Role of macrophages in the localisation of liposomes in lymph nodes after subcutaneous administration

The macrophage ‘suicide’ technique, based on the ability of clodronate-containing liposomes to deplete lymph nodes of macrophages, was used to study the role of macrophages in lymph node localisation of subcutaneous (s.c.) administered liposomes. Reduced liposome localisation in macrophage depleted lymph nodes confirmed that phagocytosis by macrophages is an important mechanism for lymph node localisation of large (non-sized) liposomes. Depletion of macrophages had less effect on the lymph node localisation of small (about 0.1 μm) liposomes; small liposomes may reach macrophages in regions of lymph nodes not reached by large liposomes. Small liposomes may also be taken up by cells other than macrophages, such as endothelial cells lining the lymph node sinuses. Remarkably, inclusion of poly(ethyleneglycol)-distearoylethanolamine (PEG-PE) into liposomes did not reduce the degree of lymph node localisation in control lymph nodes. As macrophage depletion had a strong negative effect on the lymph node localisation of PEG-liposomes, it is concluded that, despite the presence of a PEG-coating, PEG-liposomes retained by lymph nodes are to a large extent taken up by lymph node macrophages.