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

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.

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.

A NATURAL ANTIBODY THAT REACTS IN VITRO WITH A SEDIMENTABLE CONSTITUENT OF NORMAL TISSUE CELLS : II. SPECIFICITY OF THE PHENOMENON: GENERAL DISCUSSION

Continued serological investigations of the sedimentable constituents of normal and neoplastic tissues have shown that the blood serum of normal rabbits will fix complement in mixture with saline extracts of normal rabbit tissues. The phenomenon has proved referable, not to anticomplementary effects of serum or antigen nor to so called non-specific complement fixation, but to a naturally occurring serum principle, hitherto unrecognized, which reacts specifically in vitro with a sedimentable constituent of normal tissue cells. The principle exists in the blood of nearly all adult rabbits but is absent from that of rabbits less than 1 month old. It can be salted out from serum with ammonium sulfate and is destroyed when heated at 65°C. for 20 to 30 minutes. Its titer was found to run parallel in general with that of two natural antibodies also present in normal rabbit's blood (natural Wassermann reagin, natural anti-sheep hemolysin); but absorption tests showed it to be distinct from these. Because of its properties, the serum principle has been termed the natural tissue antibody. The substance with which the natural tissue antibody reacts is regularly present in saline extracts of many normal tissues,—those of rabbits and of other species as well. Kidney and liver tissues always yield it in abundance, while spleen, brain, and testicle provide somewhat less; heart and voluntary muscle extracts contain relatively little, and non-nucleated erythrocytes and skin are practically devoid of it. The results of affinity and absorption tests indicate that it is nearly or quite the same from whatever tissue or species derived. It is readily sedimentable in the high-speed centrifuge, little or none remaining in the supernatant liquid of potent suspensions spun at 25,000 R.P.M. (45,400 g) for 1 hour. It either does not come away into alcohol or is inactivated thereby, is readily destroyed by heat (56–70°C. for 30 minutes), and diminishes notably in antigenic potency upon standing overnight in saline suspension or when the tissues containing it are kept in glycerol. Its properties suggest that it may be a protein. The implications of the findings are discussed in relation to the formation of the natural antibody and its place amongst serological phenomena, to so called "non-specific" fixation of complement and other serological complexities, and with particular reference to the character of the sedimentable constituents of normal and neoplastic tissue cells.

CERTAIN CONDITIONS DETERMINING ENHANCED INFECTION WITH THE RABBIT PAPILLOMA VIRUS

The infection of normal or hyperplastic rabbit skin with the papilloma virus can be greatly enhanced by protecting the scarified and inoculated area with a layer of paraffined gauze until healing occurs. In this way the necrosis which follows upon scarification and also the scabbing are almost entirely prevented and in consequence epithelial regeneration is usually complete within 24 hours. Not only are many susceptible cells provided to the virus far earlier than would otherwise be the case,—and collateral tests have shown that it becomes associated with them within a few hours instead of after many,—but the inoculum is itself conserved, instead of becoming largely lost amidst necrotic tissue and scab, as under ordinary circumstances. The effective titer of the virus is increased by the procedure from 10 to 100 times over that attained when hyperplastic skin is allowed to dry after inoculation. Since the results under the latter circumstances are 10 to 100 times better than those when normal skin is treated in the same way it follows that a 100- to 10,000-fold increase in the effectiveness of the virus has now been obtained.

THE NATURE OF NON-SPECIFIC INHIBITION OF VIRUS HEMAGGLUTINATION

A study of the component in serum and tissue extracts responsible for non-specific inhibition of hemagglutination with mumps virus and the PR8 and Lee strains of influenza virus has yielded the following results: 1. The inhibitory factor was found in high titer in human serum and in saline extracts of various organs procured at autopsy (lung, liver,kidney, spleen). The inhibition titers of extracts of these organs were usually higher than the serum titers, whereas the titers of muscle extracts were invariably lower. 2. Similar results were obtained with serum and tissue extracts from normal rabbits and guinea pigs. 3. The serum inhibition titers were not affected by heating to 75°C. for 30 minutes, whereas the titers of the tissue extracts were usually reduced by heating at 65°C. or 75°C. for 30 minutes. 4. Saline extracts of human and chicken red blood cells also contained an inhibitory substance in high titer, and these cells showed marked agglutination with influenza and mumps viruses. Rabbit red cells, on the other band, underwent little or no agglutination with these viruses and extracts of these cells failed to cause inhibition. Sheep red cells varied in their capacity to agglutinate and also in their yield of the inhibitory substance. 5. When the virus receptor substance was removed from chicken red cells by adsorption and elution with influenza virus, extracts of the cells no longer yielded the inhibitory factor. 6. The inhibitory substance did not neutralize influenza virus in mice and it failed to fix complement when mixed with influenza or mumps viruses. 7. Evidence was obtained that some virus was released from the inhibitory substance after incubation for 6 hours at 22°C. or 37°C. The implications of these findings are discussed.

Detection of L1, infectious virions and anti-L1 antibody in domestic rabbits infected with cottontail rabbit papillomavirus

Shope papillomavirus or cottontail rabbit papillomavirus (CRPV) is one of the first small DNA tumour viruses to be characterized. Although the natural host for CRPV is the cottontail rabbit (Sylvilagus floridanus), CRPV can infect domestic laboratory rabbits (Oryctolagus cuniculus) and induce tumour outgrowth and cancer development. In previous studies, investigators attempted to passage CRPV in domestic rabbits, but achieved very limited success, leading to the suggestion that CRPV infection in domestic rabbits was abortive. The persistence of specific anti-L1 antibody in sera from rabbits infected with either virus or viral DNA led us to revisit the questions as to whether L1 and infectious CRPV can be produced in domestic rabbit tissues. We detected various levels of L1 protein in most papillomas from CRPV-infected rabbits using recently developed monoclonal antibodies. Sensitive in vitro infectivity assays additionally confirmed that extracts from these papillomas were infectious. These studies demonstrated that the CRPV/New Zealand White rabbit model could be used as an in vivo model to study natural virus infection and viral life cycle of CRPV and not be limited to studies on abortive infections.

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.

A STUDY OF QUANTITATIVE CHANGES OF THE SHOPE RABBIT PAPILLOMA VIRUS AT THE SITE OF INOCULATION IN THE SKIN OF THE COTTONTAIL AND DOMESTIC RABBIT

By excising after varying intervals the site of inoculation of the rabbit skin injected intracutaneously with the Shope papilloma virus and titrating its virus content, it has been shown that demonstrable active virus disappears from the skin within approximately 24 hours. The disappearance of most of the virus within such a short time as 15 to 60 minutes makes a transportation of virus from the site of inoculation as a responsible factor unlikely. The rate of disappearance seems to be the same both in the domestic and in the cottontail rabbit.