CD43 is a murine T cell costimulatory receptor that functions independently of CD28

Transplantation elicits a clonally diverse CD8+ T cell response that is comprised of potent CD43+ effectors

CD8+ T cells mediate acute rejection of allografts, which threatens the long-term survival of transplanted organs. Using MHC class I tetramers, we find that allogeneic CD8+ T cells are present at an elevated naive precursor frequency relative to other epitopes, only modestly increase in number after grafting, and maintain high T cell receptor diversity throughout the immune response. While antigen-specific effector CD8+ T cells poorly express the canonical effector marker KLRG-1, expression of the activated glycoform of CD43 defines potent effectors after transplantation. Activated CD43+ effector T cells maintain high expression of the coreceptor induced T cell costimulator (ICOS) in the presence of CTLA-4 immunoglobulin (Ig), and dual CTLA-4 Ig/anti-ICOS treatment prolongs graft survival. These data demonstrate that graft-specific CD8+ T cells have a distinct response profile relative to anti-pathogen CD8+ T cells and that CD43 and ICOS are critical surface receptors that define potent effector CD8+ T cell populations that form after transplantation.

CD169+ Monocyte and Regulatory T Cell Subsets Are Associated with Disease Activity in Rheumatoid Arthritis

Disease activity in rheumatoid arthritis (RA) is influenced by activation of circulating and synovial immune cells. Regulatory T cells (Tregs) and monocytes are key cells that drive inflammation in RA. This study investigated if a relationship exists between disease activity in RA and circulating Treg and monocyte numbers and phenotypes. A potential sialic acid (Sia) mediated link between Tregs and monocytes was also probed in vitro. Peripheral blood mononuclear cells (PBMCs) were isolated from RA patient (n = 62) and healthy control (n = 21) blood using density gradient separation. Flow cytometry was used to count and phenotype Treg and monocyte subsets, and to sort healthy control Tregs for Sia cell culture experiments. The effects of Sia on activated Treg FoxP3 and NFκB expression was assessed by flow cytometry and concentrations of secreted TNFα, IL-10 and IFNγ determined by ELISA. High disease activity RA patients who were unresponsive to disease modifying anti-rheumatic drugs (n = 31), have significantly lower relative numbers (percentages) of CD4+CD25+CD127− Tregs (p < 0.01) and memory CD45RA−FoxP3+ Tregs (p < 0.01), compared to low disease activity responders (n = 24). Relative numbers of non-classical CD169+ monocytes are associated with disease activity in RA (p = 0.012). Sia reduced Treg expression of FoxP3, NFκB and cytokines in vitro. A strong association has been identified between non-classical CD169+ monocytes and post-treatment disease activity in RA. This study also indicates that Sia can reduce Treg activation and cytokine release. We postulate that such a reduction could be mediated by interaction with sialyted proteins captured by CD169+ monocytes.

Changed Expression of Cytoskeleton Proteins During Lung Injury in a Mouse Model of Streptococcus pneumoniae Infection

Infections by Streptococcus pneumoniae are a major cause of morbidity and mortality worldwide, often causing community-acquired pneumonia, otitis media and also bacteremia and meningitis. Studies on S. pneumoniae are mainly focused on its virulence or capacity to evade the host immune system, but little is known about the injury caused in lungs during a pneumococcal infection. Herein we investigated this issue comparing the proteome profile of lungs from S. pneumoniae-infected mice with control mice by means of difference gel electrophoresis (DIGE) technology. In order to obtain reliable results three biological replicas were used, and four technical replicas were carried out in each biological replica. Proteomic comparison was performed at two time points: 24 and 48 h post infection. A total of 91 proteins were identified with different abundance. We found important changes in the protein profiles during pneumococcal infection mainly associated with regulation of vesicle-mediated transport, wound healing, and cytoskeleton organization. In conclusion, the results obtained show that the cytoskeleton of the host cell is modified in S. pneumoniae infection.

An alternative mode of CD43 signal transduction activates pro-survival pathways of T lymphocytes

CD43 is one of the most abundant co-stimulatory molecules on a T-cell surface; it transduces activation signals through its cytoplasmic domain, contributing to modulation of the outcome of T-cell responses. The aim of this study was to uncover new signalling pathways regulated by this sialomucin. Analysis of changes in protein abundance allowed us to identify pyruvate kinase isozyme M2 (PKM2), an enzyme of the glycolytic pathway, as an element potentially participating in the signalling cascade resulting from the engagement of CD43 and the T-cell receptor (TCR). We found that the glycolytic activity of this enzyme was not significantly increased in response to TCR+CD43 co-stimulation, but that PKM2 was tyrosine phosphorylated, suggesting that it was performing moonlight functions. We report that phosphorylation of both Y¹⁰⁵ of PKM2 and of Y⁷⁰⁵ of signal transducer and activator of transcription 3 was induced in response to TCR+CD43 co-stimulation, resulting in activation of the mitogen-activated protein kinase kinase 5/extracellular signal-regulated kinase 5 (MEK5/ERK5) pathway. ERK5 and the cAMP response element binding protein (CREB) were activated, and c-Myc and nuclear factor-κB (p65) nuclear localization, as well as Bad phosphorylation, were augmented. Consistent with this, expression of human CD43 in a murine T-cell hybridoma favoured cell survival. Altogether, our data highlight novel signalling pathways for the CD43 molecule in T lymphocytes, and underscore a role for CD43 in promoting cell survival through non-glycolytic functions of metabolic enzymes.

Engagement of distinct epitopes on CD43 induces different co-stimulatory pathways in human T cells

Co‐receptors, being either co‐stimulatory or co‐inhibitory, play a pivotal role in T‐cell immunity. Several studies have indicated that CD43, one of the abundant T‐cell surface glycoproteins, acts not only as a potent co‐receptor but also as a negative regulator for T‐cell activation. Here we demonstrate that co‐stimulation of human peripheral blood (PB) T cells through two distinct CD43 epitopes recognized by monoclonal antibodies (mAb) CD43‐6E5 (T_6E5‐act) and CD43‐10G7 (T_10G7‐act) potently induced T‐cell proliferation. However, T‐cell co‐stimulation through two CD43 epitopes differentially regulated activation of nuclear factor of activated T cells (NFAT) and nuclear factor‐κB (NF‐κB) transcription factors, T‐cell cytokine production and effector function. T_6E5‐act produced high levels of interleukin‐22 (IL‐22) and interferon‐γ (IFN‐γ) similar to T cells activated via CD28 (T_CD28‐act), whereas T_10G7‐act produced low levels of inflammatory cytokines but higher levels of regulatory cytokines transforming growth factor‐β (TGF‐β) and interleukin‐35 (IL‐35). Compared with T_6E5‐act or to T_CD28‐act, T_10G7‐act performed poorly in response to re‐stimulation and further acquired a T‐cell suppressive function. T_10G7‐act did not directly inhibit proliferation of responder T cells, but formed stable heterotypic clusters with dendritic cells (DC) via CD2 to constrain activation of responder T cells. Together, our data demonstrate that CD43 is a unique and polarizing regulator of T‐cell function.

