Generation of active myeloid and lymphoid granule serine proteases requires processing by the granule thiol protease dipeptidyl peptidase I

Dipeptidyl peptidase I is essential for activation of mast cell chymases, but not tryptases, in mice

Dipeptidyl peptidase I (DPPI) is the sole activator in vivo of several granule-associated serine proteases of cytotoxic lymphocytes. In vitro, DPPI also activates mast cell chymases and tryptases. To determine whether DPPI is essential for their activation in vivo, we used enzyme histochemical and immunohistochemical approaches and solution-based activity assays to study these enzymes in tissues and bone marrow-derived mast cells (BMMCs) from DPPI +/+ and DPPI -/- mice. We find that DPPI -/- mast cells contain normal amounts of immunoreactive chymases but no chymase activity, indicating that DPPI is essential for chymase activation and suggesting that DPPI -/- mice are functional chymase knockouts. The absence of DPPI and chymase activity does not affect the growth, granularity, or staining characteristics of BMMCs and, despite prior predictions, does not alter IgE-mediated exocytosis of histamine. In contrast, the level of active tryptase (mMCP-6) in DPPI -/- BMMCs is 25% that of DPPI +/- BMMCs. These findings indicate that DPPI is not essential for mMCP-6 activation but does influence the total amount of active mMCP-6 in mast cells and therefore may be an important, but not exclusive mechanism for tryptase activation.

A novel vascular smooth muscle chymase is upregulated in hypertensive rats

While greater than 80% of angiotensin II (Ang II) formation in the human heart and greater than 60% in arteries appears to result from chymase activity, no cardiovascular cell-expressed chymase has been previously reported. We now describe the cloning of a full-length cDNA encoding a novel chymase from rat vascular smooth muscle cells. The cDNA encompasses 953 nucleotides, encodes 247 amino acids, and exhibits 74% and 80% homology in amino acid sequence to rat mast cell chymase I and II, respectively. Southern blot analysis indicates that the rat vascular chymase is encoded by a separate gene. This chymase was induced in hypertrophied rat pulmonary arteries, with 11-fold and 8-fold higher chymase mRNA levels in aortic and pulmonary artery smooth muscle cells from spontaneously hypertensive than in corresponding tissues from normotensive rats. We assayed the activity of the endogenous enzyme and of a recombinant, epitope-tagged chymase in transfected smooth muscle cells and showed that Ang II production from Ang I can be inhibited with chymostatin, but not EDTA or captopril. Spontaneously hypertensive rats show elevated chymase expression and increased chymostatin-inhibitable angiotensin-converting activity, suggesting a possible role for this novel enzyme in the pathophysiology of hypertension.

Purification and use of granzyme B

Granzyme B (GrB) is the primary molecular mediator of apoptosis by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. It is a unique mammalian aspartic acid-cleaving serine protease. On T cell receptor activation, GrB is released from the CTL cytoplasmic granules by exocytosis, enters the target cells and, in the presence of the granule pore-forming protein perforin, it initiates the processing of caspases and apoptosis. GrB apoptosis is also activated by adenovirus, which can effectively replace perforin. Methods for the purification and quantitation of GrB and perforin, and the preparation and titration of adenovirus, are described. In addition, methods for application of these reagents to the initiation of apoptosis in tumor target cells, with several assays for detecting GrB apoptotic activity, are detailed.

Papillon-Lefèvre syndrome: mutations and polymorphisms in the cathepsin C gene

The Papillon-Lefèvre syndrome, inherited in an autosomal recessive pattern, manifests with palmoplantar keratoderma and early, destructive periodontitis. Recently, mutations in the gene encoding cathepsin C have been disclosed in a limited number of families with Papillon-Lefèvre syndrome. We have examined two multiplex families with Papillon-Lefèvre syndrome, and evaluated the gene encoding cathepsin C for mutations. The mutation detection strategy consisted of polymerase chain reaction amplification of all seven exons and flanking intronic sequences, followed by direct nucleotide sequencing. This strategy identified two missense mutations, W39S and G301S, affecting highly conserved amino acid residues within the cathepsin C polypeptide. The affected individuals were homozygotes whereas heterozygous carriers of the mutations were clinically unaffected, confirming the recessive nature of the mutations. Addition of these cathepsin C gene mutations into the expanding Papillon-Lefèvre syndrome mutation database allows further development of genotype/phenotype correlations towards understanding this severe genodermatosis.

Ectodermal dysplasias: not only 'skin' deep

The ectodermal dysplasias (EDs) are a large and complex nosologic group of diseases; more than 170 different pathologic clinical conditions have been identified. Despite the great number of EDs described so far, few causative genes have been identified. We review EDs in the light of the most recent molecular findings and propose a new classification of EDs integrating both molecular-genetic data and corresponding clinical findings of related diseases.

Arginine-based structures are specific inhibitors of cathepsin C. Application of peptide combinatorial libraries

Novel synthetic peptide inhibitors of lysosomal cysteine proteinase cathepsin C have been designed through the use of soluble peptide combinatorial libraries. The uncovered structural inhibitory module consists of the N-terminal cluster of L-arginine residues. Its modification with D-amino acids or arginine derivatives did not increase the inhibition strength. Inhibitory potency of oligoarginines improves with the elongation of peptide chain reaching a maximum for octa-L-arginine. The oligoarginines specifically interact with the cathepsin C active site as shown by competitive-type inhibition kinetics (Ki approximately 10-5 M) and intrinsic fluorescence measurements. The inhibitory interaction of oligoarginines is established through the specific spatial contact of a net of guanidino groups in the arginine side-chains, as indicated by comparison with inhibitory action of low molecular mass guanidine derivatives (Ki approximately 10-3 M). Nonarginine polyionic compounds cannot mimic the inhibitory effect of oligoarginines. The arginine-based peptide inhibitors were selective towards cathepsin C among other cysteine proteinases tested.

Dipeptidyl peptidase I cleaves matrix-associated proteins and is expressed mainly by mast cells in normal dog airways

Dipeptidyl peptidase I (DPPI) is a cysteine protease found in many tissues, including the lung. Major cell types expressing DPPI in vitro include myelomonocytic cells, cytotoxic T cells, and mast cells. After activation and degranulation, cytotoxic T cells and mast cells secrete DPPI. With a goal of clarifying possible roles for DPPI in lung diseases, we sought to identify cells expressing DPPI in lung tissue, hypothesizing that lung mast cells are major producers of DPPI and that secreted DPPI cleaves extracellular matrix proteins. To address these hypotheses, we used immunohistochemical techniques to localize DPPI in normal dog airways, lung, and cultured mast cells, and we used purified DPPI to examine cleavage of matrix-associated proteins in vitro. We found that mast cells are the major identifiable source of DPPI in airways and that macrophages are the major source in alveoli. Within mast cells, DPPI localizes to cytoplasmic granules. We also found that DPPI endoproteolytically cleaves the extracellular matrix proteins fibronectin and collagen types I, III, and IV. The finding of DPPI in airway mast cells and its cleavage of matrix proteins suggest the possibility that DPPI plays a role in mast cell-mediated turnover of matrix proteins and in airway remodeling of chronic airway diseases such as asthma.

