The catabolic fate of nitric oxide: the nitric oxide oxidase and peroxynitrite reductase activities of cytochrome oxidase

Stimulation of cardiomyocytes to endogenously evolve nitric oxide is shown by microsensor measurements on single cells to lead to transient nitric oxide concentrations of a few hundred nanomolar. At these submicromolar concentrations, no evidence could be found for the expected reaction between nitric oxide generated and the oxymyoglobin present in the cells: nitric oxide + oxymyoglobin --> nitrate + metmyoglobin. No metmyoglobin formation was detected by electron paramagnetic resonance spectroscopy, and microsensor measurements revealed near quantitative conversion of the nitric oxide to nitrite rather than nitrate ion. Moreover, the rate of nitrite formation is shown to be too rapid to be accounted for by non-enzymatic means. The essentially quantitative and rapid catabolism of nitric oxide to nitrite ion can plausibly be explained on the basis of a cycle of reactions catalyzed by cytochrome c oxidase. It is demonstrated with the purified hemoproteins in vitro that the terminal oxidase can outcompete oxymyoglobin for available nitric oxide. It is proposed that under normal physiological and most pathological (non-inflammatory) conditions, reaction with cytochrome c oxidase is the major route by which NO is removed from mitochondria-rich cells.     

Aggregation of misfolded proteins can be a selective process dependent upon peptide composition

Intracellular aggregation of misfolded proteins is observed in a number of human diseases, in particular, neurologic disorders in which expanded tracts of polyglutamine residues play a central role. A variety of other proteins are prone to aggregation when mutated, indicating that this process is a common pathologic mechanism for inherited disorders. However, little is known about the relationship between the sequence of aggregating peptides and the specificity of intracellular accumulation. Here we demonstrate that substitution of two residues eliminates aggregation of a 111-amino acid peptide derived from the C-terminal portion of the cystic fibrosis transmembrane conductance regulator (CFTR). We also show that fusion to a reporter protein considerably alters the subcellular distribution of aggregating peptide. When fused to green fluorescent protein, the peptide containing amino acids 1370-1480 of CFTR accumulates in large perinuclear or nuclear aggregates. The same CFTR fragment devoid of green fluorescent protein localizes predominantly to discrete accumulations associated with mitochondria. Importantly, both types of accumulation are dependent on the presence of the same two amino acids within the CFTR sequence. Co-expression studies show that both CFTR-derived proteins can co-localize in large cytoplasmic/nuclear aggregates. However, neither CFTR construct accumulates in intracellular inclusions formed by N-terminal fragment of huntingtin. In addition to unique accumulation patterns, each aggregating peptide shows differences in association with chaperone proteins. Thus, our results indicate that the process of intracellular aggregation can be a selective process determined by the composition of the aggregating peptides.     

Cloning and characterization of the phosphatidylserine synthase gene of Agrobacterium sp. strain ATCC 31749 and effect of its inactivation on production of high-molecular-mass (1-->3)-beta-D-glucan (curdlan)

Genes involved in the production of the extracellular (1-->3)-beta-glucan, curdlan, by Agrobacterium sp. strain ATCC 31749 were described previously (Stasinopoulos et al., Glycobiology 9:31-41, 1999). To identify additional curdlan-related genes whose protein products occur in the cell envelope, the transposon TnphoA was used as a specific genetic probe. One mutant was unable to produce high-molecular-mass curdlan when a previously uncharacterized gene, pss(AG), encoding a 30-kDa, membrane-associated phosphatidylserine synthase was disrupted. The membranes of the mutant lacked phosphatidylethanolamine (PE), whereas the phosphatidylcholine (PC) content was unchanged and that of both phosphatidylglycerol and cardiolipin was increased. In the mutant, the continued appearance of PC revealed that its production by this Agrobacterium strain is not solely dependent on PE in a pathway controlled by the Pss(AG) protein at its first step. Moreover, PC can be produced in a medium lacking choline. When the pss(AG)::TnphoA mutation was complemented by the intact pss(AG) gene, both the curdlan deficiency and the phospholipid profile were restored to wild-type, demonstrating a functional relationship between these two characteristics. The effect of the changed phospholipid profile could occur through an alteration in the overall charge distribution on the membrane or a specific requirement for PE for the folding into or maintenance of an active conformation of any or all of the structural proteins involved in curdlan production or transport.     

