'Original antigenic sin', T cell memory, and malaria sporozoite immunity: an hypothesis for immune evasion

The "original antigenic sin" and its relevance for SARS-CoV-2 (COVID-19) vaccination

Imprinting of the specific molecular image of a given protein antigen into immunological memory is one of the hallmarks of immunity. A later contact with a related, but different antigen should not trigger the memory response (because the produced antibodies would not be effective). The preferential expansion of cross-reactive antibodies, or T-lymphocytes for that matter, by a related antigen has been termed the original antigenic sin and was first described by Thomas Francis Jr. in 1960. The phenomenon was initially described for influenza virus, but also has been found for dengue and rotavirus. The antibody dependent enhancement observed in feline coronavirus vaccination also may be related to the original antigenic sin. For a full interpretation of the effectivity of the immune response against SARS-CoV-2, as well as for the success of vaccination, the role of existing immunological memory against circulating corona viruses is reviewed and analyzed.

Expansion of Functional Myeloid-Derived Suppressor Cells in Controlled Human Malaria Infection

Malaria can cause life-threatening complications which are often associated with inflammatory reactions. More subtle, but also contributing to the burden of disease are chronic, often subclinical infections, which result in conditions like anemia and immunologic hyporesponsiveness. Although very frequent, such infections are difficult to study in endemic regions because of interaction with concurrent infections and immune responses. In particular, knowledge about mechanisms of malaria-induced immunosuppression is scarce. We measured circulating immune cells by cytometry in healthy, malaria-naïve, adult volunteers undergoing controlled human malaria infection (CHMI) with a focus on potentially immunosuppressive cells. Infectious Plasmodium falciparum (Pf) sporozoites (SPZ) (PfSPZ Challenge) were inoculated during two independent studies to assess malaria vaccine efficacy. Volunteers were followed daily until parasites were detected in the circulation by RT-qPCR. This allowed us to analyze immune responses during pre-patency and at very low parasite densities in malaria-naïve healthy adults. We observed a consistent increase in circulating polymorphonuclear myeloid-derived suppressor cells (PMN-MDSC) in volunteers who developed P. falciparum blood stage parasitemia. The increase was independent of preceding vaccination with a pre-erythrocytic malaria vaccine. PMN-MDSC were functional, they suppressed CD4⁺ and CD8⁺ T cell proliferation as shown by ex-vivo co-cultivation with stimulated T cells. PMN-MDSC reduced T cell proliferation upon stimulation by about 50%. Interestingly, high circulating PMN-MDSC numbers were associated with lymphocytopenia. The number of circulating regulatory T cells (T_reg) and monocytic MDSC (M-MDSC) showed no significant parasitemia-dependent variation. These results highlight PMN-MDSC in the peripheral circulation as an early indicator of infection during malaria. They suppress CD4⁺ and CD8⁺ T cell proliferation in vitro. Their contribution to immunosuppression in vivo in subclinical and uncomplicated malaria will be the subject of further research. Pre-emptive antimalarial pre-treatment of vaccinees to reverse malaria-associated PMN-MDSC immunosuppression could improve vaccine response in exposed individuals.

Acquisition, maintenance and adaptation of invasion inhibitory antibodies against Plasmodium falciparum invasion ligands involved in immune evasion

Erythrocyte-binding antigens (EBAs) and P. falciparum reticulocyte-binding homologue proteins (PfRhs) are two important protein families that can vary in expression and utilization by P. falciparum to evade inhibitory antibodies. We evaluated antibodies at repeated time-points among individuals living in an endemic region in Nigeria over almost one year against these vaccine candidates. Antibody levels against EBA140, EBA175, EBA181, PfRh2, PfRh4, and MSP2, were measured by ELISA. We also used parasites with disrupted EBA140, EBA175 and EBA181 genes to show that all these were targets of invasion inhibitory antibodies. However, antigenic targets of inhibitory antibodies were not stable and changed substantially over time in most individuals, independent of age. Antibodies levels measured by ELISA also varied within and between individuals over time and the antibodies against EBA181, PfRh2 and MSP2 declined more rapidly in younger individuals (≤15 years) compared with older (>15). The breadth of high antibody responses over time was more influenced by age than by the frequency of infection. High antibody levels were associated with a more stable invasion inhibitory response, which could indicate that during the long process of formation of immunity, many changes not only in levels but also in functional responses are needed. This is an important finding in understanding natural immunity against malaria, which is essential for making an efficacious vaccine.