Loss-of-function mutations in the cathepsin C gene result in periodontal disease and palmoplantar keratosis

Papillon-Lefèvre syndrome, or keratosis palmoplantaris with periodontopathia (PLS, MIM 245000), is an autosomal recessive disorder that is mainly ascertained by dentists because of the severe periodontitis that afflicts patients. Both the deciduous and permanent dentitions are affected, resulting in premature tooth loss. Palmoplantar keratosis, varying from mild psoriasiform scaly skin to overt hyperkeratosis, typically develops within the first three years of life. Keratosis also affects other sites such as elbows and knees. Most PLS patients display both periodontitis and hyperkeratosis. Some patients have only palmoplantar keratosis or periodontitis, and in rare individuals the periodontitis is mild and of late onset. The PLS locus has been mapped to chromosome 11q14-q21 (refs 7, 8, 9). Using homozygosity mapping in eight small consanguineous families, we have narrowed the candidate region to a 1.2-cM interval between D11S4082 and D11S931. The gene (CTSC) encoding the lysosomal protease cathepsin C (or dipeptidyl aminopeptidase I) lies within this interval. We defined the genomic structure of CTSC and found mutations in all eight families. In two of these families we used a functional assay to demonstrate an almost total loss of cathepsin C activity in PLS patients and reduced activity in obligate carriers.

Generation of catalytically active granzyme K from Escherichia coli inclusion bodies and identification of efficient granzyme K inhibitors in human plasma

Granzymes are granule-stored lymphocyte serine proteases that are implicated in T- and natural killer cell-mediated cytotoxic defense reactions after target cell recognition. A fifth human granzyme (granzyme 3, lymphocyte tryptase-2), renamed as granzyme K (gene name GZMK), has recently been cloned from lymphocyte tissue. For its further characterization we successfully generated catalytically active enzyme in milligram quantities per liter of Escherichia coli culture. The natural proform of granzyme K with the amino-terminal propeptide Met-Glu was expressed as inclusion bodies and converted to its active enzyme by cathepsin C after refolding of precursor molecules. Recombinant granzyme K cleaves synthetic thiobenzyl ester substrates after Lys and Arg with k(cat)/K(m) values of 3.7 x 10(4) and 4.4 x 10(4) M(-1) s(-1), respectively. Granzyme K activity was shown to be inhibited by the synthetic compounds Phe-Pro-Arg-chloromethyl ketone, phenylmethylsulfonyl fluoride, PefablocSC, and benzamidine, by the Kunitz-type inhibitor aprotinin and by human blood plasma. The plasma-derived inter-alpha-trypsin inhibitor complex, its bikunin subunit, and the second carboxyl-terminal Kunitz-type domain of bikunin were identified as genuine physiologic inhibitors with K(i) values of 64, 50, and 22 nM, respectively. Inter-alpha-trypsin inhibitor and free bikunin have the potential to neutralize extracellular granzyme K activity after T cell degranulation and may thus control unspecific damage of bystander cells at sites of inflammatory reactions.

Heparin is essential for the storage of specific granule proteases in mast cells

All mammals produce heparin, a negatively charged glycosaminoglycan that is a major constituent of the secretory granules of mast cells which are found in the peritoneal cavity and most connective tissues. Although heparin is one of the most studied molecules in the body, its physiological function has yet to be determined. Here we describe transgenic mice, generated by disrupting the N-deacetylase/N-sulphotransferase-2 gene, that cannot express fully sulphated heparin. The mast cells in the skeletal muscle that normally contain heparin lacked metachromatic granules and failed to store appreciable amounts of mouse mast-cell protease (mMCP)-4, mMCP-5 and carboxypeptidase A (mMC-CPA), even though they contained substantial amounts of mMCP-7. We developed mast cells from the bone marrow of the transgenic mice. Although these cultured cells contained high levels of various protease transcripts and had substantial amounts of mMCP-6 protein in their granules, they also failed to express mMCP-5 and mMC-CPA. Our data show that heparin controls, through a post-translational mechanism, the levels of specific cassettes of positively charged proteases inside mast cells.

Dipeptidyl peptidase I is required for the processing and activation of granzymes A and B in vivo

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-/- x 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.

Anti-viral strategies of cytotoxic T lymphocytes are manifested through a variety of granule-bound pathways of apoptosis induction

Cytotoxic T cells and natural killer cells together constitute a major defence against virus infection, through their ability to induce apoptotic death in infected cells. These cytolytic lymphocytes kill their targets through two principal mechanisms, and one of these, granule exocytosis, is essential for an effective in vivo immune response against many viruses. In recent years, the authors and other investigators have identified several distinct mechanisms that can induce death in a targeted cell. In the present article, it is postulated that the reason for this redundancy of lethal mechanisms is to deal with the array of anti-apoptotic molecules elaborated by viruses to extend the life of infected cells. The fate of such a cell therefore reflects the balance of pro-apoptotic (immune) and anti-apoptotic (viral) strategies that have developed over eons of evolutionary time.

A Secreted Proform of Neutrophil Proteinase 3 Regulates the Proliferation of Granulopoietic Progenitor Cells

Myeloid leukemia cells, the human promyelocytic cell line HL-60, and a subpopulation of normal marrow cells produce a leukemia-associated inhibitor (LAI) that reversibly downmodulates DNA synthesis of normal granulopoietic progenitor cells colony-forming unit granulocyte-macrophage (CFU-GM). We isolated an active 125-kD component of LAI from HL-60 conditioned medium (CM), subjected it to cyanogen bromide cleavage and show by amino acid sequencing of the resulting peptides that it consists of a complex of the serine proteinase inhibitor 1-antitrypsin and a 31-kD fragment that retained the S-phase inhibitory activity, but resisted sequencing. This finding suggested that the 31-kD fragment originated from one of the neutrophil serine proteases (ie, elastase, proteinase 3, or cathepsin G) produced by normal promyelocytes, as well as HL-60 cells, for storage in primary granules and partly secreted during synthesis as enzymatically inactive proforms. Immunoblot analysis showed that the 125-kD complex contained proteinase 3 (PR3), and immunoprecipitation of PR3 from HL-60 CM abrogated the S-phase inhibitory activity, whereas immunoprecipitation of cathepsin G or elastase did not. Immunoprecipitation of PR3 from CM of a subpopulation of normal marrow cells also abrogated the S-phase inhibitory effect. Furthermore, CM from rat RBL and murine 32D cell lines transfected with human PR3 both reduced the fraction of CFU-GM in S-phase with 30% to 80% at 1 to 35 ng/mL PR3, whereas CM of the same cells transfected with cathepsin G or elastase did not. Also, an enzymatically silent mutant of PR3 exerted full activity, showing that the S-phase modulatory effect is not dependent on proteolytic activity. Amino acid sequencing of biosynthetically radiolabeled PR3 showed that PR3 from transfected cells is secreted after synthesis as proforms retaining amino terminal propeptides. In contrast, mature PR3 extracted from mature neutrophils has only minor activity. The inhibitory effect of secreted PR3 is reversible and abrogated by granulocyte (G)- or granulocyte-macrophage colony-stimulating factor (GM-CSF). Experiments with highly purified CD34+ bone marrow cells suggested that PR3 acts directly on the granulopoietic progenitor cells. These observations suggest a role for PR3 in regulation of granulopoiesis, and possibly in suppression of normal granulopoiesis in leukemia.

