The role of variant surface antigens on malaria-infected red blood cells.

It has been proposed that the primary role of variant antigens appearing on the surface of red blood cells infected with malaria parasites is to mediate cytoadherence, and that the antigenic variation they display is an adaptation to avoid immune attack. Here, Allan Saul proposes that their role is the opposite: that their primary purpose is to generate an immune response, which regulates their growth and thereby establishes a chronic infection, and that the role of cytoadherence is to ensure that parasites failing to express this flag to the immune system are destroyed by the spleen.     

Origins and evolution of antigenic diversity in malaria parasites

Each year, malaria parasites cause more than 500 million infections and 0.5-3 million deaths worldwide, mostly among children under five living in sub-Saharan Africa. In contrast with several viral and bacterial pathogens, which elicit long-lived immunity after a primary infection, these parasites require several years of continuous exposure to confer partial, usually non-sterilizing immune protection. One of the main obstacles to the acquisition of antimalarial immunity is the high degree of antigenic diversity in potential target antigens, which enables parasites to evade immune responses elicited by past exposure to variant forms of the same antigen. Allelic polymorphism, the existence of genetically stable alternative forms of antigen-coding genes, originates from nucleotide replacement mutations and intragenic recombination. In addition, malaria parasites display antigenic variation, whereby a clonal lineage of parasites expresses successively alternate forms of an antigen without changes in genotype. This review focuses on molecular and evolutionary processes that promote allelic polymorphism and antigenic variation in natural malaria parasite populations and their implications for naturally acquired immunity and vaccine development.     

How protozoan parasites evade the immune response

Protozoan pathogens such as Plasmodium, Leishmania, Trypanosoma and Entamoeba are responsible for several of the most widespread and lethal human diseases. Their successful survival depends mainly on evading the host immune system by, for example, penetrating and multiplying within cells, varying their surface antigens, eliminating their protein coat, and modulating the host immune response. Immunosuppression is sometimes caused directly by parasite products and sometimes involves antigenic mimicry, which often appears in association with parasitic diseases. However, one of the most sophisticated mechanisms of evasion is the selective activation of a subset of T helper cells.     

The role of HLA antigens in the control of the cytotoxic T-cell response to Epstein-Barr virus: a family study

Epstein-Barr (EB) virus-specific, HLA-restricted cytotoxic T-cell populations have been generated in vitro from each member of a family by cocultivating peripheral blood mononuclear cells with autologous EB virus-transformed B cells, the resulting effector cells being expanded as interleukin 2-dependent T-cell lines. The cytotoxicity of each of these effector populations was tested on a large panel of EB virus-transformed target cells of known HLA type, so that the particular HLA antigens which acted as restricting elements for each cytotoxic population could be identified. There was a consistent pattern within the family of preference for certain HLA class I antigens as restricting elements of the virus-specific T-cell response. Extensive functional analysis showed that, in addition to the virus-specific lysis, each effector population mediated a cross-reactive lysis against target cells prepared from certain HLA-mismatched individuals. This cross-reactivity appeared to be directed against HLA class I alloantigens and occurred irrespective of the EB virus genome status of the target cells. Effector T-cell lines derived from different family members but with virus-specific lysis predominantly restricted through the same HLA antigen showed similar patterns of concomitant allo-cross-reactivity. This suggests that antigenic mimicry of virally altered self by alloantigens is a genuine phenomenon which may be important in channelling the human cytotoxic T-cell response to a virus through preferred self-HLA determinants.     

Changes in antigenic profile during culture of Neoparamoeba sp., causative agent of amoebic gill disease in Atlantic salmon

