Current Topics in Medicinal Chemistry - Volume 4, Issue 5, 2004

Molecules of Infectious Agents as Immunomodulatory Drugs The continuous need to control infectious agents has invariably led to the application of emerging new biological disciplines and / or methodologies to their study. Particularly pertinent, advances in immunology and molecular biology during the first decades of the latter half of the twentieth century resulted in their application to research on infectious agents during the later decades. A consequence of this is that information is now available on the structure and function of a great deal of molecules derived from a wide variety of pathogens. The perceived benefit of this was that such information could be utilised to develop novel control tools for infectious agents, in particular vaccines and drugs. However the realisation that the function of some pathogen-derived products is to aid their survival by modulating immune responses of the infected host, allied to increasing awareness that many human illnesses are associated with an aberrant immune response, has recently led to the emergence of a new concept. This concept is that molecules derived from infectious agents rather than being detrimental to man, may actually benefit him. This special edition of Current Topics in Medicinal Chemistry is thus timely and brings together articles from a number of researchers who have recently began working in this new area. The molecules under study are derived from many sources including viruses, bacteria, parasites and even (although not officially an infectious agent !), a marine sponge. They have been investigated with respect to how they interact with the various cells of the immune system and a recurring theme is the induction of a polarised immune response, which is broadly anti-inflammatory in nature. Such a response, mediated by, for example, Th-2 or T-regulatory lymphocytes producing cytokines such as IL-4, IL-10 or TGF-beta, can be predicted to be of value in preventing or ameliorating pro-inflammatory diseases and this is indeed demonstrated in various models of such diseases including arthritis, diabetes, colitis and allergy. There is thus great hope that such molecules (or derivatives of them) will ultimately find use as drugs in the treatment of human disease. In retrospect it is perhaps not surprising that molecules of infectious agents may find employment as immunomodulatory drugs. During the last few decades there has been an explosive increase in the West, of allergic and autoimmune disease. Scientists and clinicians have struggled to explain this worrying situation but recently there has been emerging support for the idea that it may reflect improved hygiene resulting in lack of exposure to childhood pathogens that normally “prime” the immune system. Whether this theory - the “Hygiene Hypothesis” will hold up remains to be seen but what is certain, is that this new field of using pathogen products as drugs is likely to expand rapidly. The reason for this is the huge amount of data emerging from genome sequencing studies. More and more infectious agents are being exposed to this procedure and hence it is surely likely that more and more immunomodulators will be discovered.

Microbes produce a wide range of molecules that can modulate eukaryotic immune responses. These include factors that subvert protective mechanisms in order to facilitate pathogen colonization and persistence. Viral, bacterial and parasite-derived molecules have been identified that can inhibit inflammatory responses. However, in addition to the plethora of microbial factors that suppress immune responses, the most potent immune activators are also of microbial origin. These include the bacterial enterotoxins, parasite-derived excretory-secretory products and viral nucleic acids. In fact, there are examples of immune modulators that can exert either stimulatory or suppressive effects depending on the mode of delivery, dose and experimental model. There is presently great interest in the therapeutic exploitation of these factors, for example as a means to stimulate enhanced immune responses to a new generation of subunit vaccines or to inhibit deleterious immune mediated diseases. This short review, describes representative microbial immunomodulators, their modes of action and the potential for therapeutic application.

Cholera toxins and heat labile enterotoxin from E. coli differ from most soluble proteins in eliciting systemic immunity both against themselves and unrelated admixed antigens, rather than tolerance following administration to a mucosal surface. Several reports have also demonstrated preferential induction of Th2-type responses when these molecules are used as adjuvants. Conversely, these proteins and their non-toxic derivatives, including the B sub-units are also able prevent and alleviate autoimmune diseases in naive and systemically immune hosts demonstrating wide-ranging effects on the immune system. The recent observation that amelioration of autoimmune disease is associated with the generation of regulatory T cells which inhibit pathogenic Th1 responses may also help to consolidate these two apparently contradictory outcomes of exposure to the cholera-like enterotoxins. Furthermore, the observation that EtxB is able to alleviate autoimmune disease in the absence of conjugation to autoantigen highlights its potential for use in the clinical setting where the target antigen is often unknown. Direct effects on T cells, B cells and APC have been demonstrated in vitro which have provided insights into how these molecules may elicit these diverse effects. Further investigation is required for elucidation of the mechanisms of action of adjuvanticity and tolerance induction by these molecules to realise their potential for use in vaccines and therapies for autoimmune disease in humans.

Falling infection rates in the developed world are being matched by a rapidly rising incidence of allergic and autoimmune diseases. This review explores the hypothesis that there is a causal link between these phenomena and that infections can prevent the onset of autoimmune disease. The hypothesis is discussed with particular reference to Type I diabetes in the NOD mouse and the ability of the helminth infection Schistosoma mansoni to prevent its onset. The article addresses the possible mechanisms that underly this protection. The effects of protective pathogen-derived agents on key cells of the innate immune system such as dendritic cells are distinct and include the production of anti-inflammatory cytokines such as IL-10. The most likely mechanisms by which these innate changes prevent the subsequent adaptive autoimmune destruction are: (1) the production of systemically high levels of cytokines that oppose the production of cytokines that drive the autoimmune process - possibly via the action of natural killer T (NKT) cells (2) the induction of regulatory T cells that inhibit the action of autoreactive cells and (3) the production of pathogen-specific T cells that are not autoreactive and compete with autoreactive cells for survival signals such as cytokines and T cell receptor ligation.

