Human Galectin-3 Research Articles

Chemo-enzymatic synthesis of LacdiNAc dimers of varying length as novel galectin ligands

A set of sixteen bivalent symmetrical and asymmetrical LacdiNAc dimers containing flexible alkyl linkers were efficiently synthesized by means of chemo-enzymatic synthesis, using the versatile potential of the Y284L mutant of human placental β1,4-galactosyltransferase-1. LacdiNAc was confirmed to be a specific ligand for human galectin-3 contrary to human galectin-1. The compounds were tested as ligands for human galectin-3 in competitive binding assays and compared to a monovalent LacdiNAc standard. Molecular modeling was performed to calculate approximate length of respective ligands and its relation to their inhibitory capacity. The best performance was observed in symmetrical compounds carrying two LacdiNAc units connected with a hydrophobic linker of sufficient length (alkyl chain n ≥ 6). Here, the IC50 value was about three times lower than that of the monovalent standard. Our results propose that hydrophobicity directed by the alkyl chain length as well as the specificity and bivalency of the LacdiNAc contribute to the inhibition potential of these ligands. Though only slightly pronounced in this case, higher multivalency is a promising feature in the design of optimized ligands for galectin-3.

β-Cyclodextrin-Bearing Gold Glyconanoparticles for the Development of Site Specific Drug Delivery Systems

Three novel gold nanoparticles containing multiple long, flexible linkers decorated with lactose, β-cyclodextrin, and both simultaneously have been prepared. The interaction of such nanoparticles with β-d-galactose-recognizing lectins peanut agglutinin (PNA) and human galectin-3 (Gal-3) was demonstrated by UV-vis studies. Gal-3 is well-known to be overexpressed in several human tumors and can act as a biorecognizable target. This technique also allowed us to estimate their loading capability toward the anticancer drug methotrexate (MTX). Both results make these glyconanoparticles potential site-specific delivery systems for anticancer drugs.

Galectin-3 Interactions with Glycosphingolipids

Galectins have essential roles in pathological states including cancer, inflammation, angiogenesis and microbial infections. Endogenous receptors include members of the lacto- and neolacto-series glycosphingolipids present on mammalian cells and contain the tetrasaccharides lacto-N-tetraose (LNT) and lacto-N-neotetraose (LNnT) that form their core structural components and also ganglio-series glycosphingolipids. We present crystallographic structures of the carbohydrate recognition domain of human galectin-3, both wild type and a mutant (K176L) that influenced ligand affinity, in complex with LNT, LNnT and acetamido ganglioside a-GM3 (α2,3-sialyllactose). Key structural features revealed include galectin-3's demonstration of a binding mode towards gangliosides distinct from that to the lacto/neolacto-glycosphingolipids, with its capacity for recognising the core β-galactoside region being challenged when the core oligosaccharide epitope of ganglio-series glycosphingolipids (GM3) is embedded within particular higher-molecular-weight glycans. The lacto- and neolacto- glycosphingolipids revealed different orientations of their terminal galactose in the galectin-3-bound LNT and LNnT structures that has significant ramifications for the capacity of galectin-3 to interact with higher-order lacto/neolacto-series glycosphingolipids such as ABH blood group antigens and the HNK-1 antigen that is common on leukocytes. LNnT also presents an important model for poly-N-acetyllactosamine-containing glycans and provides insight into galectin-3's accommodation of extended oligosaccharides such as the poly-N-acetyllactosamine-modified N- and O-glycans that, via galectin-3 interaction, facilitate progression of lung and bladder cancers, respectively. These findings provide the first atomic detail of galectin-3's interactions with the core structures of mammalian glycosphingolipids, providing information important in understanding the capacity of galectin-3 to engage with receptors identified as facilitators of major disease.

Human Milk Galectin-3 Binding Protein and Breast-feeding–associated HIV Transmission

Analysis of milk from 247 HIV-infected Zambian mothers showed that galectin-3 binding protein concentrations were significantly higher among HIV-infected mothers who transmitted HIV through breast-feeding (6.51 ± 2.12 μg/mL) than among nontransmitters but were also correlated with higher milk and plasma HIV RNA copies/mL and lower CD4+ cell counts. The association between galectin-3 binding protein and postnatal transmission was attenuated after adjustment for milk and plasma HIV load and CD4+ cell counts. This suggests that although milk galectin-3 binding protein is a marker of advanced maternal disease, it does not independently modify transmission risk.

