Selenoproteins as regulators of T cell proliferation, differentiation, and metabolism

Abstract

Selenium (Se) is an essential micronutrient that plays a key role in regulating the immune system. T cells are of particular interest due to their important role in promoting adaptive immunity against pathogens and cancer as well as regulating tolerance, all of which are influenced by dietary Se levels. The biological effects of Se are mainly exerted through the actions of the proteins into which it is inserted, i.e. selenoproteins. Thus, the roles that selenoproteins play in regulating T cell biology and molecular mechanisms involved have emerged as important areas of research for understanding how selenium affects immunity. Members of this diverse family of proteins exhibit a wide variety of functions within T cells that include regulating calcium flux induced by T cell receptor (TCR) engagement, shaping the redox tone of T cells before, during, and after activation, and linking TCR-induced activation to metabolic reprogramming required for T cell proliferation and differentiation. This review summarizes recent insights into the roles that selenoproteins play in these processes and their implications in understanding how Se may influence immunity.

1. Introduction

Selenium (Se) is a dietary trace mineral that is important for various aspects of human health [1]. The biological effects of Se are mainly exerted through its incorporation into selenoproteins as the twenty-first proteinogenic amino acid, selenocysteine (Sec) [2]. There are 25 selenoprotein encoding genes identified in humans, all but one of which also are found in mice and rats [3]. Selenoprotein family members exhibit a wide variety of functions including the control of reactive oxygen species (ROS) and cellular redox tone, regulating thyroid hormone metabolism, facilitating sperm maturation/protection, preventing neurological deficiencies, and promoting optimal immunity [4]. The expression of selenoproteins is essential for life as demonstrated by the generation of mice lacking Sec-tRNA required for translation of all selenoproteins, which leads to embryonic lethality [5]. In fact, five individual selenoproteins have been demonstrated to be essential for embryonic development using mouse models [6–10]. While it has been noted that the tolerance to loss of expression of individual selenoproteins may differ between human and mouse [11], the importance of sufficient dietary Se intake and selenoprotein expression on development and health, particular maternal Se intake during the periconceptional period, has been well described [12].

Selenoproteins are translated on conventional ribosomes using a unique mechanism involving the incorporation of Sec into nascent polypeptides at sites encoded by the UGA codon, which usually represents a stop codon [13]. This process relies on special stem-loop structures in the 3-untranslated regions (UTRs) of selenprotein mRNAs that bind to dedicated protein factors and facilitate recruitment of the Sec-tRNA. Under conditions of low Se status, the translation of selenoproteins stalls at the Sec-encoding UGA codon and both the mRNA and truncated protein get degraded through nonsense-mediated decay and destruction via C-end degrons, respectively [14, 15]. In a Se deficient individual, the brain, muscle and testes receive ‘priority’ for bioavailable Se at a cost to other tissues such as those comprising the immune system [16]. Insufficient Se intake or other factors (e.g. defects in selenoprotein gene expression or some chronic infections that deplete Se) can impair adaptive immunity, especially T cell responses that are critical for producing effective vaccine responses and fighting infections [17, 18]. Se supplementation may provide an inexpensive and effective means of reversing the impaired immunity associated with aging and the immunosuppression associated with cancer, its treatment, or with infectious agents such as HIV-1 and Mycobacterium tuberculosis that act to reduce bioavailable Se in chronically infected individuals [19–22]. However, not all types of immune responses are equivalently enhanced by Se supplementation [23, 24], which emphasizes the need for a better understanding of how Se impacts immunity. Progress has been made in revealing some of the mechanisms by which Se affects the immune system, particularly crucial roles for individual selenoproteins [25]. This review article will summarize how selenoproteins contribute to T cell function, with a focus on the latest advances and their implications in understanding how Se may influence immunity.