A Secreted Proform of Neutrophil Proteinase 3 Regulates the Proliferation of Granulopoietic Progenitor Cells

Myeloid leukemia cells, the human promyelocytic cell line HL-60, and a subpopulation of normal marrow cells produce a leukemia-associated inhibitor (LAI) that reversibly downmodulates DNA synthesis of normal granulopoietic progenitor cells colony-forming unit granulocyte-macrophage (CFU-GM). We isolated an active 125-kD component of LAI from HL-60 conditioned medium (CM), subjected it to cyanogen bromide cleavage and show by amino acid sequencing of the resulting peptides that it consists of a complex of the serine proteinase inhibitor 1-antitrypsin and a 31-kD fragment that retained the S-phase inhibitory activity, but resisted sequencing. This finding suggested that the 31-kD fragment originated from one of the neutrophil serine proteases (ie, elastase, proteinase 3, or cathepsin G) produced by normal promyelocytes, as well as HL-60 cells, for storage in primary granules and partly secreted during synthesis as enzymatically inactive proforms. Immunoblot analysis showed that the 125-kD complex contained proteinase 3 (PR3), and immunoprecipitation of PR3 from HL-60 CM abrogated the S-phase inhibitory activity, whereas immunoprecipitation of cathepsin G or elastase did not. Immunoprecipitation of PR3 from CM of a subpopulation of normal marrow cells also abrogated the S-phase inhibitory effect. Furthermore, CM from rat RBL and murine 32D cell lines transfected with human PR3 both reduced the fraction of CFU-GM in S-phase with 30% to 80% at 1 to 35 ng/mL PR3, whereas CM of the same cells transfected with cathepsin G or elastase did not. Also, an enzymatically silent mutant of PR3 exerted full activity, showing that the S-phase modulatory effect is not dependent on proteolytic activity. Amino acid sequencing of biosynthetically radiolabeled PR3 showed that PR3 from transfected cells is secreted after synthesis as proforms retaining amino terminal propeptides. In contrast, mature PR3 extracted from mature neutrophils has only minor activity. The inhibitory effect of secreted PR3 is reversible and abrogated by granulocyte (G)- or granulocyte-macrophage colony-stimulating factor (GM-CSF). Experiments with highly purified CD34+ bone marrow cells suggested that PR3 acts directly on the granulopoietic progenitor cells. These observations suggest a role for PR3 in regulation of granulopoiesis, and possibly in suppression of normal granulopoiesis in leukemia.

Active recombinant rat dipeptidyl aminopeptidase I (cathepsin C) produced using the baculovirus expression system

An active form of rat dipeptidyl aminopeptidase I (DPPI, cathepsin C) was obtained by heterologous expression in insect cells. Baculoviruses carrying a cDNA sequence encoding the entire rat DPPI precursor was used to infect High Five cells in a serum-free medium. Recombinant DPPI (rDPPI) was secreted into the medium from which it was purified by a combination of ammonium sulfate fractionation, hydrophobic interaction chromatography (HIC), and ion-exchange chromatography. A polyhistidine-tagged form of the enzyme (HT-rDPPI) was purified from the medium by immobilized metal affinity chromatography (IMAC). In vivo activation of native rat DPPI involves at least three chain cleavages per subunit and the ability of the expression system to imitate this processing was investigated. Both rDPPI and HT-rDPPI were secreted into the medium as unprocessed and inactive proenzymes and gradually converted into their active forms in the medium. This process was not completed at the time of harvest but mature enzyme processed similarly to native rat and human DPPI could be obtained by incubating the eluates from the HIC and IMAC columns at pH 4.5 and 5 degrees C for 18-40 h. The yield of purified and matured enzyme was approximately 50 mg/liter, and it was shown that rDPPI and HT-rDPPI were active against both a dipeptide-p-nitroanilide substrate and human growth hormone N-terminally extended with an Ala-Glu dipeptide.

On the Role of the Proform-Conformation for Processing and Intracellular Sorting of Human Cathepsin G

The serine protease cathepsin G is synthesized during the promyelomonocytic stage of neutrophil and monocyte differentiation. After processing, including removal of an amino-terminal propeptide from the catalytically inactive proform, the active protease acquires a mature conformation and is stored in azurophil granules. To investigate the importance of the proform-conformation for targeting to granules, a cDNA encoding a double-mutant form of human preprocathepsin G lacking functional catalytic site and amino-terminal prodipeptide (CatG/Gly201/▵Gly19Glu20) was constructed, because we were not able to stably express a mutant lacking only the propeptide. Transfection of the cDNA to the rat basophilic leukemia RBL-1 and the murine myeloblast-like 32D cl3 cell lines resulted in stable, protein-expressing clones. In contrast to wild-type proenzyme, CatG/Gly201/▵Gly19Glu20 adopted a mature conformation cotranslationally, as judged by the early acquisition of affinity to the serine protease inhibitor aprotinin, appearing before the carboxyl-terminal processing and also in the presence of the Golgi-disrupting agent brefeldin A. The presence of a mature amino-terminus was confirmed by amino-terminal radiosequencing. As with wild-type proenzyme, CatG/Gly201/▵Gly19Glu20 was proteolytically processed carboxyl-terminally and glycosylated with asparagine-linked carbohydrates that were converted into complex forms. Furthermore, it was targeted to granules, as determined by subcellular fractionation. Our results show that the initial proform-conformation is not critical for intracellular sorting of human cathepsin G. Moreover, we demonstrate that double-mutant cathepsin G can achieve a mature conformation before carboxyl-terminal processing of the proform.

© 1998 by The American Society of Hematology.