CD43-mediated IFN-γ production by CD8+ T cells promotes abdominal aortic aneurysm in mice

CD43 is a glycosylated surface protein abundantly expressed on lymphocytes. Its role in immune responses has been difficult to clearly establish, with evidence supporting both costimulatory and inhibitory functions. In addition, its contribution to disease pathogenesis remains elusive. Using a well-characterized murine model of elastase-induced abdominal aortic aneurysm (AAA) that recapitulates many key features of the human disease, we established that the presence of CD43 on T cells is required for AAA formation. Moreover, we found that IFN-γ-producing CD8(+) T cells, but not CD4(+) T cells, promote the development of aneurysm by enhancing cellular apoptosis and matrix metalloprotease activity. Reconstitution with IFN-γ-producing CD8(+) T cells or recombinant IFN-γ promotes the aneurysm phenotype in CD43(-/-) mice, whereas IFN-γ antagonism abrogates disease in wild-type animals. Furthermore, we showed that the presence of CD43 with an intact cytoplasmic domain capable of binding to ezrin-radixin-moesin cytoskeletal proteins is essential for optimal in vivo IFN-γ production by T cells and aneurysm formation. We have thus identified a robust physiologic role for CD43 in a relevant animal model and established an important in vivo function for CD43-dependent regulation of IFN-γ production. These results further suggest that IFN-γ antagonism or selective blockade of CD43(+)CD8(+) T cell activities merits further investigation for immunotherapy in AAA.

CD43 regulates the threshold for T cell activation by targeting Cbl functions

T cell (TC) activation requires the coordinated signaling of the T cell receptor (TCR) and coreceptor molecules, allowing TCs to respond to lower degrees of TCR occupancy. Coreceptor molecules set the threshold for TC activation by controlling different regulatory signaling loops. The Cbl family members prevent undesired activation of T cells by regulating TCR signals. In this report, we show that TC prestimulation by the CD43 coreceptor molecule before TCR engagement inhibits TCR-dependent c-Cbl tyrosine phosphorylation, c-Cbl interaction with the adapter molecule Crk-L and promotes Cbl-b degradation in a PKCθ-dependent manner. Consequently, the prolonged tyrosine phosphorylation and delayed degradation of ZAP-70 and of the ζ chain lead to enhanced mitogen-activated protein kinase activation and robust TC response. These data indicates that CD43-mediated signals lower the threshold for TC activation by restricting the c-Cbl and Cbl-b inhibitory effects on TCR signaling. In addition to the strength and duration of intracellular signals, our data underscore temporality with which certain molecules are engaged as yet another mechanism to fine tune TC signal quality, and ultimately immune function.

Role of unusual CD4+ CD28- T cells in acute coronary syndrome

Acute coronary syndrome (ACS) is a group of clinical symptoms that results from complete or partial occlusive thrombus, which is caused by coronary an atherosclerotic plaque rupture or erosion. According to a recent study, CD4(+) CD28(-) T cells are found in atherosclerotic plaques and the peripheral circulation blood in patients with ACS, these cells play an important role in plaque ruptures. CD4(+) CD28(-) T cells are an unusual subset of helper cells, which expand and have harmful effects in ACS. In this review, we discuss the current issues on the generation of CD4(+) CD28(-) T cells and focus on their phenotypic and functional characteristics relevant to the development of cardiovascular events. Targeting the CD4(+) CD28(-) T cells subset in ACS could provide novel therapeutic means to prevent acute life-threatening coronary events.

CD43 Expression Regulated by IL-12 Signaling Is Associated with Survival of CD8 T Cells

Background

In addition to TCR and costimulatory signals, cytokine signals are required for the differentiation of activated CD8 T cells into memory T cells and their survival. Previously, we have shown that IL-12 priming during initial antigenic stimulation significantly enhanced the survival of activated CD8 T cells and increased the memory cell population. In the present study, we analyzed the mechanisms by which IL-12 priming contributes to activation and survival of CD8 T cells.

Methods

We observed dramatically decreased expression of CD43 in activated CD8 T cells by IL-12 priming. We purified CD43^lo and CD43^hi cells after IL-12 priming and analyzed the function and survival of each population both in vivo and in vitro.

Results

Compared to CD43^hi effector cells, CD43^lo effector CD8 T cells exhibited reduced cytolytic activity and lower granzyme B expression but showed increased survival. CD43^lo effector CD8 T cells also showed increased in vivo expansion after adoptive transfer and antigen challenge. The enhanced survival of CD43^lo CD8 T cells was also partly associated with CD62L expression.

Conclusion

We suggest that CD43 expression regulated by IL-12 priming plays an important role in differentiation and survival of CD8 T cells.

CD43 processing and nuclear translocation of CD43 cytoplasmic tail are required for cell homeostasis

The sialomucin CD43 is highly expressed on most hematopoietic cells. In this study, we show that the CD43 ectodomain is shed from murine granulocytes, mast cells, and T cells, but not from macrophages. To study the significance of CD43 shedding, we constructed 2 CD43/34 chimeras in which the CD43 membrane-proximal or transmembrane domain was swapped with the corresponding domain from CD34 that is not shed from cells. Viability of cells that normally shed CD43 was negatively affected when forced to express either of the 2 CD43/34 chimeras, but toxicity was reduced when cells coexpressed wild-type CD43. The CD43 cytoplasmic tail (CD43ct) was found to translocate into the nucleus, and inhibition of either its nuclear translocation or its release by gamma-secretase was proapoptotic. Involvement of CD43 in regulation of apoptosis is consistent with our findings that CD43ct was modified by small ubiquitin-like modifier-1 and was colocalized with promyelocytic nuclear bodies. CD43-deficient cells exhibited reduced levels of promyelocytic nuclear bodies and had increased sensitivity to apoptosis induced by growth factor withdrawal or T-regulatory cell suppression. Taken together, our data indicate an essential function of CD43 processing and nuclear localization of CD43ct in cell homeostasis and apoptosis.

CD43 controls the intracellular growth of Mycobacterium tuberculosis through the induction of TNF-alpha-mediated apoptosis

Establishment of Tuberculosis infection begins with the successful entry and survival of the pathogen within macrophages. We previously showed that macrophage CD43 is required for optimal uptake and growth inhibition of Mycobacterium tuberculosis both in vitro and in vivo. Here, we explore the mechanisms by which CD43 restricts mycobacterial growth in murine macrophages. We found that although M. tuberculosis grows more readily in resting CD43-/- macrophages, priming of cells with IFN-gamma returns the bacterial growth rate to that seen in CD43+/+ cells. To discern the mechanisms by which M. tuberculosis exhibits enhanced growth within resting CD43-/- macrophages, we assessed the induction of inflammatory mediators in response to infection. We found that absence of CD43 resulted in reduced production of TNF-alpha, IL-12 and IL-6 by M. tuberculosis-infected macrophages. We also found that infected resting, but not activated CD43-/- macrophages, showed decreased apoptosis and increased necrosis. Exogenous addition of the pro-inflammatory cytokine TNF-alpha restored control of M. tuberculosis growth and induction of apoptosis to CD43+/+ levels. We propose that CD43 is involved in the inflammatory response to M. tuberculosis and, through the induction of pro-inflammatory mediators, can regulate apoptosis to control intracellular growth of the bacterium.