2. T cells drive and shape immunity

T cells are lymphocytes that play a central role in guiding adaptive immunity against pathogens and cancer and regulating tolerance. Specifically, CD4⁺ T cells constitute the topmost regulatory layer of the adaptive immune system, providing cytokine ‘help’ to CD8⁺ T cells (effector cells of cell mediated immunity) and B cells (effector lymphocyte cells of humoral immunity), thus coordinating acquired immune responses [26]. After mature, naïve T cells develop in the thymus, they circulate throughout the body until they recognize their cognate antigen on the surface of antigen presenting cells (APCs). T cell activation occurs upon engagement of the T cell receptor (TCR) on CD8⁺ cytotoxic T cells or CD4⁺ helper T cells by antigen in the context of major histocompatibility complex (MHC)-I on target cells or MHC-II on professional APCs, respectively. In addition to TCR binding to antigen-loaded MHC, both CD4⁺ T helper cells and CD8⁺ cytotoxic T cells require a number of secondary signals to become fully activated. In the case of helper T cells, the first of these is provided by CD28 [27]. This molecule on the T cell binds to one of two molecules on the APC: B7.1 (CD80) or B7.2 (CD86), thereby initiating T cell signaling that immediately expands into a complex set of pro-growth signaling pathways. Other co-stimulatory molecules on the T cell are engaged by the APC to promote full T cell activation [28]. Cytotoxic T cells are less reliant on CD28 for activation but do require signals from other co-stimulatory molecules such as CD70 and 4-1BB (CD137).

One important outcome of APC-induced TCR engagement and co-stimulation of T cells is the convergence of signals in the nucleus that activate the transcription of IL-2 mRNA. IL-2 is the main growth factor for most T cells, with induction of its transcription, translation and secretion serving to promote T cell growth in an autocrine/paracrine manner [29]. CD8⁺ T cells are also activated through their TCR to produce effector and memory cells required for optimal immunity, and IL-2 is similarly important for their full activation and proliferation [30]. The initial receptor induced signaling networks required for optimal activation and proliferation are tightly regulated. This signaling network is linked with the coordinated rewiring of cellular metabolism necessary for fulfilling the bioenergetic, biosynthetic, and redox demands of proliferating and differentiating T cells [31]. It has been shown that levels of Se regulate both cell-mediated and humoral immunity [20, 25, 32] and understanding the precise mechanisms requires the dissection of the roles played by individual selenoproteins for modulating T cell functions.

3. Expression of selenoproteins in T cells

As mentioned above in section 1.0, the family of selenoproteins consists of 25 members containing the Sec residue in humans and 24 of these are Sec-containing proteins in mice. Functions have been identified through different studies for many of the family members (Table 1). Most of these have been functionally defined as enzymes, although published data has suggested non-enzymatic functions for a select few and it is likely that some selenoproteins play multiple roles [2]. In general, the functions carried out by different selenoprotein family members seem to be consistent throughout different tissues and this is reflected in their broad expression throughout most tissues, including lymphoid tissues [2, 33]. In other words, functions for individual selenoproteins in T cells for the most part reflect those functions carried out in other cell-types. There are some exceptions such as SELENOV and TXNRD3 (restricted to testes) and some that are expressed at higher levels in certain tissues or cell-types compared to T cells. Importantly, selenoprotein levels in T cells may depend on delivery of Se through SELENOP, which is highly expressed in the liver and secreted into the plasma to transport Se to other tissues [34]. SELENOP contains 10 theoretically possible Sec residues, although number of Sec residues found under normal conditions the is less than 10 [35–37]. SELENOP binds to receptors such as apolipoprotein E receptor 2 (ApoER2) on neurons or other cells, then becomes internalized and degraded as a source of Se. It has not been determined if T cells express ApoER2 or another receptor to bind and internalize SELENOP, or if T cells predominantly take up Se metabolites for the synthesis of selenoproteins. Based on data from single cell RNAseq databases, there may be some ApoER2 expressed at higher levels in activated CD8⁺ T cells compared to CD4⁺ memory or regulatory T cells [38]. If this is truly representative of SELENOP receptor expression, this may reflect a higher degree of active selenoprotein synthesis related to the increased metabolic state of the former compared to the latter. Regardless of the method by which Se uptake occurs, T cells have been shown to express higher levels of selenoproteins in conditions of higher dietary Se. For example, feeding mice increasing dietary Se in the form of selenite was shown to increase SELENOK protein, as well as both GPX and TXNRD activity [17, 39]. Consistent with these data, a study showed that Se supplementation of human subjects increased GPX enzyme activity in lymphocytes and this correlated with increased T cell functions such as proliferative capacity and cytokine secretion [18].

