Microglia polarization in heat-induced early neural injury.

Introduction: In the occurrence and development of heat stroke (HS), factors such as hyperthermia, ischemia and hypoxia are essential to the central nervous system (CNS) inflammatory response, but the main mechanism underlying CNS inflammation remains unclear. The aim of the study was to observe the polarization of microglia in response to heat-induced early nerve injury and to explore its possible mechanism of action.

Material And Methods: To establish a heatstroke animal model in Beagle dogs, 18 Beagle dogs were divided into control (group A) and experimental groups (group B, group C and group D) according to a random numbers table. The animals in the experimental groups were placed on an electric blanket of an animal body temperature maintaining apparatus. The temperature was set at 40 ±0.5°C, and the rectal temperature was monitored every 5 min until the target body temperature was reached. Once the target temperature was reached, the dogs were transferred to an environment of 26 ±0.5°C and 60 ±0.5% humidity. Western blot analysis was used to detect the expression of microglia-specific markers CD45, iNOS, arginase, and CD206 in normal and heat-damaged brain tissues at different time points (1 h, 6 h, 24 h). The expression of CD45 and arginase was further determined by co-localization with immunofluorescence.

Results: CD45 and iNOS protein expression was detected in group A. The two protein markers in group B were significantly higher than those in group A ( < 0.05), and the protein markers in group C were still higher than those in group A ( < 0.05). There was no statistically significant difference among the animals in group A ( > 0.05). Arginase and CD206 protein expression was also detected in group A. Levels of the two protein markers in group B were higher than those in group A ( < 0.05), and the protein marker levels in group C were even higher than those in group A ( < 0.05). Further analysis of the two groups of protein markers in group D showed significantly higher levels than those in group A ( < 0.001). Immunofluorescence co-localization of CD45 and arginase showed significantly increased fluorescence density at 6 h and 24 h after thermal injury ( < 0.001).

Conclusions: After heat-induced disease, microglia were found to be active in the brain tissues of dogs. The microglia activated in the early 1-6 h of CNS injury were mainly the M1 type, which were then converted to the M2 type after 6 h. The 24 h M2 type was dominant. The relationship between M1/M2 polarization trends and early brain injury in heat-induced disease may be a key to understanding CNS injury in heat-induced disease.