revealed significant increases in peak cytotoxicity against canine melanoma and OSA targets that correlated with OS.

CONCLUSIONS

In this first-in-dog clinical trial of inhaled rhIL-15 in dogs with advanced metastatic disease, we observed promising clinical activity when administered as a monotherapy for only 14 days. These data have significant clinical and biological implications for both dogs and humans with refractory lung metastases and support exploration of combinatorial therapies using inhaled rhIL-15.

T Cell Immune Profiles of Blood and Tumor in Dogs Diagnosed With Malignant Melanoma

Investigation of canine T cell immunophenotypes in canine melanomas as prognostic biomarkers for disease progression or predictive biomarkers for targeted immunotherapeutics remains in preliminary stages. We aimed to examine T cell phenotypes and function in peripheral blood mononuclear cells (PBMC) and baseline tumor samples by flow cytometry, and to compare patient (n = 11–20) T cell phenotypes with healthy controls dogs (n = 10–20). CD3, CD4, CD8, CD25, FoxP3, Ki67, granzyme B, and interferon-γ (IFN-γ) were used to classify T cell subsets in resting and mitogen stimulated PBMCs. In a separate patient cohort (n = 11), T cells were classified using CD3, CD4, CD8, FoxP3, and granzyme B in paired PBMC and single cell suspensions of tumor samples. Analysis of flow cytometric data of individual T cell phenotypes in PBMC revealed specific T cell phenotypes including FoxP3+ and CD25+FoxP3- populations that distinguished patients from healthy controls. Frequencies of IFN-γ+ cells after ConA stimulation identified two different patient phenotypic responses, including a normal/exaggerated IFN-γ response and a lower response suggesting dysfunction. Principle component analysis of selected T cell immunophenotypes also distinguished patients and controls for T cell phenotype and revealed a clustering of patients based on metastasis detected at diagnosis. Findings supported the overall hypothesis that canine melanoma patients display a T cell immunophenotype profile that is unique from healthy pet dogs and will guide future studies designed with larger patient cohorts necessary to further characterize prognostic T cell immunophenotypes.

Cancer-Immunity Cycle and Therapeutic Interventions- Opportunities for Including Pet Dogs With Cancer

The tumor-immune interplay represents a dynamic series of events executed by cellular and soluble participants that either promote or inhibit successful tumor formation and growth. Throughout a tumor’s development and progression, the host organism’s immune system reacts by generating anti-cancer defenses through various incremental and combinatorial mechanisms, and this reactive orchestration is termed the cancer-immunity cycle. Success or failure of the cancer-immunity cycle dictates the fate of both host and tumor as winner or loser. Insights into how the tumor and host immune system continuously adapt to each other throughout the lifecycle of the tumor is necessary to rationally develop new effective immunotherapies. Additionally, the evolving nature of the cancer-immunity cycle necessitates therapeutic agility, requiring real-time serial assessment of immunobiologic markers that permits tailoring of therapies to the everchanging tumor immune microenvironment. In order to accelerate advances in the field of immuno-oncology, this review summarizes the steps comprising the cancer-immunity cycle, and underscores key breakpoints in the cycle that either favor cancer regression or progression, as well as shaping of the tumor microenvironment and associated immune phenotypes. Furthermore, specific large animal models of spontaneous cancers that are deemed immunogenic will be reviewed and proposed as unique resources for validating investigational immunotherapeutic protocols that are informed by the cancer-immunity cycle. Collectively, this review will provide a progressive look into the dynamic interplay between tumor and host immune responses and raise awareness for how large animal models can be included for developing combinatorial and sequenced immunotherapies to maximizing favorable treatment outcomes.

Evaluation of lymphocyte-specific programmed cell death protein 1 receptor expression and cytokines in blood and urine in canine urothelial carcinoma patients

Urothelial carcinoma (UC) is the most common urinary tumour in dogs. Despite a range of treatment options, prognosis remains poor in dogs. In people, breakthroughs with checkpoint inhibitors have established new standards of care for muscle-invasive bladder cancer patients and elevated levels of programmed cell death protein 1 (PD-1) suggest immune checkpoint blockade may be a novel target for therapy. The goal of this study was to determine if canine UC patients express elevated levels of lymphocyte-specific PD-1 and/or urinary cytokine biomarkers compared to healthy dogs. Paired blood and urine were evaluated in 10 canine UC patients, five cystitis patients and 10 control dogs for lymphocyte-specific PD-1 expression via flow cytometry and relative cytokine expression. In UC patients, PD-1 expression was significantly elevated on CD8⁺ lymphocytes in urine samples. UC patients had a higher CD4:CD8 ratio in their urine compared to healthy dogs, however, there was no significant variation in the CD8:Treg ratio between any group. Cystitis patients had significantly elevated levels of CD4⁺ T cells, CD8⁺ T cells and Tregs in their blood samples compared to UC patients and healthy dogs. Cytokine analysis demonstrated significant elevations in urinary cytokines (granulocyte-macrophage colony-stimulating factor, interferon-gamma [IFN-γ], interleukin (IL)-2, IL-6 IL-7, IL-8 and IL-15, IP-10, KC-like, IL-18, monocyte chemoattractant protein-1 and tumour necrosis factor-alpha). Several of these cytokines have been previously correlated with both lymphocyte-specific PD-1 expression (IFN-γ, IL-2, IL-7 and IL-15) in muscle-invasive urothelial carcinoma in humans. Our results provide evidence of urinary lymphocyte PD-1 expression and future studies could elucidate whether veterinary UC patients will respond favourably to anti-PD-1 immune checkpoint inhibitor therapy.

