Giardia duodenalis: the double-edged sword of immune responses in giardiasis

Giardiasis is one of the most common intestinal protozoan infections worldwide. The etiological agent, Giardia duodenalis (syn. Giardia lamblia, Giardia intestinalis), is a flagellated, binucleated protozoan parasite which infects a wide array of mammalian hosts (Adam, 2001). The symptoms of giardiasis include abdominal cramps, nausea, and acute or chronic diarrhea, with malabsorption and failure of children to thrive occurring in both sub-clinical and symptomatic disease (Thompson et al., 1993). Infections are transmitted by cysts which are excreted in the feces of infected humans and animals. Human giardiasis is distributed worldwide, with rates of detection between 2-5% in the developed world and 20-30% in the developing nations (Farthing, 1994). There is significant variation in the outcome of Giardia infections. Most infections are self-limiting, although re-infection is common in endemic areas and chronic infections also occur. Moreover, some individuals suffer from severe cramps, nausea and diarrhea while others escape these overt symptoms. This review will describe recent advances in parasite genetics and host immunity that are helping to shed light on this variability.

Imaging of the host/parasite interplay in cutaneous leishmaniasis

An understanding of host–parasite interplay is essential for the development of therapeutics and vaccines. Immunoparasitologists have learned a great deal from ‘conventional’ in vitro and in vivo approaches, but recent developments in imaging technologies have provided us (immunologists and parasitologists) with the ability to ask new and exciting questions about the dynamic nature of the parasite–immune system interface. These studies are providing us with new insights into the mechanisms involved in the initiation of a Leishmania infection and the consequent induction and regulation of the immune response. Here, we review some of the recent developments and discuss how these observations can be further developed to understand the immunology of cutaneous Leishmania infection in vivo.

[Approaches and problems in vaccine development against leishmaniasis]

Leishmaniasis is a major public health problem of the world and Turkey. Recently there has been increasing interest in vaccine studies among strategies for control of leishmaniasis. Recently the increase of interest in vaccine studies among leishmaniasis control strategies makes the subject more up to date. So the aim of this review is to present information about recent vaccine studies, problems and new strategies for vaccine development studies. There are 3 generations of vaccine against leishmaniasis. First-generation vaccines are killed or live attenuated parasites; second-generation vaccines are recombinant or native antigens and live genetically modified parasites (knock out and suicidal cassettes), third generation vaccines are DNA vaccines. Also vector salivary proteins, dendritic cells and non-pathogenic L. tarentolae have been used as vaccine candidates. However there is still no effective vaccine against leishmaniasis. Since polymer conjugates considerably increase immunogenicity, polymer based vaccine studies have gained importance in recent years. However, there has not been such a study for an antileishmanial vaccine yet. LPG, surface antigen of Leishmania promastigotes, and polymer conjugates may be promising in antileishmanial vaccine studies so we are carrying out a TUBITAK Project on this subject which has been given the number, 1085170SBAG-4007.

Do antenatal parasite infections devalue childhood vaccination?

On a global basis, both potent vaccine efficacy and high vaccine coverage are necessary to control and eliminate vaccine-preventable diseases. Emerging evidence from animal and human studies suggest that neglected tropical diseases (NTDs) significantly impair response to standard childhood immunizations. A review of efficacy and effectiveness studies of vaccination among individuals with chronic parasitic infections was conducted, using PUBMED database searches and analysis of data from the authors' published and unpublished studies. Both animal models and human studies suggest that chronic trematode, nematode, and protozoan infections can result in decreased vaccine efficacy. Among pregnant women, who in developing countries are often infected with multiple parasites, soluble parasite antigens have been shown to cross the placenta and prime or tolerize fetal immune responses. As a result, antenatal infections can have a significant impact on later vaccine responses. Acquired childhood parasitic infections, most commonly malaria, can also affect subsequent immune response to vaccination. Additional data suggest that antiparasite therapy can improve the effectiveness of several human vaccines. Emerging evidence demonstrates that both antenatal and childhood parasitic infections alter levels of protective immune response to routine vaccinations. Successful antiparasite treatment may prevent immunomodulation caused by parasitic antigens during pregnancy and early childhood and may improve vaccine efficacy. Future research should highlight the varied effects that different parasites (alone and in combination) can have on human vaccine-related immunity. To optimize vaccine effectiveness in developing countries, better control of chronic NTDs may prove imperative.

New insights in toxoplasmosis immunology during pregnancy. Perspective for vaccine prevention

Toxoplasma gondii is one of the few pathogens that can cross the placenta. Frequency and severity of transmission vary with gestational age. While acquired toxoplasmosis is already well explored, the control of maternal-foetal transmission of the parasite remains almost unknown. This is partly due to inherent inadequacies of animal models. This review summarises the studies which have been undertaken and shows that the mouse is a valuable model despite obvious differences to the human case. The paramount role of the cellular immune response during primary infection has been consistently shown. Surprisingly, IFN-g has a dual role in this process. While its beneficial effects in the control of toxoplasmosis are well known, it also seems to have transmission-enhancing effects within the placenta and can also directly harm the developing foetus. This shows the importance of designing vaccines which protects both mother and foetus. Therefore, it is useful to study the mechanisms of natural resistance against transmission during a secondary infection. In this setting, the process is more complicated, involving cellular, but also humoral components of the immune system. In summary, even if the whole process is far from being elucidated, important insights have been gained so far which will help us to undertake rational vaccine research.

The challenges of Chagas Disease-- grim outlook or glimmer of hope

The authors discuss the key challenges that undermine the control of Chagas disease and that must be urgently addressed to ensure long-term, sustainable control.

Ovine toxoplasmosis: transmission, clinical outcome and control

Toxoplasma gondii is a significant cause of abortion in sheep. Infection is picked up from the environment and if initiated during pregnancy may cause fetal mortality. Infected sheep remain persistently infected with tissue cysts in brain and muscle (meat), and are also immune and would not be expected to abort again. The live tachyzoite vaccine (Toxovax) protects against abortion and this allows the suggestion that it may also reduce or prevent tissue cyst development in muscle. If this were so it raises the question of whether the vaccine could be used to make meat safer for human consumption.

