Chemoattractant Receptors Research Articles

Neutrophil cell surface receptors and their intracellular signal transduction pathways

Neutrophils play a critical role in the host defense against bacterial and fungal infections, but their inappropriate activation also contributes to tissue damage during autoimmune and inflammatory diseases. Neutrophils express a large number of cell surface receptors for the recognition of pathogen invasion and the inflammatory environment. Those include G-protein-coupled chemokine and chemoattractant receptors, Fc-receptors, adhesion receptors such as selectins/selectin ligands and integrins, various cytokine receptors, as well as innate immune receptors such as Toll-like receptors and C-type lectins. The various cell surface receptors trigger very diverse signal transduction pathways including activation of heterotrimeric and monomeric G-proteins, receptor-induced and store-operated Ca2 + signals, protein and lipid kinases, adapter proteins and cytoskeletal rearrangement. Here we provide an overview of the receptors involved in neutrophil activation and the intracellular signal transduction processes they trigger. This knowledge is crucial for understanding how neutrophils participate in antimicrobial host defense and inflammatory tissue damage and may also point to possible future targets of the pharmacological therapy of neutrophil-mediated autoimmune or inflammatory diseases.

The Loss of Gnai2 and Gnai3 in B Cells Eliminates B Lymphocyte Compartments and Leads to a Hyper-IgM Like Syndrome

B lymphocytes are compartmentalized within lymphoid organs. The organization of these compartments depends upon signaling initiated by G-protein linked chemoattractant receptors. To address the importance of the G-proteins Gαi2 and Gαi3 in chemoattractant signaling we created mice lacking both proteins in their B lymphocytes. While bone marrow B cell development and egress is grossly intact; mucosal sites, splenic marginal zones, and lymph nodes essentially lack B cells. There is a partial block in splenic follicular B cell development and a 50-60% reduction in splenic B cells, yet normal numbers of splenic T cells. The absence of Gαi2 and Gαi3 in B cells profoundly disturbs the architecture of lymphoid organs with loss of B cell compartments in the spleen, thymus, lymph nodes, and gastrointestinal tract. This results in a severe disruption of B cell function and a hyper-IgM like syndrome. Beyond the pro-B cell stage, B cells are refractory to chemokine stimulation, and splenic B cells are poorly responsive to antigen receptor engagement. Gαi2 and Gαi3 are therefore critical for B cell chemoattractant receptor signaling and for normal B cell function. These mice provide a worst case scenario of the consequences of losing chemoattractant receptor signaling in B cells.

42: Signal relay by CCR2 and formylpeptide receptor-2 (Fpr2) in the recruitment of monocyte-derived dendritic cells in allergic airway inflammation

Chemoattractant receptors regulate leukocyte accumulation at sites of inflammation. In allergic airway inflammation, although a chemokine receptor CCR2 was implicated in mediating monocyte-derived dendritic cell (DC) recruitment into the lung, we previously also discovered reduced accumulation of DCs in the inflamed lung in mice deficient in formylpeptide receptor Fpr2 (Fpr2−/−). We therefore investigated the role of Fpr2 in the trafficking of monocyte-derived DCs in allergic airway inflammation in cooperation with CCR2. We report that in allergic airway inflammation, CCR2 mediated the recruitment of monocyte-derived DCs to the perivascular region, and Fpr2 was required for further migration of the cells into bronchiolar area. We additionally found that the broncho-alveolar lavage liquid from mice with airway inflammation contained both the CCR2 ligand CCL2 and an Fpr2 agonist CRAMP. Furthermore, similar to Fpr2−/− mice, in the inflamed airway of CRAMP−/− mice, DC trafficking into the peribronchiolar areas was diminished. Our study demonstrates that the interaction of CCR2 and Fpr2 with their endogenous ligands sequentially mediates the trafficking of DCs within the inflamed lung.

