Heterogeneity of gangliosides among T cell subsets

Abstract

Gangliosides are major components of highly organized membrane microdomains or rafts, yet little is known about the role of gangliosides in raft organization. This is also the case of gangliosides in TCR-mediated activation. Comprehensive structural analysis of gangliosides in the primary thymocytes and CD4⁺ T and CD8⁺ T cells was not achieved due to technical difficulties. We have found that CD8⁺ T cells express very high levels of o-series gangliosides, but on the other hand, CD4⁺ T cells preferably express a-series gangliosides. In the TCR-dependent activation, CD4⁺ T cells selectively require a-series gangliosides, but CD8⁺ T cells do require only o-series gangliosides but not a-series gangliosides. Ganglioside GM3 synthase-deficient mice lacking a-series gangliosides neither exhibited the TCR-dependent activation of CD4⁺ T nor developed ovalbumin-induced allergic airway inflammation. These findings imply that the distinct expression pattern of ganglioside species in CD4⁺ and CD8⁺ T cells define the immune function of each T cell subset.

Introduction

T cells constitute an important component of our immune system and are involved in immune surveillance and antigen recognition. Virtually all mature T cells are either CD4⁺CD8⁻ or CD4⁻CD8⁺, indicating that these surface-phenotype distinctions must be crucial for the entire T cell response. CD4⁺-expressing helper lineage T cells participate in a wide variety of immune functions including humoral immunity, allergic responses, etc. On the other hand, CD8⁺-expressing cytotoxic lineage T cells possess killing ability against cancer cells and virus-infected cells. Antigen (Ag)-specific immune responses are elicited when T cells expressing a specific Ag receptor [the T cell receptor (TCR)] interact with Ag-presenting cells (APCs) that bear a cognate Ag peptide–MHC (pMHC). This interaction is responsible for the formation of a unique “immunological synapse” [1].

The membrane fluidity is directly determined by the physicochemical characteristics of lipid molecules composing the lipid bilayer of cell membranes. However, the composition and behavior of lipids in the immunological synapse are still not completely understood. “Lipid rafts” have long been proposed to be an essential platform for TCR signaling [2]. Several recent studies have detailed the molecular components of both lipid rafts and the immunological synapse, and elegantly described the dynamic nature of molecules participating in these processes during the course of T cell activation [3–7]. However, regulatory mechanisms governing recruitment of molecules to rafts and synapse organization are less well understood.

Major lipid species existing in lipid rafts are glycosphingolipids (GSLs), sphingomyelin, and cholesterol. Involvement of sialic acid (SA)-containing GSLs, gangliosides, in the T cell activation has been demonstrated by a number of observations. For example, it has been demonstrated that the polarization of GM1 ganglioside occurs only in CD4⁺ T cells but not in CD8⁺ T cells, when TCR clustering of both T cell subsets was induced by anti-CD3 and anti-CD28 antibodies [8]. Furthermore, GM1 and GM3 gangliosides appear to define different types of rafts, which segregate to the leading pole and the trailing uropod of the polarized human T cells, respectively [9]. Cholera toxin B subunit (CTx-B) has long been utilized for the visualization and functional analysis of rafts, but the raft recognized by CTx-B represents only a part of them, indicating that rafts must be heterogeneous and be grouped into subclasses. In addition, there are big differences in the expression levels of GM1, which depend on the cell types, and cellular development and differentiation. Sometimes GM1 expression on the cell surface is much lower than the other ganglioside species.

Therefore, in order to understand the role of rafts in the course of differentiation and maturation of CD4⁺ T and CD8⁺ T cells in vivo, it is prerequisite to know the detailed ganglioside composition in the respective T cell subset. Here we propose that the functional repertoire selection from double-positive to single-positive T cells is accompanied with the selection of ganglioside expression in the individual T cell subsets. This ganglioside selection process is indispensable for the functional microcluster formation and/or lipid rafts for mature T cells.

Structural analyses of ganglioside species in the T cell subsets

Glycosphingolipids are built on a ceramide backbone comprising a long-chain amino alcohol, sphingosine, and an amide-linked fatty acid. Gangliosides are SA-containing GSLs. GM3, the simplest of the “a-series” gangliosides, is synthesized by GM3 synthase (GM3S), which catalyzes the transfer of SA to the nonreducing terminal galactose (Gal) of lactosylceramide (LacCer). GM3 is altered by GM2/GD2 synthase (GM2/GD2S) to form GM2, a downstream a-series ganglioside, or by GD3 synthase (GD3S) to form GD3, the simplest of the “b-series” gangliosides. GM2/GD2S also elongates LacCer to form GA2, the simplest precursor GSL of the “o-series” ganglioside. Each branch of GSL biosynthesis is a committed pathway (Fig. 1), then competition between enzymes at a key branch point determines the relative expression levels of o-, a-, and b-series gangliosides.

