Low Levels of Human Antibodies to Gametocyte-Infected Erythrocytes Contrasts the PfEMP1-Dominant Response to Asexual Stages in P. falciparum Malaria

Abstract

Vaccines that target Plasmodium falciparum gametocytes have the potential to reduce malaria transmission and are thus attractive targets for malaria control. However, very little is known about human immune responses to gametocytes present in human hosts. We evaluated naturally-acquired antibodies to gametocyte-infected erythrocytes (gametocyte-IEs) of different developmental stages compared to other asexual parasite stages among naturally-exposed Kenyan residents. We found that acquired antibodies strongly recognized the surface of mature asexual-IEs, but there was limited reactivity to the surface of gametocyte-IEs of different stages. We used genetically-modified P. falciparum with suppressed expression of PfEMP1, the major surface antigen of asexual-stage IEs, to demonstrate that PfEMP1 is a dominant target of antibodies to asexual-IEs, in contrast to gametocyte-IEs. Antibody reactivity to gametocyte-IEs was similar to asexual-IEs lacking PfEMP1. Significant antibody reactivity to the surface of gametocytes was observed when outside of the host erythrocyte, including recognition of the major gametocyte antigen, Pfs230. This indicates that there is a deficiency of acquired antibodies to gametocyte-IEs despite the acquisition of antibodies to gametocyte antigens and asexual IEs. Our findings suggest that the acquisition of substantial immunity to the surface of gametocyte-IEs is limited, which may facilitate immune evasion to enable malaria transmission even in the face of substantial host immunity to malaria. Further studies are needed to understand the basis for the limited acquisition of antibodies to gametocytes and whether vaccine strategies can generate substantial immunity.

P. falciparum gametocyte density and infectivity in peripheral blood and skin tissue of naturally infected parasite carriers

“…Our findings argue against a long-standing hypothesis that never had a solid evidence base or proposed mechanism. Since the deformability of erythrocytes infected with mature gametocytes is similar to that of uninfected erythrocytes (23, 40) and there is no evidence for antigens on the surface of mature gametocyte-infected erythrocytes (41, 42), it is perhaps unsurprising that gametocyte concentrations are similar in the different blood compartments. While direct skin-feeding assays tend to result in higher infectivity compared that observed in indirect feeding procedures using venous blood, our data demonstrate that any differences observed are based on technical rather than biological differences in the feeding procedure.…”

Malaria vaccine candidates displayed on novel virus-like particles are immunogenic and induce transmission-blocking activity

“…The expression of monomeric recombinant Pfs230D1M was performed in HEK293F cells as previously described [27]. Briefly, a truncated form of Pfs230 containing the first 6-cys domain of Pfs230 has previously been expressed as a monomeric recombinant protein in P .…”

“…Mature stage V gametocyte-infected erythrocytes were generated as previously described [27,29] and treated with saponin to permeabilize the infected erythrocyte membrane. Gametocytes were subsequently incubated with whole serum from rabbits immunized with Pfs230c-dS/dS VLPs, followed by an AlexaFluor 488-conjugated donkey anti-rabbit IgG (1/500) with ethidium bromide to distinguish between infected and uninfected erythrocytes (1/1000), with washing between each step.…”

Setting the stage: The initial immune response to blood-stage parasites

“…As RBCs lack MHC-I, they cannot present antigen to CD8 T cells and stimulate a cytotoxic response [2]. However, the parasite does actively transport antigens across the parasitophorous vacuole to the RBC surface where they can be recognized by B cells and induce antibody production [17,24]. As the parasite grows, the parasite-infected red blood cell (iRBC) becomes stiffer and misshapen, allowing recognition and clearance by the spleen.…”

“…When taken up in a blood meal, conditions in the mosquito midgut stimulate gametogenesis, leading to fertilization and ultimately sporozoite production and transmission through the population (Figure 1). Antibodies generated against gametocyte/gamete antigens, including Pfs230, Pfs48/45 and Pfs47, have been shown to block mosquito transmission in the mosquito midgut [21], but a selective protective immune response targeted only against immature intraerythrocytic gametocytes in the human host has not been described [22][23][24].…”