Separation and partial characterization of proteinases with substrate specificity for basic amino acids from human MOLT-4 T lymphocytes: identification of those inhibited by variable-loop-V3 peptides of HIV-1 (human immunodeficiency virus-1) envelope glycoprotein

The chemokine receptor CXCR4 is expressed within the mast cell lineage and its ligand stromal cell-derived factor-1alpha acts as a mast cell chemotaxin

In this study we provide evidence that the chemokine stromal cell-derived factor-1alpha (SDF-1alpha) acts as a mast cell chemoattractant through interactions with its receptor CXCR4 expressed on mast cell progenitors in the blood as well as on in vitro-developed and leukemic mast cells. We found expression of CXCR4 on cord blood-derived mast cells (CBMC) and on the human mast cell line HMC-1, analyzed by RNAse protection assay and flow cytometry. SDF-1alpha induced intracellular calcium mobilization in HMC-1 cells and was chemotactic for both HMC-1 cells and CBMC. The activity of SDF-1alpha was completely blocked by treating the cells with pertussis toxin, indicating the involvement of Gi-proteins in the signaling. By applying a transwell assay we could show that SDF-1alpha induces migration of a cell population in peripheral blood that is enriched for cells with the capacity to differentiate into mast cells. These findings thus suggest a mechanism by which human mast cell progenitors may be recruited from circulation into the tissue.

The three-dimensional structure of recombinant leech-derived tryptase inhibitor in complex with trypsin. Implications for the structure of human mast cell tryptase and its inhibition

The x-ray crystal structure of recombinant leech-derived tryptase inhibitor (rLDTI) has been solved to a resolution of 1.9 A in complex with porcine trypsin. The nonclassical Kazal-type inhibitor exhibits the same overall architecture as that observed in solution and in rhodniin. The complex reveals structural aspects of the mast cell proteinase tryptase. The conformation of the binding region of rLDTI suggests that tryptase has a restricted active site cleft. The basic amino terminus of rLDTI, apparently flexible from previous NMR measurements, approaches the 148-loop of trypsin. This loop has an acidic equivalent in tryptase, suggesting that the basic amino terminus could make favorable electrostatic interactions with the tryptase molecule. A series of rLDTI variants constructed to probe this hypothesis confirmed that the amino-terminal Lys-Lys sequence plays a role in inhibition of human lung tryptase but not of trypsin or chymotrypsin. The location of such an acidic surface patch is in accordance with the known low molecular weight inhibitors of tryptase.

HIV type 1 V3 peptide constructs act differently on HIV type 1 infection of peripheral blood lymphocytes and macrophages

We have previously shown that a multibranched peptide construct derived from the tip of the B clade V3 loop consensus sequence (MPBC1: [GPGRAF]8-[K]4-[K]2-K-beta A-OH), but not V3 monomer peptides, inhibit human immunodeficiency virus type 1 (HIV-1) infection and syncytium formation of CD4+ T cells from immortalized lines. Here, we show that MBPC1 attaches to normal peripheral blood lymphocytes (PBLS) and monocytes but not to erythrocytes. While treatment with 5 microM MBPC1 had no significant antiviral effect on HIV-1Ba-L infection of monocyte-derived macrophages as assessed by p24 production in culture supernatants, this dose inhibited both HIV-1Ba-L and HIV-1LAI infection of PBLs. Virus production was inhibited up to 90% when MBPC1 was added to PBLs immediately after the virus, and was inhibited about 50% when it was added after 3 days; no effect was noted when it was added 7 days postinfection. MBPC1 did not affect PBL growth or IL-2 receptor and CD4 surface expression level. These observations suggest a selective antiviral effect of MBPC1 on CD4+ T lymphocytes and they provide additional circumstantial evidence that HIV-1 enters lymphocytes and monocytes by different mechanisms.

A role for urokinase-type plasminogen activator in human immunodeficiency virus type 1 infection of macrophages

