The Leishmania infantum histone H3 possesses an extremely divergent N-terminal domain

In silico and in vitro Evaluation of Mimetic Peptides as Potential Antigen Candidates for Prophylaxis of Leishmaniosis

Antigen formulation is the main feature for the success of leishmaniosis diagnosis and vaccination, since the disease is caused by different parasite species that display particularities which determine their pathogenicity and virulence. It is desirable that the antigens are recognized by different antibodies and are immunogenic for almost all Leishmania species. To overcome this problem, we selected six potentially immunogenic peptides derived from Leishmania histones and parasite membrane molecules obtained by phage display or spot synthesis and entrapped in liposome structures. We used these peptides to immunize New Zealand rabbits and determine the immunogenic capacity of the chimeric antigen. The peptides induced the production of antibodies as a humoral immune response against L. braziliensis or L. infantum. Next, to evaluate the innate response to induce cellular activation, macrophages from the peptide mix-immunized rabbits were infected in vitro with L. braziliensis or L. infantum. The peptide mix generated the IFN-γ, IL-12, IL-4 and TGF-β that led to Th1 and Th2 cellular immune responses. Interestingly, this mix of peptides also induced high expression of iNOS. These results suggest that the mix of peptides derived from histone and parasites membrane molecules was able to mimic parasites proteins and induce cytokines important to CD4+ T cell Th1 and Th2 differentiation and effector molecule to control the parasite infection. Finally, this peptide induced an immune balance that is important to prevent immunopathological disorders, inflammatory reactions, and control the parasite infection.

Epitope Selection for Fighting Visceral Leishmaniosis: Not All Peptides Function the Same Way

Visceral leishmaniosis (VL) caused by Leishmania infantum is a disease with an increasing prevalence worldwide. Treatments are expensive, toxic, and ineffective. Therefore, vaccination seems to be a promising approach to control VL. Peptide-based vaccination is a useful method due to its stability, absence of local side effects, and ease of scaling up. In this context, bioinformatics seems to facilitate the use of peptides, as this analysis can predict high binding affinity epitopes to MHC class I and II molecules of different species. We have recently reported the use of HisAK70 DNA immunization in mice to induce a resistant phenotype against L. major, L. infantum, and L. amazonensis infections. In the present study, we used bioinformatics tools to select promising multiepitope peptides (HisDTC and AK) from the polyprotein encoded in the HisAK70 DNA to evaluate their immunogenicity in the murine model of VL by L. infantum. Our results revealed that both multiepitope peptides were able to induce the control of VL in mice. Furthermore, HisDTC was able to induce a better cell-mediated immune response in terms of reduced parasite burden, protective cytokine profile, leishmanicidal enzyme modulation, and specific IgG2a isotype production in immunized mice, before and after infectious challenge. Overall, this study indicates that the HisDTC chimera may be considered a satisfactory tool to control VL because it is able to activate a potent CD4+ and CD8+ T-cell protective immune responses.

DNA aptamers targeting Leishmania infantum H3 protein as potential diagnostic tools

Leishmaniasis is a disease caused by a parasite of the genus Leishmania that affects millions of people worldwide. These parasites are characterized by the presence of a DNA-containing granule, the kinetoplastid, located in the single mitochondrion at the base of the cell's flagellum. Interestingly, these flagellates do not condense chromatin during mitosis, possibly due to the specific molecular features of their histones. Although histones are extremely conserved proteins, kinetoplastid core histone sequences diverge significantly from those of higher eukaryotes. This divergence makes kinetoplastid core histones potential diagnostic and/or therapeutic targets. Aptamers are short single-stranded nucleic acids that are able to recognize target molecules with high affinity and specificity. Their binding capacity is a consequence of the particular three-dimensional structure acquired depending on their sequence. These molecules are currently used for detection, diagnosis and therapeutic purpose. Starting from a previously obtained ssDNA aptamer population against rLiH3 protein we have isolated two individual aptamers, AptLiH3#4 and AptLiH3#10. Next, we have performed ELONA, Western blot and slot blot assays to establish aptamer specificity and affinity for LiH3 histone. In addition, ELONA assays using peptides corresponding to overlapped sequences of LiH3 were made to map the aptamers:LiH3 interaction. Finally, different assays using aptamers were performed in order to evaluate the possibility of using these aptamers as sensing molecule to recognize the endogenous protein LiH3. Our results indicate that both aptamers have high affinity and specificity for the target and are able to detect the endogenous LiH3 histone protein in promastigotes lysates. In silico analysis reveals that these two aptamers have different potential secondary structure among them, however, both of them are able to recognize the same peptide sequences present in the protein. In conclusion, our findings indicate that these aptamers could be used for LiH3 histone detection and, in consequence, as potential biosensing molecules in a diagnostic tool for leishmaniasis.

