# Microglial process convergence onto injured axonal swellings, a human postmortem brain tissue study

## TL;DR

This study investigates how microglia interact with injured axons in human brains after traumatic brain injury, confirming a previously observed phenomenon in animals.

## Contribution

The study is the first to quantitatively measure microglial process convergence onto injured axons in human postmortem brain tissue.

## Key findings

- Microglial process convergence onto injured axonal swellings was increased in TBI cases.
- Findings are consistent with previous observations in micropigs, suggesting clinical relevance.
- MPC was not observed on intact myelinated fibers, highlighting specificity to injury.

## Abstract

Traumatic brain injury (TBI) affects millions globally, with a majority of TBI cases being classified as mild, in which diffuse pathologies prevail. Two of the pathological hallmarks of TBI are diffuse axonal injury and microglial activation. While progress has been made investigating the breadth of TBI-induced axonal injury and microglial changes in rodents, the neuroinflammatory progression and interaction between microglia and injured axons following brain injury in humans is less well understood. Our group previously investigated microglial process convergence (MPC), in which processes of non-phagocytic microglia directly contact injured proximal axonal segments, in rats and micropigs acutely following TBI. These studies demonstrated that MPC occurred on injured axons in the micropig, but not in the rat, following diffuse TBI. While it has been shown that microglia co-exist and interact with injured axons in humans post-TBI, the occurrence of MPC has not been quantitatively measured in the human brain. Therefore, in the current study we sought to validate our pig findings in human postmortem tissue. We investigated MPC onto injured axonal swellings and intact myelinated fibers in cases from individuals that sustained a TBI and control human brain tissue using multiplex immunofluorescent histochemistry. We found an increase in MPC onto injured axonal swellings, consistent with our previous findings in micropigs, indicating that MPC is a clinically relevant phenomenon that warrants further investigation.