Current Pharmaceutical Design - Volume 9, Issue 1, 2003

The innate immune system uses a series of pattern recognition receptors to detect the presence of pathogens thus allowing for rapid host defense responses to invading microbes. A key component of such receptors are the “toll-like receptors”(TLRs), which recognize a panel of microbial molecules that tend to be somewhat invariant, at least in select regions, thus permitting a relatively small number of receptors to recognize a large number of different microbes. Accordingly, this panel of TLRs bears little ability to distinguish between commensal and pathogenic microbes as such organisms generally bear far more structural similarities than differences between them. For the professional phagocytic cells classically considered to be the primary mediators of innate immunity such distinction between commensal and pathogenic microbes is not particularly important since any microbe that breaches the outer host defensive barriers to reach these phagocytes, whether doing so by a pathogenspecific or opportunistic mechanism, is likely potentially hazardous to its host. However, epithelial cells that line mucosal surfaces, thus being on the front line of host defense, also play an active role in innate immunity particularly by secreting chemokines and other immune mediators in response to pathogenic microbes. Epithelial cells have been reported to express several TLRs suggesting these receptors play a role in intestinal epithelial innate immune signaling pathways. However, since some mucosal surfaces such as the intestinal epithelium are normally densely colonized by a wide variety of microbes, the ability to distinguish the occasional pathogen from the sea of commensals presents an important challenge. This minireview considers the current findings regarding TLR expression in the intestinal epithelium and the role these receptors might serve in host defense.

DAP12 is a novel immunoreceptor tyrosine-based activation motifs (ITAM)- bearing transmembrane adapter molecule. This molecule, together with its partner receptor complex molecules including the killer cell activating receptors (KARs), myeloid DAP12 associating lectin-1 (MDL-1), triggering receptor expressed on myeloid cells 1 / 2 / 3 (TREM-1, TREM-2, TREM-3), and signal regulatory protein β1 (SIRPβ1), are expressed on the surface of NK and myeloid cells including antigen presenting cells. While the function of DAP12 and its associating molecules has just begun to be unveiled, emerging evidence suggests that these molecules play an important role in both innate and adaptive immune responses. In this review, we intend to provide an overview on what have been known and are still unknown to date about the function of these molecules based on the observations made by us and others.

Mast cells have been most widely studied in the context of allergic disease but also play a critical role in host defence against bacterial infection, most elegantly demonstrated in studies using mast cell deficient w / w v mice. There is less data available concerning the role of mast cells in defence against viral pathogens, however, mast cells have been demonstrated to be a potential reservoir of infection for several pathogens, such as HIV-1 and dengue, and capable of producing mediators following challenge with a number of viral products. Traditional mast cell mediators such as histamine, protease enzymes and leukotrienes are important for effective host responses. The cytokines and chemokines produced by mast cells in response to pathogens are known to profoundly alter the nature of the innate immune response and its effectiveness in eliminating infection. Cytokine and chemokine production by mast cells is closely regulated and may occur independently of classical mast cell degranulation. Depending upon the nature of the stimulus or type of infection, a unique profile of cytokines is induced. In this review, we will examine the role and regulation of mast cell cytokines and chemokines in the context of a number of bacterial and viral infections, emphasizing the multiple receptor mechanisms used to activate mast cells. This area of research is still in its early stages and much work remains to be done. However, understanding the unique properties of resident tissue mast cells and how their cytokine responses are regulated by pathogens or pathogen products, will provide important opportunities for the therapeutic manipulation of local immune responses.

Chronic obstructive pulmonary disease (COPD) is a common cause of morbidity and mortality. The term is heterogenous and encompasses a number of distinct but often overlapping phenotypes including chronic bronchitis, small airways obstruction, emphysema and in some individuals, a systemic component. Although there have been significant advances in understanding the pathophysiology of COPD, understanding of the role of the inflammation in the pathogenesis of the condition remains in its infancy. Indeed, cytokines that are known to orchestrate the inflammatory response in asthma and other inflammatory diseases are only beginning to be reported in COPD. In this review, we highlight the potential role of cytokines in the development of mucus hypersecretion observed in chronic bronchitis and the morphological changes observed in the small airways and airspaces contributing to the development of airflow limitation and respiratory failure respectively. We report evidence that exacerbations are linked to increased expression of pro-inflammatory cytokines and that the wasting and skeletal muscle dysfunction observed in some patients is most probably related to the presence of a systemic inflammatory response. In addition transgenic and gene therapy technology has been used to explore the temporal and co-ordinated role of cytokines in the development of COPD animal models. Enhanced understanding of the events involved in the pathogenesis of COPD will lead to the development of therapy with potential to modify the observed progressive decline in lung function and impact on the development of the illness.

Idiopathic Pulmonary Fibrosis (IPF) is a chronic interstitial lung disease which results in end-stage fibrosis. The pathogenesis is believed to be related to a dysregulation in cross-talk between inflammatory and structural cells, mediated by various cytokines, chemokines and growth factors, which are responsible for the maintenance of tissue homeostasis and which coordinate the response to injury. The large number of mediators involved and the complexity of their interaction makes it difficult to identify the factors responsible for initiation of fibrogenesis and progression to chronicity. Whether a mediator's presence in fibrotic lung is as a result of tissue injury or if it playsan active role in disease onset and progression has been partly answered by the use of transient and / or permanent transgenic and gene knock-out approaches to over-express single factors at a time. Chemokines such as interleukin-8 (IL-8), RANTES, IP-10, MIG or lymphotactin, do not appear to induce fibrosis when over-expressed in rodent lung. Amongst many tested, four cytokines and growth factors have been found to be pro-fibrotic; IL-1β, which demonstrates marked inflammation, tissue damage and chronic fibrosis, TNF-α, which induces inflammation and mild fibrosis, and GM-CSF, which induces moderate inflammation and fibrosis. A common finding with these cytokines are increased lung TGF-β levels, proportionate to the degree of fibrosis generated, while TGF-β itself causes minor inflammation but marked progressive chronic fibrosis. A growth factor ‘downstream’ from the pro-fibrotic effects of TGF-β, CTGF, is a likely critical mediator. However, over-expression of CTGF produces only mild and reversible fibrosis.

