INFECTIOUS PAPILLOMATOSIS OF RABBITS : WITH A NOTE ON THE HISTOPATHOLOGY

Establishment of successively transplantable rabbit VX2 cancer cells that express enhanced green fluorescent protein

Morphological detection of cancer cells in the rabbit VX2 allograft transplantation model is often difficult in a certain region such as serosal cavity where reactive mesothelial cells mimic cancer cells and both cells share common markers such as cytokeratins. Therefore, tagging VX2 cells with a specific and sensitive marker that easily distinguishes them from other cells would be advantageous. Thus, we tried to establish a successively transplantable, enhanced green fluorescent protein (EGFP)-expressing VX2 model. Cancer cells obtained from a conventional VX2-bearing rabbit were cultured in vitro and transfected with an EGFP-encoding vector, and then successively transplanted in Healthy Japanese White rabbits (HJWRs) (n = 8). Besides, conventional VX2 cells were transplanted in other HJWRs (n = 8). Clinicopathological comparison analyses were performed between the two groups. The success rate of transplantation was 100% for both groups. The sensitivity and specificity of EGFP for immunohistochemical detection of VX2 cells were 84.3 and 100%, respectively. No significant differences in cancer cell morphology, tumor size (P = 0.742), Ki-67 labeling index (P = 0.878), or survival rate (P = 0.592) were observed between the two. VX2 cells can be genetically altered, visualized by EGFP, and successively transplanted without significant alteration of morphological and biological properties compared to those of the conventional model.

Establishing a rabbit model of malignant esophagostenosis using the endoscopic implantation technique for studies on stent innovation

BACKGROUND

Stents are recommended in patients with dysphagia caused by esophageal stricture, but an ideal stent does not currently exist. Thus, studies on new esophageal stents are necessary, and suitable animal models are desperately needed for these studies. The aim of this study was to establish a model of malignant esophageal stricture in rabbit for studies on stent innovation.

METHODS

A total of 38 New Zealand white rabbits were used in this study. Using the endoscopic submucosal injection technique, VX2 fragments were inoculated into the submucosal layer of the rabbit thoracic esophagus, and an endoscopic follow-up was subsequently performed to observe the tumor development and progression. The self-expandable metal stents were randomly deployed in rabbits with severe esophageal stricture to investigate the safety and feasibility of the animal models for stenting.

RESULTS

An endoscopic implantation procedure for VX2 tumors was completed in 34/38 rabbits, and tumor development was confirmed in 30/34 animals. The success rate of the endoscopic implantation and tumor development were 89.4% (95% CI, 79.6% to 99.2%) and 88.2% (95% CI, 76.9% to 99.5%) respectively. During the endoscopic follow-up period, severe esophageal stricture occurred in 22/30 rabbits with a rate of 73.3% (95% CI, 57.5% to 89.1%), and 12/22 models received stent placement. During and after stent implantation, no severe stent-related complication or mortality occurred in the animal models. The rabbits that received stent placement survived longer than those without stent implantation (the mean survival time: 53.9 days versus 40.3 days, P = 0.016).

CONCLUSION

The endoscopic method is a safe and effective method for establishing a malignant esophagostenosis model in rabbits. This model can simulate the human body environment for stent deployment and is an excellent tool for the study of stent innovation for the treatment of esophageal cancer.

Comparison of endoscopic submucosal implantation vs. surgical intramuscular implantation of VX2 fragments for establishing a rabbit esophageal tumor model for mimicking human esophageal squamous carcinoma

Purpose

This study was undertaken to establish a rabbit esophageal tumor model for mimicking human esophageal squamous carcinoma (ESC) by endoscopic and surgical implantation of VX2 tumors.

Methods

Fragments of a VX2 tumour were endoscopically implanted in the submucosal layer of the thoracic esophagus of 32 New Zealand white rabbits, while 34 animals received surgical implantation into the muscular layer. Then, the animals were studied endoscopically and pathologically. The safety and efficiency of the two methods and the pathological features of the animal models were analyzed.

Results

Both the endoscopic and the surgical method had a relatively high success rate of tumor implantation [93.7% (30/32) vs. 97.1% (33/34)] and tumor growth [86.7% (26/30) vs. 81.8% (27/33)], and the variation in the results was not statistically significant (P>0.05). Compared with those produced by the surgical method, the models produced by the endoscopic method had a higher rate of severe esophageal stricture [61.5% (16/26) vs. 29.6% (8/27)] and of intra-luminal tumor growth [73.1% (19/26) vs. 37.0% (10/27)], and had a lower rate of tumor invasion of adjacent organs [53.8% (14/26) vs. 81.5% (22/27)]; all of these results were statistically significant (P<0.05). However, the difference in the survival time and the rates of tumor regional/distant metastasis [38.5% (10/26) vs. 51.8% (14/27)] between the two methods were not statistically significant (P>0.05).

Conclusion

The endoscopic and surgical methods are both safe and effective for establishment of VX2 tumors in the rabbit esophagus. The models produced by the two methods have different pathologic features mimicking that of human ESC. We recommend the models for studies on surgical procedures and minimally invasive treatments.

[Epidemiology of laryngeal and hypopharyngeal cancer in the period 1988-2012 in the material of the Otolaryngology Clinic of the Bialystok Medical University]

Malignant tumour is the second cause of death in Poland, behind cardiovascular disease. 26% of men and 23% of women die of it. Head and neck tumours are the fifth most frequent group of malignant tumours. These are mostly squamous cell carcinomas. The carcinogenic factors of tobacco smoke play an indisputable role in the pathogenesis of these tumours.

AIM OF WORK

Analysis comprising: number of patients, sex and age, site and stage of the primary tumour (T), clinical assessment of neck lymph nodes (N). The results underwent statistical analysis.

MATERIALS AND METHODS

The study involved 1313 patients who underwent surgery in the period 1988-2012 in the Otolaryngology Clinic in Bialystok (1199 men and 114 women, aged 32-86 years).

RESULTS

In the study group the proportion of men was 91.3% and women 8.7%. Histopathological verification confirmed squamous cell carcinomas in 99%. The primary tumour was most often located in the supraglottic area of the larynx (48.0%) and its clinical stage was T3 (33.5%). T1 tumours were statistically significantly more frequently found in the glottis and T4 tumours - in the hypopharynx and in the transglottic area. Swollen lymph nodes in the neck were found in 52.4% of the patients, most often N2 (37.4%). The highest percentage of patients with lymph node reaction was found in the youngest group (76.6%) and the lowest - in the eldest group (30.0%). Swollen lymph nodes were statistically relevantly more frequent in patients with supraglottic or hypopharynx tumour.

