The American Journal of Tropical Medicine and Hygiene - Volume 26, Issue 6_Part_2, 1977

On behalf of the National Institute of Allergy and Infectious Diseases, I welcome you to this Workshop on the Immunology of Parasitic Infections. I also would like to avail myself of this opportunity to thank the Naval Medical Research Institute for a splendid example of interagency cooperation in allowing the use of its facilities, including food and drink, to the participants in this workshop.

During its June 1969 meeting, the Advisory Council of the National Institute of Allergy and Infectious Diseases approved the Immunology of Parasitic Infections Special Emphasis Program.

The complexity of structure and function of parasites has made the study of immunology of these infectious agents exceptionally challenging and rewarding. Exciting opportunities for the elucidation of mechanisms and manifestations of immunological responses to parasites now exist as the result of the impressive developments in immunology in recent years.

When John David and I were planning this symposium, with the able help of Franklin Neva, Allen Cheever, and Irving Delappe, we attempted to select the participants from among the best of those now working on the Immunology of Parasitic Infections. Obviously, two principal disciplines were involved, immunology and parasitology, and most of the participants began their careers in one or the other of these disciplines. This was emphasized by a recent correspondence I had with Professor John Humphrey of the Royal Postgraduate Medical School who is now working on the immunopathogenesis of malaria. In his letter he modestly disclaimed the title of “parasitological immunologist;” in my reply, I said if he wasn't a parasitological immunologist (PI) I wasn't an immunological parasitologist (IP). Subsequently, in going over the list of participants it appears that the planning committee must have had this distinction in the back of their minds as there was an almost equal division into 13 PIs and 15 IPs.

Over the last ten years there has been a steadily increasing effort in the field of schistosome immunology, and the greater part of this activity has had the long term aim of producing an effective anti-schistosome vaccine. It seems likely that the development of such a vaccine will depend on an improved understanding of the biology of the schistosome surface. Available evidence suggests that surface antigens of the young parasite act as targets for the host's immune attack. These target antigens apparently become hidden from the host as the parasite develops; in this way host immunity is evaded.

In this paper we review the current information on the nature of the schistosomular surface and the changes which take place there as a result of host-parasite interaction.

Unlike the nematode, the schistosome is not bounded by a chitinous-like cuticle but by a syncytial layer of cytoplasm bounded by a living plasma membrane (Fig. 1) which has important physiological and absorptive functions.

The two experimental models now most widely used in the study of immunity to schistosomiasis are the mouse and rat. Each experimental host is endowed with its own set of advantages and drawbacks for this work. The model which we have chosen for our research on immunity to schistosomes is the inbred mouse. The principal advantage of this host is its full susceptibility to chronic schistosomiasis, a feature which has also made the mouse a prime model for the study of the pathology of disease. Ironically, the high susceptibility of the mouse to schistosome infection is also its major drawback as an immune model—for it is often difficult to dissociate the effects of protective immunity from those of the disease itself.

The murine model characterized by Dr. Smithers and myself at Mill Hill employed inbred mice (CBA or C57/BL) immunized by primary infection with small numbers of Schistosoma mansoni cercariae (25–35 each).

The limitations and advantages of studies in vitro It is frequently asserted that studies on immune effector mechanisms in vitro, however interesting in their own right, bear no relationship to the “real” situation in vivo. This is particularly true of parasitic diseases, for which the proponents of this point of view might use three arguments. The first is that, although a particular effector mechanism might be capable of mediating detectable damage to the parasite in vitro, the culture conditions under which the parasite is maintained are necessarily artificial and to a greater or lesser extent suboptimal. An organism which undergoes irreversible damage in vitro might be able to repair itself under more suitable conditions in vivo. Secondly, although a particular effector mechanism may be demonstrable in vitro, this mechanism may be blocked or inactivated by other processes that occur within the highly complex environment in vivo.

