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

Cottontail Rabbit Papillomavirus (CRPV) Related Animal Models for Head and Neck Cancer Research: A Comprehensive Review of the Literature

Having suitable animal models is crucial to mimic human disease states and for the successful transfer of experimental data into clinical practice. In the field of papillomavirus research, the domestic rabbit (Oryctolagus cuniculus) has served as an indispensable model organism for almost 100 years. The identification and characterization of the first papillomaviruses in rabbits, their carcinogenic potential and their immunogenicity have contributed significantly to the state of knowledge on the genetics and life cycle of papillomaviruses in general, as well as the development of antiviral strategies such as vaccination procedures. Due to the high species specificity of papillomaviruses, only rabbit papillomaviruses (RPVs) can be used for animal studies on papilloma-based tumor diseases in the rabbit. The major focus of this article is on cottontail rabbit papillomavirus (CRPV)-related rabbit squamous cell carcinoma (RSCC). A brief history outlines the discovery and generation of experimentally used RSCC tumors. A comprehensive overview of the current CRPV-associated VX2 carcinoma-based tumor models with a major focus on human head and neck squamous cell carcinoma (HNSCC) tumor models is provided, and their strengths in terms of transferability to human HNSCC are discussed.

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.

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 DISTINCTIVE SUBSTANCE ASSOCIATED WITH THE BROWN-PEARCE RABBIT CARCINOMA : I. PRESENCE AND SPECIFICITY OF THE SUBSTANCE AS DETERMINED BY SERUM REACTIONS

A substance is regularly present in saline extracts of the Brown-Pearce rabbit carcinoma growth which is capable of fixing complement specifically in mixture with the sera of certain rabbits bearing the tumor or in which this has recently retrogressed, as the foregoing experiments have shown. The substance was not demonstrable in extracts of the normal tissues, virus papillomas, or uterine cancers of rabbits, nor in extracts of rabbit tissues infected with certain viruses (vaccine virus, Virus III, fibroma virus). The sera of normal rabbits, of those immune to a variety of infectious diseases, including syphilis, vaccinia, fibromatosis, and Virus III, and of others with uterine cancers or virus-induced papillomas, failed to fix complement specifically in mixture with extracts containing the antigen of the Brown-Pearce tumor. The significance of the findings will be discussed in the succeeding paper, after consideration has been given to some of the properties of the serologically active material derived from the Brown-Pearce tumor.

STUDIES ON EPIDEMIC INFLUENZA VIRUS : THE NATURE AND PROPERTIES OF THE COMPLEMENT-FIXING ANTIGEN

Evidence is presented which indicates that influenza virus elaborates a soluble antigen. The antigen is considerably smaller than the virus and can be separated from it, but the virus has not been washed free of antigen. The properties of the antigen suggest that it is an unstable protein of relatively large size.

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.

A DISTINCTIVE SUBSTANCE ASSOCIATED WITH THE BROWN-PEARCE RABBIT CARCINOMA : II. PROPERTIES OF THE SUBSTANCE: DISCUSSION

The serologically active substance of the Brown-Pearce tumor, a complement-fixing antigen, differs notably from certain other tissue antigens (the Wassermann, Forssman, and organ- and species-specific tissue haptens, for example) in the fact that it is not effective after alcoholic extraction of the tissue containing it. Like many of the proteins and viruses it is in-activated upon heating to 65 degrees C. for 30 minutes; and, like them as well, its activity is lost upon treatment with acid (to pH 4.5 or lower) or alkali (to pH 11.5 or higher). Filtration and centrifugation experiments disclosed the fact that the antigen of the Brown-Pearce tumor passed readily through collodion membranes with average pore diameters of 383 mmicro and more, but was retained completely by those with average pore diameters of 348 mmicro and less. It was thrown down completely or almost completely upon centrifugation at 20,000 R.P.M. for one hour. The findings indicate that the Brown-Pearce tumor antigen has a large and nearly uniform particle size and weight,-as large as that of many of the viruses. They differentiate it sharply from the generality of "soluble antigens.rdquo; Upon direct comparison, the complement-fixing antigen of the Brown-Pearce tumor was found to be similar in a number of its general traits to the serologically active substance of the virus-induced papillomas of rabbits, which in turn is intimately associated or identical with the virus responsible for the papillomas. Extracts of the Brown-Pearce tumor containing the serologically active substance in quantity have given rise to no lesions, however, upon injection into normal or tarred rabbits. The significance of the findings is discussed.

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.

