Sublingual macrophage-associated ILDR2 contributes to immune tolerance via Treg induction.

The sublingual mucosa (SLM) has been used for sublingual immunotherapy (SLIT) which has the potential to induce antigen-specific immune tolerance. We previously demonstrated the CD206 macrophages that were increased in the SLM after repeated antigen exposure. These macrophages showed high expression of the gene encoding ILDR2 (Ig-like domain-containing receptor 2), an immune checkpoint molecule. Here, we found a subpopulation of SLM CD206 macrophages expressed cell surface ILDR2, using a newly developed monoclonal antibody that was specific to mouse ILDR2. ILDR2 expression in the CD206 macrophages was restricted to the SLM, and the percentage of CD206ILDR2 macrophages increased after the repeated antigen painting. RNA-seq analysis revealed that this CD206ILDR2 fraction displayed downregulated expression of pro-inflammatory genes and preferentially expressed M2 macrophage related genes. This CD206ILDR2 fraction preferentially increased Foxp3 regulatory T cells (Tregs) from naive CD4 T cell coculture in vitro, and the induction of Tregs was blocked by a neutralizing anti-TGF-β antibody. Our results demonstrated that ILDR2-expressed in the SLM CD206 macrophages contribute to immune tolerance by generating Tregs in a TGF-β dependent manner.