Different L-R ‘non-producer’ cell lines synthesized one of two types of RSV(O), distinguishable by host range. RSVα(O) has no known natural host. RSVβ(O) is infectious for Japanese quails, European pheasants and some Brown Leghorn, White Leghorn and Reaseheath I × C hybrid chickens. The host range of RSVβ(O) is distinct from other avian tumour viruses. Host susceptibility to RSVβ(O) is genetically determined in a complex and as yet unclear way. It is not correlated with response to the A, B, and C sub-groups of avian tumour viruses or with presence of the natural antigen which reacts in the COFAL test: nor is it sex-linked. RSVβ(O) is not markedly oncogenic in hamsters.

The difference between RSVα(O) and RSVβ(O) did not appear to be a heritable property of the RSV particle but depended on the type of host cell in which the virus was cloned. Synthesis of RSVβ(O) occurred only after passage through COFAL positive host cells. The possible control of the infectious properties of RSVβ(O) by a helper virus present in an elusive form in permissive hosts is discussed.

A stock of inactivated Sendai virus commonly used for cell fusion was apparently contaminated with an active avian leukosis virus.


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