Pannexin 1 Channels Link Chemoattractant Receptor Signaling to Local Excitation and Global Inhibition Responses at the Front and Back of Polarized Neutrophils

Neutrophil chemotaxis requires excitatory signals at the front and inhibitory signals at the back of cells, which regulate cell migration in a chemotactic gradient field. We have previously shown that ATP release via pannexin 1 (PANX1) channels and autocrine stimulation of P2Y2 receptors contribute to the excitatory signals at the front. Here we show that PANX1 also contributes to the inhibitory signals at the back, namely by providing the ligand for A2A adenosine receptors. In resting neutrophils, we found that A2A receptors are uniformly distributed across the cell surface. In polarized cells, A2A receptors redistributed to the back where their stimulation triggered intracellular cAMP accumulation and protein kinase A (PKA) activation, which blocked chemoattractant receptor signaling. Inhibition of PANX1 blocked A2A receptor stimulation and cAMP accumulation in response to formyl peptide receptor stimulation. Treatments that blocked endogenous A2A receptor signaling impaired the polarization and migration of neutrophils in a chemotactic gradient field and resulted in enhanced ERK and p38 MAPK signaling in response to formyl peptide receptor stimulation. These findings suggest that chemoattractant receptors require PANX1 to trigger excitatory and inhibitory signals that synergize to fine-tune chemotactic responses at the front and back of neutrophils. PANX1 channels thus link local excitatory signals to the global inhibitory signals that orchestrate chemotaxis of neutrophils in gradient fields.

Chemotactic responses of peripheral blood eosinophils to prostaglandin D2 in atopic keratoconjunctivitis

BackgroundEosinophils appear to be key cells in the pathogenesis of conjunctival inflammation in atopic keratoconjunctivitis (AKC). Chemoattractant receptor homologous molecule expressed on TH2 cells (CRTH2) mediates prostaglandin D2 (PGD2)–dependent migration of eosinophils. However, it is unclear whether CRTH2/PGD2-dependent eosinophil migration is upregulated in allergic diseases. ObjectiveTo compare the chemotactic responses of peripheral blood eosinophils to prostaglandin D2 in patients with severe AKC and healthy individuals. MethodsWe used an enzyme immunoassay system to measure PGD2 levels in tears and blood samples from healthy individuals and patients with AKC. CRTH2 expression on peripheral blood eosinophils was determined using reverse-transcriptase polymerase chain reaction (RT-PCR), flow cytometry, and an oligonucleotide array system. Chemotaxis experiments were performed using a modified Boyden chamber technique and an optical assay system. ResultsThe PGD2 concentrations were higher in tears from patients with severe AKC compared with healthy individuals. RT-PCR (severe and mild cases), flow cytometry (mild cases), and GeneChip analyses revealed a significantly higher expression of CRTH2 on peripheral blood eosinophils in patients with AKC than in healthy individuals. PGD2 and its stable metabolite 13,14-dihydro-15-keto-PGD2, a CRTH2 agonist, induced chemotaxis of eosinophils from patients with AKC; chemotaxis was significantly enhanced in eosinophils from patients with severe AKC compared with those from healthy individuals. ConclusionCRTH2 is more abundantly expressed on eosinophils from patients with AKC and promoted PGD2-dependent migration to a greater extent than in healthy individuals.

The CRTH2 agonist Pyl A prevents lipopolysaccharide‐induced fetal death but induces preterm labour

