Bacterial carriers and virus-like-particles as antigen delivery devices: role of dendritic cells in antigen presentation.

Replicating attenuated strains of intracellular bacteria like Salmonella typhimurium, Listeria monocytogenes or Mycobacterium bovis Bacille Calmette Guérin (BCG), and non-replicating virus-like-particles (VLP) consisting, for instance, of the VP1-surface component of polyoma virus offer great potential as heterologous carriers delivering foreign protein antigens for immune recognition. Moreover, attenuated S. typhimurium and L. monocytogenes strains hold also great promise as delivery vehicles for DNA vaccines. Polyoma virus-specific VLP consisting of VP1-pentamers are also of interest as carrier devices for eukaryotic expression plasmids. At first sight these different replicating and non-replicating types of vehicles have little in common, but from an immunological point of view viable bacteria and non-viable VLP are both well suited for evoking protective immune responses via several routes of vaccine administration. As these antigen carriers generate humoral and cell-mediated immunity, the heterologous antigens are not only targeted to appropriate pathways of major histocompatibility (MHC) class I and class II antigen processing and presentation, but also generate an adequate cytokine milieu for promoting antigen-specific responses. The most prominent advantage of these carrier devices is presented by their capacity to directly target antigenic proteins or DNA vaccines to immature dendritic cells (DC) along their maturation pathway. Mature DC are the key antigen presenting cell population which efficiently mediates antigen transport to organised lymphoid tissues for the initiation of T cell responses. In general, uptake of these diverse antigen delivery systems by antigen presenting cells (APC) finally lead to efficacious immune responses in the control of pathogenic microorganisms and tumours.