Antibody-dependent cell-mediated cytotoxicity (ADCC) has been intensively investigated in various fields of immunology. Its major function, however, may prove to be related to infectious or parasitic agents. In the case of a metazoan parasite such as Schistosoma mansoni as a target, a large variety of in vitro killing mechanisms have been described as variations of the classical pattern of ADCC. After the original discovery by Clegg and Smithers of a cytotoxic IgG antibody in monkeys, and its further observation in man and various animal models, many in vitro instances of cell-antibody cooperation have been evidenced. In these the antibody can bind primarily to the killer cell via its Fc receptor or to the target via its antigen binding site. The latter results in an opsonic event in which the target cell is first coated with antibody and then attacked by the killer cell.

The rat represents a unique host for Schistosoma mansoni. Although susceptible to infection, it is extremely resistant to initial exposure to S. mansoni and manifests a spontaneous dramatic decrease in parasitic burden approximately 1 mo after initial infection. Following an initial exposure to normal or irradiated S. mansoni cercariae or to certain homogenates of this organism, the rat demonstrates an anamnestic response to reexposure with increased resistance to reinfection. Because of these characteristics of high intrinsic resistance, it has been in the rat that many immunologically mediated resistance mechanisms have been most clearly demonstrated.

It is the purpose of this paper to summarize a portion of that data, obtained through the study of schistosomiasis in the rat, which has enabled a partial elucidation of immune mechanisms which are operative in schistosomiasis.

Caution must be exercised in interpreting these results. Since defense mechanisms are highly effective in the rat, analogies drawn between this species and more sensitive species such as man must be done with caution.

Cysticercosis and hydatidosis are cyclozoonotic diseases of current importance to both public and animal health authorities throughout the world. Taenia taeniaeformis in the field is cycled through rodents (rats and mice) as intermediate hosts while the cat serves as the definitive host in a predator-prey relationship. The intermediate host becomes parasitized after the ingestion of infective eggs. Taeniid parasites possess the capacity to stimulate a strong immune response to the invasive larval stages, a response which is capable of destroying an infective challenge but seems incapable of eliminating the established parasites of the primary infection. At present the mechanisms by which these parasites are able to evade the efferent arm of the immune response remain undetermined. However, the natural host-parasite relationship offered by the experimental model of taeniasis, Taenia taeniaeformis in the rat, provides a means to experimentally examine both the host's response to the parasite as well as the parasite's response to the host in an effort to gain further insight into the phenomenon of prolonged survival.

Nippostrongylus brasiliensis goes through both a tissue and a gut phase during its life cycle in the rat. It enters the body through the skin (experimentally, by subcutaneous injection) as a third stage larva. The larvae migrate rapidly to the lungs where they molt, about 30–40 h after entering the body, to the fourth larval stage. These larvae migrate up the trachea and down the esophagus to the small intestine, where they molt again (about 60–120 h after infection) to the final adult worm stage. The male and female worms do not penetrate into the mucosa but lie tightly coiled between the villi like snakes in the grass. They do not suck blood.

The rejection of Nippostrongylus brasiliensis from rats by the immune response is thought to involve several aspects of the host's immunological armory. In this report we consider the many immune responses detectable during an infection, and attempt to relate their occurrence with the fate of the parasite in primary and subsequent infections.

Trichinella spiralis is an unusual nematode parasite in two respects. First, it spends its larval and adult life in the same host, and second, it is rather non-specific in its mammalian host range. Because of the above facts, and since T. spiralis is also a human pathogen, many investigations into the immunology of the infection have been conducted over the last 50 yr.

A common feature of the infection in all hosts so far studied, save the nude athymic mouse, is that they develop some degree of acquired resistance to reinfection. The nature of the protective immunity has been the subject of many reviews.

The purpose of this review is to attempt to correlate the biological activities of the various stages of the infection with what is known about host protective mechanisms active against these worm stages.

