Epitope analysis of human T-cell response to MSP-1 of Plasmodium falciparum in malaria-nonexposed individuals

Major Histocompatibility Complex and Malaria: Focus on Plasmodium vivax Infection

The importance of host and parasite genetic factors in malaria resistance or susceptibility has been investigated since the middle of the last century. Nowadays, of all diseases that affect man, malaria still plays one of the highest levels of selective pressure on human genome. Susceptibility to malaria depends on exposure profile, epidemiological characteristics, and several components of the innate and adaptive immune system that influences the quality of the immune response generated during the Plasmodium lifecycle in the vertebrate host. But it is well known that the parasite's enormous capacity of genetic variation in conjunction with the host genetics polymorphism is also associated with a wide spectrum of susceptibility degrees to complicated or severe forms of the disease. In this scenario, variations in genes of the major histocompatibility complex (MHC) associated with host resistance or susceptibility to malaria have been identified and used as markers in host-pathogen interaction studies, mainly those evaluating the impact on the immune response, acquisition of resistance, or increased susceptibility to infection or vulnerability to disease. However, due to the intense selective pressure, number of cases, and mortality rates, the majority of the reported associations reported concerned Plasmodium falciparum malaria. Studies on the MHC polymorphism and its association with Plasmodium vivax, which is the most widespread Plasmodium and the most prevalent species outside the African continent, are less frequent but equally important. Despite punctual contributions, there are accumulated evidences of human genetic control in P. vivax infection and disease. Herein, we review the current knowledge in the field of MHC and derived molecules (HLA Class I, Class II, TNF-α, LTA, BAT1, and CTL4) regarding P. vivax malaria. We discuss particularly the results of P. vivax studies on HLA class I and II polymorphisms in relation to host susceptibility, naturally acquired immune response against specific antigens and the implication of this knowledge to overcome the parasite immune evasion. Finally, the potential impact of such polymorphisms on the development of vaccine candidate antigens against P. vivax will be studied.

Promiscuous T-cell epitopes of Plasmodium merozoite surface protein 9 (PvMSP9) induces IFN-gamma and IL-4 responses in individuals naturally exposed to malaria in the Brazilian Amazon

Plasmodium vivax merozoite surface protein (PvMSP9) stimulates both cellular and humoral immune responses in individuals who are naturally infected by this parasite species. To identify immunodominant human T-cell epitopes in PvMSP9, we used the MHC class II binding peptide prediction algorithm ProPred. Eleven synthetic peptides representing predicted putative promiscuous T-cell epitopes were tested in IFN-gamma and IL-4 ELISPOT assays using peripheral blood mononuclear cells (PBMC) derived from 142 individuals from Rondonia State, Brazil who had been naturally exposed to P. vivax infections. To determine whether the predicted epitopes are preferentially recognized in the context of multiple alleles, MHC Class II typing of the cohort was also performed. Five synthetic peptides elicited robust cellular responses, and the overall frequencies of IFN-gamma and IL-4 responders to at least one of the promiscuous peptides were 62% and 46%, respectively. The frequencies of IFN-gamma and IL-4 responders to each peptide were not associated with a particular HLA-DRB1 allelic group since most of the peptides induced a response in individuals of 12 out of 13 studied allelic groups. The prediction of promiscuous epitopes using ProPred led to the identification of immunodominant epitopes recognized by PBMC from a significant proportion of a genetically heterogeneous population exposed to malaria infections. The combination of several such T-cell epitopes in a vaccine construct may increase the frequency of responders and the overall efficacy of subunit vaccines in genetically distinct populations.

CD4 T cells from malaria-nonexposed individuals respond to the CD36-Binding Domain of Plasmodium falciparum erythrocyte membrane protein-1 via an MHC class II-TCR-independent pathway

We have studied the human CD4 T cell response to a functionally conserved domain of Plasmodium falciparum erythrocyte membrane protein-1, cysteine interdomain region-1alpha (CIDR-1alpha). Responses to CIDR-1alpha were striking in that both exposed and nonexposed donors responded. The IFN-gamma response to CIDR-1alpha in the nonexposed donors was partially independent of TCR engagement of MHC class II and peptide. Contrastingly, CD4 T cell and IFN-gamma responses in malaria-exposed donors were MHC class II restricted, suggesting that the CD4 T cell response to CIDR-1alpha in malaria semi-immune adults also has a TCR-mediated component, which may represent a memory response. Dendritic cells isolated from human peripheral blood were activated by CIDR-1alpha to produce IL-12, IL-10, and IL-18. IL-12 was detectable only between 6 and 12 h of culture, whereas the IL-10 continued to increase throughout the 24-h time course. These data strengthen previous observations that P. falciparum interacts directly with human dendritic cells, and suggests that the interaction between CIDR-1alpha and the host cell may be responsible for regulation of the CD4 T cell and cytokine responses to P. falciparum-infected erythrocytes reported previously.