Peptides derived from human galectin-3 N-terminal tail interact with its carbohydrate recognition domain in a phosphorylation-dependent manner

Galectin-3 (Gal-3) is a multi-functional effector protein that functions in the cytoplasm and the nucleus, as well as extracellularly following non-classical secretion. Structurally, Gal-3 is unique among galectins with its carbohydrate recognition domain (CRD) attached to a rather long N-terminal tail composed mostly of collagen-like repeats (nine in the human protein) and terminating in a short non-collagenous terminal peptide sequence unique in this lectin family and not yet fully explored. Although several Ser and Tyr sites within the N-terminal tail can be phosphorylated, the physiological significance of this post-translational modification remains unclear. Here, we used a series of synthetic (phospho)peptides derived from the tail to assess phosphorylation-mediated interactions with 15N-labeled Gal-3 CRD. HSQC-derived chemical shift perturbations revealed selective interactions at the backface of the CRD that were attenuated by phosphorylation of Tyr 107 and Tyr 118, while phosphorylation of Ser 6 and Ser 12 was essential. Controls with sequence scrambling underscored inherent specificity. Our studies shed light on how phosphorylation of the N-terminal tail may impact on Gal-3 function and prompt further studies using phosphorylated full-length protein.

New findings in the pathogenesis of leprosy and implications for the management of leprosy

This review focuses on recent work in leprosy pathogenesis. New research of both innate and adaptive immune responses to Mycobacterium leprae is described. The proposition that Mycobacterium lepromatosis is a new species causing leprosy is discussed. Modulation of the lipid metabolism and reprogramming of adult Schwann cells have both been suggested as mechanisms used by M. leprae to disseminate the disease. New markers associated with localized, disseminated disease or the occurrences of leprosy reactions include the human interferons, CD163, microRNA-21, NOD2, galectin-3 and toll-like receptor 4. The role of keratinocytes instead of macrophages is underlined in the pathogenesis of leprosy. Adaptive immunity reports focus on the role of T regulatory cells and cytokines secreted by T helper cells in leprosy. Finally, a newly identified species named M. lepromatosis has been detected in patients with leprosy and severe erythema nodosum leprosum. Novel biological pathways have been identified to be associated with the clinical phenotype of leprosy or the occurrence of leprosy reactions. Future work should include larger numbers of clinical samples from across the leprosy spectrum in order to give new insights in the pathogenesis and management of the disease.

Human galectin binds to multivalent arabinogalactan-protein isolated from aerial parts of Echinacea purpurea

Pressed juices from the aerial parts of Echinacea purpurea are used as unspecific immunostimulants, e.g. against common cold. For an arabinogalactan-protein (AGP) from this plant material, immunomodulating activities have been shown in vitro, such as activation of the human complement system and binding to human leucocytes (1, 2). The question arises, how high-molecular-weight arabinogalactan-proteins interact with the human immune system when taken orally. One possibility is uptake of AGPs in the area of Peyer's patches of the intestinal immune system (3). Another possible mode of action might be binding of AGPs to galectins present at the brush border membrane of the intestine. Galectins are a family of multifunctional proteins, located in several cells and tissues and characterized by a conserved carbohydrate-binding domain with affinity for β-Gal containing glycoconjugates. Using different ELISA techniques, we proved binding of AGP isolated from aerial parts of E. purpurea to human recombinant galectin 3. Galectin 3 is expressed widely in immune cells and also epithelial cells, including gastric and colon mucosa (4). Enhanced binding affinity was found after partial acid hydrolysis of the AGP, which might also happen in the human stomach.