Current status of veterinary vaccines

The major goals of veterinary vaccines are to improve the health and welfare of companion animals, increase production of livestock in a cost-effective manner, and prevent animal-to-human transmission from both domestic animals and wildlife. These diverse aims have led to different approaches to the development of veterinary vaccines from crude but effective whole-pathogen preparations to molecularly defined subunit vaccines, genetically engineered organisms or chimeras, vectored antigen formulations, and naked DNA injections. The final successful outcome of vaccine research and development is the generation of a product that will be available in the marketplace or that will be used in the field to achieve desired outcomes. As detailed in this review, successful veterinary vaccines have been produced against viral, bacterial, protozoal, and multicellular pathogens, which in many ways have led the field in the application and adaptation of novel technologies. These veterinary vaccines have had, and continue to have, a major impact not only on animal health and production but also on human health through increasing safe food supplies and preventing animal-to-human transmission of infectious diseases. The continued interaction between animals and human researchers and health professionals will be of major importance for adapting new technologies, providing animal models of disease, and confronting new and emerging infectious diseases.

Attenuated vaccines for tropical theileriosis, babesiosis and heartwater: the continuing necessity

Overwhelming evidence has accumulated of the effectiveness of immunization with live attenuated vaccines to control tick-borne diseases of livestock. Despite several disadvantages, vaccination with live attenuated organisms against tropical theileriosis, babesiosis and possibly heartwater constitutes one of the most cost-effective intervention strategies. Although great advances have been made through genomics and proteomics research, this has not yet translated into effective non-living vaccines. As a result, there is a continuing necessity to use available live vaccines in tick and tick-borne disease-control strategies adapted to conditions prevailing in many parts of the world.

Vaccine prospects for amebiasis

Entamoeba histolytica is a eukaryotic protozoan parasite and is the causative agent of amebic colitis and amebic liver abscess. Many insights into the innate and acquired immune responses to infection with E. histolytica have been made in recent years. These findings have provided a foundation for producing a vaccine that could help to prevent the initial establishment of infection in the intestinal wall. The galactose and N-acetyl-D-galactosamine-specific lectin on the surface of the ameba is an immunodominant molecule that is highly conserved and has an integral role in the stimulation of these immune responses. The structure of the lectin has been defined, and the heavy subunit with its cysteine-rich region has been demonstrated in animal models to have some efficacy as a possible vaccine agent for prevention of amebic infection. Finding an ideal animal model of amebic intestinal infection has been difficult, but the C3H mouse and severe combined immunodeficient mouse-human intestinal xenograft models have both provided valuable insights into the first line of immune defense at the mucosal wall of the colon. Providing safe food and water to all people in the developing world is a formidable task that is not achievable in the foreseeable future. However, a vaccine for amebiasis could make a significant impact on the morbidity and mortality from the disease. Many components of the ameba are immunogenic and may serve as targets for a future vaccine, including the galactose and N-acetyl-D-galactosamine lectin, the serine-rich E. histolytica protein, cysteine proteinases, lipophosphoglycans, amebapores and the 29-kDa protein.

Current status and future prospects for a vaccine against American trypanosomiasis

The clinically relevant pathognomonic consequences of human infection by Trypanosoma cruzi are dilation and hypertrophy of the left ventricle walls and thinning of the apex. The major complications and debilitating evolutionary outcomes of chronic infection include ventricular fibrillation, thromboembolism and congestive heart failure. American trypanosomiasis (Chagas disease) poses serious public healthcare and budgetary concerns. The currently available drugs, although effective against acute infection, are highly toxic and ineffective in arresting or attenuating clinical disease symptoms in chronic patients. The development of an efficacious prophylactic vaccine faces many challenges, and progress is slow, despite several years of effort. Studies in animal models and human patients have revealed the pathogenic mechanisms during disease progression, pathology of disease and features of protective immunity. Accordingly, several antigens, antigen-delivery vehicles and adjuvants have been tested in animal models, and some efforts have been successful in controlling infection and disease. This review will summarize the accumulated knowledge about the parasite and disease, as well as pathogenesis and protective immunity. The authors will discuss the efforts to date, and the challenges faced in achieving an efficient prophylactic vaccine against human American trypanosomiasis, and present the future perspectives.

[Challenge of developing anti-parasite vaccines in the tropics]

Much effort has been devoted to developing anti-parasite vaccines. The main obstacles to overcome involve problems in cultivating parasites, variability in antigens of cultivated parasites, insufficient immune responses that do not provide full protection, lack of animal models and difficulty in evaluating immune protection acquired naturally or after vaccination in populations living in endemic areas. Numerous clinical trials have been conducted and several parasite antigens, in particular against malaria, have been tested in endemic areas. Up to now no candidate vaccine has shown sufficient, long-term efficacy to justify its inclusion in public health program. However trials using anti-parasite vaccination under both experimental and field conditions clearly demonstrate that a certain level of clinical immunity against malaria, bilharziasis, and leishmaniasis.

Susceptibility of sexually mature rainbow trout, Oncorhynchus mykiss to experimental cryptobiosis caused by Cryptobia salmositica

Sexually mature rainbow trout, Oncorhynchus mykiss, were highly susceptible to cryptobiosis caused by Cryptobia salmositica. Spawning female trout were more susceptible (higher parasitaemia and mortality) than sexually mature males. Most infected female trout (seven of nine) with eggs died before or shortly after spawning; however, none of the nine infected sexually matured males or the uninfected fish died. There was no significant difference in the severity of the anaemia between infected male and female trout. All infected males developed exophthalmia, while this clinical sign was not seen in any of the infected females nor in uninfected trout. The addition of 17 beta-estradiol (at physiological level or higher) did not enhance in vitro multiplication of the Cryptobia; however, fresh plasma from sexually mature females or males when added to cultures significantly increased in vitro multiplication of the pathogen. In addition, plasma from sexually mature females were significantly better than those from males in promoting in vitro parasite multiplication. Parasite multiplication did not increase after plasma from sexually mature fish were heat inactivated.

Vaccines for amoebiasis: barriers and opportunities

Amoebiasis, infection by the protozoan parasite Entamoeba histolytica, remains a global health problem, despite the availability of effective treatment. While improved sanitation could lead to the eradication of this disease, it is unlikely that this will occur worldwide in the foreseeable future; thus alternative measures must be pursued. One approach is to develop a vaccine to prevent this deadly disease. Clinical studies indicate that mucosal immunity may provide some protection against recurrent intestinal infection with E. histolytica, but there is no clear evidence that protective immunity develops after amoebic liver abscess. Over the past decade, progress in vaccine development has been facilitated by new animal models that allow better testing of potential vaccine candidates and the application of recombinant technology to vaccine design. Oral vaccines and DNA-based vaccines have been successfully tested in animals models for immunogenicity and efficacy. There has been significant progress on a number of fronts, but there are unanswered questions regarding the effectiveness of immune responses in preventing disease in man and, as yet, no testing of any of these vaccines in humans has been performed. In addition, there are strong economic barriers to developing an amoebiasis vaccine and questions about how and where an effective vaccine would be utilized.

