Psychological stress creates an immune-suppressive environment that increases the susceptibility of aged mice to Mycobacterium tuberculosis infection

Abstract

Abstract Age is a risk factor for chronic infections, including tuberculosis (TB). A second compounding factor of TB susceptibility is psychological stress, which is particularly acute in older individuals. While the influence of psychological stress in TB is well documented, the mechanisms by which stress influences M.tb susceptibility in the elderly are almost completely unknown. To study the effects of chronic psychological stress on M.tb infection in old mice, we employed a mouse social stressor model (called social disruption stress or SDR) which involves repeated social defeat in subordinate mice. Young (3 month) and old (18 month) C57BL/6 mice were subjected to SDR and bronchial fluid (BF) and alveolar macrophages (AMs) were isolated by lung lavage. Our studies showed that SDR created an immune-suppressive lung environment in old mice, based on lower levels of pro-inflammatory cytokines and chemokines in bronchial fluid (BF) or their mRNA expression by AMs. AMs from old SDR mice compared to non-stressed mice were also deficient in their ability to mount an inflammatory cytokine response or induce MHC class II mRNA expression. To determine the effect of SDR on M.tb growth, young and old mice were subjected to SDR and infected with M.tb. At 30 days, SDR in both young and old mice reduced M.tb burden. However, this was transient as at 60 days M.tb burden was higher in old mice compared to young mice and SDR further significantly increased M.tb burden in old mice. Also we found that SDR at 60 days in old mice induced significantly higher levels of IL-10 and IL-17 mRNA in the lung, which favors growth of M.tb. In conclusion, our studies suggest that psychological stress during the initial stage of Mtb infection has long-term impact on the control of the infection.