GENERAL INTRODUCTION: PAPILLOMAVIRIDAE

PAPILLOMAVIRIDAE

PAPILLOMAVIRIDAE

Current author:
J S MUNDAY - School of Veterinary Science, Massey University, Palmerston North, New Zealand

Introduction

Papillomaviruses were previously classified along with Polyomaviruses within the Papovaviridae family. However, these viruses are now classified into the two separate families of Papillomaviridae and Polyomaviridae. As polyomaviruses are not considered pathogenic in livestock, these are not discussed further.

Papillomaviruses are associated with important diseases in bovine, ovine and equine species These viruses are considered a rare cause of disease in goats and, although pigs are infected by papillomaviruses, they have not been associated with disease in this species. In other domestic species, papillomaviruses are also associated with oral papillomas, cutaneous papillomas, cutaneous pigmented plaques, and squamous cell carcinomas (SCCs) in dogs and oral papillomas, cutaneous viral plaques/Bowenoid in situ carcinomas, SCCs, and sarcoids in cats (Table 1).22, 25

Biology of papillomaviruses

Papillomaviruses are small, non-enveloped, icosahedral viruses. Their circular double-stranded DNA genome is around 8000 base pairs long and includes five or six early (E) and two late (L) open reading frames (ORF) (Figure 1). Papillomaviruses are classified using the L1 ORF sequence. Papillomaviruses within the same genus have greater than 60 per cent L1 ORF similarity and typically demonstrate similar host, location, and behavioral characteristics. Different papillomavirus types have less than 90 per cent similarity in their L1 ORF.4 The Papillomaviridae family currently includes 49 genera16 that contain numerous different papillomavirus types that infect a wide variety of animals including mammals, reptiles, and birds (Figure 2).23 However, short sequences of additional papillomavirus types have been amplified from many domestic species suggesting that new papillomavirus types are likely to be recognized in the future. Papillomaviruses have co-evolved with their hosts over a long time and the vast majority of papillomaviruses are strictly species specific. Currently, the bovine Deltapapillomaviruses are the only papillomaviruses known to be able to infect more than one species.24, 26

Infections by papillomaviruses are generally limited to stratified epithelium. The Deltapapillomaviruses are unique because they can also infect mesenchymal cells, although these cells probably do not permit virus replication.18 Papillomaviruses can be spread by direct contact. Additionally, as these viruses are able to survive for an extended time in the environment, they can also be spread indirectly. There is some evidence that papillomaviruses may also be able to be spread via biting flies or other insect vectors.13 Once the papillomavirus comes into contact with a mucocutaneous epithelium, the presence of microabrasions allows infection of basal cells resulting in the production of small numbers (10-200) of circular papillomavirus DNA copies (episomes) which are maintained in the basal cells as they replicate.30 While basal cell replication maintains the infection, the viral life-cycle is only completed when an infected cell undergoes terminal differentiation.9 This differentiation of a basal cell normally results in the cell becoming post-mitotic with subsequent loss of the nuclear machinery. To prevent the suprabasilar cell from leaving the cell cycle, the papillomavirus produces the E6 and E7 proteins that induce cell replication. Cell replication maintains the cell nuclear machinery that is required for papillomavirus replication as well as expanding the pool of infected cells within the epithelium, greatly increasing viral replication.11 The papillomavirus capsid proteins (L1 and L2) are expressed and viral assembly occurs as the infected cell reaches the upper epithelium. Papillomavirus-laden mature keratinocytes are sloughed from the epithelial surface. Papillomaviruses do not cause cell lysis, but the normal degradation of these sloughed cells releases infectious virions.11

Pathogenesis of papillomavirus disease

The clinical presentation of a papillomavirus infection is largely determined by the degree of cell proliferation induced by the individual papillomavirus. Most papillomavirus types only mildly increase cell proliferation and papillomavirus replication occurs slowly in the absence of any visible lesions.11 Alternatively, a minority of papillomavirus types markedly increase cell replication resulting in rapid production of large numbers of viral particles. Such infections cause marked epithelial hyperplasia that is visible clinically as a papilloma (wart).21

As papillomaviruses do not cause cell lysis they are generally of minimal adverse impact to the host and papillomavirus infections often illicit only a weak immune response.10 The immune reaction is further reduced because the majority of papillomavirus proteins are only expressed within the external epithelial layers. Furthermore, some papillomavirus proteins have been found to potentially interfere with an immune response.8 When an immune reaction occurs, the response can be subdivided into humoral and cell-mediated components. The production of circulating IgG antibodies blocks entry of the papillomavirus into the basal cells preventing further infections by this papillomavirus type, although antibodies do not influence resolution of an established papillomavirus infection.14, 27 Resolution of an established infection is dependent on the development of a cell-mediated immune (CMI) response.12 If an animal has a papilloma, the CMI response decreases papillomavirus replication resulting in a reduction of the papillomavirus-induced hyperplasia and subsequent spontaneous lesion resolution. There is significant intra-individual variation in the time taken by the body to mount a CMI response. This results in significant variation in the time that an animal can have visible papillomas before these lesions resolve. Immunosuppression may also delay the resolution of papillomas.25

