Cryo-EM mapping of circumsporozoite protein epitope accessibility and visualization of antibody-induced shedding on Plasmodium falciparum sporozoites

The circumsporozoite protein (CSP) on the surface of Plasmodium falciparum sporozoites is a major malaria antigen that can be recognized by the adaptive immune system. Antibodies against CSP can offer protection against malaria following a mosquito bite. While it is known that antibodies can recognize truncated forms of Plasmodium falciparum CSP (PfCSP), the accessibility and presentation of PfCSP on the parasite itself is still unclear.

Researchers employed cryo-electron microscopy and tomography to study how antibodies against different PfCSP epitopes, such as the N-terminal domain (NTD), Junction, central repeat, and C-terminal domain (CTD), affect the PfCSP coat on whole Plasmodium falciparum sporozoites (PfSPZ). They discovered that PfCSP alone cannot be seen on the surface of the cell when imaged by cryo-EM.

However, when antibodies targeting the Junction motif and central repeats were used, a boundary layer formed by PfCSP on the sporozoite outer membrane was revealed.

Further investigation showed that antibody concentration plays a crucial role in inducing the shedding of CSP. By comparing monovalent FAb with IgG, the researchers found that antibody bivalency is essential for driving this reaction. Moreover, they confirmed that antibody against the NTD does not bind, suggesting the absence or inaccessibility of this epitope on PfCSP displayed by radiation-attenuated sporozoites.

In contrast, the CTD can be bound by antibodies without causing a CSP shedding reaction. These findings provide insights into the native context of the PfCSP coat on the parasite, thereby revealing the sporozoites' antigenic profile.