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Am J Physiol Regul Integr Comp Physiol 280: R207-R213, 2001;
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Vol. 280, Issue 1, R207-R213, January 2001

Sex steroid hormones enhance immune function in male and female Siberian hamsters

Staci D. Bilbo and Randy J. Nelson

Departments of Psychology and Neuroscience, Biochemistry and Molecular Biology, Reproductive Biology Division, The Johns Hopkins University, Baltimore, Maryland 21218-2686


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Immune function is better in females than in males of many vertebrate species, and this dimorphism has been attributed to the presence of immunosuppressive androgens in males. We investigated the influence of sex steroid hormones on immune function in male and female Siberian hamsters. Previous studies indicated that immune function was impaired in male and female hamsters housed under short-day photoperiods when androgen and estrogen concentrations were virtually undetectable. In experiment 1, animals were gonadally intact, gonadectomized (gx), or gx with hormone replacement. Females exhibited the expected increase in antibody production over males, independent of hormone treatment condition, whereas male and female gx animals exhibited decreased lymphocyte proliferation to the T cell mitogen, phytohemagglutinin (PHA) compared with intact animals, and this effect was reversed in gx hamsters following testosterone and estradiol treatment, respectively. In experiment 2, testosterone, dihydrotestosterone, and estradiol all enhanced cell-mediated immunity in vitro, suggesting that sex steroid hormones may be enhancing immune function through direct actions on immune cells. In experiment 3, an acute mitogen challenge of lipopolysaccharide significantly suppressed lymphocyte proliferation to PHA in intact males but not females, suggesting that males may be less reactive to a subsequent mitogenic challenge than females. Contrary to evidence in many species such as rats, mice, and humans, these data suggest that sex steroid hormones enhance immunity in both male and female Siberian hamsters.

proliferation; immunoglobulin G; lipopolysaccharide; Phodopus sungorus


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

A SEXUAL DIMORPHISM in immune function has been identified in many species, with females generally exhibiting enhanced immune responses to antigenic challenge compared with male conspecifics. Female mice have higher titers of immunoglobulins (e.g., IgG, IgM, and IgA; Refs. 7 and 36), as well as a higher incidence of autoimmune diseases than males (23). Additionally, females display a higher splenocyte blastogenic response to T and B cell mitogens than males (31). Women also display higher serum immunoglobulin levels than men, and they suffer from a higher incidence of autoimmune diseases such as lupus erythematosus (42) and rheumatoid arthritis compared with men (21). Helminth infections are generally more severe in male than in female vertebrate hosts (40), and males of species such as deer and moorhens often have higher parasite loads than females (16, 53).

The physiological basis for the sexual dimorphism in immune function is not well understood. Sex differences in morphology, physiology, or behavior are usually mediated by sex steroid hormones (9). Hormones may alter immunologic factors and responses, including antigen expression and presentation, and cytokine production, as well as the expression of apoptotic factors and cell death (27). The modulatory actions of sex steroid hormones may be directly mediated through receptors on immune cells. In mice, the presence of estrogen receptors on various immune cells has been demonstrated, as well as the presence of androgen receptors on T and B cells (36). Sex steroid hormones also influence the immune response in part via the thymus in rodents and the bursa of Fabricius in birds through specific androgen and estrogen binding sites (21, 47). Early work investigating the effects of sex hormones in vitro on isolated leukocytes revealed no correlation between sex steroid concentrations and immunoregulation (44). Exogenous estradiol, however, stimulates adhesion molecules and their receptors on immune cells and accessory cells (8). Estradiol also has immunoenhancing effects on antibody production in vivo, whereas testosterone has immunosuppressive effects on B and T cell differentiation, as well as macrophage activation in rats and mice (7, 43, 51). In birds, exogenous testosterone implants depress antibody production and delayed-type hypersensitivity to an antigen challenge (14). Although estradiol treatment has immunoenhancing effects on humoral immunity, both cell-mediated immunity and natural killer cell activity are depressed following estradiol treatment in rats and mice (33, 46).

