Mark E. Wilson

Social environment is associated with gene regulatory variation in the rhesus macaque immune system

Variation in the social environment is a fundamental component of many vertebrate societies. In humans and other primates, adverse social environments often translate into lasting physiological costs. The biological mechanisms associated with these effects are therefore of great interest, both for understanding the evolutionary impacts of social behavior and in the context of human health. However, large gaps remain in our understanding of the mechanisms that mediate these effects at the molecular level. Here we addressed these questions by leveraging the power of an experimental system that consisted of 10 social groups of female macaques, in which each individuals social status (i.e., dominance rank) could be experimentally controlled. Using this paradigm, we show that dominance rank results in a widespread, yet plastic, imprint on gene regulation, such that peripheral blood mononuclear cell gene expression data alone predict social status with 80% accuracy. We investigated the mechanistic basis of these effects using cell type-specific gene expression profiling and glucocorticoid resistance assays, which together contributed to rank effects on gene expression levels for 694 (70%) of the 987 rank-related genes. We also explored the possible contribution of DNA methylation levels to these effects, and identified global associations between dominance rank and methylation profiles that suggest epigenetic flexibility in response to status-related behavioral cues. Together, these results illuminate the importance of the molecular response to social conditions, particularly in the immune system, and demonstrate a key role for gene regulation in linking the social environment to individual physiology.

Timing of births and reproductive success in rhesus monkey social groups.

Analysis of six years of birth records (228 potential reproductive years) from 71 female rhesus monkeys, housed in social groups in outdoor compounds, revealed several variables which influenced the reproductive pattern. The timing of conception in this seasonally breeding species was influenced by age, parity, and reproductive outcome the previous year, but not by social rank. In contrast, high social rank did confer an advantage in higher conception and lower infant mortality rates.

Estradiol, but not raloxifene, improves aspects of spatial working memory in aged ovariectomized rhesus monkeys

Estrogen replacement therapy (ERT) alleviates many postmenopausal symptoms but whether it also benefits cognitive function remains controversial. Further, since estrogen increases the risk of breast and uterine cancers, a new class of compounds, called selective estrogen receptor modulators (SERMs) is being considered as possible alternative to ERT. The SERM raloxifene is particularly interesting because, like estrogen, it improves lipid metabolism and reduces bone loss, without adverse effects on the breast or uterus. Little is known, however, about its effect upon cognitive function. We used a rhesus monkey model of human menopause to examine the effects of ERT and raloxifene on cognitive function. We tested 5 aged females (21-24 years old) ovariectomized long-term (10-16 years) on a battery of age-sensitive tasks, including the Delayed Response (DR), the Delayed Non-Matching-to-Sample-10 min (DNMS-10 min) and the spatial-Delayed Recognition Span Test (DRST). Monkeys were tested 5 days a week on each task for 9 consecutive months, while undergoing treatments with placebo, ethinyl estradiol (EE(2)), and raloxifene in alternating 28-days blocks. EE(2) transiently enhanced the working memory component of the spatial-DRST, but did not affect performance on the other tasks of the battery. Raloxifene had no effect on cognitive performance. These findings indicate that estradiol is able to enhance some aspects of spatial working memory in aged monkeys despite many years of estrogenic deprivation. Further, they suggest that raloxifene does not affect cognitive function after long-term ovarian hormone deprivation.

Continuous expression of corticotropin-releasing factor in the central nucleus of the amygdala emulates the dysregulation of the stress and reproductive axes

An increase in corticotropin-releasing factor (CRF) is a putative factor in the pathophysiology of stress-related disorders. As CRF expression in the central nucleus of the amygdala (CeA) is important in adaptation to chronic stress, we hypothesized that unrestrained synthesis of CRF in CeA would mimic the consequences of chronic stress exposure and cause dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis, increase emotionality and disrupt reproduction. To test this hypothesis, we used a lentiviral vector to increase CRF-expression site specifically in CeA of female rats. Increased synthesis of CRF in CeA amplified CRF and arginine vasopressin peptide concentration in the paraventricular nucleus of the hypothalamus, and decreased glucocorticoid negative feedback, both markers associated with the pathophysiology of depression. In addition, continuous expression of CRF in CeA also increased the acoustic startle response and depressive-like behavior in the forced swim test. Protein levels of gonadotropin-releasing hormone in the medial preoptic area were significantly reduced by continuous expression of CRF in CeA and this was associated with a lengthening of estrous cycles. Finally, sexual motivation but not sexual receptivity was significantly attenuated by continuous CRF synthesis in ovariectomized estradiol-progesterone-primed females. These data indicate that unrestrained CRF synthesis in CeA produces a dysregulation of the HPA axis, as well as many of the behavioral, physiological and reproductive consequences associated with stress-related disorders.

