Enhancement of CD8 T-cell function through modifying surface glycoproteins in young and old mice

Structure, function, and immunomodulation of the CD8 co-receptor

Expressed on the surface of CD8+ T cells, the CD8 co-receptor is a key component of the T cells that contributes to antigen recognition, immune cell maturation, and immune cell signaling. While CD8 is widely recognized as a co-stimulatory molecule for conventional CD8+ αβ T cells, recent reports highlight its multifaceted role in both adaptive and innate immune responses. In this review, we discuss the utility of CD8 in relation to its immunomodulatory properties. We outline the unique structure and function of different CD8 domains (ectodomain, hinge, transmembrane, cytoplasmic tail) in the context of the distinct properties of CD8αα homodimers and CD8αβ heterodimers. We discuss CD8 features commonly used to construct chimeric antigen receptors for immunotherapy. We describe the molecular interactions of CD8 with classical MHC-I, non-classical MHCs, and Lck partners involved in T cell signaling. Engineered and naturally occurring CD8 mutations that alter immune responses are discussed. The applications of anti-CD8 monoclonal antibodies (mABs) that target CD8 are summarized. Finally, we examine the unique structure and function of several CD8/mAB complexes. Collectively, these findings reveal the promising immunomodulatory properties of CD8 and CD8 binding partners, not only to uncover basic immune system function, but to advance efforts towards translational research for targeted immunotherapy.

A Unified Atlas of T cell Glycophysiology

Glycans are emerging as important regulators of T cell function but remain poorly characterized across the functionally distinct populations that exist in vivo . Here, we couple single-cell analysis technologies with soluble lectins and chemical probes to interrogate glycosylation patterns on major T cell populations across multiple mouse and human tissues. Our analysis focused on terminal glycan epitopes with immunomodulatory functions, including sialoglycan ligands for Siglecs. We demonstrate that glycosylation patterns are diverse across the resting murine T cell repertoire and dynamically remodelled in response to antigen-specific stimulation. Surprisingly, we find that human T cell populations do not share the same glycoprofiles or glycan remodelling dynamics as their murine counterparts. We show that these differences can be explained by divergent regulation of glycan biosynthesis pathways between the species. These results highlight fundamental glycophysiological differences between mouse and human T cells and reveal features that are critical to consider for glycan-targeted therapies.

Sialic Acid Ligands of CD28 Suppress Costimulation of T Cells

Effector T cells comprise the cellular arm of the adaptive immune system and are essential for mounting immune responses against pathogens and cancer. To reach effector status, costimulation through CD28 is required. Here, we report that sialic acid-containing glycans on the surface of both T cells and APCs are alternative ligands of CD28 that compete with binding to its well-documented activatory ligand CD80 on the APC, resulting in attenuated costimulation. Removal of sialic acids enhances antigen-mediated activation of naïve T cells and also increases the revival of effector T cells made hypofunctional or exhausted via chronic viral infection. This occurs through a mechanism that is synergistic with antibody blockade of the inhibitory PD-1 axis. These results reveal a previously unrecognized role of sialic acid ligands in attenuation of CD28-mediated costimulation of T cells.

Human T cell glycosylation and implications on immune therapy for cancer

Glycosylation is an important post-translational modification, giving rise to a diverse and abundant repertoire of glycans on the cell surface, collectively known as the glycome. When focusing on immunity, glycans are indispensable in virtually all signaling and cell-cell interactions. More specifically, glycans have been shown to regulate key pathophysiological steps within T cell biology such as T cell development, thymocyte selection, T cell activity and signaling as well as T cell differentiation and proliferation. They are of major importance in determining the interaction of human T cells with tumor cells. In this review, we will describe the role of glycosylation of human T cells in more depth, elaborate on the importance of glycosylation in the interaction of human T cells with tumor cells and discuss the potential of cancer immunotherapies that are based on manipulating the glycome functions at the tumor immune interface.^1,2

