Identification of Myeloid Derived Suppressor Cells in Dogs with Naturally Occurring Cancer

Dogs with naturally occurring cancer represent an important large animal model for drug development and testing novel immunotherapies. However, poorly defined immunophenotypes of canine leukocytes have limited the study of tumor immunology in dogs. The accumulation of myeloid derived suppressor cells (MDSCs) is known to be a key mechanism of immune suppression in tumor-bearing mice and in human patients. We sought to identify MDSCs in the blood of dogs with cancer. Peripheral blood mononuclear cells (PBMCs) from dogs with advanced or early stage cancer and from age-matched healthy controls were analyzed by flow cytometry and microscopy. Suppressive function was tested in T cell proliferation and cytokine elaboration assays. Semi-quantitative RT-PCR was used to identify potential mechanisms responsible for immunosuppression. PBMCs from dogs with advanced or metastatic cancer exhibited a significantly higher percentage of CD11b+CD14−MHCII− cells compared to dogs diagnosed with early stage non-metastatic tumors and healthy dogs. These CD11b+ CD14−MHCII− cells constitute a subpopulation of activated granulocytes that co-purify with PBMCs, display polymorphonuclear granulocyte morphology, and demonstrate a potent ability to suppress proliferation and IFN-γ production in T cells from normal and tumor-bearing donors. Furthermore, these cells expressed hallmark suppressive factors of human MDSC including ARG1, iNOS2, TGF-β and IL-10. In summary our data demonstrate that MDSCs accumulate in the blood of dogs with advanced cancer and can be measured using this three-marker immunophenotype, thereby enabling prospective studies that can monitor MDSC burden

Human Flt3L Generates Dendritic Cells from Canine Peripheral Blood Precursors: Implications for a Dog Glioma Clinical Trial

Glioblastoma multiforme (GBM) is the most common primary brain tumor in adults and carries a dismal prognosis. We have developed a conditional cytotoxic/immunotherapeutic approach using adenoviral vectors (Ads) encoding the immunostimulatory cytokine, human soluble fms-like tyrosine kinase 3 ligand (hsFlt3L) and the conditional cytotoxic molecule, i.e., Herpes Simplex Type 1- thymide kinase (TK). This therapy triggers an anti-tumor immune response that leads to tumor regression and anti-tumor immunological memory in intracranial rodent cancer models. We aim to test the efficacy of this immunotherapy in dogs bearing spontaneous GBM. In view of the controversy regarding the effect of human cytokines on dog immune cells, and considering that the efficacy of this treatment depends on hsFlt3L-stimulated dendritic cells (DCs), in the present work we tested the ability of Ad-encoded hsFlt3L to generate DCs from dog peripheral blood and compared its effects with canine IL-4 and GM-CSF.Our results demonstrate that hsFlT3L expressed form an Ad vector, generated DCs from peripheral blood cultures with very similar morphological and phenotypic characteristics to canine IL-4 and GM-CSF-cultured DCs. These include phagocytic activity and expression of CD11c, MHCII, CD80 and CD14. Maturation of DCs cultured under both conditions resulted in increased secretion of IL-6, TNF-alpha and IFN-gamma. Importantly, hsFlt3L-derived antigen presenting cells showed allostimulatory potential highlighting their ability to present antigen to T cells and elicit their proliferation.These results demonstrate that hsFlt3L induces the proliferation of canine DCs and support its use in upcoming clinical trials for canine GBM. Our data further support the translation of hsFlt3L to be used for dendritic cells' vaccination and gene therapeutic approaches from rodent models to canine patients and its future implementation in human clinical trials

CD11b⁺CD14⁻MHCII⁻ cells suppress T cell proliferation.

<p>Facs sorted CD11b⁺CD14⁻MHCII⁻ cells isolated from a dog with osteosarcoma or healthy PBMCs were co-incubated with mitogen-stimulated CD4⁺ and CD8⁺ T cells isolated from a healthy dog for 72 hs. No stimulated cells were used as negative control. Proliferative responses were measured by ³H-thymidine incorporation from experiments performed in triplicate. CPM, counts per minute. Mean ± SEM are shown.</p

Characteristics of dogs with cancer in the study.

