Immunostaining for CD3+ T cell Figure 7 Immunostaining for CD3+ T cell. 7a: Spleen tissue from 2CLP rats serving as positive control for CD3+ T cell immunostainin 7b: left: minimal lymphocytic infiltration of the lung tissue in SO rat. Middle: moderate infiltration in a 2CLP lung rat after SV/luc administration. Right: moderate infiltration in a 2CLP lung rat not treated with SV/luc. Note that the degree of lymphocyte infiltration is similar in the middle and right panels, suggesting that it is part of the disease process rather than being induced by the vector. Role of receptor polymorphism and glycosylation in syncytium induction and host range variation of ecotropic mouse gammaretroviruses BACKGROUND: We previously identified unusual variants of Moloney and Friend ecotropic mouse gammaretroviruses that have altered host range and are cytopathic in cells of the wild mouse species Mus dunni. Cytopathicity was attributed to different amino acid substitutions at the same critical env residue involved in receptor interaction: S82F in the Moloney variant Spl574, and S84A in the Friend mouse leukemia virus F-S MLV. Because M. dunni cells carry a variant CAT-1 cell surface virus receptor (dCAT-1), we examined the role of this receptor variant in cytopathicity and host range. RESULTS: We expressed dCAT-1 or mCAT-1 of NIH 3T3 origin in cells that are not normally infectible with ecotropic MLVs and evaluated the transfectants for susceptibility to virus infection and to virus-induced syncytium formation. The dCAT-1 transfectants, but not the mCAT-1 transfectants, were susceptible to virus-induced cytopathicity, and this cytopathic response was accompanied by the accumulation of unintegrated viral DNA. The dCAT-1 transfectants, however, did not also reproduce the relative resistance of M. dunni cells to Moloney MLV, and the mCAT-1 transfectants did not show the relative resistance of NIH 3T3 cells to Spl574. Western analysis, use of glycosylation inhibitors and mutagenesis to remove receptor glycosylation sites identified a possible role for cell-specific glycosylation in the modulation of virus entry. CONCLUSION: Virus entry and virus-induced syncytium formation using the CAT-1 receptor are mediated by a small number of critical amino acid residues in receptor and virus Env. Virus entry is modulated by glycosylation of cellular proteins, and this effect is cell and virus-specific. The CAT-1 receptor mediates the entry of ecotropic gammaretroviruses into rodent cells. Virus properties that rely on receptor recognition such as host range or pathogenicity could potentially be affected by polymorphisms that alter the receptor or the receptor binding domain (RBD) of the virus. In previous studies we identified two unusual ecotropic mouse leukemia virus (MLV) variants [1, 2] . Both of these viruses have altered host range, both are cytopathic, and both have amino acid substitutions at the same site in their RBDs. Spl574 is a Moloney MLV (MoMLV) variant with the substitution S82F, and F-S MLV is a Friend MLV (FrMLV) variant with the substitution S84A. Both viruses cause the formation of large multinucleated syncytia on cells derived from the wild mouse species M. dunni two days after infection, and syncytium formation is accompanied by the accumulation of large amounts of unintegrated viral DNA [2] . These two viruses also differ from each other and from their respective parental MLVs in host range. Spl574 replicates efficiently only in M. dunni cells and very inefficiently in other mouse cells such as NIH 3T3 and SC-1 cells. F-S MLV shows no unusual pattern of infectivity in mouse cells, but is capable of infecting hamster cells that are normally resistant to ecotropic MLVs.