Aging and the immune system: An overview

The world is witnessing a rapid demographic shift towards an older population, a trend with major medical, social, economic and political implications. Aging is a multifaceted process, involving numerous molecular and cellular mechanisms in the context of different organ systems. A crucial component of aging is a set of functional and structural alterations in the immune system that can manifest as a decreased ability to fight infection, diminished response to vaccination, increased incidence of cancer, higher prevalence of autoimmunity and constitutive low-grade inflammation, among others. In addition to cell-intrinsic changes in both innate and adaptive immune cells, alterations in the stromal microenvironment in primary and secondary lymphoid organs play an important role in age-associated immune dysfunction. This article will provide a broad overview of these phenomena and point out some of their clinical and therapeutic implications.

Role of Inactive and Active Trypanosoma cruzi Trans-sialidases on T Cell Homing and Secretion of Inflammatory Cytokines

Trans-sialidase from Trypanosoma cruzi (Tc-TS) belongs to a superfamily of proteins that may have enzymatic activity. While enzymatically active members (Tc-aTS) are able to transfer sialic acid from the host cell sialyl-glycoconjugates onto the parasite or to other molecules on the host cell surface, the inactive members (Tc-iTS) are characterized by their lectinic properties. Over the last 10 years, several papers demonstrated that, individually, Tc-aTS or Tc-iTS is able to modulate several biological events. Since the genes encoding Tc-iTS and Tc-aTS are present in the same copy number, and both proteins portray similar substrate-specificities as well, it would be plausible to speculate that such molecules may compete for the same sialyl-glycan structures and govern numerous immunobiological phenomena. However, their combined effect has never been evaluated in the course of an acute infection. In this study, we investigated the ability of both proteins to modulate the production of inflammatory signals, as well as the homing of T cells to the cardiac tissue of infected mice, events that usually occur during the acute phase of T. cruzi infection. The results showed that the intravenous administration of Tc-iTS, but not Tc-aTS protected the cardiac tissue from injury caused by reduced traffic of inflammatory cells. In addition, the ability of Tc-aTS to modulate the production of inflammatory cytokines was attenuated and/or compromised when Tc-iTS was co-injected in the same proportions. These results suggest that although both proteins present structural similarities and compete for the same sialyl-glycan epitopes, they might present distinct immunomodulatory properties on T cells following T. cruzi infection.

Modulation of Cell Sialoglycophenotype: A Stylish Mechanism Adopted by Trypanosoma cruzi to Ensure Its Persistence in the Infected Host

Trypanosoma cruzi, the etiological agent of Chagas disease exhibits multiple mechanisms to guarantee its establishment and persistence in the infected host. It has been well demonstrated that T. cruzi is not able to synthesize sialic acids (Sia). To acquire the monosaccharide, the parasite makes use of a multifunctional enzyme called trans-sialidase (Tc-TS). Since this enzyme has no analogous in the vertebrate host, it has been used as a target in drug therapy development. Tc-TS preferentially catalyzes the transfer of Sia from the host glycoconjugates to the terminal β-galactopyranosyl residues of mucin-like molecules present on the parasite's cell surface. Alternatively, the enzyme can sialylate/re-sialylate glycoconjugates expressed on the surface of host cells. Since its discovery, several studies have shown that T. cruzi employs the Tc-TS activity to modulate the host cell sialoglycophenotype, thus favoring its perpetuation in the infected vertebrate. In this review, we summarize the dynamic of host/parasite sialoglycophenotype modulation, highlighting its role in the subversion of host immune response in order to promote the establishment of persistent chronic infection.

