PROBLEMS CONCERNING THE TUMOR VIRUSES 345 



Such finding suggest that a few hybrids may have acquired their father's 

 agent while in utero and this reminds the virologist of the "scrapie" virus of 

 sheep which can be transmitted by the sperm to the offspring without 

 infecting the mother (Greig, 1940). 



The routes of transmission whereby the progeny get their father's virus is 

 not as pertinent here as is the fact that the mothers are infected. These 

 females may or may not develop tumors but in either event they do transmit 

 the virus to their offspring and this revealed an unexpected path for virus 

 transmission. Persistent investigations are essential before the epidemiology 

 of the tumor viruses can be learned. 



The problem of how the virus infects adult females is unsolved. The 

 placenta is an effective barrier against transmission of the virus from an 

 infected mother to her unborn offspring. Perhaps the genital tract is receptive 

 during a stage in the estrous cycle. If the mother is infected during copulation 

 the virus must increase rapidly to infect her milk because the occurrence of 

 tumors in a single litter is used to ascertain when the mother acquired the 

 virus (Bittner, 1957b). The important thing is that the virus not only infects 

 adult females, but apparently propagates rapidly in them. The mechanisms 

 responsible for this interaction deserve consideration. 



Infection of adults suggests further investigations of a problem which is of 

 interest to virologists. The mammary tumor virus, in contrast to other 

 tumor viruses, appears to be incapable of eliciting antibodies in its natural 

 hosts but does so in heterologous hosts. The absence of demonstrable 

 antibodies in mice carrying the virus may be the result of exposure to 

 th.9 virus shortly after birth, when they are known to be in a state of 

 immunologic nonreactivity. Adults may be reactive and respond to the 

 virus by the production of antibodies. Disappearance of the virus from mice 

 of the Marsh and RIII strains suggests that the genetic constitution of the 

 test animal may play an important part in immunological responses to 

 the virus. The inability of the virus to propagate in all strains of mice 

 suggests the presence of a resistance factor; the elucidation of the nature of 

 this factor presents another interesting problem. 



Anyone familiar with this tumor virus will realize that this discussion 

 includes only a few of the problems now receiving consideration. From the 

 first, one of the most important problems has been the development of a 

 technique for detection of the virus which will dispense with the months of 

 waiting for the appearance of tumors in test animals. A method for rapid 

 quantitative assay of the virus is also desirable because the known properties 

 of the virus make it almost ideal for quantitative studies of the host-virus 

 relationship. 



A satisfactory assay technique could also assist investigations of the cell- 

 virus relationships. The virus appears to be very specific for mammary gland 



