A Novel Genetic Mouse Model for Investigating Peripheral Regulatory T Cell Functions

The intestinal immune system balances between eliminating pathogens and tolerating harmless dietary and microbiota-derived antigens. Peripheral regulatory T cells (pTregs) play a critical role in maintaining this balance, and defects in pTreg responses are a hallmark of chronic intestinal inflammation. Although the crucial molecules employed by pTregs to suppress effector immune responses have been extensively studied, it remains largely unexplored whether they rely on immune synapse interactions to deliver suppressive signals or whether they act through bystander suppression.

Distinguishing these pathways has been challenging due to a lack of suitable experimental tools. To address this gap, we developed a novel MHCII chimeric mouse model (M2ChiMice). Using morula aggregation from two donors expressing distinct MHCII alleles, we generated a mice model with two separate, parallel immune-synapse-dependent cellular networks. In this system, CD4+ T cells primed by antigen presenting cells (APCs) expressing a certain MHCII allele can further communicate via immune synapses only with cells expressing the same MHCII allele.

Recently, RORγt+ APCs were identified as a rare novel population that initiates pTreg response to gut commensals. We further generated M2ChiMice using donor cells lacking MHCII in RORγt+ APCs. This approach allowed us to create two distinct networks within the same mouse, in which one network is deprived of pTreg and the other network is intact. With this system, we can, for the first time, study of TCR-dependent pTreg suppressive functions, and are currently investigating how pTregs modulate humoral immune responses.

The M2ChiMice model represents a powerful tool for studying pTreg regulation functions. Our findings shed light on how pTregs maintain immune homeostasis through direct immune synapse communication at the tissue level.