Graffi in 1957 (34) summarized extensive studies on a myeloid leukemia virus recovered from five transplantable mouse tumors, including Sarcoma 37. This virus inoculated into newborn mice of strains Agnes-Bluhm, sg (sooty yellow) and db (dilute brown), produced granulocytic leukemia in a large percentage of the recipients. A high proportion of the induced leukemias were of the chloroleukemia type. Adult mice were found to be sensitive to the agent, but to a lesser degree. However, fractionated total-body X irradiation increased the incidence of the disease in older animals.

A lack of strict species specificity of this virus is indicated by the report of Graffi and Gimmy (35), describing the induction of "intrathoracic leukoses" in newborn rats inoculated with the agent. In a similar manner cell-free preparations of the rat leukemic tissues retained leukemogenic activity when they were reinoculated into the mouse (36).

The myeloid leukemia agent is characterized by the following physical and chemical properties: It is inactivated by heating (65° C. for 30 min.), by lyophilization, and by treatment with deoxycholate, concentrated crystallized trypsin, formalin, and ether (20 min. at 0° C.). It is resistant to treatment with the nucleases, lipase, weak trypsin solutions, and 50 percent glycerin. It is stable within the pB. range 4.5 to 9.5.

High-speed sediments of the tumor filtrates are reported to contain 10 percent ribonucleic acid (RNA) and to possess a strong adenosinetri-phosphatase (ATPase) activity. Myeloid leukemia could be induced by "nucleoprotein fractions" prepared from the filtrates by precipitation at pH. 4.5 with acetate buffer. In addition, Bielka and Graffi (37) have shown that RNA extracted from the myeloid leukemia tissues also produced the disease. An incidence of 10.3 percent was obtained when the ribonuclease (RNase) sensitive extract was inoculated into newborn Agnes-Bluhm mice.

Electron microscope observations by Graffi (38) on high-speed sediments of the tumor filtrates and on sections of virus-induced leukemic tissues revealed virus particles measuring 70 mµ and possessing a distinct inner structure of 30 mµ in diameter.

The specific antigenic nature of the Graffi agent is evidenced by the neutralization of the leukemogenic factor with heterologous (rabbit) immune serum. Rabbit antiserum against normal tissues failed to inactivate the virus, and attempts to produce a homologous antiserum have thus far been unsuccessful.

It is interesting to note that, in contrast to lymphocytic leukemia, the spleen, rather than the thymus, plays a crucial role in the histogenesis of the virus-induced myeloid leukemia. Thus Fey and Graffi (89) found that splenectomy of the very young animal had a strong inhibiting effect on the development of the leukemia, regardless of whether it was performed just prior to, or several days after, virus inoculation. On the other hand, if splenectomy was delayed 1 to 2 months after virus injection, no inhibitory effect was evident.

Kaplan Leukemia Agent

Kaplan (40) established an additional link between certain of the external carcinogens and the leukemia-inducing viruses. He observed that cell-free preparations from a radiation-induced tumor produced the same neoplasm when inoculated into nonirradiated susceptible hosts.

In this respect Lieberman and Kaplan (41) recently demonstrated the presence of a leukemogenic agent in lymphoid tumors induced in C57BL/ Ka mice by fractionated total-body X irradiation. The recovered agent induces lymphocytic tumors when it is inoculated into nonirradiated newborn isologous and compatible F1 hybrid mice. After serial passage in these animals the virus produced lymphoid neoplasms in 69 percent of the recipients with a median latent period of approximately 28 weeks.