Lipid droplets in mouse thymus express immune molecules and inhibit antigen-specific CD4+ T cell responses

Abstract Lipid droplets (LDs) are vesicles present inside a wide variety of cells. LDs play a vital role in storing neutral lipids as a source for nutrients. While LDs are readily detected intracellularly, their extracellular presence is not widely reported. The cellular origins of LDs and the functions they serve in the thymus remain largely unknown. We detected presence of LDs in the mouse thymus and lymph nodes. We have investigated the proteins expressed by mouse thymic LDs and their functional role. To facilitate this inquiry, we first developed a novel method to isolate and quantify LDs from the thymus. Isolated LDs were analyzed by flow cytometer after staining with either fluorescent-tagged antibodies against immune cell proteins, or a lipid staining orange dye oil stain scarlet 6G. We report that the thymic LDs are heterogeneous in size (range 5 to 100 μm). LDs, on their membrane, express many immune molecules, including T cell receptor αβ in significant amounts. Thymic LDs do not express MHC proteins and are functionally incompetent to serve as antigen presenters. However, when included in antigen presenting assays using bona fide antigen presenting cells, LDs inhibit CD4+ T cell responses. We conclude that LDs found in the thymus express immune molecules on their membrane. While LDs do not directly present antigens to CD4+ T cells, however, their inclusion in cell cultures inhibit antigen-specific clonal expansion of CD4+ T cells in the presence of APC and an appropriate antigen.