Immunotherapy of Equine Sarcoids—From Early Approaches to Innovative Vaccines

Horses and other equid species are frequently affected by bovine papillomavirus type 1 and/or 2 (BPV1, BPV2)-induced skin tumors termed sarcoids. Although sarcoids do not metastasize, they constitute a serious health problem due to their BPV1/2-mediated resistance to treatment and propensity to recrudesce in a more severe, multiple form following accidental or iatrogenic trauma. This review provides an overview on BPV1/2 infection and associated immune escape in the equid host and presents early and recent immunotherapeutic approaches in sarcoid management.

Immunogenicity analysis of BPV-1 positive equine sarcoid-derived cultured fibroblasts

Sarcoids are the most common equine skin tumours Although they do not metastasize, they can be locally aggressive and cause significant clinical symptoms in affected horses. Despite being common, very little is known about the host immune response and the biological mechanisms underlying persistence and recurrence of equine sarcoids. The latter reflects the need for further research in this field. This in-vitro study used sarcoid explants from horses with naturally occurring sarcoids (n = 12) to evaluate the induction of a humoral immune response directed against equine sarcoid-derived bovine papilloma-virus (BPV)- 1 infected fibroblasts using a flow cytometric crossmatch assay. The presence of antibodies against exogenous bovine serum albumin (BSA) and fibroblast-like mesenchymal stromal cells (MSCs) was also evaluated by ELISA and flow cytometry, respectively. The viral load in the sarcoid explants, the corresponding cultured sarcoid fibroblasts, and matched peripheral blood mononuclear cells (PBMCs) from affected horses were determined by quantitative BPV-1/- 2 PCR analysis. Antibodies against autologous sarcoid cells were present in six out of twelve sarcoid-affected horses. Serum from all horses showed cross reactivity with allogeneic sarcoid cells, while only a part reacted with BSA or MSCs. Screening of host PBMCs demonstrated the absence of BPV E1 nucleic acids. Statistical analysis revealed a significantly higher mean viral load in the parental sarcoid tissue compared to the low passage fibroblasts (P < 0.001). These results support the hypothesis that sarcoid-affected horses may develop antibodies recognizing tumour-specific antigens. In contrast to sarcoid explants, equine PBMCs do not seem to contain complete BPV genomes. These results provide a basis for future investigations on the clinical relevance of these antibodies.

Papillomavirus-like Particles in Equine Medicine

Papillomaviruses (PVs) are a family of small DNA tumor viruses that can induce benign lesions or cancer in vertebrates. The observation that animal PV capsid-proteins spontaneously self-assemble to empty, highly immunogenic virus-like particles (VLPs) has led to the establishment of vaccines that efficiently protect humans from specific PV infections and associated diseases. We provide an overview of PV-induced tumors in horses and other equids, discuss possible routes of PV transmission in equid species, and present recent developments aiming at introducing the PV VLP-based vaccine technology into equine medicine.

EQUINE SARCOIDS IN CAPTIVE WILD EQUIDS: DIAGNOSTIC AND CLINICAL MANAGEMENT OF 16 CASES-A POSSIBLE PREDISPOSITION OF THE EUROPEAN COHORT OF SOMALI WILD ASS (EQUUS AFRICANUS SOMALIENSIS)?

Equine sarcoids (ES) were diagnosed in 12 Somali wild asses (SWA) (Equus africanus somaliensis) from 10 different institutions of the SWA European Endangered Species Programme from 1976 to 2019. Samples of surgically excised masses, biopsies, or necropsy samples were submitted for histologic and virologic analysis. In addition, tissue samples from one onager (Equus hemionus onager), one kulan (Equus hemionus kulan), and two Hartmann's mountain zebras (HMZ) (Equus zebra hartmannae) were examined. Histology confirmed the diagnosis of ES exhibiting the typical microscopic features. Polymerase chain reaction detected bovine papillomavirus type 1 (BPV1) DNA in eight SWA samples and bovine papillomavirus type 2 (BPV2) DNA in one SWA sample. The onager, kulan, and one HMZ sample tested positive for BPV1. The other HMZ tested positive for BPV1 and BPV2. This is the first report of ES in an onager. Surgical excision was the treatment elected by most veterinarians. A follow-up survey of the cases over several years after clinical diagnosis and therapy revealed variable individual outcome with ES recurrence in four cases. Three SWA and the kulan were euthanized due to the severity of the lesions. Nine affected SWA were males with seven having a sarcoid located at the prepuce. Because a genetic disposition is a risk factor for the development of ES in horses, this may also be true for endangered wild equids with few founder animals in their studbook history. Innovative approaches regarding therapy and prevention of ES in wild equids are therefore highly encouraged.