Immune function varies according to reproductive status as well. Male deer often have higher parasite loads during the breeding season concomitant with high testosterone concentrations and exaggerated secondary sexual characteristics such as large antlers (53). In mice, minimal lymphocyte, white blood cell, and natural killer cell counts coincide with the preovulatory estradiol peak in females (20). Concomitantly, the peak blastogenic responses of spleen cells to mitogens occur during the estrous cycle when estradiol and progesterone concentrations peak (7). Castrated male mice increase thymic size, and androgen treatment decreases thymic mass (37). Castration also has profound effects on both cellular and humoral immune responses in mice; antibody production and splenocyte proliferation in response to a mitogen are increased, and the ability to overcome viral and bacterial infection is improved in castrated male mice (45). Subsequent testosterone treatment can reverse these effects and increase susceptibility to infection (50). Therefore, enhanced immune function observed in females has been hypothesized to be attributed, at least in part, to a lack of the immunosuppressive effects of androgens.

Decreased cell-mediated and humoral immune function has been reported in male (13, 52) and female (unpublished observations) Siberian hamsters (Phodopus sungorus) during simulated winter conditions in the laboratory, in which the gonads are regressed and sex steroid hormone concentrations are virtually undetectable. This observation suggests that the sex steroid hormones may enhance immune function in this species. The goal of the present study was to examine the influence of the sex steroid environment on immune function in male and female Siberian hamsters. In experiment 1, we investigated cell-mediated and humoral immune responses in intact, gonadectomized (gx), and gx + sex steroid hormone-treated males and females. In experiment 2, we investigated the direct influences of testosterone, estradiol, and dihydrotestosterone (DHT) in vitro on lymphocyte proliferation in gonadally intact male and female hamsters. In experiment 3, we investigated the influence of an acute mitogen challenge [lipopolysaccharide (LPS)] on cell-mediated immunity in intact animals 24 h postinjection. We report here that females exhibited the expected increase in antibody production over males, but this effect was observed independent of hormone treatment condition. Furthermore, both estradiol and testosterone replacement therapy enhanced lymphocyte proliferation in males and females both in vivo and in vitro. Conversely, a sexual dimorphism was observed following a direct mitogen challenge, in which LPS significantly decreased cell-mediated immunity in males but not females.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animals

Forty-two adult male and 42 female Siberian hamsters from our breeding colony were used in this study. Animals were 4-5 mo of age at the onset of the study and were housed individually in polypropylene cages in colony rooms with constant temperature and humidity of 21 ± 2°C and 50 ± 5%, respectively, with a reverse 14:10-h light-dark cycle [lights on 2400 Eastern Standard Time (EST)]. Sentinel animals were housed in the colony room and screened regularly for the presence of common rodent diseases arising from parasitic, viral, bacterial, and fungal origins (all screens during the course of the experiment were negative).

Experimental Procedures

Experiment 1. Twenty-four male and 24 female hamsters were randomly selected and assigned to one of three experimental conditions: 1) 8 male and 8 female hamsters received sham gonadectomies and 4-mm empty subcutaneous Silastic implants (ID 1.95 mm, OC 3.13 mm; Dow Corning, Midland, MI); 2) 8 male and 8 female hamsters were surgically gx under pentobarbital sodium anesthesia at 120 days of age and received subcutaneous empty Silastic implants; 3) 8 male and 8 female hamsters were surgically gx under pentobarbital sodium anesthesia at 120 days of age and received Silastic capsules implanted subcutaneously and filled with testosterone propionate (males) or 17beta -estradiol (females) crystals (Sigma Chemical, St. Louis, MO). Hormone capsules of this size have previously been reported to generate serum concentrations of testosterone and estradiol similar to gonadally intact male and female Siberian hamsters (2, 32). All hamsters were allowed to recover for 2 wk prior to subsequent procedures.

