Abstract:
Defining the T helper immunogenic content of Adenovirus type 5 (Ad5) fiber as a first step towards development of immuno-attenuated adenoviral vectors

Luisa Marcon, M.D.
Assistant Professor, Brown Medical School

Abstract

Recombinant adenoviral vectors are extremely valuable for the intracellular delivery of DNA encoding proteins for replacing defective genes and for antigens. Priming with a DNA vaccine followed by boosting with a recombinant adenoviral vector has been shown to be the most efficient immunization regimen for controlling HIV in animal models. However, pre-existing adenoviral immunity due to natural exposure to adenoviruses limits the efficiency of these vectors. Neutralizing antibodies to the Adenovirus capsid components reduce vector infectivity. A novel method is herein proposed that reduces or eliminates the T cell antigenic content of Adenoviral vectors. CD4 T cells govern both the humoral and cell-mediated adaptive immune response. Binding of peptide epitope sequences present in protein antigens with the MHC class II molecule activates CD4 T cells which can then provide help to B cells or CD8 T cells to initiate their effector functions. The peptide epitope "anchor residues" (2-3 amino acids) binding to MHC class II molecules have known specificities. Identification of the adenoviral vector T helper epitopes could then allow subsequent minimal modification at anchor positions, resulting in a loss of interaction with the MHC class II molecules.Utilizing computer algorithms designed to predict binding ligands to the eight most common human MHC class II alleles, the T helper epitope content of the Adenovirus type 5 fiber will be defined. Predicted peptide epitopes will then be synthesized and tested for binding to the MHC class II molecules in vitro with a competition binding assay. To verify the in vivo immunogenic properties of the predicted epitopes sequences, T cell responses to the selected T helper peptides will be measured in PBMC from normal individuals immune to Adenoviruses by ELISpot assays. The definition of the epitope content of the Ad fiber will be the basis for future studies that will entail amino acid modifications in the epitopes to reduce or eliminate interaction with the MHC molecules. Reduction or elimination of the T helper immunogenic content present in a molecule should mask that molecule to the adaptive arm of the immune system and prevent the de novo elicitation of an immune response or the recall of a pre-existing immune response.