Fig. 1.

Ganglio-series glycosphingolipids are synthesized from ceramide and are divided into o-, a-, and b-series species

Ganglioside expression in primary T cells has previously been studied using biochemical analyses (TLC and HPLC) of whole T cell populations including cloned T cell lines and T cell blastocytes stimulated by concanavalin A. However, whole T cell populations are truly a “mixed population”, therefore any such results would be of limited value in terms of studying characteristic features of specific T cell subsets. Previously, FACS analysis with monoclonal antibodies (mAbs) against several ganglioside demonstrated that mature peripheral CD4⁺ T cells and CD8⁺ T cells express differential species of gangliosides [10, 11]. These studies mainly demonstrated the predominance of GalNAc–GM1b in CD8T cells [10] and GD1c in CD4T cells [11].

We examined the structures of gangliosides in immature thymocytes, and the primary CD4⁺ T cells and CD8⁺ T cells isolated from cell mixtures prepared from lymph nodes and spleen by LC–MS/MS analysis (Table 1) [12]. Thymocytes consisting mainly of CD4⁺ CD8⁺ T double-positive (DP) cells expressed the six distinct species, GM1a, GM1b, GD1b, GD1c, GalNAcGM1b, and extended-GM1b, and their relative expression levels (%) were 8.4, 18.9, 29.9, 13.3, 23.0, and 6.5, respectively. The presence of these gangliosides was common in all T cell subsets, but their expression levels are remarkably different in each subset (Table 1). The expression of o-series gangliosides, GalNAcGM1b and extended-GM1b, was greatly enhanced by the differentiation from thymocytes (23.0 and 6.5 %) to CD4⁺ T (36.6 and 18.9 %) and CD8⁺ T (49.0 and 37.1 %) cells. On the other hand, the expression of another o-series ganglioside GD1c was enhanced to double in CD4⁺ T cells (20.3 %) but greatly decreased in CD8⁺ T cells (3.7 %). GD1b was expressed in a considerable amount (30 %) in thymocytes, but 6 % in CD4⁺ T and 0.8 % in CD8⁺ T cells. The expression of GM1b was maintained among T cell subsets, but GM1a was expressed in both thymocytes and CD4⁺ T cells but only trace amounts in CD8T cells. We examined the expression of GM3S and GM2/GD2S genes among T cell subsets [12]. GM2/GD2S expression was markedly increased more than 100-folds in both CD4⁺ T and CD8⁺ T cells compared to thymocytes. GM3S expression was increased to 180 % in CD4⁺ T cells and was decreased to 30 % in CD8⁺ T cells, compared to the expression in thymocytes. These gene expression patterns could partially explain the above-mentioned distinct expression profile of gangliosides during the differentiation processes from immature thymocytes to the mature CD4⁺ T and CD8⁺ T cells. In the case of GD1c, Nakamura et al. [11] reported that CD4⁺ T cells can be separated into GD1c-positive IL-2-producing Th1-like cells and GD1c-negative IL-4-producing Th2-like cells. These findings suggest that T cell subsets can be classified into the distinct functional subpopulations based on the differences of gangliosides expression profiles. Wang et al. [13, 14] reported that cross-linking of GM1 on CD4⁺ and CD8⁺ effector T cells (Teff) by galactin-1 plays a significant role in autoimmune suppression through the modulation of Ca⁺² influx via transient receptor potential channel 5 (TRPC5). They showed that resting CD4⁺ Teff and CD8⁺ Teff contain GM1 and GD1a as the major gangliosides detected by HPTLC with sialidase treatment and CTx-B binding, CD8⁺ Teff express higher levels of GD1a than CD4⁺ Teff and both GM1 and GD1a are upregulated by the activation with anti-CD3 and anti-CD28. They did not discuss the possibility that GD1a assigned in their analysis might contain extended-GM1 but this possibility is supported by the following lines of evidence; extended-GM1 is sialidase-resistant, migrates like GD1a on HPTLC developed with chloroform–methanol-water containing CaCl₂ [24], exhibits CTx-B binding activity very similar to GM1 [24] and is detected in a higher amount in CD8⁺ T cells than CD4⁺ T cells [12]. GD1a-containing NeuGc of CD4⁺ and CD8⁺ T cells was present and was a minor component, compared with GM1 and extended-GM1, in our LC–MS analysis [12]. Another point for requiring ganglioside analysis of primary native cells without any activations is that the downregulation of CMP-NeuAc hydroxylase by an activation was reported in mouse B cells [15]. The change of sialic acid expression from NeuGc to NeuAc even in T cells is possible and this is another future subject for understanding physiological functions of gangliosides in the immunocytes.