All hamsters received a single subcutaneous injection of 100 µg of the novel antigen keyhole limpet hemocyanin (KLH), suspended in 0.1 ml sterile saline, and were returned to their home cages. KLH is an innocuous respiratory protein derived from the keyhole limpet (Megathura crenulata) and was used because it generates a robust antigenic response in rodents without causing illness or sickness behaviors (e.g., inflammation or anorexia) (12). On day 10 post-KLH injection, blood (~1 ml) was drawn from the retro-orbital sinus, and a lymphocyte proliferation assay was performed. The animals received an intraperitoneal injection of sterile isotonic saline (2 ml) to prevent dehydration and were returned to their cages until 5 days later (day 15 post-KLH immunization), at which time blood (0.5 ml) was again drawn from the retro-orbital sinus. Immediately following bleeding, animals were killed by rapid cervical dislocation. Samples were allowed to clot for 1 h, the clot was removed, and the samples centrifuged at 4°C for 30 min at 3,500 rpm. Serum aliquots were aspirated and stored in sealable polypropylene microcentrifuge tubes at -80°C until assayed for antibody content. This time was chosen to capture peak IgG antibody production during the course of the immune response (28).

Lymphocyte proliferation. Cell-mediated immune function was assessed by measuring lymphocyte proliferation from whole blood in response to the T cell mitogen, phytohemagglutinin (PHA), using a colorimetric assay based on the tetrazolium salt 3-(4,5-demethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfphenyl)-2H-tetrazolium (MTS). Lymphocytes were separated from blood by Histopaque-Ficoll gradient centrifugation (5), with slight alterations. One milliliter of whole blood was layered on an equal part of Ficoll in a 5-ml centrifuge tube. After centrifugation at 2,400 rpm for 30 min, the mononuclear cells were collected in a thin, buffy layer at the interface and granulocytes and erythrocytes settled to the bottom of the tube. Separated lymphocytes were suspended in 5 ml of culture medium (RPMI 1640/HEPES) and centrifuged at 1,500 rpm for 10 min. Following this second centrifugation, cells were harvested from the bottom of the tube and again suspended in 0.5 ml of culture medium supplemented with 1% penicillin (5,000 U/ml)/streptomycin (5,000 µl/ml), 1% L-glutamine (2 mM/ml), 0.1% 2-mercaptoethanol (5 × 10-2 M/ml), and 10% heat-inactivated fetal bovine serum. Lymphocyte counts and viability were determined with a hemocytometer and trypan blue exclusion. Viable cells (which exceed 95%) were adjusted to 1 × 106 cells/ml by dilution with supplemented culture medium, and 50-µl aliquots of each cell suspension (i.e., 100,000 cells) were added to the wells of sterile flat-bottom 96-well culture plates (Falcon catalog no. 3072). PHA was diluted with culture medium to concentrations of 10, 5, 2.5, 1.25, and 0 µg/ml; 50 µl of each mitogen concentration was added to the wells of the plate containing the cell suspensions to yield a final volume of 100 µl/well (each in duplicate). Plates were incubated at 37°C with 5% CO2 for 48 h prior to addition of 20 µl of MTS/PMS solution [0.92 mg/ml of phenazine methosulfate (PMS) in sterile Dulbecco's PBS; Promega] per well. Plates were then incubated at 37°C with 5% CO2 for an additional 4 h. The optical density (OD) of each well was determined with a microplate reader (Bio-Rad, Benchmark model) equipped with a 490-nm wavelength filter. Median OD values for each set of duplicates were used in subsequent statistical analyses. Dose-response curves were constructed using group means of the median OD values at each mitogen concentration and unstimulated cultures.

