Summary Plasmodium species, the parasitic agents of malaria, invade erythrocytes to reproduce resulting in erythrocyte loss. However, a greater loss is caused by the elimination of uninfected erythrocytes, sometimes long after infection has been cleared. Using a mouse model, we found that Plasmodium infection induces the generation of anti-self antibodies that bind to the surface of uninfected erythrocytes from infected, but not uninfected, mice. These antibodies recognize phosphatidylserine, which is exposed on the surface of a fraction of uninfected erythrocytes during malaria. We find that phosphatidylserine-exposing erythrocytes are reticulocytes expressing high levels of CD47, a ‘do-not-eat-me’ signal, but the binding of anti-phosphatidylserine antibodies mediates their phagocytosis, contributing to anemia. In human patients with late post-malarial anemia, we found a strong inverse correlation between the levels of anti-phosphatidylserine antibodies and plasma hemoglobin, suggesting a similar role in humans. Inhibition of this pathway may be exploited for treating malarial anemia.

Dendritic cells (DCs) are activated by pathogens to initiate and shape immune responses. We found that the activation of DCs by , the main causative agent of human malaria, induces a highly unusual phenotype by which DCs up-regulate costimulatory molecules and secretion of chemokines, but not of cytokines typical of inflammatory responses (IL-1β, IL-6, IL-10, TNF). Similar results were obtained with DCs obtained from malaria-naïve US donors and malaria-experienced donors from Mali. Contact-dependent cross-talk between the main DC subsets, plasmacytoid and myeloid DCs (mDCs) was necessary for increased chemokine and IFN-α secretion in response to the parasite. Despite the absence of inflammatory cytokine secretion, mDCs incubated with-infected erythrocytes activated antigen-specific naïve CD4 T cells to proliferate and secrete Th1-like cytokines. This unexpected response of human mDCs to exhibited a transcriptional program distinct from a classical LPS response, pointing to unique -induced activation pathways that may explain the uncharacteristic immune response to malaria.

Malaria is a highly inflammatory disease caused by Plasmodium infection of host erythrocytes. However, the parasite does not induce inflammatory cytokine responses in macrophages in vitro and the source of inflammation in patients remains unclear. Here, we identify oxidative stress, which is common in malaria, as an effective trigger of the inflammatory activation of macrophages. We observed that extracellular reactive oxygen species ( ROS ) produced by xanthine oxidase ( XO ), an enzyme upregulated during malaria, induce a strong inflammatory cytokine response in primary human monocyte‐derived macrophages. In malaria patients, elevated plasma XO activity correlates with high levels of inflammatory cytokines and with the development of cerebral malaria. We found that incubation of macrophages with plasma from these patients can induce a XO ‐dependent inflammatory cytokine response , identifying a host factor as a trigger for inflammation in malaria. XO ‐produced ROS also increase the synthesis of pro‐ IL ‐1β, while the parasite activates caspase‐1, providing the two necessary signals for the activation of the NLRP 3 inflammasome. We propose that XO ‐produced ROS are a key factor for the trigger of inflammation during malaria.

Malaria is a highly inflammatory disease caused by the protozoan parasite Plasmodium. During the blood stage of infection, patients exhibit fever with high levels of inflammatory cytokines in their blood. However, when cells of the immune system are incubated with the parasite in vitro, their cytokine response is low. In particular, human primary dendritic cells (DCs) respond to Plasmodium falciparum-infected erythrocytes by upregulating maturation markers and chemokines but lack a substantial cytokine response. Because oxidative stress is a trigger of inflammatory cytokines in malaria and synergizes with P. falciparum to induce IL-1b secretion by macrophages, we assessed whether oxidative stress has an impact on DC maturation and function in response to P. falciparum. Using xanthine oxidase, a reactive oxygen species-(ROS) producing enzyme that is increased during malaria, we observed that exposure to extracellular ROS potentiated DC maturation in response to the parasite. Xanthine oxidase-derived ROS increased parasite-induced cytokine secretion and CD80 surface expression in DCs. This enhanced maturation phenotype boosted the DCs' ability to prime autologous naive CD4 + T cells, resulting in higher T cell proliferation in vitro. Xanthine oxidase-derived ROS did not have an effect on the cytokines produced by primed T cells. We propose that oxidative stress during malaria contributes to the inflammatory response by enhancing the magnitude of DC and CD4 + T cell responses without changing the quality.

Spontaneous maturation observed in dendritic cell (DC) cultures has been linked to their capacity to induce immune responses. Despite several recent studies, the mechanisms and signals triggering spontaneous maturation of DCs are largely unknown. We found that the absence of SWAP-70 causes spontaneous maturation of spleen- and bone marrow–derived DCs and, in vivo, of spleen-resident CD11c+CD11b+CD8α− DCs. Activation markers, cross-presentation of exogenous Ags, and activation of CD8+ T cells are much increased in Swap-70−/− DCs. Spontaneous maturation of Swap-70−/− DCs depends on cell–cell contact and does not involve β-catenin signaling. SWAP-70 is known to regulate integrin activity. Signaling through the integrin CD11b (αM) subunit increases spontaneous maturation of wild-type (wt), but not of Swap-70−/− DCs. Signaling through the CD18 (β2) subunit decreases spontaneous maturation of wt and Swap-70−/− DCs. Constitutive activation of RhoA in Swap-70−/− DCs was determined as a key mechanism causing the increased spontaneous maturation. Inhibition of RhoA early, but not late, in the activation process reduces spontaneous maturation in Swap-70−/− DCs to wt levels. Inhibition of RhoA activation during CD11b integrin activation had a significant effect only in Swap-70−/− but not in wt DCs. Together, our data suggest that integrin-mediated spontaneous maturation of wt DCs does not depend on active RhoA, whereas the increase in spontaneous maturation of Swap-70−/− DCs is supported by integrin CD11b and by hyperactive RhoA. Thus, SWAP-70 deficiency reveals two pathways that contribute to spontaneous maturation of DCs.