Equine Sarcoids-Causes, Molecular Changes, and Clinicopathologic Features: A Review

Equine sarcoid is the most common skin tumor of horses. Clinically, it occurs as a locally invasive, fibroblastic, wart-like lesion of equine skin, which has 6 clinical classes: occult, verrucose, nodular, fibroblastic, mixed, and malignant. Sarcoids may be single but multiple lesions are more frequent. The typical histological feature is increased density of dermal fibroblasts which form interlacing bundles and whorls within the dermis. Lesions are mostly persistent, resist therapy, and tend to recur following treatment. In general, sarcoids are not fatal but their location, size, and progression to the more aggressive form may lead to the withdrawal of a horse from use and serious infringement of their welfare leading to the loss of valuable animals. Bovine papillomavirus (BPV) type 1 and less commonly type 2 contribute to the development of equine sarcoid. The viral genome and proteins are detected in a high percentage of cases. Furthermore, viral oncoprotein activity leads to changes in the fibroblastic tissue similar to changes seen in other types of tumors. Equine sarcoids are characterized by a loss of tumor suppressor activity and changes allowing abnormal formation of the affected tissue, as well as y immune defense abnormalities that weaken the host's immune response. This impaired immune response to BPV infection appears to be crucial for the development of lesions that do not spontaneously regress, as occurs in BPV-infected cows.

Packaging of Rift Valley fever virus pseudoviruses and establishment of a neutralization assay method

Rift Valley fever (RVF) is an acute, febrile zoonotic disease that is caused by the RVF virus (RVFV). RVF is mainly prevalent on the Arabian Peninsula, the African continent, and several islands in the Indian Ocean near southeast Africa. RVFV has been classified by the World Organisation for Animal Health (OIE) as a category A pathogen. To avoid biological safety concerns associated with use of the pathogen in RVFV neutralization assays, the present study investigated and established an RVFV pseudovirus-based neutralization assay. This study used the human immunodeficiency virus (HIV) lentiviral packaging system and RVFV structural proteins to successfully construct RVFV pseudoviruses. Electron microscopy observation and western blotting indicated that the size, structure, and shape of the packaged pseudoviruses were notably similar to those of HIV lentiviral vectors. Infection inhibition assay results showed that an antibody against RVFV inhibited the infective ability of the RVFV pseudoviruses, and an antibody neutralization assay for RVFV detection was then established. This study has successfully established a neutralization assay based on RVFV pseudoviruses and demonstrated that this method can be used to effectively evaluate antibody neutralization.

Neoplasia in 125 donkeys (Equus asinus): literature review and a survey of five veterinary schools in the United States and Canada

A diagnosis of neoplasia was noted in 125 of 357 donkeys (35%) in our review of medical records from 5 veterinary schools in the United States and Canada. Equine sarcoid was the most common tumor in our study, accounting for 72% of all tumors and 82% of cutaneous tumors. Soft-tissue sarcomas were the second most common skin tumors. All other types of neoplasia were rare. Important differences in the occurrence of neoplasia in donkeys compared to horses included the rarity or absence of squamous cell carcinoma in any organ system and gray horse melanoma. Lymphosarcoma, the most common malignant tumor in horses, appears to be very rare in donkeys. We report several tumors in donkeys including melanocytoma, peripheral nerve sheath tumor, and gastrointestinal stromal tumor. Our data demonstrate commonalities as well as differences in neoplastic diseases of donkeys and horses. Understanding differences in carcinogenesis among these 2 closely related species can inform researchers pursuing pathogenic mechanisms of equine disease and inform veterinary diagnosticians regarding tumor prevalence.

