CD4⁺ T-cells play a central role in the adaptive immune system. Upon activation, they proliferate, traffic to inflamed sites, and acquire functions that mediate the immune response against infection and malignancy (1). These processes have significant metabolic demands and understanding how metabolites (including glucose, amino acids, and cholesterol) are modulated to meet these increased energetic demands is an urgent challenge (1). The majority of current studies refer to changes in intracellular metabolites and how they affect T-cell function. In this review, we will focus on the role of cellular lipid metabolism in the regulation of plasma membrane (PM) lipid composition and the importance of this to T-cell function—a mechanism which has only just begun to be explored (2, 3).

The T-cell PM provides a flexible interface where signals generated by cell surface receptors lead to functional outcomes, including activation, proliferation, and cytokine production. Lipids and proteins are both essential PM constituents, but while PM proteins have been widely studied, there is a gap in our knowledge about the fundamental role and regulation of lipid PM components (4). This gap impedes our understanding of how PM lipids influence immune cell function and how they could be targeted or manipulated therapeutically.

Cholesterol and glycosphingolipids (GSLs) are particularly enriched in the PM and form signaling platforms known as lipid rafts. Signaling molecules accumulate at high density in lipid rafts and they are essential for immune cell activation and function (5, 6).

Cholesterol helps to maintain lipid raft structure; the amount of cholesterol, cholesterol intermediates such as lanosterol, or oxidized cholesterol in the PM can alter lipid raft stability and affect cell function by modifying the lateral mobility of membrane receptors and signaling molecules (7–11). More specifically in T-cells, PM cholesterol has been shown to mediate T-cell receptor (TCR) clustering, inhibit spontaneous TCR activation and reduce TCR mobility in the membrane (12–14). Similarly, GSLs influence T-cell functions including TCR-mediated signaling and responsiveness to cytokine stimulation (15–18), apoptosis, and recycling/endocytosis of membrane signaling and receptor molecules (19). Changes in lipid composition affect the biophysical properties of PM lipid rafts (20). Studies also show that distinct PM lipid profiles (GSL and cholesterol content) are associated with well-defined T helper (Th) cell subsets (Th1, Th2, and Th17) (15, 17, 18, 21, 22), supporting a role for PM lipid composition in defining functional T-cell phenotypes (23). Interestingly, changes in PM lipid order, measured using the fluorescent membrane probe di-4-ANEPPDHQ, can dictate the response of T-cells to TCR stimulation. T-cells with high PM order form more stable immune synapses, proliferate robustly and favor a Th-2 phenotype whereas T-cells with lower levels of PM order form more unstable immune synapses, have reduced proliferative capacity and produce more proinflammatory cytokines. For instance, reducing PM order with the oxysterol 7-ketocholesterol is alone sufficient to alter the functional phenotype of T-cells (9).

These advances in understanding the link between PM lipids and T-cell function are supported by state-of-the-art microscopy techniques including super-resolution fluorescence microscopy that have revolutionized the visualization of PM lipids and membrane order (24–28). The increasing evidence describing defects in T-cell PM lipid rafts associated with abnormal T-cell function in autoimmunity makes this an attractive therapeutic area (29, 30).

The tightly controlled network of transcriptionally regulated lipids described above could be critical for T-cell function via maintaining lipid raft homeostasis and influencing T-cell signaling pathways as summarized in Table 1 (2).

LXRβ is the predominantly active form of LXR in T-cells (80). LXRβ influences T-cell proliferation through ABCG1-dependent regulation of intracellular cholesterol thereby affecting antigen-specific immune responses (80). It is likely that this effect is driven by reducing PM cholesterol that disrupts lipid raft-associated TCR signaling. In addition, our work identified that lipid raft-associated GSLs correlate with enhanced levels of LXRβ and LXR-modulated cholesterol trafficking proteins Niemann-Pick type C 1 and 2 (NPC1/2) in human CD4⁺ T-cells from autoimmune disease patients (15), although it remains to be elucidated whether LXR directly regulates GSLs in T-cell subsets from healthy individuals. LXR stimulation in vitro inhibits Th1 and Th17 cytokine production and induces regulatory T-cell polarization suggesting a role for LXR-driven lipid modulation in anti-inflammatory T-cell differentiation potentially by reducing PM cholesterol via increased cholesterol efflux (81, 82).

