Social interactions alter proinflammatory cytokine gene expression and behavior following endotoxin administration

Immune-Endocrine Links to Gregariousness in Wild House Mice

Social interactions are critically important for survival and impact overall-health, but also impose costs on animals, such as exposure to contagious agents. The immune system can play a critical role in modulating social behavior when animals are sick, as has been demonstrated within the context of “sickness behaviors.” Can immune molecules affect or be affected by social interactions even when animals are not sick, therefore serving a role in mediating pathogen exposure? We tested whether markers of immune function in both the blood and the brain are associated with gregariousness, quantified as number of animals interacted with per day. To do this, we used remote tracking of social interactions of a wild population of house mice (Mus musculus domesticus) to categorize animals in terms of gregariousness. Blood, hair, brain and other tissue samples from animals with extreme gregariousness phenotypes were collected. We then tested whether the levels of three important cytokines (TNF-α, IFN-γ and IL-1β) in the serum, cortex and hypothalamus of these animals could be explained by the gregariousness phenotype and/or sex of the mice. Using the hair as a long-term quantification of steroid hormones, we also tested whether corticosterone, progesterone and testosterone differed by social phenotype. We found main effects of gregariousness and sex on the serum levels of TNF-α, but not on IFN-γ or IL-1β. Brain gene expression levels were not different between phenotypes. All hair steroids tended to be elevated in animals of high gregariousness phenotype, independent of sex. In sum, elements of the immune system may be associated with gregariousness, even outside of major disease events. These results extend our knowledge of the role that immune signals have in contributing to the regulation of social behaviors outside periods of illness.

A test of the effects of androgens on immunity: No relationship between 11-ketotestosterone and immune performance in bluegill (Lepomis macrochirus)

The immunosuppressive effects of androgens are a key component of the immunocompetence handicap hypothesis (ICHH). Here, we use bluegill sunfish (Lepomis macrochirus) to test two predictions arising from this hypothesis: (1) natural circulating concentrations of the androgen 11-ketotestosterone (11-KT) will be negatively related with measures of immunity, and (2) immune stimulation will lower circulating 11-KT concentration. We found no evidence for a relationship between natural circulating 11-KT concentration and measures of immunity (lymphocyte and granulocyte counts, respiratory burst, cytokine mRNA levels), and an immune stimulation with Vibrio vaccine did not affect circulating 11-KT concentration. We also performed a meta-analysis of immune stimulation studies to help interpret our results, and report evidence suggesting that immune stimulation has weaker effects on androgen levels in fishes compared to other vertebrates. These results suggest that the ICHH may not apply to all vertebrates, although it remains premature to state what factors account for the weaker evidence in fishes that androgens are immunosuppressive.

Why are behavioral and immune traits linked?

Through behavior, animals interact with a world where parasites abound. It is easy to understand how behavioral traits can thus have a differential effect on pathogen exposure. Harder to understand is why we observe behavioral traits to be linked to immune defense traits. Is variation in immune traits a consequence of behavior-induced variation in immunological experiences? Or is variation in behavioral traits a function of immune capabilities? Is our immune system a much bigger driver of personality than anticipated? In this review, I provide examples of how behavioral and immune traits co-vary. I then explore the different routes linking behavioral and immune traits, emphasizing on the physiological/hormonal mechanisms that could lead to immune control of behavior. Finally, I discuss why we should aim at understanding more about the mechanisms connecting these phenotypic traits.

