Modulation of adaptive and innate immune responses by myeloid cells during malaria

Dendritic cells and macrophages are highly heterogenous members of the mononuclear phagocyte system, that share a common lineage precursor in their differentiation progression. They are found in virtually all tissues and have broadly unifying roles in immunity with overlapping functions such as antigen presentation, cytokine production and regulation of both the innate and adaptive arms of immunity. Dendritic cells are dedicated immune cells unrivalled for antigen presentation, T-cell activation and bridging the dual arms of immunity, with little other non-immune roles. Macrophages have a multitude of indispensable roles in embryo development and adult tissue homeostasis, in concert with their immune functions, largely imparted by potent phagocytosis. Their complex heterogeneity and functional specialization extend to numerous subpopulations and requires investigation as separate entities rather as a collective whole in steady-state, disease or infection models. Here we focused on the specific conventional DC subset, Clec9A+ CD8+ lymphoid or CD103+ non-lymphoid DCs; specialized in cross-presentation, sensors of apoptotic/dying cells and potent inducers of IL-12-mediated proinflammatory responses, in the context of chronic malaria. From the macrophage axis in acute malaria, we were interested in embryonically-seeded tissue-resident macrophages, including the CD169+ subset, found in the splenic red pulp and marginal zone, liver hepatic sinusoids, lung alveolar spaces and other tissues with tissue-specific functional specializations. To study CD8+/ CD103+ DCs, we utilized the Clec9A-DTR mouse model that allowed for specific and efficient ablation of Clec9A+ DCs. Depletion of Clec9A+ DCs resulted in a striking phenotype in the Plasmodium chabaudi chabaudi AS model of chronic malaria, characterized by higher acute blood parasitemia, multiple severe recrudescent episodes and some associated mortality. Acute-phase Th1 proinflammatory responses, T effector cytokine-producing polyfunctionality and follicular T helper effector functionality were drastically impaired in Clec9A-DTR mice. During the chronic phrase, Clec9A-deficient animals showed striking anti-Plasmodium Ig polarization to the less cytophilic IgG1, with little class-switching to other Ig isotypes; as well as persistently impaired T effector and T memory precursor generation. Hence, the absence of Clec9A+ DCs had far-reaching impacts on downstream immune effector responses to PccAS infection, indicating the key roles and indirect influences this DC subset has in shaping anti-Plasmodium immunity. To study the tissue-resident macrophage turnover during acute malaria infection, we utilized the KitMerCreMer/R26 inducible fate-mapping mouse model which labelled cKit+ bone marrow progenitors and their subsequent progeny as YFP+ in adult mice, after tamoxifen pulsing. Tissue-resident macrophages mostly self-renewed with little monocytic input in unperturbed conditions. Plasmodium yoelii 1.1GFP infection resulted in macrophage disappearance during the pre-crisis period, followed by massive influx of bone marrow-derived inflammatory monocytes that developed into inflammatory macrophages as well as differentiated to tissue-resident macrophages; and in conjunction with local proliferation of surviving embryonic-seeded macrophages, replenished the emptied splenic and liver niches. In contrast, the lung alveolar macrophages were exclusively self-renewing. Transcriptional profiling in all organs showed high degree of similarity between BM-originated replenished macrophages and bona fide tissue-resident macrophages, as well as longevity beyond resolution of infection. Our results demonstrated that Plasmodium infection resulted in efficient monocytic refilling of tissue-resident macrophage compartments with a stable dual ontogeny upon recovery, which were also able to adopt tissue-specific genetic expression profiles of bona fide embryonic-origin tissue-resident macrophages.Doctor of Philosoph