Here we investigated whether acute psychological stress could upregulate inflammatory cytokine and mood responses to a peripheral immune stimulus, Salmonella typhi vaccine, in men. To the best of our knowledge, this is the first study to directly investigate a synergistic effect of psychological and immune stressors on sickness behaviour in healthy humans. We predicted that acute stress would enhance inflammatory cytokine responses to typhoid vaccine in men and that this mechanism would lead to exaggerated infection-related sickness symptoms.
In line with our hypotheses, typhoid vaccine but not placebo induced a significant increase in participants’ serum levels of the pro-inflammatory cytokine IL-6, and exposure to acute psychological stress enhanced the IL-6 response to vaccine. This is in agreement with previous reports in animals showing that acute psychological (or heterotypic) stressors upregulate inflammatory cytokine responses to immune pathogens (Anisman et al., 2007; Gandhi et al., 2007; Gibb et al., 2008; Johnson et al., 2002
). Somewhat unexpectedly, circulating IL-6 levels were not altered in the placebo/stress group. Our group and others have previously shown that acute psychological stress increases circulating levels of IL-6 in healthy humans, reaching maximum levels at 2 h post-stress (Steptoe et al., 2007
). However, participants in this study were all male, and there is evidence that women have larger IL-6 stress responses than men (Steptoe et al., 2007
As predicted, we also found that acute psychological stress increased subjective ratings of negative mood in participants, and this effect was particularly marked in the group who had received typhoid vaccine versus placebo. The heightened mood response in the presence of both psychological and immune stress is concordant with previously reported synergistic effects of heterotypic stressors on sickness responses in animals (Anisman et al., 2007; Gandhi et al., 2007; Gibb et al., 2008; Simmons and Broderick, 2005
). However, the kinetics of the mood response and the observed lack of mood change in participants receiving typhoid vaccine alone were somewhat unexpected, based on our previous research using the same experimental model. In two separate studies of healthy young men and women, we found that typhoid vaccine induced a sharp decline in mood between 1.5 and 3 h post-vaccination, with mood levels continuing to fall until 6 h post-vaccine (Strike et al., 2004; Wright et al., 2005
). In contrast, in the current investigation, changes in negative mood occurred rapidly in the vaccine/stress group, peaking immediately post-stress (1 h post-vaccine) then returning to baseline levels by 30 min post-stress, and mood was unaltered in the vaccine/rest comparison group. Participants in the current study remained in the laboratory throughout the entire testing session, whereas in previous studies participants were free to return to their normal college schedule between assessments, and it is conceivable that this might have affected responses. Furthermore, animal evidence suggests that combined exposure to two heterotypic stressors may alter the dynamics of stress responses. Prior exposure to immobilisation stress in rats significantly increased circulating as well as central levels of IL-1β 1 h following LPS injection, but did not potentiate IL-1 β responses at later time points (Johnson et al., 2002
). This is consistent with a rapid, transient effect of psychological stress on sickness responses to typhoid vaccine in humans.
Both circulating IL-6 levels and negative mood scores were highest in the vaccine/stress group, suggesting that psychological and immune stressors synergistically increased sickness behaviour through up-regulating this cytokine. However, there was no significant relationship between serum IL-6 levels and negative mood assessed immediately post-stress. Although IL-6 is a useful index of inflammation, it acts as part of a network of several different cytokines, and is induced primarily by interleukin-1β (IL-1β). It is thus conceivable that IL-6 does not directly affect mood, but instead reflects the production of another cytokine such as IL-1β that modulates mood. Supporting this, peripheral administration of IL-1β or bacterial LPS reliably induces a variety of sickness behaviours in animals, whereas administration of IL-6 alone does not directly induce sickness behaviour but rather potentiates the effects of IL-1 (Dantzer et al., 2007; Lenczowski et al., 1999
). In line with the kinetics of our observed mood response, IL-1β expression precedes the IL-6 response to peripheral LPS (Dantzer et al., 2007
), and monocyte–macrophage levels of IL-1β increase rapidly (within 30 min) following exposure to acute psychological stress, whereas stress-induced elevations in plasma IL-6 tend to be delayed (Brydon et al., 2004; Brydon et al., 2005; von Kanel et al., 2006
). Unfortunately we do not have data on serum levels of IL-1β since it acts predominately in an autocrine/paracrine manner and is difficult to detect in healthy circulating blood (Dinarello, 1996
). Notably, analyses of the POMS subscales revealed that in the vaccine/stress group only, post-stress IL-6 levels were associated with increases in fatigue at 2 h (3 h post-vaccine). This corresponds to our previous results in women showing that IL-6 responses to typhoid vaccine correlated with a decline in mood 3 h following vaccination (Wright et al., 2005
), and suggests that IL-6 may play a more important role in later mood responses.
