| Summary: | Cytotoxic CD8 T lymphocytes kill virus-infected cells and are critical for viral clearance from the body. Cytokines, particularly those sharing the common gamma receptor chain (gamma c), play
a key role in this cytotoxic function as well as in the growth, differentiation and homeostasis
of CD8 T lymphocytes. In order to exert these biological effects, cytokine-dependent signal
transduction via the Janus kinase (Jak) / Signal Transducers and Activators of Transcription
(STAT) pathway, the phosphoinositide 3-kinase (PI3-K) and mitogen-activated protein
kinase (MAPK) pathways is required. In HIV infection however, the CD8 T lymphocytes
become defective and are characterized by impaired cytotoxicity, altered differentiation
patterns, and increased susceptibility to apoptosis. I hypothesized that impaired cytokine
responsiveness resulting from defects in cytokine-dependent signal transduction contributes
to the CD8 T cell impairment observed in HIV+ patients. I investigated the activation of the
Jak/STAT signaling pathway to cytokines in CD8 T cells from HIV+ patients. Interestingly,
these cells were responsive to IL-2, IL-4, IL-10, IL-15, and IL-21 at the level of their
respective STAT activation. However, impairment of the IL-7 / IL-7Ralpha signaling axis was
identified and characterized by a defect in STAT5 signaling. The impaired STAT5
activation correlated with a low IL-7Ralpha surface expression. The expanded population of IL-
7Ralphalow-expressing CD8 T cells, found particularly in viremic HIV+ patients, expressed
higher levels of the transcriptional repressor Growth Factor Independent-1 (GFI1) compared
to their IL-7Ralphahigh counterparts. This prompted further investigations into the role of GFI1
in IL-7Ralpha regulation in primary human CD8 T cells as a model. Though silencing of GFI1
did not modulate basal IL-7Ralpha expression, exogenous overexpression negatively regulated
IL-7Ra surface levels. The gc cytokines, IL-2, IL-4, IL-7, and IL-15, but not IL-21, were
found to efficiently suppress IL-7Ralpha expression however, only IL-4 simultaneously
upregulated GFI1 expression. RNA interference studies targeting GFI1 in IL-4 stimulated
CD8 T cells established a specific role for GFI1 in sustaining the suppression of IL-7Ralpha
expression. Furthermore, transient downregulation of GFI1 in CD8 T cells subjected to IL-
4-dependent proliferation reduced their proliferative capacity. Other functions identified for
GFI1 were in the suppression of CXCR4 and Bax expression in CD8 T cells. Studies aimed
at identifying the signal transduction pathways responsible for regulating GFI1 and IL-7Ralpha
expression revealed that IL-4-mediated downregulation of IL-7Ralpha expression required
activation of the Jak/STAT and the PI3K pathways. On the other hand, IL-4-induced
upregulation of GFI1 expression was mediated via the PI3K pathway. The JNK and P38
MAPK pathways appeared to be important as regulators of basal IL-7Ralpha expression levels,
but had no statistically significant effects on GFI1 expression. To conclude, these studies
have clarified the important biological effects of GFI1 in mature human CD8 T lymphocytes.
Furthermore, exposure to IL-4 may generate CD8 T cell populations with an exhausted
phenotype similar to those found in chronically-infected HIV+ patients, characterized by
reduced cytotoxic activity and increased IL-4 production. Thus, the IL-4 study model may
prove valuable for investigating the activity of human CD8 T cells in such chronic diseases
and those characterized by a type 2 cytokine profile.
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