Palmitoylation at Cys595 is essential for PECAM-1 localisation into membrane microdomains and for efficient PECAM-1-mediated cytoprotection

The Ig-ITIM superfamily member, PECAM-1 acts as a negative regulator of ITAM-signalling pathways in platelets involving GPVI/FcR gamma chain and FcγRIIa. This negative feedback loop involves regulation of collagen and GPVI-dependent aggregation events, platelet-thrombus-growth on immobilised collagen under flow and FcγRIIa-mediated platelet responses. In this study, we show that PECAM-1 is selectively palmitoylated involving a thioester linkage with an unpaired cysteine residue at amino acid position 595 in its cytoplasmic domain. As palmitoylation is known to target proteins to membrane microdomains, we investigated the microdomain localisation for PECAM-1 in platelets and nucleated cells. In unstimulated platelets, ∼20% of PECAM-1 is localised toTriton-insoluble microdomain fractions and it does not increase with platelet activation by collagen, collagen-related peptide, thrombin-or human-aggregated IgG. PECAM-1 is in close physical proximity with GPVI in platelet microdomains. Removal of platelet cytoskeleton prior to sucrose-density-gradient separation showed that PECAM-1 was associated with both theTriton-soluble and membrane skeleton in microdomain-associated fractions. Disruption of microdomains by membrane-cholesterol depletion resulted in loss of PECAM-1 localisation to membrane microdomains. Mutational analysis of juxtamembrane cysteine residue to alanine (C595A) of human PECAM-1 resulted in loss of palmitoylation and a sixfold decrease in association with membrane microdomains. Functionally, the palmitoylated cysteine 595 residue is required, in part, for efficient PECAM-1-mediated cytoprotection. These results show that cysteine 595 is required for constitutive association of PECAM-1 with membrane microdomains and PECAM-1-mediated cytoprotection, where it may act as a crucial regulator of signaling and apoptosis events.

Muc1 mucin limits both Helicobacter pylori colonization of the murine gastric mucosa and associated gastritis

BACKGROUND & AIMS

The MUC1 mucin is expressed on the cell surface of epithelial cells lining the gastric mucosa. Epidemiologic studies suggest that functional allelic variations in the MUC1 gene may play a role in human susceptibility to Helicobacter pylori-associated pathologies, including gastric adenocarcinoma. We have evaluated the impact of Muc1 expression on the colonization and pathogenesis of gastric Helicobacter infections.

METHODS

Wild-type and Muc1-deficient mice were infected with H pylori and colonization and gastritis levels determined. Primary gastric cells were used to examine the impact of Muc1 expression on bacterial adherence.

RESULTS

Mice lacking Muc1 were colonized by 5-fold more H pylori within 1 day of infection, and this difference was maintained for at least 2 months postinfection. Mice heterozygous for the null Muc1 allele developed intermediate bacterial colonization. Although wild-type mice developed only a mild gastritis when infected for 2 months with H pylori, Muc1(-/-) mice developed an atrophic gastritis marked by loss of parietal cells. We demonstrate H pylori adhesion to purified MUC1 and significantly increased adhesion to cultured murine Muc1 null gastric epithelial cells, suggesting that Muc1 acts as a decoy limiting binding to the cell surface.

CONCLUSIONS

Muc1 provides a protective barrier, which limits both acute and chronic colonization by H pylori, as well as playing a major role in limiting the inflammation induced by Helicobacter infection. We propose that Muc1 restricts access of H pylori to the epithelial surface, hence reducing exposure of the host to proinflammatory bacterial products.

Effectiveness of vaccinationwith recombinant HpaA fromHelicobacter pyloriis influenced by host genetic background

