Direct connections between the nervous, endocrine and immune systems allow stress-induced neuroendocrine mediators to influence the immune system. Repeated social defeat (SDR) is a well-characterized model of non-chronic social stress in male mice that induces specific immunological changes, including the induction of functional glucocorticoid resistance in monocytes and DCs and increased pro-inflammatory cytokine levels. We hypothesized that SDR-induced immunological changes would augment the adaptive immune response to a pathogenic challenge: influenza virus. As patterning of immunological memory is established by the immune response to a primary infection, we hypothesized that alterations induced during a primary response would lead to enhancement of immunological memory. The first part of this study examined SDR-induced changes in immunological memory parameters with a focus on the CD8+T cell. The second part of this study was designed to investigate potential mechanisms for memory enhancement during a primary influenza viral infection.Primary Infection: Male C57BL/6 mice underwent SDR or were undisturbed (INF) before intra-nasal infection with 1 hemeagglutinating unit (HAU) of influenza A/PR/8/34. Daily two-hour SDR cycles entailed introduction of an aggressive mouse that defeated all resident mice. Mice were infected after the sixth (final) cycle. Cells were assessed via flow cytometry using fluorescent-labeled antibodies and peptide-specific MHC I tetramers to phenotype immunodominant DbNP366 and DbPA224 CD8+T cell responses.
Exposure to SDR in the absence of influenza infection increased lung mRNA expression of IFN-gamma, which persisted until the cessation of the primary infection. Following infection, SDR mice had increased numbers of lung NP366-74CD8+T cells (p<0.05). The influx originated during the clonal expansion phase, and at day 9 post-infection included more cells expressing an activated phenotype: CD25+/CD62L(LO) NP366-74CD8+ in SDR mice compared with the INF group. No significant changes were noted between groups in the lung PA224CD8+T cell population. Interestingly, the preferential stimulation of the NP366CD8+T cell population in SDR-MEM mice inverted the conventional NP:PA immunodominance hierarchy in spleen and lung tissue. Predictably in conjunction with the influx of lung CD8+T cells, viral replication in lung tissue of SDR mice was consistently decreased when compared to INF mice.
Memory Responses: The same stress and primary infection protocol were used for all studies. After 6 weeks, resting memory parameters were examined in stressed (SDR-MEM) and non-stressed (MEM) mice without additional stressor exposure. SDR-MEM mice responded with an enhanced footpad delayed-type hypersensitivity (DTH) response to A/PR/8/34 virus during resting memory. With respect to MEM mice, SDR-MEM mice had a significantly increased frequency of DbNP366-74CD8+ T cells in lung and spleen tissues during resting memory, and an increase in the CD127HINP366-74CD8+ memory T cell population that was limited to lung parenchyma and produced more IFN-gamma upon ex-vivo peptide stimulation during resting memory. When mice were re-challenged with a 16 HAU dose of A/PR/8/34 virus, SDR-MEM mice terminated viral replication significantly earlier, and generated a larger DbNP366-74CD8+ T cell response in the lung parenchyma and airways.
In the work presented here, SDR prior to a primary influenza viral infection significantly enhanced the clonal expansion and function of the DbNP366-74CD8+T cell pool. Enhanced memory was associated with an altered cytokine milieu in the lung prior to and after primary viral infection and increased expansion of DbNP366-74CD8+T cells during the primary and secondary anti-viral responses. Early modification of the lung micro-environment by SDR may have contributed to T cell activation and more rapid clearance of virus from lung tissue in SDR mice. Social stressors should be carefully considered in the design and analysis of future studies on anti-viral immunity.