Table 1.

Summary of Selenoprotein Functions.

Selenoprotein name	Abbreviations, Other names	Functions in T cells or other cells/tissues
Glutathione peroxidase 1	GPX1, Cytosolic glutathione peroxidase	Reduces cellular H2O2 and controls the ROS levels in T cells, particularly during activation. Required for secretion of IL-2, IFNg and TNFa from activated T cells.
Glutathione peroxidase 2	GPX2, Intestinal glutathione peroxidase	Reduces peroxide in gut, but function not demonstrated in T cells.
Glutathione peroxidase 3	GPX3, Plasma glutathione peroxidase	Secreted to reduce peroxide in blood and extracellular spaces in some tissues, but not shown to be expressd by T cells.
Glutathione peroxidase 4	GPX4, Phospholipid hydroperoxide glutathione peroxidase	Anti-oxidative lipid repair enzyme localized to cytosol, mitochondria, and nucleus. Controls ROS in T cells to prevent iron-induced cellular ferroptosis, particularly during activation.
Glutathione peroxidase 6	GPX6	May interact with copper as an antioxidative defense system during heavy metal exposure, although not shown to be expressed in T cells.
Iodothyronine deiodinase 1	DIO1, D1	Important for systemic active thyroid hormone levels, but not determined to be expressed in T cells.
Iodothyronine deiodinase 2	DIO2, D2	ER enzyme important for local active thyroid hormone levels, requires further studies in T cells.
Iodothyronine deiodinase 3	DIO3, D3	Inactivates thyroid hormone, requires further studies in T cells.
Methionine-R-sulfoxide reductase B1	MSRB1, SELR, SELX	Reduces oxidized methionine (R)-sulfoxide back to methionine. Regulator of f-actin polymerization in macrophages during innate immune response, requires further investigation in T cells.
Thioredoxin reductase 1	TXNRD1, TR1	Localized to cytoplasm and nucleus. Regenerates reduced TXN to regulate redox tone over the course of T cell activation. Promotes metabolic reprograming through TXN reduction.
Thioredoxin reductase 2	TXNRD2, TR3	Localized to mitochondria and regenerates reduced TXN in this highly oxidative environment. May acts as a regulator of redox tone over the course of T cell activation.
Thioredoxin reductase 3	TXNRD3, TR2, TGR	Testes-specific enzyme that regenerates reduced TXN.
Selenoprotein F	SELENOF, Selenoprotein 15, SEP15	TXN-like oxidoreductase likely to promote protein folding and quality control in ER. May serve as a gatekeeper of cytokine secretion by T cells similar to role in plasma B cells for IgG secretion.
Selenoprotein H	SELENOH, SELH, C11orf31	Nuclear localization, involved in redox sensing and transcription. Specific role in T cells is unclear.
Selenoprotein I	SELENOI, SELI, EPT1	ER transmembrane enzyme involved in phospholipid biosynthesis and T cell activation.
Selenoprotein K	SELENOK, SELK	ER transmembrane non-enzyme protein involved in TCR-induced calcium flux in T cells. Partners with DHHC6 enzyme to palmitoylate proteins, including IP3R.
Selenoprotein M	SELENOM, SELM, SEPM	TXN-like ER-resident protein that may be involved in the regulation of body weight and energy metabolism. Not well understood in T cells.
Selenoprotein N	SELENON, SELN, SEPN1	Transmembrane protein localized to the ER. Mutations lead to multiminicore disease and other myopathies. Not well studied in T cells.
Selenoprotein O	SELENOO, SELO	Mitochondrial protein that contains a TXN-like motif suggestive of a redox enzyme. Not well studied in T cells.
Selenoprotein P	SELENOP, SEPP1, SeP, SELP, SEPP	Secreted protein that transports Se throughout the body. Likely taken up by T cells as a source of Se for selenoprotein synthesis, although no specific data confirming this role.
Selenoprotein S	SELENOS, SELS, SEPS1, VIMP	Transmembrane protein localized to ER membrane, involved in ER associated degradation. Its role in T cells is unclear.
Selenoprotein T	SELENOT, SELT	Oxidoreductase localized to the Golgi complex and ER that contains a TXN-like fold. In other cell types has been shown to regulate redox status and cell anchorage. Its role in T cells is unclear.
Selenoprotein V	SELENOV, SELV	Testes-specific expression, although its function is not well defined.
Selenoprotein W	SELENOW, SELW, SEPW1	Putative antioxidant role, particularly important in muscle growth. May serve as a marker to discriminate T cells from B cells.
Selenophosphate synthetase 2	SEPHS2, SPS2	Involved in synthesis of selenoproteins, including itself. This role in T cells is similar to other cells.

