The role of the NadR regulator during infection and its implication for the coverage of a new Meningococcus B vaccine

Background: Neisseria meningitides represents a major cause of meningitis and sepsis. The meningococcal regulator NadR was previously shown to repress the expression of the Neisserial Adhesin A (NadA) and play a major role in its phase-variation. NadA is a surface exposed protein involved in epithelial cell adhesion and colonization and a major component of 4CMenB, a novel vaccine to prevent meningococcus serogroup B infection. The NadR mediated repression of NadA is attenuated by 4-HPA, a natural molecule released in human saliva. Results: In this thesis we investigated the global role of NadR during meningogoccal infection, identifying through microarray analysis the NadR regulon. Two distinct types of NadR targets were identified, differing in their promoter architectures and 4HPA responsive activities: type I are induced, while type II are co-repressed in response to the same 4HPA signal. We then investigate the mechanism of regulation of NadR by 4-HPA, generating NadR mutants and identifying classes or residues involved in either NadR DNA binding or 4HPA responsive activities. Finally, we studied the impact of NadR mediated repression of NadA on the vaccine coverage of 4CMenB. A selected MenB strains is not killed by sera from immunized infants when the strain is grown in vitro, however, in an in vivo passive protection model, the same sera protected infant rats from bacteremia. Finally, using bioluminescent reporters, nadA expression in the infant rat model was induced in vivo at 3 h post-infection. Conclusions: Our results suggest that NadR coordinates a broad transcriptional response to signals present in the human host, enabling the meningococcus to adapt to the relevant host niche. During infectious disease the effect of the same signal on NadR changes between different targets. In particular NadA expression is induced in vivo, leading to efficient killing of meningococcus by anti-NadA antibodies elicited by the 4CMenB vaccine.

Risposta Immunitaria Epatite B specifica in pazienti affetti da Epatocarcinoma

Hepatitis B x protein (HBx) is a non structural, multifunctional protein of hepatitis B virus (HBV) that modulates a variety of host processes.Due to its transcriptional activity,able to alter the expression of growth-control genes,it has been implicated in hepatocarcinogenesis.Increased expression of HBx has been reported on the liver tissue samples of hepatocellular carcinoma (HCC),and a specific anti-HBx immune response can be detected in the peripheral blood of patients with chronic HBV.However,its role and entity has not been yet clarified.Thus,we performed a cross-sectional analysis of anti-HBx specific T cell response in HBV-infected patients in different stage of disease.A total of 70 HBV-infected subjects were evaluated:15 affected by chronic hepatitis (CH-median age 45 yrs),14 by cirrhosis (median age 55 yrs),11 with dysplastic nodules (median age 64 yrs),15 with HCC (median age 60 yrs),15 with IC(median age 53 yrs).All patients were infected by virus genotype D with different levels of HBV viremia and most of them (91%) were HBeAb positive.The HBx-specific T cell response was evaluated by anti-Interferon (IFN)-gamma Elispot assay after in vitro stimulation of peripheral blood mononuclear cells,using 20 overlapping synthetic peptides covering all HBx protein sequence.HBx-specific IFN-gamma-secreting T cells were found in 6 out of 15 patients with chronic hepatitis (40%), 3 out of 14 cirrhosis (21%), in 5 out of 11 cirrhosis with macronodules (54%), and in 10 out of 15 HCC patients (67%). The number of responding patients resulted significantly higher in HCC than IC (p=0.02) and cirrhosis (p=0.02). Central specific region of the protein x was preferentially recognize,between 86-88 peptides. HBx response does not correlate with clinical feature disease(AFP,MELD).The HBx specific T-cell response seems to increase accordingly to progression of the disease, being increased in subjects with dysplastic or neoplastic lesions and can represent an additional tool to monitor the patients at high risk to develop HCC

Studio immunologico per l'individuazione di pazienti trapiantati per cirrosi HBV relata che potrebbero sospedere la terapia con immunoglobine specifiche (HBIG)

Hepatitis B virus (HBV) recurrence after orthotopic liver transplantation (OLT) is associated with poor graft and patient survival. Treatment with HBV-specific immunoglobulins (HBIG) in combination with nucleos(t)ide analogs is effective in preventing HBV reinfection of the graft and improving OLT outcome. However, the combined immunoprophylaxis has several limitations, mainly the high cost and the lack of standard schedules about duration. So far, the identification of markers able to predict the reinfection risk is needed. Although the HBV-specific immune response is believed to play an essential role in disease outcome, HBV-specific cellular immunity in viral containment in OLT recipients is unclear. To test whether or not OLT recipients maintain robust HBV-specific cellular immunity, the cellular immune response against viral nucleocapsid and envelope-protein of HBV was assessed in 15 OLT recipients and 27 individuals with chronic and 24 subjects with self-limited HBV infection, respectively. The data demonstrate that OLT recipients mounted fewer but stronger clusters of differentiation (CD)8 T cell responses than subjects with self-limited HBV infection and showed a preferential targeting of the nucleocapsid antigen. This focused response pattern was similar to responses seen in chronically infected subjects with undetectable viremia, but significantly different from patients who presented with elevated HBV viremia and who mounted mainly immune responses against the envelope protein. In conclusion, virus-specific CD4 T cell–mediated responses were only detected in subjects with self-limited HBV infection. Thus, the profile of the cellular immunity against HBV was in immune suppressed patients similar to subjects with chronic HBV infection with suppressed HBV-DNA.

