SerpinB1 controls encephalitogenic T helper cells in neuroinflammation.

SerpinB1, a protease inhibitor and neutrophil survival factor, was recently linked with IL-17-expressing T cells. Here, we show that serpinB1 (Sb1) is dramatically induced in a subset of effector CD4 cells in experimental autoimmune encephalomyelitis (EAE). Despite normal T cell priming, Sb1-/- mice are resistant to EAE with a paucity of T helper (TH) cells that produce two or more of the cytokines, IFNγ, GM-CSF, and IL-17. These multiple cytokine-producing CD4 cells proliferate extremely rapidly; highly express the cytolytic granule proteins perforin-A, granzyme C (GzmC), and GzmA and surface receptors IL-23R, IL-7Rα, and IL-1R1; and can be identified by the surface marker CXCR6. In Sb1-/- mice, CXCR6+ TH cells are generated but fail to expand due to enhanced granule protease-mediated mitochondrial damage leading to suicidal cell death. Finally, anti-CXCR6 antibody treatment, like Sb1 deletion, dramatically reverts EAE, strongly indicating that the CXCR6+ T cells are the drivers of encephalitis.

Myeloid conditional deletion and transgenic models reveal a threshold for the neutrophil survival factor Serpinb1.

Serpinb1 is an inhibitor of neutrophil granule serine proteases cathepsin G, proteinase-3 and elastase. One of its core physiological functions is to protect neutrophils from granule protease-mediated cell death. Mice lacking Serpinb1a (Sb1a-/-), its mouse ortholog, have reduced bone marrow neutrophil numbers due to cell death mediated by cathepsin G and the mice show increased susceptibility to lung infections. Here, we show that conditional deletion of Serpinb1a using the Lyz2-cre and Cebpa-cre knock-in mice effectively leads to recombination-mediated deletion in neutrophils but protein-null neutrophils were only obtained using the latter recombinase-expressing strain. Absence of Serpinb1a protein in neutrophils caused neutropenia and increased granule permeabilization-induced cell death. We then generated transgenic mice expressing human Serpinb1 in neutrophils under the human MRP8 (S100A8) promoter. Serpinb1a expression levels in founder lines correlated positively with increased neutrophil survival when crossed with Sb1a-/- mice, which had their defective neutrophil phenotype rescued in the higher expressing transgenic line. Using new conditional and transgenic mouse models, our study demonstrates the presence of a relatively low Serpinb1a protein threshold in neutrophils that is required for sustained survival. These models will also be helpful in delineating recently described functions of Serpinb1 in metabolism and cancer.

SerpinB1 Promotes Pancreatic ß Cell Proliferation.

Although compensatory islet hyperplasia in response to insulin resistance is a recognized feature in diabetes, the factor(s) that promote ß cell proliferation have been elusive. We previously reported that the liver is a source for such factors in the liver insulin receptor knockout (LIRKO) mouse, an insulin resistance model that manifests islet hyperplasia. Using proteomics we show that serpinB1, a protease inhibitor, which is abundant in the hepatocyte secretome and sera derived from LIRKO mice, is the liver-derived secretory protein that regulates ß cell proliferation in humans, mice, and zebrafish. Small-molecule compounds, that partially mimic serpinB1 effects of inhibiting elastase activity, enhanced proliferation of ß cells, and mice lacking serpinB1 exhibit attenuated ß cell compensation in response to insulin resistance. Finally, SerpinB1 treatment of islets modulated proteins in growth/survival pathways. Together, these data implicate serpinB1 as an endogenous protein that can potentially be harnessed to enhance functional ß cell mass in patients with diabetes.

The protease cathepsin L regulates Th17 cell differentiation.