Secreted galectin-3 as a possible biomarker for the immunomodulatory potential of human umbilical cord mesenchymal stromal cells

Background aimsHuman umbilical cord-derived mesenchymal stromal cells (UC-MSCs) possess broad and potent immunomodulatory activities and have shown great potential in anti-inflammatory therapies. However, a biomarker that can be used to assess quickly and efficiently the immunomodulatory function of UC-MSCs has not been identified. Several studies have revealed that galectin-3 (Gal-3), a member of the human galectin family, is involved in the immunosuppressive function of MSCs. MethodsGal-3 gene expression in UC-MSCs was analyzed using quantitative reverse transcriptase polymerase chain reaction and Western blotting. Blocking of Gal-3 expression in UC-MSCs with small interfering RNA was employed to analyze whether the immunosuppressive function of UC-MSCs was affected. ResultsWe found that UC-MSCs expressed Gal-3 both on the cell surface and in secreted form, and the expression levels of Gal-3 did not show significant variation after cell passaging. We further showed that Gal-3 expression correlated with the immunosuppressive function of UC-MSCs because knock-down of Gal-3 expression with small interfering RNA significantly abrogated the inhibitory effects of UC-MSCs on mitogen-stimulated and alloantigen-stimulated proliferation of human peripheral blood mononuclear cells; meanwhile, the inhibitory effect of UC-MSCs was reversed by adding back recombinant Gal-3 to the co-culture systems. The inhibitory activities of human UC-MSCs were not reduced even when they were separated from human peripheral blood mononuclear cells in a transwell co-culture system, indicating that the soluble form of Gal-3 was the major effector. ConclusionsThe Gal-3 protein secreted by UC-MSCs shows good correlation with immunosuppressive potential and may serve as a possible biomarker for the potency test of UC-MSCs.

Copy-number variation of functional galectin genes: Studying animal galectin-7 (p53-induced gene 1 in man) and tandem-repeat-type galectins-4 and -9

Galectins are potent adhesion/growth-regulatory effectors with characteristic expression profiles. Understanding the molecular basis of gene regulation in each case requires detailed information on copy number of genes and sequence(s) of their promoter(s). Our report reveals plasticity in this respect between galectins and species. We here describe occurrence of a two-gene constellation for human galectin (Gal)-7 and define current extent of promoter-sequence divergence. Interestingly, cross-species genome analyses also detected single-copy display. Because the regulatory potential will then be different, extrapolations of expression profiles are precluded between respective species pairs. Gal-4 coding in chromosomal vicinity was found to be confined to one gene, whereas copy-number variation also applied to Gal-9. The example of rat Gal-9 teaches the lesson that the presence of multiple bands in Southern blotting despite a single-copy gene constellation is attributable to two pseudogenes. The documented copy-number variability should thus be taken into consideration when studying regulation of galectin genes, in a species and in comparison between species.

Human Galectin-3 Selective and High Affinity Inhibitors. Present State and Future Perspectives

Over the last decade an increasing number of studies have been published reporting on the inhibitory potency or selectivity that several types of ligands show against human galectin-3 (hGal-3). The reason for this interest lies in the many important roles galectins play both in intra and extra-cellular functions. Among galectins, galectin-3 stands out because it is the only known member of its subfamily in mammals, is small and monomeric but capable of aggregating, and is known to be involved in a large number of disease processes, from cancer to heart failure. These characteristics and roles make hGal-3 an ideal target for drugs. Since it binds β-galactosides, like the rest of the galectin family of proteins, the search and design of potent and at the same time selective inhibitors for it is not an easy task. Herein we discuss the chemical features of the most potent inhibitors described so far, as well as the structural basis of their exhibited selectivity, in order to shed light on the rational design of drugs against this target.

Fluorescent SNAP-Tag Galectin Fusion Proteins as Novel Tools in Glycobiology

Galectins,β-galactoside binding proteins, function in several physiological and pathological processes. The further evaluation of these processes as well as possible applications of galectins in diagnosis and therapy has raised high scientific interest. Therefore, easy and reliable test systems are necessary. Here we present the simple and cost-efficient production of recombinant human galectins as fusion proteins with SNAP-tag and fluorescent proteins. These constructs show binding specificities and oligomerisation properties generally comparable to recombinant galectins. Their direct fluorescence signal was utilised by ELISA-type assay and flow cytometry analysis with human and ovine mesenchymal stem cells (MSC). Flow cytometry demonstrated glycan mediated binding of His6-SNAP-YFP-Gal- 3 to both MSC types, which was specifically inhibited by lactose. Moreover, directed immobilisation by SNAP-tag technology onto benzylguanine- activated sepharose was utilised to prepare galectin affinity columns for glycoprotein analysis and purification. The SNAPtag directed coupling yielded up to three-fold higher binding capacities for the glycoprotein standard asialofetuin compared to nondirected coupled galectin suggesting improved functionality following directed coupling.

