Signal inhibitory receptor on leukocytes-1 (SIRL-1) negatively regulates the oxidative burst in human phagocytes.

ROS production is an important effector mechanism mediating intracellular killing of microbes by phagocytes. Inappropriate or untimely ROS production can lead to tissue damage, thus tight regulation is essential. We recently characterized signal inhibitory receptor on leukocytes-1 (SIRL-1) as an inhibitory receptor expressed by human phagocytes. Here, we demonstrate that ligation of SIRL-1 dampens Fc receptor-induced ROS production in primary human phagocytes. In accordance, SIRL-1 engagement on these cells impairs the microbicidal activity of neutrophils, without affecting phagocytosis. The inhibition of ROS production may result from reduced ERK activation, since co-ligation of Fc receptors and SIRL-1 on phagocytes inhibited phosphorylation of ERK. Importantly, we demonstrate that microbial and inflammatory stimuli cause rapid downregulation of SIRL-1 expression on the surface of primary neutrophils and monocytes. In accordance, SIRL-1 expression levels on neutrophils in bronchoalveolar lavage fluid from patients with neutrophilic airway inflammation are greatly reduced. We propose that SIRL-1 on phagocytes sets an activation threshold to prevent inappropriate production of oxygen radicals. Upon infection, SIRL-1 expression is downregulated, allowing microbial killing and clearance of the pathogen.

Immune inhibitory receptors: essential regulators of phagocyte function.

Phagocytes, including neutrophils, monocytes, and macrophages, play a crucial role in host defense by recognition and elimination of invading pathogens. Phagocytic cells produce reactive oxygen species (ROS), inflammatory cytokines, and chemokines, leading to bacterial killing and to recruitment and activation of additional immune cells. However, inflammatory mediators are potentially harmful for the host and their production is therefore tightly controlled by multiple regulatory mechanisms. One such mechanism is immune suppression by immune inhibitory receptors, which are increasingly acknowledged as potent regulators of the immune response. So far, research has focused on the role of these receptors in the regulation of NK cells, B cells, and T cells. Importantly, an accumulating number of inhibitory receptors have been identified on phagocytes. Here, we review the role of inhibitory receptors in the regulation of phagocyte cytokine production, migration, apoptosis, ROS production, and phagocytosis. Furthermore, we discuss the intracellular mechanisms utilized by distinct inhibitory receptors to regulate specific phagocyte functions. We demonstrate that inhibitory receptors are important regulators of the immune response, which bacteria can use to their advantage.

Signal inhibitory receptor on leukocytes-1 is a novel functional inhibitory immune receptor expressed on human phagocytes.

Myeloid cells play a crucial role in controlling infection. Activation of these cells needs to be tightly regulated, because their potent effector functions can damage host tissue. Inhibitory receptors expressed by immune cells play an important role in restricting immune cell activation. In this study, we have characterized a hitherto unidentified ITIM-bearing receptor that is highly expressed on human neutrophils and monocytes: signal inhibitory receptor on leukocytes-1 (SIRL-1). The chromosomal location of SIRL-1 is adjacent to the human leukocyte receptor complex on chromosome 19q13.4 and contains two ITIMs in its cytoplasmic tail. As a classical ITIM-bearing receptor, SIRL-1 is capable of inhibiting FcepsilonRI-mediated signaling and can recruit the Src homology 2 domain-containing phosphatases Src homology region 2 domain-containing phosphatases 1 and 2. To investigate the specific involvement of the individual ITIMs in this study, mutational analysis was performed, which revealed that both ITIMs are crucial for SIRL-1 inhibitory function and phosphatase recruitment. When primary cells were stimulated in vitro, SIRL-1(high) monocytes produce less TNF-alpha than SIRL-1(low) monocytes. Thus, SIRL-1 is a novel inhibitory immune receptor belonging to the growing family of ITIM-bearing receptors that is implied in the regulation of phagocytes.

