# RBC Storage Lesion Studies in Humans and Experimental Models of Shock
## 14 Effects of Haptoglobin Concentrate Administration  
Given this potential therapeutic target, we decided to study the effects of commercially available human Hp concentrates in our _S. aureus_ pneumonia model of septic shock with and without exchange transfusion of RBCs. To determine if the beneficial effect of Hp therapy was limited only in the setting of blood transfusion, as a control group we also studied haptoglobin infusion in canines with sepsis alone who did not receive transfusion of blood. All septic canines, regardless of whether they received Hp infusion or not, were treated with the same standard clinical interventions employed during human septic shock (e.g., antibiotic, sedation, and titrated cardiopulmonary support). Plasma CFH levels were markedly elevated during sepsis in both groups of animals, those who received blood transfusions and those who did not. When human Hp concentrate was infused over 48 h, regardless of whether the septic animal received blood transfusions or not, there was significant improvement in the metabolic profile as well as reduction in the extent of lung injury, degree of shock, and mortality rates [23] (Figure 3). In both animals receiving blood transfusions and those that did not, improvement was associated with the formation of CFH-Hp complexes, compartmentalization of CFH to the intravascular space with subsequent increased CFH clearance, as well as prevention of the release of free iron and presumably heme. However, in human septic shock, compared to this preclinical experimental model, there exists a range of lethality and hemolysis with CFH release. This variable amount of hemolysis and CFH release could potentially alter Hp's beneficial effects. Prior to designing clinical studies in human sepsis, it is important to understand how the CFH-Hp complex kinetics relate to its beneficial effects. Therefore, we performed a second Hp sepsis study in which we addressed two critical questions: (1) in a model of severe and rapidly lethal bacterial septic shock secondary to _S. aureus_ pneumonia, would Hp therapy have enough time to provide beneficial effects by reducing the availability of CFH; and (2) in a model of less severe sepsis with lower lethality, could the high levels of CFH produced by transfusing older RBCs result in toxicity from the circulation of large numbers of CFH-Hp complexes [29]? We found that in both of these circumstances, severe septic shock from _S. aureus_ pneumonia as well as increased CFH from transfusion of old blood, the mechanisms of Hp action was disrupted and the previously observed beneficial effects of Hp were no longer observed. In both situations, there was no longer increased clearance of CFH or a reduction of iron levels by Hp and all beneficial effects on outcomes were eliminated [29]. The lack of clearance of the CFH-Hp complexes during severe sepsis suggests that highly lethal infections narrow the therapeutic window for haptoglobin. Extremely elevated levels of CFH after transfusion of longer stored blood were able to saturate the Hp and generate supraphysiologic levels of CFH-Hp complexes. At these levels, transfusion of Hp conferred no beneficial effects on mortality, lung injury, or metabolic parameters in animals receiving Hp and those that did not. We speculate that high levels of CFH-Hp complexes overwhelmed the normal clearance mechanisms, eliminating Hp's previously observed beneficial effects. This study documented that clearance of CFH and iron are critical to Hp's beneficial effects during infection. When designing future clinical trials of haptoglobin use in human sepsis, it will be important to understand and account for these factors promoting CFH clearance. If successful, these trials could potentially define the role of transfused haptoglobin concentrates as a therapy for septic shock. Figure 3: Kaplan–Meier survival curves, hemodynamic assessments, and lung injury plots of septic canines receiving haptoglobin infusion therapy with and without RBC transfusion.