We have previously demonstrated that the anti-inflammatory prostaglandin 15-deoxy-Δ 12,14-prostaglandin J(2) (15dPGJ(2)) delays inflammation-induced preterm labour in the mouse and improves pup survival through the inhibition of nuclear factor-κB (NF-κB) by a mechanism yet to be elucidated. 15dPGJ(2) is an agonist of the second prostaglandin D(2) receptor, chemoattractant receptor homologous to the T helper 2 cell (CRTH2). In human T helper cells CRTH2 agonists induce the production of the anti-inflammatory interleukins IL-10 and IL-4. We hypothesized that CRTH2 is involved in the protective effect of 15dPGJ(2) in inflammation-induced preterm labour in the murine model. We therefore studied the effects of a specific small molecule CRTH2 agonist on preterm labour and pup survival. An intrauterine injection of lipopolysaccharide (LPS) was administered to CD1 mice at embryonic day 16, ± CRTH2 agonist/vehicle controls. Mice were killed at 4.5 hr to assess fetal wellbeing and to harvest myometrium and pup brain for analysis of NF-κB, and T helper type 1/2 interleukins. To examine the effects of the CRTH2 agonist on LPS-induced preterm labour, mice were allowed to labour spontaneously. Direct effects of the CRTH2 agonist on uterine contractility were examined ex vivo on contracting myometrial strips. The CRTH2 agonist increased fetal survival from 20 to 100% in LPS-treated mice, and inhibited circular muscle contractility ex vivo. However, it augmented LPS-induced labour and significantly increased myometrial NF-κB, IL-1β, KC-GRO, interferon-γ and tumour necrosis factor-α. This suggests that the action of 15dPGJ(2) is not via CRTH2 and therefore small molecule CRTH2 agonists are not likely to be beneficial for the prevention of inflammation-induced preterm labour.

Excitable behavior in amoeboid chemotaxis

Chemotaxis, the directed motion of cells in response to chemical gradients, is a fundamental process. Eukaryotic cells detect spatial differences in chemoattractant receptor occupancy with high precision and use these differences to bias the location of actin-rich protrusions to guide their movement. Research into chemotaxis has benefitted greatly from a systems biology approach that combines novel experimental and computational tools to pose and test hypotheses. Recently, one such hypothesis has been postulated proposing that chemotaxis in eukaryotic cells is mediated by locally biasing the activity of an underlying excitable system. The excitable system hypothesis can account for a number of cellular behaviors related to chemotaxis, including the stochastic nature of the movement of unstimulated cells, the directional bias imposed by chemoattractant gradients, and the observed spatial and temporal distribution of signaling and cytoskeleton proteins.

Signal Relay by CC Chemokine Receptor 2 (CCR2) and Formylpeptide Receptor 2 (Fpr2) in the Recruitment of Monocyte-derived Dendritic Cells in Allergic Airway Inflammation

Chemoattractant receptors regulate leukocyte accumulation at sites of inflammation. In allergic airway inflammation, although a chemokine receptor CCR2 was implicated in mediating monocyte-derived dendritic cell (DC) recruitment into the lung, we previously also discovered reduced accumulation of DCs in the inflamed lung in mice deficient in formylpeptide receptor Fpr2 (Fpr2(-/-)). We therefore investigated the role of Fpr2 in the trafficking of monocyte-derived DCs in allergic airway inflammation in cooperation with CCR2. We report that in allergic airway inflammation, CCR2 mediated the recruitment of monocyte-derived DCs to the perivascular region, and Fpr2 was required for further migration of the cells into the bronchiolar area. We additionally found that the bronchoalveolar lavage liquid from mice with airway inflammation contained both the CCR2 ligand CCL2 and an Fpr2 agonist CRAMP. Furthermore, similar to Fpr2(-/-) mice, in the inflamed airway of CRAMP(-/-) mice, DC trafficking into the peribronchiolar areas was diminished. Our study demonstrates that the interaction of CCR2 and Fpr2 with their endogenous ligands sequentially mediates the trafficking of DCs within the inflamed lung.

SP0142 Cell Migration in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a progressive autoimmune disease. An early event in inflammation is angiogenesis where new blood vessels invade synovial tissue facilitating a self-perpetuating infiltration of immune cells. This...