Previously we reported that IL-17(+) T cells, primarily IL-17(+) γδ cells, are increased in mice lacking the protease inhibitor serpinB1 (serpinb1(-/-) mice). Here we show that serpinB1-deficient CD4 cells exhibit a cell-autonomous and selective deficiency in suppressing T helper 17 (Th17) cell differentiation. This suggested an opposing role for one or more protease in promoting Th17 differentiation. We found that several SerpinB1-inhibitable cysteine cathepsins are induced in Th17 cells, most prominently cathepsin L (catL); this was verified by peptidase assays, active site labeling and Western blots. Moreover, Th17 differentiation was suppressed by both broad cathepsin inhibitors and catL selective inhibitors. CatL is present in Th17 cells as single chain (SC)- and two-chain (TC)-forms. Inhibiting asparagine endopeptidase (AEP) blocked conversion of SC-catL to TC-catL and increased generation of serpinb1(-/-) Th17 cells, but not wild-type Th17 cells. These findings suggest that SC-catL is biologically active in promoting Th17 generation and is counter-regulated by serpinB1 and secondarily by AEP. Thus, in addition to regulation by cytokines and transcription factors, differentiation of CD4 cells to Th17 cells is actively regulated by a catL-serpinB1-AEP module. Targeting this protease regulatory module could be an approach to treating Th17 cell-driven autoimmune disorders.

Proteinase 3-dependent caspase-3 cleavage modulates neutrophil death and inflammation.

Caspase-3-mediated spontaneous death in neutrophils is a prototype of programmed cell death and is critical for modulating physiopathological inflammatory responses; however, the underlying regulatory pathways remain ill defined. Here we determined that in aging neutrophils, the cleavage and activation of caspase-3 is independent of the canonical caspase-8- or caspase-9-mediated pathway. Instead, caspase-3 activation was mediated by serine protease proteinase 3 (PR3), which is present in the cytosol of aging neutrophils. Specifically, PR3 cleaved procaspase-3 at a site upstream of the canonical caspase-9 cleavage site. In mature neutrophils, PR3 was sequestered in granules and released during aging via lysosomal membrane permeabilization (LMP), leading to procaspase-3 cleavage and apoptosis. Pharmacological inhibition or knockdown of PR3 delayed neutrophil death in vitro and consistently delayed neutrophil death and augmented neutrophil accumulation at sites of inflammation in a murine model of peritonitis. Adoptive transfer of both WT and PR3-deficient neutrophils revealed that the delayed death of neutrophils lacking PR3 is due to an altered intrinsic apoptosis/survival pathway, rather than the inflammatory microenvironment. The presence of the suicide protease inhibitor SERPINB1 counterbalanced the protease activity of PR3 in aging neutrophils, and deletion of Serpinb1 accelerated neutrophil death. Taken together, our results reveal that PR3-mediated caspase-3 activation controls neutrophil spontaneous death.

SerpinB1 regulates homeostatic expansion of IL-17+ γδ and CD4+ Th17 cells.

SerpinB1 is an endogenous inhibitor of serine proteases recognized for its anti-inflammatory and host-protective properties. Although loss of serpinB1 in mice does not result in gross immune deregulation, serpinb1a(-/-) mice display increased mortality and inflammation-associated morbidity upon challenge with influenza virus. Here, we show that IL-17A(+) γδ and CD4(+) Th17 cells are already expanded in the lungs of serpinb1a(-/-) mice at steady-state. Both γδ and αß(+) CD4(+) CCR6(+) T cells isolated from the lungs of naive serpinb1a(-/-) mice displayed a skewed transcriptional profile relative to WT cells, including increased Th17 signature transcripts [Il17a, l17f, and Rorc (RORγt)] and decreased Th1 signature transcripts [Ifng, Cxcr3, and Tbx21 (T-bet)] in γδ T cells. In addition to the lung, IL-17A(+) γδ and CD4(+) Th17 cells were increased in the spleen of naive serpinb1a(-/-) mice, despite normal αß and γδ T cell development in the thymus. Within the γδ T cell compartment, loss of serpinb1a prompted selective expansion of Vγ4(+) and Vγ6/Vδ1(+) cells, which also displayed elevated expression of the proliferating cell nuclear antigen, Ki-67, and IL-17A. Given that serpinb1a is preferentially expressed in WT IL-17A(+) γδ and CD4(+) Th17 cell subsets vis-à-vis other T cell lineages, our findings reveal a novel function of serpinB1 in limiting untoward expansion of lymphocytes with a Th17 phenotype.

