Foot-and-mouth disease and beyond: vaccine design, past, present and future

In vitro evolution and affinity-maturation with Coliphage qβ display

The Escherichia coli bacteriophage, Qβ (Coliphage Qβ), offers a favorable alternative to M13 for in vitro evolution of displayed peptides and proteins due to high mutagenesis rates in Qβ RNA replication that better simulate the affinity maturation processes of the immune response. We describe a benchtop in vitro evolution system using Qβ display of the VP1 G-H loop peptide of foot-and-mouth disease virus (FMDV). DNA encoding the G-H loop was fused to the A1 minor coat protein of Qβ resulting in a replication-competent hybrid phage that efficiently displayed the FMDV peptide. The surface-localized FMDV VP1 G-H loop cross-reacted with the anti-FMDV monoclonal antibody (mAb) SD6 and was found to decorate the corners of the Qβ icosahedral shell by electron microscopy. Evolution of Qβ-displayed peptides, starting from fully degenerate coding sequences corresponding to the immunodominant region of VP1, allowed rapid in vitro affinity maturation to SD6 mAb. Qβ selected under evolutionary pressure revealed a non-canonical, but essential epitope for mAb SD6 recognition consisting of an Arg-Gly tandem pair. Finally, the selected hybrid phages induced polyclonal antibodies in guinea pigs with good affinity to both FMDV and hybrid Qβ-G-H loop, validating the requirement of the tandem pair epitope. Qβ-display emerges as a novel framework for rapid in vitro evolution with affinity-maturation to molecular targets.

Foot-and-mouth disease virus can induce a specific and rapid CD4+ T-cell-independent neutralizing and isotype class-switched antibody response in naïve cattle

The role of T-lymphocyte subsets in recovery from foot-and-mouth disease virus (FMDV) infection in calves was investigated by administering subset-specific monoclonal antibodies. The depletion of circulating CD4(+) or WC1(+) gammadelta T cells was achieved for a period extending from before challenge to after resolution of viremia and peak clinical signs, whereas CD8(+) cell depletion was only partial. The depletion of CD4(+) cells was also confirmed by analysis of lymph node biopsy specimens 5 days postchallenge. Depletion with anti-WC1 and anti-CD8 antibodies had no effect on the kinetics of infection, clinical signs, and immune responses following FMDV infection. Three of the four CD4(+) T-cell-depleted calves failed to generate an antibody response to the nonstructural polyprotein 3ABC but generated a neutralizing antibody response similar to that in the controls, including rapid isotype switching to immunoglobulin G antibody. We conclude that antibody responses to sites on the surface of the virus capsid are T cell independent, whereas those directed against the nonstructural proteins are T cell dependent. CD4 depletion was found to substantially inhibit antibody responses to the G-H peptide loop VP1(135-156) on the viral capsid, indicating that responses to this particular site, which has a more mobile structure than other neutralizing sites on the virus capsid, are T cell dependent. The depletion of CD4(+) T cells had no adverse effect on the magnitude or duration of clinical signs or clearance of virus from the circulation. Overall, we conclude that CD4(+) T-cell-independent antibody responses play a major role in the resolution of foot-and-mouth disease in cattle.

Intranasal immunization of guinea pigs with an immunodominant foot-and-mouth disease virus peptide conjugate induces mucosal and humoral antibodies and protection against challenge

Guinea pigs immunized intranasally with a keyhole limpet hemocyanin-linked peptide, corresponding to the prominent G-H loop of the VP1 protein of foot-and-mouth disease virus, raised substantial levels of antipeptide and virus-neutralizing antibodies in sera and of peptide-specific secretory immunoglobulin A in nasal secretions. In groups of animals immunized intranasally without adjuvant, 86 percent were fully protected upon challenge with homotypic virus. Surprisingly, animals given the peptide conjugates plus the mucosal adjuvant cholera toxin were afforded only partial protection in that primary lesions were observed in most animals, although spread to other feet was prevented. These results indicate that intranasal inoculation with the peptide offers a potential route of vaccination against foot-and-mouth disease and may be useful for eliciting protection in the upper respiratory tracts of susceptible animals.