ChIP-ping the branches of the tree: functional genomics and the evolution of eukaryotic gene regulation

Advances in the methods for detecting protein-DNA interactions have played a key role in determining the directions of research into the mechanisms of transcriptional regulation. The most recent major technological transformation happened a decade ago, with the move from using tiling arrays [chromatin immunoprecipitation (ChIP)-on-Chip] to high-throughput sequencing (ChIP-seq) as a readout for ChIP assays. In addition to the numerous other ways in which it is superior to arrays, by eliminating the need to design and manufacture them, sequencing also opened the door to carrying out comparative analyses of genome-wide transcription factor occupancy across species and studying chromatin biology in previously less accessible model and nonmodel organisms, thus allowing us to understand the evolution and diversity of regulatory mechanisms in unprecedented detail. Here, we review the biological insights obtained from such studies in recent years and discuss anticipated future developments in the field.

Mitigating an undesirable immune response of inherent susceptibility to cutaneous leishmaniosis in a mouse model: the role of the pathoantigenic HISA70 DNA vaccine

Leishmania major is the major cause of cutaneous leishmaniosis (CL) outside of the Americas. In the present study we have cloned six Leishmania genes (H2A, H2B, H3, H4, A2 and HSP70) into the eukaryotic expression vector pCMVβ-m2a, resulting in pCMV-HISA70m2A, which encodes all six pathoantigenic proteins as a single polyprotein. This expression plasmid has been evaluated as a novel vaccine candidate in the BALB/c mouse model of CL. The DNA vaccine shifted the immune response normally induced by L. major infection away from a Th2-specific pathway to one of basal susceptibility. Immunization with pCMV-HISA70m2A dramatically reduced footpad lesions and lymph node parasite burdens relative to infected control mice. Complete absence of visceral parasite burden was observed in all 12 immunized animals but not in any of the 24 control mice. Moreover, vaccinated mice produced large amounts of IFN-γ, IL-17 and NO at 7 weeks post-infection (pi), and they showed lower arginase activity at the site of infection, lower IL-4 production and a weaker humoral immune response than infected control mice. Taken together, these results demonstrate the ability of the HISA70 vaccine to shift the murine immune response to L. major infection away from an undesirable, Th2-specific pathway to a less susceptible-like pathway involving Th1 and Th17 cytokine profiles.

Evaluation of an enzyme-linked immunosorbent assay based on crude Leishmania histone proteins for serodiagnosis of human infantile visceral leishmaniasis

Human visceral leishmaniasis (VL) is routinely diagnosed by detecting IgG that specifically binds to Leishmania antigens. The enzyme-linked immunosorbent assay (ELISA) remains a widely used method. However, the biggest challenge remains the choice of antigen with the highest specificity and sensitivity. This study is aimed at assessing the diagnostic performances of crude Leishmania histone (CLH) protein-based ELISAs in Mediterranean VL patients. The CLH proteins were biochemically purified from promastigote nuclear extracts. Their reactivities were analyzed by Western blotting (WB) using rabbit polyclonal antibodies against Leishmania recombinant histones and sera from VL patients, respectively. Then, the diagnostic potential of CLH proteins was validated by the CLH-based ELISA using 42 infantile VL patients' sera and 70 control subjects. The CLH-based ELISA performance was compared to that of the soluble Leishmania antigen (SLA)- and the recombinant K39 (rK39)-based ELISAs. Analysis of the WB profile with the use of polyclonal antibodies confirmed the histone origin of low molecular mass proteins (12 to 16 kDa). All VL samples tested presented antibodies reacting against different antigen fractions; however, recognition patterns were different depending on the reactivity of each serum. CLH-based ELISA showed an excellent ability to discriminate between VL cases and healthy controls (97.6% sensitivity and 100% specificity). It had a diagnostic performance similar to that of rK39-based ELISA (97.6% sensitivity and 97.1% specificity, P = 0.5) and a better serodiagnosis accuracy than the SLA-based ELISA (85.7% sensitivity and 90% specificity, P < 0.05). Therefore, crude Leishmania histone extract could be a valuable antigen for clinical use.

