Unravelling the role of aDb2 as a novel complement receptor in lung injury - Project summary β2 integrins are leukocyte-specific heterodimeric adhesion molecules consisting of α- and β- subunits. β2 integrins consist of αLβ2, αMβ2, αXβ2 and αDβ2 with αDβ2 being cloned most recently. αDβ2 is highly homologous to αMβ2 and αXβ2, both of which are also known as complement receptors (CR3 and CR4, respectively). Although the biological role of αDβ2 still needs to be studied in depth, in vivo studies have shown that this may be a promising target for acute lung injury in sepsis. Our preliminary data and our published study showed that αDβ2 deficiency was associated with less bacterial loads and less development of acute lung injury in septic mice. αDβ2 was expressed on human and mouse injured lung in sepsis. Acute lung injury is among the leading causes of morbidity and mortality in sepsis (~40% mortality), and is managed only supportively including the use of mechanical ventilation, which by itself can cause lung injury. Thus, delineating its fundamental, biological role is of great importance to understand the mechanism of how αDβ2 contributes to the disease pathophysiology. Considering its high homology to αMβ2 and αXβ2, we hypothesize that αDβ2 would be a novel complement receptor. Our preliminary data showed that sheep erythrocytes opsonized by iC3b formed rosetting on HEK cells stably expressing αDβ2, supporting our hypothesis. In Aim 1, we will characterize the binding profile of αDβ2 to different complement fragments and delineate the contribution of αDβ2 to the binding of complement fragments by different leukocytes in vitro. We will rank order the binding of αMβ2, αXβ2 and αDβ2 to iC3b. In our preliminary data, we found that lung αDβ2 knockout neutrophils had more phagocytosis than WT counterparts from septic mice. We will delineate the underlying mechanism of αDβ2-mediated phagocytosis regulation in myeloid cell population such as neutrophils and macrophages under the hypothesis that αDβ2 serves as a negative regulator of αMβ2 and αXβ2. In Aim 2, we will determine the interaction between αDβ2 and complements in vivo using two disease models mimicking sepsis. One will be polymicrobial abdominal sepsis model, one of the most commonly used sepsis models. In this model, we will examine the role of αDβ2 in lung injury caused by extrapulmonary sepsis. Another model will be Pseudomonas aeruginosa pneumonia model, given it is associated with a high mortality. In this model, we will examine the role of the role of αDβ2 in direct lung injury. We will also examine the expression pattern of αDβ2 and complement fragments in the existing discarded lung specimens biopsied from patients with the diagnosis of acute hypoxemic lung injury and sepsis. Upon the completion of the proposed study, we will solidify that αDβ2 is a novel complement receptor in vitro and in vivo. Our future plan is to screen αDβ2 antagonists and test them in vivo to further delineate if αDβ2 would serve as a potential target for acute lung injury. The ultimate goal of the project is to intervene αDβ2 in patients with acute lung injury in sepsis.
