In Vivo Approaches Reveal a Key Role for DCs in CD4+ T Cell Activation and Parasite Clearance during the Acute Phase of Experimental Blood-Stage Malaria

Abstract

Dendritic cells (DCs) are phagocytes that are highly specialized for antigen presentation. Heterogeneous populations of macrophages and DCs form a phagocyte network inside the red pulp (RP) of the spleen, which is a major site for the control of blood-borne infections such as malaria. However, the dynamics of splenic DCs during Plasmodium infections are poorly understood, limiting our knowledge regarding their protective role in malaria. Here, we used in vivo experimental approaches that enabled us to deplete or visualize DCs in order to clarify these issues. To elucidate the roles of DCs and marginal zone macrophages in the protection against blood-stage malaria, we infected DTx (diphtheria toxin)-treated C57BL/6.CD11c-DTR mice, as well as C57BL/6 mice treated with low doses of clodronate liposomes (ClLip), with Plasmodium chabaudi AS (Pc) parasites. The first evidence suggesting that DCs could contribute directly to parasite clearance was an early effect of the DTx treatment, but not of the ClLip treatment, in parasitemia control. DCs were also required for CD4+ T cell responses during infection. The phagocytosis of infected red blood cells (iRBCs) by splenic DCs was analyzed by confocal intravital microscopy, as well as by flow cytometry and immunofluorescence, at three distinct phases of Pc malaria: at the first encounter, at pre-crisis concomitant with parasitemia growth and at crisis when the parasitemia decline coincides with spleen closure. In vivo and ex vivo imaging of the spleen revealed that DCs actively phagocytize iRBCs and interact with CD4+ T cells both in T cell-rich areas and in the RP. Subcapsular RP DCs were highly efficient in the recognition and capture of iRBCs during pre-crisis, while complete DC maturation was only achieved during crisis. These findings indicate that, beyond their classical role in antigen presentation, DCs also contribute to the direct elimination of iRBCs during acute Plasmodium infection.

Splenic differentiation and emergence of CCR5+CXCL9+CXCL10+ monocyte-derived dendritic cells in the brain during cerebral malaria

“…As these cells share various markers with inflammatory monocytes and conventional myeloid, as well as plasmocytoid DCs, some of the existing results in the literature may have missed that a significant proportion of DCs in this mouse malaria model are indeed MO-DCs. Previous studies using the P. chabaudi model identified inflammatory monocytes that express low levels of CD11c and a low frequency of CD11c + cells that were CD11b + F4/80 + (refs 19, 21). Here we provide evidences that in this model, splenic inflammatory monocytes and monocytes are progenitors of MO-DCs.…”

“…This cytokine activates macrophages to produce toxic metabolites, such as reactive nitrogen intermediates and promotes Ig switch to the IgG2a/IgG2c, which mediate host resistance to Plasmodium infection in mice1150. In addition, both inflammatory monocytes and DCs can internalize and destroy infected RBCs in the spleens and mediate resistance to P. chabaudi infection in mice1921. However, depending on experimental setting, DCs may become refractory or impaired for presenting antigen to CD4 + T and CD8 + T cells5152.…”

“…Importantly, uptake of infected erythrocytes seems to inhibit maturation and function of human DCs17, and a low number of circulating DCs is associated with impairment of antigen-specific T-cell responses in symptomatic patients infected with either Plasmodium falciparum or Plasmodium vivax , suggesting their role in resistance to human disease18. In addition, inflammatory monocytes in mouse (CCR2 + Ly6c + and F4/80 + ) and humans (CD14 + CD16 high ), as well as in DCs, are important sources of inflammatory cytokines and reactive nitrogen and oxygen intermediates that effectively destroy Plasmodium parasites within phagocytosed infected red blood cells (iRBCs)11192021.…”

Development of a Novel CD4+ TCR Transgenic Line That Reveals a Dominant Role for CD8+ Dendritic Cells and CD40 Signaling in the Generation of Helper and CTL Responses to Blood-Stage Malaria

“…As blood-stage malaria Ags are largely cell associated, the superiority of CD8 + DC in presenting MHC II-restricted malaria Ag shown in this study is likely a consequence of superiority in the capturing of malaria Ag. Indeed, a recent study has shown higher levels of P. chabaudi Ag uptake by CD8 + DC relative to CD4 + DC, when assessed at day 5 of infection (82).…”

Intravital microscopy in historic and contemporary immunology

“…In addition to the brain, the spleen is a focus in malaria research. Phagocytosis of iRBC by splenic DCs and the subsequent displacement and interaction of DCs with CD4 + ‐T cells was visualised by IVM, revealing a previously unappreciated function for DCs in direct clearance of infected cells 68 …”