T‐cell regulation through a basic suppressive mechanism targeting low‐density lipoprotein receptor‐related protein 1

Abstract

Cell adhesion is generally considered to depend on positive regulation through ligation of integrins and cytokine receptors. However, here we show that T-cell adhesion, and notably also T-cell receptor (TCR) -induced activation, are subject to constant suppression through shedding of low-density lipoprotein receptor-related protein 1 (LRP1). The broad-spectrum metalloprotease inhibitor GM6001 abrogated shedding, so inducing prominent cell surface expression of LRP1 while enhancing TCR-induced activation and adhesion to β and β integrin ligands, hence arresting the cells. Integrin ligands also inhibited shedding but the effect was less potent than that of GM6001. Unlike GM6001, integrin ligands also induced cell surface expression of full-length thrombospondin-1 (TSP170) and TSP130, which associated with LRP1, and TSP110, which did not associate with LRP1. Cell surface expression of LRP1 and TSP130 were induced exclusively in adhering cells, expression of TSP110 preferentially in non-adhering cells and expression of TSP170 correlated with T-cell motility. The pro-adhesive chemokine CXCL12 also inhibited LRP1 shedding and induced surface expression of TSP170 and TSP130 while inhibiting TSP110. Exogenous TSP-1 and ligation of CD28 inhibited shedding although less effectively than GM6001, and the inhibition through CD28 was independent of TSP-1. Small interfering RNA silencing experiments confirmed involvement of LRP1 and TSP-1 in integrin-dependent adhesion and TCR-induced activation. Hence, the poor LRP1 expression in T cells depends on shedding. Integrin ligands and CXCL12 antagonize shedding through a TSP-1-dependent pathway and ligation of CD28 antagonizes shedding independent of TSP-1. The disappearance of LRP1 from the cell surface may provide basic immunosuppression at the T-cell level.

T Cell Co-Stimulation: Inhibition of Immunosuppression?

“…A different interpretation is suggested by recent findings that some forms of co-stimulation, rather than being stimulatory, inhibits a mechanism preventing TCR-induced activation ( 15 ). The studies unveiling this role of co-stimulation were initiated by evidence that T cell motility, adhesion, and activation depend on the large transmembrane cell surface receptor low-density lipoprotein receptor-related protein 1 (LRP1) and its ligand thrombospondin-1 (TSP-1) ( 16 – 22 ).…”

“…TSP-1 deficiency in mice results in inflammation in several organs suggesting that TSP-1 has a protective role in inflammation ( 26 , 27 ). LRP1 is poorly expressed on T cells directly from the blood of healthy individuals and on T cells cultured in vitro ( 15 ). TSP-1 induces cell surface expression of LRP1 and the chemokines CXCL12 and CCL5 elicit cell surface expression of LRP1 through TSP-1 ( 17 ).…”

“…TSP-1 also binds to LRP1 and promotes integrin-dependent T cell adhesion ( 19 , 21 ). However, the most prominent cell surface expression and functional impact of LRP1 are unveiled by a broad-spectrum metalloprotease (MMP)/disintegrin-type metalloproteinase (ADAM) inhibitor through inhibition of shedding, indicating that LRP1 is transported to the cell surface and then released by an enzymatic mechanism ( 15 ). If cell surface LRP1 is protected by an MMP/ADAM inhibitor, and TSP-1 transport to the cell surface simultaneously is stimulated by cell contact with β1 and β2 integrin ligands, TSP-1 associates heavily with LRP1 ( 15 ).…”

“…However, the most prominent cell surface expression and functional impact of LRP1 are unveiled by a broad-spectrum metalloprotease (MMP)/disintegrin-type metalloproteinase (ADAM) inhibitor through inhibition of shedding, indicating that LRP1 is transported to the cell surface and then released by an enzymatic mechanism ( 15 ). If cell surface LRP1 is protected by an MMP/ADAM inhibitor, and TSP-1 transport to the cell surface simultaneously is stimulated by cell contact with β1 and β2 integrin ligands, TSP-1 associates heavily with LRP1 ( 15 ). This is accompanied by firm integrin-dependent T cell adhesion, which arrests the cells, although not inhibiting motility per se , and a potent enhancement of TCR-induced T cell activation.…”

“…TSP1 is a 450-kDa glycoprotein composed of three identical disulfide-linked polypeptide chains that display binding sites for various cell surface receptors ( 25 ). Co-stimulation through CD28, integrins, and CXCR4 inhibits a protease mechanism that removes LRP1 from the cell surface and antagonizes TCR-induced activation ( 15 ).…”