Phosphorylation of PD-1-Y248 is a marker of PD-1-mediated inhibitory function in human T cells.

PD-1 is a target of cancer immunotherapy but responses are limited to a fraction of patients. Identifying patients with T cells subjected to PD-1-mediated inhibition will allow selection of suitable candidates for PD-1-blocking therapy and will improve the therapeutic success. We sought to develop an approach to detect PD-1-mediated inhibitory signaling. The cytoplasmic tail of PD-1 contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) encompassing Y223 and an immunoreceptor tyrosine-based switch motif (ITSM) encompassing Y248, which is indispensable for interaction of SHP-2 and delivery of PD-1 inhibitory function. We generated an antibody specific for phosphorylated PD-1-Y248 and examined PD-1pY248+ (pPD-1) expression in human T cells. pPD-1 was upregulated by TCR/CD3 + CD28 stimulation and simultaneous PD-1 ligation. pPD-1+CD8+ T cells were identified in human peripheral blood and had impaired effector function. pPD-1+ T cells were also detected in tumor-draining lymph nodes of tumor bearing mice and in biopsies of patients with glioblastoma multiform. Detection of pPD-1+ T cells might serve as a biomarker for identification of T cells subjected to PD-1-mediated immunosuppression.

Regulation of neutrophil functions through inhibitory receptors: an emerging paradigm in health and disease.

Neutrophils are the most abundant subset of leukocytes and play a crucial role in the immune responses against the daily pathogen attacks faced by the host. Neutrophils exhibit several functions for fighting microbes, including the release of granules containing highly toxic molecules, the production of reactive oxygen species and inflammatory cytokines as well as NETosis. Therefore, immune responses mediated by neutrophils must be tightly regulated to protect the host from pathogen assaults without inducing detrimental inflammation and tissue damage. There is now compelling evidence showing that neutrophils express various inhibitory receptors that specifically control their functions. Some of these inhibitory receptors are contained in the membrane of granules and rapidly move to the cell surface upon neutrophil stimulation. This fast upregulation of inhibitory receptors is an efficient way to rapidly enhance inhibitory signals and increase the neutrophil activation threshold. However, because of their ability to attenuate the immune responses of neutrophils, the inhibitory receptors are attractive target for pathogens. This review discusses these various aspects with a particular emphasis on the regulation of neutrophil behavior through immunoreceptor tyrosine-based inhibition motif (ITIM)-bearing inhibitory receptors belonging to LILR and SIGLEC multi-gene families in humans and animal models.

The Biology and Disease Relevance of CD300a, an Inhibitory Receptor for Phosphatidylserine and Phosphatidylethanolamine.

The CD300a inhibitory receptor belongs to the CD300 family of cell surface molecules that regulate a diverse array of immune cell processes. The inhibitory signal of CD300a depends on the phosphorylation of tyrosine residues embedded in ITIMs of the cytoplasmic tail. CD300a is broadly expressed on myeloid and lymphoid cells, and its expression is differentially regulated depending on the cell type. The finding that CD300a recognizes phosphatidylserine and phosphatidylethanolamine, two aminophospholipids exposed on the outer leaflet of dead and activated cells, has shed new light on its role in the modulation of immune functions and in its participation in the host response to several diseases states, such as infectious diseases, cancer, allergy, and chronic inflammatory diseases. This review summarizes the literature on CD300a expression, regulation, signaling pathways, and ligand interaction, as well as its role in fine tuning immune cell functions and its clinical relevance.

IRF-1 regulates alternative mRNA splicing of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) in breast epithelial cells generating an immunoreceptor tyrosine-based inhibition motif (ITIM) containing isoform.

