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Summary: This review aims at giving an overview of the aetiology, genetic basis and clinical signs of equine malignant melanoma (EMM), the diagnostic work-up and the current treatment options. The established association between certain genetic features and melanoma formation in horses is reviewed. Typically, grey horses are affected with EMM. Typical predilection sites are the ventral tail, perianal region, eyelids, lips, parotid, or guttural pouches. Tumours usually grow slowly, but frequently metastasize eventually. Chemotherapy with cisplatin, 5-Fluorouracil and cimetidine are reviewed. Surgical excision of solitary small tumours can be successful, but has no systemic therapeutic effect and cannot prevent tumour recurrence. In conclusion, therapeutic options are still limited and conventional therapy is hardly curative. Current research focuses on immune modifying therapies using either tumour antigens for therapeutic vaccination or antitumoural interleukins. The most promising therapeutic approach currently under investigation is gene therapy using a xenogenic DNA vaccine coding for tumour associated antigens such as tyrosinase and gene therapy using interleukin-12 and -18.

BackgroundDeoxyribonucleic acid (DNA) vaccines are used for experimental immunotherapy of equine melanoma. The injection of complexed linear DNA encoding interleukin (IL)-12/IL-18 induced partial tumour remission in a clinical study including 27 grey horses. To date, the detailed mechanism of the anti-tumour effect of this treatment is unknown.ResultsIn the present study, the clinical and cellular responses of 24 healthy horses were monitored over 72 h after simultaneous intradermal and intramuscular application of equine IL-12/IL-18 DNA (complexed with a transfection reagent) or comparative substances (transfection reagent only, nonsense DNA, nonsense DNA depleted of CG).Although the strongest effect was observed in horses treated with expressing DNA, horses in all groups treated with DNA showed systemic responses. In these horses treated with DNA, rectal temperatures were elevated after treatment and serum amyloid A increased. Total leukocyte and neutrophil counts increased, while lymphocyte numbers decreased. The secretion of tumour necrosis factor alpha (TNFα) and interferon gamma (IFNγ) from peripheral mononuclear blood cells ex vivo increased after treatments with DNA, while IL-10 secretion decreased. Horses treated with DNA had significantly higher myeloid cell numbers and chemokine (C-X-C motif) ligand (CXCL)-10 expression in skin samples at the intradermal injection sites compared to horses treated with transfection reagent only, suggesting an inflammatory response to DNA treatment.In horses treated with expressing DNA, however, local CXCL-10 expression was highest and immunohistochemistry revealed more intradermal IL-12-positive cells when compared to the other treatment groups.In contrast to non-grey horses, grey horses showed fewer effects of DNA treatments on blood lymphocyte counts, TNFα secretion and myeloid cell infiltration in the dermis.ConclusionTreatment with complexed linear DNA constructs induced an inflammatory response independent of the coding sequence and of CG motif content. Expressing IL-12/IL-18 DNA locally induces expression of the downstream mediator CXCL-10.The grey horses included appeared to display an attenuated immune response to DNA treatment, although grey horses bearing melanoma responded to this treatment with moderate tumour remission in a preceding study. Whether the different immunological reactivity compared to other horses may contributes to the melanoma susceptibility of grey horses remains to be elucidated.Electronic supplementary materialThe online version of this article (doi:10.1186/s12917-015-0452-3) contains supplementary material, which is available to authorized users.