Most animals are asymptomatically infected by papillomaviruses.2, 3 As the immune system prevents rapid papillomavirus replication and subsequent epithelial hyperplasia, these infections do not usually result in clinical lesions.12 However, if changes in the host allow greater papillomavirus protein expression, the resultant epithelial hyperplasia can become visible as a lesion. This is illustrated by the increased rate of oesophageal papillomas observed in cattle that ingest bracken fern, a plant that causes immunosuppression.17

In humans, the major significance of papillomaviruses is the ability of the high risk Alphapapillomaviruses to cause cervical, anogenital and oral cancer.33 The human Alphapapillomaviruses have been shown to cause cancer due to the accidental integration of the papillomavirus DNA into the host DNA. This integration prevents virus replication but results in overexpression of the papillomavirus E6 and E7 genes causing rapid uncontrolled cell growth, inhibition of cell apoptosis, telomerase loss, and disruption of processes that ensure accurate assembly of replicated host DNA.11 As these cells multiply rapidly and are genetically-unstable, they are predisposed to developing additional mutations that result in neoplastic transformation.28 While there is accumulating evidence of an association between papillomavirus infection and cancer in cattle, horses, and sheep, the mechanisms of neoplastic transformation are either unknown or have been shown to be different to those of the human high risk papillomaviruses.5, 26, 32 Additionally, as some papillomavirus-associated neoplasms of livestock have been shown to contain productive infections, it appears likely that the papillomavirus DNA is not integrated in the host DNA in these cancers.

Prevention of papillomavirus disease

Papillomaviruses are infectious agents and if the agent can be avoided then papillomavirus-induced disease can be prevented. This is illustrated by the rarity of genital papillomas in cattle that are artificially inseminated. While this could suggest that papillomavirus-induced disease could be prevented by identifying and culling affected animals, the high proportion of animals that are asymptomatically infected by papillomaviruses means such a strategy is unlikely to be successful.

In humans, virus-like particle papillomavirus vaccines are used to prevent papillomavirus infection. While such vaccines have also been shown to be effective in animals,19, 31 there are important limitations to the use of vaccines to prevent papillomavirus-induced disease in livestock. Firstly, for a vaccine to prevent infection, it has to be given prior to first exposure to the papillomavirus.29 When a papilloma develops in an animal, this indicates that it is the first time that this animal has been infected by this papillomavirus type. As papillomas generally develop in young adult animals, this suggests vaccination prior to first infection could be possible. However, the time of infection for the papillomaviruses that are associated with equine sarcoids and ovine SCC is currently uncertain and it is unknown whether vaccination prior to first infection with these papillomaviruses is possible. Secondly, a vaccine has to be economically viable. As papillomas typically have little economic cost, it appears unlikely that a vaccine to prevent these self-resolving hyperplastic lesions would be commercially successful. Papillomaviruses are associated with economically-important cancers of the bladder and alimentary tract in cattle and water buffalo and the skin of sheep.1, 6 However, these cancers are strongly influenced by exposure to other co-factors making it unclear what proportion of these cancers would be prevented by a papillomavirus vaccine. Equine sarcoids and equine penile SCC also have a significant economic cost. Research is currently underway evaluating the efficacy of vaccination against these diseases and vaccines may become available in the future.15

Diagnosis of papillomavirus disease

Papillomas are hyperplastic lesions that develop as a result of papillomavirus-induced proliferation of suprabasilar cells within the epithelium. This hyperplasia results in thickening and folding of the epithelium forming an exophytic lesion. Histological examination of a papilloma can reveal the presence of papillomavirus-induced changes to the infected cells. These changes include the development of enlarged cells that have a shrunken nucleus surrounded by a clear cytoplasmic halo (koilocytes), the development of increased quantities of blue-grey smudgy cytoplasm (Figure 3), intranuclear eosinophilic inclusion bodies, or clumping of keratohyalin granules within the superficial layers of the epithelium.25 Immunohistochemistry can be used to detect papillomavirus antigen. However, immunohistochemistry detects L1 protein and lesions rarely contain immunostaining without the presence of histologically-detectible papillomavirus-induced cell changes. Molecular techniques such as PCR can detect papillomavirus DNA within the lesions and, by sequencing the amplified DNA, the papillomavirus type can be determined.