Our research group previously evaluated unactivated mouse and human T cells for the abundance of selenoprotein mRNAs, and found some commonalities between species with T cells exhibiting a hierarchy of selenoprotein transcripts [20]. Among those exhibiting the highest expression were GPX4 and GPX1, which may indicate an important role for these antioxidant enzymes in controlling levels of ROS in T cells. SELENOF mRNA was also found to be highly expressed, and this selenoenzyme has been suggested to function as an endoplasmic reticulum (ER)-resident thioredoxin-like oxidoreductase that complexes with UDP-glucose:glycoprotein glucosyltransferase to function as a gatekeeper of secreted disulfide-rich glycoproteins [40]. Given the importance of cytokine secretion by T cells, SELENOF may serve as an ER gatekeeper of those secreted proteins, although this requires experimental verification. Among those expressed at the lower levels were some interesting selenoenzymes, including SELENOI. This selenoprotein catalyzes important steps in ethanolamine phospholipid synthesis in the ER/Golgi [41], although its low expression was found to increase 2.5-fold after T cell activation (our unpublished results). This highlights the importance of considering pre- and post-activation levels of individual selenoproteins. TXNRD1 is one selenoenzyme that significantly increases during T cell activation [42]. In particular, T cell activation led to increased TXN1/TXNRD1 expression along with a downregulation of TXN-interacting protein that binds to reduced TXN and acts as a negative regulator of its redox function. Tracking the expression levels of different selenoproteins over the course of T cell activation, proliferation, differentiation, and memory cell formation may provide important insight into their roles during these stages of T cell responses. Equally important is the expression levels of selenoproteins in T cells during infections, such as the T cell-trophic HIV-1. One such study demonstrated the downregulation of four selenoproteins in HIV-infected Jurkat T cells: GPX1, GPX4, TXNRD1, and SELENOF [43]. The HIV encoded Tat protein itself expressed in Jurkat T cells also decreased these same selenoproteins, suggesting an active downregulation during infection that may be pivotal in HIV-1 survival/replication in T cells.

A series of experiments involving selenium deficient diets in mice infected with viral pathogens showed an effect of selenium deficiency on both the viral itself in terms of virulence as well as the immune cell responses to the virus [44]. One particular study utilized a mouse model with low expression of a subset of redox regulating selenoproteins including GPX-1 and TXNRD1 that were infected with influenza virus [45]. Despite some influence of these selenoprotein deficiencies leading to altered cytokine responses during infection, the lung pathology was not dramatically different compared to witld-type controls. This may suggest that other redox regulating systems (e.g. peroxiredoxins or KEAP1/NRF2) in immune cells may be able to compensate under certain selenoprotein deficient conditions. It may also highlight roles for selenoproteins in different types of infections, supported in studies showing deficiencies in individual selenoproteins (e.g. SELENOK) reducing T cell responses to viruses [39].

