Issue:Vaccines that induce T cell responses to simian immunodeficiency virus are able to reduce virus load in infected macaques. Such vaccines typically induce central memory T cells that must expand before gaining full antiviral functions. Picker and his colleagues show that a new replicating anti-SIV vaccine, based on the persistently infecting cytomegalovirus, which preferentially induces effector memory T cells in mucosal tissues, can reduce the likelihood that the macaques become infected in the first place ( pages 244–246 ). The rapid onset of massive, systemic viral replication during primary HIV or simian immunodeficiency virus (SIV) infection and the immune evasion capabilities of these viruses pose fundamental problems for vaccines that depend upon initial viral replication to stimulate effector T cell expansion and differentiation1,2,3,4,5. We hypothesized that vaccines designed to maintain differentiated effector memory T cell (TEM cell) responses5,6 at viral entry sites might improve efficacy by impairing viral replication at its earliest stage2, and we have therefore developed SIV protein-encoding vectors based on rhesus cytomegalovirus (RhCMV), the prototypical inducer of life-long TEM cell responses7,8,9. RhCMV vectors expressing SIV Gag, Rev-Tat-Nef and Env persistently infected rhesus macaques, regardless of preexisting RhCMV immunity, and primed and maintained robust, SIV-specific CD4+ and CD8+ TEM cell responses (characterized by coordinate tumor necrosis factor, interferon-γ and macrophage inflammatory protein-1β expression, cytotoxic degranulation and accumulation at extralymphoid sites) in the absence of neutralizing antibodies. Compared to control rhesus macaques, these vaccinated rhesus macaques showed increased resistance to acquisition of progressive SIVmac239 infection upon repeated limiting-dose intrarectal challenge, including four macaques who controlled rectal mucosal infection without progressive systemic dissemination. These data suggest a new paradigm for AIDS vaccine development—vaccines capable of generating and maintaining HIV-specific TEM cells might decrease the incidence of HIV acquisition after sexual exposure.

Effector memory T cell responses are associated with protection of rhesus monkeys from mucosal simian immunodeficiency virus challenge