Long synthetic peptides encompassing the Plasmodium falciparum LSA3 are the target of human B and T cells and are potent inducers of B helper, T helper and cytolytic T cell responses in mice.

We synthesized 17 long synthetic peptides (LSP) spanning the whole 200-kDa Plasmodium falciparum liver stage antigen-3 (LSA3), an antigen that induces protection in chimpanzee, and analyzed their immunogenicity in BALB/c mice and their antigenicity in individuals living in a hyper-endemic malaria area. Our findings show that both specific antibodies and T cell proliferation against most LSA3-LSP develop in malaria-exposed adults. All individuals studied had detectable antibodies against a minimum of 6 and a maximum of 15 polypeptides. It is noteworthy that antibody prevalence and titers were as high against non-repeat as repeat regions. Although the extent of T cell reactivity was lower than that observed for B cells, most of the sequences contained at least one T helper epitope, indicating that the majority of LSA3-LSP contain both B and T cell epitopes within the same sequence. Injection of LSA3-LSP with SBSA2 adjuvant in mice, showed strong immunogenicity for most of them, eliciting both T cell responses and specific antibody production. While all the peptides were immunogenic for B cells, different patterns of T cell responses were induced. These peptides were thus classified in three sets according to the levels of the T cell proliferative and of the IFN-gamma-specific responses. Importantly, antibodies and T cells against some of the LSP were able to recognize LSA3 native protein on P. falciparum sporozoites. Additionally, some LSP (44-119, 1026-1095, 1601-1712) also contained epitopes recognized by H-2(d) class I-restricted T cells. These results led to the identification of numerous domains that are highly antigenic and immunogenic within the LSA3 protein, and underline the value of the LSP approach for vaccine development.

Human antibodies against Plasmodium falciparum liver-stage antigen 3 cross-react with Plasmodium yoelii preerythrocytic-stage epitopes and inhibit sporozoite invasion in vitro and in vivo.

The Plasmodium falciparum liver-stage antigen 3 (LSA3), a recently identified preerythrocytic antigen, induces protection against malaria in chimpanzees. Using antibodies from individuals with hyperimmunity to malaria affinity purified on recombinant or synthetic polypeptides of LSA3, we identified four non-cross-reactive B-cell epitopes in Plasmodium yoelii preerythrocytic stages. On sporozoites the P. yoelii protein detected has a molecular mass similar to that of LSA3. T-cell epitopes cross-reacting with P. yoelii were also demonstrated using peripheral blood lymphocytes from LSA3-immunized chimpanzees. In contrast, no cross-reactive epitopes were found in Plasmodium berghei. LSA3-specific human antibodies exerted up to 100% inhibition of in vitro invasion of P. yoelii sporozoites into mouse hepatocytes. This strong in vitro activity was reproduced in vivo by passive transfer of LSA3 antibodies. These results indicate that the homologous epitopes may be biologically functional and suggest that P. yoelii could be used as a model to assess the antisporozoite activity of anti-LSA3 antibodies.

DNA immunization by Plasmodium falciparum liver-stage antigen 3 induces protection against Plasmodium yoelii sporozoite challenge.

DNA-based immunization of mice by Plasmodium falciparum liver-stage antigen 3 (PfLSA3), a novel highly conserved P. falciparum preerythrocytic antigen, was evaluated. Animals developed a dominant Th1 immune response (high gamma interferon T-cell responses and predominance of immunoglobulin G2a) to each of three recombinant proteins spanning the molecule. We have exploited the immunological cross-reactivity of PfLSA3 with its putative homologue on sporozoites of the rodent parasite Plasmodium yoelii, and we show for the first time that responses induced by PfLSA3 in mice significantly protect against a heterologous challenge by P. yoelii sporozoites. These results support a significant effect of DNA-induced immune responses on preerythrocytic stages.

Adjuvant is required when using Env lipopeptide construct to induce HIV type 1-specific neutralizing antibody responses in mice in vivo.

Extensive immunological studies on HIV-1 infection, the causative agent of AIDS in humans, have led to the conclusion that efficient protection against this infection should require early elicitation of neutralizing antibodies as well as cellular immune and particularly cytotoxic T lymphocyte responses. The use of synthetic peptides modified at one end by introduction of a lipidic tail is now well known to be an effective means of eliciting virus-specific cytotoxic T lymphocyte responses in vivo, both in mouse and humans. To ascertain that such a strategy can be used for vaccinal purposes, particularly against HIV-1 infection, it remains to be determined whether these molecules can also act as effective inducers of antibody responses, most of all of the neutralizing type. The present study set out to address this question by using a synthetic HIV-1 ENV lipopeptide construct, previously identified as a potent immunogen for in vivo induction of ENV-specific CTL responses in BALB/c mice. We first showed that V3 peptide-specific antibodies were effectively induced by the lipopeptide construct. However, we provided evidence that the biological activity of these antibodies, i.e., their ability to neutralize HIV-1 infectivity in vitro, was strongly influenced by the immunizing conditions and protocol, in that only those antibodies generated by the use of adjuvanted lipopeptide formulations were effective. Albeit at a slightly lower efficacy than by the intraperitoneal route, neutralizing antibodies could also be induced using the subcutaneous route. With the prospect of a human peptide vaccine in mind, we then studied the properties of different known or possibly clinically relevant adjuvants. We found that alum, the only relevant adjuvant for human use, not only provides inefficient help to the lipopeptide construct in generating neutralizing antibodies, but tends to have deleterious effects on the ability of the construct to induce CTL responses. The only protocol that gave satisfactory results in terms of the magnitude of the neutralizing antibody responses was a mineral oil-based lipopeptide formulation. When induced under those conditions, strong neutralizing activities were still present up to 8 months after the last injection.

