Phagocytosis of Plasmodium falciparum-infected human red blood cells by human monocytes: involvement of immune and nonimmune determinants and dependence on parasite developmental stage

The stage-dependent phagocytosis of Plasmodium falciparum-infected erythrocytes (IRBC) opsonized with nonimmune serum has been investigated. An average of 2.9 red blood cell (RBC) harboring ring- forms (RIRBC) and 7.5 RBC infected with trophozoites (TIRBC) or schizonts (SIRBC) were ingested per monocyte, in comparison with 0.8 noninfected RBC (NRBC) or 5 RBC oxidatively damaged with diamide. Abrogation of generation of complement component C3b or blockage of its binding to the phagocyte inhibited phagocytosis of RIRBC by 78% to 95% and of TIRBC by 25% to 50%. Blockage of immunoglobulin G (IgG) binding reduced phagocytosis of both RIRBC and TIRBC nonsignificantly by 14%. Preincubation of monocytes with phosphatidylserine (PS)-containing liposomes reduced phagocytosis of TIRBC by 22%, but had little effect on RIRBC. Residual, noncomplement, non-IgG-, and non-PS-dependent phagocytosis amounted to 6% to 18% of total phagocytosis in RIRBC and TIRBC, respectively. RIRBC bound 2.5 times more protein A and 3.1 times more anti-C3c (a stable derivative of C3b) antibodies, and TIRBC bound 20 times more protein A and 6.8 times more anti-C3c antibodies than NRBC. Phagocytosis of oxidatively damaged RBC and RIRBC are similar, whereas a higher portion of phagocytosis appears to be noncomplement- dependent and PS-suppressible in TIRBC. It is concluded that RIRBC generate recognition signals similar to those present in oxidatively damaged or senescent RBC. Extensive membrane modifications in TIRBC produce additional, hitherto undefined signals that induce much higher and qualitatively distinct phagocytosis.