Murine malaria: analysis of erythrocyte surface-bound immunoglobulin by flow microfluorimetry.

Erythrocytes from Plasmodium yoelii-infected AKR mice were analyzed for surface-bound immunoglobulin by using immunofluorescence labeling and the fluorescence-activated cell sorter (FACS). Both IgM and IgG were detected on erythrocytes, and the increase in erythrocyte surface-bound immunoglobulin paralleled the increase in parasitemia levels. However, the minimum percent of immunoglobulin-positive erythrocytes exceeded the percent of parasitized erythrocytes throughout the course of infection, and reached a maximum of 3-fold higher on day 19 post-infection. Moreover, late in the course of infection, when no parasitized erythrocytes were detectable in peripheral blood, surface immunoglobulin-positive erythrocytes were still present. Erythrocytes from infected animals were sorted with the FACS into separate populations based on fluorescence intensity (which is proportionate to the quantity of surface immunoglobulin) and then Giemsa-stained and examined for intracellular parasites. Although the cells that generally exhibited the brightest fluorescence contained the more mature developmental stages of the parasite (schizonts and late trophozoites), parasitized cells containing rings and early trophozoites as well as nonpar-asitized erythrocytes demonstrated moderate surface fluorescence. These data suggest that the parasitized erythrocyte expresses large amounts of parasite-related antigen on its surface. The finding that nonparasitized erythrocytes from infected animals bear surface immunoglobulin indicates either that infected mice develop antibodies to nonparasitized erythrocytes or that these cells also bear parasite antigen. The binding of immunoglobulin to erythrocytes of infected animals by either mechanism could contribute to the development of anemia in these animals.