FrC6 murine leukemia virus (MuLV) is a replication-competent, neuropathogenic variant derived from Friend MuLV (F-MuLV) complex. The A8 virus (a molecular clone of the FrC6 virus) induced marked spongiform degeneration in the brain similar to the FrC6 virus, but only mild lesions were found in the spinal cord. In contrast, PVC211 virus, which is also a neuropathogenic F-MuLV variant, caused marked spongiform degeneration in the spinal cord. Virus recovery from the spinal cord of A8 virus-infected rat was the same as that of PVC211-infected rat, indicating that there is no direct correlation between the titer of virus and the intensity of lesions. Furthermore, rats infected with the A8 virus at 3 weeks of age did not undergo spongiform degeneration, although recovery of high titer of virus occurred in the central nervous system (CNS). Studies using chimeric viruses between the A8 virus and nonneuropathogenic F-MuLV clone 57 also indicated that the sequences responsible for virus titers in the CNS and neuropathogenicity are different. The chimeric virus studies proved that the env gene and the LTR and/or 5' leader sequence of A8 are critical for the induction of neuropathogenicity. These sequences in A8 and PVC211 were compared, focusing in on the sites that account for neurovirulence and viral lesional tropism.

Friend murine leukemia virus (Fr-MLV) clone A8 causes spongiform neurodegeneration in the rat brain. The A8-env gene is a primary determinant of neuropathogenicity, and the 1.5-kb ClaI-HindIII fragment containing the LTR and 5' leader from A8 are additionally required for spongiosis. After replacement of the A8 enhancer region of the neuropathogenic chimera with the enhancer region of non-neuropathogenic 57, viral titer in the brain was reduced by two orders of magnitude. However, the A8 enhancer region was not responsible for the induction of spongiosis. The region responsible for neuropathogenesis was located in the 0.3-kb KpnI-AatII fragment of A8 containing the R-U5-5' leader. The chimeric virus possessing this 0.3-kb fragment of A8 and the A8-env in the 57 background induced a high rate of spongiform neurodegeneration within 7 weeks (9/9 of infected rats). Studies using cultured cells suggest that the 0.3-kb fragment influences the expression of Env protein. Furthermore, these neuropathogenic chimerae, despite low viral replication in the brain, exhibited a stronger expression of Env protein compared with that of non-neuropathogenic viruses.

Influenza viruses recognize sialoglycans as receptors. Although viruses isolated form chickens preferentially bind to sialic acid α2,3 galactose (SAα2,3Gal) glycans as do those of ducks, chickens were not experimentally infected with viruses isolated from ducks. A chicken influenza virus, A/chicken/Ibaraki/1/2005 (H5N2) (Ck/IBR) bound to fucose-branched SAα2,3Gal glycans, whereas the binding towards linear SAα2,3Gal glycans was weak. On the epithelial cells of the upper respiratory tracts of chickens, fucose-branched SAα2,3Gal glycans were detected, but not linear SAα2,3Gal glycans. The growth of Ck/IBR in MDCK-FUT cells, which were genetically prepared to express fucose-branched SAα2,3Gal glycans, was significantly higher than that in the parental MDCK cells. The present results indicate that fucose-branched SAα2,3Gal glycans existing on the epithelial cells lining the upper respiratory tracts of chickens are critical for recognition by Ck/IBR.

The cleavage products of the spike (S) protein, the S1 and $2 subunits, of the highly neurovirulent murine coronavirus (MHV) JHMV cl-2 variant were identified by immunoprecipitation of virus-infected cell lysates after treatment with urea and 2-mercaptoethanol. By this method 14 monoclonal antibodies (MAbs) raised against the S protein of the cl-2 variant were revealed to react with the S 1 subunit and one with the $2 subunit. These 14 MAbs were classified into the following three groups: (A) MAbs reactive to almost all MHV strains examined, (B) MAbs specific for the JHMV strain and (C) MAbs specific for a large S protein of the JHMV strain. All five MAbs classified in group B showed neutralization activity and four of them also showed fusion inhibition activity. Four of six MAbs in group C showed neutralizing activity to the cl-2 variant but not to the sp-4 variant, and most of them had no fusion inhibition activity. Western blot analyses showed that all of the MAbs, except for no. 2 in group A, failed to react with the denatured S and S1 proteins. All MAbs in groups A and C, with the exception of no. 19 in group A, reacted with the mildly denatured S proteins, whereas none of the MAbs in group B did. These results suggest that MAbs in group B recognized highly conformational epitopes which may be involved in the binding ofvirions to cellular receptors and the fusion activity of the virus.Coronaviruses are enveloped viruses with a genomic ssRNA of positive polarity. On the surface of coronavirus virions are morphologically characteristic projections composed of the spike (S) protein. It is known that the S protein is N-glycosylated and varies in M r from 150K to 200K, depending upon the virus strain (Siddell et al., 1983 ; Spaan et al., 1988). The S proteins of various coronaviruses are believed to be cleaved around the middle of the protein by a host cell-derived trypsin-like protease, resulting in the S 1 and $2 subunits containing the N terminus and C terminus of the precursor protein, respectively (Spaan et al., 1988). This cleavage event is considered to be important for the manifestation of biological functions resident in the S protein. In particular, fusion formation in cells infected with coronaviruses has been reported to be related to the cleavage of the S protein (Frana et al., 1985; Sturman et al., 1985). However, it has recently been shown that cleavage of the S protein is not a prerequisite for fusion formation in murine coronaviruses (Stauber et al