The mechanism of action of SREBPs is also particularly important in T-cell function as cholesterol homeostasis is critical to PM lipid raft composition and fatty acids provide an abundant T-cell energy source (83). For instance, CD8⁺ T-cells are unable to undergo clonal expansion in response to viral infection when SREBPs are not present, which can be rescued by supplementation with cholesterol (84).

All three PPAR subsets are expressed in T-cells where they are involved in both metabolic regulation and inflammation (66, 85, 86). PPAR modulation of cholesterol may play a role in regulating lipid rafts and therefore TCR signaling and their role in fatty acid oxidation likely alters T-cell energy sources. PPAR-mediated upregulation of ApoAI/II in the periphery may indirectly influence T-cell cholesterol levels via elevated HDL levels and increased cholesterol efflux. In addition, these factors have been shown to affect cell death and proliferation. Activation of PPARγ in helper T-cells suppresses proliferation, IL-2 expression and induce apoptosis (87, 88). PPARα antagonizes NF-κB in T-cells, and conversely T-cell activation results in reduced PPARα expression (85). PPARα agonists increase IL-4 secretion, inhibit interferon (IFN)-γ expression, and reduce the proliferation of human T-cell lines. Stimulation of PPARδ increases T-cell proliferation and reduces the proapoptotic effect of type 1 IFNs (86). In an experimental autoimmune disease model, PPARδ stimulation reduced IFN-γ and IL-17 secretion from T-cells (89). This suggests possible PPAR regulatory actions on T-cell differentiation through modification of lipid metabolism.

Due to the striking gender bias in autoimmunity (90) and reported differences in T-cell function, it is important to consider gender in this area of research. The two classical ERs (ERα and ERβ) exhibit differential expression; ERα is more highly expressed in T-cells than ERβ (91). Altered ER profiles could contribute to differences in PM-associated E2 signaling in T-cell subsets and between genders. Cross-talk between ERs and LXRs may also play a role in the lipid modification of T-cells and therefore function. Interestingly, there is evidence to suggest that gender and/or estrogen are able to modulate PPAR function. Dunn et al. demonstrated that male mice express more PPARα than females and that this differential expression is hormone sensitive. Furthermore genetic ablation of the PPARα gene resulted in the loss of antagonism of NF-κB, increased production of Th1 and decreased production of Th2 cytokines by T-cells. This genetic ablation in an experimental model of autoimmune encephalomyelitis increased clinical symptoms in male but not female mice (92). This suggests a sex-specific sensitivity to the protective actions of PPARα relevant to the gender bias seen in autoimmunity.

The tight network of transcriptional metabolic regulators described above provides a great opportunity for therapeutic targeting (Table 1). Because of the cross-talk between these different nuclear receptors and pathways, manipulating multiple receptors could represent an effective strategy. The SREBP pathway responds to low cholesterol, and therefore the use of statins, which inhibit the cholesterol synthesis enzyme HMG-CoA reductase, secondarily increases the activity of SREBPs in an attempt to increase cellular cholesterol and fatty acid levels. From an autoimmune perspective, statins could be used therapeutically to counter the pathogenic increase in T-cell lipid rafts through lowering membrane cholesterol. In vitro culture of T-cells with atorvastatin reduces T-cell signaling from lipid rafts, ultimately reducing IL-6 production implicated in SLE pathogenesis (29). It has been shown that statins alter the ratio of pro- and anti-inflammatory responder T-cells, inhibit Th1 differentiation and reduce the activation and migration of CD4⁺ autoreactive T-cells across the blood–brain barrier in multiple sclerosis (93–95). This finding supports an important role for cholesterol metabolism in T-cell function. Notably, simvastatin has shown promise in a phase 2 trial in people with multiple sclerosis; the drug reduced the annual rate of whole-brain atrophy without adverse side effects (96). Independent of their modulation of cholesterol, statins may also influence T-cell function through the inhibition of prenylation (geranylgeranylation or farnesylation) (97). Prenylation of GTPases of the Ras and Rac subfamilies allows their targeting to the cell membrane which is integral to TCR signaling (98, 99). Alternatively, inhibiting SREBPs may counteract overactive TCR signaling. A small molecule SREBP processing inhibitor named betulin has been shown to improve hyperlipidemia and insulin resistance and reduces atherosclerotic plaques (100). SREBP inhibition also prevents CD8⁺ T-cell expansion in response to viral infection (84). Another potential therapeutic target is the LXRs. Synthetic ligands that stimulate the activity of these receptors exist which reduce cellular and membrane cholesterol content. An example of this is the non-steroidal ligand GW3965, an LXR agonist that has been shown to modulate macrophage, dendritic cell and T-cell function (51, 80, 101). However, the value of these therapeutics has not been explored extensively in T-cells. In light of the evidence that activated ERs aid the transcriptional function of LXRs, interact with LXRs in lipid rafts in endothelial cells, and respond to oxysterols, it is plausible to hypothesize that LXR therapeutics could be more effective in premenopausal women although this is something that has not been explored to date. Synthetic LXR ligands have been investigated as anti-atherosclerotic agents in experimental models of atherosclerosis and in a human phase 1 trial (102, 103). The main obstacle encountered in the development of LXR ligands as clinical therapeutic agents in human metabolic diseases is the concomitant increase in liver triglycerides by these agents, an effect primarily mediated by LXRα (104, 105). Furthermore, LXR activation is gaining interest in the fight against cancer because of their actions on cholesterol metabolism in cancer cells coupled with their effects on cell proliferation, growth arrest and apoptosis (106). Some of these aspects have been described for CD8⁺ T-cells (80). Whether this is recapitulated in other immune cell subsets and the impact of this in female-predominant autoimmune diseases needs to be established. Altogether this emphasizes the need for a greater understanding of isoform- (LXRα vs. LXRβ) and tissue/cell type-specific effects of LXRs in health and disease.

