IntroductionHuman cytomegalovirus (HCMV) infection generally follows a subclinical course, but may lead to severe disorders in immunocompromised individuals and is a main cause of infectious congenital diseases. HCMV remains latent in immunocompetent hosts, undergoing occasional reactivation. 1 Studies in murine models revealed that an effective defense against CMV requires the participation of natural killer (NK) and T cells. 2,3 Detection of antibodies and CD8 ϩ T lymphocytes specific for HCMV antigens allow an assessment of the adaptive immune response to the pathogen. 4,5 To escape from CD8 ϩ T cells, HCMV inhibits the expression of human leukocyte antigen (HLA) class I molecules and interferes with antigen presentation using a set of glycoproteins (US2, US3, US6, US10, and US11) whose genes are clustered within the unique short (US) region of the virus genome. [6][7][8] The loss of HLA class I molecules in HCMV-infected cells impairs the engagement of inhibitory receptors and prompts the activation of NK cell effector functions; reciprocally, the virus has developed several strategies to evade NK-mediated recognition. 9 The nature of receptor-ligand interactions involved in the NK cell response to CMV-infected cells is incompletely understood. In strains of mice expressing the Ly49H receptor, NK cell functions are triggered upon recognition of the m157 mouse CMV (MCMV) glycoprotein, becoming essential to control replication; 10,11 by contrast, human activating NK cell receptors (NKRs) specific for HCMV molecules have not been identified. The involvement of activating killer immunoglobulin (Ig)-like receptors (KIRs) and natural cytotoxicity receptors (NCRs; ie, NKp46, NKp30, and NKp44) in the response to HCMV is uncertain. The interaction of the pp65 HCMV tegument protein with NKp30 has been reported to inhibit rather than to activate NK cell functions. 12 The ability of the UL16 HCMV molecule to interfere with the surface expression of NKG2D ligands, [13][14][15] and the evidence that similar evasion mechanisms operate in MCMV infection, 16,17 support an important role for this killer lectinlike receptor (KLR) in the antiviral defense. 18 Recently, the UL141 HCMV molecule has been shown to inhibit the expression in infected cells of CD155, a ligand for the DNAM-1 stimulating receptor. 19 HCMV may also escape NK-mediated surveillance by keeping inhibitory receptors for HLA class I molecules engaged. The viral UL18 molecule binds with high affinity to the ILT2 (CD85j) inhibitory receptor, 20,21 though its role in immune evasion has not been precisely elucidated. 9 On the other hand, HLA-E appears constitutively resistant to the action of US2 and US11, 22 and it also becomes refractory to the action of US6 when bound to a peptide from the leader sequence of the HCMV UL40 protein. 23 For personal use only. on May 10, 2018. by guest www.bloodjournal.org From interfere with the NK cell response by engaging the inhibitory CD94/NKG2A KLR. 25 We recently reported 26 that healthy HCMV-seropositive individuals display...

Using a three-hybrid strategy, we have identified a novel cell surface molecule which interacts with the Src homology 2 (SH2) domains of SH2 domain-containing protein tyrosine phosphatase 1 (SHP-1), termed "immune receptor expressed on myeloid cells 1" (IREM-1). The full-length cDNA coding for a polypeptide of 290 amino acids presents an extracellular single V-type Ig domain, a transmembrane region and a cytoplasmic tail with five tyrosine residues, two of which are in the context of an immunoreceptor tyrosine-based inhibitory motif. Moreover, cDNA encoding for three other splicing forms of IREM-1, named IREM-1 splice variant (Sv)1, Sv2 and Sv3 were cloned by reverse transcription (RT)-PCR. The gene encoding for IREM-1 contains nine exons, is located on human chromosome 17 (17q25.1) and is homologous to previously identified molecules termed CMRF-35 and IRp60. RT-PCR, northern blot and FACS analysis with specific monoclonal antibodies indicated that IREM-1 is expressed on monocytes, granulocytes, and myeloid leukemia cell lines. Western blot analysis confirmed the recruitment of SHP-1 to IREM-1 and demonstrated that phosphotyrosine residue 205 is the main docking site for this interaction. Finally, crosslinking of IREM-1 results in the inhibition of FcRe-induced activation. Our results indicate that IREM-1 is a novel inhibitory receptor of the Ig superfamily in myeloid cells.

The CD300e surface molecule, originally termed immune receptor expressed by myeloid cells (IREM)-2, was reported to associate with the DNAX-activating protein (DAP) 12 adaptor in co-transfected cells, and is capable of signaling. In the present report, we investigated in detail the function of CD300e in monocytes and myeloid DC (mDC) freshly isolated from peripheral blood of normal blood donors. Upon engagement by an agonistic mAb, CD300e triggered an intracellular calcium mobilization and superoxide anion O À 2 production in monocytes. Activation via CD300e provided survival signals that prevented monocyte and mDC apoptosis, triggered the production of pro-inflammatory cytokines and upregulated the expression of cell surface co-stimulatory molecules in both cell types. Moreover, CD300e activation of mDC enhanced the alloreactive response of naive T cells. Overall, our data formally support the notion that CD300e functions as an activating receptor capable of regulating the innate immune response in myeloid cells.Key words: Cell activation . Cell surface molecules . DC . Monocytes IntroductionA number of surface molecules expressed by myeloid cells are members of multigenic families that include both activating and inhibitory receptors [1,2], which contribute to establish a balance of opposite signals that control the initiation, amplitude and duration of the cell response. Activating receptors have a short cytoplasmic tail with a positively charged amino acid residue within their transmembrane region that allows their association with ITAM-bearing adaptors (e.g. DNAX-activating protein (DAP) 12, FceRIg or CD3z) [3]. Upon ligand recognition and receptor clustering, ITAM become tyrosine phosphorylated and serve as docking sites for Src homology type 2 domaincontaining protein tyrosine kinases such as ZAP-70 or Syk [4,5]. Recruitment and activation of protein tyrosine kinases and downstream effectors regulate calcium mobilization, transcriptional activation, cytokine production, migration, proliferation and/or differentiation [6]. In contrast, inhibitory receptors display a longer cytoplasmic tail characterized by the presence of ITIM. Ligand-induced clustering results in tyrosine phosphorylation of ITIM that act as docking sites for SHP-1, SHP-2 or SHIP. Upon recruitment, tyrosine phosphatases become activated and dephosphorylate key signaling mediators of activation pathways such as Syk, LAT, BLNK/SLP-76, Vav, PI3K and cytoskeletal structures, consequently downregulating the signaling cascade [6][7][8].Ã These authors share equal credit for senior authorship. 722The CD300 or immune receptor expressed by myeloid celsl (IREM) family of myeloid-associated receptors consists of at least five surface molecules that are encoded by genes located on human chromosome 17 (17q25) [9]. CD300c (CMRF-35) was the first identified but has been thus far poorly characterized [10,11]. CD300a (IRp60) was shown to associate with SHP-1 and SHP-2, delivering inhibitory signals in human NK cells, mast cells, eosinophils and granulocyt...