Unique cellular and humoral immunogenicity profiles generated by aerosol, intranasal, or parenteral vaccination in rhesus macaques

Respiratory mucosa immunization is capable of eliciting both local and distal mucosal immune responses; it is a potentially powerful yet largely unused modality for vaccination against respiratory diseases. Targeting the lower versus upper airways by aerosol delivery alters the immunogenicity profile of a vaccine, although the full extent of this impact is not well characterized. We set out to define the cellular and humoral response profiles elicited by immunization via intranasal, small aerosol droplets, and large aerosol droplets. We compared responses following adenovirus-vectored vaccination by these routes in macaques, either for the generation of primary immune responses or for the boosting of previously primed systemic responses. Aerosol delivery (4 or 10μm diameter droplets, addressing lower or upper airways, respectively) generated the highest magnitude lung CD4 and CD8 T-cell responses, reaching 10-30% vaccine-specific levels in bronchoalveolar lavage cells. In contrast, intranasal delivery was less immunogenic with >10-fold lower peak lung T-cell responses. Systemic (blood) T-cell responses were only observed following 4μm aerosol (and parenteral) immunization, while all delivery routes elicited similar humoral responses. These data demonstrate distinct immune response profiles with each respiratory tract vaccination modality and suggest that small droplet aerosol offers several immunological advantages over other respiratory routes.

Immunization with different PfAMA1 alleles in sequence induces clonal imprint humoral responses that are similar to responses induced by the same alleles as a vaccine cocktail in rabbits

Background

Antibodies to key Plasmodium falciparum surface antigens have been shown to be important effectors that mediate clinical immunity to malaria. The cross-strain fraction of anti-malarial antibodies may however be required to achieve

strain-transcending immunity. Such antibody responses against Plasmodium falciparum apical membrane antigen 1 (PfAMA1), a vaccine target molecule that is expressed in both liver and blood stages of the parasite, can be elicited through immunization with a mixture of allelic variants of the parasite molecule. Cross-strain antibodies are most likely elicited against epitopes that are shared by the allelic antigens in the vaccine cocktail.

Methods

A standard competition ELISA was used to address whether the antibody response can be further focused on shared epitopes by exclusively boosting these common determinants through immunization of rabbits with different PfAMA1 alleles in sequence. The in vitro parasite growth inhibition assay was used to further evaluate the functional effects of the broadened antibody response that is characteristic of multi-allele vaccine strategies.

Results

A mixed antigen immunization protocol elicited humoral responses that were functionally similar to those elicited by a sequential immunization protocol (p > 0.05). Sequential exposure to the different PfAMA1 allelic variants induced immunological recall of responses to previous alleles and yielded functional cross-strain antibodies that would be capable of optimal growth inhibition of variant parasites at high enough concentrations.

Conclusions

These findings may have implications for the current understanding of the natural acquisition of clinical immunity to malaria as well as for rational vaccine design.

Chapter 5. Potential contribution of sero-epidemiological analysis for monitoring malaria control and elimination: historical and current perspectives

Anti-malarial antibody responses represent an individual's history of exposure to the disease and, as age sero-conversion rates, reflect cumulative malaria exposure in a population. As such these antibody responses are an alternate measure of malaria transmission intensity and have potential in evaluating changes in exposure. This approach was used in the 1970s to evaluate malaria control and eradication attempts in a variety of different ecological settings. These historical studies provided a wealth of information on how serological data might be used to interpret control measures. However they were limited by a lack of standardized antigens and reproducible high-throughput assays. Current techniques using recombinant antigens with a range of immunogenicities, high-throughput enzyme-linked immunosorbent assays (ELISA) and statistical analysis allow a more robust examination of how serological parameters can be used to evaluate factors affecting malaria transmission. Here we present a review of the historical data and use it to assess the serological contribution to monitoring malaria elimination.

MIG (CXCL9) is a more sensitive measure than IFN-gamma of vaccine induced T-cell responses in volunteers receiving investigated malaria vaccines

For many years the IFN-γ ex vivo ELISPOT has been a major assay for assessing human T-cell responses generated by malaria vaccines. The ELISPOT assay is a sensitive assay, but an imperfect correlate of protection against malaria. Monokine induced by gamma (MIG), or CXCL9, is a chemokine induced by IFN-γ and has the potential to provide amplification of the IFN-γ signal. MIG secretion could provide a measure of bio-active IFN-γ and a functional IFN-γ signalling pathway. We report that detecting MIG by flow cytometry and by RT-PCR can be more sensitive than the detection of IFN-γ using these methods. We also find that there is little inter-individual variability in MIG secretion when detected by flow cytometry and that the MIG assay may be used to estimate the amount of bio-active IFN-γ present. Measurement of MIG alongside IFN-γ may provide a fuller picture of Th1 type responses post-vaccination.