Immunonutrition limits pro-inflammatory and pro-oxidant effects of radiochemotherapy in cancer patients.

e17022 Background: Peri-operative immunonutrition improves digestive surgical outcomes of cancer patients reducing complications and length of stay. Radiochemotherapy (RCT) generates deleterious effects on nutritional and immune status limiting functional capacities of patients. This study aimed to evaluate if immunonutrition enriched in arginine, EPA & DHA and nucleotides could limit these side effects in Head&Neck (HN), and oesophageal cancer patients. Methods: A double-blind randomized clinical trial was driven with two groups receiving either an Immune Enteral Nutrition (IEN, n=13, Impact, Nestlé) or a Standardized Enteral Nutrition (SEN, n=15) during all RCT (5-7 weeks). Nutritional status, plasma antioxidant capacity (TEAC, FRAP) and functional capacities (WHO score, Karnofsky index) were assessed at the beginning (Db) and at the end (De) of RCT. Immune functions were assessed by leukocytes phenotyping, ex vivo PMN ROS production, and PBMC cytokines & eicosanoids (PGE2) secretion and gene expression. Results: At De vs Db, a significant improvement in total body weight (+2.1kg) was observed in IEN patients as well as albuminemia and NRI in IEN malnourished patients. TEAC and FRAP were increased (+100µM EqTrolox) in IEN patients and higher at De than for SEN patients. Functional capacities were maintained upper the autonomy threshold only in IEN patients. The PMN density of chemo-attractant receptors (CD62L, CD15) and ROS production (+30%) were significantly increased in IEN patients at De vs Db. PBMC PGE2 secretion was unchanged at De in IEN patients and lesser than for SEN patients (65.9±56.8 vs 106.8±59.3 ng/ml, p<0,05). Folding ratios (De/Db) of PBMC mRNA expression were strongly increased for antioxidant response genes (catalase, gpx1&4, sod1&2) and for bactericidal NOX system (cybA, rac1&2). Conclusions: Immunonutrition in adjunction to RCT contributes to maintain functional capacities of patients through improvement of nutritional, anti-oxidant and immune status. Immunonutrition could provide a higher bactericidal capacity and limit inflammatory and oxidative responses induced by RCT in immune cells of HN and oesophageal cancer patients. Clinical trial information: NCT00333099.

CCRL2, a fringe member of the atypical chemoattractant receptor family

The term atypical chemoattractant receptors is generally used to refer to a subset of G-protein-coupled receptors devoid of chemotactic activity and characterized by the ability to scavenge chemotactic factors from the inflammatory milieu. However, emerging evidence suggests that this class of receptors is heterogeneous in function. In this Viewpoint, we discuss the properties of CCRL2, a molecule devoid of ligand scavenging functions and suggested to regulate leukocyte recruitment by alternative mechanisms.

A new chemoattractant protein found by PBMC screening platform (P6347)

Abstract To find new human gene encoding protein which has potential to be chemokine like or chemokine, our laboratory has established the peripheral blood mononuclear cell (PBMC) chemoattractant platform, which includes four steps. First, we screened out 212 genes potential to be cytokines from the genes library in which the genes’ cytokine-related function was unknown or unreported. And then, we constructed the eukaryotic vectors expressing the candidate genes. And third, the proteins encoded by the candidate genes, which could be secreted, were chosen. At last, the secretive proteins were applied on the PBMC chemoattractant platform to find the protein which had the chemoattractant ability. The PBMC chemoattractant platform was based on the Boyden Chamber Assay. Among the proteins which were potential to be chemokine or chemokine like, one protein was found having the ability to chemoattract peripheral blood monocyte and lymphocyte, which was low expressed in normal tissue and detected in human breast cancer and prostate cancer. What’s more, we have found that CCR2B was the protein’s chemoattractant receptor in vitro. In consideration of CCR2B is normally expressed in lots of cancer sites, the protein which we have found may play some kind role in the procession of cancer development.