The trans-sialidase, the major Trypanosoma cruzi virulence factor: Three decades of studies

Chagas' disease is a potentially life-threatening disease caused by the protozoan parasite Trypanosoma cruzi. Since the description of Chagas'disease in 1909 extensive research has identified important events in the disease in order to understand the biochemical mechanism that modulates T. cruzi-host cell interactions and the ability of the parasite to ensure its survival in the infected host. Exactly 30 years ago, we presented evidence for the first time of a trans-sialidase activity in T. cruzi (T. cruzi-TS). This enzyme transfers sialic acid from the host glycoconjugates to the terminal β-galactopyranosyl residues of mucin-like molecules on the parasite's cell surface. Thenceforth, many articles have provided convincing data showing that T. cruzi-TS is able to govern relevant mechanisms involved in the parasite's survival in the mammalian host, such as invasion, escape from the phagolysosomal vacuole, differentiation, down-modulation of host immune responses, among others. The aim of this review is to cover the history of the discovery of T. cruzi-TS, as well as some well-documented biological effects encompassed by this parasite's virulence factor, an enzyme with potential attributes to become a drug target against Chagas disease.

Acute infection of mice with Clostridium difficile leads to eIF2α phosphorylation and pro-survival signalling as part of the mucosal inflammatory response

The current study sought to delineate the gene expression profile of the host response in the caecum and colon during acute infection with Clostridium difficile in a mouse model of infection, and to investigate the nature of the unfolded protein response in this process. The infected mice displayed a significant up-regulation in the expression of chemokines (Cxcl1, Cxcl2 and Ccl2), numerous pro-inflammatory cytokines (Ifng, Il1b, Il6, and Il17f), as well as Il22 and a number of anti-microbial peptides (Defa1, Defa28, Defb1, Slpi and Reg3g) at the site(s) of infection. This was accompanied by a significant influx of neutrophils, dendritic cells, cells of the monocyte/macrophage lineage and all major subsets of lymphocytes to these site(s). However, CD4 T cells of the untreated and C. difficile-infected mice expressed similar levels of CD69 and CD25. Neither tissue had up-regulated levels of Tbx21, Gata3 or Rorc. The caeca and colons of the infected mice showed a significant increase in eukaryotic initiation factor 2α (eIF2α) phosphorylation, but neither the splicing of Xbp1 nor the up-regulation of endoplasmic reticulum chaperones, casting doubt on the full-fledged induction of the unfolded protein response by C. difficile. They also displayed significantly higher phosphorylation of AKT and signal transducer and activator of transcription 3 (STAT3), an indication of pro-survival signalling. These data underscore the local, innate, pro-inflammatory nature of the response to C. difficile and highlight eIF2α phosphorylation and the interleukin-22-pSTAT3-RegIIIγ axis as two of the pathways that could be used to contain and counteract the damage inflicted on the intestinal epithelium.

Ex vivo enzymatic treatment of aged CD4 T cells restores cognate T cell helper function and enhances antibody production in mice

Previous in vitro studies showed that CD4 T cells from old mice have defects in TCR signaling, immune synapse formation, activation, and proliferation. We reported that removing a specific set of surface glycoproteins by ex vivo treatment with O-sialoglycoprotein endopeptidase (OSGE) can reverse many aspects of the age-related decline in CD4 T cell function. However, the specific mechanism by which this process occurs remains unclear, and it is unknown whether this enzymatic treatment can also restore important aspects of adaptive immunity in vivo. By using an in vivo model of the immune response based on adoptive transfer of CD4 T cells from pigeon cytochrome C-specific transgenic H-2(k/k) TCR-Vα(11)Vβ(3) CD4(+) mice to syngeneic hosts, we demonstrate that aging diminishes CD28 costimulatory signals in CD4 T cells. These age-associated defects include changes in phosphorylation of AKT and expression of glucose transporter type I, inducible T cell costimulatory molecule, and CD40L, suggesting that the lack of CD28 costimulation contributes to age-dependent loss of CD4 function. All of these deficits can be reversed by ex vivo OSGE treatment. Blocking B7-CD28 interactions on T cells prevents OSGE-mediated restoration of T cell function, suggesting that changes in surface glycosylation, including CD28, may be responsible for the age-related costimulation decline. Finally, we show that the age-related decline in CD4 cognate helper function for IgG production and long-term humoral immunity can also be restored by OSGE treatment of CD4 T cells prior to adoptive transfer.