4. T cell proliferation

4.1. Effective T cell proliferation relies on effective cellular signaling and metabolic shift

T cell responses to antigens presented in the context of MHC and co-stimulation progress through three main stages: expansion, contraction, and memory cell formation [46]. The full expansion of T cells is critical for generating a successful and long-lasting response against most antigens, with sub-optimal or inappropriate signals early in the response leading to a condition where the anergic cells are refractory to further stimulation. TCR signal transduction events that trigger T cell activation are initiated almost immediately following engagement by the APC, whereas metabolic and phenotypic changes of the T cells may not occur until minutes to hours later [28, 47]. One study focused on human subjects with compound heterozygous defects in the SECISBP2 gene that reduces synthesis of the 25 known human selenoproteins, and results showed impaired T cell proliferation and abnormal peripheral blood mononuclear cell cytokine secretion [48]. There remains much to be revealed regarding how selenoproteins function to regulate key signaling events, how they may impinge upon cell cycle decisions, or their roles in shaping the metabolic shift occurring during T cell activation and proliferation. However, some progress has been made in this area as discussed below (Figure 1).

Figure 1.

Roles identified for selenoproteins in T celll biology

4.1.1. Selenoproteins and TCR-induced cell calcium flux

Regulated increases in cytosolic calcium concentrations in T cells upon TCR engagement and throughout the activation process controls their proliferative capacity as well as effector functions. The mechanisms involved in TCR-induced calcium flux has been described in detail elsewhere [49], but the essential steps are as follows: 1) TCR engagement initiates a cascade of protein phosphorylation events that converge in the activation of PLCγ at the plasma membrane, which then cleaves phosphatidylinositol-4,5-bisphosphate into the plasma membrane-associated lipid diacylglycerol and the soluble head group inositol-1,4,5-trisphosphate (IP₃); 2) IP₃ binds IP₃R on the ER membrane and this causes the IP₃R channel to open, thereby releasing calcium stored in the ER into the cytoplasm; 3) this depletion of calcium stores from the ER is sensed by the calcium-binding domain of STIM molecules (particularly STIM1), which triggers STIM aggregation and movement to junctional spaces where the ER and plasma membrane come into close contact (~within 10–25 nm); 4) STIM oligomers contact the CRAC channel, ORAI-1, to trigger its opening in the plasma membrane that results in entry of extracellular calcium to raise the cytosolic calcium content to millimolar levels; 5) This increased calcium concentration in the cytosol activates several calcium-dependent pathways including calcineurin and NFAT, which serve to fully activate transcriptional and metabolic pathways required for optimal T cell activation.

Our published results of murine CD4⁺ T cells showed that increasing levels of dietary Se led to increased TCR-induced calcium flux [17]. Subsequent studies revealed that homozygous deletion of SELENOK impaired calcium flux in TCR-activated T cells, as well as in chemokine stimulated T cells [39]. SELENOK knockout T cells were deficient in proliferation and migration. Subsequently, mechanistic investigations revealed that SELENOK complexes with DHHC6 (letters represent the amino acids aspartic acid, histidine, histidine, and cysteine in the catalytic domain) in the ER membrane to catalyze the palmitoylation of tetrameric IP₃R in a manner that stabilizes this calcium channel [50]. In the absence of SELENOK-dependent palmitoylation on 3 different cysteine residues of the IP3R monomer, the IP₃R tetrameric complex becomes unstable and is degraded through the proteasome. The precise mechanism of SELENOK in the reaction was shown to involve the Sec residue of SELENOK, which acts to stabilize the palmitoyl-DHHC6 intermediate by reducing hydrolyzation of the thioester bond until transfer of the palmitoyl group to the Cys residue on the target protein could occur [51]. In this manner, SELENOK itself is not a selenoenzyme, but partners with the DHHC6 enzyme to increase its catalytic efficiency. Given the localization of several other selenoproteins in the ER and some evidence of their importance in calcium homeostasis in other cell-types or tissues [52–54], there may be roles for additional selenoproteins regulating TCR-induced calcium flux that are crucial for T cell activation and proliferation. However, most of these other ER proteins are oxidoreductase enzymes and therefore would affect calcium flux in T cells through a mechanism different from SELENOK [54].