Neuroendocrine-immune circuits, phenotypes, and interactions

Multidirectional interactions among the immune, endocrine, and nervous systems have been demonstrated in humans and non-human animal models for many decades by the biomedical community, but ecological and evolutionary perspectives are lacking. Neuroendocrine-immune interactions can be conceptualized using a series of feedback loops, which culminate into distinct neuroendocrine-immune phenotypes. Behavior can exert profound influences on these phenotypes, which can in turn reciprocally modulate behavior. For example, the behavioral aspects of reproduction, including courtship, aggression, mate selection and parental behaviors can impinge upon neuroendocrine-immune interactions. One classic example is the immunocompetence handicap hypothesis (ICHH), which proposes that steroid hormones act as mediators of traits important for female choice while suppressing the immune system. Reciprocally, neuroendocrine-immune pathways can promote the development of altered behavioral states, such as sickness behavior. Understanding the energetic signals that mediate neuroendocrine-immune crosstalk is an active area of research. Although the field of psychoneuroimmunology (PNI) has begun to explore this crosstalk from a biomedical standpoint, the neuroendocrine-immune-behavior nexus has been relatively underappreciated in comparative species. The field of ecoimmunology, while traditionally emphasizing the study of non-model systems from an ecological evolutionary perspective, often under natural conditions, has focused less on the physiological mechanisms underlying behavioral responses. This review summarizes neuroendocrine-immune interactions using a comparative framework to understand the ecological and evolutionary forces that shape these complex physiological interactions.

Gadd45b is an epigenetic regulator of juvenile social behavior and alters local pro-inflammatory cytokine production in the rodent amygdala

Precise regulation of the epigenome during perinatal development is critical to the formation of species-typical behavior later in life. Recent data suggests that Gadd45b facilitates active DNA demethylation by recruiting proteins involved in base excision repair (BER), which will catalyze substitution of 5-methyl-cytosine (5mC) for an unmodified cytosine. While a role for Gadd45b has been implicated in both hippocampal and amygdalar learning tasks, to the best of our knowledge, no study has been done investigating the involvement of Gadd45b in neurodevelopmental programming of social behavior. To address this, we used a targeted siRNA delivery approach to transiently knock down Gadd45b expression in the neonatal rat amygdala. We chose to examine social behavior in the juvenile period, as social deficits associated with neurodevelopmental disorders tend to emerge in humans at an equivalent age. We find that neonatal Gadd45b knock-down results in altered juvenile social behavior and reduced expression of several genes implicated in psychiatric disorders, including methyl-CpG-binding protein 2 (MeCP2), Reelin, and brain derived neurotrophic factor (BDNF). We furthermore report a novel role for Gadd45b in the programmed expression of α2-adrenoceptor (Adra2a). Consistent with Gadd45b's role in the periphery, we also observed changes in the expression of pro-inflammatory cytokines interleukin-6 (Il-6) and interleukin-1beta (Il-1beta) in the amygdala, which could potentially mediate or exacerbate effects of Gadd45b knockdown on the organization of social behavior. These data suggest a prominent role for Gadd45b in the epigenetic programming of complex juvenile social interactions, and may provide insight into the etiology of juvenile behavioral disorders such as ADHD, autism, and/or schizophrenia.

When is it socially acceptable to feel sick?

Disease is a ubiquitous and powerful evolutionary force. Hosts have evolved behavioural and physiological responses to disease that are associated with increased survival. Behavioural modifications, known as 'sickness behaviours', frequently involve symptoms such as lethargy, somnolence and anorexia. Current research has demonstrated that the social environment is a potent modulator of these behaviours: when conflicting social opportunities arise, animals can decrease or entirely forgo experiencing sickness symptoms. Here, I review how different social contexts, such as the presence of mates, caring for offspring, competing for territories or maintaining social status, affect the expression of sickness behaviours. Exploiting the circumstances that promote this behavioural plasticity will provide new insights into the evolutionary ecology of social behaviours. A deeper understanding of when and how this modulation takes place may lead to better tools to treat symptoms of infection and be relevant for the development of more efficient disease control programmes.

Lipopolysaccharide-induced sickness behavior in lactating rats decreases ultrasonic vocalizations and exacerbates immune system activity in male offspring

OBJECTIVE

The present study analyzed the effects of lipopolysaccharide (LPS) on maternal behavior during lactation and possible correlations with changes in emotional and immune responses in offspring.

METHODS

Lactating rats received 100 μg/kg LPS, and the control group received saline solution on lactation day (LD) 3. Maternal general activity and maternal behavior were observed on LD5 (i.e. the day that the peak of fever occurred). In male pups, hematological parameters and ultrasonic vocalizations (USVs) were assessed on LD5. At weaning, an additional dose of LPS (50 µg/kg, i.p.) was administered in male pups, and open-field behavior, oxidative burst and phagocytosis were evaluated.