On the Role of the Proform-Conformation for Processing and Intracellular Sorting of Human Cathepsin G

The serine protease cathepsin G is synthesized during the promyelomonocytic stage of neutrophil and monocyte differentiation. After processing, including removal of an amino-terminal propeptide from the catalytically inactive proform, the active protease acquires a mature conformation and is stored in azurophil granules. To investigate the importance of the proform-conformation for targeting to granules, a cDNA encoding a double-mutant form of human preprocathepsin G lacking functional catalytic site and amino-terminal prodipeptide (CatG/Gly201/▵Gly19Glu20) was constructed, because we were not able to stably express a mutant lacking only the propeptide. Transfection of the cDNA to the rat basophilic leukemia RBL-1 and the murine myeloblast-like 32D cl3 cell lines resulted in stable, protein-expressing clones. In contrast to wild-type proenzyme, CatG/Gly201/▵Gly19Glu20 adopted a mature conformation cotranslationally, as judged by the early acquisition of affinity to the serine protease inhibitor aprotinin, appearing before the carboxyl-terminal processing and also in the presence of the Golgi-disrupting agent brefeldin A. The presence of a mature amino-terminus was confirmed by amino-terminal radiosequencing. As with wild-type proenzyme, CatG/Gly201/▵Gly19Glu20 was proteolytically processed carboxyl-terminally and glycosylated with asparagine-linked carbohydrates that were converted into complex forms. Furthermore, it was targeted to granules, as determined by subcellular fractionation. Our results show that the initial proform-conformation is not critical for intracellular sorting of human cathepsin G. Moreover, we demonstrate that double-mutant cathepsin G can achieve a mature conformation before carboxyl-terminal processing of the proform.

© 1998 by The American Society of Hematology.

Regulated expression, processing, and secretion of dog mast cell dipeptidyl peptidase I

Dipeptidyl peptidase I (DPPI) is a cysteine protease found predominantly in myelomonocytic cells, cytotoxic T-cells, and mast cells. Recent studies identify an intracellular role for mast cell-DPPI (MC-DPPI) by activating prochymase and protryptase to their mature forms. To better define MC-DPPI and to explore the possibility of extracellular roles, we purified MC-DPPI from mastocytoma cells. We found the dog C2 mastocytoma cell line to be the richest source yet described for DPPI, purifying up to 200 microg of enzyme per g of cells. Dog MC-DPPI has an Mr of approximately 175,000 and consists of four subunits, each composed of a propeptide, light chain, and heavy chain. The heavy chain is N-glycosylated and is heterogeneously processed to three different forms. NH2-terminal sequences of the heavy chain and propeptide are identical to those predicted from a cDNA clone we sequenced from a mastocytoma cDNA library. The dog cDNA-derived sequence is 86% identical to that of human DPPI. Dog mastocytoma cells incubated with 12-O-tetradecanoylphorbol-13-acetate increase expression of MC-DPPI mRNA. MC-DPPI maintains its activity for dipeptide substrates at a neutral to alkaline pH. Cells stimulated with ionophore or substance P secrete MC-DPPI in parallel with the granule-associated mediators tryptase and histamine. Thus, dog mastocytoma cells secrete DPPI that is active at the pH of extracellular fluids, suggesting that MC-DPPI may act outside the cell.

Dipeptidyl Peptidase I and Granzyme A Are Coordinately Expressed During CD8+ T Cell Development and Differentiation

Dipeptidyl peptidase I (DPPI) is a granule protease that plays a requisite role in processing the proenzyme form of the CTL granule serine proteases (granzymes). This study assesses DPPI mRNA and enzyme expression during T lymphocyte ontogeny and CTL differentiation. The most immature CD3−CD4−CD8− thymocytes were found to express >40-fold higher levels of DPPI mRNA, although levels of DPPI enzymatic activity in CD3−CD4−CD8− thymocytes were only modestly higher than those seen for CD4+CD8+ or CD4+CD8− thymocytes. More mature CD8+CD4− thymocytes and CD8+ splenocytes expressed significantly higher levels of DPPI mRNA and enzymatic activity than CD4+CD8+ or CD4+CD8− thymocytes. Granzyme A mRNA expression was observed in DPPI expressing CD3−CD4−CD8− and CD8+CD4− thymocytes and was also observed in CD8+CD4− splenocytes; however, expression was not observed in CD4+CD8+ or CD4+CD8− thymocytes. Both DPPI mRNA and granzyme A mRNA expression in CD8+ T cells decreased to very low or undetectable levels during the first 48 h after allostimulation in MLCs. However, peak levels of both DPPI and granzyme A expression were observed later in the course of CD8+ T cell responses to alloantigen, with DPPI mRNA expression peaking on either day 3 or day 4 and granzyme A expression peaking at the end of a 5-day MLR. These data indicate that DPPI is expressed at all stages of T cell ontogeny and differentiation in which granzyme A mRNA is detected; consequently, DPPI appears to be available for the processing and activation of granzyme A during both CD8+ T cell development and differentiation.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Cytotoxic T lymphocytes and natural killer cells together comprise the means by which the immune system detects and rids higher organisms of virus-infected or transformed cells. Although differing considerably in the way they detect foreign or mutated antigens, these cells utilize highly analogous mechanisms for inducing target cell death. Both types of effector lymphocytes utilize two principal contact-dependent cytolytic mechanisms. The first of these, the granule exocytosis mechanism, depends on the synergy of a calcium-dependent pore-forming protein, perforin, and a battery of proteases (granzymes), and it results in penetration by effector molecules into the target cell cytoplasm and nucleus. The second, which requires binding of FasL (CD95L) on the effector cell with trimeric Fas (CD95) molecules on receptive target cells, is calcium independent and functions by generating a death signal at the inner leaflet of the target cell membrane. Exciting recent developments have indicated that both cytolytic mechanisms impinge on an endogenous signaling pathway that is strongly conserved in species as diverse as helminths and humans and dictates the death or survival of all cells.

Production of fully active recombinant murine granzyme B in yeast

Granzyme B (GzmB) is a neutral serine protease found in cytotoxic lymphocytes; this enzyme is critically involved in delivering the rapid apoptotic signal to susceptible target cells. GzmB has been difficult to study and has not yet been produced in non-mammalian systems because of the complex processing events that are thought to be required for its activation. In this report, we have successfully produced fully active, soluble recombinant GzmB (rGzmB) in a yeast-based system by fusing GzmB cDNA in frame with yeast alpha-factor cDNA, using the yeast KEX2 signal peptidase to release the processed enzyme into the supernatant of yeast cultures. We expressed the proenzyme form of GzmB as well and determined that pro-GzmB is efficiently converted to its active form by the cysteine proteinase dipeptidyl peptidase I. The fully processed enzyme was able to hydrolyze the synthetic substrate N-t-butyloxycarbonyl-L-alanyl-L-alanyl-L-aspartyl (Boc-Ala-Ala-Asp) thiobenzyl ester with a kcat of 17 s-1 and catalytic efficiency kcat/Km of 181,237 M-1 S-1; the recombinant enzyme is therefore at least twice as active as purified native GzmB. In addition, the recombinant enzyme hydrolyzes Boc-Ala-Ala-Met thiobenzyl ester with a kcat of 3.2 S-1 and a catalytic efficiency kcat/Km of 65,306 M-1 S-1. Purified rGzmB can also cleave the putative substrate caspase-3 into its signature p20/p10 forms. Unlike caspases, rGzmB is not sensitive to inhibition by several peptide-based inhibitors, including Ac-DEVD-CHO, Ac-YVAD-CMK, and ZIETD-FMK, as well as Zn2+ (a known inhibitor of caspase-3). Structural studies of rGzmB may allow us to better understand the substrate specificity of this enzyme and to design better inhibitors.