4.1.2. Selenoproteins are important for shaping redox tone during TCR signaling

The redox environment in different organelles within T cells is tightly regulated, and ROS-mediated signaling events are required for driving T cell activation, proliferation, and differentiation [55, 56]. Upon TCR-induced activation of T cells, there are two important processes that lead to increased cellular ROS. The first involves NADPH oxidase (NOX) enzymes that generate cytoplasmic ROS rapidly after TCR engagement, which is often referred to as TCR-induced oxidative burst. NOX2 (also called gp91phox), which is the catalytic subunit of this superoxide-generating enzymatic complex, has been shown to be particularly important in T cells. After T cell stimulation, NOX cytosolic subunits translocate to the cell membrane, where they bind to the transmembrane subunits of the enzyme complex, p22phox and NOX2, to transfer of electrons from NADPH to molecular oxygen [57]. Predominantly studied as an enzymatic subunit in phagocytes and endothelial cells, NOX2 was shown to be expressed in T cells and crucial for generating the oxidative burst observed downstream of TCR stimulation [58, 59]. NOX2 was subsequently demonstrated to be an important immunomodulator of T cell activation, and hydrogen peroxide generated by dismutation of NOX2-generated superoxide demonstrated to promote T cell proliferation and also inhibit the suppressive capacity of T regulatory cells (Tregs) [60].

The second important ROS-generating process during T cell activation involves mitochondrial oxidative phosphorylation (OXPHOS). T cells with reduced production of mitochondrial ROS display low IL-2 production and impaired antigen-specific proliferation, with distinct TCR-triggered signaling nodes implicated in proper ROS-dependent activation [56]. ROS are tightly controlled by antioxidants and metabolic rewiring during T cell activation, both of which act to influence ROS levels and antioxidant capacity [61]. Because hydrogen peroxide permeates membranes, it is likely that this is the major ROS derived from mitochondria that impacts signaling in other cellular compartments. However, this process needs to proceed at the correct pace and level to promote T cell activation. For example, excessive ROS production following ablation of de novo glutathione (GSH) synthesis suppresses the activity of mammalian target of rapamycin (mTOR) and the expression of transcription factors NFAT and c-MYC [62, 63], the latter of which control metabolic reprogramming following T cell activation [64].

Because GPX1 and GPX4 are important for detoxifying hydrogen and lipid peroxides, respectively, they are considered important in regulating redox tone during T cell activation. In fact, these are among the highest selenoprotein mRNAs detected in T cells [20]. Our studies found that increased dietary Se led to increased GPX activity in T cells, and higher Se also led to higher levels of TCR-induced oxidative burst [17]. This seems counter-intuitive given that higher GPX activity should lead to less ROS, given that these selenoenzymes carry out reactions using GSH to reduce ROS. However, the increased GPX activity detected during TCR-induced activation may rise in conjunction with higher ROS to keep levels from hitting a critical point that triggers cellular damage during the oxidative burst. This idea was supported in a study using the T cell mitogen Con A injected into GPX1 knockout mice, where it was shown that GPX1 was required for secretion of IL-2 along with IFNγ and TNFα from activated T cells [65].

It has also been shown that higher dietary Se increases TXNRD activity to maintain reducing equivalents and further protect T cells from potentially detrimental effects of TCR-induced increases in ROS [17]. TXNRD1 and 2 enzymes play critical roles in redox regulation, with TXNRD1 localized to the cytosol and TXNRD2 residing in the mitochondria [66, 67]. Both selenoenzymes use NADPH to reduce TXN that is in turn used by several cellular enzymes as a cofactor in dithiol–disulfide exchange reactions. Regeneration of reduced TXN by TXNRDs is a major mechanism by which a reduced environment is maintained within cells, particularly serving to maintain reduced cysteine groups. This is crucial during T cell activation since hydrogen peroxide efficiently oxidizes protein thiols and generates disulfide bonds [68]. Free thiols increase in T cells with increasing dietary Se, and T cell activation and proliferation both benefit from a shift in the redox tone toward a reduced environment promoted through TXNRD bioactivity [17]. This is consistent with results from a study using porcine splenocytes stimulated with TCR-activating antibodies, where both the GPX and TXNRD activities were increased accompanied by increased proliferation and IL-2 production [69, 70]. Also, data from T cells deficient in selenoprotein synthesis using T cell-specific ablation of the Sec-tRNA gene showed that selenoprotein-deficient T cells exhibited oxidant hyperproduction that suppressed TCR-induced proliferation [71].

