Current Protein and Peptide Science - Volume 7, Issue 4, 2006

I would like to thank Prof. Ben M. Dunn, Editor in Chief of CPPS, for supporting my proposal in organizing this special issue on Immune receptors for glycoconjugates. The aim of this issue is to highlight the rapid progress in our understanding of glycoconjugate-mediated communication in the context of immune responses. Only after this project was completed did I recognize what a precious chance it was for me to serve the community of glycobiology and immunology. In retrospect, I am so thankful to all the authors and reviewers for their excellent contribution. Many of them sacrificed weekends or holidays, as evidenced by my records of dates in receiving emails. I am greatly encouraged by the brilliance of every article after these synergized efforts. I have very much enjoyed the learning from each author and each reviewer. The selected topics have been based on my personal view of the important questions. These include the dendritic cell lectins, the lectins that interact with HIV, the glycopeptide antigens, the immune responses to bacteria polysaccharides, and the glycolipid antigens. These articles cover the carbohydrate recognition in both the innate immunity and the adaptive immunity. In every article, the power of biochemistry and glyco-chemistry is highlighted. I hope the concepts and strategic designs proposed by our authors will be helpful for the readers in understanding the rules of glyco-immunology. Glycoconjugates are extremely structurally diverse, with the unique central dogma being storage of information through the action of glyco-enzymes instead of genetic codes. My sincere hope is that readers will be excited enough by these articles to follow this area of research closely, and perhaps will even join us in making contributions to this very promising field of study.

Human C-type lectin receptors (CLRs) characteristically bind glycosylated ligands in a Ca2+-dependent way via their carbohydrate recognition domain (CRD). Their carbohydrate preference is dependent on the amino acid sequence in the CRD domain and on the ability and flexibility of the CRD domain to accommodate sugar moieties that are located at different distances from each other in the glycoconjugate. Although microbial and vertebrate cells are able to produce similar polysaccharide chains, the density of carbohydrates on microbes is much higher compared to vertebrate cells. Despite this difference, carbohydrates present on both cell types can be recognized by the CLRs. These receptors are predominantly expressed by antigen presenting cells such as dendritic cells. In addition to the Toll-like receptor family, CLRs function as pattern recognition receptors by recognizing glycosylated patterns on pathogens. This usually results in internalization of the pathogen, lysosomal degradation and subsequent loading of pathogen-derived peptides into major histocompatibility complex molecules for antigen presentation. However, several pathogens have developed ways to exploit the CLRs to evade immune eradication by for example escaping from the lysosomal degradation pathway or by inducing anti-inflammatory cytokines. When CLRs bind endogenous glycosylated ligands they mediate several processes like cellcell adhesion and clearance of aberrant cells like tumor cells or apoptotic cells.

Adaptive humoral immunity to extracellular bacteria is largely mediated by antibody specific for both protein and polysaccharide antigens. Proteins and polysaccharides are biochemically distinct, and as a result are processed differently by the immune system, leading to different mechanistic pathways for eventual elicitation of specific Ig isotypes. Much of our current knowledge concerning the parameters underlying anti-protein and anti-polysaccharide Ig responses have come from studies using soluble, purified antigens. However, the lessons learned from these studies are not entirely applicable to the mechanisms underlying physiologic anti-protein and anti-polysaccharide Ig responses to intact bacteria. Specifically, unlike isolated, soluble antigens, intact bacteria are complex particulate immunogens in which multiple protein and polysaccharide antigens, and bacterial adjuvants (e.g. Toll-like receptor ligands) are co-expressed, indeed often physically linked. In this review, data from a series of recent studies are discussed in which heat-killed, intact Streptococcus pneumoniae was used as an immunogen to study the mechanisms underlying in vivo anti-protein and antipolysaccharide Ig isotype induction. An unexpected role for CD4+ T cells and dendritic cells for induction of IgG antipolysaccharide responses by intact bacteria is discussed, and shown to have distinct mechanistic features from those that mediate anti-protein responses. The further role of cytokines, Toll-like receptors, and B cell receptor signaling in mediating these responses, and its implications for the effectiveness of anti-pneumococcal, polysaccharide-based vaccines, is also discussed.

