We know that a lengthening list of viruses is involved in the etiology of animal tumors, as shown in table 1. The exciting agent of each of these tumors is a specific virus separable from all others. Nine of the neoplastic diseases result immediately or finally in unequivocal malignancy. Contrariwise, 8 of the viruses collectively induce a broad spectrum of tumors that do not satisfy all criteria of malignancy. As you see, this list of tumorigenic viruses has grown since 1951. Much new information came from use of newborn animals in 3 laboratories. In 1951, Gross (12) showed that cell-free extracts prepared from^leukemic tissue from mice of a strain with a high incidence of spontaneous leukemia (AKR) would, when injected into newborn mice of the C3H (Bittner) strain with low leukemic incidence, produce leukemia months later in the injected mice. The induction of leukemia could be repeated with extracts of leukemic tissues of the injected C3H mice. These results have been confirmed abundantly. Next, Gross (IS) and Stewart (14) reported independently that injection of newborn mice with an extract of leukemic tissue resulted in development of parotid-gland carcinomas and/or fibrosarcomas. This work added notably to the list of mammalian tumor viruses initiated in 1936 by Bittner (15), with the demonstration that an agent with all properties of a virus could produce mammary cancer when transferred to newborn mice at their first feeding. Recently, Stewart and her associates (16) have transferred cell-free extracts from leukemic mice to tissue cultures, and after repeated passage in culture, harvested an agent responsible for induction of a variety of solid tumors in recipient mice. This "polyoma" virus likewise must be injected into newborn mice. These 4 instances have demonstrated unequivocal association of a virus with a malignant tumor or tumors. Bittner (17) and Muhlbock (18) have emphasized hormonal influences rather than virus as the "direct" cause of the mouse mammary cancer. This distinction would, in my view, be quite significant if it were demonstrable that the tumor cells never contained virus. It is inconceivable that virus might infect hormone-producing cells in the body, and so impair or modify their function that carcinogenic hormones were produced. But since virus is shed into the milk, and in many instances is recoverable in high titer from the mammary tumors, the distinction is not readily substantiative. According to my earlier review of the possible origin of cancer, there is room for viruses, hormones, and many other factors in etiology. Our primary need is not to decide which is causative, but how each is causative.

Table 1. Viral Tumors Of Animals

1908 Fowl leukemia (Ellerman and Bang)*

1938 Frog kidney carcinoma (Lucke)*

1911 Fowl sarcoma (Rous)*

1951 Equine cutaneous papilloma (Cook

1920 Bovine papilloma (Magalhaes)

and Olson)

1932 Canine oral papilloma (DeMonbreun

1951 Mouse leukemia (Gross)*

and Goodpasture)

1953 Mouse parotid tumor (Gross) *

1932 Rabbit fibroma (Shope)

1953 Squirrel fibroma (Kilham, Herman,

1933 Rabbit papilloma (Shope)

and Fisher)

1933 Fowl lymphoma (Furth)*

1955 Deer fibroma (Shope, Mangold,

1936 Rabbit oral papilloma (Parsons and

MacNamara, and Dumbell)

Kidd)

1956 Mouse leukemia (Friend)*

1936 Mouse mammary carcinoma (Bitt-

1957 Polyoma (Stewart and Eddy)*

ner)*

*Neoplastic virus disease which becomes malignant.

Dependence on newborn animals for demonstration of etiologic activity of these tumor viruses suggests that the development of cancer or leukemia under ordinary conditions reflects the inability of an animal's immune mechanisms to resist an agent encountered neonatally. The unborn or newborn animal is unable to generate immunity to a wide variety of agents, living or otherwise. Since constitutional defect of the newborn does not explain susceptibility to a neonatally encountered agent in later life, it has been theorized that the neonatal encounter results in a continued state of specific unresponsiveness to the particular antigens contained in the agent. That is, "actively acquired tolerance" ensures that particular neoplastic diseases can be transmitted from parent to progeny, but not from the individual in mature life. It does not appear that such tolerance need be absolute. Gross (19) showed that tissue extracts of normal C3H mice produced a significant number of parotid tumors in C3H mice inoculated when newborn. Here the important factor in the development of parotid tumors was not the presence of agent in recipient mice, for both donor and recipient mice had that; the significant difference was the inability of newborn mice ultimately to resist added virus encountered during the first days of life. The difference could be explained by acquired immunological tolerance. Woolley (20) provided other evidence to support this suggestion when he found that normal mice given sufficient cortisone to depress immune response also developed parotid tumors. Kaplan (21) showed that a variety of mice with a low natural incidence of leukemia, the C57BL strain, developed leukemia of thymic origin after fractional X irradiation. If thymus gland from an unirradiated C57BL mouse was transplanted into a thymectomized preirradiated C57BL mouse, leukemia again developed from cells of the unirradiated transplanted thymus. If, however, the thymectomized irradiated mouse was given C57BL mouse bone marrow before transplantation of normal unirradiated thymus gland, leukemia usually did not develop. In these experiments, since leukemia developed from unirradiated cells, it could not be explained as a result of somatic mutation by mutagenic action of X rays. Development of leukemia in the mice given sufficient irradiation to destroy the immune response is explained most readily by the inability of the irradiated animal to resist a tumor agent introduced with the unirradiated thymus cells. Protective action of injected bone marrow would be expected, since it is known that such injection may repair capacity for immune response of irradiated hosts through repopulation of damaged bone marrow. These and other experiments demonstrate that tumor viruses may exist as temporary residents of animals without necessary induction of tumors or leukemia, though the viruses are demonstrably capable when introduced into immunologically inadequate hosts. A tumor virus even may be transmitted from generation to generation, vertical transmission, without manifestation of its presence.

