The Receptor Is Up and Its Pathway Is Down in MS T Cells

When a signalling pathway runs down, the usual first suspect is the receptor. In CD4+ T cells from people with relapsing-remitting multiple sclerosis (RRMS) the opposite holds. NR1H2, the gene encoding LXRβ, is one of the most strongly upregulated genes Martin-Gutierrez and colleagues found, at a fold change of 2.95 and a p-value of 3.77×10⁻²³, while the pathway that receptor drives runs in the other direction. Their explanation puts the problem upstream of the receptor, in the supply of the lipid ligands that switch it on. This post follows that argument and the evidence assembled for it, in CD4+ T cells sorted from 10 RRMS patients sampled before their first disease-modifying therapy and 10 healthy controls.

What LXR Does, and Why the Isotype Matters Here
The liver X receptors, LXRα encoded by NR1H3 and LXRβ by NR1H2, are lipid-activated nuclear receptors that govern lipid metabolism and cholesterol homeostasis. Both are switched on by oxidized derivatives of cholesterol, the oxysterols, and by intermediates of cholesterol biosynthesis, and they regulate genes involved in cholesterol efflux and uptake, fatty acid biosynthesis, phospholipid remodelling and glycosphingolipid biosynthesis. LXRβ is the predominant isotype in T cells, which is why the genes regulated by the common synthetic ligand GW3965 are expected to be predominantly LXRβ-regulated in CD4+ T cells, and why a study of LXR in T cells is effectively a study of LXRβ.

Receptor Up, Output Down
Despite the increased expression of LXRB, the downstream LXR-regulated pathway and the LXR target genes were downregulated, and LXR binding motifs turned up among the genes downregulated in RRMS. Of the LXR-responsive genes previously established in healthy human CD4+ T cells, 26 were up and 40 were down in patients. Two that fell are directly about cholesterol handling: IDOL, which regulates cholesterol uptake by degrading the low-density lipoprotein receptor, and LDLR itself. Two that did not move are the classic LXR targets ABCA1 and ABCG1, which the authors read as those targets being more sensitive to LXRβ activation specifically, given that LXRβ is the dominant isotype in these cells.

The Ligand-Supply Argument
LXRs are ligand-activated transcription factors, so a receptor with no ligand produces nothing regardless of how much of it is present. The authors build their case from a known piece of regulatory logic: endogenous LXR ligands block entry of SREBF1 into the nucleus, so if oxysterols are scarce, SREBF1 activity should rise on its own, without LXR inducing it. That is what the data show, with SREBF1 upregulated at a fold change of 2.23 and SREBF2 upregulated alongside it. The genes that make and move oxysterols are disturbed in the same direction. CYP27A1, an enzyme of oxysterol synthesis, was down 8.08-fold (p = 2.19×10⁻³), the largest fold change in that panel, while the oxysterol-binding protein-like transporters split, with OSBPL7 and OSBPL8 raised and OSBPL9, OSBPL10 and OSBPL1A lowered. The authors read the set as a change in oxysterol availability and metabolism inside the cell.

Which Downstream Genes Pay for It
The LXR targets reduced in patients are the ones tied to cellular lipid metabolism. IDOL falls, as does UGCG, the rate-limiting enzyme for glycosphingolipid biosynthesis, down 1.36-fold, and SCD, a fatty acid desaturase. Genes in cellular cholesterol homeostasis move the other way, among them EEPD1, which positively regulates ABCA1-mediated cholesterol efflux. Several of the differentially expressed genes have prior links to neurological disease, including SPHK2, P2RX7, SMPD4 and ASAH1. Among the T-cell-specific LXR targets the largest shifts were CEMP1 up 4.90-fold and S100A9 down 7.58-fold, with CD14 down 5.40-fold and C5AR1 down 3.84-fold.

Ruling Out Two Alternatives
The obvious objection is that these are simply activated T cells, and activation changes lipid genes. The authors tested it directly by comparing their RRMS-versus-control fold changes against resting-versus-activated fold changes in healthy CD4+ T cells. The two patterns did not resemble each other, with most genes regulated differently in the two comparisons. A cluster did overlap, genes downregulated in both RRMS and receptor-stimulated cells, including HLA-DMB, A2M, FGR, TGFB1 and MRAS along with sterol regulation, and SREBF1 and SREBF2 were upregulated in both datasets. So activation contributes to part of the picture without accounting for it. A second alternative also failed. Serum from patients did alter T-cell lipid content in culture, but serum cytokines from RRMS patients were not different from healthy controls, while serum lipid metabolites were significantly altered and significantly correlated with the lipid metabolism pathway genes. That puts the serum effect in the lipids rather than in circulating inflammation.

Two Ligand Experiments, and Two Boundaries
If the problem is ligand supply, then supplying ligand should help, and it partly did. The synthetic agonist GW3965 reduced IL-17A expression inside the cell and reduced proliferation while increasing IL-4 production in patient CD4+ T cells, with interferon-γ production unaffected. The same effect was recapitulated using 24S-hydroxycholesterol, an endogenous oxysterol agonist, which is the more informative of the two because it tests the hypothesis with the molecule class the cells are proposed to be short of. The membrane rescue was partial in a way the authors state plainly: GW3965 significantly reduced cholesterol in patient T cells but did not influence glycosphingolipid levels and had an inconsistent effect on membrane lipid order. A separate observation points the same way from the clinic. Comparing LXR-regulated gene expression before and after three months of interferon-β treatment found four lipid metabolism genes, UGCG, IDOL, LDLR and EEPD1, moving back toward healthy control levels, which the authors suggest may contribute to that drug's effect on CD4+ T cells. Two boundaries close the picture. Neither glycosphingolipid nor cholesterol levels correlated with age, body mass index, vitamin D levels, EDSS or number of relapses, all at p above 0.23. And the T-cell membrane lipid changes were not seen in patients with secondary progressive MS, which the authors suggest may reflect differences in pathogenic mechanisms between the two forms.

Disclaimer: This blog post is based on the cited study and is intended for informational purposes only. It is not intended to provide medical advice. Please consult with a healthcare professional for any health concerns.

Reference:
Martin-Gutierrez, L., Waddington, K. E., Maggio, A., Coelewij, L., Oppong, A. E., Yang, N., Adriani, M., Nytrova, P., Farrell, R., Pineda-Torra, I., & Jury, E. C. (2024). Dysregulated lipid metabolism networks modulate T-cell function in people with relapsing-remitting multiple sclerosis. Clinical and Experimental Immunology, 217(2), 204–218. https://doi.org/10.1093/cei/uxae032