Abstract
The repeat region of the Plasmodium falciparum circumsporozoite protein (CSP) is a major vaccine antigen because it can be targeted by parasite neutralizing antibodies; however, little is known about this interaction. We used isothermal calorimetry and X-ray crystallography to analyze the binding of the Plasmodium-neutralizing 2A10 antibody to CSP. Strikingly, we found that the repeat region of CSP is bound by multiple antibodies and that this multivalent interaction drives the affinity of this antibody. Because the CSP protein can cross-link multiple B cell receptors (BCRs) we hypothesized that the B cell response might be T-independent. However, by sequencing the BCRs of CSP-repeat specific cells we found that these cells underwent somatic hypermutation and affinity maturation indicative of a T-dependent response. Interestingly, the BCR repertoire of responding B cells was limited suggesting that the structural simplicity of the repeat may limit the breadth of the immune response.
Author Summary Vaccines aim to protect by inducing the immune system to make molecules called antibodies that can recognize molecules on the surface of invading pathogens. In the case of malaria, our most advanced vaccine candidates aim to make antibodies that recognize the circumsporozoite protein molecule on the surface of the invasive parasite stage called the sporozoite. In this report we use X-ray crystallography to determine the structure of CSP-binding antibodies at the atomic level. We use other techniques such as isothermal titration calorimetry to examine how this antibody interacts with the CSP molecule. Strikingly, we found that each CSP molecule could bind 6 antibodies. This finding has implications for the immune response and may explain why high titers of antibody are needed for protection. Moreover because the structure of the CSP repeat is quite simple we determined that the number of different kinds of antibodies that could bind this molecule are quite small. However those antibodies can become quite high affinity as a result of a process called affinity maturation that allows the body to learn how to make improved antibodies specific for pathogen molecules. These data show that while it is challenging for the immune system to recognize and neutralize CSP, it should be possible to generate viable vaccines targeting this molecule.
