Showing papers by "Ajit Varki published in 2011"

Letter to the Glyco-Forum: Since there are PAMPs and DAMPs, there must be SAMPs? Glycan “self-associated molecular patterns” dampen innate immunity, but pathogens can mimic them

Abstract: The ∼500-million-year-old adaptive immune system detects foreign (“non-self”) epitopes via B cell-derived antibodies and/or T cell receptor interactions with major histocompatibility complex (MHC)/peptide complexes (Hedrick 2004). Cells of the more ancient innate immune system display receptors that detect foreign glycans, for example, fungal glycan recognition by the macrophage mannose receptor (Stahl and Ezekowitz 1998) or by circulating collectins and pentraxins (Bottazzi et al. 2010). The latter field was revolutionized by definition of “pathogen-associated molecular patterns” (PAMPs; Medzhitov and Janeway 1997), microbial products that can be detected by pattern recognition receptors (PRRs), particularly the Toll-like receptors (TLRs; Beutler 2009), Nod-like receptors (Davis et al. 2011) and dendritic cell receptors such as C-type lectins (Geijtenbeek et al. 2004). Many PAMPs are glycoconjugates (e.g., bacterial lipo-oligosaccharides) or glycan-based polymers (e.g., bacterial peptidoglycans), including bacterial DNA or viral RNA (which are (deoxy)ribose-based polymers). The innate immune system also recognizes “danger-associated molecular patterns” (DAMPs; Matzinger 2002; Chen and Nunez 2010), molecules released during tissue damage, such as heat-shock proteins, high mobility group box 1 (Lotze and Tracey 2005), hyaluronan (HA) fragments (Taylor and Gallo 2006), glycosaminoglycan (GAG)-bearing matrix proteoglycans (Moreth et al. 2010) and certain crystals (Martinon et al. 2009), all of which originate from damaged host cells or matrices. Signals initiated by DAMPs and PAMPs are transduced via similar pathways, activating innate immune inflammatory responses.

Potential impact of the non-human sialic acid N-glycolylneuraminic acid on transplant rejection risk.

Abstract: A major obstacle to clinical applications of xenotransplantation is the expression of immunogenic xenoantigens that provide targets for immune recognition of xenografts, leading to activation of host immunity and consequent rejection or poor engraftment. Among the best known xenoantigens is the “αGal” epitope (Galα1−3Galβ1−(3)4GlcNAc-R, where R is an underlying glycoconjugate) characterized by Galili and colleagues. This epitope is widely expressed by most mammals other than old world primates and recognized by abundant circulating human anti-α-Gal antibodies (1). Such antibodies are universally induced after birth in humans via exposure to gut bacteria bearing similar epitopes (2), and the resulting difficulties in xenotransplantation (3) have even encouraged production of alpha1,3-galactosyltransferase gene-knockout pigs (4) as a potential solution.

Evolutionary Forces Shaping the Golgi Glycosylation Machinery: Why Cell Surface Glycans Are Universal to Living Cells

Abstract: Despite more than 3 billion years since the origin of life on earth, the powerful forces of biological evolution seem to have failed to generate any living cell that is devoid of a dense and complex array of cell surface glycans. Thus, cell surface glycans seem to be as essential for life as having a DNA genetic code, diverse RNAs, structural/functional proteins, lipid-based membranes, and metabolites that mediate energy flux and signaling. The likely reasons for this apparently universal law of biology are considered here, and include the fact that glycans have the greatest potential for generating diversity, and thus evading recognition by pathogens. This may also explain why in striking contrast to the genetic code, glycans show widely divergent patterns between taxa. On the other hand, glycans have also been coopted for myriad intrinsic functions, which can vary in their importance for organismal survival. In keeping with these considerations, a significant percentage of the genes in the typical genome are dedicated to the generation and/or turnover of glycans. Among eukaryotes, the Golgi is the subcellular organelle that serves to generate much of the diversity of cell surface glycans, carrying out various glycan modifications of glycoconjugates that transit through the Golgi, en route to the cell surface or extracellular destinations. Here I present an overview of general considerations regarding the selective forces shaping evolution of the Golgi glycosylation machinery, and then briefly discuss the common types of variations seen in each major class of glycans, finally focusing on sialic acids as an extreme example of evolutionary glycan diversity generated by the Golgi. Future studies need to address both the phylogenetic diversity the Golgi and the molecular mechanisms for its rapid responses to intrinsic and environmental stimuli.

