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^hi 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 CD206^hiILDR2⁺ macrophages increased after the repeated antigen painting. RNA-seq analysis revealed that this CD206^hiILDR2⁺ fraction displayed downregulated expression of pro-inflammatory genes and preferentially expressed M2 macrophage related genes. This CD206^hiILDR2⁺ 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.