INTRAVASCULAR LUNG MACROPHAGES PLAY AN ESSENTIAL ROLE IN LIPID ENTRAPMENT AND THE INFLAMMATORY TISSUE REACTION SEEN AFTER LONG-TERM LIPID-BASED PARENTERAL-NUTRITION IN PIGS - AN ULTRASTRUCTURAL-STUDY

To detect possible interactions between lipid-based total parenteral n utrition (TPN) substrates and mononuclear phagocytes, ultrastructural in vitro and in vivo studies were carried out on material from pigs. M ononuclear phagocytes isolated from peripheral blood, phagocytosed lip id after incubation with 1 mg/ml Intralipid(R) for 24 h. Similary, lip id was taken up by intravascular macrophages in the lungs and liver af ter central venous administration of TPN containing 2.3 g/kg body weig ht/day of Intralipid(R) for 5-7 weeks. Lipid accumulation was almost e xclusively found intravascularly in the lungs and liver, and not in ma crophages obtained from bronchoalveolar lavage fluid. A morphometric s tudy of the lung capillaries showed that the macrophages in TPN animal s had increased in size and number, and occupied a larger portion of t he capillary lumina. The macrophages appeared to be activated, but the endothelial lining was well preserved. Free intravascular lipid dropl ets had a diameter both in vitro and in vivo of about 0.5 mu m, indica ting good stability of the emulsion. We suggest that the lipid uptake stimulates the macrophages and thereby plays a role in the lung tissue inflammation seen in response to long-term lipid-based TPN in pigs.