Malaria vaccine

Summary Among infectious diseases caused by protozoa, malaria is still the greatest killer of children. Mortality in adults living in endemic areas is significantly lower because they frequently acquire partial or complete immunity to the major pathogen,Plasmodium falciparum. This natural protection indicates that vaccination may be possible, and the first candidate antigens were cloned with the use of human immune sera as probes. Genetic and biochemical analysis of the parasite proteins revealed that they are polymorphic, and frequently gene sequences were discovered which were specific for a particular parasite isolate, which eliminated most antigens for purposes of vaccine development. The most promising candidate antigens today are the major surface proteins of sporozoites and blood stage parasites. However, the immune response against those is not sufficient for complete protection, and additional, intensive research is necessary to identify new molecules to be included in a vaccine cocktail against malaria. The current spread of the disease due to increasing drug resistance of parasites and mosquito vectors emphasizes the urgent need for a vaccine.     

Molecular parasitology: progress towards the development of vaccines for malaria, filariasis, and schistosomiasis

Advances in molecular biology have allowed for the identification of potential vaccine candidates against several parasitic diseases. Antigens from various life stages of Plasmodium and Schistosoma species and filarial worms have been cloned, sequenced and tested as vaccines. Results to date in animal models have been promising. Modest levels of protection against experimental human malaria have been obtained using both sporozoite and blood-stage antigens. However, a greater understanding of the mechanisms which lead to immunity against parasites is required before effective vaccines can be developed.     

Molecular parasitology: Progress towards the development of vaccines for malaria,filariasis, and schistosomiasis

Summary Advances in molecular biology have allowed for the identification of potential vaccine candidates against several parasitic diseases. Antigens from various life stages ofPlasmodium andSchistosoma species and filarial worms have been cloned, sequenced and tested as vaccines. Results to date in animal models have been promising. Modest levels of protection against experimental human malaria have been obtained using both sporozoite and blood-stage antigens. However, a greater understanding of the mechanisms which lead to immunity against parasites is required before effective vaccines can be developed.     

Our impasse in developing a malaria vaccine

Malaria presents a challenge to world health that to date has been beyond the abilities of researchers to conquer. This critique presents some of the strategies employed by the parasite to overcome immunity and the immunological challenges that we face to develop vaccines. A conclusion is that a vaccine must identify novel antigens or epitopes that are not normally immunogenic and which are therefore not under immune pressure and most likely to be conserved between different strains. Such antigens are most likely to be targets of cellular immunity. The case for a whole parasite blood stage vaccine is presented based on these premises.     

Immunoglobulin-binding proteins in ticks: new target for vaccine development against a blood-feeding parasite

Humans have a long history of trying to control ticks. At first, attempts focused on modifying the habitat, whereas later efforts relied heavily on the use of chemicals. Current research is directed at finding a vaccine against ticks. A strategy of targeting 'concealed antigens' succeeded with the first commercialised vaccine against the cattle tick Boophilus microplus. However, vaccine development against other tick species appears unsatisfactory to date. Vaccination depends on a specific antibody-mediated immunoreaction that damages the parasite. Immunoglobulin molecules of vertebrate hosts can pass through gut barriers into the haemolymph of ectoparasites while retaining antibody activity. Research on the ixodid tick Rhipicephalus appendiculatus revealed that host immunoglobulin-G in the parasite was excreted via salivation, during feeding. Immunoglobulin-binding proteins in tick haemolymph and salivary glands are thought to be responsible for such excretion. The discovery of an immunoglobulin excretion system in ticks indicates that they have a highly developed mechanism to protect themselves from their host's antibody attack. Such a mechanism questions whether immunization strategies will be effective against ticks, unless they circumvent or disable the ticks' immunoglobulin excretion system.     