Histone genes in trypanosomatids

Histone genes in Trypanosomatids are of considerable interest because these flagellates do not condense their chromatin during mitosis. In contrast to higher eukaryotes, histone genes in Trypanosomatids are found on separate chromosomes, and their transcripts are polyadenylated. Sequence similarity of Trypanosomatid core histones with those of higher eukaryotes is found predominantly in the globular region; the N-terminal is highly divergent. Finally, in general, Trypanosomatid histones H1 are of low molecular weight, bearing closest homology to the C-terminal region of the higher eukaryote histones H1. These features constitute interesting targets for a rational approach to the study of these protozoa, as discussed here by Norbel Galanti and colleagues.

Leishmania donovani Ran-GTPase interacts at the nuclear rim with linker histone H1

Ran-GTPase regulates multiple cellular processes such as nucleocytoplasmic transport, mitotic spindle assembly, nuclear envelope assembly, cell-cycle progression and the mitotic checkpoint. The leishmanial Ran protein, in contrast with its mammalian counterpart which is predominately nucleoplasmic, is localized at the nuclear rim. The aim of the present study was to characterize the LdRan (Leishmania donovani Ran) orthologue with an emphasis on the Ran–histone association. LdRan was found to be developmentally regulated, expressed 3-fold less in the amastigote stage. LdRan overexpression caused a growth defect linked to a delayed S-phase progression in promastigotes as for its mammalian counterpart. We report for the first time that Ran interacts with a linker histone, histone H1, in vitro and that the two proteins co-localize at the parasite nuclear rim. Interaction of Ran with core histones H3 and H4, creating in metazoans a chromosomal Ran-GTP gradient important for mitotic spindle assembly, is speculative in Leishmania spp., not only because this parasite undergoes a closed mitosis, but also because the main localization of LdRan is different from that of core histone H3. Interaction of Ran with the leishmanial linker histone H1 (LeishH1) suggests that this association maybe involved in modulation of pathways other than those documented for the metazoan Ran–core histone association.

Cell-cycle-dependent translation of histone mRNAs is the key control point for regulation of histone biosynthesis in Leishmania infantum

The cell-cycle-dependent expression of the four core histones (H2A, H2B, H3 and H4) has been studied in the protozoan parasite Leishmania infantum. For that purpose, the cell cycle was arrested by incubation of promastigotes with the DNA synthesis inhibitor hydroxyurea, which induced an accumulation of cells stalled in G1 phase. Hydroxyurea release resulted in a semi-synchronous entry into the cell cycle, as determined by flow cytometry. The steady-state levels of histone mRNAs in the G1, S and G2/M phases were found to be constant along the cell cycle. However, the levels of histone synthesis increased when parasites enter the S phase, in agreement with previous results showing that histone synthesis in Leishmania is tightly coupled with DNA replication. In addition, we analysed the distribution of histone mRNAs on polyribosomes at different stages of the cell cycle by separation of cytoplasmic RNAs in sucrose gradients. Remarkably, a drastic change in the polysome profiles of histone mRNAs was observed during the progression from G1 to S phase. Thus, in the S phase, histone mRNAs are present in ribosome-bound fractions, but in the G1 phase, the histone transcripts are exclusively found in the ribosome-free fractions. These results support a regulatory model in which the cell-cycle-regulated synthesis of histones in Leishmania is controlled through a reversible interaction between translational repressors and histone mRNAs.

Histone H3 and H3.3 variants in the protozoan pathogens Plasmodium falciparum and Toxoplasma gondii

Histones constitute the fundamental component of chromatin and participate in the regulation of gene expression by virtue of covalent modifications to their N-terminal domains. The discovery that histone-modifying enzymes are targeted by the antiprotozoal agent apicidin has prompted further investigation of gene expression regulation in protozoan parasites; consequently, several chromatin remodeling homologues with unusual features have been isolated. To facilitate investigation of these chromatin remodeling homologues using parasite-specific substrates, we sought to clone and characterize histone H3 from two medically significant pathogens in the phylum Apicomplexa: Plasmodium falciparum (malaria) and Toxoplasma gondii (opportunistic pathogen of immunocompromised individuals). Like most eukaryotic organisms, these parasites each contain at least two histone H3 variants, termed H3 and H3.3. Sequence analysis reveals the Apicomplexan H3 proteins harbor novel and rare features. Expression and purification of recombinant H3 variants will provide species-specific substrate for the analysis of the histone-modifying machinery of these parasites.

