Immune responses to the circumsporozoite protein of Plasmodium falciparum in relation to HLA-DR type

Human leukocyte antigen class II alleles influence levels of antibodies to the Plasmodium falciparum asexual-stage apical membrane antigen 1 but not to merozoite surface antigen 2 and merozoite surface protein 1

The apical membrane antigen 1 (AMA1), merozoite surface antigen 2 (MSA2), and merozoite surface protein 1 (MSP1) are asexual-stage proteins currently being evaluated for inclusion in a vaccine for Plasmodium falciparum. Accordingly, it is important to understand factors that control antibody responses to these antigens. Antibody levels in plasma from residents of Etoa, Cameroon, between the ages of 5 and 70 years, were determined using recombinant AMA1, MSA2, and the N-terminal region of MSP1 (MSP1-190L). In addition, antibody responses to four variants of the C-terminal region of MSP1 (MSP1(19)) were assessed. Results showed that all individuals produced antibodies to AMA1, MSA2, and MSP1-190L; however, a proportion of individuals never produced antibodies to the MSP1(19) variants, although the percentage of nonresponders decreased with age. The influence of age and human leukocyte antigen (HLA)-DRB1/DQB1 alleles on antibody levels was evaluated using two-way analysis of variance. Age was correlated with levels of antibodies to AMA1 and MSP1(19) but not with levels of antibodies to MSA2 and MSP1-190L. No association was found between a single HLA allele and levels of antibodies to MSA2, MSP1-190L, or any of the MSP1(19) variants. However, individuals positive for DRB1*1201 had higher levels of antibodies to the variant of recombinant AMA1 tested than did individuals of all other HLA types. Since the effect was seen across all age groups, HLA influenced the level but not the rate of antibody acquisition. This association for AMA1, combined with the previously reported association between HLA class II alleles and levels of antibodies to rhoptry-associated protein 1 (RAP1) and RAP2, indicates that HLA influences the levels of antibodies to three of the five vaccine candidate antigens that we have evaluated.

Protection from Plasmodium berghei infection by priming and boosting T cells to a single class I-restricted epitope with recombinant carriers suitable for human use

The desirability of inducing cytotoxic T cell responses to defined epitopes in humans has led to the development of a variety of recombinant delivery systems. Recombinant protein particles derived from a yeast retrotransposon (Ty) and the modified Ankara vaccinia (MVA) virus can deliver large epitope strings or even whole proteins. Both have previously been administered safely in humans. Immunization with recombinant Ty and MVA containing a single Plasmodium berghei class I-binding epitope provided 95% sterile protection against malaria in mice. The sequence of immunization, Ty followed by MVA, was critical to elicit high levels of IFN-gamma-producing cells and protection. The reciprocal sequence (MVA/TY) or homologous boosting was not protective. Both constructs (Ty and MVA) contain the H-2Kd-restricted pb9 CTL epitope from the circumsporozoite protein of P. berghei among a string of 8-15 human P. falciparum-derived CTL epitopes restricted through 7 common HLA alleles as well as widely recognized CD4 T cell epitopes. Thus, the novel recombinant Ty/MVA prime/boost combination with these constructs provides a safe alternative for evaluation for human vaccination against P. falciparum malaria.

Infectious diseases and immunity: special reference to major histocompatibility complex

Human leukocyte antigens (HLAs) are an inherent system of alloantigens, which are the products of genes of the major histocompatibility complex (MHC). These genes span a region of approximately 4 centimorgans on the short arm of human chromosome 6 at band p 21.3 and encode the HLA class I and class II antigens, which play a central role in cell-to-cell interaction in the immune system. These antigens interact with the antigen-specific cell surface receptors of T lymphocytes (TCR) thus causing activation of the lymphocytes and the resulting immune response. Class I antigens restrict cytotoxic T-cell (CD8+) function thus killing viral infected targets, while class II antigens are involved in presentation of exogenous antigens to T-helper cells (CD4+) by antigen presenting cells (APC). The APC processes the antigens, and the immunogenic peptide is then presented at the cell surface along with the MHC molecule for recognition by the TCR. Since the MHC molecules play a central role in regulating the immune response, they may have an important role in controlling resistance and susceptibility to diseases. In this review we have highlighted studies conducted to look for an association between HLA and infectious diseases; such studies have had a variable degree of success because the pathogenesis of different diseases varies widely, and most diseases have a polygenic etiology.

The presence of the HLA class II allele DPB1*0501 in ethnic Thais correlates with an enhanced vaccine-induced antibody response to a malaria sporozoite antigen

In this study, we examined the correlation between the frequency of allelic variants of the class II human leukocyte antigen (HLA) DR, DQ and DP gene loci and the quantitative humoral immune response observed in 71 Thai volunteers, subsequent to vaccination with a conjugated subunit vaccine. This vaccine was designed to induce antibodies directed against the immunodominant repeat region of the Plasmodium falciparum circumsporozoite (CS) protein. The presence of the DPB1*0501, a relatively common allele in Asian populations, was found to be associated with high vaccine-induced CS repeat-specific antibody responses in the volunteers. Given the increasing focus on the use of subunit vaccines in the control of infectious diseases, consideration of the influence of class II allele frequencies in ethnically diverse recipient populations may be important.

Lymphocyte response in vitro to Plasmodium falciparum merozoite antigens in donors from a holoendemic area

Crude merozoite antigens from Plasmodium falciparum were used to evaluate the proliferative response of peripheral mononuclear cells (PBMCs) from 114 inhabitants of the village of Dielmo (Senegal, West Africa) exposed continuously to malaria transmission. The high or low responses to merozoite antigens obtained in lymphocyte stimulation assays were correlated with the presence or absence of parasites, IFN-gamma production and HLA phenotype. The high responders produced high levels of IFN-gamma, in contrast to the low responders, most of whom did not secrete IFN-gamma (23/27). Among others, the two HLA phenotypes HLA-B51 and HLA-DR1 were significantly associated with a high response (P < 0.05).

The immune recognition of malaria antigens

Owing to the demonstration that the immune response of inbred mice to some defined malaria antigens is influenced by the major histocompatibility complex (MHC), and the finding that only a minority of individuals living in malaria-endemic areas appear to recognize such antigens, there are fears that synthetic subunit malaria vaccines will be poorly immunogenic in a substantial proportion of the target population. Such fears have been reinforced by the results of the first two human malaria vaccine trials. In this review Eleanor Riley, Olle Olerup and Marita Troye-Blomberg summarize the experimental evidence for MHC-related genetic restriction of malaria immunity and discuss some alternative explanations for nonresponsiveness in populations living in malaria-endemic areas.

Intrinsic individual differences in circumsporozoite antibody response at a hyperendemic malaria focus

Antibody to the repetitive domain of the Plasmodium falciparum circumsporozoite (CS) protein was measured in blood collected over 3 consecutive months from 224 residents of a hyperendemic focus in Thailand. An enzyme-linked immunosorbent assay incorporating the protein ([NANP]15 + NVDP)2 was used. More than 22% of adults produced high antibody titres in all 3 months, even though transmission had stopped. Malaria prevalence in these high-responders was the same as in the rest of the population and the number of consanguineous high-responders was no higher than would be expected by chance. The data suggest that individuals at prolonged, equal risk of malaria inoculation display intrinsic differences in their ability to produce CS antibody.