RESULTS

A reduction in the time in which dams retrieved the pups was observed, whereas no effects on maternal aggressive behavior were found. On LD5, a reduction of the frequency of USVs was observed in pups, but no signs of inflammation were found. At weaning, an increase in immune system activity was observed, but no differences in open-field behavior were found.

CONCLUSION

These results indicate that inflammation in lactating mothers disrupted mother/pup interactions and may have produced short- and long-term effects on pup behavior as well as biological pathways that modulate inflammatory responses to bacterial endotoxin challenge in pups.

Rodent models of depression: neurotrophic and neuroinflammatory biomarkers

Rodent models are an indispensable tool for studying etiology and progress of depression. Since interrelated systems of neurotrophic factors and cytokines comprise major regulatory mechanisms controlling normal brain plasticity, impairments of these systems form the basis for development of cerebral pathologies, including mental diseases. The present review focuses on the numerous experimental rodent models of depression induced by different stress factors (exteroceptive and interoceptive) during early life (including prenatal period) or adulthood, giving emphasis to the data on the changes of neurotrophic factors and neuroinflammatory indices in the brain. These parameters are closely related to behavioral depression-like symptoms and impairments of neuronal plasticity and are both gender- and genotype-dependent. Stress-related changes in expression of neurotrophins and cytokines in rodent brain are region-specific. Some contradictory data reported by different groups may be a consequence of differences of stress paradigms or their realization in different laboratories. Like all experimental models, stress-induced depression-like conditions are experimental simplification of clinical depression states; however, they are suitable for understanding the involvement of neurotrophic factors and cytokines in the pathogenesis of the disease—a goal unachievable in the clinical reality. These major regulatory systems may be important targets for therapeutic measures as well as for development of drugs for treatment of depression states.

Increased activity correlates with reduced ability to mount immune defenses to endotoxin in zebra finches

When suffering from infection, animals experience behavioral and physiological alterations that potentiate the immune system's ability to fight pathogens. The behavioral component of this response, termed "sickness behavior," is characterized by an overall reduction in physical activity. A growing number of reports demonstrate substantial flexibility in these sickness behaviors, which can be partially overcome in response to mates, intruders and parental duties. Since it is hypothesized that adopting sickness behaviors frees energetic resources for mounting an immune response, we tested whether diminished immune responses coincided with reduced sickness behaviors by housing male zebra finches (Taeniopygia guttata) in social conditions that alter their behavioral response to an endotoxin. To facilitate our data collection, we developed and built a miniaturized sensor capable of detecting changes in dorsoventral acceleration and categorizing them as different behaviors when attached to the finches. We found that the immune defenses (quantified as haptoglobin-like activity, ability to change body temperature and bacterial killing capacity) increased as a function of increased time spent resting. The findings indicate that when animals are sick attenuation of sickness behaviors may exact costs, such as reduced immune function. The extent of these costs depends on how relevant the affected components of immunity are for fighting a specific infection.

Mood disturbance after infection

OBJECTIVE

An aetiological link between acute infection and major depression has long been hypothesized, and is increasingly gaining recognition within contemporary literature. This review aims to examine the evidence for such a link, specifically between acute, self-limiting infection and major depression, and to summarize the current understanding of pathophysiological mechanisms underlying this link.

METHODS

Relevant articles were sourced via an online search of published literature from Embase, MEDLINE, PsycINFO and PubMed using a variety of search terms including mood disorder, depression, infection and inflammation. Additionally, a search for articles from the bibliographies of retrieved papers was conducted.

RESULTS

Findings from retrospective studies suggest an association between infection and subsequent mood disturbance, including major depression. This association has been confirmed by studies employing prospective observational or experimental challenge designs. The available evidence supports a multifactorial basis of vulnerability towards major depression in the context of acute infection. Genetic, neuroendocrine, autonomic and psychosocial factors may interact to potentiate the likelihood of a severe and prolonged depressive response to an immunological stressor in some individuals.