The gene encoding dipeptidyl aminopeptidase BI from Pseudomonas sp. WO24: cloning, sequencing and expression in Escherichia coli

We have isolated the dipeptidyl aminopeptidase BI (DAP BI) gene from the plasmid library of Pseudomonas sp. WO24 chromosomal DNA by the enzymatic plate assay using a chromogenic substrate. The DAP BI gene, designated dap b1, was further subcloned and sequenced. Sequence analysis of an approx. 3-kb fragment revealed an open reading frame of 2169 nucleotides, which was assigned to the dap b1 gene by N-terminal and internal amino acid sequences. The predicted amino acid sequence of DAP BI containing a serine protease Gly-X-Ser-X-Gly consensus motif displays extensive homologies to the several proteases belonging to the prolyl oligopeptidase family, a novel serine protease family possessing the catalytic triad with a specific array of Ser, Asp and His in this order, which is the hallmark of the member of this family including DAP IV. The dap b1 gene was expressed in Escherichia coli and the expressed enzyme was purified about 230-fold with 2.6% recovery from the cell-free extracts. The enzymatic properties such as molecular mass, substrate specificity and effect of inhibitor were similar to the native enzyme from Pseudomonas sp. WO24.

Mouse mast cell protease 9, a novel member of the chromosome 14 family of serine proteases that is selectively expressed in uterine mast cells

Mouse mast cell protease (mMCP) 1, mMCP-2, mMCP-4, and mMCP-5 are members of a family of related serine proteases whose genes reside within an approximately 850 kilobase (kb) complex on chromosome 14 that does not readily undergo crossover events. While mapping the mMCP-1 gene, we isolated a novel gene that encodes a homologous serine protease designated mMCP-9. The mMCP-9 and mMCP-1 genes are only approximately 7 kb apart on the chromosome and are oriented back to back. The proximity of the mMCP-1 and mMCP-9 genes now suggests that the low recombination frequency of the complex is due to the closeness of some of its genes. The mMCP-9 transcript and protein were observed in the jejunal submucosa of Trichinella spiralis-infected BALB/c mice. However, in normal BALB/c mice, mMCP-9 transcript and protein were found only in those mast cells that reside in the uterus. Thus, the expression of mMCP-9 differs from that of all other chymases. The observation that BALB/c mouse bone marrow-derived mast cells developed with interleukin (IL) 10 and c-kit ligand contain mMCP-9 transcript, whereas those developed with IL-3 do not, indicates that the expression of this particular chymase is regulated by the cytokine microenvironment. Comparative protein structure modeling revealed that mMCP-9 is the only known granule protease with three positively charged regions on its surface. This property may allow mMCP-9 to form multimeric complexes with serglycin proteoglycans and other negatively charged proteins inside the granule. Although mMCP-9 exhibits a >50% overall amino acid sequence identity with its homologous chymases, it has a unique substrate-binding cleft. This finding suggests that each member of the chromosome 14 family of serine proteases evolved to degrade a distinct group of proteins.

Molecular cloning, chromosomal localization, and expression of murine dipeptidyl peptidase I

Dipeptidyl peptidase I (DPPI) is a lysosomal cysteine protease that catalyzes the sequential removal of dipeptides from the amino termini of various protein substrates. We have isolated a cDNA coding for murine DPPI from mouse thymus and spleen cDNA libraries. The deduced amino acid sequence codes for a protein of 462 amino acid residues; comparison of this deduced sequence with that of rat and human DPPI revealed 90.1% and 77.8% identity, respectively. Using DPPI cDNA, we obtained two BAC (Bacterial Artificial Chromosome) clones that contained the murine DPPI locus. The DPPI gene consists of seven exons and 6 introns, and spans approximately 20 kilobases. Using fluorescence in situ chromosome hybridization, we localized murine DPPI to chromosome 7D3-E1.1. We determined that DPPI protein is widely distributed in mouse tissues, although its relative abundance varies from tissue to tissue. In contrast to previous reports, we show here that DPPI mRNA and protein levels and enzymatic activity are unchanged during in vitro T cell activation, implying that this enzyme is not rate-limiting for granzyme processing.

Human dipeptidyl-peptidase I. Gene characterization, localization, and expression

Dipeptidyl-peptidase I, a lysosomal cysteine proteinase, is important in intracellular degradation of proteins and appears to be a central coordinator for activation of many serine proteinases in immune/inflammatory cells. Little is known about the molecular genetics of the enzyme. In the present investigation the gene for dipeptidyl-peptidase I was cloned and characterized. The gene spans approximately 3.5 kilobases and consists of two exons and one intron. The genomic organization is distinct from the complex structures of the other members of the papain-type cysteine proteinase family. By fluorescence in situ hybridization, the gene was mapped to chromosomal region 11q14.1-q14.3. Analysis of the sequenced 5'-flanking region revealed no classical TATA or CCAAT box in the GC-rich region upstream of cap site. A number of possible regulatory elements that could account for tissue-specific expression were identified. Northern analyses demonstrated that the dipeptidyl-peptidase I message is expressed at high levels in lung, kidney, and placenta, at moderate to low levels in many organs, and at barely detectable levels in the brain, suggesting tissue-specific regulation. Among immune/inflammatory cells, the message is expressed at high levels in polymorphonuclear leukocytes and alveolar macrophages and their precursor cells. Treatment of lymphocytes with interleukin-2 resulted in a significant increase in dipeptidyl-peptidase I mRNA levels, suggesting that this gene is subjected to transcriptional regulation. The results provide initial insights into the molecular basis for the regulation of human dipeptidyl-peptidase I.

Biosynthesis, processing and sorting of neutrophil proteins: insight into neutrophil granule development

Neutrophil granulocytes are specialized phagocytic cells that carry a collection of granules for regulated secretion, each with distinct constituents. The granules can be classified as azurophil (primary), developed first, followed in time by specific (secondary) granules gelatinase granules, and secretory vesicles. Stage- and tissue-specific transcription factors govern the successive expression of genes for granule proteins to allow storage of the gene products in these organelle categories whose packaging is separated in time. Many of the granule proteins, in particular those of the heterogeneous lysosome-like azurophil granules, are subject to extensive post-translational proteolytic processing into mature proteins, most commonly as a post-sorting event. A selective aggregation of proteins destined for storage in granules, as discussed in this review, would facilitate their retention and eliminate a need for distinct sorting motifs on each granule protein. Aggregation of granule proteins, that are often cationic, would be assisted by the anionic serglycin proteoglycans present in neutrophils. The antibacterial granule proteins can serve as models for antibiotics and some of them possess a potentially useful therapeutic ability to bind and neutralize endotoxin. Because aberrant expression of transcription factors regulating the synthesis of granule proteins is often found in leukemia, the clarification of mechanisms regulating the timed expression of granule proteins will shed light on the maturation block in myeloid leukemias.