Molecular MR imaging of neovascular progression in the Vx2 tumor with αvβ3-targeted paramagnetic nanoparticles

PURPOSE

To assess the dependence of neovascular molecular magnetic resonance (MR) imaging on relaxivity (r1) of αvβ3-targeted paramagnetic perfluorocarbon (PFC) nanoparticles and to delineate the temporal-spatial consistency of angiogenesis assessments for individual animals.

MATERIALS AND METHODS

Animal protocols were approved by the Washington University Animal Studies Committee. Proton longitudinal and transverse relaxation rates of αvβ3-targeted and nontargeted PFC nanoparticles incorporating gadolinium diethylenetrianime pentaacedic acid (Gd-DTPA) bisoleate (BOA) or gadolinium tetraazacyclododecane tetraacetic acid (Gd-DOTA) phosphatidylethanolamine (PE) into the surfactant were measured at 3.0 T. These paramagnetic nanoparticles were compared in 30 New Zealand White rabbits (four to six rabbits per group) 14 days after implantation of a Vx2 tumor. Subsequently, serial MR (3.0 T) neovascular maps were developed 8, 14, and 16 days after tumor implantation by using αvβ3-targeted Gd-DOTA-PE nanoparticles (n = 4) or nontargeted Gd-DOTA-PE nanoparticles (n = 4). Data were analyzed with analysis of variance and nonparametric statistics.

RESULTS

At 3.0 T, Gd-DTPA-BOA nanoparticles had an ionic r1 of 10.3 L · mmol(-1) · sec(-1) and a particulate r1 of 927000 L · mmol(-1) · sec(-1). Gd-DOTA-PE nanoparticles had an ionic r1 of 13.3 L · mmol(-1) · sec(-1) and a particulate r1 of 1 197000 L · mmol(-1) · sec(-1). Neovascular contrast enhancement in Vx2 tumors (at 14 days) was 5.4% ± 1.06 of the surface volume with αvβ3-targeted Gd-DOTA-PE nanoparticles and 3.0% ± 0.3 with αvβ3-targeted Gd-DTPA-BOA nanoparticles (P = .03). MR neovascular contrast maps of tumors 8, 14, and 16 days after implantation revealed temporally consistent and progressive surface enhancement (1.0% ± 0.3, 4.5% ± 0.9, and 9.3% ± 1.4, respectively; P = .0008), with similar time-dependent changes observed among individual animals.

CONCLUSION

Temporal-spatial patterns of angiogenesis for individual animals were followed to monitor longitudinal tumor progression. Neovasculature enhancement was dependent on the relaxivity of the targeted agent.

The modulation of apoptosis by oncogenic viruses

Transforming viruses can change a normal cell into a cancer cell during their normal life cycle. Persistent infections with these viruses have been recognized to cause some types of cancer. These viruses have been implicated in the modulation of various biological processes, such as proliferation, differentiation and apoptosis. The study of infections caused by oncogenic viruses had helped in our understanding of several mechanisms that regulate cell growth, as well as the molecular alterations leading to cancer. Therefore, transforming viruses provide models of study that have enabled the advances in cancer research. Viruses with transforming abilities, include different members of the Human Papillomavirus (HPV) family, Hepatitis C virus (HCV), Human T-cell Leukemia virus (HTLV-1), Epstein Barr virus (EBV) and Kaposi’s Sarcoma Herpesvirus (KSHV).

Apoptosis, or programmed cell death, is a tightly regulated process that plays an important role in development and homeostasis. Additionally, it functions as an antiviral defense mechanism. The deregulation of apoptosis has been implicated in the etiology of diverse diseases, including cancer. Oncogenic viruses employ different mechanisms to inhibit the apoptotic process, allowing the propagation of infected and damaged cells. During this process, some viral proteins are able to evade the immune system, while others can directly interact with the caspases involved in apoptotic signaling. In some instances, viral proteins can also promote apoptosis, which may be necessary for an accurate regulation of the initial stages of infection.

Viral infections of rabbits

Viral diseases of rabbits have been used historically to study oncogenesis (e.g. rabbit fibroma virus, cottontail rabbit papillomavirus) and biologically to control feral rabbit populations (e.g. myxoma virus). However, clinicians seeing pet rabbits in North America infrequently encounter viral diseases although myxomatosis may be seen occasionally. The situation is different in Europe and Australia, where myxomatosis and rabbit hemorrhagic disease are endemic. Advances in epidemiology and virology have led to detection of other lapine viruses that are now recognized as agents of emerging infectious diseases. Rabbit caliciviruses, related to rabbit hemorrhagic disease, are generally avirulent, but lethal variants are being identified in Europe and North America. Enteric viruses including lapine rotavirus, rabbit enteric coronavirus and rabbit astrovirus are being acknowledged as contributors to the multifactorial enteritis complex of juvenile rabbits. Three avirulent leporid herpesviruses are found in domestic rabbits. A fourth highly pathogenic virus designated leporid herpesvirus 4 has been described in Canada and Alaska. This review considers viruses affecting rabbits by their clinical significance. Viruses of major and minor clinical significance are described, and viruses of laboratory significance are mentioned.

Taxonomy and phylogeny of papillomaviruses: an overview and recent developments

For more than 30 years, papillomaviruses are standing in the center of medical and molecular interest as they cause several important cancers in humans. Research of the sheer unlimited number of different papillomavirus genomes, their host specificity and slow mutation rate is an important a branch of these efforts and has led to fascinating insight into the phylogeny of a virus family that can be traced back for several 100 million years.

Genotyping human papillomaviruses: development and evaluation of a comprehensive DNA microarray

GOALS

To define the analytical and clinical performance of a human papillomavirus (HPV) custom-designed microarray targeting the HPV L1 gene for viral genotyping.

METHODS

Microarray probes were designed by cataloging the genome sequence of all 120 known HPV types to generate tiling probes using eArray® software against the unique L1 capsid gene segments targeted by MY09/11 and FAP59/64 primers. The microarray (1 slide×8 arrays×60K features) synthesized in situ by inkjet printing was tested using synthetic type-specific HPV DNA and existing HPV DNA from cervical cytology. The synthetic HPV L1 segments (genotypes 6, 11, 16, 18, 31, 33, 35, 45, 53, 58, 66, 73, 83) were manufactured from sequences stored in the NCBI taxonomy database. Using the hybridization patterns of the synthetic HPV DNA as the Support Vector Machine classifier, HPV DNA from patient samples were genotyped and compared to antecedent DNA sequencing/BLAST® results for concordance.