Advances in the development of molecular tools for the control of bovine babesiosis in Mexico

The severe negative impact that bovine babesiosis has in the Mexican cattle industry has not been ameliorated basically due to the lack of safe and effective commercially available vaccines and sensitive and reliable diagnostic tests. In recent years, the Bovine Babesiosis Laboratory at the National Center for Disciplinary Research in Veterinary Parasitology-INIFAP in Morelos State, Mexico has been directing efforts towards three main research areas: (1) The development of in vitro culture-derived, improved and safer live vaccines. This has been done in two ways: using gamma-irradiated bovine serum and erythrocytes for the in vitro culture of vaccine strains, which reduces the risk of contaminating pathogens, and improving the immune response, by the addition of L. casei, a strong stimulant of the innate immune system. (2) The study of antigens considered as vaccine candidates with the goal of developing a recombinant vaccine that suits the country's needs. Knowing their degree of conservation or variation in Mexican isolates, their phylogenetic relationship and their protective, immuno-stimulatory properties, are first steps towards that goal. (3) The development of new tools for diagnosis, detection and discrimination of bovine babesiosis is the third area. Developing variants of ELISA, which are more reliable than the currently used IFAT, are a priority, and finally, taking advantage of the genomes of Babesia bigemina, and B. bovis, we are identifying genes than allow us to discriminate isolates using molecular tools.

Search for Babesia bovis vaccine candidates

Babesia bovis is a tick-borne apicomplexan pathogen that remains an important constrain for the development of cattle industries worldwide. Effective control can be achieved by vaccination with live attenuated forms of the parasite, but they have several drawbacks and thus the development of alternative subunit vaccines, either based in recombinant versions of full size proteins or in recombinant or synthetic peptides containing combinations of protective B-cell and T-cell epitopes is needed. Our current strategies for the identification of vaccine candidate antigens include the identification of functionally relevant antigens, bioinformatics, and comparative genomics using the recently sequenced B. bovis genome. These led us to the functional and immunological characterization of members of the VMSA gene family, a group of well conserved putative cysteine and serine proteases, and to the definition of a surface exposed B-cell epitope present in the Merozoite Surface Antigen-2c. Work in progress is focused in defining additional epitopes, and to determine whether they are neutralization-sensitive. These approaches might unravel useful vaccine candidates for B. bovis, and will increase our understanding of the pathogenicity mechanisms of these and related hemoparasites.

Vaccination against large Babesia species from dogs

The original observation of Sibinovic that soluble parasite antigens (SPA) of B. canis could be used to protect dogs against challenge infection formed the starting point for the development of an effective vaccine. With the advent of in vitro cultivation techniques for haemoprotozoan parasites an important tool became available for the commercial production of the vaccine antigens. A first generation vaccine was developed for dogs, but it appeared that the level of protection induced was not complete. In contrast to what was found with the SPA from serum/plasma of infected animals, protection induced with SPA from a single Babesia canis strain protected against a homologous challenge infection only. Further research led to the discovery that a combination of SPA of B. canis and SPA of B. rossi induced a broad spectrum of immunity. This improved vaccine, Nobivac Piro, not only induces protection against heterologous B. canis infection, but also against heterologous B. rossi infection.

Purification of macroschizonts of a Sudanese isolate of Theileria lestoquardi (T. lestoquardi [Atbara])

Research on malignant theileriosis is affected by the limited access to biological materials required for studies aiming at controlling the disease through the establishment of diagnostic tools and vaccines. The main aims of this work were to isolate, establish, and characterize a Theileria lestoquardi-infected cell culture (line) as a source of biological material and to generate a schizont cDNA library for further studies aiming at the identification of antigenic proteins. The T. lestoquardi isolate used originated from a sheep showing typical signs of malignant theileriosis in Atbara town in northern Sudan, and was maintained as an infected cell culture. A high-quality representative schizont cDNA library was established by isolating and purifying the schizonts using a nocodazole/aerolysin protocol followed by Percoll gradient ultracentrifugation. As a parameter to assess the quality of the schizont library, a provisional estimation of the percentage of recombinant phage clones originating from T. lestoquardi (Atbara) was undertaken. Ten clones with inserts ranging in size between 600 and 1200 bp were selected randomly, sequenced, and subjected to BLAST similarity searches. As 6 of the 10 sequenced clones showed similarities to T. parva, T. annulata, and other apicomplexan genes, it was concluded that the majority of the library phage clones originated from the parasite and not from host cell transcripts. The cDNA library will be used for screening of antigenic proteins using sera from infected sheep.

Enzymes as feed additive to aid in responses against Eimeria species in coccidia-vaccinated broilers fed corn-soybean meal diets with different protein levels

This research aimed to evaluate the effects of adding a combination of exogenous enzymes to starter diets varying in protein content and fed to broilers vaccinated at day of hatch with live oocysts and then challenged with mixed Eimeria spp. Five hundred four 1-d-old male Cobb-500 chickens were distributed in 72 cages. The design consisted of 12 treatments. Three anticoccidial control programs [ionophore (IO), coccidian vaccine (COV), and coccidia-vaccine + enzymes (COV + EC)] were evaluated under 3 CP levels (19, 21, and 23%), and 3 unmedicated-uninfected (UU) negative controls were included for each one of the protein levels. All chickens except those in unmedicated-uninfected negative controls were infected at 17 d of age with a mixed oral inoculum of Eimeria acervulina, Eimeria maxima, and Eimeria tenella. Live performance, lesion scores, oocyst counts, and samples for gut microflora profiles were evaluated 7 d postinfection. Ileal digestibility of amino acids (IDAA) was determined 8 d postinfection. Microbial communities (MC) were analyzed by G + C%, microbial numbers were counted by flow cytometry, and IgA concentrations were measured by ELISA. The lowest CP diets had poorer (P < or = 0.001) BW gain and feed conversion ratio in the preinfection period. Coccidia-vaccinated broilers had lower performance than the ones fed ionophore diets during pre- and postchallenge periods. Intestinal lesion scores were affected (P < or = 0.05) by anticoccidial control programs, but responses changed according to gut section. Feed additives or vaccination had no effect (P > or = 0.05) on IDAA, and diets with 23% CP had the lowest (P < or = 0.001) IDAA. Coccidial infection had no effect on MC numbers in the ileum but reduced MC numbers in ceca and suppressed ileal IgA production. The COV + EC treatment modulated MC during mixed coccidiosis infection but did not significantly improve chicken performance. Results indicated that feed enzymes may be used to modulate the gut microflora of cocci-vaccinated broiler chickens.

