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BIOBEHAVIORAL MECHANISMS |
From the Department of Medicine, Division of Hematology-Oncology, UCLA School of Medicine, the Norman Cousins Center, Jonsson Comprehensive Cancer Center, UCLA AIDS Institute, and UCLA Molecular Biology Institute, Los Angeles California.
Address correspondence and reprint requests to Steve W. Cole, PhD, 11-934 Factor Building, UCLA School of Medicine, Los Angeles, CA 90095-1678. E-mail:coles{at}ucla.edu
| ABSTRACT |
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Key Words: HIV-1 disease progression sympathetic nervous system HPA axis neuroendocrine catecholamine norepinephrine cortisol
Abbreviations: HPA = hypothalamus-pituitary-adrenal; SNS = sympathetic nervous system; NE = norepinephrine;SIV = Simian Immunodeficiency Virus; ART = antiretroviral therapy; CTL = cytotoxic T lymphocyte.
| INTRODUCTION |
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The Neurobiological Interface to HIV Disease Progression
The progression of HIV-1 infection to clinical immunodeficiency and opportunistic disease is driven fundamentally by viral replication in activated CD4+T lymphocytes and macrophages (11–14). HIV-1 replicates primarily in secondary lymphoid organs (e.g., lymph nodes and spleen) (15), which house more than 90% of the bodys total complement of leukocytes. From a mechanistic standpoint, understanding how psychosocial factors such as stress, depression, or temperament might influence HIV-1 disease progression essentially involves determining how CNS perceptual, interpretive, and coping processes might affect the biology of viral replication in leukocytes residing within secondary lymphoid tissues.
Two major physiologic signaling pathways are most often studied as possible mediators of biobehavioral influences on HIV-1 pathogenesis—the hypothalamic-pituitary-adrenal (HPA) axis, which could potentially affect HIV-1-infected leukocytes through the blood-borne glucocorticoid, cortisol, and the sympathetic division of the autonomic nervous system (SNS), which would affect HIV-1-infected leukocytes via the catecholamines, epinephrine, and norepinephrine (NE) (16,17). Both the HPA/cortisol and SNS/catecholamine systems are activated by stress (16,17), and both have also been linked to other psychological or social risk factors for HIV-1 disease progression such as depression, social support, psychological inhibition, and social temperament (reviewed in Refs. (4,5)). Activity of these signaling pathways has also been linked to biological indicators of HIV-1 pathogenesis in clinical natural history studies (18–23), experimental animal models (8,9), and pharmacologic intervention studies (24–29). Other physiologic signaling pathways examined as potential mediators include hormones from the growth and gonadal axes and peptide neurotransmitters such as substance P (30,31). However, much less is known about how those factors might affect HIV-1 replication and disease pathogenesis. As a result, this review focuses primarily on HPA and SNS regulation, and readers are referred to more comprehensive reviews for consideration of other potential mediators (5,32).
Both cortisol from the HPA axis and epinephrine from the SNS can reach HIV-1-infected leukocytes via blood plasma perfusion of secondary lymphoid organs, and during leukocyte recirculation through the vasculature. NE spillover from SNS innervation of the vasculature can also signal leukocytes in both of those compartments, but this catecholamine can also reach virally infected leukocytes via SNS innervation of secondary lymphoid organs (33,34). T lymphocytes and macrophages harbor receptors for both glucocorticoids and catecholamines, and those signaling molecules are known to affect several aspects of leukocyte function, including cellular activation, cytokine production, cell trafficking and chemotaxis, and immune effector responses (35,36). Effects of neural mediators on cell localization are particularly pertinent because two key receptors for chemotactic molecules, CCR5 and CXCR4, also facilitate entry of HIV-1 viral particles into human cells (37–39) and show relationships to biobehavioral risk factors and signaling pathways (3,21,40–44). Understanding how HPA and SNS activity might affect HIV-1 disease progression thus involves understanding how the net effect of these multiple host cell factors, the ability of the virus to complete its full replication cycle and disseminate infection to other cells.