Mammalian galectins bind Galactoseβ1–4Fucose disaccharide, a unique structural component of protostomial N-type glycoproteins

Galactoseβ1–4Fucose (Galβ1–4Fuc) is a unique disaccharide exclusively found in N-glycans of protostomia, and is recognized by some galectins of Caenorhabditis elegans and Coprinopsis cinerea. In the present study, we investigated whether mammalian galectins also bind such a disaccharide. We examined sugar-binding ability of human galectin-1 (hGal-1) and found that hGal-1 preferentially binds Galβ1–4Fuc compared to Galβ1–4GlcNAc, which is its endogenous recognition unit. We also tested other human and mouse galectins, i.e., hGal-3, and -9 and mGal-1, 2, 3, 4, 8, and 9. All of them also showed substantial affinity to Galβ1–4Fuc disaccharide. Further, we assessed the inhibitory effect of Galβ1–4Fuc, Galβ1–4Glc, and Gal on the interaction between hGal-1 and its model ligand glycan, and found that Galβ1–4Fuc is the most effective. Although the biological significance of galectin−Galβ1–4Fuc interaction is obscure, it might be possible that Galβ1–4Fuc disaccharide is recognized as a non-self-glycan antigen. Furthermore, Galβ1–4Fuc could be a promising seed compound for the synthesis of novel galectin inhibitors.

Opposing Mcl-1, the GALIG proapoptotic gene is upregulated as neutrophils die and underexpressed in Acute Myeloid Leukemia cells

GALIG gene expression induces apoptosis in cultured cells through a pathway still under investigation. It is highly expressed in leukocytes but weakly detectable in bone marrow, suggesting a role in the myeloid lineage homeostasis. We show here that GALIG-induced cell death is counteracted by the overexpression of MCL-1, a pro-survival member of the Bcl2 family. Moreover, during spontaneous neutrophil apoptosis, a substantial increase in GALIG gene expression is observed: GALIG still opposes MCL-1. Finally, in bone marrow and peripheral blood cells from patients with Acute Myeloid Leukemia type 2, the level of GALIG transcripts is massively down-regulated when compared to their normal counterparts, while MCL-1 is expressed to the same extent. These data suggest that GALIG could be a key player in the cell death pathway involved in leukocytes homeostasis and myeloid malignancies.

Synthesis and Characterization of Sulfated Gal‐β‐1,3/4‐GlcNAc Disaccharides through Consecutive Protection/Glycosylation Steps

We have developed an expeditious procedure to yield large amounts of orthogonally protected Gal-β1,3/4-GlcNAc, which allowed for the systematic introduction of a sulfate group onto the C3/C6 positions of Gal and/or the C6 position of GlcNAc. In particular, the disaccharide precursors were prepared in five or six steps and high overall yield from para-tolyl-6-O-tert-butyldiphenylsilyl-1-thio-β-D-galactopyranoside. After deprotection and sulfation steps, the final products were characterized by using several NMR methods to unambiguously confirm the location of each introduced sulfate group and they were examined for their binding specificity of human galectin-1 and galectin-8.

Abstract 38: Galectin-9D5 is a prognostic marker in early stage non-small cell lung cancer.