Neoplasia associated with papillomavirus infection often contains no histological or immunohistochemical evidence of papillomavirus infection and evidence of papillomavirus involvement is derived from observations of progression from a papilloma to a neoplasm or from molecular detection of papillomavirus DNA within the neoplasm.7, 20 Due to the ubiquitous nature of papillomaviruses it can be hard to differentiate between the papillomavirus causing neoplastic transformation of infected cells and the papillomavirus being an ‘innocent bystander’ present within the lesion, but not influencing neoplastic transformation of the cells.22

Table 1 Summary of papillomaviruses and their predominant associated lesions. It should be noted that not all papillomavirus types within a genus will cause all associated lesions listed. For example, bovine papillomavirus (BPV) -4 is the only Xipapillomavirus type associated with oesophageal papillomas in cattle. OaPV, Ovis aries papillomavirus; EcPV, Equus caballus papillomavirus; SsPV, Sus scrofa papillomavirus; CPV, Canis familiaris papillomavirus, FcaPV, Felis catus papillomavirus;

Species

Papillomavirus genus

Papillomavirus types

Predominant associated lesions

Cattle and water buffalo

Delta

BPV-1, -2, -13, -14

Cutaneous fibropapilloma
Upper alimentary fibropapilloma
Urinary bladder neoplasia*

Xi

BPV-3, -4, -6, -10, -11, -12

Cutaneous papilloma
Upper alimentary papilloma
Upper alimentary squamous cell carcinoma

Epsilon

BPV-5, -8

Cutaneous fibropapilloma
Cutaneous papilloma

Dyoxi

BPV-7

Teat papilloma

Sheep

Delta

OaPV-1, -2

Cutaneous fibropapilloma

Dyolambda

OaPV-3

Cutaneous squamous cell carcinoma

Goats

Phi

ChPV-1

No associated lesions

Horses

Zeta

EcPV-1

Cutaneous papilloma

Dyoiota

EcPV-2, -4, -5

Penile papilloma
Penile squamous cell carcinoma
Oral squamous cell carcinoma
Aural plaque
Vulval plaque

Dyorho

EcPV-3, -6, -7

Aural plaque

 

Delta

 

BPV-1, -2, -13

 

Equine sarcoid

Pigs

Dyodelta

SsPV-1

No associated lesions

Dogs

Lambda

CPV-1, -6

Oral papilloma
Cutaneous papilloma

Tau

CPV-2, -7, -13, -17, -19

Cutaneous papilloma
Oral squamous cell carcinoma

Chi

CPV-3, -4, -5, -8, -9, -10,   -11, -12, -14, -15, -16

Viral pigmented plaque
Cutaneous squamous cell carcinoma

Cats

Lambda

FcaPV-1

Oral papilloma

Dyotheta

FcaPV-2

Viral plaque/bowenoid in situ carcinoma
Cutaneous squamous cell carcinoma

Unclassified

FcaPV-3, -4

Viral plaque/bowenoid in situ carcinoma
Basal cell carcinoma

Delta

BPV-14

Feline sarcoid

* These cancers are strongly influenced by co-factors and the precise role of the papillomavirus in cancer development is currently unclear.

Figure 1 Schematic genomic organisation of bovine papillomavirus (BPV)-14. Figure from Munday JS et al. Veterinary Microbiology 2015., 177, 289-95 (with permission).

Figure 2 Unrooted maximum likelihood phylogeny based on a 2965 bp concatenated nucleotide alignment of E1, E2, L1 and L2 ORF sequence 70 papillomavirus types of different species and genera. Accession numbers for the sequences used are included. Abbreviations used include human papillomavirus, HPV; Mesocricetus auratus papillomavirus (Hamster oral papillomavirus), MaPV; Micromys minutus papillomavirus (old harvest mouse papillomavirus), MmPV; Mus musculus papillomavirus, MmuPV; Mastomys coucha papillomavirus, McPV; Apodermus sylvaticus papillomavirus, AsPV; Canine papillomavirus, CPV; Bovine papillomavirus, BPV; Phocoena spinipinnis papillomavirus, PhsPV; Bettongia penicillata papillomavirus, BpPV; Macaca fascicularis papillomavirus, MfPV; Macaca mulatta papillomavirus, RPV; Felis catus papillomavirus, FcaPV; Equus caballus papillomavirus, EcPV; Multimammate rat papillomavirus, MnPV; Psittacus erithacus timneh papillomavirus, PePV; Fringilla coelebs papillomavirus, FcPV; Ovis aries papillomavirus, OaPV, Odocoileus virginianus papillomavirus, OvPV; Alces alces papillomavirus, AaPV; Oryctolagus cuniculus papillomavirus (Rabbit oral papillomavirus), OcPV; Sylvilagus floridanus papillomavirus (Cottontail rabbit (Shope) papillomavirus), SfPV. The papillomavirus genera are also listed. Internal branches are coloured based on inferred bootstrap support values, as determined by 1000 replicates usin RAxML. The scale bar indicates the genetic distance (nucleotide substitutions per site).  Figure from Munday JS et al. Veterinary Microbiology 2017, 207:50-55 (with permission).

Figure 3 Photomicrograph of a canine cutaneous papilloma. Note that the papillomavirus-induced hyperplasia of the epidermis has resulted in thickening and folding of the epidermis. Due to the influence of the papillomavirus E6 and E7 proteins, keratinocytes throughout the epidermis have retained their nuclei. A characteristic papillomavirus-induced change is the presence of numerous enlarged cells with blue-grey smudged cytoplasm. Haematoxylin and Eosin, 200x.

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