Characterization of the immune response to Leishmania infantum recombinant antigens

Leishmaniases have a high prevalence in tropical countries. In order to improve existing diagnostic systems based on total Leishmania proteins, and to identify antigen candidates for vaccine development, an intensive search for the identification of antigens was performed using molecular biology techniques. In this study, the immune response to three L. infantum recombinant antigens was evaluated. Upon stimulation with KMP11, mononuclear cells from leishmaniasis patients produced high levels of IL-10, while a predominant IFN-gamma production could be observed in cultures stimulated with H2A and soluble Leishmania antigen. All the recombinant antigens induced very little IL-5. KMP11 decreased IFN-gamma production by 48% in cultures of peripheral blood mononuclear cells from cutaneous leishmaniasis patients who had been stimulated with soluble Leishmania antigen. Furthermore, antibodies to KMP11 were detected in the sera from all patients with visceral leishmaniasis and in the majority of the sera from patients with cutaneous leishmaniasis or individuals with asymptomatic L. chagasi infection. Thus, KMP11 is recognized by cells and sera of patients with different clinical forms of leishmaniasis, and KMP11, through IL-10 production, proved to be a potent antigen in modulating type 1 immune response.

Expression of a novel Leishmania gene encoding a histone H1-like protein in Leishmania major modulates parasite infectivity in vitro

We describe identification and characterization of a novel two-copy gene of the parasitic protozoan Leishmania that encodes a nuclear protein designated LNP18. This protein is highly conserved in the genus Leishmania, and it is developmentally regulated. It is an alanine- and lysine-rich protein with potential bipartite nuclear targeting sequence sites. LNP18 shows sequence similarity to H1 histones of trypanosomatids and of higher eukaryotes and in particular with histone H1 of Leishmania major. The nuclear localization of LNP18 was determined by indirect immunofluorescence and Western blot analysis of isolated nuclei by using antibodies raised against the recombinant protein as probes. The antibodies recognized predominantly a 18-kDa band or a 18-kDa-16-kDa doublet. Photochemical cross-linking of intact parasites followed by Western blot analysis provided evidence that LNP18 is indeed a DNA-binding protein. Generation of transfectants overexpressing LNP18 allowed us to determine the role of this protein in Leishmania infection of macrophages in vitro. These studies revealed that transfectants overexpressing LNP18 are significantly less infective than transfectants with the vector alone and suggested that the level of LNP18 expression modulates Leishmania infectivity, as assessed in vitro.

Genes coding structural proteins in the Leishmania braziliensis complex

Acidic ribosomal P1 and P2b proteins, referred to as P proteins, and histone H3 are reported for first time in the Leishmania braziliensis complex. Deoxyribonucleic acid analysis and multiple sequence alignment suggest that both P proteins may maintain their structural function in the ribosomal stalk, in spite of the high rate of mutations detected. The deduced amino acid sequence of protein P1 showed 51% identity with Trypanosoma cruzi protein P1 and protein P2b showed 61% identity with T. cruzi protein P2b. Another conserved protein, L. (Viannia) braziliensis histone H3, showed 82% and 70% identity with histone H3 of L. (Leishmania) infantum and T. cruzi, respectively. The N-terminal end of this histone is divergent in comparison with the consensus eukaryotic sequence. Their predicted tridimensional structure was designed.

Evolutionarily conserved proteins as prominent immunogens during Leishmania infections

Many Leishmania antigens have been identified as members of conserved protein families, such as the acidic ribosomal proteins, the histones and the heat-shock proteins; despite this, they elicit specific immune responses. Furthermore, homologues of many of these antigens are immune targets in other infectious diseases and systemic autoimmune diseases. Here, Jose Mar a Requena, Carlos Alonso and Manuel Soto review this class of widely distributed antigens, which they call 'panantigens'. They also propose a model to explain the prominent immunogenicity of these antigens during Leishmania infection, on the basis of the fact that many panantigens are constituents of multicomponent complexes in the cell. The elucidation of the pathways by which Leishmania antigens are processed and presented to effector cells from the host immune system will shed light on the immunopathology of leishmaniasis and help in the development of protective immunotherapies.