Deletion of PI3K-p85alpha gene impairs lineage commitment, terminal maturation, cytokine generation and cytotoxicity of NK cells

Class IA phosphotidylinositol-3-kinases (PI3Ks) are a family of p85/p110 heterodimeric lipid kinases that are important in regulating signaling events in B and T cells. However, their role in natural killer (NK) cells is not understood. Here, using mice that lack the regulatory p85alpha subunit and its alternatively spliced variants p55alpha/p50alpha (collectively termed as p85alpha(-/-)), we defined the role of PI3K in NK cell development and function. p85alpha(-/-) mice had impaired lineage commitment leading to reduced NK cellularity in the bone marrow and liver. p85alpha(-/-) NK cells showed a defective Ly49 subset specification and a decreased expression of CD43. Lack of p85alpha severely reduced the NK-mediated cytotoxicity against tumor cells representing 'induced-self' and 'missing-self'. More importantly, NKG2D and NK1.1 receptor-mediated cytokine and chemokine generation was significantly compromised in p85alpha(-/-) NK cells. These results reveal a previously unrecognized role of p85alpha in the development, terminal maturation, cytokine/chemokine generation and tumor clearance of NK cells.

The actin cytoskeleton in T cell activation

T cell cytoarchitecture differs dramatically depending on whether the cell is circulating within the bloodstream, migrating through tissues, or interacting with antigen-presenting cells. The transition between these states requires important signaling-dependent changes in actin cytoskeletal dynamics. Recently, analysis of actin-regulatory proteins associated with T cell activation has provided new insights into how T cells control actin dynamics in response to external stimuli and how actin facilitates downstream signaling events and effector functions. Among the actin-regulatory proteins that have been identified are nucleation-promoting factors such as WASp, WAVE2, and HS1; severing proteins such as cofilin; motor proteins such as myosin II; and linker proteins such as ezrin and moesin. We review the current literature on how signaling pathways leading from diverse cell surface receptors regulate the coordinated activity of these and other actin-regulatory proteins and how these proteins control T cell function.

The case for extrathymic development of vaginal T lymphocytes

The vaginal tract mucosa is populated by a small, yet phenotypically diverse and functionally significant, subset of T cells that plays a major role in local cell-mediated immunity. Although phenotypic and functional characteristics of vaginal T cells have received some attention in recent years, little is known about the development of this cell population. In this mini review, the developmental origins of vaginal T cells are traced from published work related to vaginal T cells, the vaginal mucosa environment and vaginal tract infection animal models. A CD3(+)TCR(+)CD2(+)CD5(+)B220(-) (CD3(+)B220(-)) subpopulation, which is mostly CD4(+), makes up 30-40% of vaginal T lymphocytes. This population consists of a TCRalphabeta(+) subset and TCRgammadelta(+) subset. While CD3(+)B220(-)TCRalphabeta(+) vaginal T cells exhibit phenotypic and functional properties consistent with that of peripheral T cells, CD3(+)B220(-)TCRgammadelta(+) vaginal T cells exhibit unique phenotypic and functional features that set them apart from other TCRgammadelta(+) T cell subsets populating the periphery or other mucosal areas. The vaginal mucosa is populated also by CD3(+)TCRalphabeta(+)CD4(-)/8(-)B220(+)CD2(-)CD5(-) T cells (CD3(+)B220(+)) whose relative predominance increases significantly in systemic T cell deficiency. This subset is generally unresponsive to TCR-mediated stimuli and expresses high levels of CD25, perhaps indicative of a regulatory role. Current data suggest that, while CD3(+)B220(-) vaginal T cells are mostly thymic in origin, CD3(+)TCRalphabeta(+)B220(+) cells are exclusively extrathymic.

T-cell activation in the intestinal mucosa

The vast majority of peripheral T cells exist as resting lymphocytes until a signal for activation has been received. In response to antigen, this activation involves ligation of the T-cell receptor (TCR) and signal transmission through the CD3 complex, which then initiates a cascade of intracellular events that lead to the expression of genes used in T-cell activation. T-cell activation also requires soluble mediators in the form of cytokines and chemokines that regulate the process in both positive and negative ways, and costimulatory signals received in conjunction with TCR/CD3 signaling are important in the activation of T cells. Unlike T cells in other peripheral immune compartments, small and large intestinal intraepithelial lymphocytes (IELs) bear some but not all properties of activated T cells, suggesting that they constitute a large population of 'partially activated' effector cells. Thus, regulation of the IEL activation process must be held in tight check, yet it must be ready to respond to foreign antigen rapidly and effectively. We discuss how costimulatory molecules may hold the key to controlling IEL activation through a multiphase process beginning with cells that have already entered into the early stage of activation.

Galectin-1 binds different CD43 glycoforms to cluster CD43 and regulate T cell death

Galectin-1 kills immature thymocytes and activated peripheral T cells by binding to glycans on T cell glycoproteins including CD7, CD45, and CD43. Although roles for CD7 and CD45 in regulating galectin-1-induced death have been described, the requirement for CD43 remains unknown. We describe a novel role for CD43 in galectin-1-induced death, and the effects of O-glycan modification on galectin-1 binding to CD43. Loss of CD43 expression reduced galectin-1 death of murine thymocytes and human T lymphoblastoid cells, indicating that CD43 is required for maximal T cell susceptibility to galectin-1. CD43, which is heavily O-glycosylated, contributes a significant fraction of galectin-1 binding sites on T cells, as T cells lacking CD43 bound approximately 50% less galectin-1 than T cells expressing CD43. Although core 2 modification of O-glycans on other glycoprotein receptors is critical for galectin-1-induced cross-linking and T cell death, galectin-1 bound to CD43 fusion proteins modified with either unbranched core 1 or branched core 2 O-glycans and expression of core 2 O-glycans did not enhance galectin-1 binding to CD43 on T cells. Moreover, galectin-1 binding clustered CD43 modified with either core 1 or core 2 O-glycans on the T cell surface. Thus, CD43 bearing either core 1 or core 2 O-glycans can positively regulate T cell susceptibility to galectin-1, identifying a novel function for CD43 in controlling cell death. In addition, these studies demonstrate that different T cell glycoproteins on the same cell have distinct requirements for glycan modifications that allow recognition and cross-linking by galectin-1.

Modulation of MOG 37-50-specific CD8+ T cell activation and expansion by CD43

Several recent reports have described an effector role for CD8(+) T cells during EAE. We have previously demonstrated reduced disease incidence and severity in CD43(-/-) mice following MOG immunization, and attributed this attenuation in disease progression to the effects of CD43 deficiency on CD4+ T cells. Here, we extend those studies to examine the effects of the loss of CD43 on MOG-specific CD8+ T cells. A reduced frequency of MOG-specific CD8+ T cells following immunization was observed in CD43(-/-) mice relative to wild-type controls, as demonstrated by intracellular cytokine and MHC tetramer staining. In addition, adoptive transfer of CD8+ MOG 35-55-primed LN cells from CD43(-/-) mice resulted in significantly attenuated EAE induction as compared to recipients of wild-type CD8+ MOG-primed cells. Analysis of intracellular signaling intermediates revealed a deficiency in the ability of MOG-specific CD8+ T cells to phosphorylate ERK in response to antigen. These results characterize an important role for CD43 during the activation and expansion of autoreactive MOG-specific CD8+ T cells.

