The impact of age on vitamin D receptor expression, vitamin D metabolism and cytokine production in ex vivo Rhodococcus equi infection of equine alveolar macrophages.

Rhodococcus equi (R. equi), a pneumonia-causing intracellular bacterium, results in significant morbidity and mortality in young foals, while healthy adult horses rarely develop disease. Survival and replication within alveolar macrophages (AMφ) are the hallmarks of R. equi's pathogenicity. The vitamin D receptor (VDR) and its ligand, the active vitamin D metabolite 1,25(OH)2D, are important in immune responses to intracellular bacteria. The vitamin D/VDR pathway regulates the downstream production of cytokines in infected human AMφ. The immunomodulatory role of the vitamin D/VDR pathway in equine leukocytes is unknown. The objective of the current study was to determine the impact of R. equi infection and age on synthesis of 1,25(OH)2D, VDR expression, and cytokine production in an ex vivo model of R. equi infection in equine AMφ. AMφ were collected from ten healthy foals at 2-, 4- and 8-weeks old and from nine healthy adult horses once via bronchoalveolar lavage. AMφ were mock infected (CONTROL) or infected with a virulent laboratory strain of R. equi for 7 days (INFECTED). VDR expression was determined via RT-qPCR from cell lysates. 1,25(OH)2D and cytokines were measured in cell supernatant by immunoassays. VDR expression was impacted by age (P = 0.001) with higher expression in AMφ from 8-week-old foals than from 2-week-old foals and adults. There was no significant effect of infection in foal AMφ, but in adults, relative VDR expression was significantly lower in INFECTED AMφ compared to CONTROL AMφ (P = 0.002). There was no effect of age or infection on 1,25(OH)2D concentration (P > 0.37). Mean TNFα production was significantly higher from INFECTED compared to CONTROL AMφ from 4- and 8-week-old foals and adults (P < 0.005). Mean IFNγ production was significantly higher from AMφ from foals at 8-weeks-old compared to 2-weeks-old (P = 0.013) and higher from INFECTED AMφ than from CONTROL AMφ in foals at 4-weeks-old and in adults (P < 0.027). The proportion of samples producing IL-1ß and IL-10 was also significantly higher from INFECTED compared to CONTROL AMφ isolated from 4-week-old foals (P < 0.008). Similarly, in adult samples, IL-17 was produced from a greater proportion of INFECTED compared to CONTROL samples (P = 0.031). These data document age-associated changes in VDR expression and cytokine production in equine AMφ in response to R. equi infection. This preliminary investigation supports the need for further research to fully elucidate if the vitamin D pathway has an immunomodulatory role in the horse.

Age-related changes in vitamin D metabolism and vitamin D receptor expression in equine alveolar macrophages: A preliminary study.

The vitamin D receptor (VDR)-vitamin D axis modulates pulmonary immunity in people but its role in equine immunity is unknown. Bacterial pneumonia causes high morbidity/mortality in foals and alveolar macrophages (AMφ) are important for pulmonary defenses. Age-related variations in vitamin D-mediated function of AMφ might contribute to the foal's susceptibility to pneumonia. Our aim was to assess the impact of age on equine vitamin D metabolism and VDR expression in AMφ. AMφ and plasma was collected from healthy foals (2, 4 and 8 weeks old) and adult horses (once). AMφ VDR expression was determined via RT-qPCR and plasma vitamin D metabolites quantified via immunoassays. Data were analyzed with linear mixed models. Inactive-vitamin D metabolite concentrations were lowest in foals at 2 weeks and lower at 2 and 4 weeks compared to adults (P < 0.001). Active-vitamin D metabolite concentrations were higher in foals than adults (P < 0.05). VDR expression was detected in AMφ in all animals and was highest in 2-week-old foals. Vitamin D metabolism and AMφ VDR expression are impacted by age in horses. This may have immunological consequences in foals given the key role that the VDR-vitamin D axis has in pulmonary immunity in other species.

A Common Practice of Widespread Antimicrobial Use in Horse Production Promotes Multi-Drug Resistance.