KLH ELISA. Humoral immune function was assessed by measuring secondary antibody responses to a novel antigen challenge. Serum anti-KLH IgG concentrations were assayed using an enzyme-linked immunosorbent assay (ELISA). Microtiter plates (Costar no. 9018, Corning) were coated with antigen, incubated overnight at 4°C with 0.5 mg/ml KLH in sodium bicarbonate buffer, washed the next day with PBS containing 0.05% Tween 20 (PBS-T), and then blocked with 5% nonfat dry milk in PBS-T overnight at 4°C to reduce nonspecific binding. The day of the assay, plates were washed again with PBS-T. Thawed serum samples were diluted 1:20 with PBS-T, and 150 µl of each serum dilution were added in duplicate to the wells of the antigen-coated plates. Positive control samples (pooled sera from hamsters previously determined to have high levels of anti-KLH antibody, similarly diluted with PBS-T) and negative control samples (pooled sera from KLH-naive hamsters, diluted with PBS-T) were added in duplicate to each plate; plates were sealed, incubated at 37°C for 3 h, and then washed with PBS-T. Secondary antibody (alkaline phosphatase-conjugated-anti-mouse IgG diluted 1:500 with PBS-T; Cappel, Durham, NC) was added to the wells, and the plates were sealed and incubated for 1 h at 37°C. Plates were again washed with PBS-T, and 150 µl of the enzyme substrate p-nitrophenyl phosphate (1 mg/ml in diethanolamine substrate buffer; Sigma Chemical) was added to each well. Plates were protected from light during the enzyme-substrate reaction, which was terminated after 20 min by adding 50 µl of 1.5 M NaOH to each well. The OD of each well was determined using a plate reader (Bio-Rad) equipped with a 405-nm wavelength filter, and the average OD for each set of duplicate wells was calculated. To minimize intra-assay variability, the average OD for each sample is expressed as a percent of its plate positive control OD for statistical analyses.

Experiment 2

In vitro testosterone assay. Ten male and 10 female hamsters were lightly anesthetized with methoxyflurane vapors (Metofane; Schering-Plough, Union, NJ). Blood (~1 ml) was drawn from the retro-orbital sinus, and a lymphocyte proliferation assay was performed as described above in experiment 1, with the following modifications: testosterone stock (Sigma catalog no. T-6147) was dissolved in 95% EtOH and further suspended in supplemented culture media to yield a final concentration of 3, 30, or 300 ng/ml. The first concentration corresponds to the physiological range of testosterone in this species (41). All wells received 50 µl of cells and 50 µl of one dose of testosterone, with each animal receiving all doses to determine the presence of a dose response. Plates were incubated as described previously, and the OD of each well was determined. Median OD values for each set of duplicates were used in subsequent statistical analyses. Dose-response curves were constructed using group means of the median OD values at each testosterone concentration and unstimulated cultures.

In vitro estradiol and DHT assay. Animals were again bled from the retro-orbital sinus (~1 ml), and a lymphocyte proliferation assay was performed as described above in experiment 1, with the following modifications: 17beta -estradiol stock (Sigma catalog no. E-2758) and DHT stock (Sigma catalog no. D-5027) were dissolved in 95% EtOH and further suspended in supplemented culture media to yield final concentrations of 0.3, 3, or 30 ng/ml, and 3, 30, or 300 ng/ml, respectively. The first concentration in each dose range corresponds to the physiological ranges of estradiol and DHT, respectively, in this species (41). All wells received 50 µl of cells and 50 µl of one dose of estradiol or DHT, with each animal receiving all doses of each hormone to determine the presence of a dose response. Median OD values for each set of duplicates were used in subsequent statistical analyses. Dose-response curves were constructed using group means of the median OD values at each estradiol and DHT concentration and unstimulated cultures.

In vitro cholesterol control assay. To control for the possibility that the hormones of interest were acting as nonspecific mitogenic substances in vitro, animals were again bled from the retro-orbital sinus (~1 ml), and a lymphocyte proliferation assay was performed as described above in experiment 1, with the following modifications: cholesterol stock (Sigma catalog no. C-3045) was dissolved in 95% EtOH and further suspended in supplemented culture media to yield final concentrations of 3, 30, or 300 ng/ml. All wells received 50 µl of cells and 50 µl of one dose of cholesterol, with each animal receiving all doses of cholesterol to determine the presence of a dose response. Median OD values for each set of duplicates were used in subsequent statistical analyses. Dose-response curves were constructed using group means of the median OD values at each cholesterol concentration and unstimulated cultures.

