Neutrophils are essential for host defence against invading pathogens. They engulf and degrade microorganisms using an array of weapons that include reactive oxygen species, antimicrobial peptides, and proteases such as cathepsin G, neutrophil elastase and proteinase 3. As discussed in this Review, the generation of mice deficient in these proteases has established a role for these enzymes as intracellular microbicidal agents. However, I focus mainly on emerging data indicating that, after release, these proteases also contribute to the extracellular killing of microorganisms, and regulate non-infectious inflammatory processes by activating specific receptors and modulating the levels of cytokines.

The leukocyte response in inflammation is characterized by an initial recruitment of polymorphonuclear leukocytes (PMN) preceding a second wave of monocytes to the site of injury or infection. In the mouse, 2 populations of monocytes have been identified, Gr1 ؊ CCR2 ؊ CX3CR1 hi resident monocytes and Gr1 ؉ CCR2 ؉ CX3CR1 lo inflammatory monocytes. Here, intravital microscopy of the musculus cremaster and a subcutaneous air pouch model were used to investigate a possible link between PMN extravasation and the subsequent emigration of inflammatory monocytes in response to local stimulation with PAF. In mice that were made neutropenic by injection of a PMNdepleting antibody, the extravasation of inflammatory monocytes, but not resident monocytes, was markedly reduced compared with mice with intact white blood cell IntroductionPolymorphonuclear leukocytes (PMN) dominate the initial leukocyte influx to sites of acute infection and inflammation. 1 This first wave of PMN extravasation precedes a second wave of monocyte extravasation. Recruited PMN are thought to trigger this cellular switch by releasing soluble factors that initiate monocyte recruitment, 2-4 much of which may be mediated by ready-made PMN granule proteins deposited at the site of inflammation. 5,6 Indeed, supernatants of activated PMN from patients with specific granule deficiency lacking proteins in their primary, secondary, and tertiary granules show a reduced capacity to attract monocytes despite normal monocyte chemotaxis in vitro to other stimuli. 7 After this initial observation, several PMN-derived granule proteins with monocyte-chemotactic activity were identified, among them LL-37, cathepsin G, human neutrophil peptide 1-3 (HNP1-3, ␣-defensins), and heparin-binding protein (HBP, also known as CAP37 and azurocidin). [8][9][10][11] Their action was found to be pertussis toxin (PTx)-sensitive and several receptors were suggested to mediate the chemotactic effect. [11][12][13] Peripheral blood monocytes constitute a heterogeneous population of circulating leukocytes in both humans 14 and mice. 15 In the murine blood, 2 monocyte subsets can be distinguished based on their expression of CX3CR1, CCR2, and Gr1. Whereas resident monocytes (Gr1 Ϫ CCR2 Ϫ CX3CR1 hi ) home to noninflamed tissues, inflammatory monocytes (Gr1 ϩ CCR2 ϩ CX3CR1 lo ) are predominantly recruited to sites of inflammation by mechanisms involving CCR2. 15 These inflammatory monocytes were recently shown to be of critical importance in diverse inflammatory and infectious diseases. [16][17][18][19] In this study, we investigated the significance of the initial PMN efflux for the subsequent extravasation of monocytes. Our results demonstrate that PMN seed granule proteins in the tissue which contribute to mobilization specifically of inflammatory monocytes. Functionally, the PMN-dependent invasion of inflammatory monocytes results in a more vigorous immune response as shown by enhanced cytokine release and bacterial clearance at the site of inflammation. Methods AnimalsWild-type C57BL/...

Dipeptidyl peptidase I (DPPI) is a lysosomal cysteine protease that has been implicated in the processing of granzymes, which are neutral serine proteases exclusively expressed in the granules of activated cytotoxic lymphocytes. In this report, we show that cytotoxic lymphocytes derived from DPPI؊/؊ mice contain normal amounts of granzymes A and B, but these molecules retain their prodipeptide domains and are inactive. Cytotoxic assays with DPPI؊/؊ effector cells reveal severe defects in the induction of target cell apoptosis (as measured by [ 125 I]UdR release) at both early and late time points; this defect is comparable to that detected in perforin؊/؊ or granzyme A؊/؊ ؋ B؊/؊ cytotoxic lymphocytes. DPPI therefore plays an essential role in the in vivo processing and activation of granzymes A and B, which are required for cytotoxic lymphocyte granule-mediated apoptosis.

Recent studies have suggested that the retention of selectable marker cassettes (like PGK-Neo, in which a hybrid gene consisting of the phosphoglycerate kinase I promoter drives the neomycin phosphotransferase gene) in targeted loci can cause unexpected phenotypes in ''knockout'' mice due to disruption of expression of neighboring genes within a locus. We have studied targeted mutations in two multigene clusters, the granzyme B locus and the ␤-like globin gene cluster. The insertion of PGK-Neo into the granzyme B gene, the most 5 gene in the granzyme B gene cluster, severely reduced the normal expression of multiple genes within the locus, even at distances greater than 100 kb from the mutation. Similarly, the insertion of a PGK-Neo cassette into the ␤-globin locus control region (LCR) abrogates the expression of multiple globin genes downstream from the cassette. In contrast, a targeted mutation of the promyelocyte-specific cathepsin G gene (which lies just 3 to the granzyme genes in the same cluster) had minimal effects on upstream granzyme gene expression. Although the mechanism of these long distance effects are unknown, the expression of PGK-Neo can be ''captured'' by the regulatory domain into which it is inserted. These results suggest that the PGK-Neo cassette can interact productively with locus control regions and thereby disrupt normal interactions between local and long-distance regulatory regions within a tissue-specific domain.

