Clonal evolution and TCR specificity of the human T-FH cell response to Plasmodium falciparum CSP

T follicular helper (T-FH) cells play a crucial role in the development of long-lived, high-quality B cell responses after infection and vaccination. However, little is known about how antigen-specific T-FH cells clonally evolve in response to complex pathogens and what guides the targeting of different epitopes. Here, we assessed the cell phenotype, clonal dynamics, and T cell receptor (TCR) specificity of human circulating T-FH (cT(FH)) cells 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 a small number of epitopes in Pf circumsporozoite protein (PfCSP), the primary sporozoite surface protein and well-defined vaccine target. Human leukocyte antigen (HLA) restrictions and differences in TCR generation probability were associated with differences in the epitope targeting frequency and indicated the potential of amino acids 311 to 333 in the Th2R/T* region as a T cell supertope. But most of vaccine-induced anti-amino acid 311 to 333 TCRs, including convergent TCRs with high sequence similarity, failed to tolerate natural polymorphisms in their target peptide sequence, thus demonstrating that the T-FH cell response was limited to the vaccine strain. These data suggest that the high parasite diversity in endemic areas will limit boosting of the vaccine-induced T-FH cell response by natural infections. Our findings may guide the further design of PfCSP-based malaria vaccines able to induce potent T helper cell responses for broad, long-lasting antibody responses.