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CUTTING EDGE |



*
Experimental Immunology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD;
Intramural Research Support Program-Scientific Applications International Corporation, Frederick, Laboratory of Molecular Immunoregulation, National Cancer Institute, Frederick Cancer Research and Development Center, Frederick, MD;
HIV Unit, Wilford Hall Medical Center, Lackland Air Force Base, TX.
| Abstract |
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were observed in all
patients. The reduction in STATs seen in vivo and in vitro after HIV
infection may contribute to the loss of T cell function in HIV disease. | Introduction |
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A second factor that could account for the impairment of immune function in HIV-infected individuals is the profound alteration in cytokine secretion patterns seen in vivo and in vitro after HIV infection (11, 12, 13). The cytokines that a cell secretes typically depend upon the signals it receives from other cytokines in the medium. Most cytokines transduce biologic signals from cell surface receptors to the nucleus through the JAK-STAT signaling pathway (14, 15, 16). JAK kinases constitutively associate with the intracellular domains of cytokine receptors, and ligation with cytokine leads to JAK activation. STAT transcription factors are recruited to the activated cytokine receptor-JAK kinase complex where they are tyrosine phosphorylated. The phosphorylated STATs then translocate to the nucleus where they promote transcription of genes that control cell growth, differentiation, and maintenance of cellular homeostasis. Seven STAT genes, STATs 14, two closely related isoforms of STAT5, A and B, and STAT6 have been identified to date (16, 17). Of all the STATs, STAT5 is used most extensively in the signaling pathways of immunologically relevant cytokines, including IL-2, -3, -5, -7, -9, -15, and granulocyte-macrophage (GM)-CSF (18). Recently, we demonstrated that STAT5A and STAT5B protein and message were strongly depressed in tumor-bearing mice, while the expression of other members of the STAT family were unaltered (19). The selective reduction of STAT5 was associated with a loss in T and B cell function, suggesting that the STAT5 signaling pathway might play a crucial role in the tumor-dependent immunodeficiency. These observations raised the possibility that STAT down-regulation might also occur in other immunosuppressive conditions.
Therefore, in the current report we studied the expression of STAT proteins in T cells from normal donors following in vitro infection with HIV-1 and in T cells from HIV-infected patients. We show that in vitro HIV-1 infection can directly induce a selective down-modulation of STAT5B and that T cells from HIV-infected individuals have decreased expression of both STAT5A and -B.
| Materials and Methods |
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PBMC were prepared from buffy coats from normal National Institutes of Health Blood Bank donors by Ficoll/Hypaque separation (20). PBL from HIV-infected patients were isolated by Ficoll-Hypaque separation and plastic adherence (21). CD3+ T cells were enriched from PBL by negative selection using a mAb mixture plus complement (Lymphokwik, One Lambda Inc., Los Angeles, CA). Patients were classified according to the Walter Reed guidelines for the staging of HIV-infected individuals (22). The use of human blood samples in these studies was approved by the Institutional Review Boards of both institutions.
Viral stocks
Two HIV-1 isolates were used in this study. HIV-1BZ167 was grown in human PHA blasts, as previously described (23). HIV-1Ba-L was grown in monocyte-derived macrophages. Cell-free supernatants were titered for p24 core Ag by ELISA (AIDS Vaccine Program, NCI, Frederick, MD).
HIV infection
PBMC (2 x 106/ml) were stimulated with 2 µg/ml PHA (Sigma, St. Louis, MO) for 3 days at 37°C, and washed twice. PHA blasts (105/sample) were incubated for 2 h with HIV-1BZ167 or HIV-1Ba-L (172 and 570 tissue culture ID50, respectively) and washed three times. Infected cells (1 x 106/ml) were incubated 3 or 8 days in medium supplemented with 10 U/ml of recombinant IL-2 (Boehringer Mannheim, Indianapolis, IN). Medium in the 8-day cultures was replaced 4 days after infection. HIV-1 infection was followed by ELISA for HIV-1 p24.
Immunoprecipitations
The immunoprecipitation procedure has been described previously (19). Briefly, 5 x 106 cells/group were incubated for 1 h at 4°C in Triton X-100 (1%) lysing buffer and spun. Supernatants were incubated with anti-STAT polyclonal Ab and adsorbed with protein A-Sepharose beads (Pharmacia, Piscataway, NJ). Adsorbed proteins were analyzed by SDS-PAGE under reducing conditions on a 7.5% homogeneous PhastGel (Pharmacia) followed by Western blotting with the same Ab used for immunoprecipitation. Proteins were detected by enhanced chemiluminescence.
| Results |
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We previously showed that immunosuppression in tumor bearing mice
was accompanied by a loss in STAT5 expression (19). To determine
whether HIV infection would elicit a similar effect, PHA blasts from
healthy, HIV-negative donors were infected with the primary HIV-1
isolate, BZ167, and STAT expression was assessed by immunoprecipitation
and Western blotting 3 and 8 days after infection (Fig. 1
). By day 8, when viral replication was
well advanced (Table I
), STAT5B
expression was strikingly reduced. By contrast, STATs 1, 2, 3 and 5A
were not significantly altered. No loss in STAT5B was observed 3 days
after infection when viral replication was significantly less,
indicating that STAT5B modulation occurred late in the infection cycle.