Amoebic gill disease (AGD), the most serious infectious disease affecting farmed salmon in Tasmania, is caused by free-living marine amoeba Neoparamoeba sp. The parasites on the gills induce proliferation of epithelial cells initiating a hyperplastic response and reducing the surface area available for gaseous exchange. AGD can be induced in salmon by exposure to freshly isolated Neoparamoeba from AGD infected fish, however cultured Neoparamoeba are non-infective. We describe here antigenic differences between freshly isolated and in vitro cultured parasites, and within individual isolates of the parasite cultured under different conditions. Immunoblot analysis using polyclonal antisera, revealed differences in the antigen profiles of two cultured isolates of Neoparamoeba sp. when they were grown on agar versus in liquid medium. However, the antigen profiles of the two isolates were very similar when they were grown under the same culture conditions. Comparison of these antigen profiles with a preparation from parasites freshly isolated from infected gills revealed a very limited number of shared antigens. In addition monoclonal antibodies (mAbs) raised against surface antigens of cultured parasites were used in an indirect immunofluorescence assay to assess the expression of specific surface antigens of Neoparamoeba sp. after various periods in culture. Significant changes in antigen expression of freshly isolated parasites were observed after 15 days of in vitro culture. The use of mAb demonstrated progressive exposure/expression of individual antigens on the surface of the freshly isolated parasites during the period in culture.     

Antigenic analysis of gamonts of Hepatozoon canis purified from leukocytes

Hepatozoon canis is a tick-borne apicomplexan parasite of dogs that infects neutrophils and parenchymal tissues. To study the antigenic characteristics of this parasite, a technique was devised for the purification of gamonts from peripheral blood neutrophils. White blood cells were separated on Ficoll-Hypaque density gradients and the gamonts were released from the host neutrophils by nitrogen cavitation. The blood used for purification originated from dogs with natural or experimental infections of H. canis with a parasitemia of 1.4-33%. The number of parasites collected ranged from 1.5 X 10(6) to 4.2 X 10(7). Portions of purified gamonts were separated and examined under phase and scanning electron microscopy, and the remaining purified parasites were then used as a source of antigens to characterize the humoral immune response by western blot analysis. Serum antibodies from infected dogs recognized more than 15 gamont antigens, and the antigenic patterns observed with sera from naturally and experimentally infected dogs were nearly similar. Four immunodominant protein bands of relative molecular weights of 107, 88, 63, and 28 kDa were recognized by all of the sera examined. The technique applied here for the isolation of host cell-free gamonts will facilitate studies on antigenic composition and immune responses against H. canis and on antigenic relationships between Hepatozoon from different host species and geographic regions.     

Vaccination with carbohydrate peptide mimotopes promotes anti-tumor responses.

Tumor-associated carbohydrate (TAC) antigens are important targets in cancer vaccine efforts. Carbohydrates are, however, frequently poor immunogens, in that they are T-cell-independent antigens. Molecular mimicry of TAC by peptides is an alternative approach to generating anti-carbohydrate immune responses. Here we demonstrate that peptide mimotopes can elicit antibody responses that cross-react with representative human TAC antigens. Primary immunization with such a multiple antigenic peptide, along with QS-21 as adjuvant, elicits cytotoxic antibodies reactive with naturally occurring forms of TAC expressed on tumor cells, and vaccination of mice with peptide mimotopes reduced tumor growth and prolonged host survival in a murine tumor model.     

Molecular Mimics of the Tumour Antigen MUC1

A key requirement for the development of cancer immunotherapy is the identification of tumour-associated antigens that are differentially or exclusively expressed on the tumour and recognized by the host immune system. However, immune responses to such antigens are often muted or lacking due to the antigens being recognized as “self”, and further complicated by the tumour environment and regulation of immune cells within. In an effort to circumvent the lack of immune responses to tumour antigens, we have devised a strategy to develop potential synthetic immunogens. The strategy, termed mirror image phage display, is based on the concept of molecular mimicry as demonstrated by the idiotype/anti-idiotype paradigm in the immune system. Here as ‘proof of principle’ we have selected molecular mimics of the well-characterised tumour associated antigen, the human mucin1 protein (MUC1) from two different peptide phage display libraries. The putative mimics were compared in structure and function to that of the native antigen. Our results demonstrate that several of the mimic peptides display T-cell stimulation activity in vitro when presented by matured dendritic cells. The mimic peptides and the native MUC1 antigenic epitopes can cross-stimulate T-cells. The data also indicate that sequence homology and/or chemical properties to the original epitope are not the sole determining factors for the observed immunostimulatory activity of the mimic peptides.     