RESULTS

16 cytology-derived HPV DNA samples and 13 synthetic type-specific HPV DNA samples were tested singly, in duplicate, or in combination on 40 arrays. The synthetic HPV DNA hybridization patterns were found to be uniquely distinctive to serve well as a classifier of unknown HPV-containing specimens. For the 16HPV DNA+ samples classified, 15 were concordant with DNA sequencing results. In 6/16 (38%) samples, the microarray hybridization pattern revealed ≥2 concurrent HPV infections.

CONCLUSION

The novel "HPV Array" was sensitive and specific for detecting single and multiple infections. This proof-of-principle project demonstrated the accuracy and advantages of microarray technology for HPV genotyping.

The biology of papillomavirus latency

The presence of viral DNA in the absence of disease has suggested that papillomaviruses, like many other viruses, can exist as latent infections in the skin or other epithelial sites. In animal models, where detailed investigation has been carried out, papillomavirus DNA can be found at sites of previous infection following immune regression, with the site of latent infection being the epithelial basal layer. Such studies suggest that immune surveillance can restrict viral gene expression in the basal and parabasal layers without efficiently suppressing viral genome replication, most probably through the action of memory T-cells in the skin or dermis. Although gradual papillomavirus genome loss appears to occur over time at latent sites, immunosuppression can arrest this, and can lead to an elevation in viral genome copy number in experimental systems. In addition to immune-mediated latency, it appears that a similar situation can be achieved following infection at low virus titres and/or infection at epithelial sites where the virus life cycle is not properly supported. Such silent of asymptomatic infections do not necessarily involve the host immune system and may be controlled by different mechanisms. It appears that virus reactivation can be triggered by mechanical irritation, wounding or by UV irradiation which changes the local environment. Although the duration of papillomavirus latency in humans is not yet known, it is likely that some of the basic principles will resemble those elucidated in these model systems, and that persistence in the absence of disease may be the default outcome for at least some period of time following regression.

A recombinant cottontail rabbit papillomavirus genome for ectopic expression of genes in cells infected with virus in vivo

The objective of this study was to construct a cottontail rabbit papillomavirus (CRPV) genome that would co-express a gene of choice and the viral genome simultaneously. Using this construct, the effects of the ectopic expression of diverse viral or cellular genes on PV-infected cells can be examined to elucidate which genes are essential for tumor formation. CRPV-pLAIIdelXba1, which lacks the major portion of L2 (designated the XbaI fragment), has been previously shown to fully retain the ability to induce tumors, and this ability was confirmed in this study. Insertion of the XbaI fragment in an antisense orientation did not change the efficiency of tumor induction. An SV40 overexpression cassette that originated from pSG5 and contains a more diverse multiple cloning site (MCS) was cloned into CRPV-Xba1-mcs, a CRPV genome based on CRPV-pLAIIdelXba1 that contains an additional MCS inserted via XbaI digestion. Additionally, the L1 ATG initiation codon of this construct, designated CRPV-Xba1-oe-WT, was mutated to avoid unnecessary L1 protein expression, which produced the CRPV-Xba1-oe-L1mut construct. Injection of these constructs into two New Zealand White rabbits and monitoring of tumor growth for two to six months showed that CRPV-Xba1-oe-WT induced tumors at 1/10 and 1/10 of the injection sites in two animals, while the control injections in each rabbit induced tumors at 3/10 and 4/10 injection sites, respectively. However, CRPV-Xba1-oe-L1mut induced tumors at 3/10, 6/10, 7/12 and 11/12 sites in four injected animals, and the control injections induced tumor growth in these animals at 6/10, 10/10, 12/12 and 12/12 of the injected sites, respectively. Thus, CRPV-Xba1-oe-L1mut could potentially be used to conduct overexpression experiments in vivo that can be used to measure the negative or positive influences of ectopically expressed foreign or HPV genes on tumor growth.

Role of viruses in oral squamous cell carcinoma

The etiology of oral squamous cell carcinoma (OSCC) is complex and involves many factors. The most clearly defined risk factors are smoking and alcohol, which substantially increase the risk of oral SCC. However, despite this clear association, a substantial proportion of patients develop OSCC without exposure to them, emphasizing the role of other risk factors such as genetic susceptibility and oncogenic viruses. Some viruses are strongly associated with OSCC while the association of others is less frequent and may depend on co-factors for their carcinogenic effects. Therefore, the exact role of viruses must be evaluated with care in order to improve the diagnosis and treatment of OSCC.

Alternative splicing in human tumour viruses: a therapeutic target?

Persistent infection with cancer risk-related viruses leads to molecular, cellular and immune response changes in host organisms that in some cases direct cellular transformation. Alternative splicing is a conserved cellular process that increases the coding complexity of genomes at the pre-mRNA processing stage. Human and other animal tumour viruses use alternative splicing as a process to maximize their transcriptomes and proteomes. Medical therapeutics to clear persistent viral infections are still limited. However, specific lessons learned in some viruses [e.g. HIV and HCV (hepatitis C virus)] suggest that drug-directed inhibition of alternative splicing could be useful for this purpose. The present review describes the basic mechanisms of constitutive and alternative splicing in a cellular context and known splicing patterns and the mechanisms by which these might be achieved for the major human infective tumour viruses. The roles of splicing-related proteins expressed by these viruses in cellular and viral gene regulation are explored. Moreover, we discuss some currently available drugs targeting SR (serine/arginine-rich) proteins that are the main regulators of constitutive and alternative splicing, and their potential use in treatment for so-called persistent viral infections.

Anal warts and anal intradermal neoplasia

For the last five millennia we have been dealing with the annoyance of verrucas. Anogenital human papillomavirus (HPV) infection is the most common sexually transmitted disease in the United States and is increasing in incidence. As in other gastrointestinal conditions, HPV infection can lead to a stepwise transition from normal cells to dysplastic cells and then to invasive anal cancer. Knowledge of the natural history of HPV infection, risk factors, diagnostic tools, and therapeutic methods gives us the tools to adequately prevent, evaluate, treat, and counsel our patients. In this review, the authors detail the diagnosis, management, and treatment of anal condyloma and anal intraepithelial neoplasia with a focus on prevention, early detection, and treatment using current data and technology.

