# Investigating associations between serum inflammatory cytokines at the time of second mild traumatic brain injury with acute neurological signs, axonal injury and behavioural outcomes in male Sprague–Dawley rats

## TL;DR

This study explores how inflammation after a first brain injury affects outcomes when a second injury occurs in rats.

## Contribution

The study provides new evidence linking cytokine levels at the time of a second injury to reduced injury severity in repeated mild traumatic brain injury.

## Key findings

- Higher interleukin-6 and interleukin-13 levels at second injury were linked to fewer neurological signs.
- Cytokine levels like interleukin-10 and tumor necrosis factor-alpha were elevated after a single injury.
- No other cytokine levels correlated with post-injury outcomes after correction for multiple comparisons.

## Abstract

Mild traumatic brain injury is a risk factor to sustaining future mild traumatic brain injuries and increased symptom severity and duration following a second mild traumatic brain injury. Inflammation during neurobiological recovery is hypothesized to influence susceptibility to poorer outcomes after repetitive mild traumatic brain injury. Here, we investigated whether the inflammatory response during neurobiological recovery is related to susceptibility to increased functional and biological deficits following re-injury. To investigate this, we collected serum 1, 3, 7 or 14 days after mild traumatic brain injury in male Sprague–Dawley rats and measured levels of circulating inflammatory cytokines using the MesoScale Discovery MESO QuickPlex SQ 120MM platform to quantify interferon-gamma, interleukin-1-beta, interleukin-4, interleukin-5, interleukin-6, interleukin-10, interleukin-13, keratinocyte chemoattractant/human growth-related oncogene and tumour necrosis factor-alpha. Immediately following this blood collection, rats were given a second mild traumatic brain injury to assess associations between cytokine levels at time of second mild traumatic brain injury with behavioural outcomes, neurofilament light levels, and ex vivo diffusion tensor imaging in the 28 days following second injury. After a single mild traumatic brain injury, interleukin-10, interleukin-13, interleukin-4 and tumour necrosis factor-alpha were elevated 3 days post-injury while interleukin-10 and tumour necrosis factor-alpha levels were elevated 14-days post-injury. Furthermore, higher levels of interleukin-6 and interleukin-13 at the time of a second mild traumatic brain injury were associated with a reduced number of acute neurological signs of mild traumatic brain injury following the second injury. There were no other significant correlations between circulating cytokine levels and post-injury outcomes following correction for multiple comparisons. These findings provide initial, hypothesis-generating evidence that higher levels of circulating inflammatory cytokines at the time of a second mild traumatic brain injury may be associated with decreased susceptibility to a second mild traumatic brain injury, highlighting the complex role of inflammation in repeated mild traumatic brain injury.

Brand et al. report that levels of inflammatory cytokines at the time of a second mild traumatic brain injury are associated with signs of injury severity following the second mild traumatic brain injury. The findings offer new insights into the relationship between inflammation and susceptibility to repeated brain injury.

Graphical Abstract