Antibody‐dependent ingestion of P. falciparum merozoites by human blood monocytes

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Abstract
The binding of malarial antibodies to peripheral blood monocytes and the ability of these armed monocytes to attach to and ingest P. falciparum merozoites and schizont infected erythrocytes was evaluated by an in vitro assay. Monocytes from normal unsensitized subjects were preincubated with sera from individuals with various states of immunity to malaria and sera from normal controls. A marked difference in the level of merozoite phagocytosis was observed depending on immune status of the individuals whose sera were tested, but not on the antibody levels measured by fluorescence or precipitation tests. By protein-A-sepharose fractionation of these sera it appeared that the merozoite phagocytosis was mediated by Ig of the IgG class. Immunoglobulins eluted from these preincubated monocytes were able to bind to the parasites as detected by indirect fluorescent test. Similar assays performed with different strains and antibodies from various geographical areas indicated that the merozoite recognition and ingestion was not strain specific. The monocyte-Ig cooperation was effective in the phagocytosis of merozoites but not of schizont infected erythrocytes or normal erythrocytes. Some degree of adhesion to the schizonts was recorded. Apparently, human blood monocytes can be specifically armed in vitro by cytophilic IgG with antimalarial specificity, and that such an effect is able to enhance markedly the clearance of free parasites but not of intact schizonts.