Creation of rabbit bone and soft tissue tumor using cultured VX2 cells

BACKGROUND

To create rabbit VX2 bone tumors, it is surgically less demanding to implant VX2 cell suspensions than minced tumor fragments. A VX2 cell line that can be expanded using standard cell culture techniques might provide an unlimited supply of cells needed to create these bone tumors. Therefore, the aim of the present study was to establish a VX2 cell line and verify its tumorigenicity in an athymic mouse and rabbit animal model.

MATERIALS AND METHODS

Minced VX2 tumor fragments were allowed to grow as a monolayer in 10 mL Dulbecco's modified Eagle medium/nutrient mixture F-12 (1:1) supplemented with 10% fetal calf serum and passaged multiple times. The tumorigenecity of the cultured VX2 cells were tested in athymic mice (intradermal tumor development) and in New Zealand white rabbits (bone and soft tissue tumor model).

RESULTS

The VX2 cells proliferated rapidly in tissue culture flasks containing Dulbecco's modified Eagle medium/nutrient mixture F-12 medium supplemented with 10% fetal bovine serum. After reaching confluence, the VX2 cells can only be subcultured when plated at a greater density (e.g., at a dilution of 1:1). All 6 athymic mice developed tumors within 15 d of VX2 cell suspension implantation. In the rabbits, the VX2 cells were able to produce tumors in muscle tissue and in the distal femurs but not in the proximal tibia.

CONCLUSIONS

VX2 cell lines can be successfully created from VX2 tumor fragments and passaged multiple times. In contrast to previous reports, the VX2 cells grown in vitro are capable of maintaining their tumorigenecity. However, successful tumor growth might depend on the initial number of cells implanted and the use of extracellular matrices for tumor proliferation.

100 years of Rous sarcoma virus

The discovery of Rous sarcoma virus, which was reported by Peyton Rous in the Journal of Experimental Medicine 100 years ago, opened the field of tumor virology. It showed that some cancers have infectious etiology, led to the discovery of oncogenes, and laid the foundation for the molecular mechanisms of carcinogenesis. Rous spent his entire research career at The Rockefeller Institute, and he was the JEM's longest serving editor. Here, we comment briefly on the life of this remarkable scientist and on the importance of his discoveries.

Considerations for two inoculation methods of rabbit hepatic tumors: Pathology and image features

The rabbit VX2 hepatic tumor model has been widely used for the investigation of liver cancer. The present study was designed to compare two inoculation methods in order to identify an improved modality for establishing rabbit hepatic tumors. A total of 44 rabbits were randomly divided into 2 groups. All inoculations were performed by open laparotomy. In group 1, 22 rabbits received a VX2 tumor cell suspension in a volume of 0.2 ml, which was slowly injected into the rabbit liver parenchyma using an 18-gauge needle. In group 2, a 0.2-ml suspension of small, minced tumor fragments of a VX2 carcinoma, inserted into an 18-gauge needle, was implanted into the liver parenchyma. The tumor survival ratio, size and extrahepatic metastases were evaluated and compared using computed tomography (CT)-scan, angiography and pathological specimens. At 2 weeks following implantation, successful liver tumor growth was achieved in 14 out of the 22 rabbits in group 1 (63.6%) and 21 out of the 22 rabbits in group 2 (95.5%) (P<0.05). Further sub-analysis revealed that single nodular tumors in the liver were observed in 4 out of the 14 rabbits in group 1 (28.6%) and 14 out of the 21 rabbits in group 2 (66.7%), and that multinodular tumors in the liver were observed in 10 out of the 14 rabbits in group 1 (71.4%) and 7 out of the 21 rabbits in group 2 (33.3%) (P<0.05). Extrahepatic metastases were observed in 5 out of the 14 rabbits in group 1 (35.7%) and 1 of the 21 rabbits in group 2 (4.8%) (P<0.05). Imaging findings of hepatic tumors were in concordance with histopathology. In conclusion, direct implantation of VX2 tumor fragments into the liver using a fine needle achieved a higher success rate than injecting VX2 tumor cells. The rabbit hepatic tumors inoculated by this method may therefore be a more suitable animal model for investigating liver cancer.

The early history of tumor virology: Rous, RIF, and RAV

One hundred years ago Peyton Rous recovered a virus, now known as the Rous sarcoma virus (RSV), from a chicken sarcoma, which reproduced all aspects of the tumor on injection into closely related chickens. There followed recovery of causal viruses of tumors of different morphology from 4 more of 60 chicken tumors. Subsequent studies in chickens of the biology of the first RSV isolated moved slowly for 45 y until an assay of ectodermal pocks of the chorioallantoic membrane of chicken embryos was introduced. The inadequacies of that assay were resolved with the production of transformed foci in cultures of chicken fibroblasts. There followed a productive period on the dynamics of RSV infection. An avian leukosis virus (ALV) was found in some chicken embryos and named resistance-inducing factor (RIF) because it interferes with RSV. Its epidemiology in chickens is described. Another ALV was found in stocks of RSV and called Rous-associated virus (RAV). Cells preinfected with RAV interfere with RSV infection, but RSV does not produce infectious virus unless RAV is added during or after RSV infection. Intracellular RAV provides the infectious coat for the otherwise defective RSV. The coat determines the antigenicity, host range, and maturation rate of RSV. RSV particles carry reverse transcriptase, an enzyme that converts their RNA into DNA and allows integration into the cell's DNA, where it functions as a cellular gene. This was the bridge that joined the biological era to the molecular era. Its relation to oncogenes and human cancer is discussed.

History of human papillomavirus, warts and cancer: what do we know today?

Human papillomavirus has been a cause of infection in humans for thousands of years. The history of papillomaviruses, knowledge of their causative role in benign and malignant disease, and their structural characteristics have led to the development of vaccines to prevent cervical and anogenital cancers. Many questions remain unanswered before HPV vaccines can be optimised; however, the concept of virtual eradication of cervical cancer is not impossible, and remains a realistic aspiration.

