Claudio Costantini

Neutrophils in the activation and regulation of innate and adaptive immunity

Neutrophils have long been viewed as the final effector cells of an acute inflammatory response, with a primary role in the clearance of extracellular pathogens. However, more recent evidence has extended the functions of these cells. The newly discovered repertoire of effector molecules in the neutrophil armamentarium includes a broad array of cytokines, extracellular traps and effector molecules of the humoral arm of the innate immune system. In addition, neutrophils are involved in the activation, regulation and effector functions of innate and adaptive immune cells. Accordingly, neutrophils have a crucial role in the pathogenesis of a broad range of diseases, including infections caused by intracellular pathogens, autoimmunity, chronic inflammation and cancer.

Evidence for a cross-talk between human neutrophils and Th17 cells

Interleukin-17A (IL-17A) and IL-17F are 2 of several cytokines produced by T helper 17 cells (Th17), which are able to indirectly induce the recruitment of neutrophils. Recently, human Th17 cells have been phenotypically characterized and shown to express discrete chemokine receptors, including CCR2 and CCR6. Herein, we show that highly purified neutrophils cultured with interferon-gamma plus lipopolysaccharide produce the CCL2 and CCL20 chemokines, the known ligands of CCR2 and CCR6, respectively. Accordingly, supernatants from activated neutrophils induced chemotaxis of Th17 cells, which was greatly suppressed by anti-CCL20 and anti-CCL2 antibodies. We also discovered that activated Th17 cells could directly chemoattract neutrophils via the release of biologically active CXCL8. Consistent with this reciprocal recruitment, neutrophils and Th17 cells were found in gut tissue from Crohn disease and synovial fluid from rheumatoid arthritis patients. Finally, we report that, although human Th17 cells can directly interact with freshly isolated or preactivated neutrophils via granulocyte-macrophage colony-stimulating factor, tumor necrosis factor-alpha, and interferon-gamma release, these latter cells cannot be activated by IL-17A and IL-17F, because of their lack of IL-17RC expression. Collectively, our results reveal a novel chemokine-dependent reciprocal cross-talk between neutrophils and Th17 cells, which may represent a useful target for the treatment of chronic inflammatory diseases.

An aging pathway controls the TrkA to p75NTR receptor switch and amyloid β‐peptide generation

Aging of the brain is characterized by marked changes in the expression levels of the neurotrophin receptors, TrkA and p75NTR. An expression pattern in which TrkA predominates in younger animals switches to one in which p75NTR predominates in older animals. This TrkA‐to‐p75NTR switch is accompanied by activation of the second messenger ceramide, stabilization of β‐site amyloid precursor protein‐cleaving enzyme‐1 (BACE1), and increased production of amyloid β‐peptide (Aβ). Here, we show that the insulin‐like growth factor‐1 receptor (IGF1‐R), the common regulator of lifespan and age‐related events in many different organisms, is responsible for the TrkA‐to‐p75NTR switch in both human neuroblastoma cell lines and primary neurons from mouse brain. The signaling pathway that controls the level of TrkA and p75NTR downstream of the IGF1‐R requires IRS2, PIP3/Akt, and is under the control of PTEN and p44, the short isoform of p53. We also show that hyperactivation of IGF1‐R signaling in p44 transgenic animals, which show an accelerated form of aging, is characterized by early TrkA‐to‐p75NTR switch and increased production of Aβ in the brain.

A reversible form of lysine acetylation in the ER and Golgi lumen controls the molecular stabilization of BACE1

The lipid second messenger ceramide regulates the rate of beta cleavage of the Alzheimers disease APP (amyloid precursor protein) by affecting the molecular stability of the beta secretase BACE1 (beta-site APP cleaving enzyme 1). Such an event is stimulated in the brain by the normal process of aging, and is under the control of the general aging programme mediated by the insulin-like growth factor 1 receptor. In the present study we report that BACE1 is acetylated on seven lysine residues of the N-terminal portion of the nascent protein. This process involves lysine acetylation in the lumen of the ER (endoplasmic reticulum) and is followed by deacetylation in the lumen of the Golgi apparatus, once the protein is fully mature. We also show that specific enzymatic activities acetylate (in the ER) and deacetylate (in the Golgi apparatus) the lysine residues. This process requires carrier-mediated translocation of acetyl-CoA into the ER lumen and is stimulated by ceramide. Site-directed mutagenesis indicates that lysine acetylation is necessary for nascent BACE1 to leave the ER and move ahead in the secretory pathway, and for the molecular stabilization of the protein.