HLA-DRB1*1501 risk association in multiple sclerosis may not be related to presentation of myelin epitopes

Susceptibility to multiple sclerosis (MS) is associated genetically with human leucocyte antigen (HLA) class II alleles, including DRB1*1501, DRB5*0101, and DQB1*0602, and it is possible that these alleles contribute to MS through an enhanced ability to present encephalitogenic myelin peptides to pathogenic T cells. HLA-DRB1*1502, which contains glycine instead of valine at position 86 of the P1 peptide-binding pocket, is apparently not genetically associated with MS. To identify possible differences between these alleles in their antigen-presenting function, we determined if T-cell responses to known DRB1*1501-restricted myelin peptides might be diminished or absent in transgenic (Tg) DRB1*1502-expressing mice. We found that Tg DRB1*1502 mice had moderate to strong T-cell responses to several myelin peptides with favorable DRB1*1501 binding motifs, notably myelin oligodendrocyte glycoprotein (MOG)-35-55 (which was also encephalitogenic), proteolipid protein (PLP)-95-116, and MOG-194-208, as well as other PLP and MOG peptides. These peptides, with the exception of MOG-194-208, were also immunogenic in healthy human donors expressing either DRB1*1502 or DRB1*1501. In contrast, the DRB1*1502 mice had weak or absent responses to peptides with unfavorable DRB1*1501 binding motifs. Overall, none of the DRB1*1501-restricted myelin peptides tested selectively lacked immunogenicity in association with DRB1*1502. These results indicate that the difference in risk association with MS of DRB1*1501 versus DRB1*1502 is not due to a lack of antigen presentation by DRB1*1502, at least for this set of myelin peptides, and suggest that other mechanisms involving DRB1*1501 may account for increased susceptibility to MS.

A simple screening method for detecting bindings between oligopeptides and HLA-DR molecules on filter papers: possible application for mapping of putative helper T-cell epitopes on MSP1 of Plasmodium falciparum

Binding capacities of synthetic peptides to HLA-DR molecules were tested on filter papers to identify putative helper T-cell epitopes on a malarial protein. The antigen tested was the merozoite surface glycoprotein 1 (MSP1) of Plasmodium falciparum, a vaccine candidate targeting the asexual erythrocytic stage. Bindings between synthetic oligopeptides and HLA-DR molecules were tested. Such bindings were not non-specific, and a known helper T-cell epitope peptide showed positive binding to the restricting HLA-DR molecule. By using this screening system, we observed the unequal distribution of HLA-DR-binding peptides in 10 out of 17 MSP1 blocks tested. Block #6 of MSP1 seemed to show the highest frequency in the positive binding; on the other hand, blocks #1 and #17, both of which were thought to be vaccine candidate regions, contained fewer HLA-DR binding peptides. This was not inconsistent with the results that block #17 was less stimulatory to peripheral T cells than block #6. The peptides with positive binding to HLA-DR showed actual epitope activities when we tested peptide-driven proliferation of human bulk T-cell lines, and association between the two parameters was statistically significant (P<0.001). For more detailed information for vaccine development, peptides with both IgG- and HLA-DR binding activities were mapped in block #17 of MSP1. Together with these results, we demonstrate that our simple screening system seems to provide essential information for vaccine development through uncovering locations of putative epitopes for human helper T cells.

Characterization of conserved T- and B-cell epitopes in Plasmodium falciparum major merozoite surface protein 1

Vaccines for P. falciparum will need to contain both T- and B-cell epitopes. Conserved epitopes are the most desirable, but they are often poorly immunogenic. The major merozoite surface protein 1 (MSP-1) is currently a leading vaccine candidate antigen. In this study, six peptides from conserved or partly conserved regions of MSP-1 were evaluated for immunogenicity in B10 congenic mice. Following immunization with the peptides, murine T cells were tested for the ability to proliferate in vitro and antibody responses to MSP-1 were evaluated in vivo. The results showed that one highly conserved sequence (MSP-1#1, VTHESYQELVKKLEALEDAV; located at amino acid positions 20 to 39) and one partly conserved sequence (MSP-1#23, GLFHKEKMILNEEEITTKGA; located at positions 44 to 63) contained both T- and B-cell epitopes. Immunization of mice with these peptides resulted in T-cell proliferation and enhanced production of antibody to MSP-1 upon exposure to merozoites. MSP-1#1 stimulated T-cell responses in three of the six strains of mice evaluated, whereas MSP-1#23 was immunogenic in only one strain. Immunization with the other four peptides resulted in T-cell responses to the peptides, but none of the resulting peptide-specific T cells recognized native MSP-1. These results demonstrate that two sequences located in the N terminus of MSP-1 can induce T- and B-cell responses following immunization in a murine model. Clearly, these sequences merit further consideration for inclusion in a vaccine for malaria.