Mechanisms of cellular invasion by intracellular parasites

Numerous disease-causing parasites must invade host cells in order to prosper. Collectively, such pathogens are responsible for a staggering amount of human sickness and death throughout the world. Leishmaniasis, Chagas disease, toxoplasmosis, and malaria are neglected diseases and therefore are linked to socio-economical and geographical factors, affecting well-over half the world’s population. Such obligate intracellular parasites have co-evolved with humans to establish a complexity of specific molecular parasite–host cell interactions, forming the basis of the parasite’s cellular tropism. They make use of such interactions to invade host cells as a means to migrate through various tissues, to evade the host immune system, and to undergo intracellular replication. These cellular migration and invasion events are absolutely essential for the completion of the lifecycles of these parasites and lead to their for disease pathogenesis. This review is an overview of the molecular mechanisms of protozoan parasite invasion of host cells and discussion of therapeutic strategies, which could be developed by targeting these invasion pathways. Specifically, we focus on four species of protozoan parasites Leishmania, Trypanosoma cruzi, Plasmodium, and Toxoplasma, which are responsible for significant morbidity and mortality.     

Schistosome vaccines.

Schistosomiasis control currently relies primarily on chemotherapy which is both expensive and temporary. There is an urgent need for an effective vaccine. Studies in animal models and man have demonstrated the existence of protective immunity. Antibody-dependent cell-mediated cytotoxicity mechanisms involving eosinophils and macrophages have been implemented in destruction of the parasites. Antigens expressed on the surface of the schistosomulum are among the targets of protective immune responses. Vaccines comprising recombinant antigens are now being tested in vivo for their capacity to evoke protective responses. Live oral vaccines based on attenuated Salmonella expressing schistosomular surface antigens are being developed.     

Schistosome vaccines

Summary Schistosomiasis control currently relies primarily on chemotherapy which is both expensive and temporary. There is an urgent need for an effective vaccine. Studies in animal models and man have demonstrated the existence of protective immunity. Antibody-dependent cell-mediated cytotoxicity mechanisms involving eosinophils and macrophages have been implemented in destruction of the parasites. Antigens expressed on the surface of the schistosomulum are among the targets of protective immune responses. Vaccines comprising recombinant antigens are now being tested in vivo for their capacity to evoke protective responses. Live oral vaccines based on attenuatedSalmonella expressing schistosomular surface antigens are being developed.     

Progress in molecular parasitology

Substantial progress has been made in the last ten years in understanding the structural and functional organization of parasitic protozoa and helminths and the complex physiological relationships that exist between these organisms and their hosts. By employing the new powerful techniques of biochemistry, molecular biology and immunology the genomic organization in parasites, the molecular basis of parasite's variation in surface antigens and the biosynthesis, processing, transport and membrane anchoring of these and other surface proteins were extensively investigated. Significant advances have also been made in our knowledge of the specific and often peculiar strategies of intermediary metabolism, cell compartmentation, the role of oxygen for parasites and the mechanisms of antiparasitic drug action. Further major fields of interest are currently the complex processes which enables parasites to evade the host's immune defense system and other mechanisms which have resulted in the specific adaptations which enabled parasites to survive within their host environments. Various approaches in molecular and biochemical parasitology and in immunoparasitology have been proven to be of high potential for serodiagnosis, immunoprophylaxis and drug design.     

Progress in molecular parasitology

Summary Substantial progress has been made in the last ten years in understanding the structural and functional organization of parasitic protozoa and helminths and the complex physiological relationships that exist between these organisms and their hosts. By employing the new powerful techniques of biochemistry, molecular biology and immunology the genomic organization in parasites, the molecular basis of parasite's variation in surface antigens and the biosynthesis, processing, transport and membrane anchoring of these and other surface proteins were extensively investigated. Significant advances have also been made in our knowledge of the specific and often peculiar strategies of intermediary metabolism, cell compartmentation, the role of oxygen for parasites and the mechanisms of antiparasitic drug action. Further major fields of interest are currently the complex processes which enables parasites to evade the host's immune defense system and other mechanisms which have resulted in the specific adaptations which enabled parasites to survive within their host environments. Various approaches in molecular and biochemical parasitology and in immunoparasitology have been proven to be of high potential for serodiagnosis, immunoprophylaxis and drug design.This paper is based on a review presented at a workshop on Molecular Parasitology, organized by the Swiss Society of Tropical Medecine and Parasitology at the University of Neuchâtel, March 1985.     

