SUMMARYWe have compared endogenous proviruses in DNA of chickens of 11 breeds by means of Southern's technique. Many of the endogenous virus loci found were missing from the genomes of the extensively studied white leghorn chickens, although some of the proviruses and especially ev-I appeared to be widely spread among different chickens. Most of the proviruses were similar to Rous-associated virus (RAV-O) as judged by the EcoRI digestion patterns. Several genetically different proviruses were found in the DNA of 28 brown leghorns. They contain neither ev-1 provirus in their genome, nor any other one common for all individuals. All four Italian partridge-coloured chickens examined appeared to be free from ALV-related sequences in their DNA showing the possibility of normal life without known endogenous proviruses. Possible causes of inter-breed differences of chicken endogenous proviruses are discussed.
Experiments were carried out to test the possibility of tumour production in adult recipients of different mouse strains by syngeneic embryonic tissue cultures exposed to, and not transformed by several oncogenic viruses. Tumour‐producing effect of embryonic tissue cultures exposed to C‐Z strain of RSV was observed in 100% of A, C3H‐H2p, AKR, C57BL/Sn, CC57W and in a small proportion of CC57BR mice, but not in B10.D2 and BALB/c mice. However, introduction of infected embryonic tissue cultures, originating from the resistant strains of mice, into F1 hybrids between the opposite strains resulted in tumour production. It was demonstrated that resistance to tumour production of the three strains mentioned above was not mediated by the strong immune reaction against the cells introduced.
Tumour‐producing effect by infected but untransformed cells was also seen with the C‐Z strain in hamsters, with the S‐R strain of RSV in mice, and with the 5D strain in hamsters but not in mice.
Attempts to produce tumours in mice by the introduction of syngeneic embryonic tissue culture exposed to SV40 and neurotropic strain (WSN) of influenza A viruses were negative.
Cultures of cells from embryos of Af, C3H-Hg and (CBAT6T6 x Af)F, mice rapidly developed oncogenicity for syngeneic animals after exposure to RSV (Carr-Zilber strain). The tumours of mice were virogenic and contained the group-specific antigen of avian sarcoma-leukosis complex. However, when the infected cells became transformed in morphology, they lost their oncogenicity for syngeneic animals.The high oncogenicity of Af mouse embryonic cells infected with Carr-Zilber strain of RSV for the syngeneic animals was described in our previous communication (Kryukova et al., 1968).The tumours were virogenic, i.e. the introduction of the intact cells into chicks produced Rous sarcomas. After transplantation of these virogenic tumours into chicks and passing through chicks the resulting virus could induce tumours in mice. The transformed cultures appeared to be non-oncogenic for syngeneic Af mice. The further investigation of this phenomenon is described in this paper.
MATERIAL AND METHODS
Preparation of virus (Carr-Zilber strain)Virus-containing chicken tumour extract, diluted 1 :3 and centrifuged at 400 x g for 20 min, was injected intramuscularly into 7to 10-day-old White Leghorn chicks. Five or 6 days after injection the tumours were excised, pooled, homogenized, frozen in a dry-iceacetone mixture, and then stored at -30" C.
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