Plasmodium falciparum gametocytes induce innate immunity in bone marrow derived macrophages

BACKGROUND-AIM Malaria is a parasitic vector-born disease causing millions of cases every year. Gametocytes (GCT) are the stage of Plasmodium which transmit the disease, when taken up by a mosquito vector during a blood meal. GCT develop in five stages: stage I-IV are sequestered in different organs, mainly in the bone marrow, where they differentiate being released in the circulation only at stage V. The importance of innate immunity to malaria is well recognized. However, most of the studies refer to the asexual blood stage responsible for the symptoms of the disease. The aim of the present study was to investigate the innate immune responses to Plasmodium falciparum (Pf) GCT. METHODS GCT were obtained from the Pf transgenic strain 3D7elo1-pfs16-CBG99 expressing the luciferase CBG99 under the control of the GCT-specific promoter pfs16. Bone marrow derived macrophages (BMDM) were immortalised from mice, wild type (WT) or knock down for MyD88 (MyD88-KO), an adaptor protein essential for Toll-like receptors (TLRs) signal transduction. Human peripheral blood mononuclear cells (PBMCs) were isolated by density centrifugation (Ficoll- Paque). Cells were incubated with early (II-III) or late (V) GCT for 2h in the presence or not of Cytochalasin D, an inhibitor of actin polymerization and thus of phagocytosis. GCT phagocytosis by BMDM was demonstrated by Giemsa staining. A novel bioluminescent measurement of internalised GCT was set up using the transgenic strain expressing luciferase. The activation of macrophages or PBMCs was evaluated by measuring the production of TNF-α, MIP-2 and IL-10 by ELISA and of nitric oxide by Griess assay. RESULTS This is one of the first demonstration that GCT are phagocytised by murine BMDM. Moreover, late, but not early GCT, induce the production of TNF-α, nitric oxide (NO) or of the inflammatory chemokine MIP-2 by BMDM. The production of NO and TNF-α is TLR-dependent, being decreased in MyD88-KO versus WT cells. The production of TNF-α from PBMCs in response to late GCT is dependent on phagocytosis, since it is abolished by cytochalasin D. CONCLUSIONS This work represents one of the first evidences of involvement of TLRs in the inflammatory response to mature GCT.