Epidemiologic evidence strongly suggests that improved standards of living are associated with an increased incidence of immune system-mediated disease. Allergy, autoimmunity, and within the focus of our laboratory, idiopathic inflammatory bowel disease, most notably Crohn's disease and ulcerative colitis, and progression of chronic gastritis to gastric cancer, are all mediated by proinflammatory immune responses induced by known or unknown antigens. A popular theory, known as the ‘hygiene hypothesis’ [1], suggests that improved health standards achieved through sanitation and vaccination, may in part be responsible for the apparent increase in immune system-mediated disease due to decreasing microbial and parasitic infections in humans, particularly in children. As antigenic exposure of children to infectious agents, especially parasites, has rapidly decreased, it is suspected that normally protective counter-regulatory Th2-type immune responses fail to develop, increasing the risk for aberrant pro-inflammatory responses in otherwise genetically pre-disposed individuals. This hypothesis has stimulated significant interest in development of animal models of Th1- and Th2-mediated disease to test this paradigm. This review illustrates some of the exciting evidence that Th1- mediated pathology in mouse models of helicobacter disease and diabetes is ameliorated by concurrent anti-inflammatory Th2 responses to parasite antigens and that initial application of these principles is benefiting human patients. The results from developing animal models of human disease not only support the hygiene hypothesis but also have led to novel therapies using parasite antigens to stimulate anti-inflammatory Th2-type responses to restore homeostasis in patients with aberrant Th1-type immune-mediated disease.

Parasite molecules offer unique advantages for the treatment of immunologicical disorders, and several candidate molecules have been shown to be effective. In our studies, it was shown that a factor inducing immunoglobulin E from filarial nematode parasites was suppressive in animal models of immunological disorders such as allergy and insulin dependent diabetes mellitus (IDDM). The Th1 / /Th2 paradigm of CD4+ T helper cell subsets can provide the basis for the development of new types of drugs and of novel strategies for the treatment of allergic and autoimmune disorders by parasite molecules. In our experimental system, parasite molecules from a filarial nematode parasite led to the downregulation of the allergic reaction in animal models. In the majority of hosts, infection with helminths is associated with markedly reduced cellular immune reactions and polarization of T cell responses to Th2 and Th3 types. Some studies have suggested that the stimulation of host immunoregulatory networks with parasite molecules leading to the synthesis of antiinflammatory cytokines (interleukin10, transforming growth factor-beta (TGF-β) and others) can provide new therapy for immunological disorders. It is known that parasites produce some types of molecule that mimic host molecules such as CD40 ligand, TGF-β and macrophage migration inhibitory factor. These molecules are also candidates for medicinal agents. This review describes many of the latest possibilities in this field and shows how they can be best put to use for the development of medicinal agents, molecular target identification, and for prioritization.

Filarial nematodes achieve long-term infection via modulation of the host immune system. Although human infection can result in severe pathology, the majority of infected individuals exhibit little evidence of this. Analysis of the immune response during infection indicates that the apparently healthy majority have an anti-inflammatory phenotype and it has been speculated that this may contribute to maintenance of host health. Recent data suggest that parasite-derived molecular secretions contribute to the anti-inflammatory phenotype and we have thus characterised a major filarial nematode secreted glycoprotein, ES-62. This molecule has been found to possess broad immunomodulatory activities that are in general, anti-inflammatory. It has long been recognised that several autoimmune disorders including rheumatoid arthritis (RA) exhibit reduced incidence and severity in geographic regions in which filarial nematodes are endemic. Furthermore, it has been speculated that these two observations are causally linked. However, molecular explanations for such an association have not been forthcoming. Although the aetiology of RA is unknown most data suggest that it is mediated via a pro-inflammatory immune response associated with excess cytokine production. Given that ES-62 is antiinflammatory, we hypothesised that it might possess activity against diseases like RA. Indeed we found that subcutaneous injection of ES-62 prevented initiation of collagen-induced arthritis (CIA) and also suppressed progression of established disease. Ex vivo analyses demonstrated that these effects were due to inhibition of TNF-α production and reversal of collagen specific TH-1 responses. The nematode product was also found to inhibit pro-inflammatory cytokine release in vitro in synovial cells derived from RA patients. ES-62 thus represents a parasite-derived immunomodulator with significant therapeutic potential.

Although T cells were previously believed to recognize only peptide antigen associated with the major histocompatibility complex (MHC), recent studies have shown that there are unique T cells specialized for recognition of lipid or glycolipid antigens bound to the MHC class I-like CD1 molecules (CD1a, b, c or d). Among these lipid-specific T cells, CD1d-restricted T cells, also referred to as natural killer (NK) T cells, are of special interest as a target of drug development, since their role in immunoregulation has been indicated in various physiological or disease conditions including autoimmunity. They are unique in their homogeneous ligand specificity for α-glycosylated sphingolipid and secrete large amounts of regulatory cytokines shortly after T cell receptor (TCR) engagement. The first glycolipid identified as an NKT cell ligand was α-galactosylceramide (α-GalCer) derived from marine sponges. α-GalCer exhibits significant immunomodulatory effects by stimulating NKT cells. However, we found that an altered analogue of α-GalCer with a shorter sphingosine chain (OCH), is more useful than α-GalCer for treatment of autoimmune disease models, because of its ability to selectively induce IL-4, a key cytokine for control of autoimmunity. As such, altered glycolipid ligands (AGL) of α-GalCer appear to be promising reagents for treatment of human autoimmune diseases.