Increased surfactant protein D fails to improve bacterial clearance and inflammation in serpinB1-/- mice.

Previously, we described the protective role of the neutrophil serine protease inhibitor serpinB1 in preventing early mortality of Pseudomonas aeruginosa lung infection by fostering bacterial clearance and limiting inflammatory cytokines and proteolytic damage. Surfactant protein D (SP-D), which maintains the antiinflammatory pulmonary environment and mediates bacterial removal, was degraded in infected serpinB1-deficient mice. Based on the hypothesis that increased SP-D would rescue or mitigate the pathological effects of serpinB1 deletion, we generated two serpinB1(-/-) lines overexpressing lung-specific rat SP-D and inoculated the mice with P. aeruginosa. Contrary to predictions, bacterial counts in the lungs of SP-D(low)serpinB1(-/-) and SP-D(high) serpinB1(-/-) mice were 4 logs higher than wild-type and not different from serpinB1(-/-) mice. SP-D overexpression also failed to mitigate inflammation (TNF-α), lung injury (free protein, albumin), or excess neutrophil death (free myeloperoxidase, elastase). These pathological markers were higher for infected SP-D(high)serpinB1(-/-) mice than for serpinB1(-/-) mice, although the differences were not significant after controlling for multiple comparisons. The failure of transgenic SP-D to rescue antibacterial defense of serpinB1-deficient mice occurred despite 5-fold or 20-fold increased expression levels, largely normal structure, and dose-dependent bacteria-aggregating activity. SP-D of infected wild-type mice was intact in 43-kD monomers by reducing SDS-PAGE. By contrast, proteolytic fragments of 35, 17, and 8 kD were found in infected SP-D(low)serpinB1(-/-), SP-D(high) serpinB1(-/-) mice, and serpinB1(-/-) mice. Thus, although therapies to increase lung concentration of SP-D may have beneficial applications, the findings suggest that therapy with SP-D may not be beneficial for lung inflammation or infection if the underlying clinical condition includes excess proteolysis.

A serpinB1 regulatory mechanism is essential for restricting neutrophil extracellular trap generation.

NETosis (neutrophil extracellular trap [NET] generation), a programmed death pathway initiated in mature neutrophils by pathogens and inflammatory mediators, can be a protective process that sequesters microbes and prevents spread of infection, but it can also be a pathological process that causes inflammation and serious tissue injury. Little is known about the regulatory mechanism. Previously, we demonstrated that serpinb1-deficient mice are highly susceptible to pulmonary bacterial and viral infections due to inflammation and tissue injury associated with increased neutrophilic death. In this study, we used in vitro and in vivo approaches to investigate whether SerpinB1 regulates NETosis. We found that serpinb1-deficient bone marrow and lung neutrophils are hypersusceptible to NETosis induced by multiple mediators in both an NADPH-dependent and -independent manner, indicating a deeply rooted regulatory role in NETosis. This role is further supported by increased nuclear expansion (representing chromatin decondensation) of PMA-treated serpinb1-deficient neutrophils compared with wild-type, by migration of SerpinB1 from the cytoplasm to the nucleus of human neutrophils that is coincident with or preceding early conversion of lobulated (segmented) nuclei to delobulated (spherical) morphology, as well as by the finding that exogenous human recombinant SerpinB1 abrogates NET production. NETosis of serpinb1-deficient neutrophils is also increased in vivo during Pseudomonas aeruginosa lung infection. The findings identify a previously unrecognized regulatory mechanism involving SerpinB1 that restricts the production of NETs.

Critical role of serpinB1 in regulating inflammatory responses in pulmonary influenza infection.

