Mordmueller, B. | Search bioRxiv

bioRxiv2021

Clonal evolution and specificity of the human T follicular helper cell response to Plasmodium falciparum circumsporozoite protein

T follicular helper (TFH) cells play a crucial role in the development of long-lived, quality-improved B cell responses after infection and vaccination. However, little is known about their clonal evolution. Here we assessed the cell phenotype, clonal dynamics, and TCR specificity of human circulating TFH (cTFH) cells at monoclonal level during successive malaria immunizations with radiation-attenuated Plasmodium falciparum (Pf) sporozoites. Repeated parasite exposures induced a dynamic, polyclonal cTFH response with high frequency of cells specific to the Pf circumsporozoite protein (PfCSP), the main surface protein of sporozoites and a validated vaccine target. Repeated immunizations were required to induce detectable PfCSP-reactive cTFH cell responses to a small number of epitopes. HLA-restrictions and differences in TCR generation probability explain the high targeting frequency of the polymorphic Th2R/T* region over the conserved T1 epitope. The vast majority of anti-Th2R/T* TCRs failed to tolerate natural polymorphisms in their target peptide sequence suggesting that parasite diversity limits natural boosting of the cTFH cell response in endemic areas and protection from non-vaccine strains. Among convergent anti-Th2R/T* TCRs with high sequence similarity, subtle differences in CDR3 composition discriminated cross-reactive from non-cross-reactive cTFH cells. Thus, our study provides deep molecular and cellular insights into the kinetics, fine specificity and HLA-restrictions of the anti-cTFH cell response that are of direct relevance for the design of PfCSP-based malaria vaccines by guiding the selection of PfCSP peptides that induce optimal B cell help.

immunology↗

bioRxiv2020

Mapping of safe and early chemo-attenuated live Plasmodium falciparum immunization identifies immune signature of vaccine efficacy

Potent protection against malaria can be induced by attenuated live-immunization with Plasmodium falciparum (Pf) sporozoites (SPZ). However, a better understanding of the critical processes involved in the establishment of protective immunity is needed. We explored the safety and vaccine efficacy of early chemo-attenuation of PfSPZ under atovaquone-proguanil (AP). AP caused early arrest of P. berghei liver stages. Despite the absence of replication, robust protection in mice correlated with parasite-specific effector-memory CD8+ T-cell responses. In a phase I clinical trial a single dose of AP prevented Pf infections in the liver of adult, human subjects who received three doses of 5.12x104 or 1.5x105 PfSPZ by direct venous inoculation combined with oral AP. However, only 2 of 8 (25%) and 2 of 10 (20%), respectively, were protected against controlled human malaria infection (CHMI) 10 weeks after the last vaccine dose, despite levels of IgG antibodies to the Pf circumsporozoite protein (PfCSP) comparable to those achieved in fully protected volunteers after immunization with 5.12x104 PfSPZ with chloroquine chemoprophylaxis active only against subsequent blood stages. We identify lower IgG recognition of the secreted liver stage-specific antigens LISP2 and LSA1 and the multi-stage antigen MSP5 as immune signatures of inferior vaccine efficacy compared to PfSPZ with chloroquine chemoprophylaxis. In conclusion, we show that immune signatures of liver stage antigens, but neither an established rodent malaria model nor concentrations of antibodies against the major surface protein of sporozoites, permit prediction of vaccine efficacy. Thus, this study provides a clear rationale for the development of live sporozoite vaccination protocols that boost exposure to Pf liver stage antigens. Significance StatementOur research demonstrates that attenuation of liver infection of high doses of Plasmodium falciparum sporozoites by concomitant single-dose administration of atovaquone-proguanil is safe in humans. However, vaccine efficacy was modest when compared to an identical protocol using chloroquine that acts only on the subsequent blood infection. Immune signatures of secreted P. falciparum liver stage antigens, but neither an established rodent malaria model nor concentrations of sporozoite antibodies, permit prediction of vaccine efficacy.

immunology↗