Alternating vector immunizations encoding pre-erythrocytic malaria antigens enhance memory responses in a malaria endemic area

A heterologous prime-boost strategy has been developed to potently induce T cell responses to pre-erythrocytic malaria antigens. Efficacy in the field is likely to depend on both peak immunogenicity and the durability of responses. To improve both immunogenicity and durability of responses, 54 adult males from a malaria endemic area were immunized with different vaccination regimens, systematically varying antigenic insert and the number and sequence of component vaccinations. The component vaccinations were recombinant attenuated viruses, either fowlpox (FP) 9 or modified vaccinia virus Ankara (MVA). These were recombinant for either of two pre-erythrocytic malaria antigens (multiple epitope-thrombospondin-related adhesion protein, ME-TRAP, or circumsporozoite antigen (CS). ELISPOT assays were used to measure the effector and resting memory T cell responses. Sequence, antigen insert and number of vaccinations influenced immunogenicity, but the novel alternating vector immunizations generated the largest resting memory T cell populations. Effector responses were maintained at 84% of the peak response after 270 days. This durability of response is unprecedented. Classical prime-boost vaccination responses were at 5% of the peak after 270 days. Vaccines administered by heterologous prime-boost regimes are being developed for diverse pathogens and cancer. These data suggest these vaccines should also be administered by alternating vector regimens in clinical development.

CD4 T cells from malaria-nonexposed individuals respond to the CD36-Binding Domain of Plasmodium falciparum erythrocyte membrane protein-1 via an MHC class II-TCR-independent pathway

We have studied the human CD4 T cell response to a functionally conserved domain of Plasmodium falciparum erythrocyte membrane protein-1, cysteine interdomain region-1alpha (CIDR-1alpha). Responses to CIDR-1alpha were striking in that both exposed and nonexposed donors responded. The IFN-gamma response to CIDR-1alpha in the nonexposed donors was partially independent of TCR engagement of MHC class II and peptide. Contrastingly, CD4 T cell and IFN-gamma responses in malaria-exposed donors were MHC class II restricted, suggesting that the CD4 T cell response to CIDR-1alpha in malaria semi-immune adults also has a TCR-mediated component, which may represent a memory response. Dendritic cells isolated from human peripheral blood were activated by CIDR-1alpha to produce IL-12, IL-10, and IL-18. IL-12 was detectable only between 6 and 12 h of culture, whereas the IL-10 continued to increase throughout the 24-h time course. These data strengthen previous observations that P. falciparum interacts directly with human dendritic cells, and suggests that the interaction between CIDR-1alpha and the host cell may be responsible for regulation of the CD4 T cell and cytokine responses to P. falciparum-infected erythrocytes reported previously.

Protective cellular immunity against P. falciparum malaria merozoites is associated with a different P7 and P8 residue orientation in the MHC–peptide–TCR complex

Developing a logical and rational methodology for obtaining vaccines, especially against the main parasite causing human malaria (P. falciparum), consists of blocking receptor-ligand interactions. Conserved peptides derived from proteins involved in invasion and having high red blood cell binding ability have thus been identified. Immunization studies using Aotus monkeys have revealed that these peptides were neither immunogenic nor protection inducing. When modified in their critical binding residues, previously identified by Glycine scanning, some of these peptides were immunogenic and non-protection inducers; others induced short-lived antibodies whilst a few were both immunogenic and protection inducing. However, very few of these modified high activity binding peptides (HABPs) reproducibly induced protection without inducing antibody production, but with high cytokine liberation, suggesting that cellular mechanisms had been activated in the protection process. The three-dimensional structure of these peptides inducing protection without producing antibodies was determined by 1H-NMR. Their HLA-DRbeta1* molecule binding ability was also determined to ascertain association between their 3D structure and ability to bind to Major Histocompatibility Complex Class-II molecules (MHC-II). 1H Nuclear Magnetic Resonance analysis and structure calculations clearly showed that these modified HABPs inducing protective cellular immune responses (but not producing antibodies against malaria) adopted special structural configuration to fit into the MHC II-peptide-TCR complex. A different orientation for P7 and P8 TCR contacting residues was clearly recognized when comparing their structure with modified peptides, which induced high antibody titers and protection, suggesting that these residues are involved in activating the immune system associated with antibody production and protection.