Much progress has been made in understanding the relationship between cytokines, T cell development, and the maintenance of memory T cells by examining the immune response to respiratory viral infections. Most of these studies have examined the T cell response to viruses that cause acute infection of limited duration, and have focused on the interplay between cytokines and individual responses by T cell subsets. This reductionism approach has been useful to piece together the puzzle of the host-immune response to respiratory virus infection, and has added to the holistic view of the networks involved in homeostatic control of T cell development and maintenance. This review addresses aspects of T cell biology that constitute the response to respiratory viral infections.

Mycobacterium avium is a human pathogen that causes infection in immunocompetent as well as immunocompromised patients. Infection is acquired both by the respiratory and gastrointestinal routes, and bacterial invasion of mucosal epithelial cells is characteristic. M. avium crosses the mucosal barrier without triggering substantial inflammatory response. Once in the intestinal submucosa or in the alveolar space M. avium infects macrophages. Intracellular bacteria block the production of cytokines involved in the host response against the infection, such as TNF-α and IL-12, and suppress antigen presentation by the macrophage. Innate response against the infection is effective to certain extent but the ability of the bacterium to remain “silent” for a period of time prevents neutrophil and NK cells from effectively controlling the establishing of the infection. CD4+ T cells as well as CD8+ T cells are activated, although only CD4+ T cells appear to be effective in inducing anti-M. avium activity in macrophages. M. avium-specific CD8+ T cells undergo apoptosis early in the infection. Therefore, the immune mechanisms of the host and bacterial strategies for survival are complex and fascinating.

Diseases caused by Chlamydia trachomatis infection and its sequelae represent major public health concerns worldwide. In order to rationally develop an effective vaccine to chlamydial infection, we need a better understanding of the mechanisms underlying the protective and pathological immune responses to chlamydial antigens. Recent studies in chlamydial immunobiology have demonstrated a close link between cytokine production patterns and the type of immune responses to this pathogen. In particular, IL-10 has been found to be associated with susceptibility to chlamydial infection and the typical pathological changes caused by the infection such as granuloma formation and fibrosis. Although Th1 type delayed hypersensitivity (DTH) is associated with protective immunity, the Th2 type DTH documented in interferon (IFN)-γ gene knockout (KO) mice fails to control chlamydial infection. The Th2 type DTH is characterized by eosinophil and neutrophil infiltration and is associated with high levels of IL-4 and IL-5 production. The capability of dendritic cell (DC) in initiating T cell response to Chlamydia has been shown in studies using cultured DCs and a DC line. Further studies should be performed to elucidate the role played by DCs in natural chlamydial infection and the potential involvement of DC subsets in directing immune responses to chlamydial infection.

Several clinical trials have attempted to treat sepsis by blocking certain aspects of the inflammatory response. Tumor necrosis factor and interleukin 1 have been specific targets for inhibition but none of the trials have been successful. These trials were started on the basis of preclinical trials that suggested these would be effective. There were three lines of evidence to support the idea of cytokine inhibition. First, patients with increased levels of cytokines are more likely to die. Second, experimental animal models demonstrated that blocking the cytokines would improve outcome. Third, injection of purified, recombinant cytokines would cause both organ injury and death in experimental animals. Several additional aspects of the inflammatory response have been discovered since these trials were initiated. Included among these potential new targets are interleukin 18 and HMG-1. However, before new clinical trials are started there must be careful consideration of why previous interventions were not effective. The concept of blocking a single elevated cytokine may be too simple to deal with the complex problem of sepsis. As patients move through different phases of the septic response, there may be intervals when it is appropriate to inhibit multiple cytokines while at other times it may be appropriate to augment the immune response.

Autoimmune diseases frequently develop as a result of an abnormal activation of autoreactive T cells, excessive production of proinflammatory cytokines, particularly by CD4+ Th1 cells, and subsequent tissue destruction. Cytokine-dependent immunotherapy can be applied to alter the balance between Th1 and Th2 cell activity, or proinflammatory versus immunosuppressive cytokine profiles. Cytotoxic T lymphocyte (CTL) and / or macrophage activity can also be suppressed. Gene transfer offers numerous advantages for the in vivo delivery of cytokines or their receptors for immunotherapeutic use. We have relied on the injection of naked plasmid DNA into skeletal muscle to deliver therapeutic genes. In particular, we have successfully used this approach to deliver neutralizing cytokine receptors such as interferon γ (IFNγ)-receptor-Ig fusion proteins or anti-inflammatory cytokines such as transforming growth factor β-1 (TGF-β1) and interleukin 4 (IL-4). Intramuscular gene therapy is effective in protecting against several experimental autoimmune diseases including insulin-dependent diabetes mellitus (IDDM), experimental allergic encephalomyelitis (EAE), and systemic lupus erythematosus (SLE). Another promising approach involves DNA vaccination by plasmid-based codelivery of genes encoding an autoantigen and either a cytokine or other immunomodulatory molecule. Plasmid vectors offer interesting advantages over viral vectors, since they are simple to produce, nonimmunogenic and non-pathogenic. They can be repeatedly administered with relatively prolonged periods of expression in vivo, ranging from weeks to months after each injection. Plasmid-based intramuscular gene transfer has great therapeutic potential in the areas of autoimmune and inflammatory disorders.