Individual differences in the peripheral immune system promote resilience versus susceptibility to social stress

Abstract

Depression and anxiety disorders are associated with increased release of peripheral cytokines; however, their functional relevance remains unknown. Using a social stress model in mice, we find preexisting individual differences in the sensitivity of the peripheral immune system that predict and promote vulnerability to social stress. Cytokine profiles were obtained 20 min after the first social stress exposure. Of the cytokines regulated by stress, IL-6 was most highly up-regulated only in mice that ultimately developed a susceptible behavioral phenotype following a subsequent chronic stress, and levels remained elevated for at least 1 mo. We confirmed a similar elevation of serum IL-6 in two separate cohorts of patients with treatment-resistant major depressive disorder. Before any physical contact in mice, we observed individual differences in IL-6 levels from ex vivo stimulated leukocytes that predict susceptibility versus resilience to a subsequent stressor. To shift the sensitivity of the peripheral immune system to a pro- or antidepressant state, bone marrow (BM) chimeras were generated by transplanting hematopoietic progenitor cells from stress-susceptible mice releasing high IL-6 or from IL-6 knockout (IL-6(-/-)) mice. Stress-susceptible BM chimeras exhibited increased social avoidance behavior after exposure to either subthreshold repeated social defeat stress (RSDS) or a purely emotional stressor termed witness defeat. IL-6(-/-) BM chimeric and IL-6(-/-) mice, as well as those treated with a systemic IL-6 monoclonal antibody, were resilient to social stress. These data establish that preexisting differences in stress-responsive IL-6 release from BM-derived leukocytes functionally contribute to social stress-induced behavioral abnormalities.

Keywords: anxiety; depression; interleukin-6; leukocytes; stress.

Conflict of interest statement

Conflict of interest statement: This work was supported by a research grant from Janssen Pharmaceuticals.

Figures

Fig. 1.

Individual differences in the peripheral immune system predict behavioral responses to RSDS. (A) A within-subjects time course of plasma levels of IL-6 indicated a significant interaction between time point of blood draw and behavioral phenotype (F4,19 = 10.83, P < 0.001, control = 9, susceptible = 7, resilient = 6). Post hoc analysis indicated no significant differences in IL-6 levels at baseline (P > 0.05). Twenty minutes after the first defeat, animals that developed a susceptible phenotype had higher circulating levels of IL-6 than control or resilient mice (P < 0.001). Forty-eight hours after the last defeat, susceptible mice had higher levels of IL-6 than resilient or control mice (P < 0.001). (B) Patients with a diagnosis of treatment-resistant MDD (n = 19) scored higher on the QIDS than healthy controls (n = 18) (t35 = 15.68, P < 0.0001, two tailed). (C) Patients with treatment-resistant MDD had higher circulating levels of IL-6 than healthy controls (t35 = 3.17, P < 0.01, two tailed). (D) Thirty-five days after the last defeat, susceptible mice injected with saline (n = 7) had basal elevations in serum levels of IL-6 compared with controls (saline, n = 13, Imipramine, n = 13) and resilient (n = 6) mice (F4,45= 3.66, P < 0.05) but not susceptible mice injected with Imipramine (n = 11, P > 0.05). (E) Twenty-four hours after the last day of CVS, mice showed significant elevations of IL-6 (t12 = 2.48, P < 0.05, two tailed). (F) Mice exposed to the nonphysical social stress of witness defeat (witness = 10, control = 11) demonstrated elevations in IL-6 30 d after the last stressor, when serum levels were measured immediately after the social interaction test (t19 = 3.1, P < 0.01, two tailed). Bar graphs display mean ± SEM. # denotes a significant interaction. * denotes a significant difference in phenotype. For t tests, * denotes a significant difference between means.

Fig. 2.

Leukocyte profiles before stress. (A) Before RSDS, mice that displayed a susceptible phenotype at the end of RSDS had more circulating leukocytes than mice that displayed a resilient phenotype (t36 = 3.03, P < 0.01, two tailed; susceptible, n = 25, resilient, n = 13). (B) Prestress circulating leukocytes before social defeat stress negatively correlated with the social interaction ratio following RSDS (r = –0.44, P < 0.01). (C) Before RSDS, leukocytes from susceptible animals released more IL-6 than from resilient animals when stimulated with LPS (F1,36 = 15, P < 0.001; susceptible, n = 25, resilient, n = 13). (D) Prestress levels of IL-6 released by leukocytes in response to LPS negatively correlated with the social interaction ratio following RSDS (r = –0.48, P < 0.01). Bar graphs display mean ± SEM. # denotes a significant interaction. * denotes a significant difference in phenotype. Circles denote individual animals. For t tests, * denotes a significant difference between means.

