title: Body temperature during hibernation is highly correlated with a decrease in circulating innate immune cells in the brown bear (Ursus arctos): a common feature among hibernators?
abstract: BACKGROUND
  Hibernation involves periods of severely depressed metabolism (torpor) and decreases in body temperature (Tb). Small arctic mammals (<5kg), in which Tb generally drop drastically, display leukopenia during hibernation. This raised the question of whether the decreased leukocyte counts in mammalian hibernators is due to torpor per se or is secondary to low Tb. The present study examined immune cell counts in brown bears (Ursus arctos), where torpor is only associated with shallow decreases in Tb. The results were compared across hibernator species for which immune and Tb data were available.
  
  
  RESULTS
  The white blood cell counts were determined by flow cytometry in 13 bears captured in the field both during summer and winter over 2 years time. Tb dropped from 39.6±0.8 to 33.5±1.1°C during hibernation. Blood neutrophils and monocytes were lower during hibernation than during the active period (47%, p= 0.001; 43%, p=0.039, respectively), whereas no change in lymphocyte counts was detected (p=0.599). Further, combining our data and those from 10 studies on 9 hibernating species suggested that the decline in Tb explained the decrease in innate immune cells (R(2)=0.83, p<0.0001).
  
  
  CONCLUSIONS
  Bears have fewer innate immune cells in circulation during hibernation, which may represent a suppressed innate immune system. Across species comparison suggests that, both in small and large hibernators, Tb is the main driver of immune function regulation during winter dormancy. The lack of a difference in lymphocyte counts in this context requires further investigations.
authors.affiliation: Department of Clinical Medicine, School of Health and Medical Sciences, Örebro University, SE-701 82 Örebro, Sweden.
authors.forename: Berolla
authors.initials: B
authors.lastname: Sahdo
authors.forename: Alina L
authors.initials: AL
authors.lastname: Evans
authors.forename: Jon M
authors.initials: JM
authors.lastname: Arnemo
authors.forename: Ole
authors.initials: O
authors.lastname: Fröbert
authors.forename: Eva
authors.initials: E
authors.lastname: Särndahl
authors.forename: Stéphane
authors.initials: S
authors.lastname: Blanc
mesh_terms: D000818:Animals; D001831:Body Temperature / Q000502:physiology*; D005434:Flow Cytometry; D006605:Hibernation / Q000276:immunology / Q000502:physiology*; D007113:Immunity, Innate / Q000502:physiology*; D007958:Leukocyte Count; D008297:Male; D009000:Monocytes / Q000166:cytology; D009504:Neutrophils / Q000166:cytology; D010101:Oxygen Consumption / Q000502:physiology; D012621:Seasons; D001503:Ursidae / Q000097:blood* / Q000502:physiology
keywords: Brown bear; Hibernation; Innate immunity; Leukocytes; Torpor.; Ursus arctos