A thermoresistant strain of feline immunodeficiency virus (FIV) with an altered cytopathic effect and a restricted cell tropism

A thermoresistant strain, designated m41, of feline immunodeficiency virus (FIV) was selected after 31 successive passages of chronically infected IRC4 cells at 41 °C. The wild-type virus (wt) which served as a control was cultivated the same number of times at 37 °C. In Crandell feline kidney cells (CrFK), the replication of m41 was similar at 37 °C and 41 °C, whereas wt multiplied only at 37 °C. Furthermore, m41 was more resistant than the wt strain at temperatures ranging from 37 to 56 °C. Syncytia formation was observed with m41 when the CrFK were incubated at 41 °C whereas neither m41 nor wt produced syncytia at 37 °C. The level of replication of wt and m41 on feline lymphoid primary cells at 37 °C was similar. In contrast to wt, m41 was unable to infect bone marrow macrophages. Since one or several mutations in the envelope (env) gene could be involved in changes of cell fusion properties and of cellular tropism, the nucleotide sequence of the env gene derived from wt and m41 respectively was determined. Ten mutations were found in the env gene of m41, thus leading to 9 amino acid modifications in the envelope glycoproteins. These results suggest that structural modifications of the viral envelope proteins are prerequisites for the replication of a thermoresistant FIV strain at elevated temperature and are correlated with the newly acquired viral phenotype.