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

Abstract

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 naïve 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 γ 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.

Coimmunization with Preerythrocytic Antigens alongside Circumsporozoite Protein Can Enhance Sterile Protection against Plasmodium Sporozoite Infection

“…If this is the case here, then a more extensive characterization of the antibody response with a focus on specific “vulnerable” epitopes would be required to describe the antibody response patterns that lead to protection. Further, it is probable that specific characteristics of the antibody response, such as avidity, antibody maturation, and IgG subclass distribution, may be more directly associated with protection ( 47 ) and provide mechanistic insights into protective responses, and these characteristics will be an intense focus in our follow-up studies. However, we cannot rule out that the lack of correlation between antibody titers and protection status may be indicative of a more prominent role for cellular immunity, as has been observed in other similar systems ( 9 , 48 – 50 ).…”

“…BALB/cJ mice (purchased from Jackson Laboratories, Bar Harbor, ME) were selected for this study, which enabled the use of our optimized P. yoelii immunization and mosquito-bite challenge model, which has been developed and validated by us previously ( 24 , 47 , 59 ). Mice were intramuscularly immunized twice—2 weeks apart—with 20-μg total protein doses containing 13 μg of the candidate immunogens or control (gp120) and 7 μg of PyCSP, or 20 μg control protein in 20% Adjuplex adjuvant (Empirion LLC, Columbus, OH).…”

Ultra-low volume intradermal administration of radiation-attenuated sporozoites with the glycolipid adjuvant 7DW8-5 completely protects mice against malaria

“…In a final set of experiments, we therefore investigated if prime-and-trap would still be effective if RAS were administered in the presence of high titers of potent pre-existing spz neutralizing antibodies. RAM2 is a spz neutralizing monoclonal antibody (mAb) that binds PyCSP with high affinity and induces high rates of sterile protection against mosquito bite challenge in mice [ 41 ]. Here, we examined the impact of immunizing in the presence of high titers of RAM2.…”

“…PyCSP binding antibodies in mouse serum were determined by direct ELISA as previously described [ 41 ]. Blood was collected via submental bleed; serum was isolated and frozen.…”

“…RAM2 spz-invasion blocking antibodies: RAM2 monoclonal antibodies were kindly provided by Noah Sather at Seattle Children's Research Institute. RAM2 antibodies were produced and puri ed as described [41]. For spz-invasion studies, mice were injected IP with 150 µg of RAM2 or matched isotype control 24 hours before RAS immunization.…”