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Introduction: Experimental animal studies provided evidence for a synergistic effect of immunological and psychological stressors on subsequent sickness behaviours. Up to now, little corroborating evidence for such synergy exists for humans, in whom it may provide a mechanism leading to the expression of functional somatic symptoms. The aim of the present study was to determine an interaction between stress(-vulnerability) and an immunological activation on experimental pain sensitivity, i.e., pressure pain threshold and tolerance in healthy humans., Methods: In healthy female participants (n=25, mean age 22.3 years), negative affectivity (NA) and experienced stress were assessed by questionnaire before receiving a Salmonella typhi vaccine or saline control in a randomized blinded cross-over design. Pressure pain threshold was assessed at the lower back and calves and pain tolerance was assessed at the thumbnail, before and six hours after each injection., Results: Vaccination induced leukocytosis (+100%) and increased serum IL-6 (+670%). NA predicted decreased pain tolerance after vaccination (β=-.57, p=.007), but not after placebo (β=.25, p=.26). Post-hoc analyses also demonstrated an association with administration order., Discussion: NA moderated the effects of inflammation on pain tolerance. This finding is consistent with a synergistic model whereby inflammation may lower the threshold for pain reporting in individuals with increased vulnerability for somatic symptom reporting., (Copyright © 2014 Elsevier Inc. All rights reserved.)

It has been suggested that dysregulation of immune-to-brain communication plays a role in the biopsychological process underlying medically unexplained symptoms (MUS). Immune and non-immune stressors can both be involved in the activation of the central sickness-behavioural-system leading to complaints like malaise, pain and fatigue. We hypothesized increased pro-inflammatory and/or reduced anti-inflammatory cytokine activity to exist in MUS patients. Twenty-seven participants (4 male; 23 female) with heterogeneous MUS were compared with 27 healthy controls (6 male; 21 females). Blood samples were analysed for leukocyte subset cell counts, in vitro T-cell mitogen-stimulated cytokine production (IL-2, IL-4, IL-5, IL-6, IL-10, TNF-alpha and IFN-gamma) and in vitro monocyte cytokine release (IL-1beta, IL-6, IL-8, IL-10 and TNF-alpha) in response to increasing concentrations of LPS. No significant group differences were found for any of the cytokines measured. One unexpected exception was an elevation in the number of circulating B and NK-cells in participants high on MUS. Nonetheless, no support was found for the hypothesized immunological dysregulation in peripheral blood leukocyte function of MUS patients.