Monkeypox virus (MPV) is a virulent human pathogen that has gained increased attention because of its potential use as a bioterrorism agent and inadvertent introduction into North America in 2003. The US outbreak also provided an important opportunity to study MPV-specific T cell immunity. Although MPV-specific CD4 ؉ and CD8 ؉ T cells could recognize vaccinia virus (VV)-infected monocytes and produce inflammatory cytokines such as IFN␥ and TNF␣, they were largely incapable of responding to autologous MPV-infected cells. Further analysis revealed that, unlike cowpox virus (CPV), MPV did not interfere with MHC expression or intracellular transport of MHC molecules. Instead, MPV-infected cells were capable of preventing T cell receptor (TcR)-mediated T cell activation in trans. The ability to trigger a state of nonresponsiveness represents a unique MHC-independent mechanism for blocking antiviral T cell activation and inflammatory cytokine production and is likely an important attribute involved with viral dissemination in the infected host. Although MPV does not spread efficiently by human-to-human contact (12)(13)(14), it serves as an important model for smallpox (15-18) and shares several key features of pathogenesis. For instance, unlike vaccinia (VV) (19), both VAR and MPV disseminate through their infected hosts mainly by a cell-associated viremia (15,(20)(21)(22)(23). Moreover, evasion of host immune responses is well documented; VAR infection of previously vaccinated humans and MPV infection of non-human primates can result in infectious virus persisting for prolonged periods of time as an asymptomatic infection in apparently healthy individuals (24-32).The mechanisms underlying these forms of immune evasion are not well understood. Many viruses employ a battery of immune evasion strategies (33-38) and poxviruses in particular are equipped to evade antiviral cytokines, chemokines, and/or antigen presentation (35, 39). We have shown that cowpox virus (CPV) interferes with intracellular transport of MHC class I, a process that correlated with evasion of antiviral CD8 ϩ T cell responses by CPV (40). It was recently demonstrated that CPV open reading frame 203 retains MHC class I in the ER (41), and, because MPV encodes a close homologue of CPV203, we expected to find a similar mechanism of immune evasion by MPV. In contrast, we observed that MPV did not down-regulate MHC class I, but instead used a mechanism of evasion that inhibited CD4 ϩ and CD8 ϩ T cell activation after cognate interactions with MPV-infected cells. This mechanism of abrogating local T cell responses may avoid systemic immune suppression, while at the same time protecting the viral reservoir from immune surveillance. Identification of the factor or factors involved with MPV-induced T cell inhibition could prove useful for developing new biologics aimed at preventing or alleviating T cell-mediated diseases (42, 43).
Results
Antiviral CD4 ؉ and CD8 ؉ T Cells Recognize VV-Infected Monocytes butNot MPV-Infected Monocytes. Several human HLA-bindi...