CD43-independent augmentation of mouse T-cell function by glycoprotein cleaving enzymes

Previous work has shown that the function of mouse CD4+ T cells can be augmented by an enzyme, O-sialoglycoprotein endopeptidase (OSGE), which cleaves surface CD43, suggesting the idea that the high levels of glycosylated CD43 found on T cells from aged mice may contribute to immune senescence. New results now show that OSGE improves T-cell function even in mice lacking CD43, showing that other glycoproteins must contribute to the OSGE effect on function. Evaluation of other enzymes found two whose ability to stimulate CD4 activation was higher in aged than in young T cells. One of these, PNGase F, is a glycosidase specific for N-linked glycans, and the other, ST-Siase(2,3) from Salmonella typhimurium, is specific for alpha2,3-linked terminal sialic acid residues. Parallel lectin-binding experiments showed that removal of alpha2,3-linked sialic acid residues vulnerable to PNGase F and ST-Siase(2,3) was also greater in old than in young T cells. The preferential ability of PNGase F and ST-Siase(2,3) to improve the function of T cells from aged mice may involve cleavage of glycoproteins containing alpha2,3-linked sialic acid residues on N-linked or O-linked glycans or both.

TCR-Dependent Cell Response Is Modulated by the Timing of CD43 Engagement

Binding of Ag by the Ag receptor in combination with other stimuli provided by costimulatory receptors triggers the expansion and differentiation of T lymphocytes. However, it is unclear whether the time when costimulatory molecules interact with their counterreceptors with regards to Ag recognition leads to different T cell responses. Provided that the coreceptor molecule CD43 is a very abundant molecule evenly distributed on the membrane of T cell surface protruding 45 nm from the cell, we hypothesized that CD43 is one of the first molecules that interacts with the APC and thus modulates TCR activation. We show that engaging CD43 before or simultaneously with the TCR inhibited Lck-Src homology 2 domain containing phosphatase-1 interaction, preventing the onset of a negative feedback loop on TCR signals, favoring high levels of IL-2, cell proliferation, and secretion of proinflammatory cytokines and chemokines. In contrast, the intracellular signals resulting of engaging the TCR before CD43 were insufficient to induce IL-2 production and cell proliferation. Interestingly, when stimulated through the TCR and CD28, cells proliferated vigorously, independent of the order with which molecules were engaged. These results indicate that CD43 induces a signaling cascade that prolongs the duration of TCR signaling and support the temporal summation model for T cell activation. In addition to the strength and duration of intracellular signals, our data underscore temporality with which certain molecules are engaged as yet another mechanism to fine tune T cell signal quality, and ultimately immune function.

Upregulation of ICOS on CD43+ CD4+ murine small intestinal intraepithelial lymphocytes during acute reovirus infection

Murine intestinal intraepithelial lymphocytes (IELs) can be classified according to expression of a CD43 glycoform recognized by the S7 monoclonal antibody. In this study, we examined the response of S7+ and S7- IELs in mice during acute reovirus serotype 3 (Dearing strain) infection, which was confirmed by virus-specific real-time PCR. In vivo proliferation increased significantly for both S7- and S7+ IELs on day 4 post-infection as determined by BrdU incorporation; however, expression of the inducible costimulatory (ICOS) molecule, which peaked on day 7 post-infection, was upregulated on S7+ CD4+ T cells, most of which were CD4+8- IELs. In vitro ICOS stimulation by syngeneic peritoneal macrophages induced IFN-gamma secretion from IELs from day 7 infected mice, and was suppressed by treatment with anti-ICOS mAb. Additionally, IFN-gamma mRNA increased in CD4+ IELs on day 6 post-infection. These findings indicate that S7- and S7+ IELs are differentially mobilized during the immune response to reovirus infection; that the regulated expression of ICOS is associated with S7+ IELs; and that stimulation of IELs through ICOS enhances IFN-gamma synthesis during infection.

Selective upregulation of immune regulatory and effector cytokine synthesis by intestinal intraepithelial lymphocytes following CD43 costimulation

The involvement of the CD43 molecule in the activation of mouse small intestinal intraepithelial lymphocytes (IELs) has been studied using a panel of twenty-two regulatory and effector immune response analytes. In the absence of stimulation in vitro, IELs produced low levels of CCL5 only. Upon CD3 stimulation, the activity of seven of twenty-two analytes was elevated relative to unstimulated cultures, including several proinflammatory cytokines and chemokines. Notably, CD3 stimulation in the presence of CD43 costimulation resulted in elevated levels of five analytes (interleukin-2, interferon-gamma, CCL5, granulocyte colony-stimulating factor, and granulocyte-monocyte colony-stimulating factor) above that produced by CD3 stimulation alone. That CD43 costimulation was responsible for elevated cytokine/chemokine activity was confirmed at the transcriptional level by real-time PCR for IFN-gamma and CCL5, and by ELISA for IFN-gamma. These findings open the way to a better understanding of the process by which T cells are activated in the intestinal epithelium.

A glycoprotein endopeptidase enhances calcium influx and cytokine production by CD4+ T cells of old and young mice

Many of the downstream signaling defects observed in aged T cells are believed to be the result of very early events involving the initial interaction between T cells and antigen-presenting cells. Recent findings suggest that this interaction is hindered by glycosylated surface macromolecules, including CD43, on the T cell surface. Treatment of CD4+ T cells by O-sialoglycoprotein endopeptidase (OSGE), which cleaves glycosylated forms of CD43, restores the ability of cells from aged mice to form immunological synapses and to express early activation markers. Here we show that OSGE enhances Ca2+ influx in T cells from CB6F1 mice, and enhances their ability to produce IL-2, IL-4, IL-5, IL-6, IL-10, IL-13 and IFNgamma at the mRNA level, and IL-2 and IFNgamma at the protein level, in the first 6 h after activation. Although OSGE has little effect on synapse formation in CD4+ T cells from young mice, our new data show that OSGE increases the production of most cytokines by young as well as old T cells. Secretion of the T(h)2 cytokine, IL-4, was altered only slightly by OSGE treatment, suggesting that the removal of OSGE-sensitive surface molecules may have differential effects on T(h)1 and T(h)2 cytokines. These data support a model in which O-glycosylated surface proteins inhibit CD4+ lymphocyte activation in both young and old mice, and in which such glycoproteins contribute to the age-related decline in cytokine production.

T-cell activation in the curious world of the intestinal intraepithelial lymphocyte

In conventional terms, when T cells encounter appropriate stimuli, they are induced to undergo molecular and physical changes that confer upon them a state of activation. Once initiated, activation generally results in a state of full T-cell responsiveness in an all-or-none manner. Uniquely, however, the intestinal intraepithelial lymphocytes (IELs) bear features that are decidedly different from those of T cells located throughout other immunological compartments in that they exhibit some but not all properties of activated T cells, yet they can be induced to move further into activation provided appropriate costimulatory signals have been received. IEL costimulatory molecules some of which are constitutively expressed, whereas others are upregulated following T-cell receptor (TCR)/CD3 stimulation appear to hold the key to determining the nature and magnitude of the activational process. A system of activation such as this in the intestine would be expected to have great immunological protective value for the host because it would provide an untrammeled process of T-cell activation at a barrier site where the level of antigen exposure is consistently high. Clearly, however, mechanisms must be in place to insure that the IEL activation process is not inadvertently breached. These and other issues central to the operational workings of the intestinal immune system are elaborated in this article, and a model is presented in which IEL activation can be viewed as a layered, three-stage activational process.