A number of pathways have been implicated in stressor-induced sickness responses. Psychological stress activates the sympathetic nervous system (SNS) and hypothalamic–pituitary–adrenal (HPA) axis, resulting in elevated blood pressure and heart rate and increased circulating levels of the ‘stress hormones’ catecholamines and glucocorticoids, which modulate inflammatory cytokine production through binding to their respective receptors on peripheral immune cells (Black and Garbutt, 2002
). At the same time, peripheral cytokines (IL-1, IL-6, TNFα) released during infection and/or psychological stress, can feedback to the brain to exert potent effects on neuroendocrine function and neurotransmitter metabolism (Dantzer et al., 2007
). In animals, cytokines and/or stressor-induced behavioural changes are associated with increased neuroendocrine activity, as well as altered neurotransmitter turnover in brain regions essential to the regulation of emotion, motor function and reward (Anisman and Merali, 2003; Dunn, 2006
). Notably, these responses are significantly more pronounced in animals concurrently exposed to psychological and immune stressors (Anisman et al., 2007; Gandhi et al., 2007; Gibb et al., 2008; Merali et al., 1997; Song et al., 1999
), suggesting that these pathways may mediate the synergistic effects of heterotypic stressors on sickness. In humans, hyperactivity of neuroendocrine pathways and disrupted neurotransmitter metabolism have been observed in clinically depressed people (Hayley et al., 2005
) and in patients undergoing IFN-α therapy, where they correlate with development of specific sickness symptoms (Capuron and Miller, 2004
). In the current study, there were no differences between vaccine and placebo conditions in either cardiovascular or neuroendocrine responses to stress. However, in the vaccine/stress group, participants with greater increases in IL-6 had larger systolic blood pressure responses to the Speech task, elevated post-stress salivary MHPG, and prolonged increases in systolic BP following tasks, suggesting that hyperactivity of the SNS may mediate the heightened inflammatory and mood responses in this group. Supporting this idea, catecholamines increase plasma levels of IL-6 in rodents, whereas in vivo blockade of the autonomic nervous system or pre-treatment with β-adrenergic receptor (β-AR) antagonists attenuates stress-induced increases in plasma IL-6 (Johnson et al., 2005; Mohamed-Ali et al., 2001
) and prevents potentiation of LPS-induced macrophage cytokine production by foot-shock stress in animals (Broug-Holub et al., 1998
). Whether β-AR antagonists would prevent the synergistic effects of typhoid vaccine and psychological stress observed in our study remains to be investigated.
Stress-induced enhancement of the innate immune response to infection and consequent sickness symptoms can be thought of as an evolutionary adaptive mechanism, designed to conserve energy and combat infection during times of adversity such as wounding due to a predator-prey encounter (Hart, 1988
). However, in the context of ongoing inflammation, these effects may become detrimental. In humans, stressful life events have been linked to the onset and exacerbation of illness symptoms in a number of chronic inflammatory conditions where cytokines are elevated, including rheumatoid arthritis, cardiovascular disease, multiple sclerosis, asthma and psoriasis (Black and Garbutt, 2002; Kemeny and Schedlowski, 2007; Perry et al., 2007
). Notably, depressive symptoms are prevalent in many of these conditions, and are predictive of increased disease morbidity and mortality (Evans et al., 2005; Irwin and Miller, 2007
This investigation was carried out in a sample of healthy male university students, and results may not generalise to other populations. Blood was sampled at a single post-stress time point (2 h), making it difficult to determine when the peak cytokine response occurred, and a more detailed cytokine profile including earlier time points would have been useful. Cytokine and mood responses were relatively small. However, this is consistent with the fact that Salmonella typhi
vaccine is a mild inflammatory stimulus compared to other endotoxin models (Krabbe et al., 2005; Wright et al., 2005
). The correlation between IL-6 and systolic BP responses was dependent on extreme responders, so corroboration in a larger data set would be desirable. We also did not measure peripheral neurotransmitter levels and it will be important to carry out additional studies to investigate the potential contribution of these pathways, as well as the mechanisms underlying differences between our current findings and previous observations using the same inflammatory model.
Taken together, our results provide added insight into the biological mechanisms through which psychological and immune stressors interact to exacerbate sickness behaviour in humans. Improved understanding of these mechanisms may inform the development of new psychopharmacological interventions to combat the depressive symptoms that commonly accompany inflammatory disease, thereby improving patient outcomes.