Neva: I wanted to make a comment upon schistosome immunity relative to the discussion this morning which focused upon events at the surface or tegument. We should remember that the schistosome is also ingesting blood constituents and spewing them forth. Perhaps some immune mechanisms might operate within the worm. This might explain reduction of egg output later in the course of infection.

Smithers: It would seem doubtful that any immune mechanism would operate against the schistosome gut until the worm has passed the lung stage. During these early stages, it would appear that the gut is not functional; certainly no red blood cells are ingested until the hepatic portal system is reached.

In the adult worm, the pH of the gut is exceedingly low—pH 2.8 I believe. In this high acidity I would doubt whether antibodies would be effective. There is evidence that Gram-negative bacteria can live happily within the gut of the adult worm although outside the worm, in the host's bloodstream, they are rapidly destroyed.

It is a great privilege to introduce the subject of schistosome immunopathogenesis before a group of workers which includes some of the most distinguished contributors to this field. It is because of their efforts that we know more about the pathogenesis of schistosomiasis today than of other parasitic diseases. Yet, we still do not know nearly enough!

In order not to encroach upon the forthcoming talks—of which I have not received summaries—this introduction will highlight some key problems of human schistosome pathology, with incursions into pertinent experimental work already accomplished or yet to be done. To provide such perspective is not an easy task, but I rely on our later speakers, especially on Ken Warren, to complete or correct these sketchy opening remarks.

Human perspective is essential in schistosomiasis research for several reasons. Man is the principal natural host of the schistosome species we are studying.

The initiation of schistosome infection by dermal penetration by cercariae, and their differentiation into schistosomula, provide the first interface of the host-parasite relationship. Overt, clinical consequences result from this interaction most often following combinations of species of schistosomes and hosts which are incompatible in regard to the development of mature, definitive host infections. The most common such situations result from human exposure to some of the schistosomes of waterfowl. It has been observed that these lesions, termed schistosome dermatitis or “swimmer's itch,” intensify upon repeated exposure to cercariae, and involve infiltrates of mixed cell populations. Severe dermatitis seldom follows the exposure of humans or other compatible definitive hosts to the cercariae of schistosome species which develop in man. However, numerous experimental studies have established that penetration by either Schistosoma mansoni, S. haematobium, or S. japonicum does not go wholly unnoticed within such hosts.

The role in immune inflammation of basophils, mast cells, and their contained vasoactive amines is most generally appreciated in immediate hypersensitivity reactions, such as allergic responses mediated by IgE antibodies. However, there is growing recognition of a role for these elements of immediate reactions in prolonged and delayed hypersensitivity responses in which thymus derived T lymphocytes (T cells) and anaphylactic antibodies are potentially both involved.

Mast cells, whose origins are uncertain, are the principal vasoamine-containing cells of the tissues, while basophils are bone marrow-derived cells and are the principal vasoamine cells of the blood. In mice (which appear to lack basophils) the onset and full development of delayed type hypersensitivity (DTH) responses seems to require a critical communication from T cells to mast cells to vascular endothelium, via vasoamines. In humans and guinea pigs it is now recognized that delayed-in-time immune processes which are governed by interaction of antigens with T cells and/or with anaphylactic antibodies can induce basophils to leave the blood and infiltrate the tissues.

With the current burgeoning of interest in the immunology of parasitic disease, parasite immunochemistry is finally beginning to come of age. In the 1960s immunochemical research was primarily concerned with identifying serological antigens. Methods such as immunodiffusion and immunoelectrophoresis found acceptance among those interested in schistosomiasis and application to that parasitic infection. In the late 1970s emphasis has fallen on the purification of parasite antigens with special emphasis on their characterization and biological properties. Considerable progress on fractionating protozoal antigens has been made, culminating in the elegant studies of Cross et al. characterizing the molecular structure of the trypanosome surface coat antigens. In non-protozoal helminth systems, studies of Echinococcus granulosus have identified the immunodominant antigens which have proven useful in the serodiagnosis of human disease. However, research on the major helminth pathogen of man, Schistosoma, has neither equalled the molecular elegance of trypanosome work nor achieved the practical consequences of Echinococcus immunochemistry.

