Frank Petersen

The β‐thromboglobulins and platelet factor 4: blood platelet‐derived CXC chemokines with divergent roles in early neutrophil regulation

The recruitment of neutrophil granulocytes to sites of tissue injury is one of the earliest events during host defense. Several chemotactic cytokines belonging to the CXC subfamily of chemokines are thought to be implicated in this kind of response. Especially those CXC chemokines that are stored in blood platelets and become immediately released upon activation are likely to dominate neutrophil‐dependent host defense at the onset of inflammation. The major platelet‐derived CXC chemokines are the β‐thromboglobulins and platelet factor 4 (PF‐4), which are both released into the blood at micromolar concentrations. The availability as well as the functional activity of these mediators appear to be subject to tight control by diverse regulatory mechanisms. These include proteolytic processing of chemokine precursors, oligomer formation, and the differential usage of neutrophil‐expressed receptors. Herein we review our work on early neutrophil regulation by PF‐4, the β‐thromboglobulin neutrophil‐activating peptide 2 (NAP‐2) and its major precursor connective tissue‐activating peptide III (CTAP‐III). We moreover propose a model to assess the contribution by either of these chemokines to coordinated recruitment and activation of neutrophils in response to acute tissue injury. J. Leukoc. Biol. 67: 471–478; 2000.

Platelet‐derived CXC chemokines: old players in new games

Although platelet factor 4 (PF-4) and the beta-thromboglobulin (beta-TG) proteins represent the first chemokines to be discovered, their functional roles in host defense became clear only recently. Residing in platelets as storage proteins and becoming released into the blood at very high concentrations, these mediators appear to fulfill different and complementary tasks as first-line mediators in the recruitment and activation of leukocytes, as well in the regulation of tissue repair. Whereas both proteins are structurally closely related members of the CXC chemokine subfamily, they are subject to quite dissimilar regulatory mechanisms controlling their generation and their spectrum of biological activities. Thus, proteolytic processing of inactive precursors plays a decisive role in whether the beta-TG proteins will act as stimulatory or inhibitory agents in neutrophil activation via the G protein-coupled receptors CXCR-1 and 2. PF-4, existing as a single molecular form, is largely resistant to proteolytic modification, but its interaction with an unusual receptor(s) on leukocytes (a proteoglycan) appears to depend on its oligomeric state. There is growing evidence that both chemokines may interfere with each other at various regulatory levels to promote coordinated cell activation. Moreover, recent findings suggest novel and unexpected activities for these chemokines, which may extend our view on early host defense.

Platelet-derived chemokines in vascular biology

Undoubtedly, platelets are key elements in the regulation of thrombosis and haemostasis. Along with their primary task to prevent blood loss from injured vessels, platelets have emerged as regulators of a variety of processes in the vasculature. Multiple challenges, from the contact and adhesion to subendothelial matrix after injury of the vessel wall, to interactions with blood cells in inflammatory conditions, result in platelet activation with concomitant shape change and release of numerous substances. Among these, chemokines have been found to modulate several processes in the vasculature, such as atherosclerosis and angiogenesis. In particular, the chemokines connective tissue activating protein III (CTAP-III) and its precursors, or truncation products (CXCL7), platelet factor 4, (PF4, CXCL4) and its variant PF4alt (CXCL4L1) or regulated upon activation and normal T cell expressed and secreted (RANTES, CCL5), have been investigated thoroughly. Defined common properties as their aptitude to bind glycosaminoglycans or their predisposition to associate and form homooligomers are pre-requisites for their role in the vasculature and function in vivo. The current review summarizes the development of these single chemokines, and their cooperative effects that may in part be dependent on their physical interactions.

Extracellular ATP Induces Cytokine Expression and Apoptosis through P2X7 Receptor in Murine Mast Cells

Extracellular ATP and other nucleotides act through specific cell surface receptors and regulate a wide variety of cellular responses in many cell types and tissues. In this study, we demonstrate that murine mast cells express several P2Y and P2X receptor subtypes including P2X7, and describe functional responses of these cells to extracellular ATP. Stimulation of bone marrow-derived mast cells (BMMC), as well as MC/9 and P815 mast cell lines with millimolar concentrations of ATP, resulted in Ca2+ influx across the cellular membrane and cell permeabilization. Moreover, brief exposures to ATP were sufficient to induce apoptosis in BMMCs, MC/9, and P815 cells which involved activation of caspase-3 and -8. However, in the time period between commitment to apoptosis and actual cell death, ATP triggered rapid but transient phosphorylation of multiple signaling molecules in BMMCs and MC/9 cells, including ERK, Jak2, and STAT6. In addition, ATP stimulation enhanced the expression of several proinflammatory cytokines, such as IL-4, IL-6, IL-13, and TNF-α. The effects of ATP were mimicked by submillimolar concentrations of 3-O-(4′-benzoyl)-benzoyl-benzoyl-ATP, and were inhibited by pretreatment of mast cells with a selective blocker of human and mouse P2X7 receptor, 1[N,O-bis(5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine, as well as oxidized ATP. The nucleotide selectivity and pharmacological profile data support the role for P2X7 receptor as the mediator of the ATP-induced responses. Given the importance of mast cells in diverse pathological conditions, the ability of extracellular ATP to induce the P2X7-mediated apoptosis in these cells may facilitate the development of new strategies to modulate mast cell activities.

