The killer cell lectin-like receptor G1 (KLRG1) is a unique inhibitory receptor expressed on a phenotypically mature subset of resting NK cells as well as subsets of T cells in naive mice. In vivo, pathogenic immune system activation induces dramatic changes in the expression patterns of KLRG1 among the different cell subsets. In order to enhance our understanding of KLRG1 signaling properties and to clarify the functions of KLRG1 on these cells, we identified the broadly expressed N-cadherin molecule as a ligand for KLRG1. We further demonstrate that a second member of this superfamily of adhesion molecules, E-cadherin, binds to KLRG1. Additionally, we show that upon phosphorylation of the immunoreceptor tyrosine-based inhibitory motif (ITIM) tyrosine, KLRG1 recruits both SHIP-1 and SHP-2 but not SHP-1. We also delineate the key KLRG1 ITIM amino acid residues required for optimal association with these phosphatases. Finally, we demonstrate that KLRG1 engagement can inhibit sub-optimal TCR signaling. Taken together, our results indicate that KLRG1 may differentially regulate NK cell and T cell functions through the association with different ligands as well as the recruitment of distinct phosphatases.

KLRG1 is an inhibitory receptor expressed on a subset of mature T and NK cells. Recently, E-, N-, and R-cadherin have been identified as ligands for KLRG1. Cadherins are a large family of transmembrane or membrane-associated glycoproteins that were thought to only bind specifically to other cadherins to mediate specific cell-to-cell adhesion in a Ca 2؉ -dependent manner. The consequences of cadherin KLRG1 molecular interactions are not well characterized. Here, we report that the first 2 extracellular domains of cadherin are sufficient to initiate a KLRG1-dependent signaling. We also demonstrate that KLRG1 engagement inhibits cadherin-dependent cellular adhesion and influences dendritic cell secretion of inflammatory cytokines, thereby exerting immunosuppressive effects. Consistent with this, engagement of cadherin by KLRG1 molecule induces cadherin tyrosine phosphorylation. Therefore, KLRG1/cadherin interaction leads to the generation of a bidirectional signal in which both KLRG1 and cadherin activate downstream signaling cascades simultaneously. Taken IntroductionEpithelial cadherins (E-cadherins), neural cadherins (N-cadherins), and retinal cadherins (R-cadherins) are part of the classical cadherins. These ubiquitously expressed cell adhesion molecules are a large family of transmembrane or membrane-associated glycoproteins comprising an extracellular domain containing 5 cadherin repeats (EC1-5) responsible for cell-to-cell interactions, a transmembrane domain, and a cytoplasmic domain that is linked to the actin cytoskeleton. Typically, cadherins mediate calciumdependent homophilic adhesion, thereby promoting association of cells expressing the same cadherin family members to form adherens junctions. 1,2 The formation of adherens junctions is dependent on the association of cadherin's cytoplasmic tail with ␤-catenin and its partners. 1 Numerous biologic processes, including homeostasis and embryogenesis, rely on the selective adherence of one adhesion molecule to another through precise intermolecular interactions. 3 The spatiotemporal regulation of cadherin expression and function are vital to tissue morphogenesis, providing a basis for the formation of epithelial layers of the skin and intestine. [4][5][6] Aside from their homophilic adhesion mode, E-, N-, and R-cadherins have been recently reported to bind in a heterophilic manner with killer cell lectin-like receptor G1 (KLRG1). [7][8][9] KLRG1 is a transmembrane inhibitory receptor belonging to the C-type lectin-like superfamily that contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic domain. The molecule was first identified in the rat basophilic leukemia cell line RBL-2H3 and was originally termed mast cell function-associated Ag (MAFA). [10][11][12][13][14][15] In mice and humans, this well-conserved receptor is found on subsets of T and natural killer (NK) cells. [16][17][18][19][20][21][22][23][24] Cells that express KLRG1 include the most mature and recently activated NK cells as well as effector/memory T cells. [...

Checkpoint blockade therapy is effective against many cancers; however, new targets need to be identified to treat patients who do not respond to current treatment or demonstrate immune escape. Here, we showed that blocking the inhibitory receptor Killer cell lectin-like receptor G1 (KLRG1) enhances anti-tumor immunity mediated by NK cells and CD8 + T cells. We found that loss of KLRG1 signaling alone significantly decreased melanoma and breast cancer tumor growth in the lungs of mice. In addition, we demonstrated that KLRG1 blockade can synergize with PD-1 checkpoint therapy to increase the therapeutic efficacy compared to either treatment alone. This effect was even observed with tumors that do not respond to PD-1 checkpoint therapy. Double blockade therapy led to significantly decreased tumor size, increased frequency and activation of CD8 + T cells, and increased NK cell frequency and maturation in the tumor microenvironment. These findings demonstrate that KLRG1 is a novel checkpoint inhibitor target that affects NK and T cell anti-tumor immunity, both alone and in conjunction with established immunotherapies.