Age-related changes in CD8 T cell homeostasis and immunity to infection

Studies of CD8 T cell responses to vaccination or infection with various pathogens in both animal models and human subjects have revealed a markedly consistent array of age-related defects. In general, recent work shows that aged CD8 T cell responses are decreased in magnitude, and show poor differentiation into effector cells, with a reduced arsenal of effector functions. Here we review potential mechanisms underlying these defects. We specifically address phenotypic and numeric changes to the naïve CD8 T cell precursor pool, the impact of persistent viral infection(s) and inflammation, and contributions of the aging environment in which these cells are activated.

Age-related defects in the cytoskeleton signaling pathways of CD4 T cells

It has been postulated that the cytoskeleton controls many aspects of T cell function, including activation, proliferation and apoptosis. Recent advances in our understanding of F-actin polymerization and the Ezrin-Radixin-Moesin (ERM) family of cytoskeleton signal proteins have provided new insights into immunological synapse formation during T cell activation. During aging there is a significant decline of T cell function largely attributable to declines in activation of CD4 T cells and defects in the formation of the immunological synapse. Here we discuss recent progress in the understanding of how aging alters F-actin and ERM proteins in mouse CD4 T cells, and the implications of these changes for the T cell activation process.

Ex vivo enzymatic treatment of aged CD4 T cells restores antigen-driven CD69 expression and proliferation in mice

Declines in immune function have been associated with declines in the function of naïve CD4 T cells. In vitro studies of naïve CD4 T cells in TCR-specific transgenic AND mice have shown age-related defects in immunosynapse formation, activation, proliferation and cytokine production. Previous work has also documented age-related alteration in the glycosylation of surface proteins involved in TCR signaling, and shown that enzymatic treatments to remove specific surface glycoproteins can restore in vitro function in CD4 cells from aged mice. Here an adoptive transfer system shows that a large percentage of naïve CD4 T cells from old mice fail to express CD69 and expand in antigen-primed mice, but these declines in CD69 and expansion can be restored by ex vivo pretreatment of the T cells with the bacterial enzyme O-sialoglycoprotein endopeptidase (OSGE). OSGE treatment also repairs the age-dependent loss of CD69 expression after in vivo activation.

Trypanosoma cruzi subverts host cell sialylation and may compromise antigen-specific CD8+ T cell responses

Upon activation, cytotoxic CD8(+) T lymphocytes are desialylated exposing beta-galactose residues in a physiological change that enhances their effector activity and that can be monitored on the basis of increased binding of the lectin peanut agglutinin. Herein, we investigated the impact of sialylation mediated by trans-sialidase, a specific and unique Trypanosoma transglycosylase for sialic acid, on CD8(+) T cell response of mice infected with T. cruzi. Our data demonstrate that T. cruzi uses its trans-sialidase enzyme to resialylate the CD8(+) T cell surface, thereby dampening antigen-specific CD8(+) T cell response that might favor its own persistence in the mammalian host. Binding of the monoclonal antibody S7, which recognizes sialic acid-containing epitopes on the 115-kDa isoform of CD43, was augmented on CD8(+) T cells from ST3Gal-I-deficient infected mice, indicating that CD43 is one sialic acid acceptor for trans-sialidase activity on the CD8(+) T cell surface. The cytotoxic activity of antigen-experienced CD8(+) T cells against the immunodominant trans-sialidase synthetic peptide IYNVGQVSI was decreased following active trans-sialidase-mediated resialylation in vitro and in vivo. Inhibition of the parasite's native trans-sialidase activity during infection strongly decreased CD8(+) T cell sialylation, reverting it to the glycosylation status expected in the absence of parasite manipulation increasing mouse survival. Taken together, these results demonstrate, for the first time, that T. cruzi subverts sialylation to attenuate CD8(+) T cell interactions with peptide-major histocompatibility complex class I complexes. CD8(+) T cell resialylation may represent a sophisticated strategy to ensure lifetime host parasitism.

