A RESIDENT POPULATION OF APC WITHIN THE MUSCULARIZED VESSEL WALLS OF THE RAT LYMPHATIC NETWORK

The gut lymphatic network provides a unique site for the intestinal absorption of substances and their transport via lymphatics through the lymph nodes to blood. Professional antigen‐presenting cells (APC) capture, process & display antigen complexes with MHCII on their surface, to initiate the learned immune response. Classically APC are thought to capture antigen in the interstitium, migrate to lymph capillaries enter and travel via lymph flow to the node. However, the muscularized lymphatics downstream are also in a unique position to carry & monitor foreign antigens in lymph. We propose that APC in the collecting lymphatic wall can monitor antigens carried in lymph. We used MHCII as a marker to measure APC distribution & morphology in isolated rat mesenteric pre‐nodal (PreL), post‐nodal (PstL) lymphatics, and thoracic duct (TD). Results 1. APCs reside in the walls of all lymphatics, with more in PreL than downstream segments; 2. PreL APC had more & longer cell processes that were localized to the endothelium/subendothelium (to sense lymph antigens), the richest cytoplasm & the most irregular cell shape; 3. TD APC had the least & shortest processes, poorest cytoplasm & least irregular shape. The data provide the evidence for a new model of antigen processing in APC within the muscularized lymphatics & indicate an important role for the collecting lymphatics during immune responses.