<p>Characteristics of dogs with cancer in the study.</p

Immunophenotyping gating strategy and morphological analysis for MDSC identification in peripheral blood of dogs.

<p>PBMCs from healthy dogs and dogs with cancer were stained for the myeloid marker CD11b, monocytic marker CD14 and MHC II. (A) Representative flow cytometric analysis of forward and side scatter and gated CD11b⁺CD14⁻MHCII⁻ cells from dogs with advanced or metastatic tumors compared to dogs with early stage non-metastatic tumors and healthy control dogs. Plots are representative of dog with advanced metastatic hemangiosarcoma (top), early stage bladder transitional cell carcinoma (middle) and a healthy dog. (B) FACS sorted CD11b⁺CD14⁻MHCII⁻ cells were stained with diff-quick for cell morphology evaluation. A representative example of polymorphonuclear granulocyte morphology of CD11b⁺CD14⁻MHCII⁻ cells is shown at 63× magnification.</p

Percentages of circulating CD11b⁺CD14⁻MHCII⁻ cells in dogs with correlates with clinical tumor stage.

<p>(A) Analysis of average CD11⁺CD14⁻MHCII⁻ population frequency in dogs with advanced stage or metastatic tumors (n = 30) compared with early stage non-metastatic tumors (n = 15) and control dogs (n = 18). There was a significantly higher percentage of CD11b⁺CD14⁻MHCII⁻ cells in dogs with advanced cancer versus early stage non-metastatic tumors and healthy dogs (36.04% vs. 9.40% and10.24%, respectively. B) Average CD11b⁺CD14⁻MHCII⁻ population frequency in the major cancer subtypes: advanced stage or metastatic sarcomas (n = 18), early stage non-metastatic sarcomas (n = 6), advanced stage or metastatic carcinomas (n = 7) early stage non-metastatic carcinomas (n = 7), advanced stage or metastatic mast cell tumors (n = 5) and early stage non-metastatic mast cell tumors (n = 2) compared with control dogs (n = 18). Significantly elevated percentages were detected in all advanced tumors subtypes relative to early stage tumors and healthy dogs. Percentages of CD11b⁺CD14⁺MHCII⁻ cells were not significant between groups (* indicates P<0.001). Mean ± SEM are shown.</p

CD11b⁺CD14⁻MHCII⁻ cells express MDSC-derived immunosuppressive factors.

<p>RT-PCR analysis of FACS purified CD11b⁺CD14⁻MHCII⁻ cells detected expression of ARG1 and iNOS2, as well TGF-β and IL-10 immunosuppressive cytokines. ARG-1 expression was not detected in normal PMNs. CD11b⁺CD14⁻MHCII⁻ cells were isolated from the peripheral blood of a dog with osteosarcoma and PMNs were isolated from a healthy dog. NRT, RNA template in the absence of reverse transcriptase. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0033274#s3" target="_blank">Results</a> are representative three experiments.</p

CD11b⁺CD14⁻MHCII⁻ cells suppress T cell proliferation and cytokine elaboration.

<p>CD11b⁺CD14⁻MHCII⁻ cells were sorted from peripheral blood sample of dogs with cancer and then co-cultured with autologous PBMCs (A, B) or healthy dog PBMCs (C) in the presence of mitogen for 72 hs. Representative examples from a total of eight dogs are shown. The graphs represent proliferative responses after addition of CD11b⁺CD14⁻MHCII⁻ isolated from a single dog with squamous cell carcinoma (3A), prostatic adenocarcinoma (3B) and osteosarcoma (3C). Non-stimulated PBMCs were used as negative control and PBMCs stimulated in absence of CD11b⁺CD14⁻MHCII⁻ cells were used as positive control for proliferation. PBMCs were also co-incubated with PMNs, to control for presence of additional cells (3C, 3D). Proliferative responses were measured by ³H-thymidine incorporation. CPM, counts per minute. Amount of IFN-γ secretion in the co-culture was determined using canine specific IFN-γ ELISA assay (3D). All experiments were performed in triplicate. Mean ± SEM are shown.</p