PKCtheta is required for the activation of human T lymphocytes induced by CD43 engagement

The turnover of phosphoinositides leading to PKC activation constitutes one of the principal axes of intracellular signaling. In T lymphocytes, the enhanced and prolonged PKC activation resulting from the engagement of the TcR and co-receptor molecules ensures a productive T cell response. The CD43 co-receptor promotes activation and proliferation, by inducing IL-2 secretion and CD69 expression. CD43 engagement has been shown to promote phosphoinositide turnover and DAG production. Moreover, PKC activation was found to be required for the activation of the MAP kinase pathway in response to CD43 ligation. Here we show that CD43 engagement led to the membrane translocation and enzymatic activity of specific PKC isoenzymes: cPKC (alpha/beta), nPKC (epsilon and theta;), aPKC (zeta) and PKCmu. We also show that activation of PKCtheta; resulting from CD43 ligation induced CD69 expression through an ERK-dependent pathway leading to AP-1, NF-kappaB activation and an ERK independent pathway promoting NFAT activation. Together, these data suggest that PKCtheta; plays a critical role in the co-stimulatory functions of CD43 in human T cells.

Comparative analysis of T-cell costimulation and CD43 activation reveals novel signaling pathways and target genes

The CD43 lymphocyte surface receptor is involved in the regulation of lymphocyte adhesion and activation. Many CD43 functions remain controversial or unclear, and it is not known to which extent CD43 signaling pathways are shared with or distinct from those used by the T-cell receptor (TCR). Here, we systematically compared signaling events and target gene expression induced by CD43 or T-cell costimulation in primary human peripheral T cells. These studies identify nuclear factor-κB (NF-κB) p65 serine 468 as a novel inducible phosphorylation site strongly induced by T-cell costimulation and only weakly triggered by CD43 ligation. We also identified CD43 as a novel Jun N-terminal kinase (JNK) activator and a comprehensive analysis of further signaling events suggests that both stimuli use overlapping but also distinct signaling pathways. Microarray analysis of inflammatory genes shows 1 group of genes coregulated by both stimuli and 2 further groups of target genes affected solely by costimulation or primarily by CD43. (Blood. 2004;104:3302-3304)

Maximum Immunobioactivity of Murine Small Intestinal Intraepithelial Lymphocytes Resides in a Subpopulation of CD43+ T Cells

CD43 has been linked to many function-associated T cell activities. Using mAbs that recognize two different CD43 determinants, we show that, although mouse small intestinal intraepithelial lymphocytes (IELs) expressed the CD43 core molecule reactive with mAb R2/60, only about one-half of the total IELs-including some but not all of the TCRalphabeta and TCRgammadelta cells-expressed the CD43 S7(-) reactive determinant. CD43 S7(+) IELs secreted more IL-2, IL-4, IL-10, IL-17, and IFN-gamma following anti-CD3 stimulation, and were >4-fold more cytotoxic in fresh isolates and >16-fold more cytotoxic after anti-CD3 stimulation, than S7(-) IELs. S7(+) but not S7(-) IELs from the ileum of IL-10(-/-) mice spontaneously produced IFN-gamma. In vivo BrdU uptake by IELs in non-Ag-primed mice was greatest in the S7(+) population, indicating that significantly more S7(+) IELs than S7(-) IELs undergo cell expansion under normal homeostatic conditions. DNA microarray analyses showed that S7(+) IELs expressed higher levels of genes associated with activated T cells, whereas S7(-) IELs expressed genes used in the regulation of NK cells. These findings define two functionally distinct populations of IELs based on CD43 expression independent of TCR class, and they identify a subset of IELs that may serve as a target to better control intestinal inflammation.

Expression characteristics and stimulatory functions of CD43 in human CD4+ memory T cells: analysis using a monoclonal antibody to CD43 that has a novel lineage specificity

We have used HSCA-2, an mAb that recognizes a sialic acid-dependent epitope on the low molecular mass (approximately 115-kDa) glycoform of CD43 that is expressed in resting T and NK cells, to examine the expression characteristics and stimulatory functions of CD43 in human CD4+ memory T cells. Having previously reported that the memory cells that respond to recall Ags in a CD4+ CD45RO+ T cell population almost all belong to a subset whose surface CD43 expression levels are elevated, we now find that exposing these same memory T cells to HSCA-2 mAb markedly increases their proliferative responsiveness to recall Ags. We think it unlikely that this increase in responsiveness is a result of CD43-mediated monocyte activation, especially given that the HSCA-2 mAb differs from all previously used CD43 mAbs in having no obvious binding specificity for monocyte CD43. Predictably, treatment with HSCA-2 mAb did not lead to significant recall responses in CD4+ CD45RO+ T cells, whose CD43 expression levels were similar to or lower than those of naive cells. Other experiments indicated that the HSCA-2 mAb was capable of enhancing the proliferative responsiveness of CD4+ memory T cells that had been exposed to polyclonal stimulation by monocyte-bound CD3 mAb and could also act in synergy with CD28 mAb to enhance the responsiveness of CD4+ T cells to CD3 stimulation. Taken together, these findings suggest that the CD43 molecules expressed on CD4+ memory T cells may be capable of enhancing the costimulatory signaling and hence providing accessory functions to TCR-mediated activation processes.

CD43 regulation of T cell activation is not through steric inhibition of T cell-APC interactions but through an intracellular mechanism

CD43 is a large heavily glycosylated protein highly expressed on T cells and actively excluded from the immunological synapse through interactions with ezrin-radixin-moesin proteins. Due to its size and charge, it has been proposed that the CD43 ectodomain acts as a physical barrier to T cell–APC interactions. We have addressed this hypothesis by studying the effect of reconstituting CD43 mutants into the hyperproliferative CD43^−/− T cells. Reintroduction of full-length CD43 reversed the CD43^−/− T cell hyperproliferation. Interestingly, despite the lack of exclusion from the interaction site, a mutant containing the CD43 ectodomain on a glycosylphosphatidylinositol linkage was ineffective. Additionally, T cell–APC conjugate formation was not affected by this ectodomain-only construct. In contrast, CD43^−/− T cell hyperproliferation was reversed by an intracellular-only CD43 fused to the small ectodomain of hCD16. Mutation of this intracellular-only CD43 such that it could not move from the T cell–APC contact site had no further affect on proliferation than the moveable CD43 but did dramatically reduce interleukin-2 production. Thus, the exclusion of the CD43 intracellular region from the immunological synapse is required for CD43 regulation of interleukin-2 production, but the presence of the cytoplasmic tail, independent of its location, is sufficient to reverse CD43^−/− T cell hyperproliferation.

Regulation of CD43-induced U937 homotypic aggregation

CD43 (leukosialin, sialophorin), a prominent component of the hemopoietic cell surface, has an enigmatic role in cell-cell interaction. The observation that CD43 ligation triggers homotypic aggregation of monoblastoid U937 cells has permitted analysis of this: CD43-induced aggregation was distinguishable from CD29- (also known as beta1 integrin) or CD98- (also known as 4F2, or fusion-related protein 1) induced aggregation, with different energy requirements and with partial dependence on beta2 integrins. Previous studies have focused on the role of CD43 ligation in tyrosine phosphorylation. However, in the homotypic adhesion assay, although there is initial tyrosine phosphorylation, protein tyrosine kinase inhibitors did not block aggregation. Therefore, other signaling pathways were examined. CD43 ligation induced protein tyrosine dephosphorylation, and protein tyrosine phosphatase inhibitors blocked aggregation. Activation of MAP kinases was not necessary. Cytoskeletal inhibitors amplified aggregation. Protein kinase C (PKC) inhibitors amplified aggregation, implicating PKC as a negative regulator. CD43 ligation up-regulated surface adhesion molecules and enhanced CD29- and CD98-induced aggregation. Thus, CD43 participation in cell-cell adhesion is under stringent control, involving both surface events and several different intracellular signaling pathways, acting together to regulate the process. These mechanisms add a further dimension to the potential role of CD43 in tissue immune responses.