Co-expression of the collagen receptors leukocyte-associated immunoglobulin-like receptor-1 and glycoprotein VI on a subset of megakaryoblasts.

BACKGROUND: The collagen receptor glycoprotein VI generates activating signals through an immunoreceptor tyrosine-based activating motif on the co-associated Fc receptor gamma chain. Leukocyte-associated immunoglobulin-like receptor-1 also ligates collagen but generates inhibitory signals through immunoreceptor tyrosine-based inhibitory motifs. Thus far, the cellular expression of glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1 appears mutually exclusive. DESIGN AND METHODS: Using flow cytometry, we studied expression of collagen receptors on differentiating human megakaryocytes. CD34(+) cells were isolated from umbilical cord blood and matured to megakaryocytes in vitro. Freshly isolated bone marrow cells were used to study primary megakaryocytes. Upon cell sorting, cytospins were made to examine cytological characteristics of differentiation. RESULTS: Megakaryocyte maturation is accompanied by up-regulation of glycoprotein VI and down-regulation of leukocyte-associated immunoglobulin-like receptor-1. Interestingly, both in cultures from hematopoietic stem cells and primary cells obtained directly from bone marrow, we identified a subset of morphologically distinct megakaryocytes which co-express glycoprotein VI and leukocyte-associated immunoglobulin-like receptor-1. CONCLUSIONS: This is the first report of a primary cell that co-expresses these collagen receptors with opposite signaling properties. Since megakaryocytes mature in the collagen-rich environment of the bone marrow, these findings may point to a role for leukocyte-associated immunoglobulin-like receptor-1 in the control of megakaryocyte maturation/migration.

Therapy with 2'-O-Me Phosphorothioate Antisense Oligonucleotides Causes Reversible Proteinuria by Inhibiting Renal Protein Reabsorption.

Antisense oligonucleotide therapy has been reported to be associated with renal injury. Here, the mechanism of reversible proteinuria was investigated by combining clinical, pre-clinical, and in vitro data. Urine samples were obtained from Duchenne muscular dystrophy (DMD) patients treated with drisapersen, a modified 2'O-methyl phosphorothioate antisense oligonucleotide (6 mg/kg). Urine and kidney tissue samples were collected from cynomolgus monkeys (Macaca fascicularis) dosed with drisapersen (39 weeks). Cell viability and protein uptake were evaluated in vitro using human conditionally immortalized proximal tubule epithelial cells (ciPTECs). Oligonucleotide treatment in DMD patients was associated with an increase in urinary alpha-1-microglobulin (A1M), which returned to baseline following treatment interruptions. In monkeys, increased urinary A1M correlated with dose-dependent accumulation of oligonucleotide in kidney tissue without evidence of tubular damage. Furthermore, oligonucleotides accumulated in the lysosomes of ciPTECs and reduced the absorption of A1M, albumin, and receptor-associated protein, but did not affect cell viability when incubated for up to 7 days. In conclusion, phosphorothioate oligonucleotides appear to directly compete for receptor-mediated endocytosis in proximal tubules. We postulate that oligonucleotide-induced low molecular weight proteinuria in patients is therefore a transient functional change and not indicative of tubular damage.

Corrigendum: CD90(+) Stromal Cells are Non-Professional Innate Immune Effectors of the Human Colonic Mucosa.

[This corrects the article on p. 307 in vol. 4, PMID: 24137162.].

CD90(+) Stromal Cells are Non-Professional Innate Immune Effectors of the Human Colonic Mucosa.