The practice of prophylactic administration of a macrolide antimicrobial with rifampin (MaR) to apparently healthy foals with pulmonary lesions identified by thoracic ultrasonography (i.e., subclinically pneumonic foals) is common in the United States. The practice has been associated epidemiologically with emergence of R. equi resistant to MaR. Here, we report direct evidence of multi-drug resistance among foals treated with MaR. In silico and in vitro analysis of the fecal microbiome and resistome of 38 subclinically pneumonic foals treated with either MaR (n = 19) or gallium maltolate (GaM; n = 19) and 19 untreated controls was performed. Treatment with MaR, but not GaM, significantly decreased fecal microbiota abundance and diversity, and expanded the abundance and diversity of antimicrobial resistance genes in feces. Soil plots experimentally infected with Rhodococcus equi (R. equi) and treated with MaR selected for MaR-resistant R. equi, whereas MaR-susceptible R. equi out-competed resistant isolates in GaM-treated or untreated plots. Our results indicate that MaR use promotes multi-drug resistance in R. equi and commensals that are shed into their environment where they can persist and potentially infect or colonize horses and other animals.

Pharmacokinetics of the anticonvulsant levetiracetam in neonatal foals.

BACKGROUND: Seizures are a common manifestation of neurological disease in the neonatal foal and are an important cause of morbidity and mortality in this population. Current antiepileptic options are effective, but often have undesirable adverse effects, short duration of action and high cost. Levetiracetam has an ideal safety and pharmacokinetic profile in multiple species, including the adult horse, and may be a safe and cost-effective alternative anticonvulsant in neonatal foals. Due to differences in drug disposition and clearance dosages in neonates, dosing recommendations in other species or adult horses cannot be extrapolated to foals. OBJECTIVE: To establish the pharmacokinetic profile of single-dose i.v. and intragastric administration of levetiracetam in healthy neonatal foals. STUDY DESIGN: Randomised crossover experimental study. METHODS: Levetiracetam was administered as a single dose to six healthy foals (ages 1-10 days) at a dose of 32 mg/kg bwt i.v. or intragastrically. Plasma levetiracetam concentrations were measured using a validated HPLC protocol. RESULTS: After i.v. administration to healthy foals, levetiracetam had a mean (±s.d.) elimination half-life of 7.76 ± 0.51 h, a mean systemic clearance of 61.67 ± 10.96 (mL/h/kg) and a mean apparent volume of distribution at steady state of 0.670 ± 0.124 (L/kg). Following intragastric administration, levetiracetam had a peak concentration of 38.34 ± 7.42 mg/L and time to achieve peak concentration was 0.875 (0.5-1.5) h. Mean bioavailability for IG administration was excellent (103.04 ± 14.51%). No significant differences in pharmacokinetic variables between routes and order of administration were observed. MAIN LIMITATIONS: Small sample size and single-dose administration. CONCLUSIONS: Levetiracetam has excellent intragastric bioavailability in foals and is predicted to maintain plasma concentrations at or above the proposed target concentration with twice daily i.v. or oral administration. Once-daily administration may be possible in some foals based on the therapeutic range recommended in other species.

Bioavailability and tolerability of nebulised dexamethasone sodium phosphate in adult horses.

BACKGROUND: Nebulisation of the injectable dexamethasone sodium phosphate (DSP) would offer an inexpensive way of delivering a potent corticosteroid directly to the lungs of horses with asthma. However, this approach would be advantageous only if systemic absorption is minimal and if the preservatives present in the formulation do not induce airway inflammation. OBJECTIVE: To investigate the bioavailability of nebulised DSP and determine whether it induces airway inflammation or hypothalamic-pituitary-adrenal (HPA) axis suppression in healthy adult horses. STUDY DESIGN: Randomised crossover experiment. METHODS: Dexamethasone sodium phosphate was administered to six healthy adult horses at a dose of 5 mg q. 24 h for 5 days via nebulised, or intravenous (i.v.) routes. Plasma dexamethasone concentrations were measured by UPLC/MS-MS to calculate bioavailability. Cytological examination of bronchoalveolar fluid was performed at baseline and after the last dose of DSP. A validated chemiluminescent immunoassay was used to measure basal serum cortisol concentrations. RESULTS: After nebulisation to adult horses, dexamethasone had a mean (±s.d.) maximum plasma concentration of 0.774 ± 0.215 ng/mL and systemic bioavailability of 4.3 ± 1.2%. Regardless of route of administration, there was a significant decrease in the percentage of neutrophils in bronchoalveolar lavage fluid over time. During i.v. administration, basal serum cortisol concentration decreased significantly from baseline to Day 3 and remained low on Day 5. In contrast, basal serum cortisol concentration did not change significantly during administration via nebulisation. MAIN LIMITATIONS: Small sample size and short period of drug administration. CONCLUSIONS: Dexamethasone sodium phosphate administered via nebulisation had minimal systemic bioavailability and did not induce lower airway inflammation or HPA axis suppression in healthy horses.