Recombinant human proteinase 3, the Wegener's autoantigen, expressed in HMC-1 cells is enzymatically active and recognized by c-ANCA

We developed a stable expression system for conformationally intact recombinant human PR3 (rPR3) using the human mast cell line HMC-1. Like in U937 cells, the rPR3 is processed from a 34 kDa precursor to the 29 kDa mature form, primarily as the result of oligosaccharide trimming. The rPR3 binds [3H]DFP and hydrolyzes the substrate N-methoxysuccinyl-Ala-Ala-Pro-Val-pNA. The enzymatic activity is inhibited by greater than 95% by alpha 1-PI. The rPR3 and the enzymatically inactive mutant rPR3-S176A are both packaged in granules. Thus, proteolytic autoprocessing is not required for PR3's targeting to granules. This rPR3 is the first to be recognized by most c-ANCA from WG patients and all anti-PR3 ANCA that were detected by standard anti-PR3 specific ELISA. This expression system for rPR3 represents a versatile tool for the analysis of its intracellular processing, structure-function relationships and interaction with autoantibodies.

Antibiotic proteins of polymorphonuclear leukocytes

The polymorphonuclear leukocyte (PMN) plays an essential role in the innate defense of the mammalian host against bacterial invaders. Responding chemotactically, the PMN delivers a complex antibiotic arsenal to sites of infection. Among these cytotoxic systems is an array of antimicrobial proteins and peptides that the PMN directs at microorganisms both before (i.e. extracellularly) and after sequestration into a phagocytic vacuole. In addition to their microbicidal capacity, several of these proteins bind to and neutralize the endotoxic activity of Gram-negative bacterial lipopolysaccharides (LPS). In this review the principle features of these antibiotic proteins are briefly summarized with emphasis on their possible actions in biological settings. In many instances, additional functions independent of cytotoxicity have been described raising the possibility that some of these proteins subserve multiple roles in inflammation.

Expression of human recombinant granzyme A zymogen and its activation by the cysteine proteinase cathepsin C

Human granzyme A is one of the serine proteinases present in the granules of cytotoxic T lymphocytes and natural killer cells. Granzymes are synthesized as inactive proenzymes with an amino-terminal prodipeptide, which is processed during transport of granzymes to the cytotoxic granules, where they are stored as active proteinases. In this study, we explored the possibility of producing recombinant granzymes. Recombinant human granzyme A zymogen was expressed in several eukaryotic cell lines (HepG2, Jurkat, and COS-1) after infection with a recombinant vaccinia virus containing full-length granzyme A cDNA. Immunoblot analysis of cell lysates showed that all infected cells produced a disulfide-linked homodimer of identical molecular weight as natural granzyme A. Infected HepG2 cells produced the largest amount of this protease (approximately 160 times more than lymphokine activated killer (LAK) cells). The recombinant protein only had high mannose type oligosaccharides as did the natural protein. Although infected HepG2 and COS cells contained high granzyme A antigen levels, lysates from these cells did not show any granzyme A proteolytic activity. However, the inactive proenzyme could be converted into active granzyme A by incubation with the thiol proteinase cathepsin C (dipeptidyl peptidase I). This study is the first to demonstrate expression of an active recombinant human cytotoxic lymphocyte proteinase and conversion of inactive progranzyme A into an active enzyme by cathepsin C. We suggest that a similar approach can be used for the production of other granzymes and related proteinases.

Post-transcriptional regulation of chymase expression in mast cells. A cytokine-dependent mechanism for controlling the expression of granule neutral proteases of hematopoietic cells

Although all mouse mast cells are derived from a common progenitor, these effector cells exhibit tissue-specific differences in their expression of the chymase family of serine proteases whose genes reside on chromosome 14. Immature bone marrow-derived mast cells (mBMMC), developed in vitro with interleukin (IL) 3-enriched medium, were cultured in the presence or absence of IL-10 to determine at the molecular level how the expression of the individual chymases is differentially regulated. As assessed by RNA blot analysis, mBMMC contain high steady-state levels of the transcript that encodes mouse mast cell protease (mMCP) 5, but not the homologous chymase transcripts that encode mMCP-1, mMCP-2, or mMCP-4. Nevertheless, nuclear run-on analysis revealed that these cells transcribe all four mast cell chymase genes. IL-10 elicited high steady-state levels of the mMCP-2 transcript, and pulse-chase experiments revealed that the half-life of the mMCP-2 transcript in mBMMC maintained in the presence of IL-10 is approximately 4-fold longer than that in replicate cells subsequently cultured in medium without IL-10. Reverse transcription-polymerase chain reaction/nucleotide sequence analysis demonstrated that mBMMC cultured in the absence or presence of IL-10 correctly process mMCP-2 pre-mRNA. Experiments with cycloheximide and actinomycin D indicated that IL-10 induces expression of a trans-acting factor(s) that stabilizes the mMCP-2 transcript or facilitates its processing. The discovery that the expression of certain chymases in mBMMC is regulated primarily at the post-transcriptional level provides a basis for understanding the mechanism by which specific cytokines dictate expression of the chromosome 14 family of serine proteases in cells that participate in inflammatory processes.

A novel dipeptidyl aminopeptidase from Pseudomonas sp. strain WO24

An activity similar to that of dipeptidyl aminopeptidase I (DAP I) which releases dipeptide from Gly-Arg-p-nitroanilide (Gly-Arg-pNA) was detected in a Pseudomonas sp. An enzyme was isolated and purified about 400-fold by a series of column chromatographies. The enzyme, named DAP BI (DAP from bacteria, type I), was revealed to be homogeneous by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and isoelectric focusing. The molecular mass was estimated to be 82 kDa by SDS-PAGE and 65 kDa by gel filtration, suggesting that the enzyme may be a monomer. The enzyme had an isoelectric point of 4.7. It is optimally active at pH 9.0. The Km and Vmax of the enzyme for Gly-Arg-pNA were 0.25 mM and 195 micromol/min/mg, respectively. The purified enzyme did not hydrolyze Gly-Phe-pNA, which was also a substrate for DAP I, whereas it hydrolyzed Arg-Arg-4-methoxy-beta-naphthylamide (Arg-Arg-MNA), a model substrate for DAP III. The Km and Vmax for Arg-Arg-MNA were 0.019 mM and 145 micromol/min/mg, respectively. This purified enzyme can also catalyze the removal of Asp-Arg from the N termini of angiotensins I and II. The enzyme activity was completely inhibited by Zn(II) (0.5 mM), tosyl-L-Lys-chloromethyl ketone (0.1 mM), and leupeptin (0.1 mM) and partially inhibited by Co(II) (0.5 mM) and chymostatin (0.1 mM), whereas the enzyme was not affected by general serine protease inhibitors (phenylmethylsulfonyl fluoride and diisopropylfluorophosphate) and thiol protease inhibitors. The substrate specificity, classification of catalytic site, and other enzymatic properties demonstrate that this enzyme is distinct from the previously described mammalian DAPs I and III and Saccharomyces cerevisiae DAP III. These results indicate that DAP BI may be a new type of the DAP family.