Relationship between Social Factors and Pituitary-Adrenocortical Activity in Female Rhesus Monkeys (Macaca mulatta)

Psychosocial stress in humans has been related to the occurrence or progression of certain diseases and a positive social environment has been shown, in some cases, to ameliorate this effect. In many experimental studies changes in serum cortisol levels have been used as an endocrine-defined stress response, however, the variation in this measure under unchanging conditions in socially housed animals is not well documented. The present study was designed to examine the relationship between specific social behaviors and concentrations of serum cortisol in female rhesus monkeys. Subjects were members of one of two social groups. One was an established long-term (8 years) group with members which had from one to many kin, while the second was a group formed 5 months prior to the onset of the present study composed of initially unfamiliar animals. Blood samples were collected weekly and a total of 16 hr of behavioral data was collected on each subject. For purposes of analyses, the year-long study was organized according to calendar quarters (3 months) with cortisol concentrations averaged and behavioral categories cumulated. When regression techniques were applied, it is of note that neither group nor dominance rank was a predictor of cortisol levels. However, when a correlation analysis was applied to each group separately to assess the effect of dominance rank, rank significantly correlated with cortisol levels in the established but not in the recently formed group. In each quarter, analyses revealed behavioral categories which combined to account for a significant proportion of the variance in cortisol levels, but these behaviors were not identical over the four quarters. For three of the four quarters analyzed, and for the year as a whole, an affiliative behavioral category was predictive of cortisol levels in addition to agonistic behavioral categories. These data suggest that cortisol levels are influenced by not only negative interactions, such as receiving bites, but also by positive interactions such as receiving grooms. Results also showed that reconciliatory behaviors occurred significantly more frequently following dyadic agonistic episodes in the recently formed versus the long-term group. The higher rate of reconciliatory behaviors exhibited by the recently formed group may account for the lack of a dominance rank/cortisol relationship.

Formation of a new social group of unfamiliar female rhesus monkeys affects the immune and pituitary adrenocortical systems

Social stress associated with the formation of a new group of rhesus monkeys resulted in increased basal cortisol secretion and significant decreases in immunological parameters. Eight adult female rhesus monkeys, all of which had been raised in social groups, but with no common social history, were simultaneously introduced into an outdoor enclosure along with an adult male. Behavioral data were collected during the introduction and over 9 weeks thereafter. Blood samples were collected prior to and at intervals for 9 weeks following formation. The establishment of a dominance hierarchy, apparent within 48 h, was accomplished with no serious fighting and a complete absence of wounding or trauma. Overall, the group showed a significant increase in cortisol and a significant decrease in the absolute number of total lymphocytes and CD4+ and CD8+ T cells at 24 h postformation, but not thereafter. However, when partitioned into high and low dominance rank, differences in CD4+ and CD8+ T cells were evident for up to 9 weeks with low ranking subjects showing significantly lower values. The housing condition of the subjects immediately prior to introduction, either indoors in individual caging or outdoors in social groups, may have influenced behavior, rank acquisition, and possibly differences in immune parameters. These data demonstrate that social group formation is a potent psychosocial stressor in primates, since stress-sensitive changes were observed in the absence of serious aggression and wounding.

