Host fibrinogen stably bound to hemozoin rapidly activates monocytes via TLR-4 and CD11b/CD18-integrin: a new paradigm of hemozoin action

Natural hemozoin (nHZ), conservatively prepared after schizogony, consists of crystalline ferriprotoporphyrin-IX dimers from undigested heme bound to host and parasite proteins and lipids. Phagocytosed nHZ alters important functions of host phagocytes. Most alterations are long-term post-phagocytic effects. Here we show that host fibrinogen (FG) was constantly present (at approx. one FG per 25,000 HZ-heme molecules) and stably bound to nHZ isolated from plasma-cultured parasites. FG was responsible for the rapid 100-fold stimulation of ROS production and 50-fold increase in the level of TNF and Monocyte-Chemotactic-Protein-1(MCP-1) by monocytes. Those effects, starting within minutes after nHZ-cell contact, were due to interaction of FG with FG-receptors Toll-like-Receptor-4 (TLR4) and integrin CD11b/CD18. Receptor blockage by specific MAbs, or removal of FG from nHZ abrogated the effects. nHZ- opsonizing IgGs contribute to the stimulatory response but are not essential for FG-effects. Immediate increase in ROS and TNF may switch on previously described long-term effects of nHZ, largely due to (and recapitulated by) HZ-generated lipoperoxidation products 15-HETE and 4-hydroxynonenal. The FG/HZ-effects mediated by TLR4/integrins represent a novel paradigm of nHZ activity, and may allow expansion of nHZ effects to non-phagocytic cells-such as endothelia and airway epithelia-and lead to a better understanding of organ pathology in malaria.