Surface Polysaccharide Mutants Reveal that Absence of O Antigen Reduces Biofilm Formation of Actinobacillus pleuropneumoniae.

Actinobacillus pleuropneumoniae is a Gram-negative bacterium belonging to the Pasteurellaceae family and the causative agent of porcine pleuropneumonia, a highly contagious lung disease causing important economic losses. Surface polysaccharides, including lipopolysaccharides (LPS) and capsular polysaccharides (CPS), are implicated in the adhesion and virulence of A. pleuropneumoniae, but their role in biofilm formation is still unclear. In this study, we investigated the requirement for these surface polysaccharides in biofilm formation by A. pleuropneumoniae serotype 1. Well-characterized mutants were used: an O-antigen LPS mutant, a truncated core LPS mutant with an intact O antigen, a capsule mutant, and a poly-N-acetylglucosamine (PGA) mutant. We compared the amount of biofilm produced by the parental strain and the isogenic mutants using static and dynamic systems. Compared to the findings for the biofilm of the parental or other strains, the biofilm of the O antigen and the PGA mutants was dramatically reduced, and it had less cell-associated PGA. Real-time PCR analyses revealed a significant reduction in the level of pgaA, cpxR, and cpxA mRNA in the biofilm cells of the O-antigen mutant compared to that in the biofilm cells of the parental strain. Specific binding between PGA and LPS was consistently detected by surface plasmon resonance, but the lack of O antigen did not abolish these interactions. In conclusion, the absence of the O antigen reduces the ability of A. pleuropneumoniae to form a biofilm, and this is associated with the reduced expression and production of PGA.

Anti-myelin antibodies modulate clinical expression of childhood multiple sclerosis.

Anti-myelin basic protein (MBP) antibodies in pediatric-onset MS and controls were characterized. Serum samples were obtained from 94 children with MS and 106 controls. Paired CSF and serum were obtained from 25 children with MS at time of their initial episode of acute demyelinating syndrome (ADS). Complementary assays were applied across samples to evaluate the presence, and the physical binding properties, of anti-MBP antibodies. While the prevalence and titers of serum anti-MBP antibodies against both immature and mature forms of MBP were similar in children with MS and in controls, binding characteristics and formal Surface Plasmon Resonance (SPR) studies indicated surprisingly high binding affinities of all pediatric anti-MBP antibodies. Serum levels of anti-MBP antibodies correlated significantly with their CSF levels, and their presence in children with MS was associated with significantly increased risk of an acute disseminated encephalomyelitis-like initial clinical presentation. While antibodies to both immature and mature forms of MBP can be present as part of the normal pediatric humoral repertoire, these anti-myelin antibodies are of surprisingly high affinity, can access the CNS during inflammation, and have the capacity to modulate disease expression. Our findings identify an immune mechanism that could contribute to the observed heterogeneity in spectrum of clinical presentations in early-onset MS.

Apolipoprotein(a) enhances platelet responses to the thrombin receptor-activating peptide SFLLRN.

Elevated levels of lipoprotein(a) [Lp(a)] are correlated with an increased risk of atherosclerotic disease. We examined the effect of recombinant apolipoprotein(a) [r-apo(a)] and Lp(a) on responses of washed human platelets, prelabeled in the dense granules with [14C]serotonin and suspended in Tyrode's solution, to ADP and the thrombin receptor-activating peptide SFLLRN. No effect of the 17 kringle (K), 12K, or 6K r-apo(a) derivatives (at concentrations of 0.35 and 0.7 micromol/L) or Lp(a) (up to 0.1 micromol/L) on primary ADP-induced platelet aggregation was observed. In contrast, weak platelet responses stimulated by 7.5 micromol/L SFLLRN were significantly enhanced by the r-apo(a) derivatives; eg, 0.7 micromol/L 17K r-apo(a) increased aggregation from 15+/-4% to 58+/-6%, release of [14C]serotonin from 9+/-3% to 36+/-6%, and formation of thromboxane A2, measured as its stable metabolite thromboxane B2, from 7+/-1 to 29+/-5 ng/10(9) platelets (n=3; P<0.04 to 0.015). Significant enhancement of aggregation and release of granule contents was observed at a concentration of 17K r-apo(a) as low as 0.175 micromol/L. Purified Lp(a) (0.25 to 0.1 micromol/L) also enhanced SFLLRN-induced aggregation and release in a dose-dependent manner. Although plasminogen (0.7 and 1.5 micromol/L) and low density lipoprotein (0.025 to 0.1 micromol/L) both exhibited potentiating effects on SFLLRN-mediated platelet aggregation, the magnitude of the responses was less than that observed with either the r-apo(a) derivatives or Lp(a). The enhanced responses of platelets via the protease-activated receptor- thrombin receptor in the presence of Lp(a) may contribute to the increased risk of thromboembolic complications of atherosclerosis associated with this lipoprotein.

