A functional crosstalk between circulating follicular helper 2 T cells and memory B cells drives anti-Plasmodium vivax antibodies

BACKGROUND: The induction of long-term humoral immune responses depends upon the interaction between T follicular helper (Tfh) and B cells in germinal centers, leading to development of memory B cells (MBCs) and class-switched antibodies. Expansion and activation of circulating Tfh2 (cTfh2) cells were detected in both vivax and falciparum malaria subjects. However, how these cells help B cells generate anti-malarial immunity is still unclear. Here, we assessed the breadth and competency of antibody responses from P. vivax subjects and related them to the frequency and activation status of cTfh2 subset. We also demonstrated the ability of P. vivax antigen to trigger cTfh2 cell activation and the function of cTfh2 cells to help MBCs secrete antibodies. METHODOLOGY/PRINCIPAL FINDINGS: Of 40 subjects with acute P. vivax malaria, 23 were seropositive for anti-PvDBPII antibodies (High and Low responders). Three High Responders (HRs) produced inhibitory antibodies against PvDBPII-human erythrocyte binding. An expansion of cTfh2 cells was detected in seropositive subjects. While their frequency did not differ significantly between High and Low Responders (LRs), the expression of co-stimulatory molecule ICOS in cTfh2 cells was higher in HRs. Activation of cTfh2 cells was specifically stimulated by PvDBPII antigen. In cTfh2-MBC co-cultures, proliferation and activation of cTfh2 cells were detected after receiving signal from MBCs. These activated cTfh2 cells then promoted MBC differentiation into antibody secreting cells (ASCs) which secreted anti-PvDBPII IgG. A decrease in cTfh2 cell activation was observed upon the addition of IFN-γ to the co-cultures. Importantly, cTfh2 cells played a role in producing anti-malarial specific antibodies. CONCLUSIONS/SIGNIFICANCE: This study demonstrated that activation of cTfh2 cells, marked by the upregulation of ICOS molecules, was notably observed in subjects who produced high titers of anti-PvDBPII antibodies in response to P. vivax infection. This was also seen in a few subjects who produced high levels of antibody with inhibitory function. The PvDBPII antigen specifically stimulated cTfh2 cell proliferation and activation. Additionally, interactions between cTfh2 cells and MBCs promoted both cTfh2 activation and anti-PvDBPII antibody secretion.