Chairman

Dr. Donald Krogstad
(1993-1995, Member 1991-1992)
Department of Tropical Medicine
Tulane School of Public Health and Tropical Medicine
Tulane University
1501 Canal Street, Suite 505
New Orleans, Louisiana 70112

Chairman

Dr. Somei Kojima
(1992-   , Member 1988-1992)
Professor
The Institute of Medical Science
University of Tokyo
4-6-1 Shirokanedai, Minato-ku
Tokyo 108, Japan

Dr. Donald Harn (1994-1997)
Department of Tropical Health
Harvard School of Public Health
665 Huntington Avenue
Boston, Massachusetts 02115

Dr. James W. Kazura (1992-1995)
Division of Geographic Medicine
Case Western Reserve University School of Medicine
2109 Adelbert Road, W137
Cleveland, Ohio 44106-4983

Dr. Carole A. Long (1993-1996)
Department of Microbiology and Immunology
Hahnemann University
MS 405
Broad and Vine
Philadelphia, Pennsylvania 19102-1192

Dr. Fujiro Sendo (1992-   )
Professor
Faculty of Medicine
Yamagata University
2-2-2 Iida-Nishi
Yamagata 990-23, Japan

Dr. Mamoru Suzuki (1990-   )
Professor
Faculty of Medicine
Gunma University
3-39-22 Showa-machi, Maebashi
Gunma 371, Japan

Dr. Kazuyuki Tanabe (1995-   )
Professor
Osaka Institute of Technology
5-16-1 Ohmiya, Asahi-ku,
Osaka 535, Japan

Parasitic Diseases Panels USJCMSP

• Epidemiology
• Vector Biology and Control
• Parasite Biology
• Host-Parasite Interactions
• Pathology
• Immunology
• Biochemistry
• Chemotherapy.

Special emphasis will be given to studies designed to explore the role of membrane chemistry and antigen differentiation in infection and immunity. In addition, the Panels are concerned with supply of parasites and infected animals to investigators and with availability of research workers in these fields.

Five-Year Summary

Broad Goals

Parasitic diseases was one of the original areas of interest designated at the inception of the USJCMSP in 1965. At that time, the Parasitic Diseases Panels were directed to focus their activities on the two important vector-borne helminthiases, schistosomiasis and filariasis. During the early 1980's, the Panels' mandate was expanded to include malaria and other vector-borne protozoan diseases. These diseases continue to inflict substantial suffering on a large portion of the world's population today, even as they did in 1965 and for thousands of years before that. In some cases, e.g., malaria, the situation has perhaps even worsened, as strains resistant to previously useful drugs emerged and continue to spread.

The Parasitic Diseases Panels focus on promotion and encouragement of cooperative international research on these diseases, concentrating on multidisciplinary approaches to study the complex biology of parasites and their relationships to both invertebrate vector and mammalian host. A variety of basic and applied research studies have been carried out in the context of the program. These focus on the diverse areas of parasite biology, host-parasite interactions, pathology, immunology, biochemistry, chemotherapy, epidemiology, and vector biology. This research aims to control the major parasitic diseases of man through the development of vaccines, diagnostics, new or improved drugs, and the more effective control of vectors.

Progress/Accomplishments

Substantial progress has been made in identifying candidate vaccine molecules, applying cytokine and other immunotherapies to the treatment of parasitic diseases, understanding parasite biochemical mechanisms that may serve as potential drug targets, applying recent technological advances to the development of new diagnostic assays, and applying ecologic as well as molecular approaches to the development of new vector control strategies.

Major advances have been made in the development of vaccines for malaria and schistosomiasis. Vaccines against several stages of the malaria parasite are being tested in preclinical or clinical studies. These include a transmission blocking vaccine for Plasmodium falciparum, Pfs25, which has been produced in recombinant form and functions through an antibody-dependent mechanism. A 230 kDa parasite-induced liver specific antigen (LSA-1) has been identified that is recognized by lymphocytes from volunteers protected by immunization with irradiated sporozoites. This antigen could form the basis of a vaccine against exoerythrocytic stages. With regard to vaccines against blood stages of the parasite, antibody to the carboxyl-terminal region of the mouse malaria P. yoelii merozoite surface protein-1 (MSP-1) provides protective immunity against otherwise lethal infection. This sequence has been cloned and expressed, and studies have determined that the elicitation of protective immunity is conformation dependent. Other studies have examined the variability of important epitopes from sporozoite and merozoite antigens. These are being considered as vaccine candidates against parasites isolated from different geographic regions.