P-, E-, and L-selectin mediate migration of activated CD8+ T lymphocytes into inflamed skin

P- and E-selectin mediate CD4+ Th1 cell migration into the inflamed skin in a murine contact hypersensitivity model. In this model, not only CD4+ T cells but also CD8+ T cells infiltrate the inflamed skin, and the role of CD8+ type 1 cytotoxic T (Tc1) cells as effector cells has been demonstrated. Here we show that in mice deficient in both P- and E-selectin, the infiltration of CD8+ T cells in the inflamed skin is reduced, suggesting the role of these selectins in CD8+ T cell migration. We directly studied the role of selectins using in vitro-generated Tc1 cells. These cells are able to migrate into the inflamed skin of wild-type mice. This migration is partially mediated by P- and E-selectin, as shown by the reduced Tc1 cell migration into the inflamed skin of mice deficient in both P- and E-selectin or wild-type mice treated with the combination of anti-P-selectin and anti-E-selectin Abs. During P- and E-selectin-mediated migration of Tc1 cells, P-selectin glycoprotein ligand-1 appears to be the sole ligand for P-selectin and one of the ligands for E-selectin. P- and E-selectin-independent migration of Tc1 cells into the inflamed skin was predominantly mediated by L-selectin. These observations indicate that all three selectins can mediate Tc1 cell migration into the inflamed skin.     

CD1 molecules efficiently present antigen in immature dendritic cells and traffic independently of MHC class II during dendritic cell maturation

Upon exposure to Ag and inflammatory stimuli, dendritic cells (DCs) undergo a series of dynamic cellular events, referred to as DC maturation, that involve facilitated peptide Ag loading onto MHC class II molecules and their subsequent transport to the cell surface. Besides MHC molecules, human DCs prominently express molecules of the CD1 family (CD1a, -b, -c, and -d) and mediate CD1-dependent presentation of lipid and glycolipid Ags to T cells, but the impact of DC maturation upon CD1 trafficking and Ag presentation is unknown. Using monocyte-derived immature DCs and those stimulated with TNF-alpha for maturation, we observed that none of the CD1 isoforms underwent changes in intracellular trafficking that mimicked MHC class II molecules during DC maturation. In contrast to the striking increase in surface expression of MHC class II on mature DCs, the surface expression of CD1 molecules was either increased only slightly (for CD1b and CD1c) or decreased (for CD1a). In addition, unlike MHC class II, DC maturation-associated transport from lysosomes to the plasma membrane was not readily detected for CD1b despite the fact that both molecules were prominently expressed in the same MIIC lysosomal compartments before maturation. Consistent with this, DCs efficiently presented CD1b-restricted lipid Ags to specific T cells similarly in immature and mature DCs. Thus, DC maturation-independent pathways for lipid Ag presentation by CD1 may play a crucial role in host defense, even before DCs are able to induce maximum activation of peptide Ag-specific T cells.     

Attenuation of experimental allergic encephalomyelitis in complement component 6-deficient rats is associated with reduced complement C9 deposition, P-selectin expression, and cellular infiltrate in spinal cords

The role of Ab deposition and complement activation, especially the membrane attack complex (MAC), in the mediation of injury in experimental allergic encephalomyelitis (EAE) is not resolved. The course of active EAE in normal PVG rats was compared with that in PVG rats deficient in the C6 component of complement (PVG/C6(-)) that are unable to form MAC. Following immunization with myelin basic protein, PVG/C6(-) rats developed significantly milder EAE than PVG/C rats. The anti-myelin basic protein response was similar in both strains, as was deposition of C3 in spinal cord. C9 was detected in PVG/C rats but not in PVG/C6(-), consistent with their lack of C6 and inability to form MAC. In PVG/C6(-) rats, the T cell and macrophage infiltrate in the spinal cord was also significantly less than in normal PVG/C rats. There was also reduced expression of P-selectin on endothelial cells, which may have contributed to the reduced cellular infiltrate by limiting migration from the circulation. Assay of cytokine mRNA by RT-PCR in the spinal cords showed no differences in the profile of Th1 or Th2 cytokines between PVG/C and PVG/C6(-) rats. PVG/C rats also had a greater increase in peripheral blood white blood cell, neutrophil, and basophil counts than was observed in the PVG/C6(-). These findings suggest that the MAC may have a role in the pathogenesis of EAE, not only by Ig-activated MAC injury but also via induction of P-selectin on vascular endothelium to promote infiltration of T cells and macrophages into the spinal cord.     