Effect of sialic acid loss on dendritic cell maturation

Sialic acids are key structural determinants and contribute to the functionality of a number of immune cell receptors. Previously, we demonstrated that differentiation of human dendritic cells (DCs) is accompanied by an increased expression of sialylated cell surface structures, putatively through the activity of the ST3Gal.I and ST6Gal.I sialyltransferases. Furthermore, DC endocytosis was reduced upon removal of the cell surface sialic acid residues by neuraminidase. In the present work, we evaluate the contribution of the sialic acid modifications in DC maturation. We demonstrate that neuraminidase-treated human DCs have increased expression of major histocompatibility complex (MHC) and costimulatory molecules, increased gene expression of specific cytokines and induce a higher proliferative response of T lymphocytes. Together, the data suggest that clearance of cell surface sialic acids contributes to the development of a T helper type 1 proinflammatory response. This postulate is supported by mouse models, where elevated MHC class II and increased maturation of specific DC subsets were observed in DCs harvested from ST3Gal.I(-/-) and ST6Gal.I(-/-) mice. Moreover, important qualitative differences, particularly in the extent of reduced endocytosis and in the peripheral distribution of DC subsets, existed between the ST3Gal.I(-/-) and ST6Gal.I(-/-) strains. Together, the data strongly suggest not only a role of cell surface sialic acid modifications in maturation and functionality of DCs, but also that the sialic acid linkages created by different sialyltransferases are functionally distinct. Consequently, with particular relevance to DC-based therapies, cell surface sialylation, mediated by individual sialyltransferases, can influence the immunogenicity of DCs upon antigen loading.

Characterizing the glycome of the mammalian immune system

The outermost layer of all immune cells, the glycocalyx, is composed of a complex mixture of glycoproteins, glycolipids and lectins, which specifically recognize particular glycan epitopes. As the glycocalyx is the cell's primary interface with the external environment many biologically significant events can be attributed to glycan recognition. For this reason the rapidly expanding glycomics field is being increasingly recognized as an important component in our quest to better understand the functioning of the immune system. In this review, we highlight the current status of immune cell glycomics, with particular attention being paid to T- and B-lymphocytes and dendritic cells. We also describe the strategies and methodologies used to define immune cell glycomes.

Immunosupportive therapies in aging

The primary role of the immune system is to protect the organism against pathogens, but age-associated alterations to immunity increase the susceptibility of the elderly to infectious disease. The exact nature of these changes is still controversial, but the use of screening procedures, such as the SENIEUR protocol to exclude underlying illness, helped to better characterize the changes actually related to physiological aging rather than pathology. It is generally agreed that the most marked changes occur in the cellular immune response reflecting profound alterations in T cells. Much of this is due to thymic involution as well as changes in the proportions of T cell subpopulations resulting from antigen exposure, and altered T cell activation pathways. However, a body of data indicates that innate immune responses, including the critical bridge between innate and adaptive immunity, and antigen presenting capacity are not completely resistant to senescence processes. The consequences of all these alterations are an increased incidence of infections, as well as possibly cancers, autoimmune disorders, and chronic inflammatory diseases. The leading question is what, if anything, can we do to prevent these deleterious changes without dangerously dysregulating the precarious balance of productive immunity versus immunopathology? There are many potential new therapeutic means now available to modulate immunosenescence and many others are expected to be available shortly. One main problem in applying these experimental therapies is ethical: there is a common feeling that as ageing is not a disease; the elderly are not sick and therefore do not require adventurous therapies with unpredictable side-effects in mostly frail individuals. Animal models are not helpful in this context. In this chapter we will first briefly review what we think we know about human immunosenescence and its consequences for the health status of elderly individuals. We will then discuss possible interventions that might one day become applicable in an appropriate ethical environment.