Heightened cholesterol efflux by lymph node macrophages facilitates their transfer of large lymph-borne antigen to resident dendritic cells

About This Grant

PROJECT SUMMARY High-density lipoproteins (HDL) perform the essential function of removing excess insoluble cholesterol from cells. The process is so important that abnormal HDL levels were recently linked to all-cause mortality including for non-cardiovascular diseases such as cancer and to the severity of infectious disease. However, due to the focus on HDL in cardiovascular disease, what cell types and biological processes require homeostatic cholesterol efflux remains unknown. Humans lacking two cholesterol transporters that facilitate cholesterol efflux to HDL, present with yellowed and lipid-laden lymphoid tissues without developing premature cardiovascular disease. Preliminary data shows that specific deletion of these transporters in the macrophages causes mice to develop profound lipid accumulation in the medullary sinus of the lymph node even on a diet without excess cholesterol. Furthermore, fluorescently tagged HDL developed in the Randolph laboratory is enriched in the medullary sinus and binds to macrophages. Medullary sinus macrophages (MSMs) filter molecules draining from tissues in the lymph, including apoptotic cell products. The capture of particulate and apoptotic debris would introduce excess cholesterol to MSMs but also provide MSMs with a complete survey of antigens in the lymph. Current dogma states that antigen is acquired and presented by migratory dendritic cells (DCs) or can be directly captured by resident DCs if it is small enough to enter the T cell zone of the lymph node (LN). Studies of viral infection found that DCs resident to the lymph node were essential to produce a CD8 T cell response, but it is unclear how these resident DCs could acquire antigens that are too large to enter the T cell zone. This fellowship proposes that MSMs capture particulate antigens and transfer them to resident DCs but rely on HDL-mediated cholesterol efflux to clear accumulating cholesterol. Previous work showed that macrophages in the spleen directly transfer antigen to DCs and that LN macrophages are required for developing a tumor-protective CD8 T cell response against injected dead cell-associated antigen independent of migratory DCs. The proposed work will test whether MSMs transfer large model antigens to resident DCs in the lymph node using novel genetic approaches to target MSMs and imaging approaches to capture the interactions between MSMs and DCs. It will also define interactions between LN macrophages and HDL for the first time using a novel fluorescently tagged HDL and asses the contribution of dead cell cholesterol to MSMs. MSMs in tumor-draining LNs have an elevated signature of lipid handling, thus the proposed study will test if mice with cholesterol efflux-impaired macrophages are unable to form a cytotoxic T cell response against an immunogenic tumor. The proposed fellowship will take place under the mentorship of Gwendalyn Randolph an expert in macrophages, DCs, lymphatics, and HDL with support from a committee of experts in the exceptional environment of the Washinton University Immunology program. The proposed fellowship will train the awardee to be a skilled immunologist with an interdisciplinary twist that will aid in understanding complex biology at the root cause of disease in their future scientific career.