Characterization of a Neutrophil Cell Surface Glycosaminoglycan That Mediates Binding of Platelet Factor 4

Platelet factor 4 (PF-4) is a platelet-derived α-chemokine that binds to and activates human neutrophils to undergo specific functions like exocytosis or adhesion. PF-4 binding has been shown to be independent of interleukin-8 receptors and could be inhibited by soluble chondroitin sulfate type glycosaminoglycans or by pretreatment of cells with chondroitinase ABC. Here we present evidence that surface-expressed neutrophil glycosaminoglycans are of chondroitin sulfate type and that this species binds to the tetrameric form of PF-4. The glycosaminoglycans consist of a single type of chain with an average molecular mass of ∼23 kDa and are composed of ∼85–90% chondroitin 4-sulfate disaccharide units type CSA (→4GlcAβ1→3GalNAc(4-O-sulfate)β1→) and of ∼10–15% di-O-sulfated disaccharide units. A major part of these di-O-sulfated disaccharide units are CSE units (→4GlcAβ1→3GalNAc(4,6-O-sulfate)β1→). Binding studies revealed that the interaction of chondroitin sulfate with PF-4 required at least 20 monosaccharide units for significant binding. The di-O-sulfated disaccharide units in neutrophil glycosaminoglycans clearly promoted the affinity to PF-4, which showed a K d ∼ 0.8 μm, as the affinities of bovine cartilage chondroitin sulfate A, porcine skin dermatan sulfate, or bovine cartilage chondroitin sulfate C, all consisting exclusively of monosulfated disaccharide units, were found to be 3–5-fold lower. Taken together, our data indicate that chondroitin sulfate chains function as physiologically relevant binding sites for PF-4 on neutrophils and that the affinity of these chains for PF-4 is controlled by their degree of sulfation.

Platelet Factor 4 Inhibits Proliferation and Cytokine Release of Activated Human T Cells

Platelet factor 4 (PF-4), a platelet-derived CXC chemokine, has been shown to induce the differentiation of monocytes into a subset of macrophages that lack the expression of HLA-DR Ag. This suggests a potential role for PF-4 in the modulation of monocyte-dependent T cell activation. Using an Ag-specific stimulation model in which T cells were cocultured with monocytes in the presence of recall Ags, we could show that under these conditions PF-4-treatment caused a strong decrease of T cell proliferation as well as of IFN-γ release. However, inhibition of T cell functions such as proliferation, IL-2 release, and IL-2 mRNA production did also occur when isolated T cells were activated in the absence of monocytes with immobilized Abs directed against CD3 in combination with cross-linked anti-CD28 Abs. The effect could be reversed when low concentrations of exogenous IL-2 instead of anti-CD28 were used as a costimulus in combination with anti-CD3 Abs. Further evidence for direct modulation of T cell function by PF-4 was obtained by the detection of specific binding sites for the chemokine on the surface of these cells. Taken together, our results show that specific binding of PF-4, resulting in the down-regulation of the IL-2-release correlates with the inhibition of functions in activated T cells.

Platelet Factor 4/CXCL4 Induces Phagocytosis and the Generation of Reactive Oxygen Metabolites in Mononuclear Phagocytes Independently of Gi Protein Activation or Intracellular Calcium Transients

Platelet factor 4 (PF-4), a platelet-derived CXC chemokine, is known to prevent human monocytes from apoptosis and to promote differentiation of these cells into HLA-DR− macrophages. In this study, we investigated the role of PF-4 in the control of acute monocyte proinflammatory responses involved in the direct combat of microbial invaders. We show that PF-4 increases monocyte phagocytosis and provokes a strong formation of oxygen radicals but lacks a chemotactic activity in these cells. Compared with FMLP, PF-4-induced oxidative burst was later in its onset but was remarkably longer in its duration (lasting for up to 60 min). Furthermore, in PF-4-prestimulated cells, FMLP- as well as RANTES-induced burst responses became synergistically enhanced. As we could show, PF-4-mediated oxidative burst in monocytes does not involve Gi proteins, elevation of intracellular free calcium concentrations, or binding to CXCR3B, a novel PF-4 receptor recently discovered on endothelial cells. Moreover, we found that PF-4 acts on macrophages in a dual manner. On the one hand, very similar to GM-CSF or M-CSF, PF-4 treatment of monocytes generates macrophages with a high capacity for unspecific phagocytosis. On the other hand, short term priming of GM-CSF-induced human macrophages with PF-4 substantially increases their capability for particle ingestion and oxidative burst. A comparable effect was also observed in murine bone marrow-derived macrophages, indicating cross-reactivity of human PF-4 between both species. Taken together, PF-4 may play a crucial role in the induction and maintenance of an unspecific immune response.

