Fig. 4

Antibodies promote complement-fixation on P. falciparum sporozoites, which enhances antibody-mediated traversal inhibition and can lead to sporozoite death. a Antibody samples from malaria-exposed (PNG and Kenyan) and malaria-naïve (Melbourne) individuals were tested for the ability to fix human C1q and C3 to 3D7 P. falciparum sporozoites by ELISA and Western blot. ELISA samples (top panel) were tested in duplicate, and the mean and range were graphed (C1q-fixation data is from two independent experiments). Western blot sporozoites (bottom panel) were incubated with human antibody samples and normal human serum (NHS, active complement), and then washed and processed for Western blotting under reduced conditions. Any complement proteins that had deposited on the sporozoite surface were detected using C1q- and C3-specific antibodies, and the sporozoite surface antigen, CSP, was used as a loading control. b,c In vitro traversal inhibition of freshly dissected sporozoites incubated with HC-04 cells, in the presence of NHS and heat-inactivated human serum (HIS). Each condition was tested in duplicate, and the mean and range were graphed. b Traversal inhibition of NF54 sporozoites treated with rabbit anti-CSP IgG (compared to rabbit pre-immune IgG). c Traversal inhibition of NF54 and NF166.C8 sporozoites treated with malaria-exposed Kenyan pool (compared to malaria-naïve Melbourne pool). Results from two independent experiments are shown. d Percentage of dead 3D7 sporozoites (PI+ cells) treated with rabbit anti-CSP IgG and pre-immune IgG, in the presence of NHS or C5-depleted serum (C5dep.). The mean and range of two independent experiments are graphed. a-CSP after CSP immunization, C5dep. C5-depleted serum, CSP circumsporozoite protein, ELISA enzyme-linked immunosorbent assay, HIS heat-inactivated human serum, IgG immunoglobulin G, Melb. Melbourne, NHS normal human serum, OD optical density, PI propidium iodide, PNG Papua New Guinea, pre-imm. before CSP immunization