Schistosomiasis mansoni is basically an immunological disease due to the pathophysiological sequelae of a cell-mediated granulomatous inflammatory reaction around the eggs of the parasite trapped in the host tissues. An immunoregulatory response has been observed in chronic Schistosoma mansoni infections in experimental animals, and possibly in man, which results in modulation of the granulomatous inflammation and amelioration of the disease state. This paper will briefly review the immunopathogenesis of schistosomiasis mansoni, and then consider the observations on modulation and its mechanism. Evidence that such immunoregulatory phenomena are occurring in animal models of the other major human schistosome infections—S. japonicum and S. haematobium—will be presented, as will observations on modulation in man, its possible mechanisms and its implications.

Immunopathogenesis The major clinical manifestations of infection with the three principal species of human schistosomes are essentially related to obstruction to blood or urine flow due to inflammatory and fibrotic responses to products of the helminth.

Although the association of eosinophils with many helminthic infections is not new, their role as bona fide effector cells in the immune attack on the parasite is. New evidence comes from studies discussed earlier in which it was shown, in vitro, that human eosinophils will mediate antibody-dependent damage to schistosomula of Schistosoma mansoni; the evidence was supported by in vivo studies which demonstrated that depletion of eosinophils by anti-eosinophil serum would abolish established immunity to cercarial challenge in mice. Other studies have shown that eosinophils are intimately involved in the inflammatory reactions around schistosomula at the site of cercarial challenge in an immune animal and also around schistosomula in the lung when these organisms have been injected intravenously into immunized mice. In addition to their active role against the larvae, eosinophils have been shown recently to destroy Schistosoma mansoni eggs in vitro, and are found in abundance in granulomas induced by eggs from Schistosoma mansoni.

The eosinophil was first discovered in 1846 and ever since it has been an object of curiosity and fascination to investigators. The literature on eosinophils is vast; Schwarz's review done in 1913 contains 1,200 references. Nonetheless knowledge of the structure and of the chemical composition of the cell has continued to grow and in the past decade a series of studies have disclosed new information. In this review certain of the more recent reports dealing with the structure and biochemistry of the eosinophil will be discussed.

Cytoplasmic organelles of eosinophils The distinguishing feature of the eosinophil under the light microscope is the dense eosinophilic granulation which may completely fill the cytoplasm. When viewed under the electron microscope the eosinophil granule, as shown in Figure 1, is seen as a membrane bounded cytoplasmic organelle with a distinctive internal structure. In most cases, the eosinophil granule has an electron dense core and a less dense matrix.

Whatever the major functions of the eosinophil—whether related to cytotoxicity, modulation of inflammatory responses, or some other aspect of host defense—it is clear that intimate relationships must be established between this cell and its surroundings. These relationships necessarily involve the eosinophil's surface so that an understanding of its features, particularly those features termed its “membrane receptors,” is essential for an appreciation of the range of this cell's functional potential.

A number of recent investigations have disclosed the presence of immunoglobulin and complement receptors on eosinophils; however, there has been considerable variation in the findings. In the present study we have undertaken first to define some of the important quantitative and biologic features of these receptors which might account for the differences reported from various laboratories and then to relate our findings directly to specific cell-parasite interactions using a schistosomule-leukocyte adherence assay. Eosinophils from normal individuals and from patients with helminthic infections were studied and compared.

Peripheral blood and tissue hypereosinophilia are characteristics of diverse invasive parasitic infestations in animals and humans. As the initial development of peripheral blood eosinophilia after exposure to the parasites exhibits a latent period which is markedly reduced upon the subsequent reintroduction of parasites, an immunological reaction to parasite antigens has been considered a prerequisite for peripheral blood eosinophilia. Both humoral and cellular immunological reactions have been implicated in the peripheral blood hypereosinophilia of parasitic infestations. The oral administration of Taenia taeniaeformis eggs to rats leads to eosinophilia which reaches a peak level 3 wk after a primary infection and a heightened eosinophilia 3 to 7 days after a secondary challenge. The passive transfer of anti-T. taeniaeformis antisera, or fractions rich in antibodies of the IgG2a or IgE class, followed by the oral administration of eggs produces a peripheral blood eosinophilia exhibiting comparable features to those of challenged naturally immune rats.