Specificity of serum neutralizing antibodies induced by transient immune suppression of inapparent carrier ponies infected with a neutralization-resistant equine infectious anemia virus envelope strain

It has been previously reported that transient corticosteroid immune suppression of ponies experimentally infected with a highly neutralization resistant envelope variant of equine infectious anemia virus (EIAV), designated EIAVΔPND, resulted in the appearance of type-specific serum antibodies to the infecting EIAVΔPNDvirus. The current study was designed to determine if this induction of serum neutralizing antibodies was associated with changes in the specificity of envelope determinants targeted by serum antibodies or caused by changes in the nature of the antibodies targeted to previously defined surface envelope gp90 V3 and V4 neutralization determinants. To address this question, the envelope determinants of neutralization by post-immune suppression serum were mapped. The results demonstrated that the neutralization sensitivity to post-immune suppression serum antibodies mapped specifically to the surface envelope gp90 V3 and V4 domains, individually or in combination. Thus, these data indicate that the development of serum neutralizing antibodies to the resistant EIAVΔPNDwas due to an enhancement of host antibody responses caused by transient immune suppression and the associated increase in virus replication.

Antibodies to the N-terminal block 2 of Plasmodium falciparum merozoite surface protein 1 are associated with protection against clinical malaria

This longitudinal prospective study shows that antibodies to the N-terminal block 2 region of the Plasmodium falciparum merozoite surface protein 1 (MSP-1) are associated with protection against clinical malaria in an area of stable but seasonal malaria transmission of Ghana. Antibodies to the block 2 region of MSP-1 were measured in a cohort of 280 children before the beginning of the major malaria transmission season. The cohort was then actively monitored for malaria, clinically and parasitologically, over a period of 17 months. Evidence is presented for an association between antibody responses to block 2 and a significantly reduced risk of subsequent clinical malaria. Furthermore, statistical survival analysis provides new information on the duration of the effect over time. The results support a conclusion that the block 2 region of MSP-1 is a target of protective immunity against P. falciparum and, thus, a promising new candidate for the development of a malaria vaccine.

Polyspecific malaria antibodies present at the time of infection inhibit the development of immunity to malaria but antibodies specific for the malaria merozoite surface protein, MSP1, facilitate immunity

Serum taken from mice immune to malaria as a result of infection and drug cure, or from mice immunized with a recombinant form of the merozoite surface protein, MSP1, can provide passive protection of recipient mice against the lethal parasite, Plasmodium yoelii YM. However, recipients of MSP1-immune serum go on to develop long-term immunity, whereas recipients of serum from mice naturally immune to malaria rapidly lose their resistance to infection. We demonstrate that 'infection/cure' serum suppresses the development of both antibody and cell-mediated parasite-specific responses in recipients, whereas these develop in recipients of MSP1-specific antibodies. These data have profound implications for our understanding of the development of malaria immunity in babies who passively acquire antibodies from their mothers.

Identification of HLA-A2 restricted CD8(+) T-lymphocyte responses to Plasmodium vivax circumsporozoite protein in individuals naturally exposed to malaria

Specific CD8(-) T-lymphocyte (CTL) activity against Plasmodium pre-erythrocytic stages (P-ES) derived antigens is considered one of the most important mechanisms for malaria protection. Plasmodium vivax is the second most prevalent human malaria parasite species distributed worldwide. Although several CTL epitopes have been identified in Plasmodium falciparum P-ES derived antigens, none has been described for P. vivax to date. In this study, we analysed HLA-A*0201 specific CD8(-) T-lymphocyte responses to the P. vivax circumsporozoite (CS) protein in both malaria exposed and non-exposed populations from the Colombian Pacific Coast. First, we analysed the prevalence of HLA-A2 allele in the study populations and found that approximately 38 of the individuals expressed this molecule and that 50 of them were HLA-A*0201. We then selected, on the P. vivax CS, five peptide sequences containing the HLA-A*0201 binding motifs and used the corresponding synthetic peptides to evaluate the CD8(-) T-lymphocyte interferon (IFN)-gamma response. Peripheral blood mononuclear cells from the HLA-A*0201 donors were in vitro stimulated with these peptides and IFN-gamma production was determined by an ELISPOT assay. Specific CD8(-) T-lymphocyte responses were detected for three peptides located in the C-terminal region of the protein. Specific responses to these peptides were also detected in several individuals expressing different HLA-A*02 subtypes. The potential of these peptides to induce specific cytolysis and that of long synthetic peptides comprising these epitopes as P. vivax malaria vaccine subunits are being studied.

