Antibodies in human infectious disease

Classification epitopes in groups based on their protein family

Background

The humoral immune system response is based on the interaction between antibodies and antigens for the clearance of pathogens and foreign molecules. The interaction between these proteins occurs at specific positions known as antigenic determinants or B-cell epitopes. The experimental identification of epitopes is costly and time consuming. Therefore the use of in silico methods, to help discover new epitopes, is an appealing alternative due the importance of biomedical applications such as vaccine design, disease diagnostic, anti-venoms and immune-therapeutics. However, the performance of predictions is not optimal been around 70% of accuracy. Further research could increase our understanding of the biochemical and structural properties that characterize a B-cell epitope.

Results

We investigated the possibility of linear epitopes from the same protein family to share common properties. This hypothesis led us to analyze physico-chemical (PCP) and predicted secondary structure (PSS) features of a curated dataset of epitope sequences available in the literature belonging to two different groups of antigens (metalloproteinases and neurotoxins). We discovered statistically significant parameters with data mining techniques which allow us to distinguish neurotoxin from metalloproteinase and these two from random sequences. After a five cross fold validation we found that PCP based models obtained area under the curve values (AUC) and accuracy above 0.9 for regression, decision tree and support vector machine.

Conclusions

We demonstrated that antigen's family can be inferred from properties within a single group of linear epitopes (metalloproteinases or neurotoxins). Also we discovered the characteristics that represent these two epitope groups including their similarities and differences with random peptides and their respective amino acid sequence. These findings open new perspectives to improve epitope prediction by considering the specific antigen's protein family. We expect that these findings will help to improve current computational mapping methods based on physico-chemical due it's potential application during epitope discovery.

Cryptic properties of a cluster of dominant flavivirus cross-reactive antigenic sites

A number of flaviviruses are important human pathogens, including yellow fever, dengue, West Nile, Japanese encephalitis, and tick-borne encephalitis (TBE) viruses. Infection with or immunization against any of these viruses induces a subset of antibodies that are broadly flavivirus cross-reactive but do not exhibit significant cross-neutralization. Nevertheless, these antibodies can efficiently bind to the major envelope protein (E), which is the main target of neutralizing and protective antibodies because of its receptor-binding and membrane fusion functions. The structural basis for this phenomenon is still unclear. In our studies with TBE virus, we have provided evidence that such cross-reactive antibodies are specific for a cluster of epitopes that are partially occluded in the cage-like assembly of E proteins at the surfaces of infectious virions and involve-but are not restricted to-amino acids of the highly conserved internal fusion peptide loop. Virus disintegration leads to increased accessibility of these epitopes, allowing the cross-reactive antibodies to bind with strongly increased avidity. The cryptic properties of these sites in the context of infectious virions can thus provide an explanation for the observed lack of efficient neutralizing activity of broadly cross-reactive antibodies, despite their specificity for a functionally important structural element in the E protein.

Comparative effects of two neutralizing anti-respiratory syncytial virus (RSV) monoclonal antibodies in the RSV murine model: time versus potency

Respiratory syncytial virus (RSV) is the leading viral pathogen responsible for bronchiolitis and pneumonia in infants and young children worldwide. We have previously shown in the mouse model that treatment with an anti-RSV neutralizing monoclonal antibody (MAb) against the F glycoprotein of RSV, palivizumab, decreased lung inflammation, airway obstruction, and postmethacholine airway hyperresponsiveness. MEDI-524, or Numax, is a new MAb derived from palivizumab with enhanced neutralizing activity against RSV. We compared the effects of these two MAbs on different markers of disease severity using the murine model of RSV infection. BALB/c mice were intranasally inoculated with RSV A2. Palivizumab or MEDI-524 was administered once at either 24 h before or 48 h after RSV inoculation. Regardless of the time of administration, all treated mice showed significantly decreased RSV loads in bronchoalveolar lavage samples measured by plaque assay. Only MEDI-524 given at -24 h significantly decreased lung RSV RNA loads on days 5 and 28 after RSV inoculation. Pulmonary histopathologic scores, airway obstruction, and postmethacholine airway hyperresponsiveness were significantly reduced in mice treated with MEDI-524 at 24 h before inoculation, compared with untreated controls and the other regimens evaluated. MEDI-524 was superior to palivizumab on several outcome variables of RSV disease assessed in the mouse model: viral replication, inflammatory and clinical markers of acute disease severity, and long-term pulmonary abnormalities.