A novel CMKLR1 small molecule antagonist suppresses experimental autoimmune demyelinating disease (P5179)

Abstract Experimental autoimmune encephalomyelitis (EAE) is a widely studied mouse model that shares many clinical and histological features with multiple sclerosis (MS). Chemokine-like receptor-1 (CMKLR1) is a chemoattractant receptor that is expressed by key effector leukocytes in EAE and MS, including macrophages, subsets of dendritic cells, and natural killer cells. We previously showed that clinical and histological EAE is less severe in CMKLR1-deficient (CMKLR1 KO) mice compared with wild-type. In this study, we sought to identify small molecular CMKLR1 inhibitors that recapitulate the CMKLR1 KO phenotype in EAE. We identified α-naphthoyl ethyltrimethylammonium iodide (α-NETA) as a selective CMKLR1 small molecule antagonist that inhibits chemerin-stimulated β-arrestin2 association with CMKLR1, as well as chemerin-triggered CMKLR1+ cell migration. Structure-activity relationship studies indicated that the trimethylamine region is critical for CMKLR1 potency. Prophylactic dosing with α-NETA (i.e. prior to the onset of clinical signs) significantly delayed the onset of EAE induced in C57BL/6 mice by active immunization with myelin oligodendrocyte glycoprotein peptide 35-55. In addition, α-NETA treatment significantly reduced mononuclear cell infiltrates within the CNS. This study provides proof-of-concept data that pharmaceutical targeting of CMKLR1:chemerin interactions may be beneficial in preventing or treating MS.

Pathogenic role for chemerin receptor CMKLR1 in experimental pulmonary fibrosis (P5138)

Abstract Pulmonary fibrosis (PF) is a debilitating interstitial lung disease with few treatment options. The pathogenesis of idiopathic PF likely involves recurring microinjuries to the alveolar epithelium, leukocyte infiltration and inflammation, and an aberrant pro-fibrotic wound-healing response. CMKLR1 is a chemoattractant receptor for chemerin that is expressed on NK cells and other immune cells implicated in PF. Our goal was to determine the role of CMKLR1 in the pathogenesis of bleomycin-induced pulmonary inflammation and fibrosis. During the inflammatory phase of the model, neutrophilic infiltration of the airways was significantly increased in CMKLR1-deficient mice compared to wild-type (WT). During the fibrotic phase, CMKLR1-deficient mice developed more severe pulmonary fibrosis than WT as measured by lung collagen content and histopathology. The levels of key pro-inflammatory and pro-fibrotic cytokines (TNFα, IFNγ, IL-10, IL-17, and TGFβ) were significantly elevated in interstitial lung tissue from bleomycin-treated CMKRL1 KO mice. There was also a significant reduction in the recruitment of NK cells into the airways in CMKLR1 KO mice during the fibrotic phase. We hypothesize that chemerin and CMKLR1 play a protective role in lung fibrosis by dampening the initial inflammatory response and by recruiting anti-fibrotic leukocytes. Augmentation of the CMKLR1/chemerin system may therefore provide a novel approach to engage anti-fibrotic leukocytes for PF therapy.

Inhibition of antigen-induced airway inflammation and hyperresponsiveness in guinea pigs by a selective antagonist of “chemoattractant receptor homologous molecule expressed on Th2 cells” (CRTH2)

Chemoattractant receptor homologous molecule expressed on T helper type 2 cells (CRTH2) is a PGD2 receptor found on eosinophils, basophils, and Th2 type T cells which exhibits chemotaxis and functions in activation cascades. However, while a number of CRTH2 antagonists, including ramatroban, are known to exert activity in certain animal models, activity in a guinea pig model of EA-induced airway hyperresponsiveness has not been demonstrated. The newly developed CRTH2 antagonist ASP5642 has shown antagonistic activity against human and guinea pig CRTH2 in previous studies and has also been found effective in treating guinea pig models of airway inflammation and airway hyperresponsiveness. While previous studies have used animals such as rats and mice to evaluate CRTH2 antagonist effects, ours is the first attempt to evaluate CRTH2 function in a guinea pig asthma model, which may prove useful in evaluating the compound’s effects in humans, given the comparable airway function between the two species taken together, these data from the present study strongly suggest the utility of ASP5642 in investigating the role of CRTH2 in inflammatory responses and as a drug treatment for human asthma.

