Masahiro Kajima

Spontaneous Leukemia in Germfree AK Mice.

Summary Lymphatic leukemia developed spontaneously in “germfree” AK mice. The characteristics of disease were the same as those observed in the conventional control AK mice. The data indicate that the leukemia agent was transmitted to progeny by congenital route(s).

Leukemia in Germfree Rats.

Summary Germfree rats of 3 strains (Wistar, Fischer and Sprague-Dawley) were examined for leukemogenic potential, (a) Particles of indefinite nature were observed only in 2 of 10 Wistar rats, (b) Leukemia could not be induced in the rats by whole-body administered X-rays, and (c) germfree Sprague-Dawley rats were as susceptible to Gross passage A leukemia virus as conventional rats.

Virus-like particles in cultures of McCoy cells.

Virus-like particles similar to types A and C tumor viruses as classified by Bernhard and others, were consistently found in association with the human synovial cell line identified as the McCoy cell. It was occasionally possible to observe cylindrical structures among the spherical virus-like particles in the cytoplasmic vacuoles of the cells. The significance of the virus-like particles remains to be demonstrated.

Viral Status of Germfree Rodents; Present and Future

The materials used in experimentation must be clearly defined as to their purity. Experimental animals are standardized by specifying species, strain, sex, age, diet, housing and microflora; for these factors are important for obtaining clear results and for correct interpretations of the results obtained. Considerable efforts have been directed toward producing and controlling animals as pure materials for experimentation. Germfree (GF) rodents that are caesarean-derived into a sterile isolator, fed a sterilized diet and maintained aseptically, are desirable subjects for some types of experimentation because they have proven to be free from bacteria, protozoa, fungi and mycoplasma (1, 2, 3, 4). In the field of virology, possibly other fields (such as oncology, serology, and pathology) it is important to obtain “Virus-defined” or “virus-free” animals as reliable experimental tools.

7, 12-Dimethylbenz(α) Anthracene-Induced Malignant Lymphoma in Germfree Mice

It has been well-established that conventional rodents that received a single subcutaneous injection of 7,12-Dimethylbenz(α)-anthracene (DMBA) develop local and/or distant neoplasms(1–10). The age of treated rodents, the dose of carcinogen, the difference in species of animals used and other factors (sex) seems to influence the incidence and type of DMBA-induced tumors(1, 2,5,6,9). The incidence of malignant lymphoma in conventional mice is increased by subcutaneous injection of small dose of DMBA at birth (1–4). On the base of previous works by others, however, there is no such high incidence of lymphoma in other rodents, such as the rat(7, 11), hamster (8, 9) and rabbit(lO) similarly treated with DMBA. At present, it is unknown what factor(s) makes this difference in response of mice distinct from other species to DMBA-injection at birth, although it can be assumed that leukemia virus widely present in “normal” laboratory mice may be responsible for this phenomenon.

Rous Sarcoma Virus in Germfree Rats

Seven years ago, there was general agreement among investigators that germfree animals were free of all microbial agents. Since then, extensive surveys, based on serological and tissue culture procedures have failed to reveal the presence of specific viral agents in them (1, 2, 3). Additional examinations, however, revealed virus-like particles in the thymic tissues of germfree mice, which were identified as leukemia virus when mice of the same strains were exposed to leukemogenic doses of x-rays (4, 5). Thus far, no mouse strain (germfree or conventional) has been found free of leukemia virus (6). It is very possible that those viruses which are transmitted to progeny by “vertical” or congenital route(s) would be found in germfree mice which had been derived from virus-contaminated stock. Thus far, three viral agents have been detected in germfree mice: leukemia virus, mammary tumor virus, and lymphocytic choriomeningitis virus (7, 8); and it is likely that others will be added to this list. For these reasons we feel that the “germfree” mouse cannot serve as an experimental subject to delineate the exclusive role of viruses in carcinogenesis, since a virus-free mouse is not yet available.