BACKGROUND: Interferon regulatory factor-1 (IRF-1) is a master regulator of IFN-γ induced gene transcription. Previously we have shown that IRF-1 transcriptionally induces CEACAM1 via an ISRE (Interferon-Stimulated Response Element) in its promoter. CEACAM1 pre-mRNA undergoes extensive alternative splicing (AS) generating isoforms to produce either a short (S) cytoplasmic domain expressed primarily in epithelial cells or as an ITIM-containing long (L) isoform in immune cells. METHODS: The transcriptional and molecular mechanism of CEACAM1 minigenes AS containing promoter ISREs mutations in the breast epithelial, MDA-MB-468, cell line was detected using flow cytometry. In addition, transcriptome sequencing was utilized to determine whether IRF-1 could direct the AS of other genes as well. Tumor xenografts were used to evaluate CEACAM1 isoform expression on the leading edge of breast tumor cells. RESULTS: In the present study, we provide evidence that CEACAM1's promoter and variable exon 7 cross-talk allowing IRF-1 to direct AS events. Transcriptome sequencing shows that IRF-1 can also induce the global AS of genes involved in regulation of growth and differentiation as well as genes of the cytokine family. Furthermore, MDA-MB-468 cells grown as tumor xenografts exhibit an AS switch to the L-isoform of CEACAM1, demonstrating that an in vivo inflammatory milieu is also capable of generating the AS switch, similar to that found in human breast cancers Mol Cancer 7:46, 2008. CONCLUSIONS: The novel AS regulatory activities attributed to IRF-1 indicate that the IFN-γ response involves a global change in both gene transcription and AS in breast epithelial cells.

Growth factor receptor-bound protein 2 contributes to (hem)immunoreceptor tyrosine-based activation motif-mediated signaling in platelets.

RATIONALE: Platelets are anuclear cell fragments derived from bone marrow megakaryocytes (MKs) that safeguard vascular integrity but may also cause pathological vessel occlusion. One major pathway of platelet activation is triggered by 2 receptors that signal through an (hem)immunoreceptor tyrosine-based activation motif (ITAM), the activating collagen receptor glycoprotein (GP) VI and the C-type lectin-like receptor 2 (CLEC-2). Growth factor receptor-bound protein 2 (Grb2) is a ubiquitously expressed adapter molecule involved in signaling processes of numerous receptors in different cell types, but its function in platelets and MKs is unknown. OBJECTIVE: We tested the hypothesis that Grb2 is a crucial adapter protein in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets. METHODS AND RESULTS: Here, we show that genetic ablation of Grb2 in MKs and platelets did not interfere with MK differentiation or platelet production. However, Grb2-deficiency severely impaired glycoprotein VI-mediated platelet activation because of defective stabilization of the linker of activated T-cell (LAT) signalosome and activation of downstream signaling proteins that resulted in reduced adhesion, aggregation, and coagulant activity on collagen in vitro. Similarly, CLEC-2-mediated signaling was impaired in Grb2-deficient platelets, whereas the cells responded normally to stimulation of G protein-coupled receptors. In vivo, this selective (hem)immunoreceptor tyrosine-based activation motif signaling defect resulted in prolonged bleeding times but affected arterial thrombus formation only after concomitant treatment with acetylsalicylic acid, indicating that defective glycoprotein VI signaling in the absence of Grb2 can be compensated through thromboxane A2-induced G protein-coupled receptor signaling pathways. CONCLUSIONS: These results reveal an important contribution of Grb2 in (hem)immunoreceptor tyrosine-based activation motif signaling in platelets in hemostasis and thrombosis by stabilizing the LAT signalosome.

Inhibition of TLR signaling by a bacterial protein containing immunoreceptor tyrosine-based inhibitory motifs.

The protein Tir (translocated intimin receptor) in enteric bacteria shares sequence similarity with the host cellular immunoreceptor tyrosine-based inhibition motifs (ITIMs). Despite the importance of Tir in pedestal formation, relatively little is known about the role of Tir and its ITIMs in the regulation of the host immune response. Here we demonstrate that Tir from enteropathogenic Escherichia coli (EPEC) interacted with the host cellular tyrosine phosphatase SHP-1 in an ITIM phosphorylation-dependent manner. The association of Tir with SHP-1 facilitated the recruitment of SHP-1 to the adaptor TRAF6 and inhibited the ubiquitination of TRAF6. Moreover, the ITIMs of Tir suppressed EPEC-stimulated expression of proinflammatory cytokines and inhibited intestinal immunity to infection with Citrobacter rodentium. Our findings identify a previously unknown mechanism by which bacterial ITIM-containing proteins can inhibit innate immune responses.