V101L of human formyl peptide receptor 1 (FPR1) increases receptor affinity and augments the antagonism mediated by cyclosporins

Genetic variation plays a major role in drug response variability. CsA (cyclosporin A), a widely used immunosuppressive agent, is a specific antagonist for FPR1 (formyl peptide receptor 1), which is an important G-protein-coupled chemoattractant receptor in the innate immune system. In order to study the variable responses of cyclosporins to different FPR1 mutants, we investigated the distribution of human FPR1 haplotypes among 209 healthy Han Chinese subjects. The haplotype pattern in Han Chinese were characterized on the basis of five SNPs (single nucleotide polymorphisms), including rs5030878 (p.T11I), rs2070745 (p.V101L), rs5030880 (p.R190W), rs1042229 (p.N192K) and rs867228 (p.A346E). Receptor binding affinity of cyclosporins to FPR1 haplotypes was assessed using N-formyl-Nle-Leu-Phe-Nle-Tyr-Lys–FITC in CHO-Gα16 cells stably transfected with cDNAs encoding the top 12 FPR1 haplotypes in the Han Chinese. Variants of FPR1 carrying a single amino acid substitution of leucine for valine at position 101 (p.Leu101) displayed significantly higher pKi values for CsA and CsH (cyclosporin H), indicative of an improved receptor affinity. The polymorphism of FPR1 p.Leu101 also enhanced the inhibitory effects of cyclosporins on fMLF (N-formyl-methionyl-leucyl-phenylalanine)-induced activities, including calcium mobilization, cell chemotaxis and MAPK (mitogen-activated protein kinase) phosphorylation. These results point to a possible complication for clinical use of CsA in patients carrying the p.Leu101 allele of FPR1.

Reactivation of Desensitized Formyl Peptide Receptors by Platelet Activating Factor: A Novel Receptor Cross Talk Mechanism Regulating Neutrophil Superoxide Anion Production

Neutrophils express different chemoattractant receptors of importance for guiding the cells from the blood stream to sites of inflammation. These receptors communicate with one another, a cross talk manifested as hierarchical, heterologous receptor desensitization. We describe a new receptor cross talk mechanism, by which desensitized formyl peptide receptors (FPRdes) can be reactivated. FPR desensitization is induced through binding of specific FPR agonists and is reached after a short period of active signaling. The mechanism that transfers the receptor to a non-signaling desensitized state is not known, and a signaling pathway has so far not been described, that transfers FPRdes back to an active signaling state. The reactivation signal was generated by PAF stimulation of its receptor (PAFR) and the cross talk was uni-directional. LatrunculinA, an inhibitor of actin polymerization, induced a similar reactivation of FPRdes as PAF while the phosphatase inhibitor CalyculinA inhibited reactivation, suggesting a role for the actin cytoskeleton in receptor desensitization and reactivation. The activated PAFR could, however, reactivate FPRdes also when the cytoskeleton was disrupted prior to activation. The receptor cross talk model presented prophesies that the contact on the inner leaflet of the plasma membrane that blocks signaling between the G-protein and the FPR is not a point of no return; the receptor cross-talk from the PAFRs to the FPRdes initiates an actin-independent signaling pathway that turns desensitized receptors back to a signaling state. This represents a novel mechanism for amplification of neutrophil production of reactive oxygen species.