Table 1.

Structures of ganglioside species found in mouse primary T cells

Structure	Biosynthetic pathwaya	Trivial name	Relative expression level (%)
			Thy	CD4T	CD8T
Galβ1-3GalNAcβ1-4(SAα2-3)Galβ1-4Glcβ1-1’Cer	a-series	GM1a	8.4	4.1	0.8
SAα2-3Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1-1’Cer	o-series	GM1b	18.9	14.4	8.6
Galβ1-3GalNAcβ1-4(SAα2-8SAα2-3)Galβ1-4Glcβ1-1’Cer	b-series	GD1b	29.9	5.7	0.8
SAα2-8SAα2-3Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1-1’Cer	o-series	GD1c	13.3	20.3	3.7
GalNAcβ1-4(SAα2-3)Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1-1’Cer	o-series	GalNAcGM1b	23	36.6	49
Galβ1-4GalNAcβ1-4(SAα2-3)Galβ1-3GalNAcβ1-4Galβ1-4Glcβ1-1’Cer	o-series	extended-GM1b	6.5	18.9	37.1

Structures of gangliosides and their relative distribution among T cell subsets are the summary of Fig. 4b and Fig. S4 in ref [12]

Thy thymocytes

^aBiosynthetic pathway of each series gangliosides is illustrated in Fig. 1

We were also interested in the role of fatty acid acyl chain lengths of gangliosides in the organization of ganglioside-dependent rafts in T cell lineages [12]. As shown in Fig. 2, the relative expression levels of ceramide moiety carrying d18:1–16:0 in all gangliosides significantly decreased in CD4⁺ T and CD8⁺ T cells compared to those of thymocytes. On the other hand, ceramides with more hydrophobic nature, d18:1–22:0 and d18:1–24:0, in almost all gangliosides tend to increase in CD4⁺ T and CD8⁺ T cells. The contribution of the long and saturated acyl chains of glycosphingolipids to raft formation has been appreciated [16]. These glycosphingolipids allow close packing of the lipids resulting in a high melting temperature (Tm). Self-aggregates of sphingolipids form a separate phase that is less fluid (liquid-ordered) than the bulk liquid-disordered phospholipids. These observations suggest that the hydrophobicity of individual ganglioside is increased in peripheral primary T cells to create the rigid raft structure necessary for TCR signaling. It has been demonstrated that lactosylceramide, having a C24 fatty acid chain, is crucial for association of the lyn molecule, which is responsible for GSL-mediated outside-in signal transduction [17].

Fig. 2.

Alteration of fatty acid moieties of gangliosides in thymocytes (Thy) and primary CD4⁺ T and CD8⁺ T cells [12]

Distinct selectivity of gangliosides required for CD4⁺ T and CD8⁺ T cell activation

The role of gangliosides in TCR signaling and T cell activation has been studied using inhibitors of GSL biosynthesis [18]. We previously demonstrated that the treatment of Jurkat cells with d-threo–1–phenyl–2–decanoylamino–3–morpholino–1–propanol (d-PDMP), an inhibitor of glucosylceramide synthase [19], preferentially reduces the level of GSLs in rafts without affecting SM and cholesterol levels, and the alteration of GSL content neither impairs signal transduction via the T cell receptor nor the protein distribution in lipid rafts. However, d-PDMP treatment resulted in the upregulation of TCR signaling via GPI-anchored protein [19]. On the other hand, it has been reported that the treatment of human peripheral blood lymphocytes with this inhibitor attenuates TCR signaling and T cell activation as indicated by a reduced expression of markers of T cell activation and a reduced secretion of IFN-γ [20].