Influence of strong CD4 epitope on long-term virus-specific cytotoxic T cell responses induced in vivo with peptides.

Free unmodified peptides are poor immunogens for cytotoxic T lymphocytes (CTL) in mice, unless they are injected with adjuvant. Although it is generally accepted that CD4+ Th cells are essential for CTL priming with peptides, it is not clear how long sequences devoid of any CD4 epitope, or strict CD8 epitopic sequences too short to be presented in a MHC class II-restricted fashion, can generate such responses. We thus have examined the extent to which the immunization protocol affects the need for a CD4 epitope. Since peptides are potentially important for vaccination, we also examined the duration of the CTL responses using a set of peptides that contained a CD4 epitope, or a CD8 epitope, or both epitopes in the same sequence, and compared immunization protocols previously found to induce CTL responses with peptides. The in vivo injection protocol had a marked effect, since the same CD8 sequence could generate a CTL response in a Th-dependent or Th-independent fashion, depending on the protocol used. The T cell help provided by natural CD4-CD8 sequences was inefficient in Th-dependent CTL priming and CTL generation required help from a stronger exogenous CD4 peptide. The peptides could be injected either as a single tandem CD8-CD4 peptide or as a mixture of two separate peptides. Th-independent CTL responses primed in other conditions proved to be as strong as Th-dependent responses, at least when the animals were killed shortly after the last injection. However, only CTL responses generated together with specific Th cells persisted for several months. Moreover, the efficacy of CTL persistence seemed to be correlated with the strength of the CD4 epitope for priming. This has important implications for the design of peptide vaccines.

Comparative efficiency of simple lipopeptide constructs for in vivo induction of virus-specific CTL.

We have previously shown that virus-specific CTL responses can be elicited in vivo by injecting, without adjuvant, 12-40 amino acid-long peptides, modified in C-terminal position by a simple lipidic amino acid. In this paper, we have studied the chemical accessibility, and the ability to induce in mice a CTL response, of a series of lipopeptides derived from the HIV-1 env (312-327) or (302-335) sequences. We showed that a single modification of these peptides by a lipidic amino acid, preferably in C-terminal position, results in the ability to reproducibly induce, without adjuvant, a relevant CTL response. No clear discrimination appeared concerning the nature of the lipidic modification. Our findings indicate that modification of a relatively long peptide by a N epsilon-palmitoyl-L-Lysylamide can be achieved by conventional methods of synthesis and characterization, offering the possibility to develop low-cost synthetic vaccines in models in which the CTL component is of importance.

Long-lasting anti-viral cytotoxic T lymphocytes induced in vivo with chimeric-multirestricted lipopeptides.

Cytotoxic T lymphocytes (CTL) play a major role in protective immunity against viral diseases. However, the antigenic formulations that can be used in vaccinations able to generate virus-specific CTL responses in vivo have yet to be defined. We have previously shown that HIV-1-specific CTL can be elicited in mice by injecting without adjuvant a synthetic peptide of the envelope glycoprotein that has been modified by the addition of a simple lipid tail to the end of the sequence (lipopeptide). The present study set out to address the question of whether such immunogens may be appropriate for preparing a human synthetic vaccine. We first showed that CTL were effectively induced by lipopeptides when given s.c. or i.p. We evidenced that the in vivo induction required stimulation of a concomitant specific T helper cell response, implying the presence of at least one CD4 epitope in the synthetic sequence used. Bearing in mind the particular properties that would be required in a prospective human peptide vaccine, we conceived a strategy in which a virus-specific CTL response could be generated in mice of different haplotypes using a single lipopeptide. We therefore tested a lipopeptide construct that integrated a synthetic sequence having three colinear epitopes of the influenza virus nucleoprotein, each restricted to a different H-2 haplotype. We found that a CTL response could be elicited to all three epitopes of this chimeric multirestricted lipopeptide construct. Finally, we have attempted to estimate the duration of the responses; strong CTL activities were still present up to six months after the last injection. These findings indicate that this approach may be suitable for developing a synthetic vaccine for human use.