Urokinase-type plasminogen activator (uPA), a proteinase which activates plasminogen by cleaving at -CPGR(arrow downward)V-, was shown to cleave the V3 loop in recombinant gp120 of human immunodeficiency virus type 1 (HIV-1) IIIB and MN strains, as well as a synthetic, cyclized peptide representing the clade B consensus sequence of V3. Proteolysis occurred at the homologous -GPGR(arrow downward)A-, an important neutralizing determinant of HIV-1. It required soluble CD4 and was prevented by inhibitors of uPA but not by inhibitors of likely contaminating plasma proteinases. It was accelerated by heparin, a known cofactor for plasminogen activation. In immune capture experiments, tight binding of uPA to viral particles, which did not depend on CD4, was also demonstrated. Active site-directed inhibitors or uPA diminished this binding, as did a neutralizing antibody to V3. Addition of exogenous uPA to the laboratory-adapted IIIB strain of HIV-1, the macrophage-tropic field strains JR-CSF and SF-162, or a fresh patient isolate of indeterminate tropism, followed by infection of macrophages with the various treated viruses, resulted in severalfold increases in subsequent viral replication, as judged by yields of reverse transcriptase activity and p24 antigen, as well as incorporation, as judged by PCR in situ. These responses were reversible by inhibitors or antibodies targeting the proteinase active site or the V3 loop. We propose that uPA, a transcriptionally regulated proteinase which is upregulated when macrophages are HIV infected, can be bound and utilized by the virus to aid in fusion and may be an endogenous component that is critical to the infection of macrophages by HIV-1.

T-cell membrane-associated serine protease, tryptase TL2, binds human immunodeficiency virus type 1 gp120 and cleaves the third-variable-domain loop of gp120. Neutralizing antibodies of human immunodeficiency virus type 1 inhibit cleavage of gp120

It has been suggested that the third variable domain (V3) loop of human immunodeficiency virus type 1 (HIV-1) gp120 has to interact with a cell-surface-associated protease(s) that acts as a cofactor after binding of gp120 to the CD4 receptor during entry of HIV-1 into susceptible cells. We isolated the membrane-associated serine protease, tryptase TL2, from human CD4-positive lymphocytes. This enzyme specifically binds gp120 through interaction with its V3 domain. To investigate the role of tryptase TL2 in HIV infection, we examined the affinity of the interaction and the proteolytic susceptibility of various recombinant gp120 expressed in mammalian cells to the enzyme, and we determined the cleavage sites. Tryptase TL2 bound gp120 with an apparent dissociation constant of 38 nM. The affinity was lower than that of gp120 for CD4 which suggests that gp120 initially binds to CD4, followed by interaction with tryptase TL2 which is localized close to CD4 on the cell surface. After binding, tryptase TL2 cleaved recombinant gp120 expressed in mammalian cells into two protein species of 70 kDa and 50 kDa but did not cleave gp120 expressed in insect cells, which indicates that the structure of the oligosaccharides linked to the polypeptide backbone of gp120 affects the proteolytic susceptibility. Cleavage was specifically inhibited by a neutralizing antibody against the V3 loop. Cleavage-site determination revealed that tryptase TL2 cleaved gp120 at various sites in the V3 in a strain-dependent manner. The amino acid variability at the cleavage site(s) in almost all HIV-1 isolates was restricted to amino acids which are susceptible to the chymotryptic and/or tryptic activities of tryptase TL2.

Inhibition of U-937 membrane-associated cathepsin G by GP120 (IIIB) and V3 loop-derived peptides from several strains of HIV-1

A cell surface-associated cathepsin G has been reported to be a possible complementary factor for HIV-1 infection of U-937 cells. The effect of recombinant gp120 (IIIB) and a series of V3 loop peptides derived from the sequence of different strains of HIV on the activity of U-937 cathepsin G was assayed. The sequence on the N-terminal side of the highly conserved GPGRAF V3 loop segment was required for interaction with cathepsin G. The inhibition was stable for several hours and there was no cleavage of the peptides derived from the HIV-1(IIIB) strain. Recombinant gp120 (IIIB) also remained uncleaved after incubation with cathepsin G for 3 h, but some cleavage occurred, generating 2 fragments (50 kDa and 70 kDa), after 16 h. Linear peptides derived from HIV-1 Mal, ELI, MN, CDC4 and SF162 strains, and consensus V3 peptides all had inhibitory properties towards cathepsin G, although they were significantly cleaved after one hour. The cleavage site was at the carboxy-terminus of Tyr323 which is conserved in all these HIV-1 strains but not in HIV-1(IIIB). There was no cleavage at the Arg residue of the GPGRAF sequence, whatever the V3 peptide sequence, the amount of proteinase, or the incubation time. We conclude that the inhibition of membrane-associated cathepsin G of U-937 cells by the gp120 V3 loop of HIV-1 does not occur via a Kunitz-type mechanism, and that the proteinase-V3 loop interaction does not result in a significant cleavage of the V3 loop, though it has been suggested that this event is required for the entry phase of the virus.