Experiment 3

To investigate the influence of an acute mitogen challenge on subsequent lymphocyte proliferation in intact males and females, eight male and eight female hamsters received a 0.1-ml ip injection of 50 µg LPS (catalog no. L-3755; Sigma Chemical) suspended in sterile isotonic saline, at 1200 EST. Twenty-four hours post-LPS injections, blood (~1 ml) was drawn from the retro-orbital sinus, and a lymphocyte proliferation assay was performed as described previously in experiment 1. These animals were tested concurrently with untreated intact males and females in experiment 1, and blood was run in the same assay. These data are presented as a separate experiment for the purposes of data analysis only. The time period for LPS injections was based on in vitro studies of T cell mitogen-induced proliferation following an in vivo challenge with LPS (38, 39). Previous studies have quantified low and high doses of LPS administration in rodents (0.1 and 100 µg/100 g, respectively) (10, 15, 29, 48). Doses as low as 0.1 µg in mice result in the production of cytokines and the activation of the immune system (15, 48), whereas higher doses induce fever and sickness behaviors such as anorexia and lethargy in a dose-dependent manner (10, 29).

Statistical Analyses

Cell-mediated immunity in experiments 1 and 3 was assessed by comparing each animal's lymphocyte proliferation rate in response to an optimal concentration of 5 µg PHA. Data were analyzed across groups using a two-way ANOVA. Cell-mediated immunity in experiment 2 was assessed by comparing each animal's lymphocyte proliferation rate in response to each concentration of testosterone, estradiol, DHT, and cholesterol. Data were analyzed for males and females across dosage groups using repeated-measures, one-way ANOVAs. Humoral immunity was assessed by comparing IgG antibody production at an optimal concentration of 1:40 (serum:PBS-T) across animals. Data were analyzed using a two-way ANOVA. Post hoc comparisons for all analyses between pairwise means were conducted using Dunn's method tests, and differences between group means were considered statistically significant if P < 0.05.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Experiment 1

Cell-mediated immunity. Lymphocyte proliferation in response to PHA was significantly suppressed in ovariectomized (ovx) females compared with intact females, and this suppression was reversed in ovx females that received subcutaneous estradiol implants (P < 0.05). Similarly, lymphocyte proliferation was suppressed in castrated males compared with intact males, and this suppression was reversed by testosterone treatment (P < 0.05; Fig. 1A).


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  		Fig. 1.   A: lymphocyte proliferation to phytohemagglutinin (PHA; represented as absorbance units) in females and males within each treatment condition (n = 8); values are means ± SE. Intact, sham surgery; Ovx, ovariectomized; Ovx + E, ovariectomized with estradiol implants; Cast, castrated; and Cast + T, castrated with testosterone implants. B: serum anti-KLH IgG titers 15 days following immunization with KLH in females vs. males within each treatment condition (n = 8); values are means ± SE. KLH, keyhole limpet hemocyanin. *Significant difference, P < 0.05.

Humoral immunity. Females as a group had significantly higher concentrations of anti-KLH IgG on day 15 post-KLH injection than all male groups (P < 0.05; Fig. 1B). Neither females nor males differed according to hormone treatment (P > 0.05).

Experiment 2

Lymphocyte proliferation was significantly higher in cells stimulated in vitro with testosterone at all doses (3, 30, and 300 ng/ml) over basal conditions in males and females (P < 0.05; Fig. 2A). There was no significant dose response and no differences between males and females. Similarly, lymphocyte proliferation was significantly higher in cells stimulated in vitro with estradiol at all doses (0.3, 3, and 30 ng/ml) over basal conditions in males and females (P < 0.05; Fig. 2B). There was no significant dose response; however, proliferation was significantly higher in females compared with males at the physiological concentration of 0.3 ng/ml. Lymphocyte proliferation was significantly higher in cells stimulated in vitro with DHT at all doses (3, 30, and 300 ng/ml). There was no significant dose response and no differences between males and females (P > 0.05; data not shown). Cholesterol had no significant influences on lymphocyte proliferation in males or females at all three doses (3, 30, and 300 ng/ml; data not shown).