Biosynthesis and processing of proteinase 3 in U937 cells. Processing pathways are distinct from those of cathepsin G

Proteinase 3 is a human polymorphonuclear leukocyte serine proteinase that degrades elastin in vitro and causes emphysema when administered by intratracheal insufflation into hamsters. Proteinase 3, stored in the azurophilic granules, is expressed in progenitor cells of myeloid origin. In the present study, the biosynthesis, processing, and intracellular transport of the enzyme was investigated in the human myelomonocytic cell line U937. Proteinase 3 is initially identified as a 35-kDa precursor and converted into the 29-kDa mature form within 3 h. By using a combination of techniques including amino-terminal sequencing, we identified the 35-kDa form as a zymogen containing an activation dipeptide but lacking the amino-terminal 25 residues, presumably the result of cleavage by a signal peptidase. Tunicamycin treatment and alkalinization of acidic cell compartments with NH4Cl did not prevent the processing of the proteinase 3 zymogen into the mature form, suggesting that the enzyme is targeted to the cytoplasmic granules by a mechanism other than the mannose 6-phosphate receptor. Brefeldin A inhibited the zymogen processing, suggesting that the dipeptide cleavage occurred in a post-Golgi organelle. The enzyme responsible for the removal of the dipeptide is a cysteine proteinase since E-64d, a class-specific inhibitor, prevented processing. However, treatment of cells with a dipeptidyl peptidase I inhibitor, Gly-Phe-diazomethyl ketone and with the lysosomotropic agents, NH4Cl and chloroquine, did not prevent dipeptide cleavage, indicating that the processing enzyme for proteinase 3 is not dipeptidyl peptidase I. In contrast, Gly-Phe-diazomethyl ketone inhibited cleavage of the dipeptide from cathepsin G. This indicates that processing of proteinase 3 is distinct from that of cathepsin G. Proteinase 3 is also processed at the COOH-terminal extension. Cleavage takes place next to Arg-222, suggesting that a trypsin-like proteinase is involved in the COOH-terminal processing.

Human cathepsin G lacking functional glycosylation site is proteolytically processed and targeted for storage in granules after transfection to the rat basophilic/mast cell line RBL or the murine myeloid cell line 32D

The neutral protease cathepsin G belongs to a family of hematopoietic serine proteases stored in the azurophil granules of the neutrophil granulocyte. To investigate the function of asparagine-linked carbohydrates in neutrophil serine proteases, we constructed a mutant cDNA, coding for human cathepsin G deficient of a functional glycosylation site, for use in a transgenic cellular model. Wild type and mutant cDNA were stably expressed in the rat basophilic/mast cell line RBL and in the murine myeloblast-like cell line 32D. Biosynthetic labeling, followed by immunoprecipitation, SDS-polyacrylamide gel electrophoresis, and fluorography, showed that carbohydrate-deficient cathepsin G was synthesized as a 29-kDa proform in both cell lines. The proform was proteolytically processed into a stable form with an apparent molecular mass of 27.5 kDa, indicating removal of the carboxyl-terminal prodomain. The mutant cathepsin G was enzymatically activated as determined by acquisition of affinity to aprotinin, a serine protease inhibitor. As for wild type cathepsin G, small amounts of the unprocessed form of the mutated enzyme were released from the cells, while the major part was transferred to a granular compartment as demonstrated by subcellular fractionation. Thus, neither processing leading to enzymatic activation nor granular sorting was obviously affected by the lack of oligosaccharides on the mutant cathepsin G. Our results therefore indicate that glycosylation is not essential for these processes. In addition to the previously utilized cell line RBL, we propose the 32D cell line as a suitable cellular model for transgenic expression of human neutrophil serine proteases.

Packaging of proteases and proteoglycans in the granules of mast cells and other hematopoietic cells. A cluster of histidines on mouse mast cell protease 7 regulates its binding to heparin serglycin proteoglycans

Mouse mast cell protease 7 (mMCP-7) is a tryptase stored in the secretory granules of mast cells. At the granule pH of 5.5, mMCP-7 is fully active and is bound to heparin-containing serglycin proteoglycans. to understand the interaction of mMCP-7 with heparin inside and outside the mast cell, this trytase was first studied by comparative protein modeling. The "pro" form of mMCP-7 was then expressed in insect cells and studied by site-directed mutagenesis. Although mMCP-7 lacks known linear sequences of amino acis that interact with heparin, the three-dimensional model of mMCP-7 revealed an area on the surface of the folded protein away from the substrate-binding site that exhibits a strong positive electrostatic potential at the acidic pH of the granule. In agreement with this calculation, recombinant pro-mMCP-7 bound to a heparin-affinity column at pH 5.5 and readily dissociated from the column at pH > 6.5. Site-directed mutagenesis confirmed the prediction that the conversion of His residues 8,68, and 70 in the positively charged region into Glu prevents the binding of pro-mMCP-7 to heparin. Because the binding requires positively charged His residues, native mMCP-7 is able to dissociate from the protease/proteoglycan macromolecular complex when the complex is exocytosed from bone marrow-derived mast cells into a neutral pH environment. Many hematopoietic effector cells store positively charged proteins in granules that contain serglycin proteoglycans. The heparin/mMCP-7 interaction, which depends on the tertiary structure of the tryptase, may be representative of a general control mechanism by which hematopoietic cells maximize storage of properly folded, enzymatically active proteins in their granules.

Regulation of the activity of human chymase during storage and release from mast cells: the contributions of inorganic cations, pH, heparin and histamine

Chymase, the major chymotryptic proteinase of human mast cells, can be released in substantial quantities following mast cell activation. As this enzyme is stored in the secretory granules in its fully active form, we have investigated various factors which might regulate its activity in storage and upon release. Chymase was purified from human skin by high salt extraction, cetylpyridinium chloride precipitation, heparin agarose affinity chromatography and gel filtration. Neither the addition of Mg2+ or Ca2+ (0.3-10 mM) nor their sequestration by EDTA had any effect on the rate of cleavage of the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide. Monovalent cations (Na+,K+) enhanced enzyme activity, but only at non-physiological concentrations (0.5-3.0 M), suggesting an ionic strength effect. At constant I = 0.15, enzyme activity was strongly pH-dependent: at pH 5.5 (the approximate pH of the mast cell granule) the activity was only 10% of that at pH 7.5 (the approximate pH of the extracellular space). Heparin, which is stored with chymase in the mast cell granule, accentuated this difference by enhancing activity at pH 7.5 by 33% and depressing it a pH 5.5 by 40%. Histamine at concentrations up to 50 mM (I = 0.15) had little effect on chymase activity at either pH, although high concentrations did attenuate the actions of heparin. It is concluded that pH and the interaction with heparin are central to the regulation of chymase activity within the granule and following release.