Human metapneumovirus persists in BALB/c mice despite the presence of neutralizing antibodies

Human metapneumovirus (HMPV) has emerged as an important human respiratory pathogen causing upper and lower respiratory tract infections in young children and older adults. Recent epidemiological evidence indicates that HMPV may cocirculate with respiratory syncytial virus, and HMPV infection has been associated with other respiratory diseases. In this study, we show that BALB/c mice are susceptible to HMPV infection, the virus replicates in the lungs with biphasic growth kinetics in which peak titers occur at days 7 and 14 postinfection (p.i.), and infectious HMPV can be recovered from lungs up to day 60 p.i. In addition, we show that genomic HMPV RNA can be detected in the lungs for >/=180 days p.i. by reverse transcription-PCR; however, neither HMPV RNA nor infectious virus can be detected in serum, spleen, kidneys, heart, trachea, and brain tissue. Lung histopathology revealed prevalent mononuclear cell infiltration in the interstitium beginning at day 2 p.i. and peaking at day 4 p.i. which decreased by day 14 p.i. and was associated with airway remodeling. Increased mucus production evident at day 2 p.i. was concordant with increased bronchial and bronchiolar inflammation. HMPV-specific antibodies were detected by day 14 p.i., neutralizing antibody titers reached >/=6.46 log(2) end-point titers by day 28 p.i., and depletion of T cells or NK cells resulted in increased HMPV titers in the lungs, suggesting some immune control of viral persistence. This study shows that BALB/c mice are amenable for HMPV studies and indicates that HMPV persists as infectious virus in the lungs of normal mice for several weeks postinfection.

Generation of an HFRS patient-derived neutralizing recombinant antibody to Hantaan virus G1 protein and definition of the neutralizing domain

Hantaan virus (HTNV) in the Hantavirus genus, family Bunyaviridae, is the major cause of severe hemorrhagic fever with renal syndrome (HFRS). We prepared a combinatorial phage display library of human Fabs to HTNV from RNA extracted from the blood lymphocytes of a convalescent HFRS patient. We selected two G1 glycoprotein-specific clones and one nucleocapsid protein (N)-specific clone from the Fab library for further studies. The human Fab antibodies were converted to IgG form in baculovirus/insect cells system by using cassette vectors that we developed earlier. Characterization of the recombinant antibodies revealed that the two G1-specific IgGs, could bind to and neutralize HTNV but not Seoul virus (SEOV). The N-specific IgG did not neutralize either HTNV or SEOV. Sequence analysis revealed that the two G1-specific clones differed by only one predicted amino acid in their complementarity determining regions, CDR3. Epitope mapping studies were carried out with one of the two G1-specific clones and synthetic peptides representing portions of HTNV G1. Results indicated that the recombinant antibody recognizes the core amino acid sequence LTKTLVIGQ, which is found near the C-terminus of HTNV G1. These results are the first to define a neutralizing epitope on the G1 protein of HTNV using an antibody derived from an HFRS patient.

Calcium phosphate nanoparticles induce mucosal immunity and protection against herpes simplex virus type 2

Previously we reported that calcium phosphate nanoparticles (CAP) represented a superior alternative to alum adjuvants in mice immunized with viral protein. Additionally, we showed that CAP was safe and elicited no detectable immunoglobulin E (IgE) response. In this study, we demonstrated that following mucosal delivery of herpes simplex virus type 2 (HSV-2) antigen with CAP, CAP adjuvant enhanced protective systemic and mucosal immunity versus live virus. Mice were immunized intravaginally and intranasally with HSV-2 protein plus CAP adjuvant (HSV-2+CAP), CAP alone, phosphate-buffered saline, or HSV-2 alone. HSV-2+CAP induced HSV-specific mucosal IgA and IgG and concurrently enhanced systemic IgG responses. Our results demonstrate the potency of CAP as a mucosal adjuvant. Furthermore, we show that systemic immunity could be induced via the mucosal route following inoculation with CAP-based vaccine. Moreover, neutralizing antibodies were found in the sera of mice immunized intranasally or intravaginally with HSV-2+CAP. Also, the results of our in vivo experiments indicated that mice vaccinated with HSV-2+CAP were protected against live HSV-2 infection. In conclusion, these preclinical data support the hypothesis that CAP may be an effective mucosal adjuvant that protects against viral infection.

Progressive epitope-blocked panning of a phage library for isolation of human RSV antibodies

Epitope-blocked panning is an approach to mining antigen-specific diversity from phage display antibody libraries. Previously, we developed and used this method to recover a neutralizing antibody to respiratory syncytial virus (RSV) by blocking a dominant response to a nonneutralizing epitope on a recombinant derivative of the viral F antigen. We have extended this approach to the blocking of multiple epitopes simultaneously, which led to the recovery of new antibodies of different specificity, including one new neutralizing activity. A phage display Fab library was selected on recombinant F antigen in the presence of three representative antibodies recovered in the unblocked and subsequent single-blocked panning procedures. Restriction endonuclease fingerprinting of 13 F+ clones revealed seven unique Fabs. DNA sequence analysis of five of these clones revealed five new light chains in combination with different heavy chains, three of which were very similar or identical to Fabs previously isolated from this library. The blocking antibodies did not compete with the new Fabs, demonstrating effective masking of their binding sites in the panning procedure. Conversely, these Fabs did show variable inhibition of two of the blocking antibodies suggesting a close proximity or interdependence of their epitopes. One of the antibodies did inhibit virus infection, albeit with modest potency. These results demonstrate that epitope-blocked panning is a self-progressing approach to retrieving diverse antibodies from phage libraries.