Leukocyte recruitment in inflammation is critical for host defense, but excessive accumulation of inflammatory cells can lead to tissue damage. Neutrophil-derived serine proteases (cathepsin G [CG], neutrophil elastase [NE], and proteinase 3 [PR3]) are expressed specifically in mature neutrophils and are thought to play an important role in inflammation. To investigate the role of these proteases in inflammation, we generated a mouse deficient in dipeptidyl peptidase I (DPPI) and established that DPPI is required for the full activation of CG, NE, and PR3. Although DPPI(-/-) mice have normal in vitro neutrophil chemotaxis and in vivo neutrophil accumulation during sterile peritonitis, they are protected against acute arthritis induced by passive transfer of monoclonal antibodies against type II collagen. Specifically, there is no accumulation of neutrophils in the joints of DPPI(-/-) mice. This protective effect correlates with the inactivation of neutrophil-derived serine proteases, since NE(-/-) x CG(-/-) mice are equally resistant to arthritis induction by anti-collagen antibodies. In addition, protease-deficient mice have decreased response to zymosan- and immune complex-mediated inflammation in the subcutaneous air pouch. This defect is accompanied by a decrease in local production of TNF-alpha and IL-1 beta. These results implicate DPPI and polymorphonuclear neutrophil-derived serine proteases in the regulation of cytokine production at sites of inflammation.

Objective. Caspase 1, a known cysteine protease, is a critical component of the inflammasome. Both caspase 1 and neutrophil serine proteases such as proteinase 3 (PR3) can process pro-interleukin-1␤ (proIL-1␤), a crucial cytokine linked to the pathogenesis of rheumatoid arthritis. This study was undertaken to establish the relative importance of caspase 1 and serine proteases in mouse models of acute and chronic inflammatory arthritis.Methods. Acute and chronic arthritis were induced in caspase 1 ؊/؊ mice, and the lack of caspase 1 was investigated for its effects on joint swelling, cartilage metabolism, and histopathologic features. In addition, caspase 1 activity was inhibited in mice lacking active cysteine proteases, and the effects of dual blockade of caspase 1 and serine proteases on arthritis severity and histopathologic features were evaluated.Results. Conclusion. Caspase 1 deficiency does not affect neutrophil-dominated joint inflammation, whereas in chronic arthritis, the lack of caspase 1 results in reduced joint inflammation and cartilage destruction. These findings suggest that inhibitors of caspase 1 are not able to interfere with the whole spectrum of IL-1␤ production, and therefore such inhibitors may be of therapeutic value only in inflammatory conditions in which limited numbers of neutrophils are present.Cytokines, such as interleukin-1␤ (IL-1␤), that are produced by cells of the innate immune system are induced in response to a variety of pathogen-or damage-associated molecular patterns. Because of its potent inflammatory properties, IL-1␤ can be deleterious if released in high amounts in various sites of the body (1). Both the production and activity of IL-1␤ are tightly regulated at several levels, including during transcription and translation (2), conversions of the inactive

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Recent evidence suggests that protease release by neutrophils in the bone marrow may contribute to hematopoietic progenitor cell (HPC) mobilization. Matrix metalloproteinase-9 (MMP-9), neutrophil elastase (NE), and cathepsin G (CG) accumulate in the bone marrow during granulocyte colony-stimulating factor (G-CSF) treatment, where they are thought to degrade key substrates including vascular cell adhesion molecule-1 (VCAM-1) and CXCL12. To test this hypothesis, HPC mobilization was characterized in transgenic mice deficient in one or more hematopoietic proteases. Surprisingly, HPC mobilization by G-CSF was normal in MMP-9-deficient mice, NE ؋ CG-deficient mice, or mice lacking dipeptidyl peptidase I, an enzyme required for the functional activation of many hematopoietic serine proteases. Moreover, combined inhibition of neutrophil serine proteases and metalloproteinases had no significant effect on HPC mobilization. VCAM-1 expression on bone marrow stromal cells decreased during G-CSF treatment of wild-type mice but not NE ؋ CG-deficient mice, indicating that VCAM-1 cleavage is not required for efficient HPC mobilization. G-CSF induced a significant decrease in CXCL12␣ protein expression in the bone marrow of Ne ؋ CG-deficient mice, indicating that these proteases are not required to down-regulate CXCL12 expression. Collectively, these data suggest a complex model in which both protease-dependent and -independent pathways may contribute to HPC mobilization. IntroductionThe use of hematopoietic progenitor cells (HPCs) to reconstitute hematopoiesis following myeloablative therapy has significantly improved the clinical outcome for patients with a variety of diseases. Recently, mobilized peripheral blood HPCs instead of bone marrow-derived HPCs have been used because of reduced engraftment times and relative ease of collection. Although the great majority of HPCs reside within the bone marrow, a small number of HPCs also circulate in the peripheral blood. The number of circulating HPCs can be dramatically increased, or mobilized, by a wide variety of stimuli including hematopoietic growth factors, chemotherapy, and chemokines. 1,2 Currently, granulocyte colony-stimulating factor (G-CSF) is the most commonly used agent to mobilize HPCs because of its potency and lack of serious toxicity. However, the mechanisms that mediate G-CSF-induced HPC mobilization are incompletely understood.We previously showed that expression of the G-CSF receptor (G-CSFR) on HPCs is not required for their mobilization into the blood in response to G-CSF. 3 This observation suggests that G-CSF induces HPC mobilization indirectly through the generation of trans-acting signals. Recent studies suggest that hematopoietic proteases released by neutrophils into the bone marrow microenvironment may represent such a signal. A highly proteolytic microenvironment is induced in the bone marrow during HPC mobilization by G-CSF. 4 In particular, matrix metalloproteinase-9 (MMP-9 or gelatinase B), neutrophil elastase (NE), and cathepsin G (CG) accumulate...