The CD43 coreceptor molecule recruits the zeta-chain as part of its signaling pathway

CD43 is an abundant cell surface sialoglycoprotein implicated in hemopoietic cell adhesion and activation. Cell stimulation through CD43 results in recruitment of different signaling proteins, including members of the Src family kinases, Syk, phospholipase Cgamma2, the adapter protein Shc, the guanine nucleotide exchange factor Vav, and activation of protein kinase C. In this study, we report that in human T lymphocytes, the zeta-chain is part of the CD43 signaling pathway. Upon CD43 engagement, the zeta-chain was tyrosine-phosphorylated, generating docking sites for tyrosine-phosphorylated zeta-associated protein of 70 kDa and Vav. In vitro kinase assays suggested that zeta-associated protein of 70 kDa could account for the kinase activity associated with the zeta-chain following CD43 engagement. Cross-linking CD43 on the surface of the Lck-deficient JCaM.1 cells failed to phosphorylate the zeta-chain and associated proteins, suggesting that Lck is a key element in the CD43 signaling pathway leading to zeta phosphorylation. CD43 engagement with beads coated with anti-CD43 mAb resulted in concentration of the zeta-chain toward the bead attachment site, but interestingly, the distribution of the T cell Ag receptor complex remained unaffected. Recruitment of the zeta-chain through CD43-mediated signals was not restricted to T lymphocytes because phosphorylation and redistribution of the zeta-chain was also observed in NK cells. Our results provide evidence that the zeta-chain functions as a scaffold molecule in the CD43 signaling pathway, favoring the recruitment and formation of downstream signaling complexes involved in the CD43-mediated cell activation of T lymphocytes and other leukocytes such as NK cells.

hnRNP-K and Pur(alpha) act together to repress the transcriptional activity of the CD43 gene promoter

CD43 is an abundant, heavily glycosylated molecule expressed specifically on the surface of leukocytes and platelets. When leukocytes are at rest, CD43 acts to prevent both homotypic and heterotypic interactions. However, during leukocyte activation CD43 expression is repressed, facilitating the intercellular contact required for chemotaxis, phagocytosis, aggregation, adhesion to endothelium, and transendothelial migration. Consequently, CD43 repression plays a vital role both in innate and acquired immunity. Here we report that a dramatic down-regulation of CD43 mRNA levels occurs during activation of the leukocytic cell line K562. This repression coincides with repression of the transcriptional activity of the CD43 gene promoter. We have determined that heterogeneous nuclear ribonucleoprotein K (hnRNP-K) and Pur(alpha) act together to mediate repression of the CD43 promoter during K562 activation. The hnRNP-K molecule and Pur(alpha) bind single-stranded DNA. Therefore, exposure of single-stranded structures within the CD43 promoter probably plays a major role in effecting CD43 repression.

Sweet 'n' sour: the impact of differential glycosylation on T cell responses

The fate and functional activity of T lymphocytes depend largely on the precise timing of gene expression and protein production. However, it is clear that post-translational modification of proteins affects their functional properties. Although modifications such as phosphorylation have been intensely studied by immunologists, less attention has been paid to the impact that changes in glycosylation have on protein function. However, there is considerable evidence that glycosylation plays a key role in immune regulation. We will focus here on examples in which differential glycosylation affects the development, survival or reactivity of T cells.

Engagement of CD43 enhances human immunodeficiency virus type 1 transcriptional activity and virus production that is induced upon TCR/CD3 stimulation

Human immunodeficiency virus type 1 (HIV-1) transcriptional activity is regulated by several cytokines and T cell activators. CD43 (sialophorin) is a sialoglycoprotein expressed on the surface of a wide variety of blood cells including T lymphocytes. Several studies have shown that CD43 ligation induces proliferation and activation of human T lymphocytes. We were thus interested in defining whether CD43-mediated signaling events can modulate the life cycle of HIV-1. We demonstrate here that CD43 cross-linking potentiates HIV-1 promoter-driven activity and virus production that is seen following the engagement of the T-cell receptor (TCR).CD3 complex. This effect is independent of the CD28 co-stimulatory molecule and is mediated by both NF-kappaB and NFAT transcription factors. A number of signal transducers known to be involved in the TCR/CD3-dependent signal transduction pathway, including p56(lck), p36(lat), and SLP-76, as well as capacitative entry of calcium, are crucial for the noticed CD43 co-stimulatory effect. Calcium mobilization studies indicate that a synergy is occurring between CD43- and TCR/CD3-mediated signaling events leading to an augmented calcium release. These data suggest that CD43 can be seen as a co-stimulatory cell surface constituent that can modulate HIV-1 expression in T lymphocytes.

Memory functions and death proneness in three CD4+CD45RO+ human T cell subsets

We propose a classification of human CD4(+)CD45RO(+) memory T cells into three new subsets based on cell surface expression levels of CD43. The first subset consists of cells whose CD43 expression is relatively high; this subset also contains the highest proportion of recall Ag-reactive precursors, and its constituent cells respond far more strongly than cells in either of the other subsets to immobilized CD3 Ab in addition to secreting substantially more IFN-gamma and IL-4. Cells of the second subset express similar levels of CD43 to naive cells, and they also respond weakly to TCR-mediated stimuli as judged by either their ability to proliferate or capacity for cytokine production. The third subsets consists of cells whose CD43 expression levels are clearly down-regulated; its cells appear to be anergic to TCR-mediated stimuli, and when examined ex vivo many of them appear to be undergoing either spontaneous apoptosis via a caspase-independent pathway or Fas-mediated apoptosis via a caspase-dependent pathway, even in the resting state. An analysis of telomere lengths revealed that the typical telomere of a cell in the second subset was significantly longer than the typical telomere in the first or third subset. Taken together, these results appear to indicate that CD4(+)CD45RO(+) T cells fall into three functionally differing subsets, one being a subset of cells with fully matured memory phenotype, a second being a less mature subset of cells that retain longer telomeres and whose memory functionality is marginal, and a third consisting of anergic cells that give every appearance of being death-prone and/or in the process of dying.

Dynamic regulation of T cell immunity by CD43

During a viral response, Ag-specific effector T cells show dramatically increased binding by the mAb 1B11 and the lectin peanut agglutinin (PNA). We investigated the contribution of CD43 expression to 1B11 and PNA binding as well as its role in generation and maintenance of a CD8 T cell response. Analysis of CD43(-/-) mice revealed no increased 1B11 binding and reduced PNA binding on virus-specific CD8 T cells from -/- mice compared with +/+ mice. Furthermore, we examined the role of CD43 in the kinetics of an immune response. We show that CD43 expression modestly effects generation of a primary virus-specific CD8 T cell response in vivo but plays a more significant role in trafficking of CD8 T cells to tissues such as the brain. More interestingly, CD43 plays a role in the contraction of the immune response, with CD43(-/-) mice showing increased numbers of Ag-specific CD8 T cells following initial expansion. Following the peak of expansion, Ag-specific CD8 T cells from -/- mice show similar proliferation but demonstrate increased Bcl-2 levels and decreased apoptosis of Ag-specific effector CD8 T cells in vitro. Consistent with a delay in the down-modulation of the immune response, following chronic viral infection CD43(-/-) mice show increased morbidity. These data suggest a dynamic role of CD43 during an immune response: a positive regulatory role in costimulation and trafficking of T cells to the CNS and a negative regulatory role in the down-modulation of an immune response.