BACKGROUND: Excessive inflammatory host response increases morbidity and mortality associated with seasonal respiratory influenza, and highly pathogenic virus strains are characterized by massive infiltration of monocytes and/or macrophages that produce a storm of injurious cytokines. METHODS: Here, we examined the role in respiratory influenza of serpinB1, an endogenous inhibitor of the serine proteases elastase, cathepsin G, and proteinase-3, increasingly recognized as regulators of inflammation. RESULTS: After challenge with high-dose surfactant protein-D (SP-D)-sensitive influenza A/Philadelphia/82 (H3N2), serpinB1(-/-) mice died earlier and in greater numbers than did wild-type mice. Sublethally infected animals suffered increased morbidity, delayed resolution of epithelial injury, and increased immune cell death. Viral clearance and SP-D/SP-A upregulation were unimpaired and so were early virus-induced cytokine and chemokine burst and influx of large numbers of neutrophils and monocytes. Whereas initial cytokines and chemokines rapidly cleared in wild-type mice, TNF-α, IL-6, KC/CXCL1, G-CSF, IL-17A, and MCP-1/CCL2 remained elevated in serpinB1(-/-) mice. Monocyte-derived cells were the dominant immune cells in influenza-infected lungs, and those from serpinB1(-/-) mice produced excessive IL-6 and TNF-α when tested ex vivo. Pulmonary γδ T-cells that produced IL-17A were also increased. CONCLUSIONS: Because viral clearance was unimpaired, the study highlights the critical role of serpinB1 in mitigating inflammation and restricting pro-inflammatory cytokine production in influenza infection.

A substrate-based approach to convert SerpinB1 into a specific inhibitor of proteinase 3, the Wegener's granulomatosis autoantigen.

The physiological and pathological functions of proteinase 3 (PR3) are not well understood due to its close similarity to human neutrophil elastase (HNE) and the lack of a specific inhibitor. Based on structural analysis of the active sites of PR3 and HNE, we generated mutants derived from the polyvalent inhibitor SerpinB1 (monocyte/neutrophil elastase inhibitor) that specifically inhibit PR3 and that differ from wt-SerpinB1 by only 3 or 4 residues in the reactive center loop. The rate constant of association between the best SerpinB1 mutant and PR3 is 1.4 × 107 M⁻¹ · s⁻¹, which is ∼100-fold higher than that observed with wt-SerpinB1 and compares with that of α1-protease inhibitor (α1-PI) toward HNE. SerpinB1(S/DAR) is cleaved by HNE, but due to differences in rate, inhibition of PR3 by SerpinB1(S/DAR) is only minimally affected by the presence of HNE even when the latter is in excess. SerpinB1(S/DAR) inhibits soluble PR3 and also membrane-bound PR3 at the surface of activated neutrophils. Moreover, SerpinB1(S/DAR) clears induced PR3 from the surface of activated neutrophils. Overall, these specific inhibitors of PR3 will be valuable for defining biological functions of the protease and may prove useful as therapeutics for PR3-related inflammatory diseases, such as Wegener's granulomatosis.

SerpinB1 protects the mature neutrophil reserve in the bone marrow.

SerpinB1 is among the most efficient inhibitors of neutrophil serine proteases--NE, CG, and PR-3--and we investigated here its role in neutrophil development and homeostasis. We found that serpinB1 is expressed in all human bone marrow leukocytes, including stem and progenitor cells. Expression levels were highest in the neutrophil lineage and peaked at the promyelocyte stage, coincident with the production and packaging of the target proteases. Neutrophil numbers were decreased substantially in the bone marrow of serpinB1(-/-) mice. This cellular deficit was associated with an increase in serum G-CSF levels. On induction of acute pulmonary injury, neutrophils were recruited to the lungs, causing the bone marrow reserve pool to be completely exhausted in serpinB1(-/-) mice. Numbers of myeloid progenitors were normal in serpinB1(-/-) bone marrow, coincident with the absence of target protease expression at these developmental stages. Maturation arrest of serpinB1(-/-) neutrophils was excluded by the normal CFU-G growth in vitro and the normal expression in mature neutrophils of early and late differentiation markers. Normal absolute numbers of proliferating neutrophils and pulse-chase kinetic studies in vivo showed that the bone marrow deficit in serpinB1(-/-) mice was largely restricted to mature, postmitotic neutrophils. Finally, upon overnight culture, apoptosis and necrosis were greater in purified bone marrow neutrophils from serpinB1(-/-) compared with WT mice. Collectively, these findings demonstrate that serpinB1 sustains a healthy neutrophil reserve that is required in acute immune responses.

