Oral Presentation BACPATH 2017

Immunological assessment of the nature of binding of protective antibodies to the Gram-negative cell envelope reveals a complex role of LPS O-chain in restricting protection from vaccination (#37)

Vassiliy N Bavro 1 , Coral Domínguez-Medina 2 , Anna E Schager 2 , Marisol Pérez-Toledo 3 , Charlotte N Cook 2 , Saeeda Bobat 2 , James C Gumbart 4 , Faye C Morris 5 , Amanda E Rossiter 5 , Ian R Henderson 5 , Calman A McLennan 6 , Paul A Barrow 7 , Erin Logan 8 , William G Horsnell 8 , Constantino López-Macías 3 , Adam F Cunningham 2
  1. School of Biosciences, University of Essex, Colchester, CO4 3SQ, Essex, UK
  2. Institute of Immunology and Immunotherapy, University of Birmingham, Birmingham B15 2TT, UK
  3. Mexican Institute for Social Security (IMSS), Mexico City, Mexico
  4. Georgia Institute of Technology, Atlanta, Georgia, USA
  5. Institute of Microbiology and Infection, University of Birmingham, Birmingham, UK
  6. Jenner Institute, University of Oxford, Oxford, OX3 7DQ, UK
  7. School of Veterinary Medicine and Science, University of Nottingham, Nottingham, UK
  8. Institute of Infectious Disease and Molecular Medicine, University of Cape Town, Cape Town, South Africa

Antibodies that specifically target surface-exposed or secreted antigens can effectively protect after natural infection and vaccination. While a multitude of antigens is recognised by antibodies following infection, most of them do not confer protection. Understanding the nature and targets of protective antibodies to Gram-negative bacteria is important to develop a new generation of vaccines against these increasingly antibiotic-resistant pathogens.

Following our identification of the outer-membrane protein (OMP) OmpD from nontyphoidal Salmonella as a protective antigen, here we report our studies on how antibody to OmpD protects and what its potential strengths and limitations as a vaccine against multiple Salmonella serovars are. Understanding of the protection afforded by antibodies requires taking into account the wider spatial context and interplay of the membrane and lipopolysaccharide (LPS) O-chain alongside the proteinateous antigens. To address this we generated the first of its kind full-atomistic molecular dynamics (FA-MD) model of the outer membrane, which revealed an unexpected complexity of antigen-antibody interaction associated with the LPS-O-chain layer.

Our data suggests that the native, trimeric OmpD creates a tunnel in the LPS-O-chain layer of sufficient size to allow epitope binding on the bacterial surface by the Fab-region of antibody, potentially explaining why antibodies to OmpD can be protective. However, surprisingly, OmpD from S. Typhimurium does not cross-protect against bacteria expressing conserved OmpD-variants that differ by a single amino-acid. Notably, loss of the O-chain restores bacterial susceptibility to killing possibly by unmasking covered epitopes.

Further analysis of structural and FA-MD modelling corroborated with in vivo and in vitro studies of immune response to Salmonella infection in mice suggests a central role of LPS O-chain in modulating the protective antigenic properties of OMP-based epitopes. Our model allows a testable a priori prediction of the antibody-binding properties of surface antigens and will be helpful in identifying protective antigens for vaccines.