Cortisol and the HPA Axis
Several studies have documented relationships between psychological factors and circulating cortisol levels in HIV+ individuals (6,45–47; reviewed in 4,5,32), and cross-sectional studies have linked elevated cortisol levels to HIV-1 disease progression (48,49). One longitudinal natural history study has found elevated cortisol levels to predict subsequent onset of AIDS (18,19). However, it is unclear whether these associations reflect HPA axis mediation of psychosocial influences, or whether elevated cortisol levels instead represent a neuroendocrine response to underlying disease progression (i.e., independent of psychological influences). Consistent with the later hypothesis, several studies have shown that progressing HIV-1 infection can activate the HPA axis and alter the adrenal gland (5,49). In the most comprehensive study of HPA relationships to HIV-1 disease progression, high cortisol levels were found to be an additional risk factor for AIDS onset that was independent of psychosocial risk factors (e.g., depression) (18,19). However, studies experimentally manipulating glucocorticoid levels have not identified any consequent increase in HIV-1 plasma viral load, CD4+T lymphocyte loss, or clinical disease onset (25,27,28,50,51). In fact, several of those studies have shown that glucocorticoid elevation can actually reduce plasma HIV-1 viral load and CD4+T lymphocyte loss (25,27,28). One nonrandomized observational study has suggested that administration of pharmacologic glucocorticoids to late-stage HIV-1 patients may precipitate the onset of full-blown AIDS (likely due to the immunosuppressive effect of glucocorticoids) (52). No prospective study of HIV-1 disease progression has found HPA axis activity to be a plausible mediator of biobehavioral risk in multivariate statistical analysis, although one cross-sectional result is consistent with that hypothesis (20). A decisive demonstration of HPA axis mediation would require experimental control of cortisol levels to abrogate the risk of disease progression associated with a known psychosocial risk factor. In the absence of such data, and in light of experimental studies documenting a suppressive effect of glucocorticoids on markers of HIV-1 pathogenesis, it remains unclear whether correlations between endogenous cortisol levels and HIV-1 disease progression reflect HPA axis mediation of stress effects or simply serve as a neuroendocrine marker of underlying disease progression.
Experimental studies in cellular models have identified several molecular mechanisms by which glucocorticoids might potentially influence HIV-1 replication, including altered expression of the CXCR4 chemokine receptor that mediates cellular vulnerability to infection by some strains of HIV-1 (40,42–44), arrest of the cell cycle in the G2 phase favoring viral gene expression (53), and impaired activation of the Type I interferon system, which represents a key innate immune response to viral replication (54). However, most in vitro virology studies have shown minimal impact of glucocorticoids on overall viral replication rates (55–58). This is likely because, in addition to whatever stimulatory effects glucocorticoids have on specific elements of the HIV-1 replication cycle, they also profoundly inhibit the basic leukocyte activation signals required for productive viral gene expression (e.g., the NF-
B transcription factor) (58). Such suppressive effects on lymphocyte activation are consistent with clinical effects of glucocorticoids in suppressing HIV-1 viral load and CD4+T lymphocyte declines (25,27,28).
In vitro viral replication models do not capture the effects of adaptive immune responses to HIV-1 (e.g., cytotoxic T lymphocyte responses), which are believed to play a central role in establishing equilibrium rates of viral replication in vivo. One experimental system that does capture both basic viral replication dynamics and adaptive immune responses is the rhesus macaque model of SIV infection. Studies in this system have shown that experimentally imposed social stress can increase SIV replication and accelerate the onset of clinical immunodeficiency (8,9). However, this system also showed reduced glucocorticoid levels in stressed animals, suggesting that stress-induced acceleration of SIV progression stemmed from some mechanism other than chronically elevated glucocorticoid levels.
The present research literature provides mixed evidence regarding glucocorticoid regulation of HIV-1 replication in vitro, and no in vivo data currently support the hypothesis that increased HPA axis activity mediates observed relationships between psychosocial risk factors and HIV-1 (or SIV) disease progression in vivo.
Catecholamines and the SNS
Studies have more consistently linked increased SNS activity to HIV-1 pathogenesis. Initial research in this area was motivated by data showing accelerated disease progression in people with socially inhibited personality characteristics (59,60), which have been linked to elevated SNS activity in previous research (61). Subsequent studies directly assessed autonomic nervous system activity in HIV+ individuals, and found those with constitutively high levels of SNS activity to show elevated plasma viral load set-points and impaired virologic response to the initiation of combination antiretroviral therapy (ART) (21,23). Multivariate statistical analyses also supported the hypothesis that individual differences in SNS activity might account for much of the relationship between stress or temperament-related risk factors and individual differences in virologic or immunologic indicators of HIV-1 pathogenesis (22,23). Those findings are also consistent with data from experimental pharmacologic studies that (inadvertently) activated the cAMP/PKA signaling pathway which mediates catecholamine response and found a consequent increase in HIV-1 plasma viral load (24,26). Thus, both natural history studies and pharmacologic manipulation studies support the hypothesis that SNS activity may mediate some biobehavioral influences on HIV-1 pathogenesis.
