Differential expression of two distinct xenotropic viruses in NZB mice

Two distinct xenotropic viruses have been isolated from New Zealand Black (NZB) mice. One of these viruses (NZB-X1) is spontaneously produced by NZB and NZB hybrid mice. The second xenotropic virus (NZB-X2) is induced by treatment of NZB fibroblasts with IdU. NZB-X1 is distinct in that it possesses a unique surface glycoprotein (gp70) similar to gp70 found free in the serum of all mouse strains tested, but only associated with a virion in NZB mice. NZB-X2 has a gp70 similar to that of xenotropic viruses isolated from mouse strains other than NZB, including xenotropic viruses of NIH Swiss, AKR, NZW, C57BL6BALBc, and wild mice, although NZB-X2 gp70 shows more structural relatedness to NIH Swiss and wild mouse xenotropic gp70 than to the others. By backcrossing NZB to NFS Swiss mice, two independently segregating loci have been described which control high and low xenotropic virus expression. Analyses of virus isolates from mice possessing these segregated loci showed that the high expression locus is linked to expression of the spontaneously released NZB virus, NZB-X1. The presence of the low expression locus corresponds with the production of the second xenotropic virus released after IdU treatment, NZB-X2. Using unique structural markers for these two xenotropic viruses, we have examined the phenomena of differential expression of endogenous virus and viral antigens within the NZB mouse. We find, as reported previously, that the predominant gp70 of serum is similar to NZB-X1. However, gp70s expressed on thymus and spleen carry the determinants of NZB-X2. The expression of these gp70s was not a peculiarity of NZB mice, since others tested including C57BL6, NZW, and 129 mouse strains showed the same tissue segregation. The differential expression of these xenotropic virus gp70s noncoordinate with virus production points to differential expression of endogenous envelope genes along different differentiation pathways.