The dynamics of glutathione species and ophthalmate concentrations in plasma from the VX2 rabbit model of secondary liver tumors

Purpose. Available tumor markers have low sensitivity/specificity for the diagnosis of liver tumors. The present study was designed to evaluate the oxidoreductive status of the liver as surrogates of tumor subsistence and growth. Methods. Glutathione species (GSH:GSSG), ophthalmate (OA) concentrations, and their turnover were measured in plasma of rabbits (n = 6) in their healthy state and in the state of tumor growth after implantation of the VX2 carcinoma in their liver. Tumors were allowed to grow for a period of 14 days when rabbits were sacrificed. Livers were removed and cysteine concentration was measured in liver tissue. Results. Tumor growth was found in 100% of the rabbits. Concentration and labeling of GSH/GSSG were similar in experimental animals before and after tumor implantation and to sham animals. In contrast, OA concentration increased significantly in experimental animals after tumor implantation when compared to same animals prior to tumor implantation and to sham animals (P < .05). The concentration of cysteine, a precursor of GSH, was found to be significantly lower in the liver tissue adjacent to the tumor (P < .05). Conclusion. Disturbances in the oxidoreductive state of livers appear to be a surrogate of early tumor growth.

DMBA-induced hamster buccal pouch carcinoma and VX2-induced rabbit cancer as a model for human oral carcinogenesis

In this article, we have described and compared the advantages and disadvantages of two potential animal cancer models (the hamster buccal pouch cancer model and the VX2-induced rabbit cancer model) for human squamous cell carcinomas of the oral mucosa. Currently, no animal cancer model is perfectly applicable to human oral squamous cell carcinomas. This is because the hamster buccal pouch cancer model has a different etiology and genetic constitution compared with human oral carcinomas. In addition, the VX2-induced rabbit cancer model is not produced in situ and, consequently, its natural behavior is totally reliant on the location of transplantation. Nonetheless, with the use of these two animal cancer models together, researchers could evaluate different aspects of the cellular and molecular biological characteristics or assess potential novel treatment regimens for squamous cell carcinomas of the human oral mucosa.

Why do viruses cause cancer? Highlights of the first century of human tumour virology

The year 2011 marks the centenary of Francis Peyton Rous's landmark experiments on an avian cancer virus. Since then, seven human viruses have been found to cause 10-15% of human cancers worldwide. Viruses have been central to modern cancer research and provide profound insights into both infectious and non-infectious cancer causes. This diverse group of viruses reveals unexpected connections between innate immunity, immune sensors and tumour suppressor signalling that control both viral infection and cancer. This Timeline article describes common features of human tumour viruses and discusses how new technologies can be used to identify infectious causes of cancer.

Biodistribution and acute toxicity of naked gold nanoparticles in a rabbit hepatic tumor model

There is a paucity of data regarding the safety of administering solid gold nanoparticles (AuNPs) in large animal tumor models. We assessed the acute toxicity and biodistribution of 5 nm and 25 nm solid AuNPs in New Zealand White rabbits (n = 6 in each) with implanted liver Vx2 tumors 24 h after intravenous injection. Gold concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP) and imaged with transmission electron microscopy (TEM). There was no clinico-pathologic evidence of renal, hepatic, pulmonary, or other organ dysfunction. After 25 nm AuNP administration, the concentration of white blood cells increased after treatment (p = 0.001). Most other blood studies were unchanged. AuNPs were distributed to the spleen, liver, and Vx2 tumors, but not to other tissues. The urinary excretion of AuNPs was bimodal as measured by ICP. 25 nm AuNPs were more evenly distributed throughout tissues and may be better tools for medical therapy.

A niche for infectious disease in environmental health: rethinking the toxicological paradigm

OBJECTIVE

In this review we highlight the need to expand the scope of environmental health research, which now focuses largely on the study of toxicants, to incorporate infectious agents. We provide evidence that environmental health research would be strengthened through finding common ground with the tools and approaches of infectious disease research.

DATA SOURCES AND EXTRACTION

We conducted a literature review for examples of interactions between toxic agents and infectious diseases, as well as the role of these interactions as risk factors in classic "environmental" diseases. We investigated existing funding sources and research mandates in the United States from the National Science Foundation and the National Institutes of Health, particularly the National Institute of Environmental Health Sciences.

DATA SYNTHESIS

We adapted the toxicological paradigm to guide reintegration of infectious disease into environmental health research and to identify common ground between these two fields as well as opportunities for improving public health through interdisciplinary research.

CONCLUSIONS

Environmental health encompasses complex disease processes, many of which involve interactions among multiple risk factors, including toxicant exposures, pathogens, and susceptibility. Funding and program mandates for environmental health studies should be expanded to include pathogens in order to capture the true scope of these overlapping risks, thus creating more effective research investments with greater relevance to the complexity of real-world exposures and multifactorial health outcomes. We propose a new model that integrates the toxicology and infectious disease paradigms to facilitate improved collaboration and communication by providing a framework for interdisciplinary research. Pathogens should be part of environmental health research planning and funding allocation, as well as applications such as surveillance and policy development.

The Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor: Lessons on dysregulated angiogenesis from a viral oncogene

Tumor viruses can induce cell transformation by overcoming cellular defense mechanisms and promoting the ungoverned proliferation of infected cells. To this end, functionally related viral oncogenes have evolved in disparate viruses to over-ride key proliferative and survival intracellular pathways, thus assuring efficient viral replication and contributing to tumor formation. Indeed, the study of viral oncogenes has been a powerful tool for disclosing fundamental insights into these basic cellular processes. In this regard, the Kaposi's sarcoma-associated herpesvirus (KSHV or HHV8), the etiological agent of Kaposi's sarcoma (KS), is an exemplary model of an oncogenic virus that includes within its genome several homologues of cellular genes implicated in the regulation of cell proliferation and apoptosis. However, emerging evidence now points to a single KSHV gene, ORF74, encoding for the viral G protein-coupled receptor (vGPCR), as essential for KS development. Expressed in only a fraction of cells within KS lesions, this viral receptor induces tumorigenesis through both autocrine and paracrine mechanisms. Indeed, work from several laboratories has demonstrated that vGPCR can promote cell proliferation, enhance cell survival, modulate cell migration, stimulate angiogenesis, and recruit inflammatory cells, both in expressing cells, as well as in neighboring (bystander) cells. Examination of this powerful viral oncogene may expose novel targets for the treatment of patients with KS and could ultimately provide a unique perspective into how GPCRs, and specifically chemokine receptors, contribute to angiogenesis and tumorigenesis.