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  		Fig. 2.   A: lymphocyte proliferation (represented as absorbance units) in gonadally intact females (n = 10) and males (n = 10) following an in vitro challenge of testosterone of 0, 3, 30, or 300 ng/ml; values are means ± SE. B: lymphocyte proliferation (represented as absorbance units) in gonadally intact females (n = 10) and males (n = 10) following an in vitro challenge of estradiol of 0, 0.3, 3, or 30 ng/ml; values are means ± SE. *Significant difference from paired control, P < 0.05. **Significant difference from males at comparable dose, P < 0.05.

Experiment 3

Lymphocyte proliferation was significantly lower in LPS-treated males 24 h postinjection than females, compared with untreated intact males in experiment 1 (P < 0.05; Fig. 3). Females were not significantly influenced by LPS injections, compared with untreated intact females in experiment 1.


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  		Fig. 3.   Lymphocyte proliferation to PHA (represented as absorbance units) in gonadally intact females (n = 8) and males (n = 8) 24 h following injections of lipopolysaccharide (LPS), compared with untreated intact animals in experiment 1 (Fig. 1A); values are means ± SE. *Significant difference, P < 0.05.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Previous studies have suggested that both estradiol and testosterone have inhibitory influences on cell-mediated immunity (33, 46), whereas humoral immunity is enhanced following estradiol treatment and suppressed following testosterone treatment (7, 43, 51). Our results in Siberian hamsters both confirm and contradict these findings. Females had higher concentrations of anti-KLH IgG than males, but this effect occurred independent of the sex steroid hormone environment. Cell-mediated immunity was suppressed in ovx and castrated animals compared with intact females and males, and this effect was reversed following estradiol and testosterone treatment, respectively. Furthermore, both estradiol and testosterone, but not cholesterol, enhanced lymphocyte proliferation in vitro at all concentrations analyzed, in both males and females, suggesting that the sex steroid hormones may be exerting their effects directly and specifically on immune cells. Further support for this hypothesis is the observation that a physiological dose of estradiol enhanced proliferation in females significantly more than males, presumably because females have a higher density of estrogen receptors on lymphocytes than males, although this remains to be determined. DHT enhanced proliferation as well, suggesting that testosterone exerts its stimulatory effects through androgen receptors rather than through estrogen receptors (in an aromatized form) (18).

In experiment 3, an acute mitogen challenge of LPS in vivo significantly depressed lymphocyte proliferation to subsequent PHA in vitro in males but not females. Thus a sexual dimorphism comparable to that reported in rats and mice (7, 21, 23) was manifested only following a specific challenge. The effects of LPS on the immune response have been studied extensively using in vivo and in vitro experimental models. LPS affects the immune response to other antigens, and both enhancement and suppression have been reported after LPS exposure (25, 38, 39). For example, B cell-stimulated lymphocytes from LPS-treated mice suppress the in vitro antibody response of normal spleen cells (38). There is also evidence for T and B cell dysfunctions in vitro following LPS treatment (49). However, LPS administered some time (>48 h) before a subsequent dose can stimulate an immune response (19). The results presented here are consistent with the higher incidence of autoimmunity in females of various species, in which reactivity to repeated stressors is enhanced in females (1, 45). This result is similar as well to that observed in female cats in which a negative correlation between blood concentrations of estradiol and rate of lymphocyte apoptosis exists following stimulation with PHA but not under basal conditions (26).

