Investigating the role of persisting antigen in immunity against foot-and-mouth disease virus
Foot-and-mouth disease (FMD) is one of the most economically important diseases of livestock, characterised by fever and blister like sores on the tongue and around the mouth and on the feet, resulting in a significantly reduced yield of animal products. FMD virus (FMDV) is the causative agent of FMD and causes a highly contagious acute vesicular disease, resulting in more than 50% of cattle, regardless of vaccination status, and almost 100% of African buffalo becoming persistently infected for long periods (years) of time. After the resolution of acute infection and viraemia, in persistently infected animals, FMDV capsid proteins and/or genome are localised in the light zone of germinal centres (GC) in lymphoid tissue in cattle and African buffalo. The pattern of staining for FMDV proteins has been described to be consistent with virus binding to follicular dendritic cells (FDC) in the GC. The present thesis demonstrates that similar staining is observed in mouse spleens after acute infection with FMDV. FDC have the unique ability to trap antigen in the form of immune complexes (IC) on their surface for long periods of time. The first hypothesis to be tested in this project was that FMDV was binding to FDC as an IC, and ex vivo IC deposition assays demonstrated that FMDV could only bind in the form of IC, not solely antigen. Next, investigation using super-resolution microscopy showed significant co-localisation of FMDV antigen with complement receptor type 2 and 1, which are highly expressed on FDC, in the spleen post-infection. Furthermore, by blocking the CR2/CR1 prior to infection with FMDV, the detection of viral proteins on FDC and FMDV genomic RNA in spleen samples were significantly reduced. Crucially, blocking CR2/CR1 resulted in the induction of antibodies with significantly reduced capacity to neutralise virus and lower binding affinity to FMD virus-like particles (VLP) compared to control animals. Animals generally have long-lived neutralising antibody titres post-infection with FMDV, which contrast with the short-lived response induced by vaccination, of only 6 months. The results presented in this thesis highlight the importance of targeting FMDV antigen to FDC to stimulate potent neutralising antibody responses, which could be incorporated into future vaccine development, to increase the duration of immunity.