Evaluation of M2-like macrophage enrichment after diffuse traumatic brain injury through transient interleukin-4 expression from engineered mesenchymal stromal cells

Abstract

Appropriately modulating inflammation after traumatic brain injury (TBI) may prevent disabilities for the millions of those inflicted annually. In TBI, cellular mediators of inflammation, including macrophages and microglia, possess a range of phenotypes relevant for an immunomodulatory therapeutic approach. It is thought that early phenotypic modulation of these cells will have a cascading healing effect. In fact, an anti-inflammatory, "M2-like" macrophage phenotype after TBI has been associated with neurogenesis, axonal regeneration, and improved white matter integrity. There already exists clinical trials seeking an M2-like bias through mesenchymal stem/stromal cells (MSCs). However, MSCs do not endogenously synthesize key signals that induce robust M2-like phenotypes such as Interleukin-4 (IL-4). To enrich M2-like macrophages in a clinically relevant manner, we augmented MSCs to transiently express IL-4 via synthetic IL-4 mRNA. We observed that these IL-4 expressing MSCs indeed induce a robust M2-like macrophage phenotype and promote anti-inflammatory gene expression in a modified TBI model of closed head injury. However, here we demonstrate that acute enrichment of M2-like macrophages did not translate to improved functional or histological outcomes. This suggests that an acute enrichment of M2-like macrophages cannot solely orchestrate the neurogenesis, axonal regeneration, and improved WMI after diffuse TBI. To further understand whether dysfunctional pathways underlie the lack of therapeutic effect, we report transcriptomic analysis of injured and treated brains. Through this, we discovered that inflammation persists in spite of acute enrichment of M2-like macrophages in the brain. Last, we comment on our modified TBI model, behavioral studies, and propose that IL-4 expressing MSCs may also have relevance in other cavitary diseases or in improving biomaterial integration into tissues.