In vitro studies have shown that catecholamines can significantly enhance HIV-1 replication (21,62). Several molecular mechanisms of this effect have been identified, including up-regulated cell surface expression of the viral coreceptors, CXCR4 and CCR5 (21,41), enhanced transcription of HIV-1 genes by cellular transcription factors (21), and catecholamine-mediated suppression of Type I interferon responses to infection (54). Signal transduction analyses identified beta adrenergic receptor activation of the cAMP/PKA signaling pathway as the key mediator of catecholamine effects on HIV-1 replication, and showed that pharmacologic blockade of beta adrenergic receptors can abrogate those effects (62).
Analyses of lymph nodes from the rhesus macaque model of SIV infection have revealed a key role of SNS neurons in regulating viral replication in secondary lymphoid tissue. Initial studies found increased SIV replication adjacent to the SNS neural varicosities that release catecholamines within the lymph node parenchyma (34). Subsequent studies showed a surprising degree of behaviorally induced plasticity in the SNS innervation of lymphoid organs. Macaques subject to experimentally imposed social stress showed elevated density of catecholaminergic varicosities within the lymph node parenchyma (9). Stress also enhanced SIV replication, and that effect was attributable specifically to the increased density of catecholaminergic varicosities. Consistent with in vitro cellular models, interferon-beta gene expression was also suppressed in stressed animals, suggesting that impairment in innate antiviral responses might play a key role in the relationship between SNS innervation and viral replication. A recent study has also shown that pharmacologically induced enhancement of SNS innervation density is associated with increased SIV gene expression in macaque lymph nodes (29), providing an experimental link between modulation of the sympathetic nervous system and viral replication in vivo. Figure 2 provides a theoretical model that integrates existing results from human clinical studies relating SNS activity to indicators of HIV-1 pathogenesis (21,23), in vitro analyses of catecholamine effects on HIV-1 replication (21,41,54,62), and experimental analyses of the SIV lymphoid tissue model in vivo (9,29,34). In this system, SNS activity is hypothesized to enhance viral replication by inhibiting the activity of Type I interferons (9,54), which increases viral replication through multiple mechanisms including impaired resistance to viral gene expression (via inhibition of the interferon-mediated antiviral state) (54) and enhanced cellular vulnerability to infection (via disinhibited expression of the viral coreceptors CCR5 and CXCR4, which occurs under physiologic conditions (63–66), but not in artificially stimulated cells (67)). In conjunction with immune activation (e.g., via proinflammatory cytokines or ligation of the T cell receptor), these factors facilitate viral replication and thereby accelerate disease pathogenesis.
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Summary and Implications
Current empirical data are generally consistent with the hypothesis that SNS activity might mediate biobehavioral influences on HIV-1 pathogenesis, but the role of the HPA axis is less clear. In vitro model systems have shown catecholamine effects on cellular vulnerability to infection (CCR5 and CXCR4 expression), viral gene expression (transcription factor activation), and innate antiviral responses (Type I interferons). Statistical mediation analyses from clinical natural history studies find that variations in SNS activity are plausible mediators of relationships between psychosocial risk factors and HIV-1 replication (22,23), and experimental analyses of the SIV model identify a key role for stress-induced remodeling of SNS innervation in lymphoid tissues as an influence on viral replication (9,29,34). In terms of Figure 1, existing data support the both the individual binary relationships A (22,23) and B (21,54,62), and their simultaneous transitive relationship linking psychosocial risk factors to indicators of HIV-1 pathogenesis (22,23).