Abstract Background: Galectins are proteins with a conserved carbohydrate recognition domain and a high affinity for beta-galactosides on glycoconjugates. They are differentially expressed in many pathologies, including cancer. Recent findings suggest a prognostic potential of galectin-1 and -3 in lung cancer as increased expression is frequently associated with enhanced malignancy and poor patient survival. However, prognostic values of the other 13 members of the galectin family in lung cancer remain elusive. We performed extensive galectin expression profiling and determined the prognostic value of all known human galectins in early stage non-small cell lung cancer (NSCLC) patients. Methods: A retrospective study was performed on primary tumors from 93 early stage NSCLC patients with a follow-up of at least 5 years. The mRNA expression levels of all human galectins in NSCLC tumor tissue were determined by qPCR and correlated with overall survival. Additionally, tissue microarrays (TMAs) comprising of both normal and tumor lung tissue were stained for galectin-9 and staining frequency was scored in epithelial cells and endothelial cells (EC). Results: Galectin mRNA expression in NSCLC was shown to be confined to galectin-1, -3, -4, -7, -8, and -9. Patients with galectin-1 expression levels above median showed significant shorter overall survival (43.5 (95% CI 32.4-54.6) vs. 72.2 (59.3-89.0) months; p=0.039, HR=1.5). The expression of galectin-3 was not prognostic in our patient group. Only one other galectin was found to be of prognostic value, i.e. a splice variant of galectin-9 lacking exon 5 (galectin-9D5). Patients with galectin-9D5 expression levels below median have shorter overall survival (49.2 (34.1-64.2) vs. 67.3 (55.0-79.6) months; p=0.007, HR=2.2). Overall patient survival was even worse when patients with high galectin-1 and low galectin-9D5 levels were compared to all other patients (30.0 (16.2-43.9) vs. 70.8 (58.6-83.0) months; p=0.002, HR=4.6). Immunohistochemical staining of TMAs for total galectin-9 revealed no differences between epithelial cells of normal and tumor tissue. However, galectin-9 protein levels were significantly increased in tumor EC compared to normal EC. Conclusion: Comprehensive galectin expression profiling identified a spice variant of galectin-9, i.e. galectin-9D5, as a novel prognostic marker in early stage NSCLC. Interestingly, whereas low galectin-9D5 mRNA expression levels predict poor patient survival, we observed an increased galectin-9 protein expression in tumor EC compared to normal EC. Further research is warranted to unravel role of galectin-9 in angiogenesis and immune regulation in NSCLC. Since it has been shown that adjuvant chemotherapy can improve the survival of patients with resected stage II-IIIa NSCLC, determining the galectin expression profile might serve as a tool in predicting prognosis and treatment selection for individual patients. Citation Format: Victor L. Thijssen, Iris A. Schulkens, Roy Heusschen, Vivian van den Boogaart, Robert-Jan van Suylen, Thang V. Pham, Anne-Marie C. Dingemans, Arjan W. Griffioen. Galectin-9D5 is a prognostic marker in early stage non-small cell lung cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 38. doi:10.1158/1538-7445.AM2013-38

Understanding the specificity of human Galectin-8C domain interactions with its glycan ligands based on molecular dynamics simulations.

Human Galectin-8 (Gal-8) is a member of the galectin family which shares an affinity for β-galactosides. The tandem-repeat Gal-8 consists of a N- and a C-terminal carbohydrate recognition domain (N- and C-CRD) joined by a linker peptide of various length. Despite their structural similarity both CRDs recognize different oligosaccharides. While the molecular requirements of the N-CRD for high binding affinity to sulfated and sialylated glycans have recently been elucidated by crystallographic studies of complexes with several oligosaccharides, the binding specificities of the C-CRD for a different set of oligosaccharides, as derived from experimental data, has only been explained in terms of the three-dimensional structure for the complex C-CRD with lactose. In this study we performed molecular dynamics (MD) simulations using the recently released crystal structure of the Gal-8C-CRD to analyse the three-dimensional conditions for its specific binding to a variety of oligosaccharides as previously defined by glycan-microarray analysis. The terminal β-galactose of disaccharides (LacNAc, lacto-N-biose and lactose) and the internal β-galactose moiety of blood group antigens A and B (BGA, BGB) as well as of longer linear oligosaccharide chains (di-LacNAc and lacto-N-neotetraose) are interacting favorably with conserved amino acids (H53, R57, N66, W73, E76). Lacto-N-neotetraose and di-LacNAc as well as BGA and BGB are well accommodated. BGA and BGB showed higher affinity than LacNAc and lactose due to generally stronger hydrogen bond interactions and water mediated hydrogen bonds with α1-2 fucose respectively. Our results derived from molecular dynamics simulations are able to explain the glycan binding specificities of the Gal-8C-CRD in comparison to those of the Gal-8N -CRD.

