Probes for TNF

Immunological characterization of the Fiebig-equivalent stages of SHIVAD8-EO infection showed that, despite the observed differences in progression of infection between the challenge groups, the timing of virus-specific CD8+ T cell responses, as well as the viral load and virus distribution in the LNs, was concordant between the groups when assessed by Fiebig-equivalent staging. LN SHIVAD8-EO RNA+ cells were initially detected in both follicular and extrafollicular areas and mostly preceding peak plasma viremia at Fiebig-equivalent stage II, in agreement with previous studies (31, 41-44). Their levels increased proportionally to viral load in plasma, and seemed to decrease at Fiebig-equivalent stage VI when strong LN SIV-specific CD8+ T cell responses were detected. These responses, which appeared to develop at later stages than in the peripheral blood of acutely HIV-1-infected subjects (45), were predominantly characterized by production of the highly proinflammatory cytokine TNF and expression of CD107a, indicative of degranulation. In addition, levels of fCD8+ T cells, previously shown to be highly cytolytic ex vivo and able to mediate killing of HIV-infected target cells in vitro (31, 46), were higher at Fiebig-equivalent stage VI than stage V. LN NK cell levels also transiently increased at Fiebig-equivalent stage V. Thus, our data are consistent with the notion that, after peak plasma viremia during Fiebig-equivalent stages V and VI of SHIVAD8-EO infection, cytotoxic immune responses develop in the LNs and may lead to lysis of virus-infected cells and release of virions. In fact, LN SHIVAD8-EO RNA+ virions were almost absent at Fiebig-equivalent stage II when SHIVAD8-EO RNA+ cells, likely producing virions (47), were already present but CD8+ T cell responses were undetectable. However, virions were highly abundant in the follicles during Fiebig-equivalent stage VI when CD8+ T cell responses were stronger. Although there was no significant correlation between the levels of LN SHIVAD8-EO RNA+ cells and the virus-specific function of LN CD8+ T cells in our study, possibly because of limited sample size and data variation between animals, previous studies reported in situ observation of CD8+ T cells expressing T cell intracellular antigen-1 (TIA-1) or perforin (48, 49) and increased activation of cytotoxic cells (50) in the LNs of HIV-1-infected subjects. Furthermore, CD8+ T cells with HIV- or SIV-specific killing activity in vitro were detected in the LNs, splenic white pulp, and tonsil GCs of HIV-1-infected subjects (46, 51), and in the LNs of SIV-infected NHPs (31, 52, 53). Also, NK cells were shown to accumulate in the LNs of African green monkeys during nonpathogenic SIV infection, where they played an important role in the control of viral replication (54).