GM-CSF treatment for Crohn's disease: a stimulating new therapy?

Crohn's disease (CD) has been classically viewed as an overactive intestinal immune response to the normal constituents of the gut flora. Most therapeutic strategies to date have tried to suppress this overactive adaptive immune response. Recently, a novel, rather alternative therapeutic strategy has been proposed, wherein the approach is to stimulate the innate immune system with growth factors. This review will take a closer look at this unconventional hypothesis and the data that support it, and will place the information in the context of some of the other biological and experimental therapies currently under consideration for the treatment of CD.     

Possible drug targets for celiac disease

Celiac disease (CD) is an intestinal disorder caused by an altered immune response against wheat gluten, a common dietary antigen, and related cereal proteins. Both CD4+ and CD8+ T cells have a role in inducing the intestinal damage, although recent studies have also pinpointed the involvement of the innate immune response in CD pathogenesis. So far, the only available treatment for CD is the strict avoidance of gluten in the diet, but the poor compliance and the associated complications demand alternative therapies. During the last decade, the knowledge of genetic, molecular and cellular mechanisms leading to CD pathogenesis made great progress. The improved understanding of gluten peptides activating either adaptive or innate immune response, of HLA restriction molecules, as well as of cytokines that mediate most of the inflammatory reactions, opens several new promising perspectives for therapeutic intervention. This review discusses both molecular and cellular strategies to treat CD, including the use of proteolytic enzymes active on gluten peptides, antibodies neutralising IL-15 and IFN-gamma, drugs targeting HLA, regulatory cytokines and T cells.     

Synthesis and biological evaluation of biotinyl hydrazone derivatives of muramyl peptides

Muramyl peptides derived from bacterial peptidoglycan have long been known for their ability to trigger host innate immune responses, including inflammation and antimicrobial defense. Muramyl peptides have also been widely studied for their role as immune adjuvants. In mammals, the nucleotide-binding oligomerization domain (Nod) proteins Nod1 and Nod2 detect distinct muramyl peptide structures and mediate their biological activity. Because of the poor immunogenicity of these small peptidoglycan derivatives, research in this field is currently limited by the lack of reagents to track or immobilize specific muramyl peptides. We present here the generation and initial biological characterization of synthetic muramyl peptides covalently coupled to dansyl or biotinyl derivatives and demonstrate that biotinyl coupling on the muramyl moiety results in derivatives that can be tracked by immunofluorescence and maintain full biological activity, as observed by their capacity to trigger Nod signaling. Moreover, using digitonin-mediated permeabilization techniques on live cells, we also demonstrate that biotinylated muramyl peptides efficiently reach the host cytosol, where they activate Nod signaling. Therefore, these derivatives represent useful probes to study the cell biology and the biochemistry of host responses to muramyl peptides.     

Possible drug targets for celiac disease

Celiac disease (CD) is an intestinal disorder caused by an altered immune response against wheat gluten, a common dietary antigen, and related cereal proteins. Both CD4⁺ and CD8⁺ T cells have a role in inducing the intestinal damage, although recent studies have also pinpointed the involvement of the innate immune response in CD pathogenesis. So far, the only available treatment for CD is the strict avoidance of gluten in the diet, but the poor compliance and the associated complications demand alternative therapies. During the last decade, the knowledge of genetic, molecular and cellular mechanisms leading to CD pathogenesis made great progress. The improved understanding of gluten peptides activating either adaptive or innate immune response, of HLA restriction molecules, as well as of cytokines that mediate most of the inflammatory reactions, opens several new promising perspectives for therapeutic intervention. This review discusses both molecular and cellular strategies to treat CD, including the use of proteolytic enzymes active on gluten peptides, antibodies neutralising IL-15 and IFN-γ, drugs targeting HLA, regulatory cytokines and T cells.     

Intestinal flora and Crohn's disease

The pathogenesis of inflammatory bowel diseases (IBD) proceeds through stages of initiation, amplification and healing. Abundant clinical and experimental data incriminate luminal bacteria or bacterial products in both the initiation and perpetuation of chronic intestinal inflammation. Macrophage and T-cell activation with accompanying inflammatory cytokine production appears to be an early event. Studies of lymphocyte responsiveness to autologous and heterologous intestinal bacteria have suggested that this activation may result from a breakdown in tolerance to the enteric flora in IBD. This lack of tolerance might be due to an imbalance between protective and aggressive commensal luminal bacterial species (dysbiosis), a decreased barrier function and/or an impaired mucosal clearance allowing the access of bacteria to the mucosal immune system and lack of regulatory mediators or cells. There is still controversy over whether the virulence traits of bacteria are expressed broadly or just in a small subset of bacteria. Individual bacterial species within the indigenous flora vary in their capacity to drive intestinal inflammation. In experimental models, some bacteria such as Bacteroides vulgatus can cause colitis alone when monoassociated in the HLA-B27 transgenic rat model. Others, including Lactobacillus and Bifidobacterium species have no proinflammatory capacity and have been used as probiotics. In patients with IBD, systematic approach to this issue is hampered by the limited knowledge of intestinal flora. Adherent-invasive Escherichia coli are a possible candidate for the onset and/or persistence of intestinal inflammation in patients with Crohn's disease, since they possess all the virulence factors that allow the bacteria to cross the intestinal barrier, to move to deep tissues, and to continuously activate macrophages. The recent identification of NOD2/CARD15 as a susceptibility gene for Crohn's disease has provided another link between the immune response to enteric bacteria and the development of mucosal inflammation. NOD2/CARD15 is composed of two caspase recruitment domain (CARD), a nucleotide-binding domain (NBD) and a leucin-rich-repeat (LRR) region. The LRR domain of NOD2/CARD15 has binding activity for bacterial peptidoglycans and its deletion stimulates the NF-kappaB pathway. The most frequent variants of NOD2/CARD15 observed in Crohn's disease tend to cluster in the LRR and its adjacent regions. This suggests that the LRR domain of CD-associated variants is likely to be impaired in its recognition of microbial components. Continuing studies are investigating the pathophysiological mechanisms induced by NOD2/CARD15 variants in the intestinal mucosa.     