Convolution of chemoattractant secretion rate, source density, and receptor desensitization direct diverse migration patterns in leukocytes

Chemoattractants regulate diverse immunological, developmental, and pathological processes, but how cell migration patterns are shaped by attractant production in tissues remains incompletely understood. Using computational modeling and chemokine-releasing microspheres (CRMs), cell-sized attractant-releasing beads, we analyzed leukocyte migration in physiologic gradients of CCL21 or CCL19 produced by beads embedded in 3D collagen gels. Individual T-cells that migrated into contact with CRMs exhibited characteristic highly directional migration to attractant sources independent of their starting position in the gradient (and thus independent of initial gradient strength experienced) but the fraction of responding cells was highly sensitive to position in the gradient. These responses were consistent with modeling calculations assuming a threshold absolute difference in receptor occupancy across individual cells of ~10 receptors required to stimulate chemotaxis. In sustained gradients eliciting low receptor desensitization, attracted T-cells or dendritic cells swarmed around isolated CRMs for hours. With increasing CRM density, overlapping gradients and high attractant concentrations caused a transition from local swarming to transient "hopping" of cells bead to bead. Thus, diverse migration responses observed in vivo may be determined by chemoattractant source density and secretion rate, which govern receptor occupancy patterns in nearby cells.

G Protein Coupled Chemoattractant Receptor Modulates the Diffusion and Membrane Binding of Activated Gα via their Association

Recent studies of G protein signaling in chemotaxis of Dictyostelium discoideum cells have revealed that G protein molecules diffuse more rapidly than their receptor, and dissociate from the membrane into the cytoplasm. Rapid diffusion of G protein on membrane and in cytoplasm would disperse its spatial information. Some specific mechanisms to prevent G protein dispersion are necessary for retaining and transmitting spatial information about a chemical gradient. To reveal the mechanisms for spatial information retention, we applied single-molecule imaging analysis to a chemoattractant receptor and its G proteins during chemotactic signaling in Dictyostellium cells. Intracellular G protein dynamics such as lateral diffusion and resident time on the membrane was observed upon chemoattractant stimulations. Before stimulation, the chemoattractant receptor and heterotrimeric G-protein exhibited lateral diffusion on membrane in a different manner in which the diffusion of the G protein is much faster than that of the receptor, demonstrating an absence of stable precoupling complexes between them. After chemoattractant stimulations, however, a substantial fraction of Gα subunits showed slowing in the diffusion with a diffusion coefficient that equaled that of the receptor. Dose dependency of the slowing response was about 300 nM chemoattractant, which corresponds to the chemoattractant-binding affinity of the receptor, suggesting a complex formation between activated Gα and the chemoattractant-bound receptor. The complex formation caused the increase in the membrane resident time of individual active Gα subunits and therefore the accumulation of Gα at the membrane region that is stimulated by a focal chemoattractant. Such a mechanism positively retains the gradient signals from the receptor leading to the signal transmission into the downstream pathways.

Differential chemotactic receptor requirements for NK cell subset trafficking into bone marrow

Responsiveness of maturing natural killer (NK) cells to chemotactic molecules directly affect their retention and relocation in selected bone marrow (BM) microenvironment during development, as well as their localization at sites of immune response during inflammatory diseases. BM is the main site of NK cell generation, providing microenvironmental signals required to sustain cell proliferation and differentiation. Drastic changes of expression and function of several chemoattractant receptors can be observed during progression from precursor NK cells to immature and mature NK cells. Indeed, the gradual decrease of CXCR4 expression parallels the increased expression of CXCR3, CCR1, and CX3CR1 and S1P5 (Sphingosine-1-phosphate receptor 5) on mature DX5+ NK cells. The chemokine CXCL12 is produced constitutively in the BM and, acting via CXCR4, is critical for retaining immature and mature NK cell subsets in the BM. During steady-state, the maintenance of NK cells into BM parenchyma depends on the equilibrium of CXCR4 retention and S1P5 mobilizing functions, as the gradient of S1P coming from the sinusoids facilitates mature NK cell egress into circulation via S1P5, when CXCR4/CXCL12-mediated retention decreases. Chemoattractants are also key factors for the response to inflammatory or infection conditions that promote mobilization of effector NK cells from storage compartments (including BM) to sites of disease or for NK cell recruitment/response during pathological conditions that affect BM integrity, including hematopoietic malignancies. In this review, we summarize what is known about the requirement for NK cell localization and exit from BM and how chemokine-mediated functions may affect BM NK cell development and immune responses.