Human Xeno-autoantibodies Against a Non-human Sialic acid Serve As Novel Serum Biomarkers and Immunotherapeutics in Cancer

Abstract: Human carcinomas can metabolically incorporate and present the dietary non-human sialic acid Neu5Gc, which differs from the human sialic acid N-acetylneuraminic acid (Neu5Ac) by 1 oxygen atom. Tumor-associated Neu5Gc can interact with low levels of circulating anti-Neu5Gc antibodies, thereby facilitating tumor progression via chronic inflammation in a human-like Neu5Gc-deficient mouse model. Here we show that human anti-Neu5Gc antibodies can be affinity-purified in substantial amounts from clinically approved intravenous IgG (IVIG) and used at higher concentrations to suppress growth of the same Neu5Gc-expressing tumors. Hypothesizing that this polyclonal spectrum of human anti-Neu5Gc antibodies also includes potential cancer biomarkers, we then characterize them in cancer and noncancer patients' sera, using a novel sialoglycan microarray presenting multiple Neu5Gc-glycans and control Neu5Ac-glycans. Antibodies against Neu5Gcα2-6GalNAcα1-O-Ser/Thr (GcSTn) were found to be more prominent in patients with carcinomas than with other diseases. This unusual epitope arises from dietary Neu5Gc incorporation into the carcinoma marker Sialyl-Tn, and is the first example of such a novel mechanism for biomarker generation. Finally, human serum or purified antibodies rich in anti-GcSTn-reactivity kill GcSTn-expressing human tumors via complement-dependent cytotoxicity or antibody-dependent cellular cytotoxicity. Such xeno-autoantibodies and xeno-autoantigens have potential for novel diagnostics, prognostics, and therapeutics in human carcinomas.

Biomedical Differences Between Human and Nonhuman Hominids: Potential Roles for Uniquely Human Aspects of Sialic Acid Biology

Abstract: Although humans are genetically very similar to the evolutionarily related nonhuman hominids (chimpanzees, bonobos, gorillas, and orangutans), comparative studies suggest a surprising number of uniquely human differences in the incidence and/or severity of biomedical conditions. Some differences are due to anatomical changes that occurred during human evolution. However, many cannot be explained either by these changes or by known environmental factors. Because chimpanzees were long considered models for human disease, it is important to be aware of these differences, which appear to have been deemphasized relative to similarities. We focus on the pathophysiology and pathobiology of biomedical conditions that appear unique to humans, including several speculative possibilities that require further study. We pay particular attention to the possible contributions of uniquely human changes in the biology of cell-surface sialic acids and the proteins that recognize them. We also discuss the metabolic incorporation of a diet-derived nonhuman sialic acid, which generates a novel xeno-autoantigen reaction, and chronic inflammation known as xenosialitis.

Siglec-F inhibition reduces esophageal eosinophilia and angiogenesis in a mouse model of eosinophilic esophagitis.

Abstract: Objectives:Eosinophilic esophagitis (EoE) is a disorder characterized histologically by tissue eosinophilia. Sialic acid–binding immunoglobulin-like lectin (Siglec-F) is a receptor highly expressed on mouse eosinophils and mediates eosinophilic apoptosis. We investigated whether administrati

Sexual selection by female immunity against paternal antigens can fix loss of function alleles

Abstract: Humans lack the common mammalian cell surface molecule N-glycolylneuraminic acid (Neu5Gc) due to a CMAH gene inactivation, which occurred approximately three million years ago. Modern humans produce antibodies specific for Neu5Gc. We hypothesized that anti-Neu5Gc antibodies could enter the female reproductive tract and target Neu5Gc-positive sperm or fetal tissues, reducing reproductive compatibility. Indeed, female mice with a human-like Cmah(−/−) mutation and immunized to express anti-Neu5Gc antibodies show lower fertility with Neu5Gc-positive males, due to prezygotic incompatibilities. Human anti-Neu5Gc antibodies are also capable of targeting paternally derived antigens and mediate cytotoxicity against Neu5Gc-bearing chimpanzee sperm in vitro. Models of populations polymorphic for such antigens show that reproductive incompatibility by female immunity can drive loss-of-function alleles to fixation from moderate initial frequencies. Initially, the loss of a cell-surface antigen can occur due to drift in isolated populations or when natural selection favors the loss of a receptor exploited by pathogens, subsequently the same loss-of-function allele can come under sexual selection because it avoids being targeted by the female immune system. Thus, we provide evidence of a link between sexual selection and immune function: Antigenicity in females can select against foreign paternal antigens on sperm and rapidly fix loss-of-function alleles. Similar circumstances existed when the CMAH null allele was polymorphic in ancestral hominins, just before the divergence of Homo from australopithecines.

