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We compared the amino acid sequences of groups of receptor (CD46) downregulating and nondownregulating measles virus (MV) hemagglutinins (Hs) and identified seven group-specific differences as candidates for the mediation of the observed differential effects. Using site-directed mutagenesis, we mutated the chosen amino acids of the H of MV-strain WTF (WTF-H), a nondownregulating H, and Introduced the corresponding amino acids of Edmonston-H (Edm-H), a downregulating H. We identified four amino acids, 211G, 243R, 451V, and 481Y, which influenced the downregulative function when introduced into WTF-H. The double mutation 451V and 481Y in WTF-H led to a degree of CD46 downregulation comparable to that of Edm-H. Conversely, introducing amino acids 451E and 481N into Edm-H resulted in a loss of the downregulative function. These results indicate that these amino acids play a decisive role in the H-CD46 interaction.

Measles virus infection of the central nervous system in the murine model of experimental measles virusinduced encephalitis is successfully controlled by virus-specific T-helper lymphocytes. T cells from BALB/c mice that are resistant to measles virus encephalitis proliferate well against measles virus in vitro, and bulk cultures recognize viral nucleocapsid and hemagglutinin as well as fusion proteins. The measles virus-specific T cells secrete large amounts of interleukin 2 (IL-2), gamma interferon (IFN-␥), and tumor necrosis factor alpha (TNF-␣) but no IL-4, IL-6, or IL-10, and hence the cytokine pattern is consistent with that of subtype 1 T-helper lymphocytes. In contrast, cells obtained from measles virus-infected susceptible C3H mice recognize measles virus proteins only weakly and secrete little IFN-␥ and TNF-␣. Treatment of infected mice with anti-TNF-␣ antibodies has no effect on survival or virus clearance from the brain. Upon neutralization of IFN-␥ in vivo, the phenotype of measles virus-specific T-helper cells isolatable from BALB/c mice is reversedfrom subtype 1 to subtype 2-like. Anti-IFN-␥ antibody-treated BALB/c mice are susceptible to measles virus encephalitis, and viral clearance from the central nervous system is impaired. These results indicate that IFN-␥ plays a significant role in the control of measles virus infection of the central nervous system.

In measles virus (MV) infection in humans, meninigitis and encephalitis are important complications. However, little is known of the pathogenesis of MV encephalitis, in particular about the role of the immune response. We have examined the role of cytotoxic T lymphocytes (CTL) in a mouse model of MV-induced encephalitis. We report here that the resistance of inbred strains of mice to MV-induced encephalitis correlated with the major histocompatibility complex (MHC) haplotype and that only resistant mouse strains mounted an effective CTL response to MV. Mice with low susceptibility to MV infection, such as the BALB/c strain (H-2"), generated CTL, whereas the highly susceptible strains, C3H (H-2k) and C57BV6 (H-2?), revealed very poor CTL responses. MV-induced CTL were usually CD8+, and the generation of these cells was independent of the route of inoculation or the time postinfection. CD4+ T cells were generally only weakly Iytic. The nudeocapsid protein was the major target antigen for CTL in BALB/c mice, although in some experiments the hemagglutinin was also recognized. CTL from C3H and C57BIJ6 mice did not lyse MV-infected target cells. However, targets infected with vaccinia virus recombinants expressing the nucleocapsid protein or bemagglutinin were lysed, but levels of cytotoxicity were still low. Experiments using target cells transfected with single MHC class I genes suggested inefficient antigen presentation of MV proteins by the MHC molecules of the H-2 and H-2? haplotypes.

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Lewis rats immunized with recombinant vaccinia virus expressing the nucleocapsid (N) protein of measles virus were protected from encephalitis when subsequently challenged by intracerebral infection with neurotropic measles virus. Immunized rats revealed polyvalent antibodies to the N protein of measles virus in the absence of any neutralizing antibodies as well as an N protein-specific proliferative lymphocyte response. Depletion of CD8+ T lymphocytes did not abrogate the protective potential of the N protein-specific cell-mediated immune response in rats, while protection could be adoptively transferred with N protein-specific CD4+ T lymphocytes. These results indicate that a CD4+ cell-mediated immune response specific for the N protein of measles virus is sufficient to control measles virus infections of the central nervous system.