Development of malaria parasites in mosquitoes fed with ookinetes suspended in defined media

Information concerning the specific nutritional requirements of malarial parasites developing in the mosquito host has been difficult to obtain, owing primarily to the complex nature of the blood meal that accompanies the parasites and the lack of success in culturing the complete invertebrate cycle of Plasmodium in vitro. The present report describes a blood-free system for infecting mosquitoes with ookinetes of Plasmodium berghei and for allowing the latter to develop into infective sporozoites. Ookinetes cultured in vitro were separated from blood proteins, suspended in defined medium, and fed to Anopheles stephensi mosquitoes through a membrane. The mosquitoes were then maintained on the same defined medium plus 5% sucrose. Infectivity of the parasites was demonstrated 17-19 days later by intracardial inoculation of the macerated mosquitoes into hamsters. This system makes it possible to evaluate nutritional factors that affect parasite development in the mosquito host under controlled conditions.     

Hsp70 in parasites: as an inducible protective protein and as an antigen

The heat shock (HS) response is a general homeostatic mechanism that protects cells and the entire organism from the deleterious effects of environmental stresses. It has been demonstrated that heat shock proteins (HSP) play major roles in many cellular processes, and have a unique role in several areas of cell biology, from chronic degenerative diseases to immunology, from cancer research to interaction between host and parasites. This review deals with thehsp70 gene family and with its protein product, hsp70, as an antigen when pathogens infect humans. Members of HSP have been shown to be major antigens of many pathogenic organisms when they experience a major temperature shift upwards at the onset of infection and become targets for host B and T cells.     

Pathogenesis of Plasmodium falciparum malaria: the roles of parasite adhesion and antigenic variation

Malaria results in up to 2.5 million deaths annually, with young children and pregnant women at greatest risk. The great majority of severe disease is caused by Plasmodium falciparum. A characteristic feature of infection with P. falciparum is the accumulation or sequestration of parasite-infected red blood cells (RBCs) in various organs, such as the brain, lung and placenta, and together with other factors is important in the pathogenesis of severe forms of malaria. Sequestration results from adhesive interactions between parasite-derived proteins expressed on the surface of infected RBCs and a number of host molecules on the surface of endothelial cells, placental cells and uninfected RBCs. Some receptors for parasite adhesion have been implicated in particular malaria syndromes, such as intercellular adhesion molecule 1 in cerebral malaria and chondroitin sulfate A and hyaluronic acid in placental infection. The principal parasite ligand and antigen on the RBC surface, P. falciparum erythrocyte membrane protein 1 encoded by a multigene family termed var, is clonally variant, enabling evasion of specific immune responses. An understanding of these host-parasite interactions in the context of clinical disease and immunity may reveal potential targets to prevent or treat severe forms of malaria. Received 25 June 2001; received after revision 22 August 2001; accepted 24 August 2001     

The macromolecular peptide-loading complex in MHC class I-dependent antigen presentation

A challenging task for the adaptive immune system of vertebrates is to identify and eliminate intracellular antigens. Therefore a highly specialized antigen presentation machinery has evolved to display fragments of newly synthesized proteins to effector cells of the immune system at the cell surface. After proteasomal degradation of unwanted proteins or defective ribosome products, resulting peptides are translocated into the endoplasmic reticulum by the transporter associated with antigen processing and loaded onto major histocompatibility complex (MHC) class I molecules. Peptide-MHC I complexes are transported via the secretory pathway to the cell surface where they are then inspected by cytotoxic T lymphocytes, which can trigger an immune response. This review summarizes the current view of the intracellular machinery of antigen processing and of viral immune escape mechanisms to circumvent destruction by the host. Received 4 October 2005; received after revision 19 November 2005; accepted 24 November 2005     