Several studies have explored the production and immunogenicity of HpaA as a potential protective antigen against Helicobacter pylori but little is known regarding its protective capabilities. We therefore evaluated the protective efficacy of recombinant HpaA (rHpaA) as a candidate vaccine antigen against H. pylori. To explore the impact of genetic diversity, inbred and outbred mice were prophylactically and therapeutically immunized with rHpaA adjuvanted with cholera toxin (CT). Prophylactic immunization induced a reduction in bacterial colonization in BALB/c and QS mice, but was ineffective in C57BL/6 mice, despite induction of antigen-specific antibodies. By contrast, therapeutic immunization was effective in all three strains of mice. Prophylactic immunization with CT-adjuvanted rHpaA was more effective when delivered via the nasal route than following intragastric delivery in BALB/c mice. However, HpaA-mediated protection was inferior to that induced by bacterial lysate. Hence, protective efficacy is inducible with vaccines containing HpaA, most relevantly shown in an outbred population of mice. The effectiveness of protection induced by HpaA antigen was influenced by host genetics and was less effective than lysate. HpaA therefore has potential for the development of effective immunization against H. pylori but this would probably entail the antigen to be one component of a multiantigenic vaccine.

Does the clinical outcome of hepatitis C infection vary with the infecting hepatitis C virus type?

Whether differences in the natural history of hepatitis C virus (HCV) can be explained by differences in the infecting HCV type is unknown. The aim of this study was to investigate whether the HCV type might influence the clinical outcome of infection. Study serum samples were assembled from 749 individuals enrolled into the UK HCV National Register from which data on clinical outcomes were extracted. HCV-RNA-positive specimens were genotyped and HCV-RNA-negative specimens serotyped. Logistic regression analysis was used to investigate the independent effect of HCV type on viral clearance by comparing patients who were HCV RNA negative (n = 86) with those who were HCV RNA positive (n = 508). The same method was used to investigate whether HCV type was associated with histological stage of liver disease. The prevalence of HCV type 1 among those who cleared infection was 69% and among those who remained HCV RNA positive was 51%: Type 1 infections were more likely to be HCV RNA negative than non-1 types (OR 0.47, 95% CI 0.29-0.78, P = 0.003). Type 1 infections were also more likely to be associated with histological stage scores above the median when compared with non-1 types (OR 2.03, 95% CI 1.07-3.83, P = 0.03). In conclusion, HCV type 1 infection was more often HCV RNA negative, suggesting that spontaneous clearance may occur more commonly with this type. Among the RNA-positive infections, type 1 infection may be more aggressive than types 2/3.

False negativity by an anti-HIV assay kit (IMx 8B32) and evaluation of its replacement (IMx 8C98)

False negativity in a commercial anti-HIV kit (IMx HIV-1/HIV-2 3rd Generation Plus (code 8B32) was investigated, and the kit that superseded it (IMx HIV-1/HIV-2 III Plus, code 8C98) was evaluated. In a comparison on 574 freshly collected anti-HIV-1-positive specimens, 97.2% were more reactive in 8C98 than in 8B32; 35.5% were more than twice as reactive and 8.5% were more than four times as reactive. In 8B32, the signal from 55 specimens selected because of weak reactivity was enhanced 1.5 to 8.8 times by preliminary heating at 56 degrees C for 30 min. The reactivity of the 55 heated sera was then similar to that of the same specimens tested without heat treatment in the 8C98 assay. Reactivity in 8B32 was also increased in 66 of 76 (at least twofold in 20) randomly chosen anti-HIV-positive serum specimens by the addition of EDTA (10 mM final concentration). One of these specimens was false negative (signal:cutoff (S:CO) ratio 0.76) in 8B32, though its reactivity was restored by addition of EDTA (S:CO ratio 9.54). These findings indicate that the inhibitory effect that originally led to false negative findings in 8B32 was probably due to complement activity, and that the same activity was present in the freshly collected specimens used here to evaluate the replacement IMx anti-HIV assay (8C98). The specimen panel employed to evaluate 8C98 included 1,892 anti-HIV-positive and 779 anti-HIV-negative specimens. There were no false negative reactions. The lowest S:CO ratio observed was 6.2 and only 17 (0.2%) anti-HIV-positive specimens gave ratios less than 10. Nine unreproducible false positive reactions arose, all possibly attributable to specimen carryover by the IMx instrument. The performance of 8C98 was also compared with that of 10 other current anti-HIV kits using 21 sets of seroconversion specimens (127 specimens in total), and five performance assessment panels (92 specimens in total) comprised mostly of single bleeds from recent seroconverters. IMx 8C98 was the second most sensitive assay. We found no evidence that the 8C98 kit was prone to the effect that had given rise to false negative results in its predecessor (8B32).