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

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 β₁ and β₂ 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.