Comparative pharmacokinetics of minocycline in foals and adult horses.

The objective of this study was to compare the pharmacokinetics of minocycline in foals vs. adult horses. Minocycline was administered to six healthy 6- to 9-week-old foals and six adult horses at a dose of 4 mg/kg intragastrically (IG) and 2 mg/kg intravenously (i.v.) in a cross-over design. Five additional oral doses were administered at 12-h intervals in foals. A microbiologic assay was used to measure minocycline concentration in plasma, urine, synovial fluid, and cerebrospinal fluid (CSF). Liquid chromatography-tandem mass spectrometry was used to measure minocycline concentrations in pulmonary epithelial lining fluid (PELF) and bronchoalveolar (BAL) cells. After i.v. administration to foals, minocycline had a mean (±SD) elimination half-life of 8.5 ± 2.1 h, a systemic clearance of 113.3 ± 26.1 mL/h/kg, and an apparent volume of distribution of 1.24 ± 0.19 L/kg. Pharmacokinetic variables determined after i.v. administration to adult horses were not significantly different from those determined in foals. Bioavailability was significantly higher in foals (57.8 ± 19.3%) than in adult horses (32.0 ± 18.0%). Minocycline concentrations in PELF were higher than in other body fluids. Oral minocycline dosed at 4 mg/kg every 12 h might be adequate for the treatment of susceptible bacterial infections in foals.

Clinical Assessment of a Point-of-Care Serum Amyloid A Assay in Foals with Bronchopneumonia.

BACKGROUND: Despite the paucity of data available, stall-side serum amyloid (SAA) assays are commonly used to make diagnostic and treatment decisions in foals with bronchopneumonia. HYPOTHESIS: Measurement of SAA concentrations can accurately differentiate pneumonic from healthy foals. ANIMALS: Fifty-four pneumonic foals between 3 weeks and 5 months of age were compared to 44 healthy controls. In addition, 47 foals on a farm endemic for R. equi infections were studied. METHODS: Serum samples were collected from pneumonic foals at hospital admission. Foals were categorized as having pneumonia caused by R. equi or by other microorganisms based on culture of a tracheobronchial aspirate. In addition, serum samples were obtained at 2-week intervals from foals born at a farm endemic for R. equi. SAA concentrations were measured by a point-of-care assay. Diagnostic performance of SAA was assessed by use of receiver operating characteristic curves. RESULTS: Concentrations of SAA in foals with bronchopneumonia were significantly (P < 0.001) higher than those of healthy foals, but 15 of 54 pneumonic foals (28%) had SAA concentrations <5 µg/mL. There was no correlation between SAA concentrations and radiographic score in foals with R. equi pneumonia. The ability of SAA to predict development of R. equi pneumonia at the endemic farm was limited with a sensitivity of 64% and a specificity of 77%. CONCLUSION AND CLINICAL IMPORTANCE: Overall, SAA concentrations are significantly higher in pneumonic than in healthy foals. However, performance of SAA in detecting pneumonic foals is limited by the high proportion of false-positive and false-negative results.

Use of Liposomal Gentamicin for Treatment of 5 Foals with Experimentally Induced Rhodococcus equi Pneumonia.

BACKGROUND: Adverse effects of, and bacterial resistance to, macrolides used to treat Rhodococcus equi infections have prompted search for clinically effective alternative antimicrobials. Liposomal gentamicin (LG) is effective against R. equi in vitro and decreases tissue concentrations of R. equi in experimentally infected mice. Effectiveness of LG treatment of foals with R. equi pneumonia, however, has not been described. HYPOTHESIS: Liposomal gentamicin is safe and effective for treating foals with R. equi pneumonia. ANIMALS: Ten foals with experimentally induced R. equi pneumonia. METHODS: Pilot treatment trial. Foals with pneumonia induced by intrabronchial instillation of R. equi were randomly allocated to receive either clarithromycin combined with rifampin (CLR + RIF) PO or LG IV, and followed by daily physical examinations and weekly thoracic ultrasonography and serum creatinine concentration determinations until the resolution of clinical signs. Treatment success was defined as the resolution of clinical signs and ultrasonographically identified pulmonary abscesses. RESULTS: All 10 foals were successfully treated. Two of 5 foals treated with LG developed azotemia within 1 week; LG was discontinued and treatment switched to CLR + RIF for these foals. None of the CLR + RIF treated foals developed azotemia. CONCLUSIONS AND CLINICAL IMPORTANCE: Liposomal gentamicin IV can be effective for treatment of R. equi pneumonia, but nephrotoxicity indicates that an alternative dosing interval or route (such as nebulization) will be needed before LG is adequately safe for clinical use. Larger comparative trials will be needed to evaluate the relative efficacy of a safer LG dosage regimen.

