Patients diagnosed with post-traumatic stress disorder (PTSD) are at a significantly elevated risk of developing comorbid inflammatory conditions, but the mechanisms underlying this predilection remain unclear. Our previous work has shown that T-lymphocytes exposed to elevated levels of norepinephrine (NE) displayed a pro-inflammatory signature reminiscent of an autoreactive phenotype. With this, we hypothesized that the increased sympathetic tone observed during psychological trauma may be promoting pro-inflammatory T-lymphocytes, which causes a predisposition to comorbid inflammatory conditions. Here, we examined the consequences of psychological trauma on splenic T-lymphocytes using a mouse model of repeated social defeat stress. Social defeat led to anxiety-like and depression-like behavior as has been previously described. The spleens of socially-defeated mice showed significant elevations of NE, tyrosine hydroxylase (TH), and acetylcholinesterase (ACHE) levels, which appeared to be due in part to increased expression within T-lymphocytes. Additionally, T-lymphocytes from stressed animals showed higher levels of pro-inflammatory cytokines and mitochondrial superoxide. Interestingly, in this model system, close associations exist within splenic T-lymphocytes amid the autonomic, inflammatory, and redox environments, but these only weakly correlate with individual behavioral differences among animals suggesting the psychological and physiological manifestations of trauma may not be tightly coupled. Last, we describe, for the first time, elevations in calprotectin levels within T-lymphocytes and in circulation of psychologically stressed animals. Calprotectin correlated with both behavioral and physiological changes after social defeat, suggesting the potential for a new biological marker and/or therapeutic target for psychological trauma and its inflammatory comorbidities.

Post-traumatic stress disorder (PTSD) is a psychiatric illness that results in an increased risk for a variety of inflammatory diseases. The exact etiology of this increased risk is unknown, and thus several animal models have been developed to investigate the neuroimmune interactions of PTSD. Repeated social defeat stress (RSDS) is an established preclinical model of psychological trauma that recapitulates certain behavioral and inflammatory aspects of human PTSD. Furthermore, RSDS has been utilized to subgroup animals into susceptible and resilient populations based on one specific behavioral phenotype ( i.e. , social interaction). Herein, we conducted an extensive investigation of circulating inflammatory proteins after RSDS and found significant elevations in various cytokines and chemokines after exposure to RSDS. When categorizing animals into either susceptible or resilient populations based on social interaction, we found no inflammatory or other behavioral differences between these subgroups. Furthermore, correlative analyses found no significant correlation between social interaction parameters and inflammation. In contrast, parameters from the elevated zero maze (EZM) demonstrated significant associations and clustering to five circulating cytokines. When animals were subdivided into susceptible and resilient populations solely based upon combined EZM performance, significant inflammatory differences were evident between these groups. Strikingly, these circulating inflammatory proteins displayed a stronger predictive ability of EZM performance compared to social interaction test performance. These findings provide new insights into inflammatory markers associated with RSDS, and the utility of EZM to effectively group RSDS-exposed mice into populations with differential levels of peripheral inflammation.

No abstract

While the role of mitochondrial metabolism in controlling T-lymphocyte activation and function is becoming more clear, the specifics of how mitochondrial redox signaling contributes to T-lymphocyte regulation remains elusive. Here, we examined the global effects of elevated mitochondrial superoxide (O2•-) on T-lymphocyte activation using a novel model of inducible manganese superoxide dismutase (MnSOD) knock-out. Loss of MnSOD led to specific increases in mitochondrial O2•- with no evident changes in hydrogen peroxide (H2O2), peroxynitrite (ONOO−), or copper/zinc superoxide dismutase (CuZnSOD) levels. Unexpectedly, both mitochondrial and glycolytic metabolism showed significant reductions in baseline, maximal capacities, and ATP production with increased mitochondrial O2•- levels. MnSOD knock-out T-lymphocytes demonstrated aberrant activation including widespread dysregulation in cytokine production and increased cellular apoptosis. Interestingly, an elevated proliferative signature defined by significant upregulation of cell cycle regulatory genes was also evident in MnSOD knock-out T-lymphocytes, but these cells did not show accelerated proliferative rates. Global disruption in T-lymphocyte DNA methylation and hydroxymethylation was also observed with increased mitochondrial O2•-, which was correlated to alterations in intracellular metabolite pools linked to the methionine cycle. Together, these results demonstrate a mitochondrial redox and metabolic couple that when disrupted may alter cellular processes necessary for proper T-lymphocyte activation.

Alterations in peripheral immune markers are observed in individuals with post-traumatic stress disorder (PTSD). PTSD is characterized in part by impaired extinction of fear memory for a traumatic experience. We hypothesized that fear memory extinction is regulated by immune signaling stimulated when fear memory is retrieved. The relationship between fear memory and the peripheral immune response was tested using auditory Pavlovian fear conditioning in mice. Memory for the association was quantified by the amount of conditioned freezing exhibited in response to the conditioned stimulus (CS), extinction and time-dependent changes in circulating inflammatory cytokines. Brief extinction training with 12 CS rapidly and acutely increased circulating levels of the cytokine interleukin-6 (IL-6), downstream IL-6 signaling, other IL-6 related pro-inflammatory cytokines. Transgenic manipulations or neutralizing antibodies that inhibit IL-6 activity did not affect conditioned freezing during the acquisition of fear conditioning or extinction but significantly reduced conditioned freezing 24 h after extinction training with 12 CS. Conversely, conditioned freezing after extinction training was unchanged by IL-6 inhibition when 40 CS were used during the extinction training session. In addition to effectively diminishing conditioned freezing, extinction training with 40 CS also diminished the subsequent IL-6 response to the CS. These data demonstrate that IL-6 released following fear memory retrieval contributes to the maintenance of that fear memory and that this effect is extinction dependent. These findings extend the current understanding for the role of the immune system in PTSD and suggest that IL-6 and other IL-6 related pro-inflammatory cytokines may contribute to the persistence of fear memory in PTSD where fear memory extinction is impaired.