Neurotrophin p75 Receptor Is Involved in Neuronal Damage by Prion Peptide-(106–126)

In this work we have investigated the molecular basis of the neuronal damage induced by the prion peptide by searching for a surface receptor whose activation could be the first step of a cascade of events responsible for cell death. By using a human neuroblastoma cell line lacking all the neurotrophin receptors and derived clones expressing the full-length or truncated forms of the low affinity neurotrophin receptor (p75NTR), we have been able to demonstrate that the neuronal death induced by the prion protein fragment PrP-(106–126) is an active process mediated by a) the binding of the peptide to the extracellular region of p75NTR, b) the signaling function of the intracytoplasmic region of the receptor, and c) the activation of caspase-8 and the production of oxidant species.

The defensive alliance between neutrophils and NK cells as a novel arm of innate immunity.

The immune system is equipped with a plethora of mechanisms that protect the host from the harmful effects of environmental insults. However, the traditional “hierarchical” view of the immune response, in which innate, “nonspecific” cells are first recruited to the site of damage, before the highly “specific”, adaptive immune response develops, has been questioned recently. First, the innate response is much more specific than recognized previously: indeed, each cell of the innate system is not only endowed with an ever‐expanding array of germ‐line‐encoded receptors, which differentiate between distinct insults, but also is modulated continuously by other leukocytes that concomitantly interact with and respond to that particular insult. The other reason is that the cells of the innate system are instrumental for the adaptive system to accomplish its function, as they can also modulate the activity of lymphocytes reciprocally during the entire course of the immune response. This complex pattern of interactions is illustrated by recent advances on the functions of PMNs, clearly showing that unexpectedly, these cells also contribute to the regulation of the host immune response by crosstalk with innate and adaptive leukocytes, including NK cells. Herein, given the peculiar role of neutrophils and NK cells in inflammation, clearance of pathogens/viral‐infected cells, and cancer immunosurveillance, we summarize the current knowledge about the mechanisms whereby neutrophils and NK cells interact and regulate the activities of one another, as well as discuss their potential implications involved in the pathogenesis of chronic, inflammatory pathologies, infections, and tumors.

PCSK9 is required for the disposal of non‐acetylated intermediates of the nascent membrane protein BACE1

We have recently identified a new form of post‐translational regulation of BACE1 (β‐site amyloid precursor protein (APP)‐cleaving enzyme 1), a membrane protein that acts as the rate‐limiting enzyme in the generation of the Alzheimer disease amyloid β‐peptide (Aβ). Specifically, BACE1 is transiently acetylated on seven lysine residues in the lumen of the endoplasmic reticulum/endoplasmic reticulum–Golgi intermediate compartment (ER/ERGIC). The acetylated intermediates of the nascent protein are able to reach the Golgi apparatus, whereas the non‐acetylated ones are retained and degraded in a post‐ER compartment. Here, we report that the serine protease PCSK9 (proprotein convertase subtilisin kexin type 9) contributes to the disposal of non‐acetylated BACE1. Both overexpression and small interfering RNA‐mediated downregulation of PCSK9 affected the levels of BACE1. The downregulation of PCSK9 affected the levels of the loss‐of‐acetylation mutants (BACE1Ala and BACE1Arg) but not those of the gain‐of‐acetylation mutant (BACE1Gln). In addition, Pcsk9−/− mice showed increased levels of BACE1 and Aβ in the brain. Finally, we found that nascent low‐density lipoprotein receptor, a known substrate of PCSK9, is also acetylated.