Structural and functional characterization of Group B Streptococcus class C sortases

In Group B Streptococcus (GBS) three structurally distinct types of pili have been discovered as potential virulence factors and vaccine candidates. The pilus-forming proteins are assembled into high-molecular weight polymers via a transpeptidation mechanism mediated by specific class C sortases. Using a multidisciplinary approach including bioinformatics, structural and biochemical studies and in vivo mutagenesis we performed a broad characterization of GBS sortase C. The high resolution X-ray structure of the enzymes revealed that the active site, located into the β-barrel core of the enzyme, is made of the catalytic triad His157-Cys219-Arg228 and covered by a loop, known as the “lid”. We show that the catalytic triad and the predicted N- and C-terminal trans-membrane regions are required for the enzyme activity. Interestingly, by in vivo complementation mutagenesis studies we found that the deletion of the entire lid loop or mutations in specific lid key residues had no effect on catalytic activity of the enzyme. In addition, kinetic characterizations of recombinant enzymes indicate that the lid mutants can still recognize and cleave the substrate-mimicking peptide at least as well as the wild type protein.

Analysis of the memory B cell response against glycoconjugate vaccines

The development of vaccines directed against polysaccharide capsules of S. pneumoniae, H. influenzae and N. meningitidis have been of great importance in preventing potentially fatal infections. Bacterial capsular polysaccharides are T-cell-independent antigens that induce specific antibody response characterized by IgM immunoglobulins, with a very low IgG class switched response and lack of capability of inducing a booster response. The inability of pure polysaccharides to induce sustained immune responses has required the development of vaccines containing polysaccharides conjugated to a carrier protein, with the aim to generate T cell help. It is clear that the immunogenicity of glycoconjugate vaccines can vary depending on different factors, e.g. chemical nature of the linked polysaccharide, carrier protein, age of the target population, adjuvant used. The present study analyzes the memory B cell (MBC) response to the polysaccharide and to the carrier protein following vaccination with a glycoconjugate vaccine for the prevention of Group B streptococcus (GBS) infection. Not much is known about the role of adjuvants in the development of immunological memory raised against GBS polysaccharides, as well as about the influence of having a pre-existing immunity against the carrier protein on the B cell response raised against the polysaccharide component of the vaccine. We demonstrate in the mouse model that adjuvants can increase the antibody and memory B cell response to the carrier protein and to the conjugated polysaccharide. We also demonstrate that a pre-existing immunity to the carrier protein favors the development of the antibody and memory B cell response to subsequent vaccinations with a glycoconjugate, even in absence of adjuvants. These data provide a useful insight for a better understanding of the mechanism of action of this class of vaccines and for designing the best vaccine that could result in a productive and long lasting memory response.

Polymerizing activity and regulation of group B Streptococcus pilus 2a sortase C1

Group B Streptococcus [GBS; Streptococcus agalactiae] is the leading cause of life-threatening diseases in newborn and is also becoming a common cause of invasive diseases in non-pregnant, elderly and immune-compromised adults. Pili, long filamentous fibers protruding from the bacterial surface, have been discovered in GBS, as important virulence factors and vaccine candidates. Gram-positive bacteria build pili on their cell surface via a class C sortase-catalyzed transpeptidation mechanism from pilin protein substrates. Despite the availability of several crystal structures, pilus-related C sortases remain poorly characterized to date and their mechanisms of transpeptidation and regulation need to be further investigated. The available three-dimensional structures of these enzymes reveal a typical sortase fold except for the presence of a unique feature represented by an N-terminal highly flexible loop, known as the “lid”. This region interacts with the residues composing the catalytic triad and covers the active site, thus maintaining the enzyme in an auto-inhibited state and preventing the accessibility to the substrate. It is believed that enzyme activation may occur only after lid displacement from the catalytic domain. In this work we provide the first direct evidence of the regulatory role of the lid, demonstrating that it is possible to obtain in vitro an efficient polymerization of pilin subunits using an active C sortase lid mutant carrying a single residue mutation in the lid region. Moreover, biochemical analyses of this recombinant mutant reveal that the lid confers thermodynamic and proteolytic stability to the enzyme. A further characterization of this sortase active mutant showed promiscuity in the substrate recognition, as it is able to polymerize different LPXTG-proteins in vitro.

Structural and functional characterization of Group B Streptococcus pilus 2b

Group B Streptococcus (GBS) is a Gram-positive human pathogen representing one of the most common causes of life-threatening bacterial infections such as sepsis and meningitis in neonates. Covalently polymerized pilus-like structures have been discovered in GBS as important virulence factors as well as vaccine candidates. Pili are protein polymers forming long and thin filamentous structures protruding from bacterial cells, mediating adhesion and colonization to host cells. Gram-positive bacteria, including GBS, build pili on their cell surface via a class C sortase-catalyzed transpeptidation mechanism from pilin protein substrates that are the backbone protein forming the pilus shaft and two ancillary proteins. Also the cell-wall anchoring of the pilus polymers made of covalently linked pilin subunits is mediated by a sortase enzyme. GBS expresses three structurally distinct pilus types (type 1, 2a and 2b). Although the mechanisms of assembly and cell wall anchoring of GBS types 1 and 2a pili have been investigated, those of pilus 2b are not understood until now. Pilus 2b is frequently found in ST-17 strains that are mostly associated with meningitis and high mortality rate especially in infants. In this work the assembly mechanism of GBS pilus type 2b has been elucidated by dissecting through genetic, biochemical and structural studies the role of the two pilus-associated sortases. The most significant findings show that pilus 2b assembly appears “non-canonical”, differing significantly from current pilus assembly models in Gram-positive pathogens. Only sortase-C1 is involved in pilin polymerization, while the sortase-C2 does not act as a pilin polymerase, but it is involved in cell-wall pilus anchoring. Our findings provide new insights into pili biogenesis in Gram-positive bacteria. Moreover, the role of this pilus type during host infection has been investigated. By using a mouse model of meningitis we demonstrated that type 2b pilus contributes to pathogenesis of meningitis in vivo.