Both tumour cells and infiltrating T-cells in equine sarcoids express FOXP3 associated with an immune-supressed cytokine microenvironment

Bovine papillomavirus (BPV) infections of equine species have a central role in the aetiology of equine sarcoids; a common benign skin tumour of horses, zebras and donkeys. Within the lesions, all of the early papillomavirus genes are expressed and promote the excessive replication of fibroblasts which characterise these tumours. Equine sarcoids differ from BPV induced fibro-papillomas of cattle (the natural host of BPV), in that they do not produce high amounts of virus particles, do not usually regress spontaneously and do not sero-convert to BPV; features which suggest that affected horses lack an effective anti-viral immune response to BPV. Equine sarcoids contain large numbers of CD4+ CD8+ dual positive T-cells which uniformly express FOXP3, the key transcription factor of regulatory T-cells, and FOXP3 is also expressed within the BPV infected fibroblasts. Compared to healthy skin, sarcoids showed increased mRNA transcription for FOXP3 and the regulatory cytokine TGFβ. Transcription of IL17, which has been shown to have a regulatory function in human papillomavirus-associated tumours, was also elevated in equine sarcoids compared to spleen. In contrast, the levels of mRNA transcripts for effector T cell cytokines IL2, IL4 and interferon-gamma (IFNγ) were not elevated in sarcoids compared to healthy skin or spleen. Similarly neither interferon-alpha (IFNα), interferon-beta (IFNβ) nor IL12 family members were elevated in sarcoids compared to normal skin. We suggest that the regulatory cytokine micro-environment within sarcoids enables the persistence of the lesions by preventing an effective anti-viral immune response.

Bovine papillomavirus: opening new trends for comparative pathology

For many years, research on bovine papillomavirus (BPV) has contributed to the understanding of papillomavirus-induced pathology in humans and animals. The present review shows how recent studies on BPV keep providing evidence concerning key points in viral infection, such as the expression of viral proteins in lymphocytes and the occurrence of productive infections of the placenta. Studies on BPV-induced tumours also provide important information concerning the mechanisms of oncogenesis and immune evasion, as in the cases of connexin 43 down-regulation with loss of intercellular gap junctions and Toll-like receptor 4 (TLR4) down-regulation in equine sarcoids. The biological functions of viral proteins are also being further clarified, as in the case of E2, which was recently shown to load BPV genomes into host chromosomes during the S phase, a process mediated by the ChlR1 protein. In the near future, the ongoing efforts to characterize and classify additional emerging BPV types are likely to broaden even further the possibilities for research.

Equine sarcoids: Bovine Papillomavirus type 1 transformed fibroblasts are sensitive to cisplatin and UVB induced apoptosis and show aberrant expression of p53

Bovine papillomavirus type 1 infects not only cattle but also equids and is a causative factor in the pathogenesis of commonly occurring equine sarcoid tumours. Whilst treatment of sarcoids is notoriously difficult, cisplatin has been shown to be one of the most effective treatment strategies for sarcoids. In this study we show that in equine fibroblasts, BPV-1 sensitises cells to cisplatin-induced and UVB-induced apoptosis, a known cofactor for papillomavirus associated disease, however BPV-1 transformed fibroblasts show increased clonogenic survival, which may potentially limit the therapeutic effects of repeated cisplatin treatment. Furthermore we show that BPV-1 increases p53 expression in sarcoid cell lines and p53 expression can be either nuclear or cytoplasmic. The mechanism and clinical significance of increase/abnormal p53 expression remains to be established.

Phylogenetic and structural studies of a novel equine papillomavirus identified from aural plaques

Papillomaviruses (PVs) infect a wide range of animal species and show great genetic diversity. To date, excluding equine sarcoids, only three species of PVs were identified associated with lesions in horses: Equus caballus papillomavirus 1 (EcPV1-cutaneous), EcPV2 (genital) and EcPV3 (aural plaques). In this study, we identified a novel equine PV from aural plaques, which we designated EcPV4. Cutaneous samples from horses with lesions that were microscopically diagnosed as aural plaques were subjected to DNA extraction, amplification and sequencing. Rolling circle amplification and inverse PCR with specific primers confirmed the presence of an approximately 8 kb circular genome. The full-length EcPV4 L1 major capsid protein sequence has 1488 nucleotides (495 amino acids). EcPV4 had a sequence identity of only 53.3%, 60.2% and 51.7% when compared with the published sequences for EcPV1, EcPV2 and EcPV3, respectively. A Bayesian phylogenetic analysis indicated that EcPV4 clusters with EcPV2, but not with EcPV1 and EcPV3. Using the current PV classification system that is based on the nucleotide sequence of L1, we could not define the genus of the newly identified virus. Therefore, a structural analysis of the L1 protein was carried out to aid in this classification because EcPV4 cause lesion similar to the lesion caused by EcPV3. A comparison of the superficial loops demonstrated a distinct amino acid conservation pattern between EcPV4/EcPV2 and EcPV4/EcPV3. These results demonstrate the presence of a new equine PV species and that structural studies could be useful in the classification of PVs.

