The Role of TLR-9 in Macrophages in the Development of T1DM

Background

DNA including self DNA binds to Toll-like receptor 9 (TLR9) as a nucleotide-sensing receptor. TLR9 is expressed mostly on antigen presenting cells (APCs) and plays an important role in host immune response to pathogens as well as to self antigens. We have previously shown that TLR9-deficient NOD mice are protected from developing T1DM. It is not clear which type of APC contributes to the disease protection. As macrophages are important APCs, the aim of this study was to investigate the role of TLR9 in macrophages in T1DM development.

Methods

We generated TLR9^fl/fl/Lys-Cre NOD mice that have a macrophage-specific deletion of TLR9 using the Cre/loxP system – lysozyme promoter driven Cre gene expression together with floxed TLR9 gene expression. The mouse that expresses only floxed TLR9 gene was used as control. We characterized the immune cell subsets and observed the mice for spontaneous T1DM development.

Results

Macrophage-specific deletion of TLR9 affected the phenotype of different subsets of immune cells. We observed an increase in naïve CD8⁺ T cells, regulatory CD4⁺ T cells and CD5⁺CD1d⁺ B cells in different peripheral lymphoid tissues as well as increased TNFα-expressing macrophages, but IL-10 expressing dendritic cells (DCs) were decreased in pancreatic draining lymph nodes (PLN). Splenic macrophages and DCs from TLR9^fl/fl/Lys-Cre mice showed impaired antigen presentating function compared to control TLR9^fl/fl mice. In spite of these immune alterations, TLR9^fl/fl/Lys-Cre mice developed a similar incidence of diabetes to the control TLR9^fl/fl mice.

Conclusion

Whilst deletion of TLR9 can alter the phenotype and function of immune cells it is not required for spontaneous T1DM development.