Carboxyl-terminal prodomain-deleted human leukocyte elastase and cathepsin G are efficiently targeted to granules and enzymatically activated in the rat basophilic/mast cell line RBL

The hematopoietic neutral serine proteases leukocyte elastase and cathepsin G are synthesized as inactive precursors, but become activated by removal of an amino-terminal dipeptide and are stored in granules. Moreover, the pro forms of elastase and cathepsin G show carboxyl-terminal prodomains of 20 and 11 amino acids, respectively, which are not present in the mature enzymes. To investigate mechanisms of processing, activation, and granular targeting, we have utilized transgenic expression of myeloid serine proteases in the rat basophilic/mast cell line RBL-1 (Gullberg, U., Lindmark, A., Nilsson, E., Persson, A.-M., and Olsson, I. (1994) J. Biol. Chem. 269, 25219-25225). Leukocyte elastase was stably expressed in RBL-1 cells, and the translation products were characterized by biosynthetic labeling followed by immunoprecipitation, SDS-polyacrylamide gel electrophoresis, and fluorography. Processing of a main pro form of 34 kDa into mature 31- and 29-kDa forms was demonstrated. Translocation of mature forms to granule-containing fractions was shown by subcellular fractionation experiments. The processed forms were enzymatically active, judging by the occurrence of amino-terminal processing demonstrated by radiosequence analysis, the acquisition of affinity for the protease inhibitor aprotinin, and the appearance of elastase activity in transfected RBL cells. To investigate the function of the carboxyl-terminal prodomains, deletion mutants of leukocyte elastase and cathepsin G lacking the carboxyl-terminal extension were constructed and transfected into RBL cells. Our results show that as full-length proteins, the deletion mutants were converted to active enzymes and transferred to granules with kinetics similar to that of wild-type enzymes. We conclude that human leukocyte elastase and cathepsin G are converted into enzymatically active forms when expressed in RBL cells and targeted for storage in granules; the carboxyl-terminal prodomains are necessary neither for enzymatic activation nor for targeting to granules in RBL cells.

Mast cell procarboxypeptidase A. Molecular modeling and biochemical characterization of its processing within secretory granules

Previously, we characterized murine mast cell procarboxypeptidase A (MC-proCPA) as an inactive zymogen. To investigate the mechanisms for this lack of enzymatic activity and the processing of the zymogen to the active form, we now have performed molecular modeling of the tertiary structure of murine MC-proCPA based on the x-ray crystallographic structures of porcine pancreatic procarboxypeptidases A and B. Our model predicts that MC-proCPA retains a high degree of structural similarity to its pancreatic homologues. The globular propeptide physically blocks access to the fully formed active site of the catalytic domain and contains a salt bridge to the substrate-binding region that precludes docking of even small substrates. Based on consideration of the predicted tertiary structure and charge field characteristics of the model, the activation site (between GluA94 and Ile1) appears to be highly exposed even after MC-proCPA binds to secretory granule proteoglycans. Based on the steady-state levels of MC-proCPA versus MC-CPA, cycloheximide inhibition of protein synthesis, and brefeldin A blockage of protein sorting, we show that MC-proCPA is processed rapidly in murine mast cell line KiSV-MC14 with a half-life of 26 +/- 5 min (mean +/- S.D., n = 3), and the processing occurs within the secretory granules. The enzyme responsible for this processing may be a thiol protease since treatment of the KiSV-MC14 with 200 microM E-64d, a selective thiol-protease inhibitor, increases MC-proCPA by 2.7 +/- 0.2-fold (mean +/- S.D., n = 3) within 6 h of application.

Structure and biogenesis of lytic granules

Lytic granules are specialized secretory organelles which appear after activation of CTLs and NK cells. The lytic granules contain a series of proteins that mediate target cell destruction after secretion from the cell. In addition, these organelles serve as the lysosomes of these lymphocytes. At the EM level three types of granules with distinct regions are distinguished. Intriguingly, lytic and lysosomal proteins are localized in distinct regions. This is particularly interesting because lysosomal and lytic proteins can use the same sorting mechanisms to be targeted to this compartment. We favor the idea that a combination of sorting mechanisms result in this final segregation: the MPR receptor sorts both lysosomal proteins and granzymes from the Golgi complex, but a second event, such as selective aggregation with proteoglycans, then results in the segregation of lytic and lysosomal proteins in the granule. Lytic granules provide a way to store and simultaneously secrete the lytic proteins in a highly specific fashion. The granules are able to move along microtubules using a kinesin-like motor, and thus can cluster at the site of membrane contact with a target cell. Once polarized, the granules exocytose their contents, using a molecular machinery that is as yet poorly defined. Understanding the machinery involved in both functions of the lytic granules will provide ways to control the action of cytotoxic lymphocytes, ultimately in clinical situations.

Activation of thrombin-inactivated single-chain urokinase-type plasminogen activator by dipeptidyl peptidase I (cathepsin C)

Single-chain urokinase-type plasminogen activator (scu-PA) is inactivated by thrombin, which cleaves the peptide bond between Arg156 and Phe157. In a search for potential activators of thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T), we found that the lysosomal aminopeptidase dipeptidyl-peptidase I or cathepsin C efficiently activates tcu-PA/T. Cathepsin C was as active towards tcu-PA/T as the bacterial proteinase thermolysin and about 300-times more active than plasmin. The activation by cathepsin C proceeded in a concentration-dependent and time-dependent manner with a pH optimum between 5 and 7. Furthermore, the effect of cathepsin C was inhibited by cystatin and stimulated by cysteine, typical for the action of a thiol proteinase. As no degradation of the tcu-PA/T molecule by cathepsin C was visible on SDS/PAGE, we suggest that activation of tcu-PA/T occurs by cleavage between Lys158-Ile159 and removal of the two N-terminal amino acid residues (Phe157-Lys158) of the B chain of tcu-PA/T. We conclude that both thrombin and dipeptidyl-peptidases like cathepsin C might play a regulatory role in the plasminogen-plasmin system by inactivating scu-PA and activating tcu-PA/T, respectively.

Cytotoxic lymphocytes require granzyme B for the rapid induction of DNA fragmentation and apoptosis in allogeneic target cells

We have generated H-2b mice with a homozygous null mutation in the granzyme (gzm) B gene. Gzm B is a neutral serine protease with Aspase activity that is found only in the granules of activated cytolytic T cells, natural killer cells, and lymphokine-activated killer cells. Gzm B-/- mice develop normally and have normal hematopoiesis and lymphopoiesis. In vitro, cytotoxic T lymphocytes (CTL) derived from gzm B-/- animals are able to induce 51Cr release from allotarget cells, but with reduced efficiency. However, gzm B-/- CTL have a profound defect in their ability to induce rapid DNA fragmentation and apoptosis in allogeneic target cells. This defect is kinetic since DNA fragmentation is partially compensated and 51Cr release is completely rescued with long incubation times. We conclude that gzm B serves a critical and nonredundant role for the rapid induction of target cell DNA fragmentation and apoptosis by alloreactive cytotoxic T lymphocytes.