Costimulation of host T lymphocytes by a trypanosomal trans-sialidase: involvement of CD43 signaling

Trans-sialidase is a membrane-bound and shed sialidase from Trypanosoma cruzi, the protozoan parasite responsible for Chagas disease. We investigated the role of soluble trans-sialidase on host CD4+ T cell activation. Trans-sialidase activated naive CD4+ T cells in vivo. Both enzymatically active and inactive recombinant trans-sialidases costimulated CD4+ T cell activation in vitro. Costimulation resulted in increased mitogen-activated protein kinase activation, proliferation, and cytokine synthesis. Furthermore, active and inactive trans-sialidases blocked activation-induced cell death in CD4+ T cells from T. cruzi-infected mice. By flow cytometry, inactive trans-sialidase bound the highly sialylated surface Ag CD43 on host CD4+ T cells. Both costimulatory and antiapoptotic effects of trans-sialidases required CD43 signaling. These results suggest that trans-sialidase family proteins are involved in exacerbated host T lymphocyte responses observed in T. cruzi infection.

Transient resistance to B16F10 melanoma growth and metastasis in CD43-/- mice

CD43, the major transmembrane sialoglycoprotein of neutrophils, monocytes, T lymphocytes and platelets, is highly glycosylated and its high sialic acid content contributes to the strongly negative charge of cells. In this study the role of CD43 in melanoma development was addressed using CD43 -/- mice (null mutated for the corresponding gene or knockout [KO]). Growth of B16F10 melanoma was retarded in the KO mice compared with the wild-type CD43+/+ control (WT). A marked difference in lung colonization and other metastatic foci was observed in the KO and WT mice up to 15 days after intravenous injection of tumour cells. The initial resistance of KO mice was reversed with time, and in the long term there was no difference in the survival rate of the two animal groups. Transient resistance was attributed to increased adhesion of thrombin-activated platelets and leukocytes to melanoma and endothelial cells in KO mice. In the KO mice tumour emboli were found in the central portion of the lung more than at the lung periphery immediately after intravenous injection, in contrast to the WT mice. Activation of melanoma adhesion receptors by thrombin or TRAP stimulated lung colonization in WT but not KO mice. Therefore, the correlation of tumour embolism and metastasis in short-term experiments depends on the nature and stability of interactions between the tumour and the blood/endothelial cells of the host.

Exclusion of CD43 from the immunological synapse is mediated by phosphorylation-regulated relocation of the cytoskeletal adaptor moesin

Formation of the immunological synapse requires TCR signal-dependent protein redistribution. However, the specific molecular mechanisms controlling protein relocation are not well defined. Moesin is a widely expressed phospho-protein that links many transmembrane molecules to the cortical actin cytoskeleton. Here, we demonstrate that TCR-induced exclusion of the large sialoprotein CD43 from the synapse is an active event mediated by its reversible binding to moesin. Our results also reveal that relocalization of moesin is associated with changes in the phosphorylation status of this cytoskeletal adaptor protein. Finally, these findings raise the possibility that the change in moesin localization resulting from TCR engagement modifies the overall topology of the lymphocyte membrane and facilitates molecular interactions at the site of presenting cell contact.

ERM-dependent movement of CD43 defines a novel protein complex distal to the immunological synapse

The large mucin CD43 is actively excluded from T cell/APC interaction sites, concentrating in a membrane domain distal to the site of TCR engagement. The cytoplasmic region of CD43 was necessary and sufficient for this antipodal movement. ERM cytoskeletal adaptor proteins colocalized with CD43 in this domain. An ERM dominant-negative mutant blocked the distal accumulation of CD43 and another known ERM binding protein, Rho-GDI. Inhibition of ERM function decreased the production of IL-2 and IFNgamma, without affecting PKC(theta) focusing or CD69 upregulation. These results indicate that ERM proteins organize a complex distal to the T cell/APC interaction site and provide evidence that full T cell activation may involve removal of inhibitory proteins from the immunological synapse.

CD43 potentiates CD3-induced proliferation of murine intestinal intraepithelial lymphocytes

The involvement of CD43 in cell proliferation of murine intestinal intraepithelial lymphocytes (IEL) has been studied in in vitro CD3-stimulated cell cultures. In the presence of either IL-2 or IL-15, CD3 stimulation of IEL resulted in low levels of proliferation as measured by thymidine incorporation, whereas no proliferation occurred upon CD3 stimulation in the absence of cytokines. The combination of both cytokines to IEL cultures synergistically enhanced CD3-induced proliferation by approximately threefold that of cultures supplemented with either cytokine alone. Most importantly, however, proliferation of IEL was significantly greater when CD3 stimulation occurred in conjunction with CD43 triggering, indicating that CD43 functions as a coactivational signal for murine IEL. These findings indicate that a spectrum of potential proliferative responses exist among murine IEL depending on the types and combinations of signals received, and that because under normal conditions murine IEL are largely devoid of CD28 expression, a classical T-cell coactivational molecule, the capacity for high-level IEL proliferation may reside with CD43.

Validity of the two-signal model for activation of CD28-deficient T lymphocytes: quantitative characterization of an alternative costimulatory function of dendritic cells

The observation that primary T-dependent immune responses are generated in mice lacking CD28, the only receptor definitively shown to costimulate naive T cells, has led to ambiguity as to whether costimulation is absolutely required for initiation of T-cell responses. In this report, in vitro analysis of the relationship between cell density and proliferation demonstrates that activation of CD28-/- T cells to immobilized T-cell receptor (TCR)-alpha monoclonal antibodies (MoAb) depends on costimulatory signals provided by other cells in culture and occurs only at cell densities sufficient to permit these intercellular interactions. These signals are necessary even under TCR triggering conditions that obviate the CD28 requirement. Dendritic cells (DCs) provide the necessary costimulation in vitro and prime T cells in vivo in CD28-/- mice. Single-cell and limiting dilution analyses indicate that individual T cells from normal and CD28-/- mice produce equivalent interleukin (IL)-2 in response to DCs. However, half as many T cells produce IL-2 when only the CD28-independent pathway is used. Nonetheless, CD28-/- T cells produce sufficient IL-2 to support clonal expansion comparable to that of CD28+/+ T cells, which may account for the equally robust in vivo responses initiated by DCs in normal and CD28-deficient animals.

