There are many viruses known to cause viral hemorrhagic fevers in humans. The mechanisms causing hemorrhage are likely to vary among viruses. Some viruses, such as Marburg virus, are directly cytopathic to infected endothelial cells, suggesting infection of endothelial cells alone can cause hemorrhage. On the other hand, there are viruses which infect endothelial cells without causing any cytopathic effects, suggesting the involvement of host immune responses in developing hemorrhage. Typical examples of these include viruses of the hantavirus species. We hypothesize that impairment of endothelial cell's defense mechanisms against cytotoxic CD8+ T cells is the mechanism of capillary leakage in hantavirus pulmonary syndrome and hemorrhagic fever with renal syndrome, which may be common to other viral hemorrhagic fevers. CD8+ T cells may be a potential target for therapy of some viral hemorrhagic fevers.

Hantavirus infection causes two human diseases, hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. The typical feature of these diseases is increased permeability in microvascular beds in the kidneys and the lungs, respectively. The mechanism of capillary leakage, however, is not understood. Some evidence suggests that hantavirus disease pathogenesis is immunologically mediated by cytotoxic T lymphocytes and other immune cells in target organs producing inflammatory cytokines. In this study we examined the roles of virus-specific cytotoxic T lymphocytes in increased permeability of human endothelial cells infected with hantavirus. We used a human CD8(+) hantavirus-specific cytotoxic T lymphocyte line, 1A-E2, specific for the HLA-A24-restricted epitope in Sin Nombre and Puumala virus G2 protein, and the human endothelial cell line, EA.hy926 that expresses HLA-A24 molecule. The cytotoxic T lymphocyte line recognized and lysed target cells infected with Sin Nombre virus, and in transwell permeability assays increased permeability of EA.hy926 cell monolayer infected with Sin Nombre virus or recombinant adenovirus expressing the Sin Nombre virus G2 protein. These results suggest that cytotoxic T lymphocyte activity contribute to capillary leakage observed in patients with hantavirus pulmonary syndrome or hemorrhagic fever with renal syndrome.

The efficiency of prime-boost vaccinations on the induction of T-cell responses to Sin Nombre virus nucleocapsid protein expressed by recombinant vaccinia virus, replication-deficient adenovirus, and plasmid DNA in mice was quantitated by the number of epitope-specific interferon-gamma-producing T cells and cytotoxic T-lymphocyte activity induced. In prime-boost immunizations, all combinations that included the recombinant adenovirus induced a much higher number of epitope-specific interferon-gamma-producing T cells than did other combinations. A single immunization of the recombinant adenovirus was able to induce similarly high levels of epitope-specific interferon-gamma-producing cells, despite the fact that the recombinant adenovirus produces less amount of the Sin Nombre virus nucleocapsid protein.

Sin Nombre virus (SNV) causes hantavirus pulmonary syndrome (HPS), with a high rate of mortality in humans who are infected by the transmission of virus from the natural rodent host. In humans, cytotoxic T lymphocytes (CTL) specific for SNV appear to play an important role in the pathogenicity of HPS. There is a correlation between the frequencies of SNV-specific CTLs and the severity of HPS disease. In order to create a mouse model to study the role of SNV-specific T cells in vivo, T cell responses to SNV nucleocapsid (N) protein in B6.PL Thy1(a)/Cy mice (H-2(b)) immunized with plasmid DNA or recombinant vaccinia virus expressing SNV N protein were examined. Four peptides, NC94-101, NC175-189, NC217-231 and NC331-345, were recognized by CD8(+) T cells in CTL and ELISPOT assays in SNV N-immunized mice. Interestingly, two of these epitopes are located in the central region of the SNV N protein, where several human CD8(+) T-cell epitopes have been defined in Puumala virus and SNV. CTL lines specific for these four epitopes were cross-reactive to corresponding Puumala virus peptides, but only one of them was cross-reactive to Hantaan virus peptides. These results will enable the analysis of the roles of CTL in immunopathology of HPS in experimental mouse models of HPS.