The stability and maturation of the H2A histone mRNAs from Trypanosoma cruzi are implicated in their post-transcriptional regulation

We have recently described that the Trypanosoma cruzi histone H2A genes are actively transcribed as two sized classes of polyadenylated transcripts and that they differ in the 3'-UTRs due to the insertion of a partial SINE sequence in the 3'-end of some of H2A gene units. The expression of the H2A genes in the non-replicative trypomastigote forms is very low, whereas in the replicative forms, there is significant and constitutive transcription of the H2A genes probably regulated in a posttranscriptional way and associated to DNA replication. The data presented in this paper reveal that in epimastigotes, the steady-state levels of the H2A mRNAs are determined by controlling the stability of the messengers in the cytoplasm, most likely mediated by a nuclease attack. The data also indicate that there must be an additional control, associated to the parasite growth phase, which may act at the maturation step of the transcripts. The data suggest, moreover; that the cytoplasmic level of the H2A protein might be involved in the regulation of its own synthesis by controlling translation of existing messengers.

Nuclear and genome organization of Trypanosoma brucei

In this article, Klaus Ersfeld, Sara Melville and Keith Gull review current understanding of the structural organization of the nucleus of Trypanosoma brucei, and summarize recent data pertinent to the organization of its genome. Until recently, the cell biology of the trypanosome nucleus and issues of DNA organization and gene expression have often been treated as separate themes. However, recent work emphasizes the need for a more holistic approach to understanding these aspects of the biology of this parasite.

Control mechanisms of the H2A genes expression in Trypanosoma cruzi

In a previous report we have described that the T. cruzi histone H2A gene is encoded in two independent gene clusters located in a single chromosome. In the present paper we show that both gene cluster are actively transcribed as two sized classes of polyadenylated mRNAs demonstrating, moreover, the existence of alternative splicing sites and microheterogeneities at the polyadenylation site. We also describe that while the expression of the H2A genes in the non replicative trypomastigote forms is only residual, in the replicative forms there is constitutive transcription of these genes and that the transcription is not associated to DNA replication. The data show, moreover, that in the replicative forms the steady state levels of the H2A mRNAs are controlled at a post-transcriptional level which is associated to DNA replication.

Molecular cloning and analysis of expression of the Leishmania infantum histone H4 genes

In the present work, we describe the sequence, organization and expression of histone H4 genes in the protozoan parasite Leishmania infantum. The predicted L. infantum histone H4 is a polypeptide of 100 amino acids with a molecular mass of 11.5 kDa. Comparison of the amino acid sequence of Leishmania histone H4 with the rest of histone H4 sequences indicates that this is the most divergent sequence reported to date. The genomic distribution analysis of histone H4 genes indicates that there must be up to seven gene copies. A single size-class histone H4 mRNA of 0.6 kb was detected, whose level dramatically decreases from logarithmic to stationary phase. However, the Leishmania histone H4 mRNAs do not decrease in abundance following treatment with inhibitors of DNA synthesis, suggesting a regulation by a replication-independent mechanism.

An unusual histone H3 specific for early macronuclear development in Euplotes crassus

Characterization of the histone H3 genes of the ciliated protozoan Euplotes crassus indicates that one gene functions only during the sexual phase of the life cycle. Maximum expression of this gene, as judged by transcript accumulation, correlates with DNA replications leading to polytenization of the micronuclear chromosomes before massive DNA elimination, which produces a transcriptionally active macronucleus. Transcripts of the other gene accumulate primarily during vegetative growth and in the sexual phase of the life cycle during replication phases not related to polytenization. Although both histone H3 genes encode proteins that are fairly divergent in sequence at the amino terminus, the meiotic/polytene-specific histone H3 contains two insertions in the amino terminus that increase the size of the protein by 15 amino acids. Analysis of micrococcal nuclease digests of chromatin using hybridization probes specific for micronuclear vs. macronuclear sequences indicates that a change in nucleosomal spacing correlates with the maximal expression of the meiotic/polytene-specific histone H3 gene. Thus, we surmise that this unusual histone H3 may play a key role in targeting DNA sequences for either transcriptional activation and retention in the macronucleus or heterochromatization and elimination.