Immune responses at the intestinal mucosa must allow for host protection whilst simultaneously avoiding inappropriate inflammation. Although much work has focused on the innate immune functionality of hematopoietic immune cells, non-hematopoietic cell populations - including epithelial and stromal cells - are now recognized as playing a key role in innate defense at this site. In this study we examined the innate immune capacity of primary human intestinal stromal cells (iSCs). CD90(+) iSCs isolated from human colonic mucosa expressed a wide array of innate immune receptors and functionally responded to stimulation with bacterial ligands. iSCs also sensed infection with live Salmonella typhimurium, rapidly expressing IL-1 family cytokines via a RIPK2/p38MAPK-dependent signaling process. In addition to responding to innate immune triggers, primary iSCs exhibited a capacity for bacterial uptake, phagocytosis, and antigen processing, although to a lesser extent than professional APCs. Thus CD90(+) iSCs represent an abundant population of "non-professional" innate immune effector cells of the human colonic mucosa and likely play an important adjunctive role in host defense and immune regulation at this site.

Constitutive, but not challenge-induced, interleukin-10 production is robust in acute pre-pubescent protein and energy deficits: new support for the tolerance hypothesis of malnutrition-associated immune depression based on cytokine production in vivo.

The tolerance model of acute (i.e., wasting) pre-pubescent protein and energy deficits proposes that the immune depression characteristic of these pathologies reflects an intact anti-inflammatory form of immune competence that reduces the risk of autoimmune reactions to catabolically released self antigens. A cornerstone of this proposition is the finding that constitutive (first-tier) interleukin(IL)-10 production is sustained even into the advanced stages of acute malnutrition. The IL-10 response to inflammatory challenge constitutes a second tier of anti-inflammatory regulation and was the focus of this investigation. Weanling mice consumed a complete diet ad libitum, a low-protein diet ad libitum (mimicking incipient kwashiorkor), or the complete diet in restricted daily quantities (mimicking marasmus), and their second-tier IL-10 production was determined both in vitro and in vivo using lipopolysaccharide (LPS) and anti-CD3 as stimulants of innate and adaptive defences, respectively. Both early (3 days) and advanced (14 days) stages of wasting pathology were examined and three main outcomes emerged. First, classic in vitro systems are unreliable for discerning cytokine production in vivo. Secondly, in diverse forms of acute malnutrition declining challenge-induced IL-10 production may provide an early sign that anti-inflammatory control over immune competence is failing. Thirdly, and most fundamentally, the investigation provides new support for the tolerance model of malnutrition-associated inflammatory immune depression.

Effector/memory T cells of the weanling mouse exhibit Type 2 cytokine polarization in vitro and in vivo in the advanced stages of acute energy deficit.

Our objective was to determine whether the polarizing cytokine profile of the effector/memory T-cell compartment reflects the profound decline of cell-mediated inflammatory competence that characterizes acute prepubescent malnutrition. Weanling C57BL/6J mice were permitted free access to a complete purified diet, free access to an isocaloric low-protein diet or restricted intake of the complete diet for 14 days. First, interleukin (IL)-4 and interferon (IFN)-gamma concentrations generated in vitro by splenic and nodal effector/memory T cells were assessed following exposure to plate-bound anti-CD3. Second, net systemic production of IFN-gamma and IL-4 by the effector/memory T-cell compartment was assessed by the in vivo cytokine capture assay following anti-CD3 stimulation. In vitro stimulation generated less IFN-gamma (P=.002) but more IL-4 (P=.05) by T cells from the restricted-intake group relative to the age-matched control group. Similarly, in vivo stimulation generated low serum levels of antibody-captured IFN-gamma in the restricted-intake group vis-à-vis the age-matched control group (P=.01), while the IL-4 response was sustained (P=.39). By contrast, the 14-day low-protein model exhibited no change in T-cell cytokine signature either in vitro or in vivo. However, following extended consumption of the low-protein diet (26 days), carcass energy losses exceeded those of the 14-day protocol and serum levels of in vivo antibody-captured IFN-gamma were low after anti-CD3 challenge relative to the age-matched control group (P=.02), while levels of captured IL-4 remained unaffected (P=.07). Acute weanling malnutrition elicits a Type 2 polarizing cytokine character on the part of the effector/memory T-cell compartment, but only in the most advanced stages of energy decrement.