Until about 1990 there was general consent about the assumption that only protein and peptide antigens have the capacity of CD4+ or CD8+ T-cell stimulation. Since about ten years evidence is now accumulating that carbohydratepeptide epitopes do play a role in classical MHC-mediated immune responses. This holds true for glycopeptides, where the glycan chain is short and not located at an "anchor residue " needed for MHC interaction. T-cell recognition of Oglycosylated peptides is potentially of high biomedical significance, because it can mediate the immune protection against microorganisms, the vaccination in anti-tumor therapies, but also some aspects of autoimmunity. The epithelial type 1 transmembrane mucin MUC1 is established as a marker for monitoring recurrence of breast cancer and is a promising target for immunotherapeutic strategies to treat cancer by active specific immunization. Natural human immune responses to the tumor-associated glycoforms of the mucin indicate that antibody reactivities are more directed to glycopeptide than to non-glycosylated peptide epitopes. To overcome the weak immunogenicity of the natural target, heavily O-glycosylated MUC1, the question was addressed whether O-linked glycans remain intact during processing in the MHC class II pathway and interfere with endosomal processing and peptide presentation. Attempts were made to define on a biochemical level the structural requirements for an efficient endosomal proteolysis catalyzed by cathepsin L in antigen-presenting cells. Evidence based on work with CD4+ T-hybridomas confirms that O-glycopeptides can be effectively presented to T-cells and that glycans can form integral parts of the TCR defined epitopes. Similar approaches are currently followed in the MHC class I pathway which aim at the identification of immunogenic glycopeptides generated by immunoproteasomes.

Approximately half of the molecular mass of gp120, the receptor-binding envelope protein of human immunodeficiency virus (HIV), consists of N-linked glycans. Nearly half of these glycans are of the high mannose type. These high mannose glycans furnish a rich forest of mannose residues on the virus surface making HIV a prime target for interaction with mannose-specific lectins of the immune system. This review focuses on the known interactions between gp120 and immune system lectins some of which HIV appears to exploit. The effect of variation in glycosylation of gp120, especially with respect to clades of HIV, on binding of immune system lectins is highlighted.

The mechanistic studies on immune recognition of carbohydrates have been paved by the synergized advances in identifying the precise sugar structures recognized by the immune system, in analyzing the cellular and humoral components bearing the receptors for glycoconjugates, and production of the biological relevant carbohydrate epitopes by synthetic chemistry. In our current studies on natural antigenic glycolipids, we have found that the activation as well as the development of natural killer T cells (NKT) is guided by the information provided by glycolipid metabolism pathways in antigen presenting cells (APC). Based on genetic data and cellular immunological assays, we propose a neutral glycosphingolipid isoglobotrihexosylceramide, iGb3, as one of the candidates recognized by NKT cells under pathophysiological conditions such as cancer and auto-immune disease. New immunotherapy approaches might be explored by interfering with glycolipid metabolism or by directly supplementing rationally designed glycolipids.

The exchange of information between cells represents an important regulatory mechanism for cellular activities. Such regulation processes mainly occur by hydrophilic compounds, unable to penetrate the cell membrane. Accordingly such signals have to be transmitted into the cell that is performed by transmembrane receptors. The widespread group of G-protein coupled receptors plays a decisive role in extracellular signal recognition and transition into cellular response. The importance of this interaction is evidently shown by the severe diseases that correlate with dysfunction of the interaction between ligand and G-protein coupled receptor. The development of drugs against these diseases needs the comprehension of signal recognition and transition as well as the understanding of intracellular signal pathways. In this review, we describe concepts and methods to identify the structure-activity relationships of G-protein coupled peptide receptors and their successful application. Furthermore we provide an insight into peptide based drug design. Examples are taken from the field of CGRP, orexin and growth hormone secretagogue receptor ligands.

Rho proteins belong to the Ras superfamily of small GTPases and function as binary switches that shuttle between active and inactive states based on the nature of bound guanine nucleotide. Three sets of regulatory proteins, namely, guanine dissociation inhibitors, guanine exchange factors, and GTPase activating proteins (GAPs) control the balance between active and inactive Rho proteins. There are more than 70 RhoGAPs encoded in the human genome. The RhoGAP family is distinguished by the presence of the RhoGAP domain. However, the majority of RhoGAPs contain multiple additional domains. There are as many as eight domains in some of these proteins. The modular structure of GAPs is important for their interaction with other proteins. A significant number of RhoGAPs have been shown to be present in altered abundance in a variety of human cancers or cell lines. The ability of RhoGAPs to modulate Rho mediated signaling pathways may lend themselves as targets for small molecule therapeutic agents against cancer.