THE RECOVERABILITY OF VIRUS FROM PAPILLOMAS PRODUCED THEREWITH IN DOMESTIC RABBITS

By preliminary preparation of the skin in ways that render it hyperplastic the presence of infective virus can be demonstrated in extracts of domestic rabbit papillomas which yield no growths when inoculated by the ordinary methods and which for this reason have been supposed to contain no virus. The amount of virus recovered by the method outlined in the present work, however, is small when compared with the yield obtained in most instances from comparable cottontail rabbit papillomas. The yield is greatly influenced not only by the virus strain used to produce the growths but by the individual rabbit host. Although virus has been obtained from papillomas produced in domestic rabbits by all of the virus strains tested, a total of 21 thus far, only about one-fourth of these strains are readily to be procured again from the growths they cause and the others are demonstrable only in hosts in which the conditions are favorable for reasons unknown. An experimental comparison of the capacity of suspensions of papilloma tissue from domestic and cottontail rabbits to elicit specific antibodies has shown that the titers attained are approximately proportional to the amount of infective virus demonstrable in the suspensions. The findings as a whole indicate that far less virus exists in infective or antigenic form in the papillomas of domestic rabbits than in those of cottontail rabbits.

A VIRUS-INDUCED MAMMALIAN GROWTH WITH THE CHARACTERS OF A TUMOR (THE SHOPE RABBIT PAPILLOMA) : II. EXPERIMENTAL ALTERATIONS OF THE GROWTH ON THE SKIN: MORPHOLOGICAL CONSIDERATIONS: THE PHENOMENA OF RETROGRESSION

The injection of Scharlach R into the skin about rabbit papillomas resulting from virus inoculation causes them to invade the underlying tissue and form large, fleshy masses beneath the surface. Histologically these appear malignant, and they frequently invade the blood vessels. Covering young papillomas with a layer of collodion causes them to burrow downwards with result in discoid masses which enlarge progressively, both by expansive growth beneath the epidermis and by invasion. Such masses, like the nodules resulting from implantation, have the papillae turned toward their interior, the apparent reverse of the condition of affairs when the growth is situated on the skin surface. The reasons for this are analyzed. The peculiarities of the host influence skin papillomas not a little, as is plain from the forms they assume; but the epithelial changes induced by the virus take a single direction, and no significant variations from type have been encountered. Local or generalized retrogression of the experimentally induced papilloma is not uncommon. The histological alterations that take place are identical with those attending retrogression of the epidermoid tumors, and the reactive changes taking place in the surrounding tissue are also like those about such tumors. The slowing and cessation of growth that occur secondarily in the case of virus-induced skin papillomas are associated with the formation under them of a dense layer of connective tissue, and to this their behavior is attributable. Similar findings have been often recorded for tumors, notably for the epidermoid cancers produced in rabbits by tarring.

THE DETECTION OF A "MASKED" VIRUS (THE SHOPE PAPILLOMA VIRUS) BY MEANS OF IMMUNIZATION : RESULTS OF IMMUNIZATION WITH MIXTURES CONTAINING VIRUS AND ANTIBODY

A study has been made of the immunization procedure described by Shope, with particular reference to the detection of "masked" papilloma virus by means of it. Papilloma extracts were frequently encountered which, though non-pathogenic, elicited the specific antiviral antibody and induced resistance to the virus upon injection intraperitoneally into normal rabbits. The results of the immunization experiments were often complicated, however, by the effects of extravasated antibody, which had accumulated in various amounts in many of the papillomas and was consequently present in extracts of them together with "masked" virus. The extravasated antibody was often sufficient to render extracts of domestic rabbit papillomas non-antigenic; and sometimes, when present in excess, its passive transfer conferred resistance to reinfection with the virus. The conclusion seems warranted that only positive immunization findings can be interpreted with certainty. Negative results provide no decisive evidence as to whether "masked" virus is or is not present in the injected material, unless the amount of extravasated antibody also present is known. The findings may have a bearing on the negative outcome of immunization experiments with extracts of the cancers deriving from the natural papillomas of cottontails. Crude suspensions of domestic rabbit papillomas, which contain little or no virus demonstrable by ordinary methods, are far less antigenic than extracts of the natural growths of wild rabbits, which contain virus in quantity. In explanation of the finding the possibility seems worthy of attention that domestic rabbit papillomas may contain much less virus than the growths of cotton tails, the natural hosts of the virus.

INFLUENCE OF EXTRANEOUS PROTEIN AND VIRUS CONCENTRATION ON THE INACTIVATION OF THE RABBIT PAPILLOMA VIRUS BY X-RAYS

The pronounced resistance to the x-rays manifested by the papilloma virus in ordinary suspensions is due to the protecting influence of extraneous matter and also in considerable degree to the amount of virus present in the preparation. Two to 4 million r were required to inactivate the virus contained in the crude papilloma extracts prepared for the present work, whereas 100,000 r or less was enough to inactivate comparable concentrations of virus after extraneous matter had been excluded by repeated differential centrifugation. The addition of normal rabbit serum or crystalline egg albumin to purified suspensions of virus was found to increase greatly the amount of irradiation required to inactivate the virus. Furthermore the percentage destruction of virus by a given amount of irradiation increases as the concentration is decreased by dilution with saline or buffer solutions. As little as 3,000 r will inactivate much of the virus in very dilute suspensions. The complement-binding antigen of papilloma virus suspensions is also inactivated by x-rays, but requires a somewhat larger amount of irradiation than necessary to destroy the infectivity of the suspensions. The effects of irradiation on the antiviral antibody present in the blood of animals which have become immune to the virus—an antibody that specifically fixes complement in mixture with the papilloma virus—are also conditioned by extraneous material. 250,000 to 500,000 r had only a slight effect on the antibody in whole serum, while this amount of irradiation completely inactivated comparable amounts of antibody in preparations partially purified by precipitation with ammonium sulfate. As a whole the findings indicate that under certain conditions of purity and concentration most of the radiation does not act by direct hits on virus or antibody particles, but indirectly by ionizing or exciting some other molecules present in the exposed suspension, which then react with the virus or antibody molecules.

A COMPARISON OF VIRUS-INDUCED RABBIT TUMORS WITH THE TUMORS OF UNKNOWN CAUSE ELICITED BY TARRING

Tarring the ears of rabbits of one sort with a single kind of tar evoked epidermal tumors of a few sharply defined types, namely ordinary papillomas, carcinoids, carcinomas, and "frill horns." These last, relatively infrequent, are now recognized for the first time. The carcinoids have proved to be the expression of a spurious malignancy of papillomas, resulting from intercurrent influences, and they were wholly dependent upon these for their threatening aspect and behavior. Chief amongst such influences was continued tarring. It had the effect of establishing the papillomas, stimulated their proliferation, complicated their morphology, and rendered some of them disorderly, aggressive, and anaplastic. It brought all of the tissues of the ears into an excitable state, and often this state endured long after the skin had apparently returned to normal. The characters of the papilloma-carcinoids and of the frill horns were so different and distinctive as to imply the action of differing, specific causes. The papillomas were very like those induced with the Shope virus, and hence a point-to-point comparison was made of their manifestations, including the derivation of carcinomas from them. This comparison demonstrated that the unknown cause of the tar papillomas provoked neoplastic phenomena which were identical in all essential respects with those due to the virus. To suppose, for experimental purposes, that the papillomas which tarring elicits are caused by a virus rendered pathogenic by this procedure, is to demand least of the unknown. Yet it does not follow that they must be due to a virus.