To investigate functional roles of gangliosides in T cell development and immunity, we utilized two types of gene-targeted mice with interrupted ganglioside biosynthesis (Fig. 1). GM3S null mice lack all a- and b-series gangliosides, resulting in LacCer accumulation and compensatory increase of o-series GSLs [21]. GM2/GD2S null mice lack all o-series GSLs and all elongated a- and b-series gangliosides, then they accumulate LacCer and express only GM3 and GD3 gangliosides [22].

GM3S null and GM2/GD2S null mice did not reveal any obvious alteration in the cellularity of the lymphoid organs, indicating no gross defects in T cell development despite the altered ganglioside expression. However, CD4⁺ T cells (but not CD8⁺ T cells) from GM3S null mice exhibited severe defects in their proliferative response to the stimulation with an anti-CD3/anti-CD28 antibody mixture, but maintained a normal response to a phorbol myristate acetate (PMA) plus ionomycin (Io) treatment. Conversely, CD8⁺ T cells (but not CD4⁺ T cells) from GM2/GD2S null mice exhibited similar severe defects in TCR-mediated activation when stimulated with the antibody mixture [12]. Following the TCR stimulation, both IL-2 and IFNγ production were severely reduced in both GM3S null CD4⁺ T cells and GM2/GD2S null CD8⁺ T cells. Responses to an alloantigen, a superantigen, and a specific antigen peptide were similar to those observed with the anti-CD3/anti-CD28 mixture. In contrast to T cells, splenic B cells isolated from GM3S null and GM2/GD2S null mice exhibited no obvious alterations in proliferative responses to various concentrations of anti-IgM antibody and lipopolysaccharide [12].

Staining with CTx-B, commonly used to detect GM1a and visualize rafts, has demonstrated that CD8⁺ T cells express higher levels of GM1a in the rafts than CD4⁺ T cells [23]. However, this toxin also reacts to other gangliosides including fucosylated GM1a and extended-GM1b, both of which have a monosialo–ganglio–triose structure, Galβ1–3GalNAcβ1–4(SAα2–3)Galβ1– [24]. In fact, we were able to detect two CTx-B-reactive gangliosides, GM1a and extended-GM1b, in T cells with different quantities in individual T cell subsets [12]. In WT CD4⁺ T cells, GM1a was expressed in both thymocytes and CD4⁺ T cells but only trace amounts in CD8⁺ T cells. Thus, extended-GM1b was expressed much more than GM1a in WT CD8⁺ T cells as shown in Table 1. The thymocytes and peripheral CD4⁺ T and CD8⁺ T cells of GM3S null mice lacked a-series GM1a and expressed o-series extended-GM1b, while the cells from GM2/GD2S null mice did not express either (Fig. 1). The presence of a variety of rafts with different gangliosides in a single cell has been suggested [8, 9]. It has been reported that cross-linking of gangliosides using CTx-B or a homologue, the heat-labile enterotoxin of Escherichia coli, can induce apoptosis in CD8⁺ T cells but not in CD4⁺ T cells [25]. The apoptosis is caused by activation of NF-κB and c-Myc via the induction of caspase-dependent signaling [26]. While this process is known to proceed in a Fas- and p55 tumor necrosis factor receptor-independent pathway [27], the events in rafts following the ganglioside cross-linking remain undetermined. Considering the difference in the expression of CTx-B-binding gangliosides between CD4⁺ T cells and CD8⁺ T cells, the apoptosis caused by CTx-B cross-linking possibly involve extended-GM1b rafts but not GM1a rafts. This suggests that each T cell subset has unique rafts in the plasma membrane and that these rafts provide distinct functions in different intracellular events following receptor-mediated stimulation.

In the experiments to determine which gangliosides have an essential role in TCR-mediated activation of T cell subsets, CD4⁺ T cells and CD8⁺ T cells from WT, GM3S null, and GM2/GD2S null mice were pretreated with gangliosides followed by anti-CD3/anti-CD28 stimulation [12]. Proliferative responses of GM3S null CD4⁺ T cells were restored by the pretreatment with any of the a-series gangliosides (GM3, GM1, and GD1a), but were not restored with any of o- or b-series gangliosides such as GM1b or GD1b (Fig. 3). Conversely, the proliferative responses of GM2/GD2S null CD8⁺ T cells were restored by the pretreatment with any of the o-series gangliosides (GA2, GA1, and GM1b), but not with any of a- or b-series gangliosides. Importantly, the gangliosides themselves had no effect without the proliferative stimulation. These results indicate that o-series ganglioside expression is essential for TCR-mediated activation of CD8⁺ T cells and a-series ganglioside is indispensable for that of CD4⁺ T cells. These data suggest that different species of gangliosides are essentially required by CD4⁺ T cells and CD8⁺ T cells in terms of TCR-mediated activation (Fig. 3) [12].