The tissue level of the organization of the parasite-host system]

The paper represents a review and analysis of different trends in the evolution of tissue parasitism. Data are given on different ways of departure of parasites from the host's immune response, inhibition of its immunity, antigenic mimicry, capsule formation. The paper considers in detail the mechanism of origin, structure and functions of capsules, which are induced by various groups of parasites in vertebrate and invertebrate hosts. Uniformity in the structure of capsules, which are formed in different host-parasite systems, suggests the universality of mechanisms ensuring their formation.     

Understanding original antigenic sin in influenza with a dynamical system

Original antigenic sin is the phenomenon in which prior exposure to an antigen leads to a subsequent suboptimal immune response to a related antigen. Immune memory normally allows for an improved and rapid response to antigens previously seen and is the mechanism by which vaccination works. I here develop a dynamical system model of the mechanism of original antigenic sin in influenza, clarifying and explaining the detailed spin-glass treatment of original antigenic sin. The dynamical system describes the viral load, the quantities of healthy and infected epithelial cells, the concentrations of naïve and memory antibodies, and the affinities of naïve and memory antibodies. I give explicit correspondences between the microscopic variables of the spin-glass model and those of the present dynamical system model. The dynamical system model reproduces the phenomenon of original antigenic sin and describes how a competition between different types of B cells compromises the overall effect of immune response. I illustrate the competition between the naïve and the memory antibodies as a function of the antigenic distance between the initial and subsequent antigens. The suboptimal immune response caused by original antigenic sin is observed when the host is exposed to an antigen which has intermediate antigenic distance to a second antigen previously recognized by the host''s immune system.     

Why Helicobacter pylori has Lewis antigens

In mimicry with human gastric epithelial cells, the lipopolysaccharide of Helicobacter pylori expresses Lewis blood group antigens. Recent data suggest that molecular mimicry does not promote immune evasion, nor does it lead to induction of autoantibodies, but that H. pylori Lewis X mediates adhesion to gastric epithelial cells and is essential for colonization.     

A new small cell lung cancer (SCLC)-specific marker discovered through antigenic subtraction of neuroblastoma cells

Small cell lung cancer (SCLC) is an aggressive form of lung cancer associated with cigarette smoking and presently accounts for approximately 20% of all lung cancer cases. SCLC cells derive from a neuroendocrine origin and therefore their antigenic profile coincides, to a great extent, with that of neuroendocrine cells. Multiple attempts to generate SCLC-specific MoAbs during the past decade have failed because all SCLC-specific MoAbs isolated also react against neuroendocrine tissues or normal immune cells. Cross-reactivity with normal antigens raises safety concerns due to the inevitable toxicity of such interactions and the dreaded effects. The concept of DIAAD trade mark ( Differential Immunization for Antigen and Antibody Discovery) provides for an immune response that can be effectively focused on cancer antigens. The object is to overcome obstacles resulting from an antigenic hierarchical pattern biased towards a response to dominant antigens in order to induce a robust immune response to cancer antigens. Cancer antigens are weak or nonimmunogenic molecules. Due to the fact that the immune system responds more strongly to immunodominant antigens than to weak immunogenic antigens, cancer cell proliferation is unencumbered. DIAAD employs protocols of induction of tolerance and immunity, conducted in sequential order to "biologically subtract" the immune response of dominant antigens expressed by normal cells. This biological subtraction is achieved in a laboratory animal by first eliminating the immune response to the normal cells or closely related cancer cells, followed by immunization of the same laboratory animal with diseased cells. This procedure directs the immune response exclusively towards antigens expressed by the diseased and not the normal cells. Our objective was to use DIAAD to generate monoclonal antibodies specific to SCLC antigens that are not shared by neuroendocrine cells by contrasting a pool of human SCLC cell lines with a pool of human neuroendocrine cancer cell lines. Four monoclonal antibodies reacted strongly and exclusively with SCLC cells and identified a membrane molecule comprising a single chain glycoprotein. Two of four antibodies were selected for a detailed analysis that revealed a narrow tissue specificity of antigen expressed by colon, lung, and pancreatic cancers (less than 20% staining was found on breast, ovarian and prostate cancer). These antibodies did not bind to various other cancers such as kidney, carcinoid, lymphoma, sarcoma, adrenal, liver, melanoma, seminoma, leiomyoma, basal cell cancer, or undifferentiated cancer. The epitope recognized by the selected MoAbs was destroyed with the removal of carbohydrates from SCLC cells. This result does not exclude the possibility of protein-carbohydrate cooperation in epitope recognition. However, it strongly suggests the pivotal role of carbohydrates in antibody binding to this molecule. Upon binding to the extracellular molecule on SCLC cells, the antibodies were shown to internalize. A low or insignificant level of internalization was recorded following incubation of the antibodies with neuroendocrine-derived tumors. The capacity of these antibodies to internalize upon binding the extracellular receptors renders them potential candidates for prodrug or immunotoxin-targeted therapeutics. In a qualitative experiment involving immunoaffinity purification, the SCLC antigen was shown to be differentially detected in sera of SCLC patients. Plans are being generated to explore the possible utility of this novel SCLC-specific antigen recognized by the above MoAbs as a new biomarker for early diagnosis of the disease, as well as for therapeutic intervention for SCLC.     