Future schistosome vaccines most likely will include antigens from the infective or migrating larval stages of the parasite. Among the antigens showing promise in preclinical studies are the IrV5 antigen of Schistosoma mansoni, which is recognized by sera from mice multiply immunized with attenuated parasites, the integral membrane protein Sm23, and the glycolytic enzyme triose phosphate isomerase of both S. japonicum and S. mansoni paramyosin. Recombinant or synthetic forms of all of these antigens are being tested for their ability to generate protective reactivity in murine models. Paramyosin also has shown promise as a vaccine against Brugia malayi in the jird model.

Recent advances in cytokine research have proven extremely beneficial to the study of parasitic diseases. This area of research was the topic of a minisymposium held during the 1994 meeting in Yamagata. The TH1 immune pathway has been implicated in the development of protective immunity against S. mansoni. Here, interferon-y activating macrophage or endothelial effector cells produce toxic nitrogen oxides that kill parasite larvae. Such TH1 cytokine responses also are protective against Leishmania and Trypanosoma cruzi parasites. In these infections, cytokine products of the alternative TH2 pathway appear to participate in pathology and disease. In these diseases, cytokines such as IL-12 promote the TH1 pattern of response and thus may be beneficial in immunotherapy or as vaccine adjuvants. Conversely, TH2 type responses seem to be protective against other helminthic infections such as Brugia malayi and Angiostrogylus cantonensis.

A minisymposium at the 1993 Baltimore meeting focused on advances in drug development for parasitic diseases. Because of the emergence of chloroquine resistant malaria, many Japanese and U.S. investigators have increased their efforts to identify new or improved chemotherapeutics for this parasite. Chloroquine and other aminoquinoline antimalarials inhibit the polymerization of free heme to hemozoin. This heme polymerase activity may be the biologically important initial step in the mode of action of aminoquinoline against the parasite. Compounds that inhibit chloroquine efflux from resistant P. falciparum in vitro enhance chloroquine accumulation by otherwise resistant parasites in the P. chabaudi model. Various structure-function studies are yielding additional information concerning the biological activity of chloroquine. New drug targets may include the two aspartic proteases and one cysteine protease that account for the hemoglobin-degrading activity of the digestive vacuole in plasmodia. Inhibitors of these enzymes have been developed and are being tested as potential antimalarials. The chemical basis of artemisinin action, which appears to center on the endoperoxide linkage, is being studied for the development of improved analogs. Artemisinin action depends on binding of the drug to hemin, followed by free radical release and alkylation of parasite proteins. 5-Fluoro-orotic acid is active against P. falciparum in vitro and against P. yoelii in vivo. It acts by inhibiting thymidylate synthase activity. Construction of an artificial gene encoding the dihydro-folate reductase part of the dihydrofolate reductase-thymidylate synthase complex of P. falciparum in E. coli, using synthetic gene fragments, may aid in development of novel drugs by computer drug design. Desferroxamine B, an iron chelator, inhibits the growth of P. falciparum in vitro, clears parasitemia in semi-immune persons with mild infection, and enhances the rate of parasite clearance and recovery from coma in cerebral malaria. Thus, iron chelators that may be given orally are under development as adjuncts to standard malaria chemotherapy. Two approaches have been used to develop protease inhibitors that are potentially useful clinically: 1) screening of known protease inhibitors and 2) computer-aided design of new inhibitors based on the graphic analysis of known parasite enzymes. This strategy has revealed several putative inhibitors of the cercarial serine protease of S. mansoni. These inhibitors have sufficient activity to be of potential clinical interest. Because of the availability of useful drugs for filariasis, chemotherapeutic studies of these parasitic infections are further along in clinical trials. For example, comparison of a single-dose regimen of diethylcarbamazine (DEC) to single-dose ivermectin indicated that DEC was as effective in terms of reduction of microfilaremia after 18 months and as safe for treatment of Brancroftian filariasis. Repetitive administration of ivermectin was found to reduce transmission of Onchocerca volvulus in endemic areas of Central America.

One disadvantage of research on certain human parasites has been the lack of useful laboratory animal models. It has recently been found that mice lacking T cells (nude mice) or those lacking both T and B cells (scid mice) are permissive for human filarial infection. Using scid mice with a NOD (non-obese diabetic) background to permit immunologic reconstitution with human splenocytes, it has now been possible to elicit human antibodies in vivo during murine Brugia malayi infection. BALB/c mice immunized with O. volvulus antigen developed a keratitis with eosinophils and disruption of collagen lamellae when challenged intracorneally with the same antigen. This may facilitate studies on the immunologic basis of the pathology of onchocerciasis. Recent studies indicate it is possible to grow P. falciparum in vivo using the NOD-scid mouse. With splenectomy and daily infusion of human erythrocytes, the infection can be sustained for several weeks and then can be transmitted by Anopheles mosquitoes. This, in conjunction with earlier studies that described the growth of extracellular stages of P. falciparum in vitro, may significantly hasten the development of new control strategies for this most virulent type of human malaria. In addition, two groups have now developed primate models for the study of cerebral malaria using the non-human parasite P. coatneyi. Biological and clinical features of these models indicate that they will be very useful in studies on the pathology of malaria.