4.1.3. Selenoproteins and T cell metabolism

Naïve T cells, memory T cells, and Tregs predominantly rely on fatty acid oxidation (FAO) and oxidative phosphorylation (OXPHOS) to meet their relatively low energy needs and, perhaps more importantly, their low needs for biomolecular precursors [72]. Upon TCR triggered activation of naïve T cells, a metabolic shift occurs to meet energy and metabolite demands, involving a rapid increase in glucose uptake to feed the upregulation of aerobic glycolysis, although mitochondrial metabolism through OXPHOS remains important [73]. Thus, systemic Se levels and expression of certain selenoproteins including GPX1 and SELENOP that affect insulin sensitivity and glucose metabolism and thereby influence available glucose to drive TCR-induced metabolic reprogramming may be considered indirect regulators of T cell metabolic reprogramming [74, 75]. The anabolic demands in activated T cells suggest that selenoproteins involved in biosynthetic pathways would be intrinsically important for T cell proliferation. Related to this issue, our laboratory is currently investigating the role of SELENOI as a key enzyme in ethanolamine phospholipid synthesis, and there appears to be an upregulation of this enzyme during T cell activation required for the metabolic reprogramming that occurs as a result of TCR-induced activation (our unpublished results), although further investigation is required to determine the precise role of this selenoprotein.

While the crucial role of the TXN/TXNRD system of maintaining redox balance in T cells has been explored using several mouse models, the TXNRD redox regulating enzymes have also been linked to metabolic demands in activated T cells. For example, deletion of TXNRD1 prevents expansion the CD4⁻CD8⁻ thymocyte population and terminates T cell development [42], while TXNRD2^−/− thymocyte numbers were not different compared to wild-type controls [76]. Deletion of TXNRD1 in mature CD4⁺ T cells led to a failure in cells to expand following viral and parasite infection, and the requirement of this selenoenzyme for metabolic reprogramming was determined to be the mechanism of both thymocyte and peripheral T cell impairments [42]. Mechanistically, TXNRD1 was required for the last step of nucleotide biosynthesis, serving as the key enzyme that donates reducing equivalents to ribonucleotide reductase. Without this ability to repair DNA-damage in activated T cells along with de novo synthesis of deoxyribonucleotides, cell cycle arrest was induced. In these studies, the TCR signal itself was not as affected by TXNRD1 deficiency, and this included oxidative burst. This supports the notion that, unlike GPXs, the TXN/TXNRD system does not provide a direct check on the rapid rise in ROS required for T cell activation. Rather, the reducing power of this system is critical for both the regulation of redox tone and the promotion of the metabolic reprogramming that supports cellular proliferation and activation. The central role of the TXN/TXNRD system in controlling cell growth for both prokaryotes and eukaryotes is well described in a comprehensive review [77].

4.2. Ferroptosis in T cells

Programmed cell death via apoptosis plays a critical role in different facets of T cell biology including thymocyte development, preventing autoreactivity through peripheral tolerance, and in promoting the contraction phase of T cell responses [78]. In addition to apoptosis, ferroptosis is another form of regulated cell death driven by accumulation of lipid-based reactive oxygen species and the major enzyme involved in preventing ferroptosis is GPX4 [79]. T cell-specific GPX4 knockout mice were generated using CD4-cre crossed with GPX4 floxed mice to study the role of this enzyme in T cell development and function [80]. Interestingly, GPX4-deficient thymocytes effectively proceeded through developmental stages in the thymus similar to controls, suggesting that apoptosis rather than ferroptosis is the main mechanism of programmed death during T cell development. In contrast, in the spleen the CD8⁺ T cell numbers were lower than the controls. Both CD4⁺ and CD8⁺ T cells deficient in GPX4 failed to survive when adoptively transferred into a congenic mouse, accumulating lipid peroxides and dying by ferroptosis. The roles of ferroptosis in regulating T cell biology has not been extensively studied, and the precise role of GPX4 in regulating this mode of programmed cell death requires further investigation.