CONCLUSION

Mood disturbance is likely to have a host-protective role in the context of an acute sickness response to infection. However, this usually adaptive and reversible response may progress in some vulnerable individuals into a more sustained and severe pattern of behavioural and physiological changes of major depression. Further research is needed to delineate the factors that predispose, precipitate and perpetuate depression in the context of acute infective illness. Such insights will inform effective prevention and treatment strategies.

The impact of exposure to a novel female on symptoms of infection and on the reproductive axis

BACKGROUND/AIM

Sickness behaviors are the behavioral alterations animals exhibit during the course of an infection, often accompanied by reduced reproductive activity. Adopting sickness behaviors may aid in overcoming the infection, by diverting energy from routine activities towards enhancement of the immune system. Nonetheless, sickness behaviors are plastic, being influenced by specific environmental and social circumstances. Here, we tested whether the presentation of a novel female to males suffering from a simulated infection could impact the behavioral effects of sickness, the reproductive axis, or both.

METHODS

Male zebra finches were housed in isolation and injected intramuscularly with lipopolysaccharide or saline. Behaviors were recorded before (3 h before injection) and after (3.5 h after injection) addition of a novel female to the cage for 30 min. Four hours after injection, we collected the brain and testis for the analysis of important reproductive axis modulators, gonadotropin-releasing hormone, and gonadotropin-inhibitory hormone, and to quantify gene expression of a proinflammatory cytokine involved in the regulation of sickness behaviors [interleukin (IL)-1β]. Testosterone was quantified in the plasma.

RESULTS

The presence of a novel female diminished sickness behaviors and induced alterations in the reproductive axis within 30 min, with no associated changes in brain gene expression of IL-1β. Social environment itself altered brain gene expression of IL-1β.

CONCLUSIONS

Male zebra finches prioritize the opportunity to mate versus investment in recovery from an infection, as determined by reduced expression of sickness behaviors when a potential mate was present. The behavioral effects of IL-1β appear to be context dependent in this species.

Variation in melatonin receptors (Mel(1a) and Mel(1b)) and androgen receptor (AR) expression in the spleen of a seasonally breeding bird, Perdicula asiatica

Daily variation in the peripheral level of melatonin plays a major role in integrating reproduction and environmental information for seasonally breeding birds. However, the variation in immunity and reproduction has never been assessed in any avian species on a 24 h time scale. Therefore, to understand the relationship between immune function and reproductive phases in a seasonally breeding bird, Perdicula asiatica, the Indian jungle bush quail, we studied the daily variation of melatonin and testosterone levels along with expression of their receptors Mel(1a), Mel(1b), and androgen receptor in the spleen during the reproductively active phase. Immunocytochemistry for the melatonin receptors Mel(1a) and Mel(1b) presented a differential distribution pattern. Western blot of splenic protein suggested a daily rhythm of melatonin receptors, while acrophases for the two melatonin receptors Mel(1a) and Mel(1b) differed by 4 h, suggesting that the expression of the receptors may peak at different times, causing more of either Mel(1a) or Mel(1b) to be available at a particular time to mediate function. The circulatory melatonin level correlated with percentage stimulation ratio of splenocytes and plasma interleukin-2 level, but did not correlate with testosterone or androgen receptor, suggesting that melatonin could be a major hormone imparting a time-of-day effect on the modulation of immune function in a seasonally breeding bird during the reproductively active phase.

Acute-phase responses vary with pathogen identity in house sparrows (Passer domesticus)

Pathogens may induce different immune responses in hosts contingent on pathogen characteristics, host characteristics, or interactions between the two. We investigated whether the broadly effective acute-phase response (APR), a whole body immune response that occurs in response to constitutive immune receptor activation and includes fever, secretion of immune peptides, and sickness behaviors such as anorexia and lethargy, varies with pathogen identity in the house sparrow ( Passer domesticus ). Birds were challenged with a subcutaneous injection of either a glucan at 0.7 mg/kg (to simulate fungal infection), a synthetic double-stranded RNA at 25 mg/kg (to simulate viral infection), or LPS at 1 mg/kg (to simulate a gram-negative bacterial infection), and then body mass, core body temperature changes, sickness behaviors, and secretion of an acute-phase protein, haptoglobin, were compared. Despite using what are moderate-to-high pyrogen doses for other vertebrates, only house sparrows challenged with LPS showed measurable APRs. Febrile, behavioral, and physiological responses to fungal and viral mimetics had minimal effects.