Data regarding HPA mediation are less consistent. In vitro analyses suggest that glucocorticoids can influence selected molecular processes involved in viral replication, but glucocorticoids have not been found to substantially enhance overall viral replication rates either in vitro or in the SIV animal model in vivo. In fact, glucocorticoid elevation has been linked to reduced viral replication in both of those systems (8,52). Similarly mixed results emerge from human clinical studies. Some observational natural history analyses have linked high cortisol levels to subsequent AIDS onset (18,19), but several studies have also shown that experimental elevation of glucocorticoid levels can reduce viral load and CD4+T cell loss (25,27,28). No corelational mediation analyses currently support the hypothesis that variations in HPA activity serve as biological intermediates between psychosocial risk factors and long-term individual differences in HIV-1 disease progression. It is conceivable that such mediating relationships do exist, but are obscured by sampling limitations or failure to assess glucocorticoid activity at the postreceptor level. In the absence of positive empirical support, however, the role of HPA activity as a mediator of psychosocial influences on HIV-1-related disease remains uncertain. Some evidence supports the binary relationship A in Figure 1 (6,45–47; as reviewed in 4,5,32), but data relevant to the B relationship are contradictory, and no prospective data currently support a transitive influence through A and B simultaneously.
To the extent that data continue to support a role for SNS activity in driving HIV-1 pathogenesis, catecholamine signaling could constitute an appealing target for behavioral or pharmacologic interventions. SNS activity may also serve as a functional neural biomarker to assess the impact of potential behavioral interventions on HIV-1 biology. This could provide for rapid assessment of intervention impact, and rational selection of psychosocial or pharmacologic interventions to protect HIV+ individuals from the detrimental effects of stress on disease progression. A similar case might be made for suppressing HPA activity based on natural history data, but results from clinical pharmacologic interventions and the SIV animal model raise the possibility that such suppression might instead enhance disease pathogenesis.
Several potential mechanisms of neural influence on HIV-1 pathogenesis are not considered in the current research literature, and might constitute productive targets for future research. No published data are available on potential effects of the parasympathetic nervous system on HIV-1 disease pathogenesis or progression (i.e., no prospective clinical analyses, and no experimental studies assessing effects of the parasympathetic neurotransmitter acetylcholine on viral replication). A great deal also remains to be learned about the interface between neural/endocrine activity and the adaptive immune response to HIV-1. The studies outlined above suggest that neural dynamics can directly regulate viral replication and the innate antiviral response, but little information is available regarding their impact on the adaptive immune response to HIV-1. Cytotoxic T lymphocyte (CTL) responses are believed to play a key role in regulating long-term viral replication set-points (68), but no published research has examined the relationship between psychosocial characteristics or neural/endocrine parameters and HIV-specific CTL activity (ascertained through PubMed in May 2007). Activated CD8+T lymphocytes can also inhibit HIV-1 replication via a soluble "CD8 antiviral factor" (69), but no biobehavioral studies have examined this parameter as a potential mechanism of differential disease progression. Another immune parameter that has received little attention in the HIV biobehavioral literature, but looms large (and, unfortunately, still distant) on the public health horizon, is the vaccine-induced immune response. Current efforts to develop a prophylactic vaccine to HIV-1 are hampered by difficulties in eliciting durable CTL responses in the absence of viral replication, and by difficulties in identifying specific antibodies that neutralize a broad range of viral strains. However, the development of an HIV vaccine may represent the most effective means for controlling the global pandemic (68,70). Given the inhibitory effects of stress and social isolation on other vaccine-induced immune responses (71–76), it is critical that mechanisms of those effects be elucidated to help optimize the efficacy of HIV vaccine candidates.
A great deal has been discovered in the past 25 years about the mechanisms by which psychosocial factors might potentially influence the basic biology of HIV infection. We now understand much about how HIV-1 causes disease, and it is increasingly clear how biobehavioral processes might affect those processes. What remains to be accomplished is the translation of these basic insights into effective clinical interventions that can suppress viral pathogenesis over the long periods of time necessary to preserve the health and well-being of HIV+ individuals. Antiretroviral medications slow, but do not stop, HIV-1 replication (77). Studies carried out in the context of ART suggest that biobehavioral dynamics continue to influence underlying viral replication rates as well as virologic and immunologic response to treatment (21,23). As the other articles in this special issue emphasize (1–3), a broad array of psychosocial factors could potentially influence HIV-1 disease progression. Modulation of HIV-1 replication by the neuroendocrine system and by penetrating innervation of lymphoid tissue provides new insights into biobehavioral regulation of infectious disease, and may suggest novel targets for health-protective intervention.
| NOTES |
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Supported by NIH R01 AI52737 and NCI R01 CA116778, and the James L. Pendelton Charitable Trust. Dedicated to George F. Solomon, sorely missed.
DOI:10.1097/PSY.0b013e3181773bbd
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