Fig. 3.

Activation of CD4⁺ T and CD8⁺ T cells upon TCR-mediated stimulation. Purified peripheral CD4⁺ T and CD8⁺ T cells from WT, GM3S null, and GM2/GD2S null mice were stimulated with anti-CD3 antibody plus anti-CD28 antibody in the presence or absence of the indicated ganglioside [12]

Why do CD4⁺ T cells and CD8⁺ T cells require the expression of distinct gangliosides for proper immune function? Both cells share a part of molecular mechanisms for TCR-mediated signaling, but certainly do have different cellular and molecular modifications. CD4 and CD8 are considered to localize to lipid rafts by palmitoylation at their juxtamembrane region [28, 29], yet raft localization seems not to be determined by lipid modification alone [29, 30]. To ensure that CD4 and CD8 undergo proper intracellular trafficking and successful localization on the plasma membrane, it might be vital for CD4/CD8 to interact with rafts carrying a specific ganglioside. CD28, a B7 type co-stimulatory molecule, provides functional differences between CD4⁺ T cells and CD8⁺ T cells [31, 32]. In human and mouse CD4⁺ T cells, CD28 promotes the clustering of CTx-B-detectable rafts at the immunological synapse through its downstream signaling molecule protein kinase Cθ [33]. However, CD8⁺ T cells do not re-orient CTx-B-detectable rafts to the T cell–antigen-presenting cell (APC) interface during TCR activation [8, 34].

Another aspect to consider is the physicochemical properties of sugar residues of gangliosides on plasma membranes. Sonnino and his colleagues [35] proposed that the segregation of one ganglioside with respect to the other in plasma membranes is a spontaneous process explained on the basis of the different geometrical properties of ganglioside headgroup. Based on their assumption, the external galactose of GM1a is quite free. In fact, the disaccharide Galß1–3GalNAcß is characterized by two pairs of torsional angles of +30°, +14°, and +30°, −20°, respectively. Instead, the trisaccharide GalNAcß1–4(Neu5Acα2–3)Galß is a rigid structure in which each linkage is defined by a single pair of torsional angles because of the stabilization by a hydrogen bond between the GalNAc amide portion and the Neu5Ac carboxyl group [36]. In GM1b, the conformational properties should be completely different. All the linkages are free to rotate, each being defined by at least two pairs of torsional angles. This means that the Gal–GalNAc–(Neu5Ac) Gal of GM1a is defined by two conformers, while the Neu5Ac–Gal–GalNAc–Gal of GM1b is defined by eight conformers (Fig. 4). Thus, the volume requested by GM1b to stay in the membrane should be much bigger than that of GM1a. It is expected that geometrical properties of GM1a and GM1b are quite different and a bigger influence on a positive membrane curvature is brought by GM1b than by GM1a.

Fig. 4.

Dynamic of the ganglioside glycosidic linkages as a function of the primary and secondary structure of gangliosides. Dynamics is reflected as a number of conformers (see details in text)

Considering these results and discussion, we speculate that ganglioside compositions in rafts required for the signal transduction mediated by TCR and co-stimulatory molecules are unique and different in CD4⁺ T cells and CD8⁺ T cells.

Selective suppression of CD4⁺ T cell in GM3S null mice resulted in amelioration of experimental asthma

