Stress-induced redistribution of immune cells--from barracks to boulevards to battlefields: a tale of three hormones--Curt Richter Award winner

Abstract

Background: The surveillance and effector functions of the immune system are critically dependent on the appropriate distribution of immune cells in the body. An acute or short-term stress response induces a rapid and significant redistribution of immune cells among different body compartments. Stress-induced leukocyte redistribution may be a fundamental survival response that directs leukocyte subpopulations to specific target organs during stress, and significantly enhances the speed, efficacy and regulation of an immune response. Immune responses are generally enhanced in compartments (e.g., skin) that are enriched with leukocytes, and suppressed in compartments that are depleted of leukocytes during/following stress. The experiments described here were designed to elucidate the: (1) Time-course, trajectory, and subpopulation-specificity of stress-induced mobilization and trafficking of blood leukocytes. (2) Individual and combined actions of the principal stress hormones, norepinephrine (NE), epinephrine (EPI), and corticosterone (CORT), in mediating mobilization or trafficking of specific leukocyte subpopulations. (3) Effects of stress/stress hormones on adhesion molecule, L-selectin (CD62L), expression by each subpopulation to assess its adhesion/functional/maturation status.

Methods: Male Sprague Dawley rats were stressed (short-term restraint, 2-120 min), or adrenalectomized and injected with vehicle (VEH), NE, EPI, CORT, or their combinations, and blood was collected for measurement of hormones and flow cytometric quantification of leukocyte subpopulations.

Results: Acute stress induced an early increase/mobilization of neutrophils, lymphocytes, helper T cells (Th), cytolytic T cells (CTL), and B cells into the blood, followed by a decrease/trafficking of all cell types out of the blood, except neutrophil numbers that continued to increase. CD62L expression was increased on neutrophils, decreased on Th, CTL, and natural killer (NK) cells, and showed a biphasic decrease on monocytes & B cells, suggesting that CD62L is involved in mediating the redistribution effects of stress. Additionally, we observed significant differences in the direction, magnitude, and subpopulation specificity of the effects of each hormone: NE increased leukocyte numbers, most notably CD62L⁻/⁺ neutrophils and CD62L⁻ B cells. EPI increased monocyte and neutrophil numbers, most notably CD62L⁻/⁺ neutrophils and CD62L⁻ monocytes, but decreased lymphocyte numbers with CD62L⁻/⁺ CTL and CD62L⁺ B cells being especially sensitive. CORT decreased monocyte, lymphocyte, Th, CTL, and B cell numbers with CD62L⁻ and CD62L⁺ cells being equally affected. Thus, naïve (CD62L⁺) vs. memory (CD62L⁻) T cells, classical (CD62L⁺) vs. non-classical (CD62L⁻) monocytes, and similarly distinct functional subsets of other leukocyte populations are differentially mobilized into the blood and trafficked to tissues by stress hormones.

Conclusion: Stress hormones orchestrate a large-scale redistribution of immune cells in the body. NE and EPI mobilize immune cells into the bloodstream, and EPI and CORT induce traffic out of the blood possibly to tissue surveillance pathways, lymphoid tissues, and sites of ongoing or de novo immune activation. Immune cell subpopulations appear to show differential sensitivities and redistribution responses to each hormone depending on the type of leukocyte (neutrophil, monocyte or lymphocyte) and its maturation/functional characteristics (e.g., non-classical/resident or classical/inflammatory monocyte, naïve or central/effector memory T cell). Thus, stress hormones could be administered simultaneously or sequentially to induce specific leukocyte subpopulations to be mobilized into the blood, or to traffic from blood to tissues. Stress- or stress hormone-mediated changes in immune cell distribution could be clinically harnessed to: (1) Direct leukocytes to sites of vaccination, wound healing, infection, or cancer and thereby enhance protective immunity. (2) Reduce leukocyte traffic to sites of inflammatory/autoimmune reactions. (3) Sequester immune cells in relatively protected compartments to minimize exposure to cytotoxic treatments like radiation or localized chemotherapy. (4) Measure biological resistance/sensitivity to stress hormones in vivo. In keeping with the guidelines for Richter Award manuscripts, in addition to original data we also present a model and synthesis of findings in the context of the literature on the effects of short-term stress on immune cell distribution and function.

