Differential effect and interaction of monocytes, hyperimmune sera, and immunoglobulin G on the growth of asexual stage Plasmodium falciparum parasites

Using a flow cytometry-based parasite growth inhibition assay (GIA) and an antibody-dependent cellular inhibition (ADCI) assay, we have assessed the d ifferential effect and interaction of monocytes, immune sera, and purified immunoglobulins from Kenyan adults on the growth of Plasmodium falciparum p arasites in vitro. We found that monocytes from 14 different normal, health y, non-malaria-exposed donors had varying effects on parasite growth, i.e., inhibition or enhancement of parasitemia, suggesting heterogeneity in anti -parasitic activities of monocytes from individual donors. Twenty-two serum samples collected from clinically immune adults from western Kenya inhibit ed growth of P. falciparum after 48 hr in culture. In contrast, all IgG pre parations, except one, purified from the same serum samples enhanced parasi te growth. In ADCI experiments, of the 22 purified IgG samples used, 11 sho wed ADCI activities with specific growth inhibition (SGI) of more than 10%, with the highest at 27.6%, and the remaining 11 IgG samples had an SGI of less than 10%. Our results also showed that the ratio of IgG1 to IgG3 antib odies, as determined by an indirect immunofluorescence assay, was higher in the high ADCI response group than in the low response group, suggesting th at a higher concentration of IgG1 antibodies with a higher IgG1/IgG3 ratio might be associated with ADCI activities. The present study has resulted in the development of simple, reproducible how cytometry-based GIA and ADCI a ssays, and also provides baseline information for further investigation of the role of ADCI activity in naturally acquired immune protection against m alaria.