Pharmacokinetics of danofloxacin and N-desmethyldanofloxacin in adult horses and their concentration in synovial fluid.

The objectives of this study were to investigate the pharmacokinetics of danofloxacin and its metabolite N-desmethyldanofloxacin and to determine their concentrations in synovial fluid after administration by the intravenous, intramuscular or intragastric routes. Six adult mares received danofloxacin mesylate administered intravenously (i.v.) or intramuscularly (i.m.) at a dose of 5 mg/kg, or intragastrically (IG) at a dose of 7.5 mg/kg using a randomized Latin square design. Concentrations of danofloxacin and N-desmethyldanofloxacin were measured by UPLC-MS/MS. After i.v. administration, danofloxacin had an apparent volume of distribution (mean ± SD) of 3.57 ± 0.26 L/kg, a systemic clearance of 357.6 ± 61.0 mL/h/kg, and an elimination half-life of 8.00 ± 0.48 h. Maximum plasma concentration (Cmax ) of N-desmethyldanofloxacin (0.151 ± 0.038 µg/mL) was achieved within 5 min of i.v. administration. Peak danofloxacin concentrations were significantly higher after i.m. (1.37 ± 0.13 µg/mL) than after IG administration (0.99 ± 0.1 µg/mL). Bioavailability was significantly higher after i.m. (100.0 ± 12.5%) than after IG (35.8 ± 8.5%) administration. Concentrations of danofloxacin in synovial fluid samples collected 1.5 h after administration were significantly higher after i.v. (1.02 ± 0.50 µg/mL) and i.m. (0.70 ± 0.35 µg/mL) than after IG (0.20 ± 0.12 µg/mL) administration. Monte Carlo simulations indicated that danofloxacin would be predicted to be effective against bacteria with a minimum inhibitory concentration (MIC) ≤0.25 µg/mL for i.v. and i.m. administration and 0.12 µg/mL for oral administration to maintain an area under the curve:MIC ratio ≥50.

Pulmonary pharmacokinetics of desfuroylceftiofur acetamide after nebulisation or intramuscular administration of ceftiofur sodium to weanling foals.

REASONS FOR PERFORMING STUDY: Administration of ceftiofur sodium via nebulisation has been recommended for the treatment of bronchopneumonia in horses, despite the lack of pharmacokinetic and safety data. OBJECTIVES: To compare concentrations of desfuroylceftiofur acetamide (DCA) in plasma and pulmonary epithelial lining fluid (PELF) of foals after nebulisation or i.m. administration of ceftiofur sodium and to determine if nebulisation of ceftiofur sodium induces airway inflammation. STUDY DESIGN: Randomised experimental study. METHODS: Six weanling foals received ceftiofur sodium (2.2 mg/kg bwt daily for 5 doses) by the i.m. route and 6 foals received the same dose by nebulisation. Concentrations of DCA in plasma and PELF were measured after Doses 1 and 5, and differential cell counts were performed on bronchoalveolar lavage samples obtained after Dose 5. RESULTS: Foals receiving ceftiofur sodium via nebulisation had significantly lower peak concentrations (0.15 ± 0.12 vs. 6.15 ± 0.75 mg/l) and area under the curve (1.26 ± 0.96 vs. 37.63 ± 4.01 mg●h/l) in plasma compared with those receiving the drug by the i.m. route. In contrast, foals receiving ceftiofur sodium via nebulisation had significantly higher peak concentrations (4.52 ± 2.91 vs. 0.73 ± 0.73 mg/l) and area under the curve (24.14 ± 14.09 vs. 5.91 ± 3.28 mg●h/l) in PELF compared with those receiving the drug by the i.m. route. Cell concentration and differential cell count in bronchoalveolar lavage fluid of foals nebulised with ceftiofur sodium were not significantly different from those of foals nebulised with saline. CONCLUSIONS: Administration of ceftiofur sodium via nebulisation is well tolerated and DCA concentrations in PELF remain above the minimum inhibitory concentration of the drug required to inhibit the growth of 90% of Streptococcus zooepidemicus for approximately 24 h after administration. Nebulised ceftiofur sodium warrants further investigation for the treatment of bacterial infections of the lower respiratory tract in horses.