CANCERS DERIVING FROM THE VIRUS PAPILLOMAS OF WILD RABBITS UNDER NATURAL CONDITIONS

The naturally occurring virus papillomas of western cottontail rabbits become malignant occasionally. The cancers derive from the papilloma cells, that is to say from elements already rendered neoplastic by the virus and still infected therewith. Papillomas produced with the virus in jack rabbits and snowshoe rabbits become cancerous in the same way but much more frequently, as is the case in domestic rabbits also. To all three species the virus is foreign. The character of the cancers of the wild rabbits is described and the relation of the virus to them discussed on the basis of experimental findings. The facts support the view that the cancers result from virus variation, this in many instances being but slight.

IDENTITY OF "INHIBITOR" AND ANTIBODY IN EXTRACTS OF VIRUS-INDUCED RABBIT PAPILLOMAS

The "inhibitor" demonstrable in extracts of the virus-induced rabbit papillomas is identical with the antiviral antibody found in the blood of hosts bearing the growths. The conditions in these latter are frequently favorable to its extravasation in considerable amount into them. Its significance and its influence upon the recovery of virus from the papillomas are discussed.

INDUCED ANTIBODIES THAT REACT IN VITRO WITH SEDIMENTABLE CONSTITUENTS OF NORMAL AND NEOPLASTIC TISSUE CELLS : PRESENCE OF THE ANTIBODIES IN THE BLOOD OF RABBITS CARRYING VARIOUS TRANSPLANTED CANCERS

Antibodies were found in the blood of certain rabbits carrying one or another of four transplanted cancers (Brown-Pearce and V2 carcinomas; RSI and Kato sarcomas) which will fix complement in vitro in mixture with saline extracts of various normal and neoplastic rabbit tissues—including liver, kidney, spleen, and the four tumors mentioned—and chick embryo tissue as well. These antibodies, which have been called induced tissue antibodies, are similar to the natural antibodies previously described (2) in that they react with those constituents of the various tissue cells that prove readily sedimentable in the high speed centrifuge; they differ from the natural antibodies in being absent from the blood of normal rabbits and in withstanding 65° C. for 30 minutes. Certain quantitative differences suggest that the induced tissue antibodies have somewhat various affinities, depending in part upon the type of neoplasm carried by the host. They may perhaps be consequent on antigenic differences between the sedimentable constituents of the tumor cells and those of the new hosts; for they were not found in the blood of rabbits carrying papillomas and cancers composed of the animals' own cells, and not in that of rabbits in which multiple vaccinia or fibroma virus lesions had recently regressed. The characters of the sedimentable constituents of normal and neoplastic tissue cells, as revealed thus far by chemical, morphological, and serological studies, have recently been discussed (2,8). In this relation, it has seemed essential to recognize the induced antibodies here described, particularly since they may complicate serological studies aimed at disclosing distinctive sedimentable substances in tissue cells. In an associated paper experiments are reported which bear upon the relation between the induced tissue antibodies and an antibody that reacts specifically with a distinctive sedimentable constituent of Brown-Pearce carcinoma cells (7).

A SOLUBLE ANTIGEN OF LYMPHOCYTIC CHORIOMENINGITIS : I. SEPARATION OF SOLUBLE ANTIGEN FROM VIRUS

The virus of lymphocytic choriomeningitis can be sedimented in the ultracentrifuge and washed repeatedly; the virus retains its activity provided that a small amount of normal serum is present in the diluent. A soluble substance capable of fixing complement in the presence of immune serum can be separated from the virus. Washed virus fixes complement poorly. The serologically specific soluble antigen is widely distributed in tissues of infected guinea pigs, mice, and monkeys.

ELECTROPHORESIS OF THE COMPLEMENT-FIXING ANTIGEN OF HUMAN INFLUENZA VIRUS

1. An electrophoretic study has been made of normal mouse serum, influenza mouse serum, and influenza mouse lung suspension. The mobilities of the protein fractions present have been determined at various pH's by the optical method. 2. The pH-mobility curve of the soluble (complement-fixing) antigen present in the lung suspension has been determined analytically by sampling and application of the complement fixation test. The results show that the complement-fixing antigen has a mobility definitely smaller than that of serum albumin and close to that of a globulin, with an isoelectric point close to pH 5.

