Ex Vivo Cytokine and Memory T Cell Responses to the 42-kDa Fragment of Plasmodium falciparum Merozoite Surface Protein-1 in Vaccinated Volunteers

Abstract

A number of blood-stage malaria Ags are under development as vaccine candidates, but knowledge of the cellular responses to these vaccines in humans is limited. We evaluated the nature and specificity of cellular responses in healthy American volunteers vaccinated with a portion of the major merozoite surface protein-1 (MSP1) of Plasmodium falciparum, MSP142, formulated on Alhydrogel. Volunteers were vaccinated three times with 80 μg of either MSP142-FVO/Alhydrogel or MSP142-3D7/Alhydrogel. Cells collected 2 wk after the third vaccination produced Th1 cytokines, including IFN-γ and IL-2 following Ag stimulation, and greater levels of the Th2 cytokines IL-5 and IL-13; the anti-inflammatory cytokine IL-10 and the molecule CD25 (IL-2Rα) were also detected. The volunteers were evaluated for the MSP142–FVO or MSP142-3D7 specificity of their T cell responses. Comparison of their responses to homologous and heterologous Ags showed ex vivo IFN-γ and IL-5 levels that were significantly higher to homologous rather than to heterologous Ags. The epitopes involved in this stimulation were shown to be present in the dimorphic MSP133 portion of the larger MSP142-3D7 polypeptide, and indirect experiment suggests the same for the MSP142–FVO polypeptide. This contrasts with B cell responses, which were primarily directed to the conserved MSP119 portion. Furthermore, we explored the maturation of memory T cells and found that 46% of vaccinees showed specific memory T cells defined as CD4+CD45RO+CD40L+ after long-term in vitro culture. The identification of human-specific CD4+ memory T cells provides the foundation for future studies of these cells both after vaccination and in field studies.

Assessment of Immune Interference, Antagonism, and Diversion following Human Immunization with Biallelic Blood-Stage Malaria Viral-Vectored Vaccines and Controlled Malaria Infection

“…However, the data clearly establish that strong and detectable IFN-g T cell responses can be primed and boosted by immunization against both component alleles, and this was independently observed in two clinical trials of the same vaccines. The observation of strong cellular responses against the MSP1 33 region (in comparison with MSP1 19 ) is in good agreement with many previous studies that have assessed T cell recognition within MSP1 42 in both animal models (20,45) and humans (46)(47)(48)(49). Note also that future studies could further assess such phenomena in the context of other T cell phenotype readouts, including different cytokine production.…”

Allele Specificity of Gamma Interferon Responses to the Carboxyl-Terminal Region of Plasmodium falciparum Merozoite Surface Protein 1 by Kenyan Adults with Naturally Acquired Immunity to Malaria

“…Also, in a study by Lee et al (24), fewer than 10% of Kenyan adults demonstrated IFN-␥ responses to individual 20-mer peptides from MSP1 19 . Lastly, in a study of five malaria-naïve adults vaccinated with the 3D7 allele of MSP1 42 , interleukin-5 (IL-5) levels from PBMCs stimulated with homologous MSP1 33 or MSP1 42 were both higher than in cells stimulated with MSP1 19 (20). Six additional volunteers were vaccinated with MSP1 42 FVO, and similarly IL-5 production with MSP1 42 was higher than with MSP1 19 .…”

“…The ability of a vaccine to induce cross-reactive cellular responses to the multiple alleles of P. falciparum encountered in the field may also be a determining factor in its success. In the study of malaria-naïve adults vaccinated with either the 3D7 or FVO allele of MSP1 42 , there was little IFN-␥ produced by PBMCs when they were stimulated with the heterologous allele compared to when they were stimulated with the homologous allele (20). A single blood stage antigen will not likely be sufficient as a stand-alone vaccine; thus, planned studies examining the nature and development of immune responses to MSP1 in Kenyan infants and children, and the involvement of these responses in protection, will be crucial to the development of a successful multistage, multiantigen vaccine candidate that can benefit children as well as adults, particularly as increased global efforts to reduce malaria transmission may reduce the strength and duration of naturally acquired immunity.…”

“…Antibodies to MSP1 19 that interfere with RBC invasion by merozoites are one of several possible mechanisms by which naturally acquired immunity and experimental MSP1 vaccines may mediate protection against blood stage infection (27,34). Studies of primates and mice immunized with MSP1 and more limited observations of residents of areas in which P. falciparum is endemic who have naturally acquired immunity and malaria-naïve human volunteers vaccinated with MSP1 42 suggest that T-cell responses to MSP1 are driven primarily by epitopes within the shed MSP1 33 fragment and that gamma interferon (IFN-␥) is important for optimal protective immunity (20,23,44,46).…”

Immunogenicity of the Merozoite Surface Protein-1 (Msp-1) of Human Plasmodium Sp.

“…The adjuvant Alhydrogel was formulated to Plasmodium falciparum MSP-1 42 , with both 3D7 and FVO alleles, and evaluated in 60 volunteers in USA. The results showed that although the cytokines IFN-γ, IL-2, IL-5, IL-10 and IL-13 were detected in the vaccinatedvolunteers, addition of other immunostimulants to both vaccines were needed as the raised anti-MSP-1 42 antibodies were insufficient to inhibit parasite growth up to protection level (70,71). Ellis et al (72) mixed the FVO and 3D7 of E. coli-expressed P. falciparum MSP-1 42, formulated with Alhydrogel and novel adjuvant CPG 7909, in order to induce immune responses that recognized the major antigenic polymorphisms.…”