Neutrophil activation and survival are modulated by interaction with NK cells

It is increasingly evident that neutrophils are able to cross-talk with other leukocytes to shape ongoing inflammatory and immune responses. In this study, we analyzed whether human NK cells may influence the survival and activation of neutrophils under co-culture conditions. We report that NK cells exposed to either IL-15 or IL-18 alone strongly protect the survival of neutrophils via the release of IFNγ and granulocyte macrophage colony-stimulating factor (GM-CSF) plus IFNγ, respectively, and cause a slight up-regulation of neutrophil CD64 and CD11b expression. In comparison, NK cells exposed to both IL-15 and IL-18 show a lesser ability to increase the survival of neutrophils but can more potently up-regulate CD64 and CD11b expression, as well as induce the de novo surface expression of CD69, in neutrophils. Analysis of the events occurring in neutrophil/NK co-cultures exposed to IL-15 plus IL-18 revealed that (i) neutrophil survival is positively affected by NK-derived GM-CSF but negatively influenced by a CD18-dependent neutrophil/NK contact, (ii) NK-derived IFNγ is almost entirely responsible for the induction of CD64, (iii) both soluble factors (primarily GM-CSF) and direct cell-cell contact up-regulate CD11b and CD69 and (iv) NK-derived GM-CSF induces the expression of biologically active heparin-binding EGF-like growth factor (HB-EGF) in neutrophils. Finally, we demonstrate that NK cells can also express HB-EGF when stimulated with either IL-2 or IL-15, yet independently of endogenous GM-CSF. Altogether, our results define a novel interaction within the innate immune system whereby NK cells, by directly modulating neutrophil functions, might contribute to the pathogenesis of inflammatory diseases.

Human neutrophils interact with both 6-sulfo LacNAc + DC and NK cells to amplify NK-derived IFNγ: role of CD18, ICAM-1, and ICAM-3

The role of neutrophils as key players in the regulation of innate and adaptive immune responses is increasingly being recognized. We report that human neutrophils establish a network with both natural killer (NK) cells and 6-sulfo LacNAc(+) dendritic cells (slanDCs), which ultimately serves to up-regulate NK-derived interferonγ (IFNγ). This network involves direct reciprocal interactions and positive amplification loops mediated by cell-derived cytokines. Accordingly, we show that after lipopolysaccharide + interleukin-2 (IL-2) or IL-15/IL-18 stimulation, neutrophils directly interact with and potentiate the activity of both slanDCs and NK cells. On the one hand, neutrophils augment the release of IL-12p70 by slanDCs via a CD18/ intercellular adhesion molecule-1 (ICAM-1) interaction that stimulates activated NK cells to produce IFNγ. IFNγ further potentiates the interaction between neutrophils and slanDCs and the release of slanDC-derived IL-12p70, thus creating a positive feedback loop. On the other hand, neutrophils directly co-stimulate NK cells via CD18/ICAM-3, leading to the production of IFNγ. Colocalization of neutrophils, NK cells, and slanDCs, as well as of IL-12p70 and IFNγ, in inflamed tissues of Crohn disease and psoriasis provides strong evidence for a novel cellular and cytokine cooperation within the innate immune system in which neutrophils act as amplifiers of NK cell/slanDC-mediated responses.

Characterization of the signaling pathway downstream p75 neurotrophin receptor involved in β-amyloid peptide-dependent cell death

The accumulation of β-amyloid (Aβ) peptide is a key pathogenic event in Alzheimer’s disease. Previous studies have shown that Aβ peptide can damage neurons by activating the p75 neurotrophin receptor (p75NTR). However, the signaling pathway leading to neuronal cell death is not completely understood. By using a neuroblastoma cell line devoid of neurotrophin receptors and engineered to express either a full-length or a death domain (DD)-truncated form of p75NTR, we demonstrated that Aβ peptide activates the mitogen-activated protein kinases (MAPKs) p38 and c-Jun N-terminal kinase (JNK). We also found that Aβ peptide induces the translocation of nuclear factor-κB (NF-κB). These events depend on the DD of p75NTR. β-Amyloid (Aβ) peptide was found not to be toxic when the above interactors were inhibited, indicating that they are required for Aβ-induced neuronal cell death. p75 neurotrophin receptor (p75NTR)-expressing cells became resistant to Aβ toxicity when transfected with dominant-negative mutants of MAPK kinases 3, 4, or 6 (MKK3, MKK4, or MKK6), the inhibitor of κBα, or when treated with chemical inhibitors of p38 and JNK. Furthermore, p75NTR-expressing cells became resistant to Aβ peptide upon transfection with a dominant-negative mutant of p53. These results were obtained in the presence of normal p38 and JNK activation, indicating that p53 acts downstream of p38 and JNK. Finally, we demonstrated that NF-κB activation is dependent on p38 and JNK activation. Therefore, our data suggest a signaling pathway in which Aβ peptide binds to p75NTR and activates p38 and JNK in a DD-dependent manner, followed by NF-κB translocation and p53 activation.