Impact of the lack of cross-presenting dendritic cells on the generation of inflammatory immune responses in experimental <em>Plasmodium berghei </em>ANKA infection and therapeutic possibilities to limit excessive brain inflammation with doxycycline

Abstract

Malaria is a vector-transmitted disease caused by unicellular parasites of the <em>Plasmodium</em> species and resulted in over 400,000 deaths in 2015, most of them in African children below the age of five. <em>Plasmodium</em> parasites are transmitted to a vertebrate intermediate host during blood-meals of female <em>Anopheles</em> mosquitoes, the definitive host of the parasite. <br /> Infections with<em> P. falciparum</em> are responsible for the severest form of malaria in humans, cerebral malaria (CM). CM is thought to be a multi-factorial process, where sequestration of infected erythrocytes and immune cells in the cerebral microvasculature and overwhelming pro-inflammatory immune responses result in severe cerebral damage. Previous research using experimental models revealed that T cells that have been primed in the spleens of infected individuals and migrated to the brain, play an important role in the development of experimental CM. However, the exact mechanisms that lead to CM in some cases of<em> P. falciparum </em>infection remain elusive. <br /> Cross-presenting dendritic cells (DCs) have been shown to be the major cell subset in priming cytotoxic CD8⁺ T cells during <em>Plasmodium</em> infection. Batf3^-/- mice, which genetically lack cross-presenting DCs, are completely protected from experimental CM. We demonstrated here, that the protection was accompanied by reduced immune cell migration to the brain and impaired cytotoxic T lymphocyte (CTL) responses in the spleens of Batf3^-/- mice after <em>P. berghei</em> ANKA-infection. Additionally, we found elevated expression of immune regulating factors in these mice, which could further contribute to the ECM-resistance in Batf3^-/- mice. <br /> In summary, these results underlined the importance of cross-presenting DCs in the pathogenesis of ECM. Furthermore, they improved our understanding of the disease development by adding important details about the alterations in the immune responses that result from the lack of cross-presenting DCs. <br /> Another important question in understanding ECM pathology is how these immune responses can be modified later in <em>Plasmodium</em> infection, before cerebral symptoms occur, in order to prevent brain inflammation. <br /> The lack of specific treatment of cerebral malaria and increasing drug resistance of <em>Plasmodium</em> species requires new approaches to fight this disease. Here, we demonstrated that the antibiotic doxycycline did not only reduce the parasitemia in <em>P. berghei</em> ANKA infected mice but also generally dampened the pro-inflammatory milieu and antigen-specific cytotoxicity of immune cells in the spleens and brains of infected animals. Different approaches in our experimental setup suggest that the reduced inflammation in doxycycline-treated mice was not exclusively dependent on doxycycline's anti-plasmodial effect. Thus, a combined treatment of doxycycline, which is relatively well-tolerated with few side effects, and another fast-acting anti-parasitic drug might offer a reasonable therapeutic option to target parasite load and cerebral inflammation at the same time.