Copyright © 2012 Elsevier Ltd. All rights reserved.

Figures

Figure 1

Acute stress induces rapid increases in circulating (plasma) norepinephrine, epinephrine, and corticosterone concentrations. Data are expressed as means ± SEM (n = 5 or 6 per time point). Statistically significant differences are indicated: * p

Figure 2

Acute stress induces rapid and significant changes in WBC, monocyte, neutrophil, lymphocyte, and Th, CTL, B, and NK cell numbers in the blood. Leukocyte numbers (1000 /μl blood) were quantified at 0 (baseline) and 6, 15, 30, 60, and 120 minutes during restraint stress. An increase in immune cell numbers relative to baseline represents mobilization of leukocytes into the blood. A decrease from peak numbers or relative to baseline represents trafficking of cells out of the blood and into tissues. Percent change relative to baseline at peak and trough is indicated. Data are expressed as means ± SEM (n=3). Statistically significant differences are indicated: * p

Figure 3

Effects of acute stress on CD62L− and CD62L+ monocyte and neutrophil numbers and CD62L expression on monocytes and neutrophils. Leukocyte numbers (1000 /μl blood) were quantified at 0 (baseline) and 6, 15, 30, 60, and 120 minutes during restraint stress. CD62L surface expression is expressed as mean fluorescence intensity (MFI) as measured on the flow cytometer. An increase in immune cell numbers relative to baseline represents mobilization of leukocytes into the blood. A decrease from peak numbers or relative to baseline represents trafficking of cells out of the blood and into tissues. Percent change relative to baseline at peak and trough is indicated. Data are expressed as means ± SEM (n=3). Statistically significant differences are indicated: * p

Figure 4

Effects of acute stress on CD62L− and CD62L+ T&NK cell, CTL, and B cells numbers and CD62L expression on the same cell types. Leukocyte numbers (1000 /μl blood) were quantified at 0 (baseline) and 6, 15, 30, 60, and 120 minutes during restraint stress. CD62L surface expression is expressed as mean fluorescence intensity (MFI) as measured on the flow cytometer. An increase in immune cell numbers relative to baseline represents mobilization of leukocytes into the blood. A decrease from peak numbers or relative to baseline represents trafficking of cells out of the blood and into tissues. Percent change relative to baseline at peak and trough is indicated. Data are expressed as means ± SEM (n=3). Statistically significant differences are indicated: * p

Figure 5

Effects of control (vehicle, VEH) or stress hormones, norepinephrine (NE), epinephrine (EPI), corticosterone (CORT), NE+EPI, NE+CORT, EPI+CORT, and NE+EPI+CORT (bars shown in this order) on WBC, monocyte, neutrophil, lymphocyte, and Th, CTL, B, and NK cell numbers in the blood. Leukocyte numbers (1000 /μl blood) were quantified 2 hours after vehicle or hormone injection in adrenalectomized animals. An increase in immune cell numbers relative to the VEH group represents mobilization of leukocytes into the blood. A decrease represents trafficking of cells out of the blood and into tissues. Data are expressed as means ± SEM (n=3 or 4 per treatment group). Statistically significant differences are indicated: * p

Figure 6

Effects of control (vehicle, VEH) or stress hormones, norepinephrine (NE), epinephrine (EPI), corticosterone (CORT), NE+EPI, NE+CORT, EPI+CORT, and NE+EPI+CORT (bars shown in this order) on CD62L− and CD62L+ monocyte and neutrophil numbers and CD62L expression on monocytes and neutrophils. Leukocyte numbers (1000 /μl blood) were quantified 2 hours after vehicle or hormone injection in adrenalectomized animals. CD62L surface expression is expressed as mean fluorescence intensity (MFI) as measured on the flow cytometer. An increase in immune cell numbers relative to the VEH group represents mobilization of leukocytes into the blood. A decrease represents trafficking of cells out of the blood and into tissues. Data are expressed as means ± SEM (n=3 or 4 per treatment group). Statistically significant differences are indicated: * p

