The acquisition of long-lived memory B cell responses to merozoite surface protein-8 in individuals with Plasmodium vivax infection

Abstract

Background: The ability of a malaria antigen to induce effective, long-lasting immune responses is important for the development of a protective malaria vaccine. Plasmodium vivax merozoite surface protein-8 (PvMSP8) has been shown to be immunogenic in natural P. vivax infections and produces both cell-mediated and antibody-mediated immunity. Thus, PvMSP8 has been proposed as a vaccine candidate following fusion with other merozoite antigens in blood stage vaccine design. Here, the long-term responses of antibodies and memory B cells (MBCs) specific to PvMSP8 in individuals were monitored in a longitudinal cohort study.

Methods: Both cross-sectional surveys and cohort studies were utilized to explore the persistence of antibody and MBC responses to PvMSP8. Antibody titers were detected in individuals with acute disease and those who recovered from an infection for 4 years. The dominant peptide epitope of PvMSP8 recognized by naturally acquired antibodies was examined to observe the durability of the post-infection antibody response. PvMSP8-specific MBCs were also in subjects 4 years post-infection using an enzyme-linked immunospot assay.

Results: The prevalence of antibodies to PvMSP8 was high during and after infection. The antibody levels in individual responders were monitored for up to 12 months post-infection and showed that most patients maintained their seropositive response. Interestingly, the anti-PvMSP8 antibody responses stably persisted in some patients who had recovered from an infection for 4 years. Positive PvMSP8-specific MBCs were also detected at 4 years post-infection. However, analysis in these individuals showed no correlation with the presence or titer of circulating antibody.

Conclusion: PvMSP8 had the ability to induce a long-term humoral immune response. The antibodies and MBCs specific for this antigen developed and persisted in subjects who acquired a natural P. vivax infection. Inclusion of the PvMSP8 antigen in blood stage vaccine design should be considered.

Keywords: Memory B cells; Merozoite surface protein 8; Plasmodium vivax.

Fig. 1

Antibody responses to PvMSP8 in the acute disease stage and after infection. A cross-sectional survey was used to explore the seroprevalence of the antibody response in symptomatic patients with a P. vivax infection (n = 40) and after recovery from infection for 3 (n = 35), 9 (n = 26), and 12 (n = 25) months compared to healthy controls (HC, n = 20). The filled circles represent the subjects that were used for the longitudinal analysis. The cut-off value was calculated from the mean plus 2 SDs of the OD for HC. AC, acute phase; HC, healthy controls

Fig. 2

Longitudinal analysis of anti-PvMSP8 responses. The longevity of anti-PvMSP8 responses is shown in a, b for P. vivax-infected patients in the 12-month cohort study (n = 16) and c for subjects in the 4-year cohort study (n = 5). The cut-off value was calculated from the mean plus 2 SDs of the OD for HC

Fig. 3

The durability of antibody responses to predominant PvMSP8 peptides. a–d The frequencies of antibody responses to PvMSP8 peptide Nos. 2, 3, 4 and 5 in patients with an acute P. vivax infection (n = 16) compared to HC (n = 16). e–g Antibody responses to PvMSP8 peptides in seropositive patients, peptide No. 2 (n = 6), peptide No. 3 (n = 7), and peptide No. 5 (n = 5), at acute infection and three follow-up times in the cohort study (AC, 3 months, 9 months and 12 months post-infection). The cut-off value was calculated as the mean plus 2 SDs of the OD for HC

Fig. 4

PvMSP8-specific MBC responses at 4 years post-infection. The numbers of specific MBCs produced in response to a PvMSP8, b tetanus toxoid (TT), and total IgG in PBMCs from subjects that persisted 4 years after the P. vivax infection were determined using an ELISPOT assay (n = 13). The frequencies of MBCs were determined by counting the number of spots that appeared per million cultured PBMCs. Each symbol represents the MBC number for one individual. The line represents the median value