Antigenic affinity of meningococci and human A- and B-group erythrocytes

The use of the modified method of isohemagglutinin adsorption by microbial antigens in experiments with the causative agent of meningococcal infection has led, for the first time, to the detection of meningococcal antigens affined to the antigens of human erythrocytes, groups A and B. The antigenic affinity of group A erythrocytes and meningococci has proved to be more pronounced in meningococcal strains isolated from the spinal fluid of patients than in cultures obtained from the nasopharynx of healthy persons. The detection of the affinity of these antigens makes it possible to explain the mechanism of differences in the susceptibility of persons with different blood groups to meningococcal infection by "antigenic mimicry".     

Compositional bias and mimicry toward the nonself proteome in immunodominant T cell epitopes of self and nonself antigens.

We investigated whether and how molecular mimicry affects the shaping of the helper T cell repertoire. We implemented an algorithm that measures the probability of mimicry between epitopes of known immunogenicity and self or nonself proteomes. This algorithm yields 'similarity profiles', which represent the probability of matching between all contiguous overlapping peptides of the antigen under examination and those in the proteome(s) considered. Similarity profiles between helper T cell epitopes (of self or microbial antigens and allergens) and human or microbial SWISSPROT collections were produced. For each antigen, both collections yielded largely overlapping profiles, demonstrating that self-nonself discrimination does not rely on qualitative features that distinguish human from microbial peptides. However, epitopes whose probability of mimicry with self or nonself prevails are, respectively, tolerated or immunodominant and coexist within the same (auto-)antigen regardless of its self/nonself nature. Epitopes (on self and nonself antigens) can cross-stimulate T cells at increasing potency as their similarity with nonself augments. Mimicry, rather than complicating self-nonself discrimination, assists in the shaping of the immune repertoire and helps define the defensive or autoreactive potential of a T cell. Being a predictor of epitope immunogenicity, it bears relevance to vaccine design.     

Babesia bovis: bovine helper T cell lines reactive with soluble and membrane antigens of merozoites.

Babesia bovis-specific T cell lines were established from cattle infected with either tick-derived or cultured parasites by stimulation of peripheral blood mononuclear cells with a crude parasite membrane fraction. Induction and enrichment of CD4+ T cells occurred over time. All cell lines responded vigorously and in a dose-dependent, MHC-restricted manner to intact merozoites, and to soluble and membrane fractions derived from merozoites by homogenization and high-speed centrifugation. Solubilization of the membrane fraction with nondenaturing zwitterionic or nonionic detergents yielded antigenic extracts which also stimulated the T cells. However, a differential response was observed, in that cell lines from one animal proliferated vigorously to the detergent extracts of the membrane fraction, whereas cell lines from a second animal proliferated only weakly to these extracts. SDS-PAGE analysis revealed common protein bands of 90 and 22 kDa in the various immunogenic fractions. Cell lines from the animal infected with cultured parasites also responded to parasite culture supernatant "exoantigens" and to the related parasite, Babesia bigemina. We conclude that antigens present in merozoite membranes and soluble parasite extracts preferentially stimulate CD4+ T cells from cattle immune to Babesia bovis. The differential pattern of response of T cell lines from different cattle suggests that more than one protein or epitope is immunodominant for T cells.     