Safety and immunogenicity of BPV-1 L1 virus-like particles in a dose-escalation vaccination trial in horses

REASONS FOR PERFORMING STUDY

Infection with bovine papillomaviruses types 1 and 2 (BPV-1, BPV-2) can lead to the development of therapy-resistant skin tumours termed sarcoids and possibly other skin diseases in equids. Although sarcoids seriously compromise the welfare of affected animals and cause considerable economic losses, no prophylactic vaccine is available to prevent this common disease. In several animal species and man, immunisation with papillomavirus-like particles (VLP) has been shown to protect efficiently from papillomaviral infection.

HYPOTHESIS

BPV-1 L1 VLPs may constitute a safe and highly immunogenic vaccine candidate for protection of horses against BPV-1/-2-induced disease.

METHODS

Three groups of 4 horses each received 50, 100 or 150 µg of BPV-1 L1 VLPs, respectively, on Days 0, 28 and 168. Three control horses received adjuvant only. Horses were monitored on a daily basis for one week after each immunisation and then in 2 week intervals. Sera were collected immediately before, 2 weeks after each vaccination and one and 2 years after the final boost and analysed by pseudovirion neutralisation assay.

RESULTS

None of the horses showed adverse reactions upon vaccination apart from mild and transient swelling in 2 individuals. Irrespective of the VLP dose, all VLP-immunised horses had developed a BPV-1-neutralising antibody titre of ≥ 1600 plaque forming units (pfu)/ml 2 weeks after the third vaccination. Eight of 10 trial horses still available for follow-up had neutralising antibody titres ≥ 1600 pfu/ml one year and ≥ 800 pfu/ml 2 years after the last immunisation.

CONCLUSION

Intramuscular BPV-1 L1 VLP vaccination in horses is safe and results in a long-lasting antibody response against BPV-1. Neutralisation titres were induced at levels that correlate with protection in experimental animals and man.

POTENTIAL RELEVANCE

BPV-1 L1 VLPs constitute a promising vaccine candidate for prevention of BPV-1/-2-induced disease in equids.

Upregulation of equine matrix metalloproteinase 1 by bovine papillomavirus type 1 is through the transcription factor activator protein-1

Equine sarcoids represent the most common skin tumours in equids worldwide, characterized by extensive invasion and infiltration of lymphatics, rare regression and high recurrence after surgical intervention. Bovine papillomavirus type 1 (BPV-1) activity is necessary for the transformation phenotype of equine fibroblasts. Among the many changes induced by BPV-1, matrix metalloproteinase 1 (MMP-1) upregulation contributes to the invasiveness of equine fibroblasts. However, it is not yet known how BPV-1 proteins regulate equine MMP-1 expression. To elucidate this mechanism, the equine MMP-1 promoter was cloned and analysed. A putative activator protein-1 (AP-1)-binding site was demonstrated to be crucial for upregulated MMP-1 promoter activity by BPV-1. BPV-1 E6 and E7 proteins increased MMP-1 promoter activity, and inhibition of BPV-1 gene expression by small interfering RNA significantly reduced the promoter activity. c-Jun and Fra-1, two components of the AP-1 transcription factor complex, were overexpressed and activated by BPV-1 in equine fibroblasts. Finally, BPV-1 E5, E6 and E7 proteins increased MMP-1 mRNA and protein expression. In conclusion, the expression of MMP-1 can be enhanced by BPV-1 oncoproteins E6 and E7 through the AP-1 transcription factor and by E5 via an indirect mechanism. These findings shed light on the mechanism of BPV-1-mediated equine fibroblast infiltration and indicate that both BPV-1 oncoproteins and AP-1 could be potential targets for equine sarcoid therapy.

p38 mitogen-activated protein kinase is crucial for bovine papillomavirus type-1 transformation of equine fibroblasts

Equine sarcoids represent the most common skin tumours in equids worldwide, characterized by extensive invasion and infiltration of lymphatics, rare regression and high recurrence after surgical intervention. Bovine papillomavirus type-1 (BPV-1) and less commonly BPV-2 are the causative agents of the diseases. It has been demonstrated that BPV-1 viral gene expression is necessary for maintaining the transformation phenotype. However, the underlying mechanism for BPV-1 transformation remains largely unknown, and the cellular factors involved in transformation are not fully understood. Previously mitogen-activated protein kinase (MAPK) signalling pathway has been shown to be important for cellular transformation. This study investigated the role of p38 MAPK (p38) in the transformation of equine fibroblasts by BPV-1. Elevated expression of phosphorylated p38 was observed in BPV-1 expressing fibroblasts due to the expression of BPV-1 E5 and E6. The phosphorylation of the MK2 kinase, a substrate of p38, was also enhanced. Inhibition of p38 activity by its selective inhibitor SB203580 changed cell morphology, reduced the proliferation of sarcoid fibroblasts and inhibited cellular invasiveness, indicating the indispensable role of p38 in BPV-1 transformation of equine fibroblasts. These findings provide new insights into the pathogenesis of equine sarcoids and suggest that p38 could be a potential target for equine sarcoid therapy.