Vaccinating against zoonotic parasitic diseases: myth or reality?

The largely unanticipated difficulties of parasite vaccine development have led us to a renewed awareness of the survival strategies evolutionarily embedded within parasites over hundreds of millions of years. We have grown to appreciate that efforts to disrupt parasite–host relationships are substantially compounded by our incomplete understanding of the complex immune responses that occur in the naturally infected host. Given the inability to transfer laboratory successes to field trials, research is leading us to conclude that genetically defined animal models may not be good predictors of the unique and disparate protective immune responses one can expect from the genetically heterogeneous populations of animals that represent the parasite's natural environment. This is further compounded by the abundance of mechanisms parasites have created for themselves to defend against immune intervention. Thus, in the never-ending saga of vaccine development, it is only appropriate that pitfalls and advancements be critiqued as they apply across parasite groups, with a look towards promising technologies that may propel this field to the level of scientific achievement once envisaged.

Regulations and procedures in parasite vaccine development

Although immunisation protocols for a wide variety of parasitic diseases have been developed, it is often questioned why these do not always reach the market. In this review information about the regulations and procedures that apply to licensing the production and marketing of medicinal preparations, especially parasite vaccines, is presented. These general regulations specify issues on product (quality, safety, efficacy and potency) and production (facilities and consistency). Vaccine developers and manufacturers have to comply with these regulations, which may involve years of research and development. Moreover, where the manufacturer claims specific features of the product, these claims have to be corroborated by (experimental) data. A series of principles has been used to develop vaccines against parasite infections varying from the use of (attenuated) live vaccines to killed vaccines and subunit vaccines. The implications of some specific regulatory issues associated with these approaches are discussed.

[Antiparasitic vaccines: where are we now?]

In more than one way, the development of antiparasitic vaccines is challenging, but major efforts have been made. A large number of clinical trials have been carried out and a few antigens have been tested in the endemic zone, especially against malaria. So far, no vaccine candidate has shown a sufficient and long-lasting effectiveness that would be useful for public health. However, the trials have shown without ambiguity that a certain level of clinical immunity against paludism, schistosomiasis or leishmaniasis could be induced by vaccination, in the experimental setting or in the field.

Recombinant protein Bd37 protected gerbils against heterologous challenges with isolates ofBabesia divergenspolymorphic for thebd37gene

The Bd37gene encoding for a glycosyl-phosphatidyl-inositol anchored protein ofBabesia divergensdisplays genetic polymorphisms among isolates. Five major polymorphic groups (clades) were shown by PCR-RFLP among differentB. divergensisolates. Each group has been characterized according to a reference Bd37 gene (Rouen87, W8843, Y5, 6303E and 1705B). Recombinant (GST fusion) protein (Bd37r) expressed from the Bd37 gene, was used as antigen in a saponin-based formulation and was able to protect gerbils, after 2 injections at low dose vaccine concentration (1 μg per dose), against a virulent challenge with theB. divergensRouen87 isolate. In spite of polymorphism ofBd37gene, Bd37r induced complete immunoprotection against challenges with each of the 5 reference isolate groups defined by PCR-RFLP.

Identification of Homologous Genes ofT. annulataProteins in the Genome ofTheileriasp. (China)

Homologues to previously described Theileria (T.) annulata genes (T. annulata surface protein [TaSP], putative T. annulata membrane protein [TaD]) were successfully amplified by polymerase chain reaction (PCR) from Theileria sp. (China) merozoite cDNA, with 88% identity to TaD; TcSP partial cDNA, 94% identity to TaSP. Moreover, homologues to a secretory protein of T. annulata (TaSE), with a sequence identity of 99% on the cDNA level (TcSE partial cDNA) and to a potential membrane protein of T. lestoquardi (Clone-5), with a sequence identity of 100% on the genomic level (Tc Clone-5) but lacking an intron at positions 1894-1928 were identified.

Bovine Babesiosis Live Vaccine Production

Gamma irradiation on bovine serum and red blood cells (RBC) allows proliferation and growth of in vitro-cultured Babesia sp., and has potential application to inactivate contaminating viruses and bacteria from the substrate. Gamma irradiation with 25 kGy in a source of (60)Co was able to inactivate infectious bovine rinotracheitis (IBR) and bovine viral diarrhea (BVD) viruses in artificially contaminated serum; besides, bacteria were also eliminated. In vitro culture of Babesia bovis (B. bovis) in modified substrate, by adding irradiated serum with (60)Co at 25 kGy was propagated from 24-well culture plates to 225 cm(2) tissue culture flasks, and percentages of parasitized erythrocytes (PPE) from 2.4% to 8.8% were obtained. Infected RBC adapted to Irrad S were transferred to the irradiated substrate in vitro culture system, by using serum irradiated at 25 kGy and RBC from 10 to 70 Gy. The PPE ranged from 3.1 to 11. Culture of Babesia bigemina (B. bigemina) was established with Irrad S (25 kGy); its propagation was achieved in tissue culture flasks reaching PPE from 0.5 to 4.3 with no statistical difference (P > 0.05) when compared to the nonirradiated control culture (1.2-4.8). B. bigemina-infected RBCs were transferred to the modified culture system by adding irradiated serum and RBC (25 kGy and 70 Gy, respectively). PPE obtained in culture flasks were from 0.8 to 4.2. The results indicate that gamma irradiation is a suitable method to inactivate potential viral contamination and eliminate bacteria from bovine serum, to produce a live attenuated vaccine through the in vitro culture.

Infectivity of Theileria parva sporozoites following cryopreservation in four suspension media and multiple refreezing: evaluation by in vitro titration

Theileria parva sporozoite stabilates are used for immunizing cattle against East Coast fever and in in vitro sporozoite neutralization assays. In this study, we attempted to identify a cheaper freezing medium and quantified the infectivity loss of sporozoites due to refreezing of stabilates, using an in vitro technique. Pools of stabilates prepared using Minimum Essential Medium (MEM), Roswell Park Memorial Institute (RPMI 1640), foetal calf serum (FCS) and phosphate-buffered saline (PBS) were compared. All were supplemented with bovine serum albumin except the FCS. RPMI 1640 was as effective as MEM in maintaining sporozoite infectivity while the infectivity in PBS and FCS reached only 59% and 67%, respectively. In a second experiment, a stabiiate based on MEM was subjected to several freeze-thaw cycles including various holding times on ice between thawing and refreezing. Refrozen stabilate gave an average sporozoite infectivity loss of 35% per cycle. The results indicate that RPMI can be used as a cheaper freezing medium for T. parva stabilates and that refrozen stabilate doses need to be adjusted for the 35% loss of infectivity.