The lack of practical methods suitable for isolation and purification of individual cell types has precluded attempts to study granulocyte antigens and their specificity. While studies on the cell surface characteristics of the lymphocytes have identified two major classes and possibly subsets, investigations of granulocyte antigens are only just beginning. A leukocyte antigen that seems to be specific for the neutrophil has been described recently. In addition, sera from mothers of infants with neonatal neutropenia, or heterologous sera raised against neutrophils, demonstrated various specific immunologic effects. The cell surface membrane of the basophil has also been found to possess specific antigenic characteristics related to IgE. As we are beginning to formulate a clearer picture of granulocyte antigens, three subsets are now being recognized; 1) histocompatibility antigens that can be demonstrated on other cells as well; 2) cell-specific antigens that characterize each subset of granulocytes, namely, basophils, neutrophils, and eosinophils and also their precursors; 3) leukemia-associated antigens that are expressed along with the malignant transformation of granulocytes.

As William Trager pointed out some time ago in a review in Science, all obligate intracellular parasites—be they viral, bacterial, or protozoan—face a common dilemma. That dilemma is to invade their host cells in a way that is not destructive of the host cell upon whose metabolic hospitality and functional well-being their own reproduction depends. Simply stated, these organisms must penetrate the plasma membranes of their host and take up residence in a suitable location in the cell's cytoplasm. Since many of the speakers in this session will address themselves to the issues of penetration and intracellular location of specific organisms, I view my task as one of trying to place these issues into a general conceptual framework.

There are three general paths an intracellular parasite might follow to gain entry into an animal cell (Fig. 1): a) direct passage of the parasite through the host cell's plasma membrane; b) fusion of the outer membrane of the parasite with the cell's plasma membrane; c) endocytosis of the parasite by the host cell.

In 1968 and during the subsequent 9 years, studies in our laboratory have revealed that mice infected with Toxoplasma gondii are resistant to a number of phylogenetically unrelated organisms, including bacteria, fungi, and viruses. In studies designed to explore the mechanisms of this resistance it was noted that peritoneal macrophages of mice infected with Toxoplasma gondii and Besnoitia jellisoni could inhibit or kill Listeria monocytogenes, Cryptococcus neoformans, and Trypanosoma cruzi, as well as T. gondii and B. jellisoni (R. McLeod and J. Remington, manuscript in preparation † ). Studies in tumor models revealed that infection with the latter two protozoa leads to increased resistance to tumors in vivo, and that peritoneal macrophages of the infected mice can inhibit or kill certain tumor cells in vitro. We review here certain of the recent results from our laboratory concerning the influence of in vivo infection with Toxoplasma on macrophage function and the role of macrophages in resistance to Toxoplasma, and attempt to place these results into perspective with results obtained by others.

Functional changes in macrophages during the immune response have been under detailed investigation for over a decade; investigation which has revealed an amazing array of activities. The macrophage has been now shown to process antigen, help and activate lymphocytes, induce extracellular cytolysis of microbes and foreign cells, synthesize and release numerous biologic substances; conduct phagocytosis, killing and digestion of microbes; respond to lymphocyte products by changes in migration, chemotaxis, metabolism and microbicidal and static actions; it can be “activated,” “armed,” and “stimulated.”

In our laboratory we have designed experimental models to examine one of these macrophage responses in detail: the response induced by lymphocyte products to effect inhibition of intracellular microbe multiplication. We have focused on this issue more for clinical than experimental reasons—the fact is that important intracellular microbes persist in tissues of the infected person for his lifetime. This persistence has been well documented in tuberculosis, and is considered a part of the life cycle in toxoplasmosis.

