The mutable vaccine for mutable viruses

Abstract

Mutable viruses, such as HIV, pose difficult obstacles to prevention and/or control by vaccination. Mutable viruses rapidly diversify in populations and in individuals, impeding development of effective vaccines. We devised the 'mutable vaccine' to appropriate the properties of mutable viruses that undermine conventional strategies. The vaccine consists of a DNA construct encoding viral antigen and regulatory sequences that upon delivery to B cells target the enzymatic apparatus of 'somatic hypermutation' causing the construct to mutate one million-times baseline rates and allowing production and presentation of antigen variants. We postulate the mutable vaccine might thus anticipate diversification of mutable viruses, allowing direct control or slowing of evolution. Initial work presented here should encourage consideration of this novel approach.

Keywords: B cell; HIV; antigen; escape variant; mutable virus; somatic hypermutation; vaccine; viral evolution; virus.

Figure 1.. A schematic model of the mutable vaccine.

The vaccine is a DNA construct encoding one or more viral antigens (HIV gp140 is shown as an example) fused with C3d and regulated by immunoglobulin promoter and enhancer sequences.

Figure 2.. Model for viral evolution and control by vaccination with the mutable vaccine.

(A) Evolution of a mutable virus and the adaptive immune response. After infection at time 0, the virus diversifies over time (time 1 and 2). Immunity reflecting lymphocyte specificity to the infecting virus is manifested at time 1 (dashed line), but the virus has already diversified, leaving variants that escape control by adaptive immunity. Further diversification (time 2) generates still more variants at time 2, some of which might be controlled by lymphocytes specific for viral variants at time 1 (dashed lines), but many variants escape control, leading to progressive viral diversification, despite T-cell and B-cell responses. (B) Potential impact of the mutable vaccine on viral evolution. The mutable vaccine administered at time 0 evokes immunity against the encoded antigen symbolized an open circle. The vaccine antigen diversifies by time 2, at which time lymphocytes responding to the initial antigen are proliferating. At time 3, infection occurs and lymphocyte effector cells can eradicate infecting virus. If infecting virus is not entirely cleared and begins to diversify by time 4, the expanded effector lymphocytes against initial variants may exert partial control, slowing viral evolution and allowing immunity specific for antigens of the infecting virus other than those encoded in the vaccine to emerge (not illustrated).

Figure 3.. Extent of mutation in the mutable vaccine.

Tumor B cells, hypermutation-competent tumor B cells (of the 18.81 lineage [43]) were transduced with the mutable vaccine and the env sequences and deduced amino acid sequences were determined in single cell clones derived from the original tumor cells following three sequential limiting dilutions [44]. (A) Sites of mutation of env in clonal B cells expressing the mutable vaccine. Arrows denote nucleotide changes from the original vaccine sequence. The type of mutation and the position in the sequence are shown. (B) Amino acid changes relative to the variable (V) domains of the Env. Arrows and numbers indicate the position of changes in the sequence. Numbering was according to the YU-2 protein (UniProtKB: locus ENV_HV1Y2, accession P35961). Adapted in part from [44].

Figure 4.. Antibody response to the mutable vaccine.

Mice vaccinated with tumor B cells expressing the mutable vaccine produce anti-ENV antibodies. The levels of anti-ENV antibodies before and after vaccination were measured using a capture ELISA and displayed as absorbance (y-axis) versus dilution (x-axis). Gray indicates post-immune sera and black pre-immune sera. The graph is representative of five independent experiments.

Figure 5.. The mutable vaccine generates protective immunity.

Immune-competent syngeneic (A) or RAG-/- γc-/- mice (B) were injected with 2 × 10⁷ tumor B cells expressing (+HIV-ENV) or not expressing the mutable vaccine (-HIV-ENV) in the peritoneum and tumor volume (in mm³) was determined on day 10. Figure 5C–E depict survival of immune-competent syngeneic (C), RAG-/- γc-/- mice (E) or B-cell-deficient mice (JH-/- κ -/-) following injection with 2 × 10⁷ tumor B cells expressing (+HIV-ENV) or not expressing the mutable vaccine (-HIV-ENV) in the peritoneum.