The primary headaches as a reflection of genetic darwinian adaptive behavioral responses

OBJECTIVE

The objective of this study is to present a view of the primary headaches as genetically determined behavioral responses consistent with sickness behavior and defense reaction, respectively.

BACKGROUND AND DESIGN

A review of the literature bearing on the behavioral, humoral, and functional imaging aspects of the primary headaches shows that migraine and cluster headache (CH) are pain conditions characterized by different behaviors during the attacks. Here it is postulated that the behavioral responses to migraine and CH are evolutionary conserved reactions consistent with sickness behavior and defense reaction.

RESULTS

The sickness behavior observed during migraine attacks is a pan-mammalian adaptive response to internal and external stressors, characterized by withdrawal and motor quiescence, sympatho-inhibition and lethargy, in which visceral pain signals a homeostatic imbalance of the body and/or brain. In contrast, the defense reaction in CH consists of a fight-or-flight reaction, with motor restlessness and agitation, in which pain is exteroceptive in kind.

CONCLUSION

These different behavioral responses are thus specific to different kinds of pain, distinguished by the behavioral significance of the pain (visceral pain in migraine vs exteroceptive pain in CH), and imply brain matrices involving different networks in the brainstem, hypothalamus, and forebrain regions that engender evolutionarily conserved adaptive genetic responses. Cytokines play an important role in their development. Predictions and limitations of the hypothesis are discussed together with implications for genetic studies on headaches.

A critical review of human endotoxin administration as an experimental paradigm of depression

The syndrome called depression may represent the common final pathway at which different aetiopathogenic processes converge. One such aetiopathogenic process is innate immune system activation. Some depressed patients have increased levels of inflammatory cytokines and other immunologic abnormalities. It is not known whether immune system activation contributes to the pathogenesis of depressive symptoms. Supporting this possibility is the observation that in both rodents and humans, exogenous immune stimuli such as endotoxin can produce symptoms that resemble depression. A new approach to depression research would be to use immune stimuli to elicit depressive symptoms in humans. Here we review each of the symptoms elicited in humans by endotoxin administration, and compare this model to two other immune depression paradigms: interferon-alpha treatment and typhoid vaccine administration, to assess to what degree endotoxin administration represents a valid model of immune depression. We also review corresponding behavioral changes in rodents and the potential molecular pathways through which immune system activation produces each symptom.

Immune and behavioral consequences of microglial reactivity in the aged brain

Bidirectional communication between the immune system and the brain is essential for mounting the appropriate immunological, physiological, and behavioral responses to immune activation. Aging, however, may impair this important bi-directional interaction. In support of this notion, peripheral infection in the elderly is associated with an increased frequency of behavioral and cognitive complications. Recent findings in animal models of aging and neurodegenerative disease indicate that microglia, innate immune cells of the brain, become primed or reactive. Understanding age- and disease-associated alterations in microglia is important because glia (microglia and astrocytes) play an integral role in propagating inflammatory signals that are initiated in the periphery. In this capacity, brain glia produce inflammatory cytokines that target neuronal substrates and elicit a sickness-behavior syndrome that is normally beneficial to the host organism. Increased reactivity of microglia sets the stage for an exaggerated neuroinflammatory cytokine response following activation of the peripheral innate immune system, which may underlie subsequent long-lasting behavioral and cognitive deficits. In support of this premise, recent findings indicate that stimulation of the peripheral immune system in aged rodents causes exaggerated neuroinflammation that is paralleled by cognitive impairment, prolonged sickness, and depressive-like complications. Therefore, the purpose of this review is to discuss the new evidence that age-associated priming of microglia could play a pathophysiological role in exaggerated behavioral and cognitive sequelae to peripheral infection.

