Various parameters of HIX virus were examined to determine its origin and its relationship to other murine C type oncornaviruses. Envelope properties of HIX virus grown in cells of several species were subjected to analyses of host range, interference, and neutralization. Cloned amphotropic HIX virus was adapted to grow in human RD cells. After 6 months in culture, the resulting virus (HIX-RD) could enter mouse cells but essentially lost the capacity of propagating in mouse cells. Interference patterns of HIX and HIX-RD were identical to each other and unrelated to murine ecotropic MuLV interference. MSV(HIX) or MSV(HIX-RD) could not penetrate HIX-, HIX-RD-, or MuX-preinfected cells. However, infection with HIX exhibited a unique one-way interference in that MSV(MuX) could penetrate and transform HIX-preinfected cells. Neutralization of HIX and HIX-RD with relatively type-specific anti-gp70 sera showed that they resembled Moloney (M)-MuLV most closely. Significant neutralization was observed also with anti-Rauscher gp70 or BALB-2 MuX gp70 sera. Both HIX derivatives were acutely susceptible to inactivation with normal mouse sera, a characteristic of xenotropic viruses. Competition radioimmunoassays were performed to determine the antigenic relationship of HIX to other MuLV types. The highly type-specific phosphorylated p12 and the relatively type-specific gag region p15 of HIX were found to be identical to M-MuLV and less related to other murine C-type oncornaviruses. The examination of HIX gp70 with type-specific anti-M-MuLV or anti-C57L MuX gp70 sera showed that it was clearly different from either virus. Tryptic peptide maps of the gag region-p15 and p30 of HIX were identical to corresponding maps of M-MuLV proteins. The gp70 of HIX was unique and different from known eco-, xeno-, and amphotropic murine C type oncornaviruses. Based on known migration patterns of characteristic trypsin- and chymotrypsin-derived peptides of various eco-, and xenotropic MuLV's, it was concluded that gp70 of HIX was related to both MuX and M-MuLV. Tryptic fingerprint maps also revealed several significant differences between parental HIX and its HIX-RD variant. Comparative hybridizations of assorted high-molecular-weight (HMW) virus RNAs with complementary DNA from HIX virus showed that, with unfractionated probes, no significant differences could be seen between HIX and M-MuLV. Based on the above, HIX virus appears to contain predominantly M-MuLV-specific information except for its envelope gene which has been presumably derived from a recombinational event involving corresponding M-MuLV and MuX nucleotide sequences.
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