Assessment of positivity in immuno-assays with variability in background measurements: a new approach applied to the antibody response to Plasmodium falciparum MSP2

Measurements of immune responses often exhibit considerable heterogeneity, making it impossible to clearly distinguish responders and nonresponders to particular antigens. Typically, in, for example, enzyme-linked immunosorbent assay (ELISA) procedures, a nonexposed control group is used to assign a cutoff value of positivity, calculated as the mean plus either 2 or 3 standard deviations (S.D.). This can cause extremely biased estimates of response rates when the background is variable, and especially when there is overlap between the distribution of the control levels and that of responders. This problem is compounded when results of assays with different background levels are compared. We illustrate this with hypothetical data sets reflecting frequent patterns seen in laboratory and epidemiological studies. We propose that such data should be analysed by statistical modelling of the ratio of numbers of test samples/control samples as a function of the readout from the assay. Rather than classifying samples dichotomously as negative or positive, this provides estimates of the prevalence of positivity lambda, and the probability, for each sample, that the measured activity is above background. Several statistical methods can provide such estimates. Analyses of simulated data sets using our preferred estimation method [a latent class model (LCM)] demonstrate that this gives more reliable results than the traditional assignment using cutoff values. We have applied this approach to the analysis of ELISA assessments of antibodies against distinct regions of the Plasmodium falciparum merozoite surface protein 2 (MSP2) in human sera from Tanzania.

Towards a blood-stage vaccine for malaria: are we following all the leads?

Although the malaria parasite was discovered more than 120 years ago, it is only during the past 20 years, following the cloning of malaria genes, that we have been able to think rationally about vaccine design and development. Effective vaccines for malaria could interrupt the life cycle of the parasite at different stages in the human host or in the mosquito. The purpose of this review is to outline the challenges we face in developing a vaccine that will limit growth of the parasite during the stage within red blood cells--the stage responsible for all the symptoms and pathology of malaria. More than 15 vaccine trials have either been completed or are in progress, and many more are planned. Success in current trials could lead to a vaccine capable of saving more than 2 million lives per year.

Central nervous system in cerebral malaria: 'Innocent bystander' or active participant in the induction of immunopathology?

Cerebral malaria (CM) is a major life-threatening complication of Plasmodium falciparum infection in humans, responsible for up to 2 million deaths annually. The mechanisms underlying the fatal cerebral complications are still not fully understood. Many theories exist on the aetiology of human CM. The sequestration hypo-thesis suggests that adherence of parasitized erythrocytes to the cerebral vasculature leads to obstruction of the microcirculation, anoxia or metabolic disturbances affecting brain function, resulting in coma. This mechanism alone seems insufficient to explain all the known features of CM. In this review we focus on another major school of thought, that CM is the result of an over-vigorous immune response originally evolved for the protection of the host. Evidence in support of this second hypothesis comes from studies in murine malaria models in which T cells, monocytes, adhesion molecules and cytokines, have been implicated in the development of the cerebral complications. Recent studies of human CM also indicate a role for the immune system in the neurological complications. However, it is likely that multiple mechanisms are involved in the induction of cerebral complications and both the presence of parasitized erythrocytes in the central nervous system (CNS) and immunopathological processes contribute to the pathogenesis of CM. Most studies examining immunopathological responses in CM have focused on reactions occurring primarily in the systemic circulation. However, these also do not fully account for the development of cerebral complications in CM. In this review we summarize results from human and mouse studies that demonstrate morphological and functional changes in the resident glial cells of the CNS. The degree of immune activation and degeneration of glial cells was shown to reflect the extent of neurological complications in murine cerebral malaria. From these results we highlight the need to consider the potentially important contribution within the CNS of glia and their secreted products, such as cytokines, in the development of human CM.