THE CARCINOGENIC EFFECT OF A PAPILLOMA VIRUS ON THE TARRED SKIN OF RABBITS : I. DESCRIPTION OF THE PHENOMENON

The Shope papilloma virus elicits carcinomas forthwith, as well as papillomas in great variety, when it is distributed by way of the blood stream to the tarred epidermis of domestic rabbits. The phenomenon will be analyzed in succeeding papers with the aid of additional instances.

THE CARCINOGENIC EFFECT OF A PAPILLOMA VIRUS ON THE TARRED SKIN OF RABBITS : II. MAJOR FACTORS DETERMINING THE PHENOMENON: THE MANIFOLD EFFECTS OF TARRING

A considerable variety of tumors, both benign and malignant, result from the localization of the rabbit papilloma virus in skin which has been prepared by repeated tarrings. They appear only in individuals highly susceptible to the action of the virus, and are more likely to be engendered by highly pathogenic inocula. No evidence has been found that differences in the potentialities of the virus entities are responsible for the diversity of the growths. This is referable to changes in the epidermal cells; and much more preliminary tarring is required to produce these changes than suffices to cause localization of the virus out of the blood stream with a resulting papillomatosis of the ordinary sort. The character of the individual anomalous tumors depends in some degree upon the extent of the preparatory changes in the cells, malignant growths being more frequent when the epidermis has been tarred for a relatively long period. All are focal or punctate in origin, and they exhibit their peculiar characters from the first, none being due to secondary alterations in ordinary papillomas. Tarring after the virus has localized in the epidermis does not significantly increase their number. They are the outcome of the state of the cells at the time of virus infection. Tarring exerts important influences in addition to changing the cells in such a way that unusual tumors result from the action of the virus. The procedure is notably effective in determining localization of the virus out of the blood stream; enables it to produce growths when otherwise it would not do so though present in the tarred skin; stimulates the proliferation of the tumors engendered; makes them disorderly and aggressive; and hastens the anaplasia of such of them as are malignant. It has similar effects upon the tar tumors, as will be demonstrated in a subsequent paper.

IMMUNOLOGICAL REACTIONS WITH A VIRUS CAUSING PAPILLOMAS IN RABBITS : III. ANTIGENICITY AND PATHOGENICITY OF EXTRACTS OF THE GROWTHS OF WILD AND DOMESTIC SPECIES: GENERAL DISCUSSION

A study has been made of the yield of virus and of the complement-binding antigen from the virus-induced papillomas of cottontail and domestic rabbits. Extracts of the discrete, naturally occurring papillomas of cottontail rabbits usually contain virus in large amount; and, as a rule, they also contain the complement-binding antigen in high titer. The confluent growths produced experimentally with the virus in some cottontails, on the other hand, often fail to yield the virus, or furnish it in small amount; and extracts of them have little if any complement-binding capacity. The sera of cottontails with massive papillomas from which the virus cannot be recovered often have high antibody titers. Many extracts were tested of the virus-induced papillomas of domestic rabbits. None contained the virus in large amount, and the majority of them failed to manifest it on sensitive test. A few fixed complement in low titer when mixed with immune sera, but most failed to do so. Crude extracts of the "non-infectious," virus-induced papillomas of domestic rabbits stimulated the formation of virus-neutralizing and complement-binding antibodies in low titer when injected intraperitoneally into normal rabbits of the same breed, but Berkefeld filtrates of the same materials proved devoid of this immunizing effect. The significance of the findings described in the three papers is discussed. The evidence as a whole favors the view that the virus stimulates the formation of the virus-neutralizing and complement-binding antibodies in vivo, and many facts indicate that it is closely associated, and in all probability identical, with the antigen that reacts with immune serum to fix complement in vitro.

THE PROGRESSION TO CARCINOMA OF VIRUS-INDUCED RABBIT PAPILLOMAS (SHOPE)

The papillomas induced in domestic rabbits with virus procured from cottontails undergo progressive changes in the direction of malignancy when they grow vigorously. From the beginning they exhibit the traits whereby tumors are characterized, and they have malignant potentialities. In seven animals of a group of ten carrying papillomas for more than 200 days, cancer has developed, and in an eighth a tumor of problematic malignancy has arisen. One of the remaining two rabbits died early in the cancer period, and the papillomas of the other eventually retrogressed. Ten cottontails with induced growths of much longer duration have not developed cancer. The malignant tumors have all been acanthomatous in type, and have arisen directly from the papillomas by graded, continuous alterations. These have often gone further after malignancy has been attained, and have eventuated in great anaplasia. Metastasis has been frequent, and transplantation to another host has proved successful. Individual growths have occurred expressive of each stage of the transformation to cancer, as if through a stabilization at this stage; yet despite the variety thus afforded, the tumors must all be looked upon as the consequence of alterations in cells of a single sort, namely epidermal cells affected by the virus, and the alterations themselves have taken a single direction. In the morphology of many of the cancers the influence of the virus is still manifest. The better the papilloma grew, the more likely was cancer to occur, and the greater was the tendency to multiple tumors. In the most favorable rabbits malignant changes took place at numerous locations in the papillomatous tissue, and were imminent at many others. Intercurrent factors had much to do with determining frank carcinosis; and when the tendency to it was not marked their influence sometimes seemed crucial. Analogous instances of a graded alteration from papilloma to cancer are frequent in human pathology. The virus that gives rise to the rabbit papillomas must be looked upon as the primary cause of the cancers developing therefrom. Whether it is their proximate cause has yet to be determined.

