Identification of genetic determinants responsible for the rapid immunosuppressive activity and the low leukemogenic potential of a variant of Friend leukemia virus, FIS-2.

An immunosuppressive variant of Friend murine leukemia virus (F-MuLV), FIS-2, induces suppression of the primary antibody response against sheep erythrocytes (SRBC) in adult NMRI mice more efficiently than the prototype F-MuLV clone 57 (cl.57). It is, however, less potent than F-MuLV cl.57 in inducing erythroleukemia upon inoculation into newborn NMRI mice. Nucleotide sequence analysis shows a high degree of homology between the two viruses. Single point mutations are scattered over both the gag and the env encoding regions. The most notable mutations are the deletion of one direct repeat and a few single point mutations occurring in the binding sites for cellular transcriptional factors in the FIS-2 long terminal repeat region (LTR). To define the genetic determinants responsible for the pathogenic properties of FIS-2, we constructed six chimeras between FIS-2 and F-MuLV cl.57. Adult mice were infected with the chimeras, and their primary antibody responses against SRBC were investigated. The results showed that the fragment encompassing the FIS-2 env encoding region SU is responsible for the increased immunosuppressive activity in adult mice. A leukemogenicity assay was also performed by infecting newborn mice with the chimeras. Consistent with the previous studies, it showed that the deletion of one direct repeat in the FIS-2 LTR is responsible for the long latent period of erythroleukemia induced by FIS-2 in newborn-inoculated mice. However, studies of cell type-specific transcriptional activities of FIS-2 and F-MuLV cl.57 LTRs using LTR-chloramphenicol acetyltransferase constructs showed that the deletion of one direct repeat does not reduce the transcriptional activity of the FIS-2 LTR. The activity is either comparable to or higher than the transcriptional activity of the F-MuLV cl.57 LTR in the different cell lines that we used, even in an erythroleukemia cell line. It seems that the high transcriptional strength of the FIS-2 LTR is not sufficient to give FIS-2 a high leukemogenic effect. This suggestion is inconsistent with the previous suggestion that the transcriptional strength of an LTR in a given cell type is correlated with the leukemogenic potential in the corresponding tissue. In other words, these data indicate that the direct repeats in the F-MuLV LTR may play other roles besides transcriptional enhancer in the leukemogenesis of F-MuLV.

Molecular cloning and characterization of an immunosuppressive and weakly oncogenic variant of Friend murine leukemia virus, FIS-2.

The FIS variant is a weakly leukemogenic, relatively strong immunosuppressive murine retrovirus which was isolated from the T helper cells of adult NMRI mice infected with Friend murine leukemia virus (F-MuLV) complex (FV). Unlike FV, it does not induce acute erythroleukemia but retains the immunosuppressive property of FV and induces suppression of the primary antibody response rapidly and persistently in adult mice. A previous study showed that the FIS variant contains two viral components, a replication-competent virus and a defective virus. In this study, we have biologically purified the FIS variant by end point dilution and we show that the replication-competent virus FIS-2 alone can induce immunosuppression as the parental FIS variant. Most newborn mice infected with FIS-2 developed erythroleukemia, but with an increased latency period compared with that of F-MuLV clone 57. In contrast, FIS-2 induced suppression of the primary antibody response and disease more rapidly than F-MuLV clone 57 in immunocompetent, adult mice. FIS-2 was further molecularly cloned and characterized. Restriction mapping and nucleotide sequence analysis of FIS-2 showed a high degree of homology between FIS-2 and F-MuLV clone 57, suggesting that FIS-2 is a variant of F-MuLV. The striking difference is the deletion of one of the tandem repeats in the FIS-2 long terminal repeat and the single point mutation in the binding sites for core-binding protein and FVa compared with the long terminal repeat of F-MuLV clone 57. Two single point mutations led to the appearance of two extra potential N glycosylation sites in the FIS-2 gag-encoded glycoprotein. Together, the results suggest that FIS-2 represents an interesting murine model to study retrovirus-induced immunosuppression on the basis of its unique combined property of low leukemogenicity and relatively strong and persistent immunosuppressive activity in adult mice.