HLA-A*0201 restricted CD8+ T-lymphocyte responses to malaria: identification of new Plasmodium falciparum epitopes by IFN-gamma ELISPOT

The role of antigen specific CD8+ T-lymphocytes in mediating protection against sporozoite-induced malaria has been well established in murine models. In humans, indirect evidence has accumulated suggesting a similar protective role for antigen-specific CD8+ T-lymphocytes. Nevertheless, the low frequency of circulating specific cells together with the lack of sensitive methods to quantify them has hampered the direct assessment of their function. Using a combination of short-term cell culture and IFN-gamma ELISPOT, we studied CD8+ T-lymphocyte responses to a panel of HLA-A*0201 binding peptides. In addition to confirming the response to already described epitopes, we also identified five new CD8+ T-lymphocyte epitopes. These epitopes are presented in pre-erythrocytic stages gene products of Plasmodium falciparum 7G8 strain and correspond to the following protein segments: circumsporozoite (CS) 64-72, 104-113, 299-308 and 403-411; liver stage antigen (LSA-1) repeat region; sporozoite surface protein 2 or thrombospondin related anonymous protein (SSP2/TRAP) 78-88 and 504-513. Four of these peptides are conserved amongst all published sequences of P. falciparum strains. We conclude that the modified IFN-gamma ELISPOT assay is a sensitive technique to monitor antigen-specific CD8+ T-lymphocyte responses in human malaria which may help in the improvement and assessment of the efficacy of malaria subunit vaccines.

ESAT-6 subunit vaccination against Mycobacterium tuberculosis

The ESAT-6 antigen from Mycobacterium tuberculosis is a dominant target for cell-mediated immunity in the early phase of tuberculosis (TB) in TB patients as well as in various animal models. The purpose of our study was to evaluate the potential of ESAT-6 in an experimental TB vaccine. We started out using dimethyl dioctadecylammonium bromide (DDA), an adjuvant which has been demonstrated to be efficient for the induction of cellular immune responses and has been used successfully before as a delivery system for TB vaccines. Here we demonstrate that, whereas immune responses to both short-term-culture filtrate and Ag85B are efficiently induced with DDA, this adjuvant was inefficient for the induction of immune responses to ESAT-6. Therefore, we investigated the modulatory effect of monophosphoryl lipid A (MPL), an immunomodulator which in different combinations has demonstrated strong adjuvant activity for both cellular and humoral immune responses. We show in the present study that vaccination with ESAT-6 delivered in a combination of MPL and DDA elicited a strong ESAT-6-specific T-cell response and protective immunity comparable to that achieved with Mycobacterium bovis BCG.

Levels of antibody to conserved parts of Plasmodium falciparum merozoite surface protein 1 in Ghanaian children are not associated with protection from clinical malaria

The 19-kDa conserved C-terminal part of the Plasmodium falciparum merozoite surface protein 1 (PfMSP119) is a malaria vaccine candidate antigen, and human antibody responses to PfMSP119 have been associated with protection against clinical malaria. In this longitudinal study carried out in an area of stable but seasonal malaria transmission with an estimated parasite inoculation of about 20 infective bites/year, we monitored 266 3- to 15-year-old Ghanaian children clinically and parasitologically over a period of 18 months. Blood samples were collected at the beginning of the study before the major malaria season in April and after the season in November. Using enzyme-linked immunosorbent assay, we measured antibody responses to recombinant gluthathione S-transferase-PfMSP119 fusion proteins corresponding to the Wellcome and MAD20 allelic variants in these samples. Prevalence of antibodies recognizing the Wellcome 19 construct containing both epidermal growth factor (EGF)-like motifs in Wellcome type PfMSP119 was about 30%. Prevalence of antibodies to constructs containing only the first EGF domain from either Wellcome or MAD20 type PfMSP119 was about 15%, whereas antibodies recognizing a construct containing only the second EGF domain of MAD20 type PfMSP119 was found in only about 4% of the donors. Neither the prevalence nor the levels of any of the antibody specificities varied significantly with season, age, or sex. Significantly, and in contrast to previous reports from other parts of West Africa, we found no evidence of an association between antibody responses to PfMSP119 and clinical protection against malaria.

MHC-restricted antigen presentation and recognition: constraints on gene, recombinant and peptide vaccines in humans

The target of any immunization is to activate and expand lymphocyte clones with the desired recognition specificity and the necessary effector functions. In gene, recombinant and peptide vaccines, the immunogen is a single protein or a small assembly of epitopes from antigenic proteins. Since most immune responses against protein and peptide antigens are T-cell dependent, the molecular target of such vaccines is to generate at least 50-100 complexes between MHC molecule and the antigenic peptide per antigen-presenting cell, sensitizing a T cell population of appropriate clonal size and effector characteristics. Thus, the immunobiology of antigen recognition by T cells must be taken into account when designing new generation peptide- or gene-based vaccines. Since T cell recognition is MHC-restricted, and given the wide polymorphism of the different MHC molecules, distinct epitopes may be recognized by different individuals in the population. Therefore, the issue of whether immunization will be effective in inducing a protective immune response, covering the entire target population, becomes an important question. Many pathogens have evolved molecular mechanisms to escape recognition by the immune system by variation of antigenic protein sequences. In this short review, we will discuss the several concepts related to selection of amino acid sequences to be included in DNA and peptide vaccines.

