Date Published: July 12, 2017
Publisher: Public Library of Science
Author(s): Blake T. Gurfein, Burcu Hasdemir, Jeffrey M. Milush, Chadi Touma, Rupert Palme, Douglas F. Nixon, Nicholas Darcel, Frederick M. Hecht, Aditi Bhargava, Taishin Akiyama.
Prolonged chronic stress has deleterious effects on immune function and is associated with numerous negative health outcomes. The spleen harbors one-fourth of the body’s lymphocytes and mediates both innate and adaptive immune responses. However, the subset of splenic lymphocytes that respond, either adaptively or maladaptively, to various stressors remains largely unknown. Here we investigated the effects of unpredictable chronic mild stress (CMS) exposure on spleen composition in male mice housed in two different caging conditions: standard caging (Cntl) and enriched environment (EE). EE-caged mice exhibited the greatest absolute number of splenocytes and CMS exposure significantly lowered splenocyte numbers in both caging conditions. Glucocorticoid production, measured by mean fecal corticosterone metabolites (FCM), was significantly lower in EE-caged mice vs. Cntl-caged mice. Surprisingly, CMS exposure resulted in an increase in mean FCM in EE-caged mice, but no significant change in Cntl-caged mice. CMS altered the splenic B:T lymphocyte ratio; it reduced the frequency of B cells, but increased the frequency of T cells in EE-caged mice. Splenocyte number and B:T lymphocyte ratio showed a negative relationship with mean FCM. EE-caged mice had a lower frequency of immature and germinal B cells than Cntl-caged mice. CMS markedly increased the frequency of immature and marginal zone B cells, but decreased the frequency of follicular B cells in both caging conditions. Mean FCM correlated positively with frequency of immature, marginal zone and germinal center B cells, but negatively with frequency of follicular B cells. To conclude, splenic immune cells, particularly B lymphocyte composition, are modulated by caging environment and stress and may prime mice differently to respond to immune challenges.
The spleen serves as a hematopoietic and secondary lymphoid organ in mice. It is comprised of two morphologically and functionally distinct regions: the white (marginal zone) and the red pulp. In the white pulp, lymphocytes and macrophages traffic through and plasma cell formation occurs upon antigen stimulation. The red pulp is largely responsible for filtering blood and removing foreign material and damaged erythrocytes. Mature B cells recirculating through the spleen are known as follicular B cells. As follicles are adjacent to T cell zones, this proximity allows for follicular B cells to participate in antigen-induced T cell-dependent responses . The marginal zone B cells can differentiate into plasma cells with a short life span and are thought to evoke innate-like immune responses. The marginal zone B cells express high CD21 levels and allow for presentation of pathogenic lipid antigens to invariant NK cells (iNKC) and thus primarily mediate T cell-independent responses to blood-born pathogens .
The spleen harbors one fourth of the body’s lymphocytes and mediates both innate and adaptive immune responses. B cells found in various zones and structures of the spleen aide in mediating either T cell-dependent or independent immune responses to pathogens. While studies have examined changes in subsets of splenic B and T cells in response to pathogen or LPS challenge, the effect of caging environment superimposed with stress on splenic lymphocytes has not been examined. We made several key observations with regard to the effects of chronic mild stress and caging environment. First, while non-stressed EE-caged mice had the greatest spleen cellularity, they were found to be most sensitive to stress-associated reductions in B lymphocyte frequency. Second, CMS exposure in EE-caged mice yielded an increase in T lymphocyte frequency and a reduction in B lymphocyte frequency and the splenic B:T lymphocyte ratio across all animals correlated negatively with FCM measures of HPA activity, suggesting its potential utility as a stress immunomodulation biomarker. Third, subsets of splenic B lymphocytes were markedly skewed by unpredictable chronic mild stress exposure, particularly immature, germinal, and follicular B lymphocytes. The findings are summarized in Table 1.
This study demonstrates that caging environment and stress exposure may prime mice to respond differently to immune challenges at the level of the spleen and that B lymphocyte composition appears to have pronounced sensitivity to these variables. This is of particular relevance to experimental designs for immunological studies involving mice and may inform clinical studies investigating the impact of stress and stress reduction on immune function.