Theileria: intracellular protozoan parasites of wild and domestic ruminants transmitted by ixodid ticks

Theileria are economically important, intra-cellular protozoa, transmitted by ixodid ticks, which infect wild and domestic ruminants. In the mammalian host, parasites infect leukocytes and erythrocytes. In the arthropod vector they develop in gut epithelial cells and salivary glands. All four intra-cellular stages of Theileria survive free in the cytoplasm. The schizont stages of certain Theileria species induce a unique, cancer-like, phenotype in infected host leukocytes. Theileria undergoes an obligate sexual cycle, involving fusion of gametes in the tick gut, to produce a transiently diploid zygote. The existence of sexual recombination in T. parva has been confirmed in the laboratory, and is presumed to contribute to the extensive polymorphism observed in field isolates. Key parameters in T. parva population dynamics are the relative importance of asymptomatic carrier cattle and animals undergoing severe disease, in transmission of the parasite to ticks, and the extent of transmission by nymphs as compared to adult ticks. Tick populations differ in vector competence for specific T. parva stocks. Recombinant forms of T. parva and T. annulata sporozoite surface antigens induce protection against parasite challenge in cattle. In future, vaccines might be improved by inclusion of tick peptides in multivalent vaccines.

Intestinal microbial ecology of broilers vaccinated and challenged with mixed Eimeria species, and supplemented with essential oil blends

Intestinal microbiota is an important component in the development of defense mechanisms in the gut mucosa. This project determined the dynamics of intestinal microbial communities (MC) of broilers vaccinated at first day of age with live oocysts of Eimeria species and fed diets supplemented with 2 specific essential oil (EO) blends, Crina Poultry (CP) and Crina Alternate (CA). Five treatments were analyzed: 1) unmedicated-uninfected (UU) control; 2) unmedicated-infected (UI) control; 3) vaccinated with Advent cocci-vaccine and without feed additive (COV) supplements; 4) vaccinated with Advent and supplemented with CP; and 5) vaccinated with Advent and supplemented with CA. The EO blends were added at 100 ppm to the same basal diets. Chicks were gavage-infected at 19 d of age with Eimeria acervulina, Eimeria maxima, and Eimeria tenella. Duodenal, ileal, and cecal samples were taken from 12 birds per treatment just before the infection and 7 d after the challenge, pooled in 6 samples, and frozen. Denaturing gradient gel electrophoresis was used to examine PCR-amplified fragments of the bacterial 16S ribosomal DNA variable region. Results are presented as percentages of similarity coefficients (SC). Dendrograms of amplicon patterns indicated MC differences due to intestinal location, feed additives, and cocci infection. The EO blends CP and CA did affect MC in all gut sections. The cocci-infection caused drastic MC population shifts in duodenal, ileal, and cecal sections (36.7, 55.4, and 36.2% SC, respectively). The CP-supplemented birds had higher SC between pre- and postchallenge MC in duodenal and ileal (73.3, 81.8%) than COV (66.4, 66.5%). However, COV broilers had the smallest changes in cecal MC after infection (79.5% SC). We concluded that cocci-vaccination causes small changes in intestinal MC, but challenge causes drastic shifts. The EO blend supplementation modulates MC in cocci-vaccinated broilers, avoiding drastic shifts after a mixed coccidia infection. Correlations between MC dynamics and host responses are discussed.

Leishmune vaccine: the newest tool for prevention and control of canine visceral leishmaniosis and its potential as a transmission-blocking vaccine

Canine visceral leishmaniosis is a life-threatening disease caused by Leishmania infantum. For quite some time, specialists in leishmaniosis have tried to develop more affordable and effective control measures against this disease. In this search, the first vaccine against canine visceral leishmaniosis was recently licensed in Brazil. In the light of recent research, the Leishmune vaccine might be seen as the newest tool for prevention and control of canine visceral leishmaniosis. Moreover, the potential of the Leishmune as a transmission-blocking vaccine has recently been demonstrated, indicating its usefulness in the control of zoonotic visceral leishmaniosis.

Comparison of different direct diagnostic methods to identify Babesia bovis and Babesia bigemina in animals vaccinated with live attenuated parasites

Blood smear examination, flow cytometry, duplex Polymerase Chain Reaction (PCR), and duplex nested PCR (nPCR) were evaluated for detection of Babesia bigemina and Babesia bovis infections in cattle vaccinated with live attenuated strains. Two groups of four cattle were immunized with either B. bigemina (Bi) or B. bovis (Bo). On day 23 post inoculation (PI), Bi cattle were vaccinated with B. bovis (BiBo) and Bo cattle were vaccinated with B. bigemina (BoBi). Babesia bigemina was first detected by blood smear examination 7.5+/-3.5 days PI in the Bi group and 32.2+/-1.7 days PI in the BoBi group. The first occurrence of B. bovis in blood smears was 8.0 days PI in the Bo group and 36.0+/-2.6 days PI in the BiBo group. Flow cytometry detected parasitized erythrocytes on day 1.7+/-1.5 and 2.2+/-1.5 PI in the Bi and Bo groups, respectively, but did not discriminate between the two Babesia spp. Duplex PCR detected B. bigemina on day 4.0+/-0.8 and 26.0+/-0.8 PI in the Bi and BoBi groups, respectively, and B. bovis on day 4.0 and 25.3+/-0.5 PI in the Bo and BiBo groups, respectively. The duplex nPCR detected B. bigemina on 3.0+/-0.8 and 25.0+/-0.0 days PI in the Bi and BoBi groups, respectively, and 4.7+/-1.7 and 27.7+/-6.2 days PI in the Bo and BiBo groups, respectively. Duplex nPCR outperformed the other tests in terms of specificity and sensitivity, indicating that it is the most useful method for identifying Babesia spp. in cattle following vaccination.

