Hypothermia reduces toll-like receptor 3-activated microglial interferon-β and nitric oxide production

Brain-selective mild hypothermia promotes long-term white matter integrity after ischemic stroke in mice

INTRODUCTION

The neuroprotective effects of hypothermia in acute ischemic stroke are well documented. However, the mechanisms involved in the effects remain to be clearly elucidated and the role of hypothermia on long-term white matter integrity after acute ischemic stroke has yet to be investigated.

AIMS

To investigate the role of mild focal hypothermia on long-term white matter (WM) integrity after transient cerebral ischemia.

RESULTS

Mild focal hypothermia treatment immediately after ischemic stroke significantly promotes WM integrity 28 days after the occlusion of the middle cerebral artery (MCAO) in mice. Higher integrity of white matter, lower activation of total microglia, less infarct volume, and better neurobehavioral function were detected in hypothermia-treated mice compared to normothermia-treated mice. Furthermore, we found that hypothermia could decrease detrimental M1 phenotype microglia and promote healthy M2 phenotype microglia. In vitro, results also indicated that hypothermia promoted oligodendrocytes differentiation and maturation after oxygen glucose deprivation.

CONCLUSION

Hypothermia promotes long-term WM integrity and inhibits neuroinflammation in a mouse model of ischemic brain injury.

The Efficacy of the Interferon Alpha/Beta Response versus Arboviruses Is Temperature Dependent

Interferon alpha/beta (IFN-α/β) is a critical mediator of protection against most viruses, with host survival frequently impossible in its absence. Many studies have investigated the pathways involved in the induction of IFN-α/β after virus infection and the resultant upregulation of antiviral IFN-stimulated genes (ISGs) through IFN-α/β receptor complex signaling. However, other than examining the effects of genetic deletion of induction or effector pathway components, little is known regarding the functionality of these responses in intact hosts and whether host genetic or environmental factors might influence their potency. Here, we demonstrate that the IFN-α/β response against multiple arthropod-vectored viruses, which replicate over a wide temperature range, is extremely sensitive to fluctuations in temperature, exhibiting reduced antiviral efficacy at subnormal cellular temperatures and increased efficacy at supranormal temperatures. The effect involves both IFN-α/β and ISG upregulation pathways with a major aspect of altered potency reflecting highly temperature-dependent transcription of IFN response genes that leads to altered IFN-α/β and ISG protein levels. Discordantly, signaling steps prior to transcription that were examined showed the opposite effect from gene transcription, with potentiation at low temperature and inhibition at high temperature. Finally, we demonstrate that by lowering the temperature of mice, chikungunya arbovirus replication and disease are exacerbated in an IFN-α/β-dependent manner. This finding raises the potential for use of hyperthermia as a therapeutic modality for viral infections and in other contexts such as antitumor therapy. The increased IFN-α/β efficacy at high temperatures may also reflect an innate immune-relevant aspect of the febrile response.

The interferon alpha/beta (IFN-α/β) response is a first-line innate defense against arthropod-borne viruses (arboviruses). Arboviruses, such as chikungunya virus (CHIKV), can infect cells and replicate across a wide temperature range due to their replication in both mammalian/avian and arthropod hosts. Accordingly, these viruses can cause human disease in tissues regularly exposed to temperatures below the normal mammalian core temperature, 37°C. We questioned whether temperature variation could affect the efficacy of IFN-α/β responses against these viruses and help to explain some aspects of human disease manifestations. We observed that IFN-α/β efficacy was dramatically lower at subnormal temperatures and modestly enhanced at febrile temperatures, with the effects involving altered IFN-α/β response gene transcription but not IFN-α/β pathway signaling. These results provide insight into the functioning of the IFN-α/β response in vivo and suggest that temperature elevation may represent an immune-enhancing therapeutic modality for a wide variety of IFN-α/β-sensitive infections and pathologies.

Mild focal hypothermia regulates the dynamic polarization of microglia after ischemic stroke in mice

Objectives The protective effects of hypothermia on acute stroke have been demonstrated in many studies. However, its underlying mechanisms have not been thoroughly elucidated. Following an ischemic stroke event, microglia undertakes an early 'healthy' M2 phenotype and gradually transform into a 'sick' M1 phenotype over time. This transformation of polarity of microglia has influence on the degree of damage following a stroke. This study investigated the effects of mild focal hypothermia on microglia polarization following ischemic stroke. Methods Transient cerebral ischemic models were created by intraluminal filament occlusion of right middle cerebral artery (MCAO) in mice for one hour. By placing an ice box under their skull, hypothermia of mice brain was initiated immediately following MCAO for 2 h. Temporal muscle temperature was recorded and maintained between 32 and 34 °C. Brain tissue loss was assessed by hematoxylin and eosin (H&E) staining 28 days after MCAO. Quantitative real-time polymerase chain reaction (qPCR) and immunostaining were used to assess phenotype of microglia in different ischemic perfusion time. Results Hypothermia reduced brain tissue loss 28 days after ischemic stroke. Hypothermia also reduced the number of CD16-positive M1 microglia and increased the numbers of CD206-positive M2 microglia following ischemic stroke. Moreover, hypothermia also led to the reduction of the M1 markers at the level of transcription, while it increased the expression of mRNA for M2 markers. Conclusions Hypothermia is protective following ischemic stroke and can reduce brain tissue loss. Moreover, hypothermia shifts the polarization of microglia from the M1 to the M2 phenotype in the ischemic mice brain. This observed biological phenomenon may partially explain the protective effects seen due to hypothermia in acute ischemic stroke.