Novel strategies to attenuate immune activation in Crohn's disease

Our understanding of the pathogenic mechanisms that underlie the chronic intestinal inflammation characteristic of Crohn's disease (CD) has advanced greatly in recent years. We now realize that effective treatment will not be achieved through non-specific generalized immunosuppression, but rather with the development of well-designed therapies that target specific immunological pathways. The success of infliximab has driven exploration into the blockade of additional pro-inflammatory cytokines, such as interleukin-12 and interferon-gamma. Novel strategies have also emerged that target other components of the inflammatory cascade, such as the migration of leukocytes to the intestine. The identification of mutations in the nod2/card15 gene that are associated with a predisposition for the development of CD has focused attention on the role of the innate immune system in CD. This has allowed a fundamentally different approach to the development of therapies for CD aimed at intensifying innate immune responses. In parallel with the development of several 'biological' agents, alternative therapeutic options have also shown promising results. The use of probiotic bacteria as both therapeutic agents and delivery vehicles for other molecules is a rapidly developing field. As the role of regulatory T cells in the pathogenesis of CD is revealed, the potential of immunotherapy is being opened up. With so many therapeutic options in sight and the ongoing elucidation of the pathogenesis of CD, it will become steadily more possible to tailor treatment to the clinicopathological and immunogenetic profile of the individual patient.     

Pandemic of atopic diseases--a lack of microbial exposure in early infancy?

Improved hygienic conditions in Western societies have reduced early microbial exposure, which has been proposed as a reason for the continuously rising prevalence of atopy and subsequent atopic diseases: atopic eczema, allergic rhinitis and asthma (The Hygiene Hypothesis of Allergy). This hypothesis is supported by immunological data showing that the immune response to microbial antigens, both pathogenic and non-pathogenic ones, is accompanied by preferential expression of cytokines that counterbalance the T-helper 2-polarized cytokine production of neonates, the continuity of which might lead to enhanced IgE production, atopy, and atopic disease. Experimental, epidemiological and clinical studies, conducted over the last decade, indicate that non-pathogenic microbes in the gut might be a major factor essential for the maturation of the human immune system to a nonatopic mode. A recent randomised, placebo-controlled trial demonstrated that perinatal administration of probiotics, cultures of potentially beneficial bacteria of the healthy gut microflora, halved the later development of atopic eczema during the first two years of life. Some putative mechanisms of action of gut commensals in host-microbe interactions have been described. Two structural components of bacteria, the lipopolysaccharide portion of Gram-negative bacteria and specified CpG motif in bacterial DNA, activate immunomodulatory genes via Toll-like receptors present e.g. on intestinal epithelial cells thus controlling physiological cytokine milieu in the gut. Probiotics have also been shown to reverse increased intestinal permeability and to reduce antigen load in the gut by degrading and modifying macromolecules. The actual preventive role of natural and genetically constructed supplementary microbes in the development of immunological diseases, like allergy, remains to be elucidated.     

Immune-non immune networks in intestinal inflammation

The intestinal mucosa forms a primary barrier providing both barrier function and immediate effective recognition of bacterial products invading the mucosa. This is of great importance for the prevention of permanent and chronic inflammation as a reaction to the commensal intestinal flora and the multitude of antigens present in the intestinal lumen. It is obvious that a tight network of specialized cell types and intense cell-cell communication is required to maintain this function and coordinate immunological reactions. Yet most publications are focused on unidirectional cause and effect-chains. Since a real integrated view on the network of cellular functions is not available or at least incomplete bidirectional immune cell interactions with epithelial cells, fibroblasts/myofibroblasts, adipocytes endothelial cells and the nervous system are reviewed in this article. Networking is certainly mediated by different effector pathways but limited resources are available to assemble a model of interactions in intestinal inflammatory diseases. However, recent development of knowledge regarding unidirectional and bidirectional effect-chains is exciting. Apart from the classical discrimination of immune cells (such as neutrophils, macrophages, and cytotoxic T cells) and non immune cells (epithelial cells, fibroblasts, adipocytes and endothelial cells) it became stunningly evident that not only the classical immune cells have the ability to track down pathogens as most of the mentioned cell types express pathogen recognition receptors (toll-like receptors, Nod2) and defense mechanisms (such as secretion of defensin).     

Siglecs in innate immunity

Siglecs are sialic acid-binding Ig-like lectins expressed in a highly specific manner, and which are implicated in signaling and adhesive functions. The CD33-related siglecs represent a distinct subgroup that is undergoing rapid evolution within the innate immune system, with the potential to trigger apoptosis and provide inhibitory signals. CD22 is a well-characterised B cell restricted siglec that has been shown to mediate both sialic acid-dependent and -independent signaling functions in B cell regulation. As endocytic receptors, siglecs provide portals of entry for certain viral and bacterial pathogens, as well as therapeutic opportunities for targeting innate immune cells in disease.