Immune activation suppresses plasma testosterone level: a meta-analysis

Females often select mates on the basis of sexual signals, which can be reliable indicators of male quality when the costliness of these signals prevents cheating. The immunocompetence handicap hypothesis (ICHH) provides a mechanistic explanation of these costs, by proposing a trade-off between immune function and sexual displays. This trade-off arises because testosterone enhances sexual signals, but suppresses immune function. Many studies have investigated the ICHH by administrating testosterone, and a recent meta-analysis found little evidence that testosterone suppressed immune function. However, another component of the ICHH, which has received less empirical interest, suggests that there may also be an interaction in the other direction, with immune activation suppressing testosterone levels. We present a meta-analysis to test for this effect. Overall, there was a strong suppressive effect of experimental immune activation on testosterone levels (r=-0.52), regardless of whether live pathogens or non-pathogenic antigens were used to challenge the immune system. The latter is important because it shows that immune activation per se suppresses testosterone levels. Thus, a trade-off between immunocompetence and sexual displays may primarily be generated by the effect of immune activation on testosterone, rather than the opposite effect that has received most attention.

Genetic resistance to infection influences a male's sexual attractiveness and modulation of testosterone

Females may be attracted to males genetically resistant to infectious diseases, and one potential mechanism for this mating bias is that such males may be better able to maintain high testosterone. To test these two hypotheses, we collected scent-marks from male house mice (Mus domesticus) genetically resistant and susceptible to Salmonella due to a single locus (Nramp 1, also known as Slc11a1). We tested whether females are more attracted to the scent-marks of resistant males, and whether such males are better able to maintain testosterone concentrations during an experimental Salmonella infection. We found that females preferred the scent-marks of genetically resistant males compared to susceptible ones; but they showed no preferences 5d after males were infected. As predicted, genetically resistant males maintained their testosterone concentrations during the experimental infection, whereas susceptible males showed a significant decline 14 d after inoculation. These differences in the males' ability to modulate testosterone, however, do not explain females' attraction to resistant males. Thus, our results indicate that females sometimes prefer males genetically resistant to infection, and they provide the first evidence that males modulate their testosterone depending upon their genetic resistance to infection; however, we found no evidence to link these two findings.

Seasonal changes in vertebrate immune activity: mediation by physiological trade-offs

Animals living in temporally dynamic environments experience variation in resource availability, climate and threat of infection over the course of the year. Thus, to survive and reproduce successfully, these organisms must allocate resources among competing physiological systems in such a way as to maximize fitness in changing environments. Here, we review evidence supporting the hypothesis that physiological trade-offs, particularly those between the reproductive and immune systems, mediate part of the seasonal changes detected in the immune defences of many vertebrates. Abundant recent work has detected significant energetic and nutritional costs of immune defence. Sometimes these physiological costs are sufficiently large to affect fitness (e.g. reproductive output, growth or survival), indicating that selection for appropriate allocation strategies probably occurred in the past. Because hormones often orchestrate allocations among physiological systems, the endocrine mediators of seasonal changes in immune activity are discussed. Many hormones, including melatonin, glucocorticoids and androgens have extensive and consistent effects on the immune system, and they change in systematic fashions over the year. Finally, a modified framework within which to conduct future studies in ecological immunology is proposed, viz. a heightened appreciation of the complex but intelligible nature of the vertebrate immune system. Although other factors besides trade-offs undoubtedly influence seasonal variation in immune defence in animals, a growing literature supports a role for physiological trade-offs and the fitness consequences they sometimes produce.

c-Fos immunoreactivity induced by intraperitoneal LPS administration is reduced in the brain of mice lacking the microsomal prostaglandin E synthase-1 (mPGES-1)