Allergic airway inflammation is tightly regulated by adaptive immunity, in which CD4⁺ T cells play a crucial role via Th2 cytokine production. We examined allergic airway responses induced by inhalation of ovalbumin (OVA) in OVA-sensitized mice (experimental asthma). OVA challenge induced extensive mucus hypersecretion, a cardinal feature of asthma, in WT but not in the GM3S null mice (Fig. 5) [12]. Although both types of mice challenged with OVA exhibited airway infiltration of inflammatory cells including eosinophils and lymphocytes, the numbers of infiltrating cells were significantly lower in GM3 null mice. Serum OVA-specific IgE levels were greatly increased on days three and five after OVA-challenge in WT mice, but not in GM3S null mice. Decreased levels of Th2 cytokines in the bronchoalveolar lavage (BAL) fluids were also observed in GM3S null mice. In adoptive transfer experiments, CD4⁺ T cells isolated from OVA-sensitized WT, GM3S, and GM3/GD2S null mice were transferred into naive WT mice, and the recipient mice were challenged with OVA. Infiltration of inflammatory cells, particularly eosinophils and lymphocytes, was suppressed in the recipient mice of CD4⁺ T cells from GM3S null mice. In the recipient mice of CD4⁺ T cells from GM2/GD2S null mice, there was no difference in the numbers of infiltrated cells compared to those of recipient mice of CD4⁺ T cells from WT mice. These results indicate that the immune function of CD4⁺ T cells in vivo is selectively deficient in GM3S null mice.

Fig. 5.

The role of gangliosides in the pathogenesis of allergic asthma. a-series gangliosides (a-GMs) in CD4⁺ T cells is critical for the signal transduction through immunological synapse, resulting in the synthesis and release of Th2 cytokines such as IL-4, IL-5, and IL-13. These cytokines play a central role in the development of airway narrowing by activating eosinophils, B cells, and mast cells, which cause mucus secretion and airway hyper-responsiveness

Recently, novel CD4⁺ T cell subsets, in addition to Th1 and Th2 cells, Th17, and regulatory T (Treg), have been described. In allergic airway inflammation, the balance between effector Th2 cells and suppressive Treg cells is skewed towards Th2 predominance [37]. Th17 cells have been suggested to contribute to neutrophilic, steroid-resistant severe asthma and to enhance Th2-mediated airway inflammation, although a role for the cells in asthma remains to be determined [38]. Reportedly, GM3S null mice exhibit a decreased number of Th17 cells skewed by in vitro culture [39]. As the adoptive transfer experiments confirmed the marked reduction of allergic responses in WT mice transferred a whole of CD4⁺ T cell population from sensitized GM3S null mice [12], we are underway to identify which CD4⁺ T cell subset is affected in GM3S null mice in vivo.

Concluding remarks

We determined the ganglioside expression patterns of primary T cells, specifically those of immature thymocytes and mature peripheral CD4⁺ T cells and CD8⁺ T cells. Our most important finding was that distinct expression patterns of gangliosides exist in CD4⁺ T cells and CD8⁺ T cells, and these define the immune function of each T cell subset. CD4⁺ T cells preferentially express a-series gangliosides, and cells isolated from GM3S null mice exhibit severe impairments in TCR-mediated cytokine production and clonal expansion, but can be rescued by exogenously added a-series gangliosides. Similarly, CD8⁺ T cells preferentially express o-series GSLs, and cells isolated from GM2/GD2S null mice exhibit severe impairments in TCR-mediated cytokine production and clonal expansion. They can be rescued by re-introducing o-series gangliosides. These results suggest that CD4⁺ T cell and CD8⁺ T cell subsets require a-series and o-series ganglioside expression, respectively, to undergo activation upon TCR ligation [12].

We propose that the repertoire selection from thymocytes to T cell subsets is accompanied by selective GSLs expression in individual T cell subsets. This GSL selection process may be indispensable for the formation of distinct and functional lipid rafts in mature T cells (Fig. 6). These findings may open up a strategy for targeting helper T cells to treat allergic inflammation including asthma by controlling ganglioside expression in lipid rafts.

Fig. 6.

Distinct difference of lipid rafts in individual T cell subsets. The repertoire selection from immature thymocytes (CD4⁺ and CD8⁺ double-positive) to mature single-positive T cell subpopulations is accompanied by selective ganglioside expression. The gene expression patterns together with ganglioside analysis confirm that CD8⁺ T cells exclusively express o-series gangliosides due to downregulation of GM3S and upregulation of GM2/GD2S expression. On the other hand, CD4⁺ T cells maintain the expression of GM1a due to the upregulation of GM3S. This suggests that each T cell subset has unique rafts composed of different ganglioside species and that these rafts provide distinct functions in different intracellular events following receptor-mediated stimulation. This ganglioside selection process may be indispensable in the formation of distinct and functional lipid rafts in mature T cells

Abbreviations

GM3S

GM3 synthase

GM2/GD2S

GM2/GD2 synthase

GSLs

Glycosphingolipids

SA

Sialic acid

CTx-B

Cholera toxin B subunit