Inhibition of T cell receptor signaling by cholesterol sulfate, a naturally occurring derivative of membrane cholesterol

Most adaptive immune responses require the activation of specific T cells through the T cell antigen receptor (TCR)-CD3 complex. Here we show that cholesterol sulfate (CS), a naturally occurring analog of cholesterol, inhibits CD3 ITAM phosphorylation, a crucial first step in T cell activation. In biochemical studies, CS disrupted TCR multimers, apparently by displacing cholesterol, which is known to bind TCRβ. Moreover, CS-deficient mice showed heightened sensitivity to a self-antigen, whereas increasing CS content by intrathymic injection inhibited thymic selection, indicating that this molecule is an intrinsic regulator of thymocyte development. These results reveal a regulatory role for CS in TCR signaling and thymic selection, highlighting the importance of the membrane microenvironment in modulating cell surface receptor activation.

Analysis of interactions between proteins and fatty acids or cholesterol using a fatty acid/cholesterol pull-down assay

Specific interactions between the polar head groups of membrane lipids and proteins have been described previously. In contrast, the specificity of the interaction between lipid acyl chains with proteins is less understood. By combining a fatty acid or cholesterol pull-down assay with Western blot analysis or mass spectrometry, we identified transmembrane and cytosolic proteins that bound preferentially to short or long acyl chains or to cholesterol. Thus, this approach allows identification of specific fatty acid-protein or cholesterol-protein interactions.

Cholesterol and sphingomyelin drive ligand-independent T-cell antigen receptor nanoclustering

The T-cell antigen receptor (TCR) exists in monomeric and nanoclustered forms independently of antigen binding. Although the clustering is involved in the regulation of T-cell sensitivity, it is unknown how the TCR nanoclusters form. We show that cholesterol is required for TCR nanoclustering in T cells and that this clustering enhances the avidity but not the affinity of the TCR-antigen interaction. Investigating the mechanism of the nanoclustering, we found that radioactive photocholesterol specifically binds to the TCRβ chain in vivo. In order to reduce the complexity of cellular membranes, we used a synthetic biology approach and reconstituted the TCR in liposomes of defined lipid composition. Both cholesterol and sphingomyelin were required for the formation of TCR dimers in phosphatidylcholine-containing large unilamellar vesicles. Further, the TCR was localized in the liquid disordered phase in giant unilamellar vesicles. We propose a model in which cholesterol and sphingomyelin binding to the TCRβ chain causes TCR dimerization. The lipid-induced TCR nanoclustering enhances the avidity to antigen and thus might be involved in enhanced sensitivity of memory compared with naive T cells. Our work contributes to the understanding of the function of specific nonannular lipid-membrane protein interactions.