Group 1 metabotropic glutamate receptor expression defines a T cell memory population during chronic Toxoplasma infection that enhances IFN-gamma and perforin production in the CNS

Within the brain, a pro-inflammatory response is essential to prevent clinical disease due to Toxoplasma gondii reactivation. Infection in the immunocompromised leads to lethal Toxoplasmic encephalitis while in the immunocompetent, there is persistent low-grade inflammation which is devoid of clinical symptoms. T cells are the dominant immune cell that prevent clinical disease, and this is mediated through the secretion of effector molecules such as perforins and IFN?. However, other extrinsic signals that promote protection and the formation of memory T cells within tissue are poorly understood. During chronic infection, there is an increase in extracellular glutamate that, due to its function as an excitatory neurotransmitter, is normally tightly controlled in the brain. To investigate glutamate's influence of CD8 T cells, single cell RNA-sequencing (scRNA-seq) was performed on brain CD8 T cells that were cell-sorted for CD3+CD8+mGluR1-mGluR5- or CD3+CD8+mGluR1+mGluR5+ expression. In addition, we also sorted on splenic CD8 T cells isolated from 3wpi mice with the aforementioned genotype for comparative analysis Overall design: We sequenced 27497 sorted CD8 T cells from Toxoplasma-infected brain (8904) or spleen (18593), 3-weeks post infection (wpi). This dataset is composed of one sorted population of CD3+CD8+mGluR1-mGluR5- brain T cells (7676), and three replicates of CD3+CD8+mGluR1+mGluR5+ brain T cells (324, 570, 334). Additionally, splenic T cells were also sorted for CD3+CD8+mGluR1-mGluR5- (16091) or CD3+CD8+mGluR1+mGluR5+ (2502) expression.

在脑部中，促炎应答对于预防刚地弓形虫（Toxoplasma gondii）再激活引发的临床疾病至关重要。免疫功能低下者感染该原虫后会引发致死性弓形虫脑炎，而免疫功能正常个体则会出现持续的轻度炎症且无临床症状。T细胞是预防临床疾病的核心免疫细胞，其保护作用通过分泌穿孔素、干扰素-γ（IFN-γ）等效应分子介导。然而，目前对促进保护性免疫及组织内记忆T细胞形成的其他外源性信号仍知之甚少。
在慢性感染期间，细胞外谷氨酸水平会升高；而谷氨酸作为兴奋性神经递质，在脑部通常受到严格调控。为探究谷氨酸对CD8 T细胞的影响，本研究对感染弓形虫后3周（3-weeks post infection，简称wpi）小鼠的脑部CD8 T细胞进行了单细胞RNA测序（single cell RNA-sequencing, scRNA-seq），并依据CD3+CD8+mGluR1-mGluR5-或CD3+CD8+mGluR1+mGluR5+的表达表型进行细胞分选。此外，本研究还分选了同期感染小鼠脾脏中分离得到的CD8 T细胞，并采用与上述脑部细胞相同的分选标准进行富集，用于比较分析。
实验整体设计：本研究共对27497个分选得到的CD8 T细胞进行了测序，这些细胞均来自感染3wpi的小鼠，其中脑组织样本含8904个细胞，脾脏组织样本含18593个细胞。本数据集包含一类CD3+CD8+mGluR1-mGluR5-脑部T细胞群（7676个细胞），以及三类CD3+CD8+mGluR1+mGluR5+脑部T细胞重复样本（细胞数分别为324、570、334）。此外，本研究还分选了符合CD3+CD8+mGluR1-mGluR5-（16091个细胞）与CD3+CD8+mGluR1+mGluR5+（2502个细胞）表达特征的脾脏T细胞。