Because gp120 and tat proteins have been found to affect immune
function (24, 25, 26, 27), we tested whether culturing PHA blasts with either
of these HIV-1 proteins would alter STAT5A or B expression. A mixture
of recombinant gp120 and tat had no effect on STAT5A or B expression,
and similar negative results were obtained when the cells were cultured
for 3 or 8 days with either tat or gp120 alone (data not shown).
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The capacity of the dual-tropic HIV-1 strain BZ167 to
down-modulate STAT5B was compared with that of the M-tropic strain
Ba-L. HIV-1BZ167 isolate (Fig. 2
A), which uses
predominantly CXCR4 and CCR3 coreceptors (D. Cohen, unpublished
observation), markedly decreased STAT5B expression but had no effect on
STAT1 or STAT5A, consistent with the results presented above. By
contrast, the Ba-L M-tropic HIV-1 strain (Fig. 2
B),
which uses primarily CCR5 as coreceptor (28), had little effect on the
expression of any of the three STATs examined. HIV-1BZ167
was significantly more virulent than HIV-1Ba-L (Table I
),
suggesting that the difference between the two strains in blocking
STAT5B expression was due to differences in their abilities to infect
the cells.
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The capacity of HIV-1BZ167 to decrease STAT5B
expression in infected PHA blasts raised the possibility that STAT
levels might also be reduced in the T cells of HIV-1-infected donors.
Purified T cells from 5 HIV-positive patients exhibiting a range of CD4
counts (Table II
) were examined for the
levels of expression of STAT5 proteins. As seen in Figure 3
, both STAT5A and STAT5B were markedly
reduced in all 5 of the HIV-positive patients (P1 through P5), as
compared with 2 HIV-negative (N1 and N2) control donors studied
simultaneously. By contrast, STAT3, STAT6, and the lower m.w. splice
variant of STAT1 (STAT1ß) showed near normal levels of protein
expression in all donors. Interestingly, the higher m.w. variant,
STAT1
, appeared to be absent in the cells from the infected
patients. No quantitative correlation between the reduction in STAT
protein and either the stage of the disease or therapy was observed in
this small group of patients. Further studies in an extended group of
patients will be necessary to elucidate the relationship between
clinical characteristics and alteration in STAT expression. Reduced
expression of STAT5A, STAT5B, and STAT1
in patients T cells
differed from in vitro HIV-1-infected cultures in which the reduction
was limited to STAT5B.
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| Discussion |
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The decrease in STAT5 and STAT1
expression in purified T cells from
HIV infected patients was remarkable because it occurred in all five
patients examined (Fig. 3
), although these patients differed with
respect to their disease stages and CD4+ T cell counts
(Table II
). In contrast to in vitro infection of PHA blasts, STAT5A,
STAT5B and STAT1
were all reduced in patients T cells. The loss of
these STATs was selective because no consistent change was detected in
the expression of STAT1ß, STAT3, and STAT6. Our preliminary finding
that STAT1
but not -ß is lost in HIV+ patients could
have profound immunologic consequences because the
and ß forms of
STAT1 are splice variants, with STAT1ß acting as a dominant negative.
Thus the loss of the
form of STAT1 could lead to abrogation of
STAT1 function, which in deficient mice produces impaired innate
immunity to viral infections (15). Since most peripheral blood T cells
are not infected in HIV+ patients (31), it is unlikely that
the loss in STAT5 and STAT1
seen in lysates from patients T cells
was due to a direct effect of the virus on STAT expression. A more
likely mechanism is that HIV-1 induced the secretion of one or more
factors that resulted in STAT down-regulation.
Decreased STAT5 expression has now been observed in two unrelated diseases that cause immunosuppression, HIV disease in humans and experimentally induced mammary carcinoma in mice (19). The role that STAT5 plays in the impairment of immune function in these diseases is not known. However, because STAT5 has a broad spectrum of cytokine usage, its loss would be expected to strongly influence the immune system. The present study is the first to suggest that regulation of STAT5 may be involved in modulating cytokine profiles and immune function in HIV-infected patients. This study, as well as the previous report demonstrating STAT5 decreases in tumor-bearing mice (19), suggests that the regulation of STAT signaling pathways may provide a molecular mechanism that the immune system uses to control its level of activity.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. David Segal, Building 10, Room 4B17, National Institutes of Health, Bethesda, MD 20892. E-mail address: ![]()
Received for publication August 13, 1997. Accepted for publication October 28, 1997.
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