Platelet-Derived Chemokines CXC Chemokine Ligand (CXCL)7, Connective Tissue-Activating Peptide III, and CXCL4 Differentially Affect and Cross-Regulate Neutrophil Adhesion and Transendothelial Migration

In this study, we have examined the major platelet-derived CXC chemokines connective tissue-activating peptide III (CTAP-III), its truncation product neutrophil-activating peptide 2 (CXC chemokine ligand 7 (CXCL7)), as well as the structurally related platelet factor 4 (CXCL4) for their impact on neutrophil adhesion to and transmigration through unstimulated vascular endothelium. Using monolayers of cultured HUVEC, we found all three chemokines to promote neutrophil adhesion, while only CXCL7 induced transmigration. Induction of cell adhesion following exposure to CTAP-III, a molecule to date described to lack neutrophil-stimulating capacity, depended on proteolytical conversion of the inactive chemokine into CXCL7 by neutrophils. This was evident from experiments in which inhibition of the CTAP-III-processing protease and simultaneous blockade of the CXCL7 high affinity receptor CXCR-2 led to complete abrogation of CTAP-III-mediated neutrophil adhesion. CXCL4 at substimulatory dosages modulated CTAP-III- as well as CXCL7-induced adhesion. Although cell adhesion following exposure to CTAP-III was drastically reduced, CXCL7-mediated adhesion underwent significant enhancement. Transendothelial migration of neutrophils in response to CXCL7 or IL-8 (CXCL8) was subject to modulation by CTAP-III, but not CXCL4, as seen by drastic desensitization of the migratory response of neutrophils pre-exposed to CTAP-III, which was paralleled by selective down-modulation of CXCR-2. Altogether our results demonstrate that there exist multiple interactions between platelet-derived chemokines in the regulation of neutrophil adhesion and transendothelial migration.

Meta-analysis reveals an association of PTPN22 C1858T with autoimmune diseases, which depends on the localization of the affected tissue

Protein tyrosine phosphatase non-receptor type 22 (PTPN22) is a strong susceptibility gene shared by many autoimmune diseases. The aim of this study was to explore the mechanisms underlying this relationship. We performed a comprehensive analysis of the association between PTPN22 polymorphism C1858T and autoimmune diseases. The results showed a remarkable pattern; PTPN22 C1858T was strongly associated with type I diabetes, rheumatoid arthritis, immune thrombocytopenia, generalized vitiligo with concomitant autoimmune diseases, idiopathic inflammatory myopathies, Graves’ disease, juvenile idiopathic arthritis, myasthenia gravis, systemic lupus erythematosus, anti-neutrophil cytoplasmic antibody-associated vasculitis and Addison’s disease. By contrast, PTPN22 C1858T showed a negligible association with systemic sclerosis, celiac disease, multiple sclerosis, psoriasis, ankylosing spondylitis, pemphigus vulgaris, ulcerative colitis, primary sclerosing cholangitis, primary biliary cirrhosis, Crohn’s disease and acute anterior uveitis. Further analysis revealed a clear distinction between the two groups of diseases with regard to their targeted tissues: most autoimmune diseases showing an insignificant association with PTPN22 C1858T manifest in skin, the gastrointestinal tract or in immune privileged sites. These results showed that the association of PTPN22 polymorphism with autoimmune diseases depends on the localization of the affected tissue, suggesting a role of targeted organ variation in the disease manifestations.

Platelet factor 4 in conjunction with IL-4 directs differentiation of human monocytes into specialized antigen-presenting cells

Recent evidence suggests that platelets are not only involved in haemostatic processes but also modulate immune responses. As antigen‐presenting cells (APC) are of crucial importance for the regulation of immunity, in this study we wanted to define the role of platelet factor 4 (PF‐4) as one of the major platelet‐derived chemokines on the transition of monocytes into APCs. Our experiments show that within 3 days PF‐4 in conjunction with IL‐4 induces a rapid differentiation of monocytes into APC. These PFAPC (PF‐4/IL‐4 differentiated APC) display unique phenotypical and functional characteristics setting them apart from macrophages and conventional dendritic cells. Functional studies revealed that PFAPC preferentially stimulated proliferation of lymphocytes and lytic NK activity while they induced only moderate cytokine responses. Beyond day 3 of differentiation, PFAPC became less immunostimulatory and maintained their capacity to phagocytose particulate material even after LPS‐induced maturation. These experiments uncover a previously unknown role for the platelet‐derived CXC‐chemokine PF‐4 in differentiation of human APC. Our data further support the newly discovered function of platelets in immunomodulation and provide new evidence for a rapid transition of monocytes into APC under the influence of inflammatory stimuli.