Lipid bodies: structural markers of inflammatory macrophages in innate immunity

Objective and design The correlation between innate immune responses and formation of cytoplasmic lipid bodies (LBs) was investigated in vivo in inflammatory macrophages from rats infected with Trypanosoma cruzi, the intracellular parasite which causes Chagas’ disease. Material and methods We used an experimental model of high-dose irradiation prior to infection, which depletes the humoral and cellular immune responses except for the phagocytic activity of macrophages. Rats, irradiated or not, were infected with T. cruzi and macrophages from different origins (peritoneum, heart, uterus) were studied by transmission electron microscopy (TEM). Results As documented by quantitative TEM, innate immune responses induced prominent formation, structural changes and intracellular interactions of LBs. LBs significantly increased their size and changed their osmiophilia in response to both infection alone and when macrophages were challenged with irradiation-induced increased parasite load. Remarkably, a consistent LB-phagolysosome association was identified. LBs were surrounding, attached to or internalized by phagolysosomes. Conclusions We demonstrated that LBs are dynamic organelles notably involved in the host response to acute T. cruzi infection, an event that may be important for pathogen control during innate immunity. Our findings highlight LBs as structural markers of the innate immune responses in phagocytic cells. Received 14 December 2005; returned for revision 12 March 2006; accepted by G. Wallace 21 March 2006