Characterization of V3 loop-binding protein(s) of Molt-4 and U937 cells

The V3 loop in gp120 of human immunodeficiency virus type 1 (HIV-1) is known as a principal neutralizing and cell-tropic determinant. Biotinylated synthetic V3 loop peptides derived from three different HIV-1 strains were used as ligands to identify the cell surface counterreceptor, which may participate in the infection of HIV-1. Two different cell lines, Molt-4 and U937, and three V3 loop peptides derived from LAVELI, HTLV-IIIMN, and HTLV-IIIB strains were used. The binding of HTLV-IIIB-derived peptide to the cell surface was confirmed using 125I-labeled surface proteins of both cell lines. The relative molecular mass of the major radioactive band on the autoradiogram was 32-33 kDa in both cell lines. A protein was purified from the plasma membrane fraction of Molt-4 cells using affinity columns coupled with three different V3 loop peptides. Two major polypeptides (32 and 33 kDa) were eluted from the affinity column. Size-exclusion chromatography showed that the protein migrated as a single peak with a molecular mass of 130 kDa. These proteins were separated by reversed-phase chromatography, which indicated that the 32-kDa protein is more hydrophobic than the 33-kDa protein in Molt-4 cells. A similar but not identical 130-kDa protein with 32- and 33-kDa polypeptides were also purified from U937 cells. These findings indicate that HIV-1 utilizes a tetrameric protein on the surface of Molt-4 and U937 cells on infection.

Mast cell proteinases and cytokines in skin inflammation

The role of mast cells in provoking immediate-type hypersensitivity reactions is well established, but their involvement in chronic inflammation and immune reactions is not so clear. Mast cells synthesize and secrete large amounts of active proteinases, including tryptase, chymase, carboxypeptidase and cathepsin G, which can rapidly process numerous biologically active peptides and proteins or their precursors. Furthermore, mast cells are able to produce a variety of cytokines such as interleukin-4 (IL-4), IL-5, IL-6, tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) which are known to be intensively involved in modulating and directing inflammatory responses in the skin. In this review, the role of mast cell proteinases and cytokines in skin inflammation is discussed.

Identification of the U-937 membrane-associated proteinase interacting with the V3 loop of HIV-1 gp120 as cathepsin G

We have purified a serine proteinase from the membrane of U-937 cells that was inhibited in a tight-binding manner by recombinant gp120 and by peptides mimicking the V3 loop of gp120 [(1993) FEBS Lett. 317, 167-172]. This proteinase has now been characterized, both structurally and functionally. It has a dual trypsin- and chymotrypsin-like specificity, and N-terminal sequence analysis of the first 32 residues indicates complete identity with leukocyte cathepsin G. Cathepsin G-like material was located at the surface of U-937 cells using a monoclonal antibody directed against leukocyte cathepsin G, and polyclonal anti-cathepsin G antibodies precipitated the purified proteinase. However, the U-937 enzyme differs slightly from commercial leukocyte cathepsin G in its apparent M(r) because of different glycosylation. No other protein structurally related to cathepsin G was found upon screening a U-937 cDNA library using several oligonucleotide probes constructed from the membrane proteinase N-terminal amino acid sequence. The possible interaction of a cathepsin G-like proteinase at the surface of U-937 cells with the V3 loop of HIV-1 gp120 is discussed.

Cross-reactivity between autoimmune anti-U1 snRNP antibodies and neutralizing epitopes of HIV-1 gp120/41

We report extensive amino acid sequence homology between HIV-1 gp120/41, and > 33% of a U1 RNA-associated splicing protein, 70K. The latter is a target of autoimmune anti-RNP antibodies in mixed connective tissue disease (MCTD). The homologies, involving dominant epitopes of 70K and neutralizing epitopes of gp120/41, are the basis for mutual antibody cross-reactivity. A key finding is that the epitope GRAFVTIG in the V3 loop of gp120 (strain IIIB) is homologous to the functionally essential U1 RNA-binding site of 70K. ELISA data reveal a mean reactivity of anti-RNP antibodies to V3 IIIB that is as high as that of HIV sera. V3 MN, containing the framework sequence G-AF-T, also cross-reacts with anti-RNP antibodies, as do hydrophilic epitopes in gp41 homologous to the COOH end of 70K. Further, there is strong cross-reactivity between HIV sera and 70K in Western blots. In contrast, antibodies from a related autoimmune disorder, Sjögren's syndrome (SS), are neither V3 nor gp41 selective. We conclude that the substantial cross-reactivities reported here are due to conserved, antigenically dominant B cell epitopes having homologous counterparts in 70K and gp120/41. Because antibody production in both MCTD and HIV-1 infection is T cell dependent, the results imply that common T cell clones are also activated in these two disease paradigms. Further exploration of the mechanisms that activate these clones, and that control their divergent fates in MCTD and AIDS, may provide new insights into immune dysregulation in HIV infection.(ABSTRACT TRUNCATED AT 250 WORDS)