Immune activation and viral burden in acute disease induced by simian immunodeficiency virus SIVsmmPBj14: correlation between in vitro and in vivo events.

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The simian immunodeficiency virus SIVsmmPBj14 (SIV-PBj14) is an atypical lentivirus that causes acute disease and death in pig-tailed macaques and in vitro replicates efficiently in resting macaque lymphocytes and activates and induces proliferation of lymphocytes. The present study was conducted to test the hypothesis that production of large quantities of SIV-PBj14 induces widespread immune activation and elaboration of cytokines which lead directly to the death of infected pig-tailed macaques. Following intravenous inoculation of pig-tailed macaques with SIV-PBj14, acute disease developed and was characterized by high levels of plasma viremia, p27gag antigenemia, tumor necrosis factor alpha, and interleukin-6 (IL-6). All animals died within 10 days of infection, at which time some animals had as many as 100% CD4+ cells in the periphery and lymphoid tissues infected. During the last few days before death, titers of infectious virus in blood increased as much as 10(5)-fold. By using dual-label immunofluorescence assays for detection of cell surface activation markers, both CD4+ and CD8+ lymphocytes were shown to express the IL-2 and transferrin receptors following either in vivo or in vitro infection with SIV-PBj14. Furthermore, in vitro infection of quiescent macaque lymphocytes by SIV-PBj14 was accompanied by proliferation of both CD4+ and CD8+ lymphocyte subsets, as measured by incorporation of [3H]thymidine. Increases in numbers of activated lymphocytes and levels of proinflammatory cytokines in plasma coincided with increased amounts of detectable virus in vivo. Clinical signs of disease and pathologic findings were most consistent with death from a shock-like syndrome, in which acute-phase inflammatory cytokines are known to play a major role. Tumor necrosis factor alpha, IL-2, and IL-6 were detected in some cultures infected with SIV-PBj14, but this finding was not consistent. When cytokines were detected, their concentrations were essentially no different from those found in control cultures infected with SIVsmm9, a prototypic strain from which SIV-PBj14 was derived. The in vivo results suggest a synergistic cycle of activation of lymphocytes and monocytes, elaboration of cytokines, and virus production that accelerates uncontrolled and culminates in death. The observed correlations between in vivo and in vitro activation events following SIV-PBj14 infection validate the use of in vitro studies to clarify lentivirus-lymphocyte interactions that may contribute to the virulence of SIV-PBj14.

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