Mapping of specific and promiscuous HLA-DR-restricted T-cell epitopes on the Plasmodium falciparum 27-kilodalton sexual stage-specific antigen

Phagocytosis and activation of bone marrow-derived macrophages by Plasmodium falciparum gametocytes

Background

The innate immune response against various life cycle stages of the malaria parasite plays an important role in protection against the disease and regulation of its severity. Phagocytosis of asexual erythrocytic stages is well documented, but little and contrasting results are available about phagocytic clearance of sexual stages, the gametocytes, which are responsible for the transmission of the parasites from humans to mosquitoes. Similarly, activation of host macrophages by gametocytes has not yet been carefully addressed.

Methods

Phagocytosis of early or late Plasmodium falciparum gametocytes was evaluated through methanol fixed cytospin preparations of immortalized mouse C57Bl/6 bone marrow-derived macrophages treated for 2 h with P. falciparum and stained with Giemsa, and it was confirmed through a standardized bioluminescent method using the transgenic P. falciparum 3D7elo1-pfs16-CBG99 strain. Activation was evaluated by measuring nitric oxide or cytokine levels in the supernatants of immortalized mouse C57Bl/6 bone marrow-derived macrophages treated with early or late gametocytes.

Results

The results showed that murine bone marrow-derived macrophages can phagocytose both early and late gametocytes, but only the latter were able to induce the production of inflammatory mediators, specifically nitric oxide and the cytokines tumour necrosis factor and macrophage inflammatory protein 2.

Conclusions

These results support the hypothesis that developing gametocytes interact in different ways with innate immune cells of the host. Moreover, the present study proposes that early and late gametocytes act differently as targets for innate immune responses.

Human Leukocyte Antigen (HLA)-DRB1*15:01 and HLA-DRB5*01:01 Present Complementary Peptide Repertoires

Human leukocyte antigen (HLA)-DR15 is a haplotype associated with multiple sclerosis. It contains the two DRB* genes DRB1*1501 (DR2b) and DRB5*0101 (DR2a). The reported anchor motif of the corresponding HLA-DR molecules was determined in 1994 based on a small number of peptide ligands and binding assays. DR2a could display a set of peptides complementary to that presented by DR2b or, alternatively, a similar peptide repertoire but recognized in a different manner by T cells. It is known that DR2a and DR2b share some peptide ligands, although the degree of similarity of their associated peptidomes remains unclear. In addition, the contribution of each molecule to the global peptide repertoire presented by the HLA-DR15 haplotype has not been evaluated. We used mass spectrometry to analyze the peptide pools bound to DR2a and DR2b, identifying 169 and 555 unique peptide ligands of DR2a and DR2b, respectively. The analysis of these sets of peptides allowed the refinement of the corresponding binding motifs revealing novel anchor residues that had been overlooked in previous analyses. Moreover, the number of shared ligands between both molecules was low, indicating that DR2a and DR2b present complementary peptide repertoires to T cells. Finally, our analysis suggests that, quantitatively, both molecules contribute to the peptide repertoire presented by cells expressing the HLA-DR15 haplotype.

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.

Molecular characterization and identification of Th1 epitopes of a Schistosoma japonicum protein similar to prosaposin

The tegument of schistosomula contains T cell antigens that might simulate the protective mechanisms of the radiation-attenuated vaccine in a mouse model of schistosomiasis. Immune mechanisms mediated by the CD4+ Th1 response are important in the RAV model. To rapidly identify Th1 epitopes in molecules from the Schistosoma japonicum schistosomula tegument, this study analyzed S. japonicum proteomics data. Preliminary experiments identified a protein similar to prosaposin (SjPSAP) from the tegument of schistosomula. We confirmed that SjPSAP was present in the tegument of the parasite using an indirect immunofluorescence assay. We then identified Th cell epitopes in SjPSAP using in silico prediction combined with experimental validation. From the SjPSAP sequence, we used several algorithms to predict 11 promiscuous Th cell epitopes that might bind to both murine and human MHC class II molecules. To validate the in silico predictions, proliferation and cytokine production profiles of spleen lymphocytes from BALB/c mice immunized with the 11 predicted peptides were measured in vitro using a modified methyl thiazolyl tetrazolium assay and flow cytometry. The results showed that 4 of the 11 predicted peptides induced a recall CD4+ Th1 response in vitro. We measured direct binding of the four peptides predicted to induce a response to antigen-presenting cells from BALB/c mice using a fluorometric method and found that the peptides bound to both I-Ad and I-Ed mouse molecules. These results demonstrated that potentially protective Th1-type epitopes in SjPSAP molecules could be identified rapidly by combining in silico prediction with experimental validation. This strategy could be a fast method for identifying Th1 epitopes in a schistosoma antigen with features such as large size or poor expression of recombinant antigens.