Longitudinal studies of neutralizing antibody responses to rotavirus in stools and sera of children following severe rotavirus gastroenteritis

Rotavirus-neutralizing antibody responses in sera and stools of children hospitalized with rotavirus gastroenteritis and then monitored longitudinally were optimally detected by using local rotavirus strains. Stool responses were highest on days 5 to 8 after the onset of diarrhea. Longitudinal monitoring suggested that serum neutralizing antibody responses were a more useful measure of severely symptomatic rotavirus infection than stool responses but that stool antibody responses may be a useful measure of rotavirus immunity.

Original antigenic sin impairs cytotoxic T lymphocyte responses to viruses bearing variant epitopes

Some viruses, including human immunodeficiency virus (HIV) and hepatitis B virus (HBV) in humans, and lymphocytic choriomeningitis virus (LCMV) in mice, are initially controlled by cytotoxic T lymphocytes (CTLs), but may subsequently escape through mutation of the relevant T-cell epitope. Some of these mutations preserve the normal binding to major histocompatibility complex class I molecules, but present an altered surface to the T-cell antigen receptor. The exact role of these so-called altered peptide ligands in vivo is not clear. Here we report that mice primed with LCMV-WE strain respond to a subsequent infection by WE-derived CTL epitope variants with a CTL response directed against the initial epitope rather than against the new variant epitope. This phenomenon of 'original antigenic sin' was initially described in influenza and is an asymmetric pattern of protective antibody crossreactivity determined by exposure to previously existing strains, which may therefore extend to some CTL responses. Original antigenic sin by CTL leads to impaired clearance of variant viruses infecting the same individual and so may enhance the immune escape of mutant viruses evolving in an individual host.

Human and murine T-cell responses to allelic forms of a malaria circumsporozoite protein epitope support a polyvalent vaccine strategy

Mouse models and a recent vaccine trial have indicated the importance of T-cell immunity to the circumsporozoite protein (CSP) of malaria sporozoites. One of the major impediments for the development of a CSP-based vaccine is that human T-cell epitopes, identified on the CSP, span regions of significant point mutational polymorphism. Studies with human and mouse T-cell clones have indicated that this polymorphism affects T-cell cross-reactivity to Th2R and Th3R, the two most polymorphic and immunodominant epitopes. We extend this observation with polyclonal human T-cell lines, from 11 donors, raised to known variants of Th2R. These lines showed limited but variable cross-reactivity with the heterologous peptides. T cells from B10.A4(R) (I-Ak) mice immunized with each of 18 natural variants of Th2R indicated a similar, limited, cross-reactivity. I-Ak competition assays showed that a number of peptides were unable to bind because of a single polymorphic residue. In both the human and mouse assays, analysis of the sequences of immunogenic cross-reactive and non-cross-reactive peptides suggested that the individual polymorphic residues affect the three-dimensional conformation of the peptide within the major histocompatibility complex (MHC) groove in an, as yet, unpredictable way. These observations argue that design of an epitope able to generate broad cross-reactivity is, to date, not possible. However, despite the limited cross-reactivity of the individual human T-cell lines, most of the donors had T-cell repertoires capable of recognizing all or nearly all of the variants tested, which supports a strategy using a multivalent vaccine.

Toxocara canis adult worm antigen induces proliferative response of healthy human peripheral blood mononuclear cells

The proliferative response of human peripheral blood mononuclear cells (PBMC) from healthy donors to Toxocara canis adult worm antigens (TcA) was examined. PBMC from all donors examined (n = 7) strongly responded to TcA in a dose-dependent fashion after six days of culture, irrespective of their serological reactivity. In contrast, cord blood mononuclear cells did not react to TcA. The proliferation of PBMC in response to TcA was completely inhibited by anti-HLA-DR antibody. Purified CD4+ T cells reconstituted with autologous irradiated antigen presenting cells (APC) vigorously proliferated in response to TcA, but this was abrogated by pretreatment of APC with paraformaldehyde. Significant IL-2, IL-3, IL-4, IL-5 and IFN-gamma mRNA expression was detected in PBMC stimulated with TcA, with expression peaking at 72 h after stimulation. IL-1 beta, IL-6, IL-10 and GM-CSF mRNA expression was also upregulated, peaking at 24 h after stimulation. Taken together, these results suggest that adult T. canis-derived antigens have the ability to activate human PBMC as conventional antigens, possibly due to their cross-reactivity, which may be involved in the host defence against helminth infection.

