Germinal center activity and B cell maturation are associated with protective antibody responses against Plasmodium pre-erythrocytic infection

Blocking Plasmodium, the causative agent of malaria, at the asymptomatic pre-erythrocytic stage would abrogate disease pathology and prevent transmission. However, the lack of well-defined features within vaccine-elicited antibody responses that correlate with protection represents a major roadblock to improving on current generation vaccines. We vaccinated mice (BALB/cJ and C57BL/6J) with Py circumsporozoite protein (CSP), the major surface antigen on the sporozoite, and evaluated vaccine-elicited humoral immunity and identified immunological factors associated with protection after mosquito bite challenge. Vaccination achieved 60% sterile protection and otherwise delayed blood stage patency in BALB/cJ mice. In contrast, all C57BL/6J mice were infected similar to controls. Protection was mediated by antibodies and could be passively transferred from immunized BALB/cJ mice into naive C57BL/6J. Dissection of the underlying immunological features of protection revealed early deficits in antibody titers and polyclonal avidity in C57BL/6J mice. Additionally, PyCSP-vaccination in BALB/cJ induced a significantly higher proportion of antigen-specific B-cells and class-switched memory B-cell (MBCs) populations than in C57BL/6J mice. Strikingly, C57BL/6J mice also had markedly fewer CSP-specific germinal center experienced B cells and class-switched MBCs compared to BALB/cJ mice. Analysis of the IgG gamma chain repertoires by next generation sequencing in PyCSP-specific memory B-cell repertoires also revealed higher somatic hypermutation rates in BALB/cJ mice than in C57BL/6J mice. These findings indicate that the development of protective antibody responses in BALB/cJ mice in response to vaccination with PyCSP was associated with increased germinal center activity and somatic mutation compared to C57BL/6J mice, highlighting the key role B cell maturation may have in the development of vaccine-elicited protective antibodies against CSP. Author summary Identifying specific features of vaccine-elicited antibody responses that are associated with protection from malaria infection is a key step toward the development of a safe and effective vaccine. Here we compared antibody and B cell responses in two mouse strains that exhibited a differential ability to generate antibodies that protect from infection challenge. We found that protection was due to the presence of vaccine-elicited antibodies and could be transferred between strains, and that the ability of antibodies to neutralize the parasite was directly linked to the strength (affinity) with which it binds CSP. Thus, we sought to understand if there were differences in the two strains in the process of B cell maturation that leads to generation of high affinity, protective antibody responses after vaccination. Overall, our comparative analysis indicates that germinal center (GC) activity, a key process in B cell maturation, was significantly diminished in the non-protected strain. Further, we observed evidence of higher levels of somatic mutation, which is a result of germinal center activity, in protected mice. Thus, our results indicate that the ability to generate protective antibody responses was linked to enhanced B cell maturation in the protected strain, providing a key clue to the type of responses that should be generated by future vaccines.

Automated summary

This study compared antibody and B cell responses in two mouse strains with different abilities to generate protective antibodies against malaria infection. The presence of vaccine-elicited antibodies was shown to be responsible for protection, and the ability of antibodies to neutralize the parasite was directly linked to their affinity to CSP. The study found that germinal center activity, a key process in B cell maturation, was significantly diminished in the non-protected mouse strain. Additionally, higher levels of somatic mutation, a result of germinal center activity, were observed in the protected mice. These results suggest that enhanced B cell maturation is linked to the ability to generate protective antibody responses, providing important information for the development of future vaccines.