mRNA PCR-based epitope chase method

The identification of specific viral and tumor antigen epitopes recognized by CD4(+) or CD8(+) T lymphocytes remains a challenge. Unfortunately, epitope mapping methods are generally costly and time-consuming. This chapter details a polymerase chain reaction (PCR)-based mRNA epitope identification method called mPEC, which is designed to rapidly and precisely identify relevant T cell epitopes recognized by previously isolated CD8(+) or CD4(+) T lymphocytes.This method is based on the use of mRNA fragments synthesized from PCR-amplified cDNA with a variety of 3'end iterative deletions. mRNA fragments are electroporated into autologous antigen-presenting cells to map the epitope in a given protein antigen. Considering mRNA's sensitivity to degradation, we also insert a control define epitope at the mRNA's 3'end to control for electroporated mRNA's integrity and capacity to be translated.

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.

Flexibility in MHC and TCR Recognition: Degenerate Specificity at the T Cell Level in the Recognition of Promiscuous Th Epitopes Exhibiting No Primary Sequence Homology

Recognition of peptide Ags by T cells through the TCR can be highly specific. In this report we show the degeneracy of Ag recognition at both MHC and TCR levels. We present evidence that unrelated promiscuous Th cell epitopes from various protein sources exhibit sufficient structural homology, despite minimal structural identity, to elicit cross-reactive proliferative responses at the bulk T cell level. This epitopic mimicry was also observed when peptide (CS.T3(378-395) and TT(830-844))-specific CD4+ T cell lines and T cell hybridoma clones were used in proliferation and Ag presentation assays. A scrambled CS.T3(378-395) peptide did not show any proliferation, indicating that the specificity of the cross-reactive responses may be linked with the primary structure of the peptides. Blocking of CS.T3(378-395)-specific CD4+ T cell proliferation by anti-MHC class II mAb showed that recognition of promiscuous T cell epitopes is largely in association with MHC class II molecules. These findings suggest that promiscuous Th epitopes may be useful in designing peptide-based vaccine constructs. At the same time these results show that at the T cell level there may be a great deal of immunological cross-reactivity between heterologous pathogens, and because of this the host's response to a pathogen may be modified by its previous experience with other unrelated pathogens.

Distinct human T cell repertoires mediate immediate and delayed-type hypersensitivity to the Trichophyton antigen, Tri r 2

The 29-kDa subtilase homologue, Tri r 2, derived from the dermatophyte fungus Trichophyton rubrum, exhibits unique immunologic characteristics in its ability to elicit immediate (IH) and delayed-type (DTH) hypersensitivity skin tests in different individuals. Thus, Tri r 2 provides a model for comparing the T cell repertoire in subjects with distinct immune responses to a single Ag. Recombinant Tri r 2 produced as a GST fusion protein in Escherichia coli stimulated strong in vitro lymphoproliferative responses in 10 IH and 10 DTH responders. Patterns of T cell epitope recognition were compared between skin test groups using 28 overlapping peptides (each in 12 replicate wells) derived from Tri r 2 to stimulate T lymphocyte proliferation in vitro. Peptide 5 (P5; aa 41-60) induced the strongest response in DTH subjects and showed the largest difference between DTH and IH responders in proliferation (mean standardized index, 2.22 and 0.82, respectively; p = 0.0047) and number of positive wells (81 vs 12). Responses to P5 were associated with diverse HLA haplotypes. These results showed that P5 contains an immunodominant epitope specifically associated with DTH and that this peptide is recognized in a permissive manner. Cross-validated linear discriminant analysis using T cell proliferative responses to two regions of Tri r 2 (aa 51-90 and 231-270) gave a 95% predictive accuracy for classification of subjects into IH or DTH groups. We conclude that different immune responses to Trichophyton are mediated by distinct T cell repertoires between individuals with IH and DTH reactions to Tri r 2.