Peroxisome proliferator-activated receptor pharmaceutical agonists including fibrates for PPARα, glitazones for PPARγ, and phenoxyacetic acid derivatives for PPARδ have therapeutic value in hypertriglyceridemia, hypoalphalipoproteinemia, and diabetes (67, 68, 107). PPARα activators reduce Th1 and increase Th2 polarization making these therapeutics attractive for the treatment of autoimmune diseases (69). PPARγ agonists have also shown promise following a study of autoimmune myocarditis in Lewis rats. A PPARγ agonist ameliorated disease severity, which was also attributed to a Th1/2 phenotypic switch (108). It will be interesting to assess the effect of PPARs on membrane lipids, especially as Th1/Th2 status has been linked to differences in PM order (9). Again, gender may play a role in the effectiveness of these treatments. Activated ERs may compete for PPAR DNA binding and there is evidence to suggest that PPAR ligands perform better under estrogen free/ER-inhibited conditions (77). Therefore, inverse to the LXR hypothesis, PPAR therapies may be of greater benefit in males and post-menopausal women. Finally, in recent years, modulation of PM lipid composition and structure, either by reducing or by increasing PM cholesterol levels, has been investigated in the treatment of cancer. Reduced PM cholesterol has been associated with increased cancer cell metastasis whereas high PM cholesterol has been linked to drug resistance. In these contexts, lipid modulating therapies combined with conventional drugs can improve the efficacy of anti-cancer treatments (109). Recently, Avasimibe, a drug that blocks free cholesterol esterification and its subsequent storage as cellular lipid droplets by inhibiting the enzyme acetyl-CoA acetyltransferase 1, increased the efficacy of checkpoint inhibitor blockade in preclinical models of melanoma and lung carcinoma. This was achieved by increased PM cholesterol leading to stronger TCR signaling and cytotoxic activity in CD8⁺ T-cells (110, 111). This supports the possibility of combining established therapeutics with lipid-modulating treatments in order to enhance efficacy and improve outcomes in a range of clinical settings.

Here, we have summarized evidence showing that manipulation of lipid metabolism in T-cells by targeting nuclear receptor transcription factors could be a promising therapeutic avenue in the treatment of autoimmune diseases. However, the cross-talk between this tight network of receptors and transcription factors will need to be considered when determining which receptors to target. We have also highlighted that gender is an important factor for consideration, thus emphasizing the relevance of these receptors in a group of immune diseases dominated by gender bias. With the advent of advanced lipidomic technologies, we anticipate that in the coming years more in depth studies on PM lipid composition and its metabolic, inflammatory and pharmacological regulation in different immune cell types including T-cells will become available. This will likely allow new opportunities to use ligands targeting these receptors/factors as adjuvant therapies in various proliferative and immunological disorders.

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

The authors declare that they have no commercial or financial relationships that could be construed as a potential conflict of interest relating to this work.