Plasma and pulmonary pharmacokinetics of desfuroylceftiofur acetamide after weekly administration of ceftiofur crystalline free acid to adult horses.

REASONS FOR PERFORMING STUDY: Current labelling for the use of ceftiofur crystalline free acid (CCFA) in horses states that 2 i.m. doses must be administered 4 days apart to provide 10 days of therapeutic coverage. A 10 day treatment regimen is not sufficient for the long-term treatment of horses with severe lung consolidation or pleuropneumonia. There are currently no data to guide an appropriate dosing interval when a longer treatment regimen is warranted. OBJECTIVES: To determine steady-state plasma and pulmonary epithelial lining fluid (PELF) concentrations of desfuroylceftiofur acetamide (DCA) after weekly i.m. administration of CCFA to adult horses. STUDY DESIGN: Experimental study. METHODS: Seven adult horses received i.m. CCFA at a dose of 6.6 mg/kg bwt on Day 0, Day 4 and every 7 days thereafter for 3 additional doses. Concentrations of DCA in plasma and PELF were measured at various time intervals. RESULTS: After weekly i.m. administration, the mean (± s.d.) steady-state peak DCA concentration in plasma (2.87 ± 1.50 µg/ml) was significantly higher than that in PELF (0.84 ± 0.53 µg/ml). Mean terminal half-lives in plasma (77.5 ± 17.5 h) and PELF (92.8 ± 59.0 h) were not significantly different. Concentrations of DCA in plasma and PELF remained in the therapeutic range for the entire dosing interval. CONCLUSIONS: After the initial 2-dose regimen 4 days apart, weekly i.m. administration of CCFA was well tolerated and resulted in plasma and PELF DCA concentrations above the minimal inhibitory concentration that inhibits growth of at least 90% of common lower respiratory tract pathogens of horses. POTENTIAL RELEVANCE: Weekly administration of CCFA would appear appropriate when a treatment regimen longer than 10 days is warranted based on clinical signs and disease severity.

Pharmacodynamic evaluation of 4 angiotensin-converting enzyme inhibitors in healthy adult horses.

BACKGROUND: Angiotensin-converting enzyme (ACE) inhibitors are used in horses with cardiovascular disorders despite the paucity of available data regarding their efficacy. HYPOTHESIS: The degree of serum ACE inhibition varies considerably between drugs. ANIMALS: Eight healthy adult horses. METHODS: Randomized prospective study. Horses were fasted overnight prior to receiving one of 4 ACE inhibitors intragastrically, administered at one of 2 dosages, using a randomized Latin square design (benazepril: 0.5 and 0.25 mg/kg; ramipril: 0.3 and 0.1 mg/kg; quinapril: 0.25 and 0.125 mg/kg; perindopril: 0.1 and 0.05 mg/kg). Serum ACE activity was measured using a kinetic spectrophotometric method. RESULTS: There was a significant effect of drug and dosage on maximum ACE inhibition (I(max)), ACE inhibition 24 hours after administration (I(24h)), and area under the curve (AUC(0-48h)). Benazepril at 0.5 mg/kg resulted in significantly higher I(max) (86.9 ± 7.0%) and I(24h) (60.3 ± 7.9%) compared to the other drugs. There was a significant decrease in indirect blood pressure (BP) over time after administration of each drug, but differences in BP were not significantly different between drugs. Pharmacodynamic variables measured after administration of benazepril to horses with free access to hay were not significantly different from those obtained after fasting. Administration of benazepril orally once daily for 7 days did not result in a cumulative effect on ACE inhibition. CONCLUSIONS AND CLINICAL IMPORTANCE: Of the ACE inhibitors tested, oral benazepril (0.5 mg/kg) is the most effective at inhibiting serum ACE activity in healthy horses.

Plasma and pulmonary disposition of ceftiofur and its metabolites after intramuscular administration of ceftiofur crystalline free acid in weanling foals.