The sexual dimorphism observed following a mitogen challenge may reflect the hypothesis that high testosterone concentrations exert a high metabolic cost on males (17, 24), and thus more energetically expensive immune responses (such as mounting a second response) are compromised. However, the general enhancement of cell-mediated immune function by sex steroid hormones is consistent with data from Siberian hamsters demonstrating a downregulation of certain parameters of immune function during simulated winter conditions in the laboratory, in which the gonads are regressed and sex steroid hormone concentrations are virtually undetectable (13, 52, and unpublished observations). Unlike laboratory strains of rats and house mice, Siberian hamsters are seasonally breeding animals and restrict breeding to the long days of summer (4, 22). Many animals have evolved to display seasonal alterations in immune function in preparation for harsh conditions during the winter (34). Because of the substantial energetic costs (11, 30), it is reasonable to consider immune function from a perspective of energetic trade offs. It is possible that cell-mediated immune function in particular is upregulated during the breeding season in these hamsters, because it is a time when parasitic transfer between animals is most likely. According to this hypothesis, it is logical to assume that this change would be sex steroid dependent, as the two mechanisms may have coevolved.

Taken together, the evidence from this study suggests that the reproductive system plays an important role in the regulation of the immune system in Siberian hamsters. It is unclear why humoral immune function was enhanced in females compared with males independent of hormonal condition. These results suggest that the organizational effects of early endocrine experience may play a significant role in forming immune responsiveness in females throughout life, and this is an area that deserves thorough investigation. Alternatively, because circulating estradiol and testosterone concentrations were not measured in animals immediately prior to gonadectomies in experiment 1, it is possible that the lingering activational effects of the sex steroids were responsible for the observed dimorphism. This explanation seems unlikely, however, because these animals were allowed to recover for 2 wk prior to subsequent experimental procedures. In contrast to several species, both estradiol and testosterone facilitated cell-mediated lymphocyte proliferation both in vivo and in vitro in Siberian hamsters. Furthermore, females do not appear to display enhanced cell-mediated immunity under basal (untreated) conditions. Rather, females demonstrated an enhanced immune response only following an acute endotoxin challenge. Future research is needed to investigate the influence of pregnancy and lactation on immune function in females to explore the possibility that immune function is upregulated during the breeding season in this species, as well as a thorough investigation of the organizational influences of the sex steroid hormones on immune function.

Perspectives

The influence of reproductive condition and sex steroid hormones on immune function in many species remains a subject of intense investigation. However, our current knowledge of sex steroid hormone effects on immune function is relatively limited to that of mice, rats, and humans (6, 23, 45). The results of the current study suggest a unique role for the sex steroids in influencing immune function in Siberian hamsters that may have evolved along with the distinct natural history of these animals. Many species exhibit alterations in immune function concomitant with changes in environmental factors such as photoperiod, temperature, and food availability (34), and seasonal changes in parasite prevalence have been well-documented for a number of mammalian species, ranging from livestock and rodents to white-tailed deer (Odocoileus virginianus), in which parasite prevalence is much greater during the spring and summer (reviewed in Ref. 35). Our data suggest that immune function in Siberian hamsters may have evolved to be enhanced by reliable changes in sex steroid hormones each year to coincide with increased parasite prevalence during breeding and reproduction. Thus future research in this area may benefit greatly from the analysis of a greater diversity of species, with specific regard to the natural history of the species under study.


    ACKNOWLEDGEMENTS

The technical contributions of Debbie Drazen, Sabra Klein, Rashmi Roy, Nancy Edwards, Brian Spar, Michelle Dumler, and Aliza Katz are greatly appreciated. We also thank Rob Wasnick and Isaac Osei-Bobie for expert animal care.


    FOOTNOTES

This study was supported by National Institutes of Health Grant MH-22201 and National Science Foundation Grant IBN-97-23420.

Address for reprint requests and other correspondence: S. D. Bilbo, Dept. of Psychology, Ohio State Univ., 7 Townshend Hall, Columbus, OH 43210 (E-mail: bilbo{at}jhu.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 24 February 2000; accepted in final form 10 August 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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Am J Physiol Regul Integr Comp Physiol 280(1):R207-R213
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