The 1992 meeting in Maebashi featured a symposium on the epidemiology of parasitic and tropical diseases. It also included discussions on malaria and schistosomiasis in China, the Philippines, and Vietnam by local representatives. These informative discussions provided an important opportunity to update the Panels on the current status of parasitic disease problems within the Southeast Asian region. The need for improved diagnostic assays is closely linked not only to epidemiological studies, but also to drug and vaccine testing. A competitive indirect ELISA method for the detection of DEC in blood has been described. It may aid in monitoring the efficacy of different treatment regimens. A fluorescent dye staining technique using acridine orange has been tested as a rapid malaria diagnostic under field conditions. DNA-based diagnostic techniques employing the polymerase chain reaction (PCR) to detect P. falciparum also have been developed. Using PCR technology to detect polymorphisms in the N-terminal region of MSP-1, it has been determined that infection with multiple strains of P. falciparum is common. It may occur in approxi-mately 40 percent of infected children in sub-Saharan Africa. A DNA-based diagnostic technique also has proven useful in distinguishing between pathogenic and nonpathogenic forms of O. volvulus in Africa.

Vector biology studies are yielding new insights into mechanisms of parasite transmission. The role of vector saliva in enhancing parasite entry into the vasculature and suppressing potentially protective host immune responses has only recently begun to be elucidated. Biochemical pathways involved in vector resistance to parasitic infection and the function of the invertebrate immune system in this interaction are being described. Efforts are underway to develop insect, and possibly even snail, transformation systems in which the genes responsible for resistance to parasite infection can be transferred to susceptible species, thereby rendering them unsusceptible to parasitic infections.

Future Goals

The Parasitic Diseases Panels, and the joint research efforts they represent, remain committed to the application of relevant scientific and biotechnologic advances to elucidate host-parasite relationships and to apply such knowledge to the development of control strategies for use in the developing world.

Selected References

1. Krogstad DJ, Suzuki M, Long CA, Aoki Y, Ishii A, James SL. Drug discovery, development and deployment: A report from the 28th Joint Conference of the U.S.-Japan Parasitic Diseases Panels. Am J Trop Med Hyg 1994; 51:384-8.
2. Slater AF, Cerami A. Inhibition by chloroquine of a novel haem polymerase enzyme activity in malaria trophozoites. Nature 1992; 355:167-9.
3. Theodos CM, Ribeiro JM, Titus RG. Analysis of enhancing effect of sand fly saliva on Leishmania infection in mice. Infect Immun 1991; 59:1592-8.
4. Nelson FK, Greiner DL, Shultz LS, Rajan TV. The immunodeficient scid mouse as a model for human lymphatic filariasis. J Exp Med 1991; 173:659-63.
5. Long CA, Daly TM, Kima P, Srivastava I. Immunity to erythrocytic stages of malarial parasites. Am J Trop Med Hyg 1994; 50:27-32.

1. Iwamura Y, Irie Y, Kominami R, Nara T, Yasuraoka K. Existence of host-related DNA sequences in the schistosome genome. Parasitology 1991; 102:397-403.
2. Nara T, Matsumoto N, Janecharut T, Matsuda H, Yamamoto K, Irimura T, Nakamura K, Aikawa M, Oswald I, Sher A, Kita K, Kojima S. Demonstration of the target molecule of a protective IgE antibody in secretory glands of Schistosoma japonicum larvae. Int Immunol 1994; 6:963-71.
3. Tanabe M, Kaneko N, Takeuchi T. Schistosoma mansoni: Higher free proline levels in the livers of infected mice. Exp Parasitol 1991; 72:134-44.
4. Yamashita T, Watanabe T, Saito S, Araki Y, Sendo F. Schistosoma japonicum soluble egg antigens activate naive B cells to produce antibodies: Definition of parasite mechanisms of immune deviation. Immunology 1993; 79:189-95.
5. Waki S, Uehara S, Kanbe K, Ono K, Suzuki M, Nariuchi H. The role of T cells in pathogenesis and protective immunity to murine malaria. Immunology 1992; 75:646-51.