Genetics of resistance to virus-induced leukemias

Antigenic diversity in Eimeria maxima and the influence of host genetics and immunization schedule on cross-protective immunity

Eimeria spp. are a group of highly successful intracellular protozoan parasites that develop within enterocytes. Eimeria maxima from the chicken is characterized by high immunogenicity (a small priming infection gives complete immunity to subsequent homologous challenge) and naturally occurring antigenically variant populations that do not completely cross-protect. In this study we examined the expression of antigenic diversity in E. maxima, as manifested by cross-strain protection in a series of inbred chicken lines. The IAH line of Light Sussex chickens and all lines of inbred White Leghorns were susceptible to primary infections with either of two strains (H and W) of E. maxima and were protected completely against challenge with the homologous strain of parasite. The extent of cross-protection against the heterologous parasite strain varied from 0 to almost 100% depending on host genetics. Interestingly, in one inbred line of chickens (line 15I) the cross-protective phenotype was directional and intensely influenced by the infection history of the host. The basis for the observed variation in cross-protection is not known, but our results suggest that the major histocompatibility complex is not a major genetic component of the phenotype. These results are discussed in relation to the number of protective antigens presented by complex pathogens and the development of immunoprotective responses in hosts of different genetic backgrounds.

In vivo generation of antigenic variants of murine retroviruses

Inoculation of adult BALB/c-H-2k (BALB.K) mice with both Gross murine leukemia virus (GV) and a biological clone derived from this virus resulted in the recovery of variant viruses which differ from GV with respect to the expression of specific epitopes associated with the env gene product, gp70. The loss of these epitopes correlated with the failure of antiserum raised in BALB.K mice against GV to neutralize variant virus although this antiserum neutralized GV. In contrast, BALB/c-H-2b (BALB.B) mice, immunized with GV, produced antibodies which neutralized both GV and the variant virus, indicating that BALB.B mice respond to epitopes distinct from those recognized by BALB.K mice. These results suggest that the selection of variant viruses resulting from in vivo passage may be related to the immunoselective pressures exerted in mice which express particular alleles of certain major histocompatibility complex (MHC)-linked genes.

Feline leukemia virus envelope protein expression encoded by a recombinant vaccinia virus: apparent lack of immunogenicity in vaccinated animals

We have constructed a recombinant vaccinia virus encoding the expression of the feline leukemia virus (FeLV) envelope gene of the Gardner-Arnstein strain of FeLV subgroup B. Human cells infected with the recombinant virus (vFeLVenv) express and process the FeLV envelope protein similarly to cells infected with authentic FeLV. The mature gp 70 protein is transported to and accumulates on the surface of vFeLVenv-infected cells. Vaccinia virus replication and FeLV gp70 accumulation was also observed in cells of feline and murine origin, albeit at levels somewhat reduced from those in human cells. Toward the goal of developing a recombinant vaccinia virus as a live vaccine for feline leukemia disease in cats, immunogenicity studies were performed in cats and mice. These experiments yielded surprising results: although animals mounted a typical virus-neutralizing antibody response to the vaccinia virus vector, we were unable to detect antibodies against FeLV gp70 in any of the vaccinated animals. A subsequent 'booster' immunization with killed FeLV was unable to elicit evidence of immunologic 'priming' by the recombinant virus. We are presently unable to explain the apparent lack of immunogenicity. These results may point to complexities involved in the development of vaccines to protect against retrovirus infection.

Association of endogenous viral loci with genes encoding murine histocompatibility and lymphocyte differentiation antigens

Several polymorphic DNA restriction endonuclease fragments hybridizing with xenotropic and ecotropic envelope virus probes map adjacent to minor histocompatibility and lymphocyte (H/Ly) antigen-encoding loci. Viral DNA restriction fragments are associated with Ly-17 on chromosome 1, H-30, H-3, and H-13 on chromosome 2, Ly-21 on chromosome 7, H-28 on chromosome 3, and H-38 (chromosomal location as yet undetermined). In each case no recombinant can be found between the H/Ly locus in question and the virus-related restriction fragment, suggesting that linkage is very tight. Although some viral loci map to locations where no H/Ly has yet been mapped, the frequency and tightness of linkage in the seven instances described, coupled with the large number of as yet unmapped H/Ly loci, suggests that the associations found are significant.