Lymphocytic choriomeningitis infection of mice as a model for the study of latent virus infection

Lymphocytic choriomeningitis virus has been used to set up a model of a latent virus infection in mice. It has been shown to be possible to induce reproducible virus infections in mice which remain completely symptom-free in spite of levels of viral growth equal to those found in the sick animal, by the inoculation of mice within a few hours of birth. This is a convenient method of producing a latent infection in the mice. The effect of the age of the mice at the time of intracerebral inoculation was studied with respect to the pattern of disease produced. Several methods were tried without success in order to induce overt disease in the latently infected animals. The virus did not cause any demonstrable cytopathogenic effect on mouse tissue and several other types of animal tissue. A slight cytopathogenic effect was observed in a strain of human cells in vitro. Virus persisted for weeks in some of the tissue cultures, without damaging the tissue but with the production of active virus. The bearing of the results obtained is discussed in relationship to current concepts of latent virus infection and particularly immunological tolerance. A concept of a special variety of latency is introduced and the name "vital" infection suggested for this.

The estimation of the number of histocompatibility genes controlling the successful transplantation of normal skin in mice

The success or failure of a transplant of tissue from one animal to another depends on the action of a number of independently acting histocompatibility genes which are present in the donor's tissues and absent from those of the host. An estimate of the number of such genes in mice can be made from the proportion of grafts which survive transplantation from either of the two parent strains to members of theirF2generation. Ifnis the number of genes, the proportion of successful grafts = (¾)n. Most previous work of this sort has used tumour tissue to provide material for grafting. A normal tissue, such as skin, has several advantages over malignant tissues which may kill the host and are difficult to observe; it is also probably more exacting in its genetic requirements. We have used single 7 mm diameter full-thickness pieces of skin transplanted orthotopically to the side of the chest as test grafts. Taking survival in an autograft-like condition 100 days after grafting as the criterion for judging success or failure, two of 120Astrain and one of 154CBAstrain grafts survived transplantation toF2generation mice. Assuming that each separate antigen is capable of causing breakdown of the graft, these figures imply that certainly not less than fifteen independently segregating genes control the fate of a transplant. But since breakdown of such 'successful' grafts was observed as late as 180 days after grafting, the estimates represents minimal values only. The survival times for the grafts are distributed widely from 10 or 11 days (the normal survival time for interstrain homografts) to more than 100 days. Both frequency distributions (forAandCBAdonors) are quite unlike the theoretical distribution for the frequency of occurrence of 0 to 15 homozygous gene pairs in theF2generation. They also differ between themselves and suggest that the alleles in theCBAstrain are less potent sources of antigens than those in theAstrain. It is not possible to equate numbers of gene differences with survival for any given number of days, but clearly the individual genes have widely differing powers of forming antigens. The process of destruction, once begun, is soon complete and resembles the breakdown process in normal interstrain homografts more closely than it does the slower, vacillating process found in mice that have acquired tolerance to foreign skin by virtue of an inoculation during embryonic life. Second set grafts are usually thrown off more rapidly than the first set but anomalous results occur occasionally. Immunoparalysis was not found even when five sets of grafts were transplanted in succession.