Relationship of proteinases and proteinase inhibitors with microbial presence in chronic lung disease of prematurity.

BACKGROUND: A proteolytic imbalance has been implicated in the development of "classical" chronic lung disease of prematurity (CLD). However, in "new" CLD this pattern has changed. This study examines the longitudinal relationship between neutrophil proteinases and their inhibitors in ventilated preterm infants and their relationship to microbial colonisation. METHODS: Serial bronchoalveolar lavage fluid was obtained from ventilated newborn preterm infants. Neutrophil elastase (NE) activity, cell counts, metalloproteinase (MMP)-9, MMP-9/TIMP-1 complex, SerpinB1 concentration and percentage of SerpinB1 and alpha(1)-antitrypsin (AAT) in complex with elastase were measured. The presence of microbial genes was examined using PCR for 16S rRNA genes. RESULTS: Statistically more infants who developed CLD had NE activity in at least one sample (10/20) compared with infants with resolved respiratory distress syndrome (RDS) (2/17). However, NE activity was present in a minority of samples, occurring as episodic peaks. Peak levels of MMP-9, MMP-9/TIMP-1 complex, percentage of AAT and SerpinB1 in complex and cell counts were all statistically greater in infants developing CLD than in infants with resolved RDS. Peak values frequently occurred as episodic spikes and strong temporal relationships were noted between all markers. The peak values for all variables were significantly correlated to each other. The presence of bacterial 16S rRNA genes was associated with the development of CLD and with elevated elastase and MMP-9. CONCLUSION: NE activity and MMP-9 appear to be important in the development of "new" CLD with both proteinase and inhibitor concentrations increasing episodically, possibly in response to postnatal infection.

Platelets amplify inflammation in arthritis via collagen-dependent microparticle production.

In addition to their pivotal role in thrombosis and wound repair, platelets participate in inflammatory responses. We investigated the role of platelets in the autoimmune disease rheumatoid arthritis. We identified platelet microparticles--submicrometer vesicles elaborated by activated platelets--in joint fluid from patients with rheumatoid arthritis and other forms of inflammatory arthritis, but not in joint fluid from patients with osteoarthritis. Platelet microparticles were proinflammatory, eliciting cytokine responses from synovial fibroblasts via interleukin-1. Consistent with these findings, depletion of platelets attenuated murine inflammatory arthritis. Using both pharmacologic and genetic approaches, we identified the collagen receptor glycoprotein VI as a key trigger for platelet microparticle generation in arthritis pathophysiology. Thus, these findings demonstrate a previously unappreciated role for platelets and their activation-induced microparticles in inflammatory joint diseases.

WASP plays a novel role in regulating platelet responses dependent on alphaIIbbeta3 integrin outside-in signalling.

The most consistent feature of Wiskott Aldrich syndrome (WAS) is profound thrombocytopenia with small platelets. The responsible gene encodes WAS protein (WASP), which functions in leucocytes as an actin filament nucleating agent -yet- actin filament nucleation proceeds normally in patient platelets regarding shape change, filopodia and lamellipodia generation. Because WASP localizes in the platelet membrane skeleton and is mobilized by alphaIIbbeta3 integrin outside-in signalling, we questioned whether its function might be linked to integrin. Agonist-induced alphaIIbbeta3 activation (PAC-1 binding) was normal for patient platelets, indicating normal integrin inside-out signalling. Inside-out signalling (fibrinogen, JON/A binding) was also normal for wasp-deficient murine platelets. However, adherence/spreading on immobilized fibrinogen was decreased for patient platelets and wasp-deficient murine platelets, indicating decreased integrin outside-in responses. Another integrin outside-in dependent response, fibrin clot retraction, involving contraction of the post-aggregation actin cytoskeleton, was also decreased for patient platelets and wasp-deficient murine platelets. Rebleeding from tail cuts was more frequent for wasp-deficient mice, suggesting decreased stabilisation of the primary platelet plug. In contrast, phosphatidylserine exposure, a pro-coagulant response, was enhanced for WASP-deficient patient and murine platelets. The collective results reveal a novel function for WASP in regulating pro-aggregatory and pro-coagulant responses downstream of integrin outside-in signalling.