Comparative Cancer Cell Signaling in Muscle-Invasive Urothelial Carcinoma of the Bladder in Dogs and Humans

Muscle-invasive urothelial carcinoma (MIUC) is the most common type of bladder malignancy in humans, but also in dogs that represent a naturally occurring model for this disease. Dogs are immunocompetent animals that share risk factors, pathophysiological features, clinical signs and response to chemotherapeutics with human cancer patients. This review summarizes the fundamental pathways for canine MIUC initiation, progression, and metastasis, emerging therapeutic targets and mechanisms of drug resistance, and proposes new opportunities for potential prognostic and diagnostic biomarkers and therapeutics. Identifying similarities and differences between cancer signaling in dogs and humans is of utmost importance for the efficient translation of in vitro research to successful clinical trials for both species.

Immunotherapy for Dogs: Still Running Behind Humans

Despite all good intentions, dogs are still running behind humans in effective cancer immunotherapies. The more effective treatments in humans, like infusions of CAR-T and NK-cells are not broadly pursued for canines due to significant costs, the rather complicated logistics and the lack of targetable surface antigens. Monoclonal antibodies are challenging to develop considering the limited knowledge about canine target antigens and about their mode of action. Although immunogenic vaccines could be less costly, this approach is hampered by the fact that cancer by itself is immuno-suppressive and any preceding chemotherapy may suppress any clinically meaningful immune response. This review - rather than providing a comprehensive listing of all available immunotherapies for dogs, aims at pointing out the issues that are holding back this field but which hopefully can be addressed so that dogs can "catch up" with what is available to humans.

Dog leukocyte antigen-88*034:01 presents nonamer peptides from canine distemper virus hemagglutinin, large polymerase, and matrix proteins

Canine spontaneous cancers may offer greater fidelity than rodent models in advancing clinical immunotherapies. Boxers in particular are distinguished as study subjects by their popularity, and high incidence of human-relevant cancers. Further, the MHC class I allele DLA-88*034:01, with a known motif, dominates the breed, facilitating discovery of shared CTL responses against mutation-origin neoepitopes by standard prediction methods. We experimentally confirmed the allomorph's binding motif by developing an MHC surface stabilization assay. The assay validated four DLA-88*034:01-presented peptides from canine distemper virus, ubiquitously administered in routine vaccines, for positive controls in future CTL studies. In turn, these viral peptides substantiated motif-based prediction for DLA-88*034:01. The study adds new tools for studying neoepitope-specific CTL in Boxers to foster canine comparative oncology.

Cross-Reactivity and Functionality of Approved Human Immune Checkpoint Blockers in Dogs

BACKGROUND

Rodent cancer models have limitations in predicting efficacy, tolerability and accompanying biomarkers of ICIs in humans. Companion dogs suffering from neoplastic diseases have gained attention as a highly relevant translational disease model. Despite successful reports of PD-1/PD-L1 blockade in dogs, no compounds are available for veterinary medicine.

METHODS

Here, we assessed suitability of seven FDA-approved human ICIs to target CTLA-4 or PD-1/PD-L1 in dogs. Cross-reactivity and blocking potential was assessed using ELISA and flow cytometry. Functional responses were assessed on peripheral blood mononuclear cells (PBMCs) derived from healthy donors (n = 12) and cancer patient dogs (n = 27) as cytokine production after stimulation. Immune composition and target expression of healthy donors and cancer patients was assessed via flow cytometry.

RESULTS

Four candidates showed cross-reactivity and two blocked the interaction of canine PD-1 and PD-L1. Of those, only atezolizumab significantly increased cytokine production of healthy and patient derived PBMCs in vitro. Especially lymphoma patient PBMCs responded with increased cytokine production. In other types of cancer, response to atezolizumab appeared to correlate with a lower frequency of CD8 T cells.

CONCLUSIONS

Cross-functionality of atezolizumab encourages reverse translational efforts using (combination) immunotherapies in companion dog tumor patients to benefit both veterinary and human medicine.