SLAM expression is not downregulated by measles virus infection

It has been reported that the signaling lymphocyte-activation molecule (SLAM), the second receptor of Measles virus (MV) on the cell surface appears to be downregulated by MV infection or by expression of MV hemagglutinin (H) (Tanaka et al., 2002; Welsteadt et al., 2004). The aim of this study was to analyze this phenomenon in more detail using a Chinese vaccine strain (S191) of MV. Flow cytometry confirmed apparent downregulation of SLAM in the virus-infected cells and in the cells transfected with a plasmid expressing viral H. Moreover, a similar effect was obtained by incubation of the cells with UV-inactivated virus or soluble viral H. Real-time quantitative PCR showed that the SLAM mRNA level remained stable during the virus infection, and Western blot analysis demonstrated that the SLAM content of total membrane proteins did not change change after the virus infection. Thus we conclude that SLAM expression is stable during the MV infection and that its apparent downregulation reported earlier and confirmed also in this study was just the result of masking of the antibody recognition sites on SLAM with MV H during the flow cytometry assay.