Evolution of highly polymorphic T cell populations in siblings with the Wiskott-Aldrich Syndrome.

Population level evolutionary processes can occur within a single organism when the germ line contains a mutation that confers a cost at the level of the cell. Here we describe how multiple compensatory mutations arose through a within-individual evolutionary process in two brothers with the immune deficiency Wiskott-Aldrich Syndrome (WAS). As a result, both brothers have T lymphocyte populations that are highly polymorphic at the locus of the germ line defect, and no single allele achieves fixation. WASP, the gene product affected in this disease, is specific to white blood cells where it is responsible for regulating actin cytoskeleton dynamics in a wide range of cellular responses. The brothers inherited a rare allele predicted to result in truncated WASP lacking the carboxy-terminal VCA domains, the region that directly catalyzes actin filament generation. Although the brothers' T cell populations are highly polymorphic, all share a corrective effect relative to the inherited allele in that they restore the VCA domain. This indicates massive selection against the truncated germ line allele. No single somatic allele becomes fixed in the circulating T cell population of either brother, indicating that a regulated step in maturation of the affected cell lineage is severely compromised by the germ line allele. Based on the finding of multiple somatic mutations, the known maturation pathway for T-lineage cells and the known defects of T cells and precursor thymocytes in mice with truncated WASP, we hypothesize that the presence of truncated WASP (WASP Delta VCA) confers an extreme disadvantage in early developing thymocytes, above and beyond the known cost of absence of full-length WASP, and that the disadvantage likely occurs through dominant negative competition of WASP Delta VCA with N-WASP, a protein that otherwise partially compensates for WASP absence in developing thymocytes.

Inflammation induces hemorrhage in thrombocytopenia.

The role of platelets in hemostasis is to produce a plug to arrest bleeding. During thrombocytopenia, spontaneous bleeding is seen in some patients but not in others; the reason for this is unknown. Here, we subjected thrombocytopenic mice to models of dermatitis, stroke, and lung inflammation. The mice showed massive hemorrhage that was limited to the area of inflammation and was not observed in uninflamed thrombocytopenic mice. Endotoxin-induced lung inflammation during thrombocytopenia triggered substantial intra-alveolar hemorrhage leading to profound anemia and respiratory distress. By imaging the cutaneous Arthus reaction through a skin window, we observed in real time the loss of vascular integrity and the kinetics of skin hemorrhage in thrombocytopenic mice. Bleeding-observed mostly from venules-occurred as early as 20 minutes after challenge, pointing to a continuous need for platelets to maintain vascular integrity in inflamed microcirculation. Inflammatory hemorrhage was not seen in genetically engineered mice lacking major platelet adhesion receptors or their activators (alphaIIbbeta3, glycoprotein Ibalpha [GPIbalpha], GPVI, and calcium and diacylglycerol-regulated guanine nucleotide exchange factor I [CalDAG-GEFI]), thus indicating that firm platelet adhesion was not necessary for their supporting role. While platelets were previously shown to promote endothelial activation and recruitment of inflammatory cells, they also appear indispensable to maintain vascular integrity in inflamed tissue. Based on our observations, we propose that inflammation may cause life-threatening hemorrhage during thrombocytopenia.

Patterns of neutrophil serine protease-dependent cleavage of surfactant protein D in inflammatory lung disease.

