Recruited monocytes modulate malaria-induced lung injury through CD36-mediated clearance of sequestered infected erythrocytes.

Pulmonary complications occur in a significant percentage of adults and children during the course of severe malaria. The cellular and molecular innate immune mechanisms that limit the extent of pulmonary inflammation and preserve lung function during severe Plasmodium infections remain unclear. In particular, the contributions to pulmonary complications by parasitized erythrocyte sequestration and subsequent clearance from the lung microvasculature by immune cells have not been clearly defined. We used the Plasmodium berghei ANKA-C57BL/6 mouse model of severe malaria to investigate the mechanisms governing the nature and extent of malaria-associated lung injury. We have demonstrated that sequestration of infected erythrocytes on postcapillary endothelial surfaces results in acute lung injury and the rapid recruitment of CCR2(+)CD11b(+)Ly6C(hi) monocytes from the circulation. These recruited cells remain in the lungs as monocyte-derived macrophages and are instrumental in the phagocytic clearance of adherent Plasmodium berghei-infected erythrocytes. In contrast, alveolar macrophages do not play a significant role in the clearance of malaria-infected cells. Furthermore, the results obtained from Ccr2(-/-), Cd36(-/-), and CD36 bone marrow chimeric mice showed that sequestration in the absence of CD36-mediated phagocytic clearance by monocytes leads to exaggerated lung pathologic features. In summary, our data indicate that the intensity of malaria-induced lung pathologic features is proportional to the steady-state levels of Plasmodium-infected erythrocytes adhering to the pulmonary vasculature. Moreover, the present work has defined a major role of recruited monocytes in clearing infected erythrocytes from the pulmonary interstitium, thus minimizing lung damage.

CD36 and Fyn kinase mediate malaria-induced lung endothelial barrier dysfunction in mice infected with Plasmodium berghei.

Severe malaria can trigger acute lung injury characterized by pulmonary edema resulting from increased endothelial permeability. However, the mechanism through which lung fluid conductance is altered during malaria remains unclear. To define the role that the scavenger receptor CD36 may play in mediating this response, C57BL/6J (WT) and CD36-/- mice were infected with P. berghei ANKA and monitored for changes in pulmonary endothelial barrier function employing an isolated perfused lung system. WT lungs demonstrated a >10-fold increase in two measures of paracellular fluid conductance and a decrease in the albumin reflection coefficient (σalb) compared to control lungs indicating a loss of barrier function. In contrast, malaria-infected CD36-/- mice had near normal fluid conductance but a similar reduction in σalb. In WT mice, lung sequestered iRBCs demonstrated production of reactive oxygen species (ROS). To determine whether knockout of CD36 could protect against ROS-induced endothelial barrier dysfunction, mouse lung microvascular endothelial monolayers (MLMVEC) from WT and CD36-/- mice were exposed to H2O2. Unlike WT monolayers, which showed dose-dependent decreases in transendothelial electrical resistance (TER) from H2O2 indicating loss of barrier function, CD36-/- MLMVEC demonstrated dose-dependent increases in TER. The differences between responses in WT and CD36-/- endothelial cells correlated with important differences in the intracellular compartmentalization of the CD36-associated Fyn kinase. Malaria infection increased total lung Fyn levels in CD36-/- lungs compared to WT, but this increase was due to elevated production of the inactive form of Fyn further suggesting a dysregulation of Fyn-mediated signaling. The importance of Fyn in CD36-dependent endothelial signaling was confirmed using in vitro Fyn knockdown as well as Fyn-/- mice, which were also protected from H2O2- and malaria-induced lung endothelial leak, respectively. Our results demonstrate that CD36 and Fyn kinase are critical mediators of the increased lung endothelial fluid conductance caused by malaria infection.