LAP, a lymphocyte activation gene-3 (LAG-3)-associated protein that binds to a repeated EP motif in the intracellular region of LAG-3, may participate in the down-regulation of the CD3/TCR activation pathway

Abstract

The threshold, extent and termination of TCR activation is controlled in part by inhibitory co‐receptors expressed on activated T cells. The lymphocyte activation gene product (LAG‐3), a ligand for MHC class II molecules co‐caps with the CD3/TCR complex and inhibits cell proliferation and cytokine secretion in response to CD3 signaling. We first investigated whether LAG‐3 is localized in activated T cells in detergent‐resistant membrane rafts enriched in glycosphingolipids and cholesterol. We showed that both LAG‐3 and MHC class II are present in the cell fraction of glycosphingolipid‐rich complexes (GSL complexes) before the assembly of the immunological synapse by CD3/TCR complex cross‐linking. Using the LAG‐3 intracytoplasmic region as bait in the yeast two‐hybrid cloning system, we next identified a novel protein termed LAP for LAG‐3‐associated protein. LAP is encoded by a 1.8‐kb RNA message in lymphocytes and encodes a 45‐kDa protein that is expressed in most tissues. We showed that LAP binds specifically in vitro and in vivo to the Glu‐Pro (EP) repeated motif present in the LAG‐3 intracytoplasmic region. LAP also binds to the EP motif of another functionally important receptor, the PDGFR. Thus, LAP is a candidate molecule for a new type of signal transduction and/or coupling of clustered rafts to the microtubule networks that could explain how negative signaling of co‐receptors may occur through molecules devoid of any immunoreceptor tyrosine‐based inhibitory motif consensus sequence.

Maturation and Activation of Dendritic Cells Induced by Lymphocyte Activation Gene-3 (CD223)

“…They serve as platforms for signal transduction and, in clustering, form the TCR-APC contact junction, termed the immunological synapse. LAG-3 was found to be associated with the TCR (25,26) and to be constitutively present in plasma membrane lipid rafts extracted from human activated T cells based on their insolubility in Triton X-100 and low buoyant density in sucrose gradients (32). It was thus possible that in the TCR-APC contact junction LAG-3 and class II collaborate in establishing the clustering of raft platforms.…”

Trafficking of LAG-3 to the Surface on Activated T Cells via Its Cytoplasmic Domain and Protein Kinase C Signaling

“…The second conserved region contains the KIEELE motif, which is essential for LAG-3 downstream signaling (10). The third conserved region contains the EP repetitive sequence that binds to LAG-3-associated protein; however, the molecular function of the EP motif remains elucidated (32). Within the cytoplasmic tail of LAG-3, two serine residues could be phosphorylated (Fig.…”

Lymphocyte Activation Gene-3 Expression Defines a Discrete Subset of HIV-Specific CD8+T Cells That Is Associated with Lower Viral Load

“…41 LAG-3 has been associated as an inhibitory marker and in immune suppression. 3,8,9,11,15,16,42,43 Given the similarities in downstream targets, it is tempting to speculate that inhibitory molecules may act antagonistically against other positive costimulation. The presence of LAG-3 + HIV-specific-positive CD8 + T cells was associated with a significantly lower HIV RNA level in our study, arguing that these cells do not have a similar regulatory effect previously ascribed to suppressor T cells.…”