Sonia Villapol describes for SIIC his article published in Glia 65(9):1423-1438, September 2017

Washington DC, EE.UU. (special for SIIC)
Microglial activation and the production of several inflammatory cytokines are involved in traumatic brain injury (TBI), and a dysregulated neuroinflammatory response is associated with neuronal cell death.

Microglial cells are resident immune cells in the brain that are activated in response to injury. Activated macrophages recruited from the periphery, in response to TBI, rapid transit to the site of injury and act as the first line of defense against infections. Differentiating these two cell populations can be accomplished via morphological and phenotypical classification. Typically, activated microglial cells show hypertrophic/bushy or longer and robust prolongations, and peripheral macrophages are displayed an amoeboid morphology containing numerous vacuoles.

This response in males and females may change over time after TBI, yet few studies have analyzed sex discrepancies in microglia phenotypes under pathological conditions. Although the majority of the experimental studies have been conducted using male animals, a few comparative studies report sexual dimorphism in microglia activation and inflammatory genes. Clinical studies are very controversial, some showed that mortality y and poor outcomes were significantly higher in women than in men with the same extent of injury, however, other seen to suggest the opposite. While it is known that females respond differently to brain injury due to hormonal and chromosomal changes, the mechanism underlying these sex differences remains elusive.

To the best of our knowledge, this represents the first study to analyze neuroinflammatory differences between males and females after TBI.

In this study, we elucidate whether the spatial-temporal profile of microglial activation and cytokine expression at several time points after TBI exhibits sex differences.

We used 9-14 week-old female and male C57BL/6 mice weighing 20-25 g. We performed moderate controlled cortical impact injury (CCI), and mice were sacrificed at 4 h, 1, 3, 7, and 30 days post-injury (dpi). Sham-injured animals received the same craniotomy without the impact injury. Immunohistochemistry analysis was performed for microglia (Iba-1) and the percentage of the area that occupied was quantified in the injured cortex, hippocampus and thalamus at several times following TBI. Morphological analysis was done to characterize the quiescent, hypertrophied, amoeboid or bushy microglial cells. Customized probes to detect mRNA for pro-inflammatory cytokines IL-1beta and TNFalpha (M1-like microglia subtype), and anti-inflammatory cytokines Arg1 and TGFbeta (M2-like microglia subtype) were used for in situ hybridization using a novel RNAscope method optimized for compatibility with subsequent immunohistochemistry for Iba-1 within the same brain tissue section.

In all sexes, we observed that microglial cell numbers are increased at 4 h after TBI, peaking at 7 dpi and decreasing at 30 dpi. Females have less Iba-1 immunoreactive cells and amoeboid microglia in the injured cortex and hippocampus at 1 dpi compared to males. At later time-points both sexes have similar levels. In addition, females have lower mRNA TNF&alpha and IL-1beta expression in the injured brain regions compared to males at 1 dpi. No sex differences were found at longer time points in the inflammatory cytokine expression around the lesion.

These findings indicate that male and female mice have different microglial activation responses/patterns after CCI injury, suggesting a rapid acute pro-inflammatory response in males compared to a diminished and delayed response in females. This observed sexual dimorphism in the acute reactive microgliosis after brain injury could represent a promising novel avenue for therapeutic strategies.

Due to the lack of female mice in preclinical studies, as far we know, this is the first study demonstrating a time course discrepancy in the morphological and phenotypical characteristics of MG/MF; population in males and females after TBI. In this study, we have shown that the injured females produce a massive amount of M1 pro-inflammatory cytokines without apparent activation and morphological changes of MG/MF; population a few hours after TBI. However, at days after injury, MG/MF; activation, infiltration of peripheral macrophages and M1- M2 cytokines expression is increased in males, accompanied by astrogliosis and cell death in perilesional brain regions. We established that the mechanism of MG/MF; activation and inflammatory gene regulation in response to TBI is distinct in males and females and dependent on their status at the time of injury. Thus, most of the pharmacological treatments that have been investigated to date should take into consideration that the inflammatory cascade has gender dimorphism and might be effective in males yet ineffective in females.

In summary, we have identified several inflammatory genes that change dramatically after brain injury. This affect is gender specific, and these genes may play a role in the observed changes in MG/MF; morphology and activation. However, research aimed at investigating the effects of sex, microglial function, cell death and are needed to elucidate the molecular and cellular mechanisms underlying hormonal and sex-related differences in the incidence and clinical course of brain injuries. Future studies will be essential for exploring personalized therapeutic treatments for TBI patients of both sexes.