Immunity to Malaria in <em>Plasmodium vivax</em> Infection: A Study in Central China

<div><h3>Background</h3><p><em>P. vivax</em> infection is characterised by relapsing fever, indicating reinfection by previously hidden parasites in the host. Relapsed infection can lead to the activation of the memory T cell pool, which may lead to protective immunity. This study aims to characterise immune responses in acute <em>P. vivax</em>-infected patients living in an area of central China characterised by only <em>P. vivax</em> infection.</p> <h3>Methodology/Principal Findings</h3><p>We conducted a cross-sectional immune-phenotypic analysis of adults using the following inclusion criteria: acute <em>P. vivax</em> infection (N = 37), a history of <em>P. vivax</em> infection (N = 17), and no known history of <em>P. vivax</em> infection (N = 21). We also conducted a 2-week longitudinal analysis following acute <em>P. vivax</em> infection, in which PBMC proliferation was measured in response to <em>P. vivax</em> and <em>P. falciparum</em> blood stage lysates. Using flow cytometry, we showed elevated memory T cells in the blood during acute <em>P. vivax</em> infection. The levels of γδ T cells were two-fold higher than those measured in naive controls. This result suggested that in the two populations, memory and γδ T cells promptly responded to <em>P. vivax</em> parasites. Interestingly, <em>P. falciparum</em> antigens stimulated T cells obtained from <em>P. vivax</em>-infected patients during a day 14-convalescence, whereas lymphocytes from the naïve control group responded to a lower degree of convalescence.</p> <h3>Conclusions/Significance</h3><p>Cell-mediated immunity during the convalescent period of the <em>P. vivax</em>-infected hosts was comprised of T cells that were specifically able to recognise <em>P. falciparum</em> antigens. Although the magnitude of the response was only half that measured after stimulation with <em>P. vivax</em> antigens, the matter of cross-antigenic stimulation is of great interest.</p>