Trypanosoma cruzi produces widespread infections in man and animals, parasitizing a variety of cell types, particularly those of the myocardium. In this and other sites, accumulation of lymphocytes and macrophages are common, leading a number of investigators to suggest a role for the macrophage in defense against T. cruzi. This view was disregarded later, when macrophages were seen harboring great numbers of parasites. In the 1950s, Pizzi et al. again stressed the importance of macrophages in T. cruzi infections. They reported on in vivo observations that macrophages from normal mice supported growth of trypomastigotes, whereas in an immunized host, macrophages in inflammatory sites seemed to be capable of destroying those forms. More recently, Hoff described an increased resistance of macrophages from mice infected with T. cruzi and BCG for culture forms of T. cruzi. We have been examining the in vitro interaction of T. cruzi with normal mouse peritoneal macrophages and with macrophages obtained from BCG- and T. cruzi-infected mice.

Receptors on the cell surface are involved in specific responses to stimuli in the cell's environment. The interaction between the cell and the environmental signals influence a diverse range of events from developmental biology and the immune response to hormonal and neural regulation. It is not surprising, then, that viruses and protozoa that have evolved with their hosts utilize similar mechanisms for attachment and invasion of host cells. In this case the parasite would have a receptor for a determinant on the host cell. This receptor on the parasite, in turn, attaches to a receptor for the parasite on the host cell. Identification of the receptors on the host cell could facilitate isolation of the parasite receptor which might be used as an immunogen for inducing protective immunity.

In this paper we will review the evidence for receptors on erythrocytes for malaria merozoites and the nature of these receptors in three primate malarias, Plasmodium knowlesi, P. vivax, and P. falciparum.

The new information and advances in malaria in the last few years, especially at the immunological and cellular level, have truly been impressive. Consequently, shifting emphasis to this area of research action has stirred expectations of new initiatives for an attack on malaria by immunologic methods. Three of the four presentations that follow will undoubtedly emphasize the new biology that characterizes current research in malaria. Therefore, I thought it appropriate at the outset to refresh our memory of some features of malarial infection in the human host—features against which the new information must contend or which it should clarify.

Let us recall that infection with all species of human malaria is initiated under natural conditions by sporozoites from an infected mosquito. In each instance pre-erythrocytic development first takes place in parenchymal cells of the liver, releasing merozoites to initiate the asexual cycle in circulating red blood cells some 6 to 30 days later in most instances, depending upon the species of parasite and other factors.

Immunity to haemoprotozoa is complex, involving several components that interact in ways that vary from one host-parasite combination to another. In recent years, most attention has been given to antibody formation, which is relatively easily measured. It has been suggested that antibodies interfere with the penetration of merozoites into erythrocytes, or opsonize parasites or parasitized erythrocytes for phagocytosis by macrophages. Our observations on human and rodent malaria and Babesia infections show that other factors must also be borne in mind.

Erythrocytes themselves differ in susceptibility to infection, and some of the factors underlying these differences are known, as will shortly be summarized. These differences are specific for each parasite, even though they do not depend upon acquired responses of immunocompetent cells.

That both T- and B-lymphocytes respond to parasite antigens is, nevertheless, clear. Cells of the B-lymphocyte lineage produce antibodies to the parasites, and ways of detecting responses to T-lymphocytes are considered later.

The clinical pattern of acquired immunity to malaria varies widely in different parasite host species. In some instances there is no effective immune response and the disease is rapidly fatal, e.g., Plasmodium knowlesi in the rhesus monkey. More commonly, however, induced immunity controls but does not eliminate the infection, which persists at low density over long periods—a sequence referred to as ‘premunition,’ e.g., P. falciparum in man. Finally, malaria may result in clinical cure, complete elimination of the parasite (sterilizing immunity) and life-long resistance to challenge, e.g., P. berghei in the rat. A given species of Plasmodium does not induce comparable immunity in different hosts, e.g., P. knowlesi produces a rapidly fatal infection in the rhesus monkey, Macaca mulatta, but mild intermittent parasitemia in the kra monkey, M. fascicularis. In addition, immunity may be age-dependent, as in the rat where the diminishing intensity of P. berghei infections is associated with altered immune responsiveness.