The manuscript presents definitive studies of surfactant protein D (SP-D) in the context of inflammatory lung fluids. The extent of SP-D depletion in bronchoalveolar lavage fluid (BALF) of children affected with cystic fibrosis (CF) is demonstrated to correlate best with the presence of the active neutrophil serine protease (NSP) elastase. Novel C-terminal SP-D fragments of 27 kDa and 11 kDa were identified in patient lavage fluid in addition to the previously described N-terminal, 35-kDa fragment by the use of isoelectrofocusing, modified blotting conditions, and region-specific antibodies. SP-D cleavage sites were identified. In vitro treatment of recombinant human SP-D dodecamers with NSPs replicated the fragmentation, but unexpectedly, the pattern of SP-D fragments generated by NSPs was dependent on calcium concentration. Whereas the 35- and 11-kDa fragments were generated when incubations were performed in low calcium (200 microM CaCl(2)), incubations in physiological calcium (2 mM) with higher amounts of elastase or proteinase-3 generated C-terminal 27, 21, and 14 kDa fragments, representing cleavage within the collagen and neck regions. Studies in which recombinant SP-D cleavage by individual NSPs was quantitatively evaluated under low and high calcium conditions showed that the most potent NSP for cleaving SP-D is elastase, followed by proteinase-3, followed by cathepsin G. These relative potency findings were considered in the context of other studies that showed that active NSPs in CF BALF are in the order: elastase, followed by cathepsin G, followed by proteinase-3. The findings support a pre-eminent role for neutrophil elastase as the critical protease responsible for SP-D depletion in inflammatory lung disease.

WASP localizes to the membrane skeleton of platelets.

Patients with Wiskott-Aldrich syndrome (WAS), an X-linked blood cell disease, suffer from severe thrombocytopenia due to accelerated loss of defective platelets. The affected gene encodes WASP, an actin regulatory protein thought to reside in the cytoplasm of resting leucocytes. In contrast, this study showed that, for platelets, one-quarter of WASP molecules fractionate in the detergent-insoluble high speed pellet characterized as the membrane skeleton, the scaffold structure that underlies the lipid bilayer and stabilizes the surface membrane. Following treatment of platelets with thrombin and stirring, which induces cytoarchitectural remodelling, WASP and other membrane skeletal components sedimented at lower g force and partitioned in the low-speed pellet. Thrombin and stirring also induced WASP tyrosine phosphorylation, a rapid activating reaction, and proteolytic inactivation by cysteine protease calpain. Both the alteration of the sedimentation profile and the proteolytic inactivation were specific for the membrane skeletal pool of WASP and were abrogated in alphaIIb beta3 integrin-deficient platelets and in normal platelets treated with an integrin antagonist. The findings demonstrate that WASP is a component of the resting platelet membrane skeleton and participates in membrane skeletal rearrangements downstream of integrin outside-in signalling. The possible implications for the platelet defect in WAS are discussed.

The neutrophil serine protease inhibitor serpinb1 preserves lung defense functions in Pseudomonas aeruginosa infection.

Neutrophil serine proteases (NSPs; elastase, cathepsin G, and proteinase-3) directly kill invading microbes. However, excess NSPs in the lungs play a central role in the pathology of inflammatory pulmonary disease. We show that serpinb1, an efficient inhibitor of the three NSPs, preserves cell and molecular components responsible for host defense against Pseudomonas aeruginosa. On infection, wild-type (WT) and serpinb1-deficient mice mount similar early responses, including robust production of cytokines and chemokines, recruitment of neutrophils, and initial containment of bacteria. However, serpinb1(-/-) mice have considerably increased mortality relative to WT mice in association with late-onset failed bacterial clearance. We found that serpinb1-deficient neutrophils recruited to the lungs have an intrinsic defect in survival accompanied by release of neutrophil protease activity, sustained inflammatory cytokine production, and proteolysis of the collectin surfactant protein-D (SP-D). Coadministration of recombinant SERPINB1 with the P. aeruginosa inoculum normalized bacterial clearance in serpinb1(-/-) mice. Thus, regulation of pulmonary innate immunity by serpinb1 is nonredundant and is required to protect two key components, the neutrophil and SP-D, from NSP damage during the host response to infection.