Social dominance and female reproductive behaviour in rhesus monkeys (Macaca mulatta)

Abstract The relationship between dominance rank and female sexual behaviour was examined in rheusus monkeys (Macaca mulatta) living in a social group. High-ranking females engaged in copulatory series as frequently as lower-ranking females. Furthermore, lower-ranking females copulated with as many available males as did high-ranking females. Social rank did appear to influence the pattern of sexual activity exhibited, in that copulatory series were more often initiated by the higher-ranking animal of the mating pair. Copulatory series involving high-ranking females were characterized by more mounts by males and were longer in duration. Higher-ranking animals interfered more often with copulatory series involving other animals, but such interference was not effective in preventing completion of the series. These data indicate that any reproductive advantage conferred to high-ranking females is not the result of sexual competition in social-living rhesus monkeys.

Social separation and reunion affects immune system in juvenile rhesus monkeys

Removal of juvenile rhesus monkeys from their natal social group to indoor individual caging resulted in increased basal cortisol secretion and significant decrements in the frequency of lymphoid subpopulations. Fourteen juvenile rhesus monkeys, which had never been removed from the group, were studied. Baseline immune and cortisol measurements were obtained before seven of the subjects were removed from social housing to standard individual cages. The remaining seven subjects, matched for age, sex, weight, and rank, remained in the social group throughout the study serving as controls. Blood samples were taken 24 hours after removal of the test subjects from the group and at specific intervals thereafter through 11 weeks. At 24 hours after the separation test subjects showed a significant increase in basal cortisol levels (40%) and a significant decrease in several immune parameters, with absolute numbers of total T cells declining 72 +/- 12%. Significant group differences in immune parameters persisted through 11 weeks. Eighteen weeks following removal, the test subjects were returned to the group which produced a cortisol rise in both test and controls at the 24-hour postreturn sample. Although there were no group differences in the frequency of lymphoid subsets 24 hours after return, some test subjects showed marked decrements which were inversely related to cortisol and were predicted by behavioral events. These data demonstrate that the removal of naive juvenile rhesus monkeys from their natal social group to individual indoor caging is a potent psychosocial stressor and that the behavioral interactions which characterize the return of the individual subjects to the natal group may predict physiological response.

Pubertal growth spurt in the female rhesus monkey: Relation to menarche and skeletal maturation

Three‐monthly measurements of tibia length, crown‐rump length (CRL), and body wieght have been made on nine indoor‐housed and six outdoor‐housed female rhesus monkeys (Macaca mulatta) from ages 17 to 41 months. In the indoor‐housed there were clear pubertal growth spurts, identifiable in individual curves, with average peaks at 22.5 months for tibia length, 23.5 months for CRL, and 24.5 for weight. Menarche occurred on average at 26.0 months. In both skeletal dimensions the amount of acceleration exceeded that which occurs in man. The relative timing of the spurts and of menarche was similar to that in man. The outdoor‐housed monkeys showed a similar spurt in tibia length, although a less easily identifiable one in crown‐rump length. The peak velocity of tibia length in these monkeys occurred on average at 30.5 months, with menarche at 32.5 months. We conclude that the pubertal growth spurt in female rhesus is very little different from that in man.

Social status alters immune regulation and response to infection in macaques

Status alters immune function in macaques Rhesus macaques experience variable levels of stress on the basis of their position in the social hierarchy. To examine how stress affects immune function, Snyder-Mackler et al. manipulated the social status of individual macaques (see the Perspective by Sapolsky). Social status influenced the immune system at multiple levels, from immune cell numbers to gene expression, and altered signaling pathways in a model of response to infection. Macaques possess a plastic and adaptive immune response wherein social subordination promotes antibacterial responses, whereas high social status promotes antiviral responses. Science, this issue p. 1041; see also p. 967 Manipulation of social status in macaques affects cell-specific immune gene regulation. Social status is one of the strongest predictors of human disease risk and mortality, and it also influences Darwinian fitness in social mammals more generally. To understand the biological basis of these effects, we combined genomics with a social status manipulation in female rhesus macaques to investigate how status alters immune function. We demonstrate causal but largely plastic social status effects on immune cell proportions, cell type–specific gene expression levels, and the gene expression response to immune challenge. Further, we identify specific transcription factor signaling pathways that explain these differences, including low-status–associated polarization of the Toll-like receptor 4 signaling pathway toward a proinflammatory response. Our findings provide insight into the direct biological effects of social inequality on immune function, thus improving our understanding of social gradients in health.