A tale of three genomes: the kinetoplastids have arrived

July 2005 marked a milestone in kinetoplastid biology research. A tour de force effort led by the Tri-Trypanosomatidae "Tritryp" genome consortium yielded the publication of three prominent kinetoplastid parasite genome sequences: Trypanosoma brucei, Trypanosoma cruzi and Leishmania major. The individual and combined comparative analyses of these three genome sequences, combined with proteomic analyses, have yielded insights into topics ranging from genome evolution and horizontal gene transfer to potential new therapeutic and vaccine targets.

Enhancement of the immunogenicity of synthetic carbohydrates by conjugation to virosomes: a leishmaniasis vaccine candidate

Novel virosomal formulations of a synthetic oligosaccharide were prepared and evaluated as vaccine candidates against leishmaniasis. A lipophosphoglycan-related synthetic tetrasaccharide antigen was conjugated to a phospholipid and to the influenza virus coat protein hemagglutinin. These glycan conjugates were embedded into the lipid membrane of reconstituted influenza virus virosomes. The virosomal formulations elicited both IgM and IgG anti-glycan antibodies in mice, indicating an antibody isotype class switch to IgG. The antisera cross-reacted in vitro with the corresponding natural carbohydrate antigens expressed by leishmania cells. These findings support the concept of using virosomes as universal antigen delivery platform for synthetic carbohydrate vaccines.

Toxoplasma gondii: DNA vaccination with bradyzoite antigens induces protective immunity in mice against oral infection with parasite cysts

Infection of the host by Toxoplasma gondii leads to an acute systemic dissemination of tachyzoites, followed by a chronic phase, in which bradyzoites, enclosed in cysts, persist in the brain, the heart, and other tissues. Among putative vaccine candidates, the bradyzoite antigens BAG1 and MAG1 look promising since they are preferentially expressed during the chronic stage of the parasite. This work focused on studying the immunogenicity of bradyzoite antigens in a mouse model of chronic toxoplasmosis. A mixture of plasmids directing the cytoplasmic expression of MAG1 and BAG1 in mammalian cells was used to immunize mice. We show here that immunized mice developed, preferentially, specific anti-MAG1 and anti-BAG1 IgG2a subclass antibodies, indicating a shift towards a Th1-like response after DNA immunization. We then demonstrated that DNA immunization followed by challenge infection elicited effective protection in mice, suggesting that bradyzoite antigens should be considered in the design of vaccines against toxoplasmosis.

Are extensive T cell epitope polymorphisms in the Plasmodium falciparum circumsporozoite antigen, a leading sporozoite vaccine candidate, selected by immune pressure?

Protective cellular immune responses depend on MHC presentation of pathogen-derived Ag fragments. MHC diversity renders this process sensitive to point mutations coding for altered amino acid sequence of the short target Ag-derived peptides epitopes. Thus, in a given host, a pathogen with an altered epitope sequence will be more likely to escape detection and elimination by the immune system. At a population level, selection by immune pressure will increase the likelihood of polymorphism in important pathogen antigenic epitopes. This mechanism of immune evasion is found in viruses and other pathogens. The detection of polymorphic hot spots in an Ag is often taken as a strong indication of its role in protective immunity. We provide evidence that polymorphisms in the T cell epitopes of a malaria vaccine candidate are unlikely to have been selected by immune pressure in the human host.

DIFFERENCES IN HSP70 EXPRESSION IN THE SPOROZOITES OF THE ORIGINAL STRAIN AND PRECOCIOUS LINES OF EIMERIA TENELLA

The levels of expression of Heat shock protein 70 (Hsp 70) in sporozoites of a wild-type parent strain and 2 precocious lines of Eimeria tenella, were compared to investigate the relationship between the heat shock proteins expressed by the parasite and virulence of the strain. Hsp70 expression was analyzed in sporozoites by immunohistochemical techniques, immunoblot, and flow cytometric analyses. One band of 70 kDa was identified and the variation of the Hsp70 expression levels was quantified by optical densitometric analyses. The results showed a significant gradual decrease in the Hsp70 expression in sporozoites of E. tenella as attenuation progressed, suggesting that the Hsp70 expressed in the excysted sporozoites of E. tenella might be involved in parasite pathogenicity. In addition, the cytoplasmic distribution of the Hsp70, which was observed in the entire sporozoites of the wild strain, was reduced to the anterior portion in the precocious lines.

Effect of butyric acid on the performance and carcass yield of broiler chickens

Short-chain fatty acids such as butyrate are considered potential alternatives to antibiotic growth promoters. The efficacy of butyric acid on performance and carcass characteristics of broiler chickens was tested in two studies. The effect of dietary butyrate on the ability to withstand coccidial oocyte challenge also was investigated. In experiment 1, male broiler chickens were fed diets supplemented with 0 or 11 ppm virginiamycin or 0.2 or 0.4% butyric acid (as mono-, di-, and triglyceride). In experiment 2, broilers were fed bacitracin methylene disalicylate or 0.1 or 0.2% butyric acid. In another trial, birds vaccinated against coccidiosis were challenged with oocytes at 21 d and examined 6 d later. In experiment 1, diet treatments had no effect on body weight gain. Feed intake of the birds fed 0.4% butyric acid was decreased (P < 0.01) compared with birds fed the nonmedicated diet during the starter period, whereas birds fed 0.2% butyric acid had similar feed intake to the control birds. In experiment 2, diet treatments did not affect the performance of broiler chicks while carcass weight and breast meat yield increased (P < 0.01) in birds fed 0.2% butyric acid. With oocyte challenge, birds that had received butyric acid before challenge showed higher growth rate following the challenge compared with birds that received nonmedicated feed. Bacitracin decreased (P < 0.05%) duodenal villi crypt depth, whereas villus length was similar in birds fed butyric acid or the nonmedicated control diet. These results show that 0.2% butyric acid can help to maintain the performance and carcass quality of broilers, especially in vaccinated birds challenged with coccidiosis.