Expression of Siglec-11 by human and chimpanzee ovarian stromal cells, with uniquely human ligands: implications for human ovarian physiology and pathology.

Abstract: Siglecs (Sialic acid-binding Immunoglobulin Superfamily Lectins) are cell surface signaling receptors of the I-type lectin group that recognize sialic acid-bearing glycans. CD33-related-Siglecs are a subset with expression primarily in cells of hematopoietic origin and functional relevance to immune reactions. Earlier we reported a human-specific gene conversion event that markedly changed the coding region for the extracellular domain of Siglec-11, associated with human-specific expression in microglia (Hayakawa T, Angata T, Lewis AL, Mikkelsen TS, Varki NM, Varki A. 2005. A human-specific gene in microglia. Science. 309:1693). Analyzing human gene microarrays to define new patterns of expression, we observed high levels of SIGLEC11 transcript in the ovary and adrenal cortex. Thus, we examined human and chimpanzee tissues using a well-characterized anti-Siglec-11 mouse monoclonal antibody. Although adrenal expression was variable and confined to infiltrating macrophages in capillaries, ovarian expression of Siglec-11 in both humans and chimpanzees was on fibroblasts, the first example of Siglec expression on mesenchyme-derived stromal cells. Cytokines from such ovarian stromal fibroblasts play important roles in follicle development and ovulation. Stable transfection of SIGLEC11 into a primary human ovarian stromal fibroblast cell line altered the secretion of growth-regulated oncogene α, interleukin (IL)-10, IL-7, transforming growth factor β1 and tumor necrosis factor-α, cytokines involved in ovarian physiology. Probing for Siglec-11 ligands revealed distinct and strong mast cell expression in human ovaries, contrasting to diffuse stromal ligands in chimpanzee ovaries. Interestingly, there was a trend of increased Siglec-11 expression in post-menopausal ovaries compared with pre-menopausal ones. Siglec-11 expression was also found on human ovarian stromal tumors and in polycystic ovarian syndrome, a human-specific disease. These results indicate potential roles for Siglec-11 in ovarian physiology and human evolution.

Pancreatic beta cell failure in obese mice with human-like CMP-Neu5Ac hydroxylase deficiency

Abstract: Type 2 diabetes is highly prevalent in human populations, particularly in obese individuals, and is characterized by progressive pancreatic β-cell dysfunction and insulin resistance. Most mammals, including Old World primates, express two major kinds of sialic acids, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), typically found at the distal ends of glycoconjugate chains at the cell surface. Humans are uniquely unable to produce endogenous Neu5Gc due to an inactivating mutation in the CMP-Neu5Ac hydroxylase (CMAH) gene. The CMAH enzyme catalyzes the generation of CMP-Neu5Gc by the transfer of a single oxygen atom to the acyl group of CMP-Neu5Ac. Here, we show that mice bearing a human-like deletion of the Cmah gene exhibit fasting hyperglycemia and glucose intolerance following a high-fat diet. This phenotype is caused not by worsened insulin resistance but by compromised pancreatic β-cell function associated with a 65% decrease in islet size and area and 50% decrease in islet number. Obese Cmah-null mice also show an ∼40% reduction in response to insulin secretagogues in vivo. These findings show that human evolution-like changes in sialic acid composition impair pancreatic β-cell function and exacerbate glucose intolerance in mice. This may lend insight into the pathogenesis of type 2 diabetes in obese humans.

Compositions and Methods for Detecting Cancer

Abstract: The invention provides sialylated glycans and antibodies that specifically bind to them. The invention's compositions and methods for using them are useful for early detection and diagnosis of cancer.

Novel glycosylated polypeptides

Abstract: Provided herein are glycosylated polypeptide compositions with substantially reduced Neu5Gc content. The glycosylated polypeptides compositions with substantially reduced Neu5Gc content can be obtained from cell sources cultured with Neu5Gc competitor or from non-human animal sources fed a diet supplemented with Neu5Gc competitor. Also provided herein are methods of treating a human subject with said compositions.

Methods for detecting cancer

Abstract: The invention provides sialylated glycans and antibodies that specifically bind to them. The invention's compositions and methods for using them are useful for early detection and diagnosis of cancer.