Surface antigens of Plasmodium falciparum-infected erythrocytes as immune targets and malaria vaccine candidates

Understanding the targets and mechanisms of human immunity to malaria caused by Plasmodium falciparum is crucial for advancing effective vaccines and developing tools for measuring immunity and exposure in populations. Acquired immunity to malaria predominantly targets the blood stage of infection when merozoites of Plasmodium spp. infect erythrocytes and replicate within them. During the intra-erythrocytic development of P. falciparum, numerous parasite-derived antigens are expressed on the surface of infected erythrocytes (IEs). These antigens enable P. falciparum-IEs to adhere in the vasculature and accumulate in multiple organs, which is a key process in the pathogenesis of disease. IE surface antigens, often referred to as variant surface antigens, are important targets of acquired protective immunity and include PfEMP1, RIFIN, STEVOR and SURFIN. These antigens are highly polymorphic and encoded by multigene families, which generate substantial antigenic diversity to mediate immune evasion. The most important immune target appears to be PfEMP1, which is a major ligand for vascular adhesion and sequestration of IEs. Studies are beginning to identify specific variants of PfEMP1 linked to disease pathogenesis that may be suitable for vaccine development, but overcoming antigenic diversity in PfEMP1 remains a major challenge. Much less is known about other surface antigens, or antigens on the surface of gametocyte-IEs, the effector mechanisms that mediate immunity, and how immunity is acquired and maintained over time; these are important topics for future research.     

Molecular diagnosis of parasites

Summary New developments in molecular biology have generated exciting possibilities for improved diagnosis of parasitic diseases. Through gene clonign and expression and peptide synthesis, defined parasite antigens can be produced in vitro for use in serodiagnosis, while nuclear hybridization techniques offer a vastly improved approach to identification of parasites in the tissue specimens of infected hosts as a means of diagnosis. Furthermore, the advent of the polymerase chain reaction technique has made it possible to increase the sensitivity of nuclear hybridization techniques, through amplification of target DNA sequences of the parasites in test material, by in situ synthesis of these sequences prior to hybridization with the diagnostic probe. Finally, through the use of monoclonal antibody technology, it is possible to design highly specific and sensitive serological assays, as well as assays for parasite antigen detection in tissue fluids and in the excreta of infected hosts, as a means of diagnosis.     

Molecular diagnosis of parasites

New developments in molecular biology have generated exciting possibilities for improved diagnosis of parasitic diseases. Through gene cloning and expression and peptide synthesis, defined parasite antigens can be produced in vitro for use in serodiagnosis, while nuclear hybridization techniques offer a vastly improved approach to identification of parasites in the tissue specimens of infected hosts as a means of diagnosis. Furthermore, the advent of the polymerase chain reaction technique has made it possible to increase the sensitivity of nuclear hybridization techniques, through amplification of target DNA sequences of the parasites in test material, by in situ synthesis of these sequences prior to hybridization with the diagnostic probe. Finally, through the use of monoclonal antibody technology, it is possible to design highly specific and sensitive serological assays, as well as assays for parasite antigen detection in tissue fluids and in the excreta of infected hosts, as a means of diagnosis.     

Casein, a prohormone with an immunomodulating role for the newborn?

Maternal colostrum and milk, the earliest food of the newborn, should not only be considered as supplying nutrients, but also as agents providing protection against aggressions from the new environment. Indeed by enzymatic digestion of the main milk proteins, the caseins, biologically active peptides are released; they may be implicated in the stimulation of the newborn's immune system. From this point of view a 'strategic active zone' has been characterized in beta-casein. A possible role of casein as a 'prohormone' for the newborn is suggested.