The sequence and organization of the core histone H3 and H4 genes in the early branching amitochondriate protist Trichomonas vaginalis

Among the unicellular protists, several of which are parasitic, some of the most divergent eukaryotic species are found. The evolutionary distances between protists are so large that even slowly evolving proteins like histones are strongly divergent. In this study we isolated cDNA and genomic histone H3 and H4 clones from Trichomonas vaginalis. Two histone H3 and three histone H4 genes were detected on three genomic clones with one complete H3 and two complete H4 sequences. H3 and H4 genes were divergently transcribed with very short intergenic regions of only 194 bp, which contained T. vaginalis-specific as well as histone-specific putative promoter elements. Southern blot analysis showed that there may be several more histone gene pairs. The two complete histone H4 genes were different on the nucleotide level but encoded the same amino acid sequence. Comparison of the amino acid sequences of the T. vaginalis H3 and H4 histones with sequences from animals, fungi, and plants as well as other protists revealed a significant divergence not only from the sequences in multicellular organisms but especially from the sequences in other protists like Entamoeba histolytica, Trypanosoma cruzi, and Leishmania infantum.

Organization, transcription and regulation of the Leishmania infantum histone H3 genes

The genomic organization and transcription of the genes encoding the histone H3 of the protozoan parasite Leishmania infantum have been studied. It was found that there are multiple copies of the histone H3 genes distributed in chromosomal bands XIX and XIV. The nucleotide sequence of two of the L. infantum H3 genes, each one located in a different chromosome, is reported. Although the nucleotide sequence of the coding region of both genes is identical, the sequence of the 3' untranslated region is highly divergent. It was found also that there exist two different size classes of histone H3 transcripts, each one derived from a different gene, and that they are polyadenylated. The steady-state level of the transcripts dramatically decreases when the parasites enter the stationary phase of growth, suggesting a mode of regulation which is linked to the proliferation status of the cell. Unlike the replication-dependent histones, the L. infantum H3 mRNA levels do not decrease after treatment with DNA synthesis inhibitors. A comparative analysis of the sensitivity of the histone mRNA levels to DNA inhibition in the parasites L. infantum and Trypanosoma cruzi revealed the existence of different control mechanisms in histone expression in these two phylogenetically related protozoan parasites.

Common features of analogous replacement histone H3 genes in animals and plants

Phylogenetic analysis of histone H3 protein sequences demonstrates the independent origin of the replacement histone H3 genes in animals and in plants. Multiple introns in the replacement histone H3 genes of animals in a pattern distinct from that in plant replacement H3 genes supports this conclusion. It is suggested that replacement H3 genes arose at the same time that, independently, multicellular forms of animals and of plants evolved. Judged by the degree of invariant and functionally constrained amino acid positions, histones H3 and H4, which form together the tetramer kernel of the nucleosome, have co-evolved with equal rates of sequence divergence. Residues 31 and 87 in histone H3 are the only residues that consistently changed across each gene duplication event that created functional replacement histone H3 variant forms. Once changed, these residues have remained invariant across divergent speciation. This suggests that they are required to allow replacement histone H3 to participate in the assembly of nucleosomes in non-S-phase cells. The abundant occurrence of polypyrimidine sequences in the introns of all replacement H3 genes, and the replacement of an intron by a polypyrimidine motif upstream of the alfalfa replacement H3 gene, suggests a function. It is speculated that they may contribute to the characteristic cell-cycle-independent pattern of replacement histone H3 genes by binding nucleosome-excluding proteins.

Human T-cell activation by 14- and 18-kilodalton nuclear proteins of Leishmania infantum

Leishmanial antigens which stimulate T lymphocytes from primed individuals may be candidates for a vaccine. We recently found a significant concordance between the humoral response specific for two proteins from Leishmania infantum promastigotes, p14 and p18, and a positive leishmanin delayed-type hypersensitivity reaction, testifying to the occurrence of cell-mediated immunity. In this communication, we describe a partial characterization of these antigens and an in vitro analysis of their capacity to activate primed human T cells. We showed, by immunofluorescent staining and through analysis of subcellular fractions by Western immunoblotting, that in stationary-phase promastigotes, p14 and p18 were located only in the parasite nuclei; in the middle of the log phase, a transitory and only weak expression outside the nucleus was detected. We then showed that p14 and p18 antigens shared a common epitope(s). Finally, we analyzed the in vitro proliferation and interleukin-2 production induced by leishmanial proteins in human peripheral blood mononuclear cells from sensitized subjects. We showed that in some individuals who have been exposed to L. infantum the specific response to the whole lysate was mostly due to the nuclear antigens. We demonstrated directly the capacity of nitrocellulose-bound p14 and p18 to activate in vitro all of the tested primed peripheral blood mononuclear cells, which contrasted with a lack of stimulatory activity of other membrane-bound leishmanial proteins. Taken together, our results suggest that an antigenic determinant(s) dominant for some individuals might exist on both antigens.