5. T helper cell differentiation

TCR-induced activation of CD4⁺ T cells leads to proliferation and differentiation into helper subsets, which forms the foundation for their ability to shape immune response and mediate host protection [81]. CD4⁺ T helper (Th) cell differentiation into Th1, Th2, Th17, Treg cells and other subtypes represents a major step in driving different types of immune responses as described elsewhere [82]. Fine tuning of ROS to levels to regulate polarization of differentiating T cells relies on engagement of lineage-specific transcription factors and the modulation of cytokine profiles [83]. For example, NOX-derived ROS modulate the function of GATA-binding protein 3 (GATA3), signal transducer and activator of transcription (STAT), and T-box transcription factor (T-bet) to collectively control T cell activation and differentiation [83, 84]. T cells from NOX-deficient animals showed a skewed Th17 phenotype, whereas NOX-intact cells exhibited a preferred Th1 response. As mentioned above in section 4.1.2, higher dietary Se leads to a strong NOX-induced oxidative burst during TCR-triggered activation. This was accompanied by a bias toward Th1 differentiation with higher Se intake [17]. Consistent with this finding, supranutritional levels of dietary Se was found to decrease lung p-STAT6⁺ Th2 cells in a mouse model of allergic airway inflammation compared to adequate Se levels [85]. These results suggest that higher selenoenzyme bioactivity in activated T cells skew T cell differentiation toward a Th1 subtype, which is opposed to development of Tregs. Similarly, when GPX1 and catalase double knockout mice were subjected to experimental colitis, increases in ROS were detected in T cells and a higher suppressor capacity of Tregs found in the double KO cells [86]. By regulating redox tone and levels of the TCR-induced oxidative burst, TXNRDs and GPXs may serve to promote Th1 immunity while minimizing Treg development. It would be important to determine the precise mechanisms by which each of these selenoenzymes function to regulate T cell differentiation and how other selenoproteins may be involved.

6. Conclusions

Meeting the required intake of Se for expressing functional selenoproteins at appropriate levels is important for optimal immunity. This is particularly true for lymphocytes due to their need for rapid proliferation and effective differentiation in response to different types of antigen challenge. T cell lymphocytes have shown defects in activation, proliferation, and differentiation under conditions of inadequate Se intake, and the roles for different selenoproteins in regulating these processes is becoming clear as new research models and tools become availability. For example, transgenic mouse models along with CRISPR/Cas9 edited cell models have allowed gain- or loss-of-function models to be established and these have provided insight into specific roles for selenoproteins in T cell biology and immunity [87]. Loss-of-function studies may be able to be applied to better understand some fundamental issues. For example, sex-differences have been shown in selenium-related autoimmune diseases [88], and T cells represent the first layer of tolerance breakage for autoimmunity. So, are these sex-differences related to specific selenoproteins expressed in T cells or other immune cells differing between male and female? There remains much to be learned regarding molecular mechanisms through which selenoproteins impact T cell functions and how this is related to dietary Se intake or to other factors such as genetic factors. Future mechanistic studies combined with much needed clinical studies will help address these gaps in knowledge and further define roles for selenoproteins in regulating T cell driven immunity.

Abbreviations

APC

antigen presenting cell

ApoER2

apolipoprotein E receptor 2

ER

endoplasmic reticulum

GSH

glutathione

GATA3

GATA-binding protein 3

GPX

Glutathione peroxidase

IP₃

inositol-1,4,5-trisphosphate

MHC

major histocompatibility complex

mTOR

mammalian target of rapamycin

NOX

NADPH oxidase

OXPHOS

oxidative phosphorylation

ROS

reactive oxygen species

Sec

selenocysteine

Se

selenium

SELENO

selenoprotein

STAT

signal transducer and activator of transcription

T-bet

T-box transcription factor

TXN

thioredoxin

TXNRD

thioredoxin reductase

TCR

T cell receptor

Treg

T regulatory

UTRs

Untranslated regions