Binding of Transmembrane Mucins to Galectin-3 Limits Herpesvirus 1 Infection of Human Corneal Keratinocytes

Epithelial cells lining mucosal surfaces impose multiple barriers to viral infection. At the ocular surface, the carbohydrate-binding protein galectin-3 maintains barrier function by cross-linking transmembrane mucins on the apical glycocalyx. Despite these defense mechanisms, many viruses have evolved to exploit fundamental cellular processes on host cells. Here, we use affinity assays to show that herpes simplex virus type 1 (HSV-1), but not HSV-2, binds human galectin-3. Knockdown of galectin-3 in human corneal keratinocytes by small interfering RNA significantly impaired HSV-1 infection, but not expression of nectin-1, indicating that galectin-3 is a herpesvirus entry mediator. Interestingly, exposure of epithelial cell cultures to transmembrane mucin isolates decreased viral infectivity. Moreover, HSV-1 failed to elute the biological counterreceptor MUC16 from galectin-3 affinity columns, suggesting that association of transmembrane mucins to galectin-3 provides protection against viral infection. Together, these results indicate that HSV-1 exploits galectin-3 to enhance virus attachment to host cells and support a protective role for transmembrane mucins under physiological conditions by masking viral entry mediators on the epithelial glycocalyx.

Structural Analysis of the Evolutionary Origins of Influenza Virus Hemagglutinin and Other Viral Lectins

Influenza virus and other viruses use host cell surface sugars as receptors. Here we show that the sugar-binding domains in influenza virus hemagglutinin and other viral lectins share the same structural fold as human galectins (host lectins). Unlike the easily accessible sugar-binding sites in human galectins, the sugar-binding sites in viral lectins are hidden in cavities. We propose that these viral lectins originated from host lectins but have evolved to use hidden sugar-binding sites to evade host immune attacks.

Design and synthesis of glycoprotein-based multivalent glyco-ligands for influenza hemagglutinin and human galectin-3

We report a facile synthesis of glycoprotein-based glyco-ligands and their binding with influenza hemagglutinin and human galectin-3. Human serum albumin (HSA) was used as the scaffold and an Asn-linked complex type N-glycan prepared from chicken eggs was used as the glycan building block. It was found that Cu(I)-catalyzed alkyne–azide cycloaddition reaction (click chemistry) between the alkyne-labeled glycan and the azide-tagged HSA led to an efficient formation of the glycoconjugates. The density of glycan ligands on the protein scaffold was readily varied by changing the molar ratios of the two reactants. Binding studies indicated that the sialylated and desialylated multivalent glycoligands could selectively bind to influenza hemagglutinin and human galectin-3, respectively, with high affinity. In the two glycan–lectin interactions, a clear multivalent effect was observed. Moreover, a cell-based assay showed that the synthetic multivalent glyco-ligands could efficiently inhibit the attachment of galectin-3 to human prostate cancer and lung cancer cell lines. This study suggests that the synthetic glycoprotein-based glyco-ligands can be useful for different applications, including blocking the function of galectin-3 in cancer metastasis.

Structure of full-lengthToxascaris leoninagalectin with two carbohydrate-recognition domains

The full-length crystal structure of Toxascaris leonine galectin (Tl-gal), a galectin-9 homologue protein, was determined at a resolution of 2.0 Å. Galectin-9 exhibits a variety of biological functions, including cell aggregation, eosinophil chemoattraction, activation and apoptosis of murine thymocytes, T cells and human melanoma cells. Similar to this galectin, Tl-gal may function as a regulatory molecule in the host immune system; however, no molecular or structural information has been reported for Tl-gal. Moreover, until now, there have been no reports of a full-length galectin structure. There are two molecules of Tl-gal per asymmetric unit in space group P2(1)2(1)2(1), and the N-terminal and C-terminal carbohydrate-recognition domains (NCRD and CCRD) of Tl-gal are composed of six-stranded β-sheets and five-stranded β-sheets with a short α-helix. The NCRD of Tl-gal resembles that of human galectin-7 and its CCRD resembles human galectin-9, but the residues in the interface and loop regions of the NCRD and CCRD are flexible and are related to interaction. Engagement of the T-cell immunoglobulin mucin-3 (Tim-3) immunoglobulin variable (IgV) domain by a galectin-9 ligand is known to be important for appropriate termination of T-helper 1 immune responses. To investigate the binding site of Tl-gal, the interaction between Tl-gal and Tim-3 was modelled. Tim-3 is docked into a major groove of the Tl-gal structure, which is larger and deeper than the minor groove. The structural information presented here will provide insight into the development of novel anti-inflammatory agents or selective modulators of immune response.