The solution phase interaction between apolipoprotein(a) and plasminogen inhibits the binding of plasminogen to a plasmin-modified fibrinogen surface.

In the present study, we assessed the binding of recombinant forms of apolipoprotein(a) [r-apo(a)] to plasminogen. Apo(a)-plasminogen interactions were demonstrated to be lysine-dependent, as they were abolished by the addition of epsilon-aminocaproic acid. Binding of r-apo(a) and plasma-derived Lp(a) to Glu-plasminogen was assessed in solution using a mutant form of recombinant plasminogen [Plg(S741C)] labeled at the active site with 5'-(iodoacetamido)fluorescein. High-affinity binding of apo(a) to plasminogen was observed with the 17-kringle r-apo(a) (Kd = 20.1 +/- 3.3 nM) as well as with plasma-derived Lp(a) (Kd = 5.58 +/- 0.08 nM). Binding studies using various truncated and mutant forms of r-apo(a) demonstrated that sequences within apo(a) kringle IV types 2-9 and the strong lysine binding site (LBS) in apo(a) kringle IV type 10 are not required for high-affinity binding to plasminogen. In all cases, the binding stoichiometry for the apo(a)-plasminogen interaction was determined to be 1:1. Binding data obtained using a 17-kringle r-apo(a) derivative lacking the protease-like domain (17KDeltaP; Kd = 3158 +/- 138 nM) indicate that sequences within the protease-like domain of apo(a) mediate its interaction with LBS in plasminogen. We determined that r-apo(a) and plasminogen bind to distinct sites on plasmin-modified fibrinogen with the concentration of plasminogen binding sites exceeding the concentration of r-apo(a) sites by a factor of 10. Furthermore, r-apo(a) is capable of inhibiting the binding of plasminogen to plasmin-modified fibrinogen surfaces, an effect which we show is attributable to the formation of a solution phase apo(a)/plasminogen complex which exhibits a greatly reduced affinity for plasminogen binding sites on plasmin-modified fibrinogen. The results of this study provide new insights into the mechanism by which apo(a) and Lp(a) may inhibit fibrinolysis, thus contributing to the atherothrombotic risk associated with this lipoprotein.

Defensin promotes the binding of lipoprotein(a) to vascular matrix.

Retention of lipoproteins within the vasculature is a central event in the pathogenesis of atherosclerosis. However, the signals that mediate this process are only partially understood. Prompted by putative links between inflammation and atherosclerosis, we previously reported that alpha-defensins released by neutrophils are present in human atherosclerotic lesions and promote the binding of lipoprotein(a) [Lp(a)] to vascular cells without a concomitant increase in degradation. We have now tested the hypothesis that this accumulation results from the propensity of defensin to form stable complexes with Lp(a) that divert the lipoprotein from its normal cellular degradative pathways to the extracellular matrix (ECM). In accord with this hypothesis, defensin stimulated the binding of Lp(a) to vascular matrices approximately 40-fold and binding of the reactants to the matrix was essentially irreversible. Defensin formed stable, multivalent complexes with Lp(a) and with its components, apoprotein (a) and low-density lipoprotein (LDL), as assessed by optical biosensor analysis, gel filtration, and immunoelectron microscopy. Binding of defensin/Lp(a) complexes to matrix was inhibited (>90%) by heparin and by antibodies to fibronectin (>70%), but not by antibodies to vitronectin or thrombospondin. Defensin increased the binding of Lp(a) (10 nmol/L) to purified fibronectin more than 30-fold. Whereas defensin and Lp(a) readily traversed the endothelial cell membranes individually, defensin/Lp(a) complexes lodged on the cell surface. These studies demonstrate that alpha-defensins released from activated or senescent neutrophils stimulate the binding of an atherogenic lipoprotein to the ECM of endothelial cells, a process that may contribute to lipoprotein accumulation in atherosclerotic lesions.