Regulation of Cbl molecular interactions by the co-receptor molecule CD43 in human T cells

CD43, one of the most abundant glycoproteins on the T cell surface, has been implicated in selection and maturation of thymocytes and migration, adhesion, and activation of mature T cells. The adapter molecule Cbl has been shown to be a negative regulator of Ras. Furthermore, it may also regulate intracellular signaling through the formation of several multi-molecular complexes. Here we investigated the role of Cbl in the CD43-mediated signaling pathway in human T cells. Unlike T cell receptor signaling, the interaction of the adapter protein Cbl with Vav and phosphatidylinositol 3-kinase, resulting from CD43-specific signals, is independent of Cbl tyrosine phosphorylation, suggesting an alternative mechanism of interaction. CD43 signals induced a Cbl serine phosphorylation-dependent interaction with the tau-isoform of 14-3-3. protein. Protein kinase C-mediated Cbl serine phosphorylation was required for this interaction, because the PKC inhibitor RO-31-8220 prevented it, as well as 14-3-3 dimerization. Moreover, mutation of Cbl serine residues 619, 623, 639, and 642 abolished the interaction between Cbl and 14-3-3. Overexpression of Cbl in Jurkat cells inhibited the CD43-dependent activation of the mitogen-activated protein kinase (MAPK) pathway and AP-1 transcriptional activity, confirming nevertheless a negative role for Cbl in T cell signaling. However, under normal conditions, PKC activation resulting from CD43 engagement was required to activate the MAPK pathway, suggesting that phosphorylation of Cbl on serine residues by PKC and its association with 14-3-3 molecules may play a role in preventing the Cbl inhibitory effect on the Ras-MAPK pathway. These data suggest that by inducing its phosphorylation on serine residues, CD43-mediated signals may regulate the molecular associations and functions of the Cbl adapter protein.

Biosynthesis and function of beta 1,6 branched mucin-type glycans

The contribution of carbohydrate structure to biomolecular, cellular, and organismal function is well-established, but has not yet received the attention it deserves, perhaps due to the complexity of the structures involved and to a lack of simple experimental methods for relating structure and function. In particular, beta1,6 GlcNAc branching plays a key functional role in processes ranging from inflammation and immune system function to tumor cell metastasis. For instance, synthesis of the core 2 beta1,6 branched structure in the mucin glycan chain by C2GnT enables the expression of functional structures at the termini of polylactosamine chains, such as blood group antigens and sialyl Lewis x. Also, IGnT can create multiple branches on the polylactosamine chain, which may serve as a mechanism for amplifying the functional potency of cell surface glycoproteins and glycolipids. The family of enzymes which creates beta1,6 branched structure in mucin glycans is proving to be quite complex, since multiple isoforms appear to exist for these enzymes, and some of the enzymes are adept at forming more than one type of beta1,6 branched structure, as in the case of C2GnT-M. Furthermore, the enzymes do not appear to be restricted to acting on mucin-type acceptor structures, but are able to act on glycolipid structures as well. Much remains to be learned regarding the specific biological niche filled by each of these enzymes and how their activities complement one another, as well as the manner in which the activities of these enzymes are regulated in the cell.

Antiadhesive function of 130-kd glycoform of CD43 expressed in CD4 T-lymphocyte clones and transfectant cell lines

Conflicting findings regarding proadhesion and antiadhesion in cell-to-cell interactions were previously reported for CD43. We examined possible differences in the role of the 130-kd glycoform and the 115-kd glycoform of CD43 in cellular adhesion in vitro. We generated a monoclonal antibody (MFT3) that discriminates between helper and nonhelper murine T-cell clones. Characterization of MFT3 with use of biochemical analysis and complementary DNA (cDNA) transfection experiments showed that it is specific for the 130-kd glycoform of CD43. T-cell clones that expressed the 130-kd CD43 glycoform showed decreased homocytic aggregation and decreased adhesion to spleen cells, B-lymphoma cell lines, and fibroblastic cell lines compared with T-cell clones negative for the 130-kd glycoform. Expression of core 2 β-1, 6-N-acetylglucosaminyltransferase (C2GnT) cDNA together with CD43 cDNA resulted in expression of both the 130-kd CD43 glycoform and the 115-kd CD43 glycoform in fibroblastic cell lines. Using these cell lines, we showed that the 130-kd glycoform but not the 115-kd glycoform of CD43 has an antiadhesive function in cellular interactions. Our findings suggest that the antiadhesive function of CD43 is primarily carried out by the 130-kd glycoform.

Antiadhesive function of 130-kd glycoform of CD43 expressed in CD4 T-lymphocyte clones and transfectant cell lines

Conflicting findings regarding proadhesion and antiadhesion in cell-to-cell interactions were previously reported for CD43. We examined possible differences in the role of the 130-kd glycoform and the 115-kd glycoform of CD43 in cellular adhesion in vitro. We generated a monoclonal antibody (MFT3) that discriminates between helper and nonhelper murine T-cell clones. Characterization of MFT3 with use of biochemical analysis and complementary DNA (cDNA) transfection experiments showed that it is specific for the 130-kd glycoform of CD43. T-cell clones that expressed the 130-kd CD43 glycoform showed decreased homocytic aggregation and decreased adhesion to spleen cells, B-lymphoma cell lines, and fibroblastic cell lines compared with T-cell clones negative for the 130-kd glycoform. Expression of core 2 β-1, 6-N-acetylglucosaminyltransferase (C2GnT) cDNA together with CD43 cDNA resulted in expression of both the 130-kd CD43 glycoform and the 115-kd CD43 glycoform in fibroblastic cell lines. Using these cell lines, we showed that the 130-kd glycoform but not the 115-kd glycoform of CD43 has an antiadhesive function in cellular interactions. Our findings suggest that the antiadhesive function of CD43 is primarily carried out by the 130-kd glycoform.

Identification and cloning of a CD43-associated serine/threonine kinase

CD43 is expressed on most hematopoetic cells and has been shown to regulate the activation and adhesion of T cells. We have cloned a serine/threonine kinase that can interact with the cytoplasmic domain of CD43. This protein is expressed in multiple tissues, including lymphoid cells. Analysis of the subcellular localization reveals it to be present in both the nucleus and the cytoplasm of the cell. The identification of this protein suggests that CD43 may mediate its biologic effects through activation of a kinase cascade, resulting in the regulation of cell growth.

CD43-mediated signals induce DNA binding activity of AP-1, NF-AT, and NFkappa B transcription factors in human T lymphocytes

Although numerous reports document a role for CD43 in T cell signaling, the direct participation of this molecule in cell activation has been questioned. In this study we show that CD43 ligation on human normal peripheral T cells was sufficient to induce interleukin-2, CD69, and CD40-L gene expression, without requiring signals provided by additional receptor molecules. This response was partially inhibited by cyclosporin A and staurosporine, suggesting the participation of both the Ca(2+) and the protein kinase C pathways in CD43 signaling. Consistent with the transient CD43-dependent intracellular Ca(2+) peaks reported by others, signals generated through the CD43 molecule resulted in the induction of NF-AT DNA binding activity. CD43-dependent signals resulted also in AP-1 and NFkappaB activation, probably as a result of protein kinase C involvement. AP-1 complexes bound to the AP-1 sequence contained c-Jun, and those bound to the NF-AT-AP-1 composite site contained c-Jun and Fos. NFkappaB complexes containing p65 could be found as early as 1 h after CD43 cross-linking, suggesting that CD43 participates in early events of T cell activation. The induction of the interleukin-2, CD69, and CD-40L genes and the participation of AP-1, NF-AT, and NFkappaB in the CD43-mediated signaling cascade implicate an important role for this molecule in the regulation of gene expression and cell function.