Innate lymphocyte function in malaria

National Institute Of Allergy And Infectious Diseases

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Abstract

Lysis of target cells by NK cells depends on two processes, which are triggered by different signals. First, degranulation of NK cells, which releases perforin and granzymes, is triggered by either Fc receptor CD16 or a combination of signals from synergistic co-activation receptors. In the case of infected erythrocytes, degranulation is induced by antibodies that trigger CD16 on NK cells. Second, tight adhesion to target cells is required. Signaling by integrin alphaLbeta2 (LFA-1) is sufficient to induce conjugation and polarization of NK cells upon binding to integrin ligands ICAM-1 or ICAM-2 on target cells. As erythrocytes do not express ICAM-1 or ICAM-2, it is not clear how NK cells bind and what receptor(s) might be involved. Using live imaging, release of parasitophorous vacuoles (PV) from infected red blood cells (iRBC) after incubation with NK cells in the presence of rabbit anti-RBC antibodies was observed. These live imaging studies were confirmed by flow cytometric analysis. Direct interaction of NK cells with iRBCs was imaged by scanning electron microscopy. These images showed tight immunological synapses between NK cells and infected RBCs and revealed extensive damage to iRBC membranes. This membrane damage results in formation of ghost RBCs and eventually release of PVs. We hypothesize that monocytes clear PVs, which could reduce the inflammatory response. To study this, PVs were isolated by mild saponin treatment and mechanical disruption of iRBC membranes. Binding of IgG isolated from plasma of Malian adults to PVs was orders of magnitude higher than binding to iRBCs. Phagocytosis of PVs by primary human monocytes was detected and was enhanced in the presence of Mali IgG. Similar results were obtained using PVs released by NK-dependent lysis of iRBCs in the presence of Mali IgG. These results support the concept that NK-mediated ADCC inflicts severe damage to the plasma membrane of iRBCs, leading to release of PVs which bind immune serum IgG and are cleared through phagocytosis by monocytes.