Background:  HIV infection is characterized by a brisk immune activation that is felt to play an important role in the CD4 depletion and dysfunction of patients with AIDS. The mechanism underlying this activation is poorly understood but is tightly correlated with viral load. In the current study we tested the hypothesis that this activation is the net product of 2 distinct pathways: the inflammatory response to HIV infection and the homeostatic response to CD4 depletion.

Methods:  To test this hypothesis, ex vivo BrdU-labeling was performed on peripheral blood mononucleara cells (PBMC) from 283 patients with different stages of HIV infection. To analyze the effects of the homeostatic response to CD4 T cell depletion, patients where divided into groups based on CD4 T cell counts in low (<300 cells/µL) and high (>800 cells/µL). To analyze the effects of the viral load, the groups were further divided into those individuals whose viral load was high (HIV RNA >10,000) or low (HIV RNA <50). The size of the groups were:  CD4 low-viral load high (n = 38), CD4 high-viral load high (n = 12), CD4 low-viral load low (n = 42) and CD4 high-viral load low (n = 69). Serum samples from selected patients were tested for levels of gamma-common cytokines (interleukin [IL] -2, -7, and -15) and for levels of inflammatory cytokines tumor necrosis factor (TNF) -α, IL-5, etc. To determine the in vivo effects of gamma-common cytokines STAT-5 phosphorylation in PMBC was measured by flow cytometry.

Results:  Levels of in vivo BrdU incorporation ranged from 0.085 to 0.639% of BrdU⁺ CD4 T cells. A multivariate analysis correcting for CD4 T cell counts and viral load, revealed that both CD4 depletion and HIV RNA levels contributed substantially to driving CD4 T cell proliferation (r² = 0.20, 0.30 separately and 0.38 combined). In contrast, the turnover of CD8 T cells was mostly a result of the viral load (r² = 0.09, 0.33 separately and 0.35 combined). Both CD4 depletion and HIV RNA levels were independently associated with an increase in circulating levels of IL-7 (r² = 0.36, 0.27 and 0.46 combined). Further supporting the role of gamma-common cytokines in the increases of CD4 T cell turnover, in vivo levels of STAT-5 phosphorylation in PBMC were seen in cells from HIV⁺ comparing to HIV^– individuals (MFI: 506 vs 317, p = 0.018).

Conclusions:  Taken together these data suggest that immune activation in the setting of HIV infection is driven by both CD4 depletion and viral load. In the case of CD4 activation this effect is evenly split between these 2 forces; in the CD8 pool it is heavily weighted toward viral load.