Reduced infection and protection from clinical signs of cerebral neosporosis in C57BL/6 mice vaccinated with recombinant microneme antigen NcMIC1

NcMIC1 is a 460 amino acid Neospora caninum microneme protein implicated in host cell adhesion and invasion processes. In this study, we assessed the potential protectivity of NcMIC1-based vaccination against experimental N. caninum infection in mice, employing both recombinant antigen vaccines and DNA vaccines. Recombinant NcMIC1 (recNcMIC1) was expressed in Escherichia coli as gluthatione-S-transferase-fusion protein. The corresponding NcMIC1 cDNA was cloned into the pcDNA3.1 expression plasmid (pcDNA-MIC1), and expression was checked in transfected Vero cells. Mice (10 animals/group) were vaccinated either with recNcMIC1 antigen suspended in Ribi-adjuvant (3 intraperitoneal injections), pcDNA-NcMIC1 (3 intramuscular injections), or pcDNA-NcMIC1 (twice intramuscularly), followed by 1 intraperitoneal recNcMIC1 antigen boost. Control groups included corresponding treatments with adjuvant, pcDNA3.1 without insert, and PBS (= infection control). All vaccinated and control groups were then challenged intraperitoneally with 2 x 10(6) N. caninum tachyzoites. Animals were inspected daily for a period of 3 wk postinfection (PI). At day 21, all animals were killed and assessed for infection. Before day 21 PI, clinical signs such as walking disorders, rounded back, apathy, and paralysis occurred in infection controls (50% of the mice), pcDNA and adjuvant controls (20% each), and the combined pcDNA-NcMIC1/recNcMIC1-treated group (30%). No clinical symptoms were observed in the recNcMIC1 and pcDNA-NcMIC1 vaccinated groups. All mice were positive for cerebral N. caninum infection as assessed by PCR of brain tissue. However, quantitative real-time PCR revealed that the infection intensity was significantly reduced in the group vaccinated with recNcMIC1 antigen. Immunohistochemistry confirmed these findings. In contrast, the infection intensity was highest in the group vaccinated with the pcDNA-NcMIC1/recNcMIC1 combination, indicating that the sequential application of the DNA vaccine and recombinant antigen had a deleterious effect. Serological analysis showed that only recNcMIC1-immunized animals generated detectable antibody levels recognizing native NcMIC1. Thus, of all protocols applied here, only recNcMIC1 vaccination appears to be suited to reduce cerebral infection in mice challenged with N. caninum tachyzoites.

Intercurrent coccidiosis and necrotic enteritis of chickens: rational, integrated disease management by maintenance of gut integrity

Coccidiosis and necrotic enteritis (NE) are globally common, sometimes intercurrent, diseases of poultry. The risk of NE, due to the Gram-positive bacterium Clostridium perfringens, has increased in recent years because of the voluntary or legally required withdrawal of the use of certain in-feed antibiotic growth promoters with anticlostridial activity. In-feed ionophorous anticoccidial drugs incidentally also possess anticlostridial activity. Such ionophores, although not banned, are usually precluded when live anticoccidial vaccines are used, potentially increasing yet further the risk of NE. This review provides information for the design of rational, integrated management strategies for the prevention and control of coccidiosis and NE in chickens by maintaining gut integrity. Because of differences in local availability of feed ingredients and national legislations regarding antibiotic growth promoters and anticoccidial vaccine licensing, no universal strategy is applicable. The diseases and their interactions are described under the headings of forms of disease, diagnosis, sources of infection, pathophysiological effects, predisposing factors, and control methods. Elements of gut integrity, which influences host predisposition and clinical responses to disease, include physical development, immune competence, gut enzyme activity, mucin production, gut flora and epithelial damage. Experimental studies of coccidiosis and NE are compared, and where possible reconciled, with field observations. Gaps in knowledge and necessary further experiments are identified. Insights are provided regarding interactions between coccidiosis, NE, and the use of live anticoccidial vaccines. Recent changes in NE prevalence in commercial flocks, and their possible causes, are discussed. The necessarily wide range of topics reviewed emphasizes the enormous complexity of this disease combination, and indicates the importance of a multidisciplinary approach in order to reduce its harmful impact on the world's poultry industry.

Vaccinations with live-attenuated Leishmania major promastigotes and challenge infection with L. major in BALB/c mice

BACKGROUND

Currently there is no vaccine available in use against any form of leishmaniases worldwide.

OBJECTIVE

To assess potential of a live-attenuated Leishmania major promastigates, for protection against a challenge infection with L. major in BALB/c mice.

DESIGN

A laboratory based study.

SETTING

Study was carried out at Centre for Biotechnology Research and Development, Kenya Medical Research Institute, Nairobi.

RESULTS

The greatest protection against challenge with L. major was seen in mice immunised with live parasites (P < 0.001) compared to vaccinations with heat killed or soluble antigens. In general, immunised mice produced high level of antileishmanial antibodies and T cell stimulation to their respective antigens.

CONCLUSIONS

Our live-attenuated parasites produced by serial sub-culture of L. major parasites 118 times showed the capacity to induce appropriate cell-mediated immune responses and protection against L. major infection in BALB/c mice. Data also suggests that these parasites do not revert to virulence when injected subcutaneously in mice.

The immunology of Leishmania infection and the implications for vaccine development

Leishmania parasites are vector-borne protozoal pathogens found in tropical and subtropical regions of both the Old and New World. These parasites can cause visceral or cutaneous disease, and the pathology of the infection is determined by both host immune factors and species/strain differences of the parasite. Dogs are an important reservoir for maintaining the population of Leishmania parasites that can lead to visceral leishmaniasis in humans, and a vaccination approach may be an effective method for reducing the numbers of infected dogs. Resistance to leishmaniasis has been consistently associated with a T helper 1 immune response, characterized by the production of IFN-gamma by the antigen-specific lymphocyte population. The development of this Th1 response has been shown to be dependent upon both cytokines and dendritic cells during T cell activation. However, the development of a Leishmania vaccine effective in preventing these chronic diseases has proven to be a challenge. Vaccine trials have focused on whole-killed or subunit vaccines with adjuvants. Newer experimental strategies involve the attenuation of the Leishmania parasite via gene deletion technologies or the expression of specific Leishmania peptides within attenuated organisms, such as Bacillus Calmette Guérin. DNA vaccines and dendritic cell potentiators, such as CpG oligodeoxynucleotides and Flt-3 ligand, are also in the early stage of development. In addition, as part of blocking the transmission cycle of leishmaniasis, several laboratories are also exploring the possibility of immunomodulating the host toward the bite of the sand fly.