The objectives of this study were to determine the plasma and pulmonary disposition of ceftiofur crystalline free acid (CCFA) in weanling foals and to compare the plasma pharmacokinetic profile of weanling foals to that of adult horses. A single dose of CCFA was administered intramuscularly to six weanling foals and six adult horses at a dose of 6.6 mg/kg of body weight. Concentrations of desfuroylceftiofur acetamide (DCA) were determined in the plasma of all animals, and in pulmonary epithelial lining fluid (PELF) and bronchoalveolar lavage (BAL) cells of foals. After intramuscular (IM) administration to foals, median time to maximum plasma and PELF concentrations was 24 h (12-48 h). Mean (± SD) peak DCA concentration in plasma (1.44 ± 0.46 µg/mL) was significantly higher than that in PELF (0.46 ± 0.03 µg/mL) and BAL cells (0.024 ± 0.011 µg/mL). Time above the therapeutic target of 0.2 µg/mL was significantly longer in plasma (185 ± 20 h) than in PELF (107 ± 31 h). The concentration of DCA in BAL cells did not reach the therapeutic level. Adult horses had significantly lower peak plasma concentrations and area under the curve compared to foals. Based on the results of this study, CCFA administered IM at 6.6 mg/kg in weanling foals provided plasma and PELF concentrations above the therapeutic target of 0.2 µg/mL for at least 4 days and would be expected to be an effective treatment for pneumonia caused by Streptococcus equi subsp. zooepidemicus at doses similar to the adult label.

Plasma pharmacokinetics, pulmonary distribution, and in vitro activity of gamithromycin in foals.

The objectives of this study were to determine the plasma and pulmonary disposition of gamithromycin in foals and to investigate the in vitro activity of the drug against Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) and Rhodococcus equi. A single dose of gamithromycin (6 mg/kg of body weight) was administered intramuscularly. Concentrations of gamithromycin in plasma, pulmonary epithelial lining fluid (PELF), bronchoalveolar lavage (BAL) cells, and blood neutrophils were determined using HPLC with tandem mass spectrometry detection. The minimum inhibitory concentration of gamithromycin required for growth inhibition of 90% of R. equi and S. zooepidemicus isolates (MIC(90)) was determined. Additionally, the activity of gamithromycin against intracellular R. equi was measured. Mean peak gamithromycin concentrations were significantly higher in blood neutrophils (8.35±1.77 µg/mL) and BAL cells (8.91±1.65 µg/mL) compared with PELF (2.15±2.78 µg/mL) and plasma (0.33±0.12 µg/mL). Mean terminal half-lives in neutrophils (78.6 h), BAL cells (70.3 h), and PELF (63.6 h) were significantly longer than those in plasma (39.1 h). The MIC(90) for S. zooepidemicus isolates was 0.125 µg/mL. The MIC of gamithromycin for macrolide-resistant R. equi isolates (MIC(90)=128 µg/mL) was significantly higher than that for macrolide-susceptible isolates (1.0 µg/mL). The activity of gamithromycin against intracellular R. equi was similar to that of azithromycin and erythromycin. Intramuscular administration of gamithromycin at a dosage of 6 mg/kg would maintain PELF concentrations above the MIC(90) for S. zooepidemicus and phagocytic cell concentrations above the MIC(90) for R. equi for approximately 7 days.

Natural killer cells express estrogen receptor-alpha and estrogen receptor-beta and can respond to estrogen via a non-estrogen receptor-alpha-mediated pathway.

Natural killer (NK) cells play a crucial role in host defense against pathogens and immune surveillance against cancer. Given that estrogens have been reported to suppress NK cell activity, we sought to elucidate the mechanisms by which estrogen mediates this effect. We demonstrate by immunocytochemical staining with estrogen receptor-alpha (ERalpha)- and estrogen receptor-beta (ERbeta)-specific antibodies that both ERalpha and ERbeta are expressed in murine NK cells. We also compared the ability of high doses of 17beta-estradiol ( approximately 800 pg/ml) to regulate NK cell activity in wild-type and estrogen receptor-alpha-deficient (ERalphaKO) mice. 17beta-estradiol elicited a significant decrease in NK cell activity in both wild-type and ERalphaKO mice (P < 0.001). These data suggest that ERbeta or possibly a novel receptor is involved in mediating estrogen action on NK cell activity and raise the potential for therapeutic modulation of NK cell activity with selective estrogen receptor modulators (SERMS).