Pharmacologically induced hypothermia attenuates traumatic brain injury in neonatal rats

Neonatal brain trauma is linked to higher risks of mortality and neurological disability. The use of mild to moderate hypothermia has shown promising potential against brain injuries induced by stroke and traumatic brain injury (TBI) in various experimental models and in clinical trials. Conventional methods of physical cooling, however, are difficult to use in acute treatments and in induction of regulated hypothermia. In addition, general anesthesia is usually required to mitigate the negative effects of shivering during physical cooling. Our recent investigations demonstrate the potential therapeutic benefits of pharmacologically induced hypothermia (PIH) using the neurotensin receptor (NTR) agonist HPI201 (formerly known as ABS201) in stroke and TBI models of adult rodents. The present investigation explored the brain protective effects of HPI201 in a P14 rat pediatric model of TBI induced by controlled cortical impact. When administered via intraperitoneal (i.p.) injection, HPI201 induced dose-dependent reduction of body and brain temperature. A 6-h hypothermic treatment, providing an overall 2-3°C reduction of brain and body temperature, showed significant effect of attenuating the contusion volume versus TBI controls. Attenuation occurs whether hypothermia is initiated 15min or 2h after TBI. No shivering response was seen in HPI201-treated animals. HPI201 treatment also reduced TUNEL-positive and TUNEL/NeuN-colabeled cells in the contusion area and peri-injury regions. TBI-induced blood-brain barrier damage was attenuated by HPI201 treatment, evaluated using the Evans Blue assay. HPI201 significantly decreased MMP-9 levels and caspase-3 activation, both of which are pro-apototic, while it increased anti-apoptotic Bcl-2 gene expression in the peri-contusion region. In addition, HPI201 prevented the up-regulation of pro-inflammatory tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and IL-6. In sensorimotor activity assessments, rats in the HPI201 treated group exhibited improved functional recovery after TBI versus controls. These data support that PIH therapy using our NTR agonist is effective in reducing neuronal and BBB damage, attenuating inflammatory response and detrimental cellular signaling, and promoting functional recovery after TBI in the developing brain, supporting its potential for further evaluation towards clinical development.

Hypothermia at 35 °C reduces the time-dependent microglial production of pro-inflammatory and anti-inflammatory factors that mediate neuronal cell death

BACKGROUND

Therapeutic hypothermia protects neurons after severe brain damage. This effect has been mainly achieved at the core temperatures of 32-34 °C; however, the optimum temperature of therapeutic hypothermia is not fully defined. Here we studied whether hypothermic culture at 35 °C had the same effects on the decrease of time-dependent expression of tumor necrosis factor (TNF)-α, interleukin (IL)-10, and nitric oxide (NO) by stimuli-activated microglia as that at 33 °C, as determined in our previous reports, and whether these factors directly induced neuronal cell death.

METHODS

We determined the levels of cytokines and NO produced by microglia cultured with adenosine triphosphate (ATP), a toll-like receptor (TLR)2 agonist (N-palmitoyl-S-(2,3-bis(palmitoyloxy)-(2R,S)-propyl)-(R)-cysteinyl-seryl-(lysyl)3-lysine, Pam(3)CSK(4)), or a TLR4 agonist (lipopolysaccharide) under mild hypothermic (33 °C), minimal hypothermic (35 °C), and normothermic (37 °C) conditions. We also determined the viability of rat neuronal pheochromocytoma PC12 cells treated with recombinant TNF-α or IL-10 or (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3, an NO donor).

RESULTS

Production of TNF-α, as well as that of IL-10 and NO were decreased by minimal hypothermia at 1.5-6, and 24-48 h, respectively, compared with normothermia, although some effects were diminished as compared with those by mild hypothermia. Exposure to TNF-α, IL-10, and NOR3 caused the death of PC12 cells in a concentration-dependent manner after 24 h.

CONCLUSION

Hypothermic culture at 35 °C decreased the production of early-phase TNF-α and late-phase IL-10 and NO from ATP- and TLR-activated microglia as observed at 33 °C, albeit with diminished effects. Moreover, these factors caused the death of neuronal cells in a concentration-dependent manner. These results suggest that the attenuation of microglial production of TNF-α, IL-10, and NO by therapeutic hypothermia leads to the inhibition of neuronal cell death. Minimal hypothermia at 35 °C may be sufficient to elicit neuroprotective effect.