Evaluation of immunosuppressants and dietary mechanisms in an experimental disease model for necrotic enteritis

Clostridium perfringens (CP) is the etiologic agent of necrotic enteritis (NE). Clinical signs of this disease include depression, decreased appetite, diarrhea, and severe necrosis of the intestinal tract. Understanding the disease progression of NE has been difficult due to its complexity and the involvement of multiple factors (dietary components, immunosuppression, and mechanical irritation of the gut) that appear to contribute to this syndrome. In the present investigation, day-of-hatch broilers were fed a 55% wheat diet and randomly assigned to 1 of 8 groups. Treatments included positive control (CP challenge only), commercial coccidia vaccine (CCV), commercial bursal disease vaccine (CBDV), or the combination of CCV and CBDV, and an appropriate negative control for each (vaccinated and not challenged). Challenged treatment groups received 10(7) cfu of CP twice daily. When compared with controls, broilers in each treatment group had increased (P < or = 0.05) lesion scores, with mean scores of 1.05 and 2.05 in the CP and CBDV + CP treatments, respectively. When compared with controls, the incidence of CP increased (P < or = 0.05) in all treatment groups (73 and 100% in the CCV + CP and CBDV + CP treatment groups, respectively). Compared with controls, percentage mortality increased (P < or = 0.05) from 2% to 26 and 34% in the CP and CBDV + CP treatment groups, respectively. Results of this study indicate that the methodology used provides a good model for studying NE.

Application of biotechnological tools for coccidia vaccine development

Coccidiosis is a ubiquitous intestinal protozoan infection of poultry seriously impairing the growth and feed utilization of infected animals. Conventional disease control strategies have relied on prophylactic medication. Due to the continual emergence of drug resistant parasites in the field and increasing incidence of broiler condemnations due to coccidia, novel approaches are urgently needed to reduce economic losses. Understanding the basic biology of host-parasite interactions and protective intestinal immune mechanisms, as well as characterization of host and parasite genes and proteins involved in eliciting protective host responses are crucial for the development of new control strategy. This review will highlight recent developments in coccidiosis research with special emphasis on the utilization of cutting edge techniques in molecular/cell biology, immunology, and functional genomics in coccidia vaccine development. The information will enhance our understanding of host-parasite biology, mucosal immunology, and host and parasite genomics in the development of a practical and effective control strategy against Eimeria and design of nutritional interventions to maximize growth under the stress caused by vaccination or infection. Furthermore, successful identification of quantitative economic traits associated with disease resistance to coccidiosis will provide poultry breeders with a novel selection strategy for development of genetically stable, coccidiosis-resistant chickens, thereby increasing the production efficiency.

Entamoeba histolytica: cDNAs cloned as 30kDa collagen-binding proteins (CBP) belong to an antioxidant molecule family. Protection of hamsters from amoebic liver abscess by immunization with recombinant CBP

A cDNA expression library of Entamoeba histolytica was screened with antiserum to native amoebic collagen binding proteins (CBPs), and two clones C13 and C7 which partially encode for the 30 kDa CBP were obtained. The sequenced clones were 90% homologous. C7 had a 69 bp deletion at the 5' end that is present in C13 and encodes for a Glu-Cys-Lys rich region and a four amino acids repeat (Glu-Lys-Glu-Cys). Purified fusion proteins from these cDNA clones were able to bind native type I collagen gels in a pH, calcium, ionic strength, and temperature dependent way. The binding of pgtC13 to collagen gel was time and temperature stable, while pgtC7 binding was not, suggesting that the deleted region in C7 is important for the binding. The clones reported here partially encode a 30 kDa CBP that also belong to an antioxidant molecule family. We demonstrated that the fusion protein pgtC13 is immunogenic and partially protective as a subunit vaccine in the hamster model of amoebic liver abscess.

Protective immunity against trypanosoma cruzi infection in a highly susceptible mouse strain after vaccination with genes encoding the amastigote surface protein-2 and trans-sialidase

Protective immunity against lethal infection is developed when BALB/c or C57BL/6 mice are immunized with plasmids containing genes from the protozoan parasite Trypanosoma cruzi. However, genetic vaccination of the highly susceptible mouse strain A/Sn promoted limited survival after challenge. This observation questioned whether this type of vaccination would be appropriate for highly susceptible individuals. Here, we compared the protective efficacy and the immune response after individual or combined genetic vaccination of A/Sn mice with genes encoding trans-sialidase (TS) or the amastigote surface protein-2 (ASP-2). After challenge, a significant proportion of A/Sn mice immunized with either the asp-2 gene or simultaneously with asp-2 and ts genes, survived infection. In contrast, the vast majority of mice immunized with the ts gene or the vector alone died. Parasitological and histological studies performed in the surviving mice revealed that these mice harbored parasites; however, minimal inflammatory responses were seen in heart and striated muscle. We used this model to search for an in vitro correlation for protection. We found that protective immunity correlated with a higher secretion of interferon- by spleen cells on in vitro restimulation with ASP-2 and the presence of ASP-2-specific CD8 cells. Depletion of either CD4 or CD8 or both T-cell subpopulations prior to the challenge rendered the mice susceptible to infection demonstrating the critical contribution of both cell types in protective immunity. Our results reinforce the prophylactic potential of genetic vaccination with asp-2 and ts genes by describing protective immunity against lethal T. cruzi infection and chronic tissue pathology in a highly susceptible mouse strain.

The development of effector and memory T cells in cutaneous leishmaniasis: the implications for vaccine development

Leishmania major infections induce the development of a CD4(+) T-helper 1 (Th1) response that not only controls the primary infection but also results in life-long immunity to reinfection. How that immunity is maintained is unknown, although because of the existence of infection-induced immunity, there has been an assumption that the development of a vaccine against leishmaniasis would be relatively easy. This has turned out not to be the case. One problem has been the finding that a large part of the immunity induced by a primary infection depends upon the presence of persistent parasites. Nevertheless, there are ample situations where immunologic memory persists without the continued presence of antigen, providing the prospect that a non-live vaccine for leishmaniasis can be developed. To do so will require an understanding of the events involved in the development of an effective protective T-cell response and, more importantly, an understanding of how to maintain that response. Here, we review work from our laboratory, describing how Th1 cells develop in L. major-infected mice, the nature of the memory T cells that provide protection to reinfection, and how that information may be utilized in the development of vaccines.

Central memory T cells mediate long-term immunity to Leishmania major in the absence of persistent parasites

Infection with Leishmania major induces a protective immune response and long-term resistance to reinfection, which is thought to depend upon persistent parasites. Here we demonstrate that although effector CD4(+) T cells are lost in the absence of parasites, central memory CD4(+) T cells are maintained. Upon secondary infection, these central memory T cells become tissue-homing effector T cells and mediate protection. Thus, immunity to L. major is mediated by at least two distinct populations of CD4(+) T cells: short-lived pathogen-dependent effector cells and long-lived pathogen-independent central memory cells. These data suggest that central memory T cells should be the targets for nonlive vaccines against infectious diseases requiring cell-mediated immunity.