Fig. 3.

BM hematopoietic progenitor cell transplant from a susceptible donor induces depression-like behavior in naïve host animals. (A) A greater percentage of cells came from the donor versus host in BM chimeras across cell types [χ2 (14, n = 58) = 24.11, P < 0.05, two tailed]. The phenotype of the donor did not alter the distribution of leukocytes in either condition [viable cells, χ2 (2, n = 58) = 0.5, P > 0.05, two tailed; T cells, χ2 (2, n = 58) = 1.08, P > 0.05, two tailed; B cells, χ2 (2, n = 29) = 0.04, P > 0.05, two tailed; monocytes, χ2 (2, n = 58) = 1.28, P > 0.05, two tailed]. (B) In a subset of animals tested, naïve host animals that received BM transplants from susceptible donors (n = 14) had higher levels of circulating leukocytes 28 d after transplant and before stress than host mice that received BM grafts from control donors (n = 15) (t25 = 2.34, P < 0.05, two tailed). (C) Donor phenotype did not alter the percentage of microglia from the host or donor measured in a subset of the animals [χ2 (1, n = 15) = 0.0, P > 0.05, two tailed]. More than 98% of the microglia were of host origin. (D) Stress-susceptible BM chimeras that underwent a subthreshold defeat (n = 7) displayed social avoidance behavior, as indicated by a significant interaction and post hoc analysis, compared with control (no defeat, n = 8; subthreshold defeat, n = 7) and stress-susceptible BM chimeras that did not undergo subthreshold defeat (n = 7), as indicated by a significant interaction (F1,25 = 6.94, P < 0.05). (E) Stress-susceptible BM chimeras that witnessed RSDS (subthreshold witness, n = 7) but not animals with transplants from the same donors that did not witness RSDS (n = 8) or control BM chimeras (no witness, n = 6; subthreshold witness, n = 8) displayed greater social avoidance behavior, as indicated by a significant interaction and post hoc analysis (F1,25 = 4.99, P < 0. 05). Bar graphs display mean ± SEM. * denotes a significant main effect of phenotype. # denotes a significant interaction between donor phenotype and exposure to subthreshold defeat/witness. For t tests, * denotes a significant difference between means.

Fig. 4.

Peripheral IL-6 controls susceptibility versus resilience to social defeat. (A) Percent of donor versus host leukocyte concentration in blood of BM chimeras [χ2(14, n = 32) = 70.76, P < 0.001, two tailed]. IL-6−/− BM chimeras had similar distribution of leukocytes in blood compared with WT BM chimeras [viable cells, χ2(2, n = 32) = 0.63, P > 0.05, two tailed; T cells, χ2(2, n = 32) = 1.02, P > 0.05, two tailed; B cells, χ2(2, n = 32) = 1.47, P > 0.05; monocytes χ2 (2, n = 32) = 1.18, P > 0.05]. (B) Constitutive IL-6−/− (IL-6−/−, n = 14; WT, n = 15) or IL-6−/− BM chimeras (IL-6−/−, n = 9; WT, n = 8) are resilient following 10 d of RSDS, as indicated by a significant main effect of IL-6 deletion (F1,43 = 9.18, P < 0.01). (C) IL-6−/− BM chimeras (n = 8) are more resilient to witness defeat (t12 = 2.21, P < 0.05, two tailed) compared with control BM chimera mice (n = 6). (D) An i.p. injection of IL-6 (n = 5), but not IgG (n = 4) mAb, blocked an increase in circulating IL-6 within 20 min of the first defeat (t7 = 4.85, P < 0.01, two tailed). (E) Daily systemic injection of IL-6 (n = 22) but not IgG (n = 23) mAb or saline (n = 28) blocked the development of social avoidance behavior (F3,72 = 7.14, P < 0.01). Bar graphs display mean ± SEM. * denotes a significant main effect of IL-6 deletion. For t test, * denotes a significant difference between means.