Immunologic and survival studies in mice immunised with cryodestroyed ascites fibrosarcoma (AFS) cells

Adult mice weighing 20 g, divided into 5 groups, were immunised with single and three cycle cryodestroyed ascites fibrosarcoma cells using different modes of immunisation with respect to dose and administration frequency. Survival against subsequent challenge with the same tumour cells and immune response given by leucocyte migration inhibition, were studied in these animals. It appears from the present study that animals with relatively low antigenic load have a significantly (P less than 0.001) high mean survival time and/or survival index compared with controls than those with relatively high antigenic load. Survival in animals immunised with three cycle cryodestroyed tumour cells is significantly (P less than 0.001) higher than that with single cycle even in face of a greater challenging tumour dose with mode of immunisation remaining the same. Preliminary observations on percentage leucocyte migration inhibition showed that there exists no difference in percentage inhibition with either viable or cryodestroyed tumour cell antigens added to migration chamber excepting in animals immunized with three cycle cryodestroyed tumour cells where percentage inhibition was significantly (P less than 0.001) greater with viable than with cryodestroyed tumour cell antigen without altering the percentage inhibition between the two groups. Factors maximising immune response and their modulating effects are discussed.     

DNA vaccine against malaria: a long way to go

Vaccination is the attempt to mimic certain aspects of an infection for the purpose of causing an immune response that will protect the individual from that infection. Malaria, a disease responsible for immense human suffering, is caused by infection with Plasmodium spp. parasites, which have a very complex life cycle--antigenically unique stages infect different tissues of the body. It is a parasitic disease for which no successful vaccine has been developed so far, despite considerable efforts to develop a subunit vaccine that offers protective immunity. Due to the spread of drug-resistant malaria, efforts to develop an effective vaccine have become increasingly critical. DNA vaccination provides a stable and long-lived source of protein vaccine capable of inducing both antibody- and cell-mediated immune responses to a wide variety of antigens. Injected DNA enters the cells of the host and makes the protein, which triggers the immune response. According to present needs, the flexibility of DNA vaccine technology permits the combination of multiple antigens from both the preerythrocytic and erythrocytic stages of malaria parasite. DNA vaccines with genes coding for different antigenic parts of malaria proteins have been created and presently some of these are undergoing field trials. The results from these trials will help to determine the likelihood of success of this technology in humans. This review presents an update of the studies carried out in malaria using DNA vaccine approach, the challenges, and the future prospects.     

How does autoimmunity cause tumor regression? A potential mechanism involving cross-reaction through epitope mimicry.

BACKGROUND: Although the exact mechanisms mediating the initiation of autoimmune diseases are unknown, sequence similarity between infectious agents and self-proteins (epitope mimicry) has been proposed as the main trigger mechanism. Interestingly, this mechanism of epitope mimicry may also evoke potent tumor immunity. Indeed, experimental data support a beneficial role of autoimmunity in some patients with cancer. Additionally, autoimmunity induced via vaccination with xenogeneic antigens was found to be effective. Thus, the ability to manipulate the immune system via immunologic cross-reactions should have important potential in both preventive and therapeutic strategies for cancer. This strategy may down the friendly established relationship between tumor tissues and the cells of the immune system.     

Induction of protective immunity against Schistosoma mansoni by a nonliving vaccine is dependent on the method of antigen presentation

A preparation of nonliving parasite antigens containing both soluble and particulate components of frozen-and-thawed invasive larvae was used to immunize C57BL/6J mice against challenge Schistosoma mansoni infection. The method of antigen presentation was observed to be critical to the ability of this preparation to induce protective immunity, because intradermal administration in conjunction with a bacterial adjuvant (BCG) resulted in strong protection against challenge parasites (51% reduction in worm burden in six experiments), whereas i.v. injection of the same antigenic preparation was completely ineffective. Induction of resistance was accompanied by specific immune responsiveness toward schistosome antigens. Protection correlated more closely with sensitization for specific delayed hypersensitivity than with elicitation of circulating antibodies to larval surface antigens or immediate hypersensitivity in these models. These results suggest that it will be possible to design a defined vaccine against S. mansoni infection, but that identification of the route of antigen presentation that most effectively elicits relevant immune effector mechanisms will be crucial to the success of any vaccination protocol involving nonliving antigens.