A TRANSPLANTABLE RABBIT CARCINOMA ORIGINATING IN A VIRUS-INDUCED PAPILLOMA AND CONTAINING THE VIRUS IN MASKED OR ALTERED FORM

A squamous cell carcinoma derived from a virus-induced rabbit papilloma has been propagated in fourteen successive groups of animals. It grows rapidly now in most individuals to which it is transplanted, killing early and metastasizing frequently. The original cancer was the outcome of alterations in epidermal cells already rendered neoplastic by the virus, and the latter, or an agent nearly related to it, has persisted and increased in the malignant tissue, as a study of the blood of the first ten groups of cancerous animals has shown. An antibody capable of specifically neutralizing the virus in vitro appeared in the blood of every new host in which the tumor enlarged progressively, and reached a titer comparable with that obtaining in animals which had long carried large papillomas. The antibody was absent from normal rabbits and those in which the cancer failed to grow. The implications of these facts are considered.

The filtration and centrifugation of the viruses of rabbit fibroma and rabbit papilloma

The diameter of the virus of rabbit fibroma has been estimated by filtration through graded membranes to be about the same as that of vaccinia (125–175 mµ) and by centrifugation to be 126–141 mµ.

That of the rabbit papilloma virus appeared by filtration to be 23–35 mµ and by centrifugation 32–50 mµ.

UNION IN VITRO OF THE PAPILLOMA VIRUS AND ITS ANTIBODY

Experiments are described which show that the rabbit papilloma virus elicits an antibody of one type only, this being capable both of neutralizing the virus and of fixing complement in mixture with it. The virus and its antibody have a powerful, specific affinity for one another, each being capable of absorbing the other in great excess when they are brought together in the test tube. The union formed by them in vitro cannot be dissociated in any demonstrable degree by dilution or centrifugation. In many respects the findings differ from those with most viruses previously studied.

THE MASKING EFFECT OF EXTRAVASATED ANTIBODY ON THE RABBIT PAPILLOMA VIRUS (SHOPE)

The foregoing experiments have shown that the causative virus is usually "masked" in the large, disorderly, fissured and inflamed papillomas of cottontails when antiviral antibody is present in quantity in their blood, though virus can be recovered as a rule from the smaller, discrete, well ordered papillomas of these rabbits, almost irrespective of the amount of antibody in the blood of the individuals bearing them. Other findings are described which indicate that the masking of the virus in the large fissured growths is due to serum antibody present in them as result of exudation or hemorrhage, which neutralizes the virus when the growths are extracted or preserved in vitro. The local conditions that favor extravasation of serum (and the accumulation of antibody) prevail as a rule in the large, confluent growths arising after virus has been sown broadcast on scarified skin, but to lesser extent or not at all in the discrete papillomas that occur naturally or as result of tattoo inoculation. The state of affairs is notably different in the papillomas of domestic rabbits. The virus is regularly masked in these, and usually masked completely, even when there is little antibody in the blood and the local conditions do not favor its extravasation into the growths. The findings indicate that something other than antibody is primarily responsible for the masking in this species.

THE FIXATION AND PROTECTION OF VIRUSES BY THE CELLS OF SUSCEPTIBLE ANIMALS

Methods were developed for a study of the relations existing between viruses and living cells. It was found that vaccinia and the virus causing the infectious fibroma of rabbits (Shope) rapidly become fixed upon tissue cells freed as individuals and submitted to virus in suspension. This happens whether the cells are alive or have been killed with heat or ultraviolet light. The virus does not come away during agitation of the cells with Tyrode solution and repeated washings with large amounts of it. The exposure to neutralizing antisera of cells carrying virus fails to affect this latter significantly if the cells are alive, whereas if they are dead the activity of the virus is nullified. Cells freed as individuals from tissue cultures of vaccinia and the Shope tumor carry these viruses in abundance through repeated washings, and, if living, protect them from the influence of a neutralizing serum, whereas killed cells exert no such protection. The findings would appear to throw light on the way in which viruses gain a foothold in the host; and they suggest reasons for the persistence of some viruses in recovered animals and for the unsatisfactory results of serum treatment instituted during the course of virus diseases. The virus causing the Shope fibroma has been successfully maintained in cultures of the growth. It is closely associated with the cells, almost none being present in the culture fluid. Certain of its other attributes have been determined. Vaccinia greatly damages the cells of cultures of rabbit embryo in which it is under propagation.

THE ACTIVATING, TRANSFORMING, AND CARCINOGENIC EFFECTS OF THE RABBIT PAPILLOMA VIRUS (SHOPE) UPON IMPLANTED TAR TUMORS

The effects of the rabbit papilloma virus upon tar tumor tissue are widely various, as the present paper and previous ones attest. It enables some of the benign tar tumors of domestic rabbits (papillomas, carcinomatoids) to establish themselves after implantation,—which they are unable to do under ordinary circumstances, being dependent upon favoring factors; and it may drive them to active proliferation without altering their morphology. Some growths it fails to influence and some it converts into virus papillomas. Often, however, it brings about cytological changes which are indicative of a combination of its influence with that of the undetermined factor motivating the original tumor. The resulting neoplasm exhibits a blend of characteristics. The virus makes some benign tar tumors become cancerous forthwith, the malignancy developing without intermediate virus papillomatosis. It can be readily imposed upon some of the squamous cell carcinomas which tar elicits in its natural host, the cottontail rabbit, and it may drive such tumors to proliferate faster, or alter them morphologically, or do both. Its stimulating effect is especially pronounced in the case of those tar cancers that are slow-growing. Since the virus can influence tar cancers markedly, one can scarcely suppose it to be devoid of effect upon the cancers of the same type which derive from the papillomas it has itself engendered. Other implications of the work are discussed.

A VIRUS-INDUCED MAMMALIAN GROWTH WITH THE CHARACTERS OF A TUMOR (THE SHOPE RABBIT PAPILLOMA) : III. FURTHER CHARACTERS OF THE GROWTH: GENERAL DISCUSSION

Experimental study of the rabbit papilloma of Shope, a growth caused by a virus, has shown that it possesses the immediate characters whereby tumors are recognized. Often it looks and acts like a malignant neoplasm. It differs from the tumors as a group, however, in its incidence which is that of an infectious process, and from other mammalian tumors in that its cause has been demonstrated. The possible bearing of the findings upon the problem of tumor causation is discussed. The morphology and behavior of the generality of tumors can no longer be taken to exclude the possibility that these are produced by extraneous, living entities. The incidence of some of the tumors at least, and the failure to demonstrate their cause can both be explained on the assumption that they are due to such entities, widely distributed in or upon the animal population but effective only under special circumstances. Present knowledge makes this assumption reasonable as a basis for further work.