Effects of extraerythrocytic hemoglobin and its components on mononuclear cell procoagulant activity

Earlier studies performed in rabbits in this laboratory showed that human hemoglobin causes leukocyte infiltration into heart and lung tissues and coagulation abnormalities including thrombosis. Since stimulation of peripheral blood mononuclear cells induces procoagulant activity, we investigated the possibility that isolated peripheral blood mononuclear cells from humans and rabbits were stimulated by hemoglobin to produce a procoagulant activity response. Stroma-free hemoglobin and its purified components were assayed for their stimulatory effect on peripheral blood mononuclear cells. The added effect of endotoxin contamination was also explored. A significant dose-dependent effect of stroma-free hemoglobin on procoagulant activity was found with both human and rabbit peripheral blood mononuclear cells, but rabbit cells were found to be 50-fold more sensitive than human cells. Hematin, methemoglobin, and purified hemoglobin Ao elicited procoagulant activity effects with human peripheral blood mononuclear cells, but isolated globin did not. We also studied the effects of these compounds on activation of complement, a known mediator of the procoagulant activity response. Stroma-free hemoglobin, hematin, purified hemoglobin Ao, and methemoglobin were all found to cleave complement fraction C3 to C3a but at concentrations much higher than those required for procoagulant activity. The data suggest that heme itself may be responsible for the procoagulant effect of stroma-free hemoglobin solutions.