Evidence has accumulated during the last decade that immunopathological mechanisms are involved in the pathogenesis of lesions in several parasitic infections. Although both humoral and cellular mechanisms may be involved, considerable progress has been made in the understanding of lesions initiated (and perpetuated?) by humoral components, mainly immune complexes (IC) and complement, in protozoal infections, particularly in malaria. I would like to stress that formation of IC, i.e., binding of antibody with relevant antigen, is a physiological mechanism and protective function of antibody, and, therefore, that the antigen-antibody complexes formed are part of the defense system. Most of the IC formed are removed by physiological routes and their presence in circulation and/or in phagocytic cells should not always be regarded as a pathological situation. However, due to the multivalent properties of IC, they may initiate harmful effects leading either to tissue injury or to impairment of otherwise effective immunological mechanisms.

Antigenic variation in trypanosomes has been the subject of several recent and comprehensive reviews. The substance of my own contribution to reviewing current highlights of this topic is being published elsewhere. In the absence of new results to comment upon, I shall in the present report restrict myself to a concise summary of the structure and possible functions of the major variant surface antigens of Trypanosoma brucei, concluding with a brief assessment of the prospects for immunoprophylaxis and improved diagnosis of the African trypanosomiases in the light of an increased understanding of antigenic variation.

It is now clear that antigenic variation in T. brucei can be largely, if not entirely, explained by the sequential expression of alternative cell surface glycoproteins. It seems likely that similar groups of glycoproteins are responsible for antigenic variation in other salivarian trypanosome species. The repertoire of alternative antigens which may be expressed by a trypanosome clone, strain or species, or with a certain geographical area, is so far undetermined but may run to several hundred.

Dr. Anthony C. Allison

Clinical Research Centre

Harrow, Middlesex, England

Dr. Philip W. Askenase

Yale University School of Medicine

New Haven, Connecticut

Dr. Frank Austen

Robert B. Brigham Hospital

Boston, Massachusetts

Dr. Wilford S. Bailey

School of Veterinary Medicine

Auburn University

Auburn, Alabama

Dr. Richard Beaudoin

Naval Medical Research Institute

Bethesda, Maryland

Dr. Barry R. Bloom

Albert Einstein College of Medicine

Bronx, New York

Dr. Dov Boros

Wayne State University

School of Medicine

Detroit, Michigan

Dr. Markley Boyer

Harvard University

School of Public Health

Boston, Massachusetts

Dr. Anthony Butterworth

Wellcome Trust Research Laboratory

Nairobi, Kenya

Dr. André R. G. Capron

Institut Pasteur

Lille, France

Dr. Monique Capron

Institut Pasteur

Lille, France

Dr. Allen W. Cheever

National Institute for Allergy and Infectious Diseases

National Institutes of Health

Bethesda, Maryland

Dr. Sheldon G. Cohen

National Institute for Allergy and Infectious Diseases

National Institutes of Health

Bethesda, Maryland

Dr. Sidney Cohen

Guy's Hospital Medical School

London, England

Dr. Daniel G. Colley

Vanderbilt University

Nashville, Tennessee

Dr. G. A. M. Cross

Medical Research Council

Molteno Institute

Cambridge, England

Dr. Raymond Cypess

New York State College of Veterinary Medicine

Cornell University

Ithaca, New York

Ms. Susan Damsel

Case Western Reserve University

Cleveland, Ohio

Dr. John R. David

Robert B. Brigham Hospital

Boston, Massachusetts

Ms. Roberta David

Robert B. Brigham Hospital