Figure 7

Effects of control (vehicle, VEH) or stress hormones, norepinephrine (NE), epinephrine (EPI), corticosterone (CORT), NE+EPI, NE+CORT, EPI+CORT, and NE+EPI+CORT (bars shown in this order) on CD62L− and CD62L+ T&NK cell, CTL, and B cells numbers and CD62L expression on the same cell types. Leukocyte numbers (1000 /μl blood) were quantified 2 hours after vehicle or hormone injection in adrenalectomized animals. CD62L surface expression is expressed as mean fluorescence intensity (MFI) as measured on the flow cytometer. An increase in immune cell numbers relative to the VEH group represents mobilization of leukocytes into the blood. A decrease represents trafficking of cells out of the blood and into tissues. Data are expressed as means ± SEM (n=3 or 4 per treatment group). Statistically significant differences are indicated: * p

Figure 8

Model of stress-induced changes in leukocyte distribution. The model integrates the hormone mediators, source organs, kinetics, direction, leukocyte subpopulation specificity, and target organs, of stress-induced changes in blood immune cell numbers (Dhabhar and McEwen, 1997), and is based on data presented here as well as that published in the literature (for review see: (Dhabhar, 1998; Dhabhar, 2009b)). The stressor can be psychological (sensing a predator), physical (exercise / running from predator) or physiological (inflammation), and generally induces the release of norepinephrine (NE), epinephrine (EPI), and corticosterone (CORT). The adaptive/salubrious profile of blood lymphocyte and monocyte (MO) redistribution during stress involves a mobilization/increase (shaded dark upward diagonal) followed by a trafficking/decrease (shaded light downward diagonal) in cell numbers (Dhabhar, 2009b; Rosenberger et al., 2009), while neutrophils show only a biphasic mobilization. In general, early (2 to 30 minutes) during stress, NE and EPI mobilize monocytes, neutrophils, and lymphocytes into the blood from “barracks” like the spleen, marginated leukocyte pool, lung, bone marrow and lymph nodes. However, some leukocyte subpopulations (e.g., CD62L− CTLs or CD62L+ B cells) that are circulating in the blood under resting conditions, may traffic to tissues soon after the beginning of stress. Later (30 to > 120 minutes), overall lymphocyte and monocyte numbers decrease as cells traffic out of the blood and into target organs that include: 1) Active “battlefields” which are sites of wounding, antigen/pathogen entry, de novo immune activation, or ongoing inflammation. 2) Homeostatic surveillance and homing pathways within organs that form the main interfaces between the internal and external environments, i.e., the skin and the mucosal-epithelial linings of the oro-digestive and urogenital tracts. 3) Back to the “barracks” which are sites such as the spleen, lung, bone marrow, lymph nodes to which many immune cells may return during stress. Decreases in blood lymphocytes and monocytes, are largely driven by CORT, and for some subpopulations (CTL & B cells) also by EPI. from the bone marrow and may be driven by NE and EPI, and CORT. It is highly likely that leukocytes with specific functional and/or maturational characteristics (e.g., monocyte vs. neutrophil vs. lymphocyte, inflammatory vs. resident monocyte, naïve vs. memory lymphocyte, mucosa vs. bone marrow directed B cell) traffic to specific targets during stress. If the stress response is accompanied by immune activation then the stress-induced leukocyte redistribution ensures that more leukocytes are present to respond to challenge at sites of ongoing or potential inflammation either by already being present at the site of attack (e.g., inflammatory monocytes, effector T cells, and antibody secreting B cells) or by being available in the blood (e.g., mobilized neutrophils). Using “real world” metaphors, we have suggested that a short-term / fight-or-flight stress response prepares the body's “army” for battle, by inducing a redistribution of the body's “soldiers” from “barracks” to “boulevards” to actual or potential “battlefields” (Dhabhar, 1998; Dhabhar, 2009b). As a result, leukocyte redistribution response is one mechanism through which a short-term stress response facilitates immuno-enhancement in compartments that are enriched with immune cells during/following stress (Dhabhar, 2009b; Dhabhar and McEwen, 1996; Viswanathan et al., 2005; Viswanathan and Dhabhar, 2005).