Development of immunity to malaria may not be an entirely active process

It has never been explained why it takes so long for humans to develop immunity to malaria, although factors such as antigenic variation, antigenic polymorphism, and poor immunological responses to critical antigens are thought to be important. Models of malaria, particularly in rodents, have not been helpful. The course of malaria infection differs considerably between humans and rodents. Mice rapidly develop immunity whereas for most humans it takes several years of exposure for this to occur. Mice typically exhibit high parasitaemias whereas humans typically do not. A significant difference in the immune response of humans and mice to malaria parasites might, in part, explain these differences. Most humans have a preexisting population of activated malaria parasite-specific T cells (cross-reactive T cells) which we have referred to as 'natural' T cells, but such cells have not been observed in mice. These cells, many of which secrete interferon-gamma, might control parasitaemia early in the infection, but a by-product of their further activation by malaria parasites might be disease symptoms. Development of immunity has been thought of as an active process--acquisition of specific antibody and effector T cell responses. However, it might in part reflect induction of tolerance of this preexisting population of disease-inducing T cells as a result of chronic parasitaemia. The initial presence of these Th1-like cells may also impede the development of a Th2-like response necessary for the production of protective antibodies. Persistent cross-reactive stimulation may significantly impede this process.

Life-spans of human T-cell responses to determinants from the circumsporozoite proteins of Plasmodium falciparum and Plasmodium vivax

The longevity of specific human memory T-cell responses is largely unknown. However, a knowledge of the duration of memory is important for understanding immunity to an organism and for planning vaccine intervention. To address this, we have examined T-cell memory to malaria by determining T-cell responses by subjects recently exposed to peptides spanning the circumsporozoite (CS) proteins of two species of malaria-causing organisms, Plasmodium falciparum and Plasmodium vivax. Responses to vivax CS peptides by exposed Thai subjects were more frequent than responses by nonexposed individuals, permitting identification of determinants seen by vivax-induced responses. At the population level, there appears to be life-long memory, as the time since individuals were exposed did not diminish responsiveness to these determinants. In contrast, falciparum-exposed subjects were largely indistinguishable from nonexposed controls in responsiveness to falciparum CS determinants. However, a single peptide (F16: DNEKLRKPKHKKLKQPGDGN) was recognized significantly more frequently by P. falciparum-exposed than nonexposed Thai subjects. T cells responsive to this peptide were CD450+ and produced gamma-interferon. In contrast to the response to the vivax determinants and the other falciparum determinants, responsiveness to F16 was undetectable or minimal 2 years after exposure. Our data provide the average life-spans of certain malaria-specific T cells and are consistent with, but do not prove, the hypothesis that antigenic persistence (in the form of P. vivax hypnozoites) correlates with persistence of human T-cell memory.

Antigenic variation of parasite-derived antigens on the surface of Babesia bovis-infected erythrocytes

The hemoparasite Babesia bovis antigenically alters the bovine erythrocyte membrane surface by expression of isolate-specific, parasite-derived polypeptides. To determine whether antigenic variation also occurred on the infected erythrocyte surface, a calf was infected once with parasitized erythrocytes carrying the C9.1 clonal line of B. bovis. In vitro cultures then were established periodically from the peripheral blood and analyzed with sequentially collected sera from the same animal. The surface reactivity of infected erythrocytes cultured from the infected animal varied over time, on the basis of reactivity in live cell immunofluorescence, surface immunoprecipitation, and panning assays. Subclones C8 and H10, established from day 41 cultures, were analyzed immunochemically. A loss of immunoreactivity was observed in antigens corresponding to the 113- and 128-kDa parasite-derived antigens of clone C9.1, demonstrating epitopic variation in these antigens; the immunochemical recognition of these antigens paralleled the results of live cell immunofluorescence and panning assays. Concomitant size polymorphism suggested polypeptide structural variation of these antigens as well. Calves infected by inoculation of infected blood or by injection of cloned parasites from in vitro cultures rapidly developed antibodies which cross-reacted among the clonal variant lines, suggesting the presence of common as well as unique epitopes. These results demonstrate that antigenic variation occurs on the surface of B. bovis-infected erythrocytes and that the parasite-derived antigens of 113 and 128 kDa compose at least a part of the antigens undergoing variation.