Bovine papillomavirus type 1 oncoprotein E5 inhibits equine MHC class I and interacts with equine MHC I heavy chain

Bovine papillomavirus type 1 is one of the aetiological agents of equine sarcoids. The viral major oncoprotein E5 is expressed in virtually all sarcoids, sarcoid cell lines and in vitro-transformed equine fibroblasts. To ascertain whether E5 behaves in equine cells as it does in bovine cells, we introduced the E5 open reading frame into fetal equine fibroblasts (EqPalF). As observed in primary bovine fibroblasts (BoPalF), E5 by itself could not immortalize EqPalF and an immortalizing gene, such as human telomerase (hTERT/hT), was required for the cells to survive selection. The EqPalF-hT-1E5 cells were morphologically transformed, elongated with many pseudopodia and capable of forming foci. Equine major histocompatibility complex class I (MHC I) was inhibited in these cells at least at two levels: transcription of MHC I heavy chain was inhibited and the MHC I complex was retained in the Golgi apparatus and prevented from reaching the cell surface. We conclude that, as in bovine cells and tumours, E5 is a player in the transformation of equine cells and the induction of sarcoids, and a potential major cause of MHC I downregulation and hence poor immune clearance of tumour cells.

Activated platelet-derived growth factor beta receptor expression, PI3K-AKT pathway molecular analysis, and transforming signals in equine sarcoids

The equine sarcoid is the most common dermatologic neoplasm reported in horses. Bovine papillomavirus (BPV) types 1 and 2 are associated with sarcoids, in which the expression of the major transforming oncoprotein (E5) is often recorded. The transformation activity of the virus is due to the binding of the E5 to the platelet-derived growth factor beta receptor (PDGFbeta-r). In the present study, we show by Western blot in 4 sarcoid samples and 3 normal equine skin samples that the PDGFbeta-r is more phosphorylated in sarcoid tissue than in normal skin (P < .001). Furthermore, the physical interaction between the activated receptor and the 85-kDa regulatory subunit (p85) of phosphatidylinositol-3-kinase (PI3K) is shown by coimmunoprecipitation. The PI3K-AKT-cyclin D3 molecular pathway downstream to the activation of the PDGFbeta-r is shown to be expressed, and the amount of the investigated molecules is higher than normal (P < .001), suggesting an activation of these effectors in sarcoids. Further, we demonstrate that phospho-JNK and phospho-JUN are more expressed in sarcoids than in normal skin. Our results provide new insights into the pathogenesis of equine sarcoids and support the validity of this in-vivo model to further characterize the molecular pathways underlying BPV E5-induced carcinogenesis.

Bovine papillomaviruses: their role in the aetiology of cutaneous tumours of bovids and equids

Bovine papillomavirus (BPV) is perhaps the most extensively studied animal papillomavirus. In cattle BPVs induce benign tumours of cutaneous or mucosal epithelia, called papillomas or warts. Cattle papillomas are benign tumours and generally regress without eliciting any serious clinical problems in the host, but occasionally persist and provide the focus for malignant transformation to squamous cell carcinoma, as in the case of cancer of the urinary bladder and cancer of the upper alimentary canal. BPV is the only papillomavirus that jumps species: the virus also infects equids, and gives rise to fibroblastic tumours called sarcoids. Sarcoids very rarely regress, more often they persist and can be locally aggressive. These tumours are the most common dermatological tumour of equids worldwide. The purpose of this review is to discuss the biology of BPV, the biology of bovine tumours and equine sarcoids, and present the current understanding of BPV in tumour pathogenesis in its natural host, cattle, and in its heterologous host, equids. Finally, the use of anti-BPV vaccines as a therapy for equine sarcoids will be discussed. Only limited information on the clinical or pathological aspects of either bovine or equine tumours will be provided as this subject has been extensively addressed previously.