Experiments on the cause of the rabbit carcinomas derived from virus-induced papillomas. I. Propagation of several of the cancers in sucklings, with etiological tests

Three out of four carcinomas arising from rims-induced, rabbit papillomas have grown well after transplantation to sucklings. Two were propagated serially, and it seems likely that all could have been maintained indefinitely had litters been available of newborn animals of the sort in which they arose. These successes are the more worthy of note because of the well-nigh uniform failure of similar growths on transfer to adults. The tumors enlarged with great rapidity in the sucklings, were extraordinarily destructive, and two of them metastaslzed within a few weeks. Many efforts were made to extract causative agents from the three carcinomas, on the assumption that these might be due to variants of the Shope virus. Highly favorable conditions for the demonstration of this latter were provided in the tests; yet their outcome was wholly negative although all of the cancers derived from papillomas caused by "recoverable" strains of virus, and although one of them appeared to be consequent upon only the slightest of alterations toward malignancy on the part of the papilloma from which it came. Extracts of another of the cancers, an anaplastic, squamous-cell carcinoma devoid of any morphological sign of the influence of the Shope virus, yielded typical virus papillomas on several occasions. The wholly negative results with the third cancer must be considered in the light of the fact that the "recoverable" strain of virus causing the papilloma from which it originated could no longer be recovered from such growths on collateral test.

Experiments on the cause of the rabbit carcinomas derived from virus-induced papillomas. II. Loss by the Vx2 carcinoma of the power to immunize hosts against the papilloma virus

Tests were made to learn whether an anaplastic, epidermal carcinoma, the Vx2, which had originated more than 8 years previously from a virus papilloma in a domestic rabbit, still rendered its hosts immune to the virus. It had done so in the first 22 successive groups of animals to which it was transferred during a period of 3(1/2) years, its growth regularly eliciting a blood antibody that neutralized the Shope virus and fixed complement in mixture with it; and on the assumption that this would continue to be the case no further observations were made for nearly 4(1/2) years more. Then direct inoculation of animals carrying the tumor in its 46th Generation showed them to be as susceptible to the virus as normal rabbits; and sera procured from hosts of the 46th, 47th, 48th, and 50th Generations failed to neutralize the virus or fix complement with it. Tests of this last sort, repeated at intervals since,-most recently with sera from animals carrying the tumor in its 73rd Generation,-have yielded consistently negative findings. Loss of the power to immunize against the papilloma virus was not attended by any perceptible change in the Vx2 carcinoma. Manifestly the antigen responsible for the immunity cannot, as such, have been the actuating cause of the tumor. Attempts were made to infect the cells providing 48th Generation cancers, by mixing them with a suspension of the papilloma virus at time of implantation, or by injecting this agent into the blood stream of rabbits in which the tumog had already begun to proliferate. Its morphology and rate of growth remained unaltered; but tests of the animals to which transfers were next made yielded what appeared to be evidence of some slight immunity to the virus.

Arthropod transmission of rabbit papillomatosis

The virus of rabbit papillomatosis, a neoplastic disease studied intensively in relation to cancer because of its tendency toward malignant transformation, has been successfully transmitted by Aedes aegypti, Rhodnius prolixus, and Triatoma infestans from papillomas induced in cottontail rabbits, and by the first species mentioned, from papillomas induced in domestic rabbits. This was accomplished by interrupted feeding, feeding after an interval of several days from the infective meal, or by application of a suspension of the mouthparts of the insects either immediately after their infective meal or several days later. Insect transmission was also successful from the satellite growths resulting from subcutaneous extensions of the typical papillomas of infected cottontails, and from certain peculiar, subcutaneous nodules, arising rapidly after infection, and not heretofore described. Although the latter growths resembled rabbit fibromas, antibodies to fibroma virus could not be demonstrated by neutralization tests with the sera of the animals involved.

Fluorescent antibody detection of the antigens of the Shope papilloma virus in papillomas of the wild and domestic rabbit

The results obtained by a fluorescent antibody study of the Shope papilloma virus in papillomas of the wild and domestic rabbit are presented. In the wild rabbit papillomas the viral antigens occurred exclusively in the nucleus and were present in the differentiating cells of the keratohyaline layers and in the keratinized layers. The antigens were not present in the deeper proliferating epithelial cells of the papillomas. The Shope viral antigens were present in very minute amounts in papillomas of the domestic rabbit, as compared with papillomas of the wild rabbit, and were only detected in the superficial keratinized layers. It is postulated that virus is present in the nuclei of the proliferating cells of the papillomas of the wild and domestic rabbit but exists there in an early stage of development, consisting mainly of nucleic acid and deficient in protein, therefore non-antigenic and not demonstrable by fluorescent antibody. The nucleic acid moiety of the virus may be infective, and the protein component may provide immunologic specificity and serve to preserve transmissibility. The protein-deficient virus can be referred to as masked virus.