Single-cell RNA sequencing reveals human leukocyte responses in humanized mice with contusive spinal cord injury
Next-generation humanized mouse models and single-cell RNA sequencing approaches enable in-depth studies into human immune cell biology. Here we used NSG-SGM3 mice engrafted with human umbilical cord haematopoietic stem cells to investigate how human immune cells respond to and/or are changed by traumatic spinal cord injury (SCI). Our behavioural studies firstly show that the benefits of immunocompromise on SCI recovery are lost with the introduction of human immune cells into these mice. Using flow cytometry and scRNAseq, we then describe the composition of their circulating immune cell repertoire, and how the human immune cell population transcriptionally changes following injury. Through comparisons of SCI with naïve and sham-operated mice, we also provide insight into the transcriptional signature of human leukocytes in association with SCI-induced systemic immune depression syndrome (SCI-IDS). We further highlight the local activating influence of the spinal cord lesion microenvironment by comparing the transcriptomes of circulating versus infiltrated human cells. We lastly applied an integrated bioinformatics approach to determine where immune responses in humanized NSG-SGM3 mice appear congruent to the native responses of human SCI patients, and where they diverge. Collectively, our study provides a valuable resource and methodological framework for the use of these mice in translational research. Overall design: To investigate human white blood cell responses to spinal cord injury, we generated humanized NSG-SGM3 mice, performed a contusive T9 spinal cord injury and sacrificed at 7 days post-injury. We collected human immune cells from the blood and spinal cords of spinal cord injured mice, as well as from the blood of naive and sham controls, and performed scRNA-seq on human white blood cells. This data was used to conduct  gene expression profiling analysis to determine how human immune cells are changed with injury and between compartments.