The aim of the present study was to investigate the impact of the deletion of the microsomal prostaglandin E synthase-1 (mPGES-1) gene on lipopolysaccharide (LPS)-induced neuronal activation in central nervous structures. The mPGES-1 catalyses the conversion of COX-derived PGH(2) to PGE(2) and has been described as a regulated enzyme whose expression is stimulated by proinflammatory agents. Using the immediate-early gene c-fos as a marker of neuronal activation, we determined whether deletion of the mPGES-1 gene altered the neuronal activation induced by LPS in structures classically recognized as immunosensitive regions. No significant difference in the c-Fos immunostaining was observed in the brain of saline-treated mPGES-1+/+, mPGES-1+/- and mPGES-1-/- mice. However, we observed that LPS-induced neuronal activation was reduced in most of the centres known as immunosensitive nuclei in mPGES-1-/- mice compared with heterozygous and wild-type mice. The decrease in the number of c-Fos positive nuclei occurred particularly in the caudal ventrolateral medulla, the medial, intermediate and central parts of the nucleus tractus solitarius, area postrema, parabrachial nucleus, locus coeruleus, paraventricular nucleus of the hypothalamus, ventromedial preoptic area, central amygdala, bed nucleus of the stria terminalis and to a lesser extent in the ventrolateral part of the nucleus tractus solitarius and rostral ventrolateral medulla. These results suggest that the mPGES-1 enzyme is strongly needed to provide sufficient PGE(2) production required to stimulate immunosensitive brain regions and they are discussed with regard to the recent works reporting impaired sickness behavior in mPGES-1-/- mice.

Immunological memory is compromised by food restriction in deer mice Peromyscus maniculatus

The immune system protects organisms against infection, but this protection presumably comes at a cost. Here, we asked whether food restriction would compromise the ability of an organism to generate an immune response on reexposure to an antigen, which would represent a functional cost of immunological memory. Immunological memory is generated when B and T lymphocytes sensitive to components of pathogens (i.e., antigens) proliferate after exposure and persist in circulation to hinder reinfection. To test the possibility that B cell memory, the component of the immune system responsible for antibody production, is expensive to maintain, secondary antibody production against a novel protein [keyhole limpet hemocyanin (KLH)] was compared in food-restricted and ad libitum-fed male deer mice ( Peromyscus maniculatus). To determine whether compromised secondary antibody production was solely due to elevated corticosterone independent of resource availability, some food-restricted and ad libitum-fed mice were subjected to unpredictable, chronic (2 h/day) restraint. Mice fed 70% of their ad libitum diet 2 wk after primary antigen challenge produced ∼95% less IgG against KLH after a second antigen challenge than mice fed ad libitum, even though all mice were fed ad libitum during the secondary antibody response period. Restraint had no effect on secondary IgG production in response to KLH, and corticosterone concentrations 1 day after food restriction did not differ between food-restricted and ad libitum-fed mice. Together, these data imply that secondary antibody responses and the benefits of immunological memory are energetically costly in this species.

Maternal aggression persists following lipopolysaccharide-induced activation of the immune system

Lactating females direct aggressive behaviors towards intruders presumably to reduce the likelihood of infanticide of their pups. Infected animals display a constellation of responses that include lethargy, anorexia, and decreased social interactions. This suite of responses is referred to as sickness behavior, and is putatively part of an adaptive strategy to aid the organism in recovery from infection. Previous work has suggested that animals can suppress the behavioral symptoms of sickness in order to engage in adaptive behaviors. To test whether adaptive nest defense is affected by illness, dams received a peripheral injection of either saline or lipopolysaccharide (LPS [50, 400, or 1000 microg/kg]), a non-replicating component of bacterial cell walls that activates the immune system. Simulated infection with LPS reduced body mass and food intake in dams and interfered with litter growth in a dose-dependent manner. Generally, nest defense was unaffected by LPS; the proportion of dams displaying maternal aggression against a male intruder, as well as the latency and duration of aggressive encounters were only suppressed at the highest LPS dose tested. Further, LPS treatment also altered non-agonistic behavior during the aggression test as indicated by reduced social investigation of the intruder and an increased time spent immobile during the session. LPS administration also significantly increased serum corticosterone concentrations in lactating females. These findings suggest that maternal aggression is not suppressed by LPS-evoked immune activation at doses that attenuate other aspects of maternal and social behavior.