Assessing the binding of four Plasmodium falciparum T helper cell epitopes to HLA-DQ and induction of T-cell responses in HLA-DQ transgenic mice

A subunit vaccine for Plasmodium falciparum malaria will need to contain well-defined T helper cell epitopes that induce protective immune responses to the parasite. One major barrier to the use of subunit vaccines is the requirement for T helper cell epitopes to be presented by the HLA class II molecules that are present in the population being vaccinated. Since the majority of malaria studies have focused on HLA-DR, little information on the role of HLA-DQ in the binding and immune response to malarial epitopes is available. This study used an in vitro peptide-binding assay to predict the extent of HLA-DQ binding of four conserved T helper cell epitopes identified from asexual-stage malaria vaccine candidate antigens. Epstein-Barr virus (EBV)-transformed human B-cell lines expressing 14 different DQ molecules (DQ2.1, -2.2, -4.1, -4.2, -5.1 to -5.3, -6.1, -6.2, -6.4, -7.1, -7.3, -8, and -9) representing all broad serological specificities, including common DQ molecules present in populations in areas where malaria is endemic, were used in the binding assay. Moreover, an HLA-DQ transgenic mouse model was employed to evaluate the correlation between the in vitro DQ binding of the peptides and the generation of in vivo immune responses following peptide immunization. This study identified two broad DQ-binding peptides, ABRA#14 and SERA#9. ABRA#14 also induced T-cell proliferation and Th1-associated cytokine production in DQ8(+) transgenic mice. The combination of peptide binding to EBV-transformed cell lines and DQ transgenic mice provides a method for identifying additional T-cell epitopes for inclusion in a vaccine.

Multispecific CD4+ T cell response to a single 12-mer epitope of the immunodominant heat-shock protein 60 of Yersinia enterocolitica in Yersinia-triggered reactive arthritis: overlap with the B27-restricted CD8 epitope, functional properties, and epitope presentation by multiple DR alleles

Yersinia heat-shock protein 60 (Ye-hsp60) has recently been found to be a dominant CD4 and CD8 T cell Ag in Yersinia-triggered reactive arthritis. The nature of this response with respect to the epitopes recognized and functional characteristics of the T cells is largely unknown. CD4+ T cell clones specific for Ye-hsp60 were raised from synovial fluid mononuclear cells from a patient with Yersinia-triggered reactive arthritis. and their specificity was determined using three recombinant Ye-hsp60 fragments, overlapping 18-mer synthetic peptides as well as truncated peptides. Functional characteristics were assessed by cytokine secretion analysis in culture supernatants after specific antigenic stimulation. Amino acid positions relevant for T cell activation were detected by single alanine substitutions within the epitopes. Fragment II comprising amino acid sequence 182-371 was recognized by the majority of clones. All these clones were specific for peptide 319-342. Th1 clones and IL-10-secreting clones occurred in parallel, sometimes with the same fine specificity. The 12-mer core epitope 322-333 is a degenerate MHC binder and is presented to some T cell clones in a "promiscuous" manner. This epitope is almost identical with a B27-restricted CTL epitope of Ye-hsp60. Cross-reactivity of Ye-hsp60-specific T cell clones with self-hsp60 was not observed. In conclusion, an interesting Ye-hsp60 T cell epitope has been identified and characterized. It remains to be determined whether this epitope is also relevant in other reactive arthritis patients.