I have departed from the cellular view of virus-induced neoplasia to emphasize these grosser aspects of virus-host relationship, since these aspects are involved in an understanding that cancer can be noninfectious in the usual sense and still be induced by a virus. On the basis of our present knowledge of viruses and on the assumption that host conditions are appropriate for manifestation of virus infection as a tumor or cancer, three alternative results of infection can be conceived: (a) Infection results in an altered cell that continues to produce virus, (b) Infection produces an altered cell, but does not permit virus reproduction, and (c) infection results in the production of altered virus with eventual destruction of the producing cell. In other words, is virus production compatible with cell proliferation, or if it is not, how are the two processes reconcilable? Two biological phenomena may provide clues. The first is the phenomenon of lysogeny (22). I have said that a virus is a code of instructions wrapped in a package. Certain bacterial viruses, upon loss of the protein coat and delivery of the nucleic acid into the bacterium, do not undergo replication of the nucleic acid or instruction code. The viral code becomes associated with the cell code, i.e., with the cell genic chromosomal structure. Spontaneously at intervals, or purposefully under the influence of an activating agent such as irradiation or nitrogen mustard, the preserved bacterial code, or provirus, detaches from the cell chromosomal structure to undergo replication and repackaging. The result is virulent, destructive virus that lyses the host bacterium and may infect and lyse other susceptible cells. On encounter with insusceptible cells again, the infecting virus re-enters the provirus state. While in this state, the virus code is undetectable as virus, and is treated by the bacterial cell as a portion of its chromosome. The code is replicated and transmitted to daughter cells just like native cell code. The second phenomenon is the infectivity of viral nucleic acid. The studies to which I refer were carried out in collaboration with Holland and McLaren (23-26). We reported, for example, that type 1 poliovirus will infect only cells of primate origin, and that this susceptibility of primate cells depends on an interaction between the protein package of the poliovirus and lipoproteins of the cell surface. Nonprimate cells are quite insusceptible to poliovirus because, lacking the cell-wall receptor, they cannot adsorb the virus. We also showed that nucleic acid (the viral instruction code) extracted with phenol from type 1 poliovirus, Coxsackie A-9, Coxsackie B-l, and ECHO-8 viruses infected ordinarily insusceptible nonprimate cells. This infectivity of the viral RNA was manifested on insusceptible cells in established continuous culture, in primary monolayer culture, on suspensions of minced tissues, and on living animals inoculated intracerebrally. Cells of rabbit, swine, mouse, guinea pig, chicken, and hamster origin could be infected equally well with enterovirus RNA. Each of the viruses so produced was identical to the virus donating the RNA. Virus produced from infecting RNA was neutralized by homotypic antiserum, resistant to ribonuclease treatment, and insusceptible to adsorption or replication by nonprimate cells, even of the strain producing the virus from RNA. In RNA-infected cell cultures, virus was produced in a single cycle, without overt cytopathic effect on nonprimate cells or disease in intracerebrally inoculated animals.

Generalizing from these two phenomena, one can suggest that viral nucleic acid, freed of its protein overcoat under influence of any of a variety of agents, could infect any mammalian cell which it encountered, and either replicate more of the virus from which it was derived or lyso-genize the recipient cell. Production of infectious virus from RNA would not entail destruction of the cell population. The lysogenic state might be abrogated years later under influence of a provocative agent such as the chemical carcinogen or radiation. These possibilities permit us to accept any of the alternatives for tumor virus-cell interaction postulated previously. As yet, we do not know whether we should associate a proliferative effect on cells with the state of apparent or inapparent infection. We do know that considerable latitude of postulation is permissible, since tumor viruses all do not act with equal speed. Rous sarcoma virus induces a fully malignant tumor in a few weeks. Rabbit skin carcinoma, in contrast, results after many months, by development from a clearly defined, preceding benign papilloma. A virus may initiate neoplasia by inducing cell proliferation, and/or increasing genetic variability of cells. To investigate these questions, we are handicapped by not having available a biological test for detection of intracellular pro-viruses, or a cell-culture test for cytomalignancy comparable to the cyto-pathogenicity test used for the study of enterovirus infection. To emphasize our need and the importance of these tests, I may remind you that it was the use of trypsin to disperse cells (27), a comparatively simple laboratory tool, that led to monolayer cultures (27), quantitative assay of animal virus particles by plaque count (28) and clonally derived mammalian cells strains (29) that led to the discovery of more than 100 new viruses that undoubtedly had existed and been productive of disease in man's environment for centuries.

This has been more a speculative than a definitive lecture on viruses. Perhaps its major lesson is that with virology as in other areas of medical science, we must get from the patient to the laboratory before we, confident of treatment or prevention, can return from the laboratory to the patient.