Suppression of experimental autoimmune encephalomyelitis using peptide mimics of CD28

The B7:CD28/CTLA-4 costimulatory pathway plays a critical role in regulating the immune response and thus provides an ideal target for therapeutic manipulation of autoimmune disease. Previous studies have shown that blockade of CD28 signaling by mAbs can both prevent and exacerbate experimental autoimmune encephalomyelitis (EAE). In this study, we have designed two CD28 peptide mimics that selectively block B7:CD28 interactions. By surface plasmon resonance, both the end group-blocked CD28 peptide (EL-CD28) and its retro-inverso isomer (RI-CD28) compete effectively with the extracellular domain of CD28 for binding to B7-1. Both the CD28 peptide mimics inhibited expansion of encephalitogenic T cells in vitro. A single administration of EL-CD28 or RI-CD28 peptide significantly reduced disease severity in EAE. Importantly, we show that either CD28 peptide mimic administered during acute disease dramatically improved clinical signs of EAE, suppressing ongoing disease. The ratio of CD80:CD86 expression was significantly lower on CD4(+) and F4/80(+) spleen cells in CD28 peptide-treated mice. Peripheral deletion of Ag-specific CD4(+) T cells occurs following in vivo blockade of CD28 with synthetic CD28 peptides.     

Acute visceral toxoplasmosis in captive dik-dik (Madoqua guentheri smithi)

Acute toxoplasmosis was diagnosed in 2 captive dik-dik (Madoqua guentheri smithi) in the Houston Zoo. Both animals became ill suddenly and died in spite of supportive therapy. Toxoplasma gondii was identified in tissues of both animals immunohistochemically, and antibodies to T. gondii were found in titers of 1:800 or more in both animals upon examination by the modified agglutination rest. The cause of death was considered to be toxoplasmic pneumonia. This is the first report of toxoplasmosis in M. g. smithi.     

Consistent patterns in the development and immunodominance of human immunodeficiency virus type 1 (HIV-1)-specific CD8+ T-cell responses following acute HIV-1 infection

Human immunodeficiency virus type 1 (HIV-1)-specific CD8+ T-cell responses generated during acute infection play a critical role in the initial control of viremia. However, little is known about the viral T-cell epitopes targeted during acute infection or about their hierarchy in appearance and relative immunodominance over time. In this study, HIV-1-specific CD8+ T-cell responses in 18 acutely infected individuals expressing HLA-A3 and/or -B7 were characterized. Detailed analysis of CD8 responses in one such person who underwent treatment of acute infection followed by reexposure to HIV-1 through supervised treatment interruptions (STI) revealed recognition of only two cytotoxic T-lymphocyte (CTL) epitopes during symptomatic acute infection. HIV-1-specific CD8+ T-cell responses broadened significantly during subsequent exposure to the virus, ultimately targeting 27 distinct CTL epitopes, including 15 different CTL epitopes restricted by a single HLA class I allele (HLA-A3). The same few peptides were consistently targeted in an additional 17 persons expressing HLA-A3 and/or -B7 during acute infection. These studies demonstrate a consistent pattern in the development of epitope-specific responses restricted by a single HLA allele during acute HIV-1 infection, as well as persistence of the initial pattern of immunodominance during subsequent STI. In addition, they demonstrate that HIV-1-specific CD8+ T-cell responses can ultimately target a previously unexpected and unprecedented number of epitopes in a single infected individual, even though these are not detectable during the initial exposure to virus. These studies have important implications for vaccine design and evaluation.