Towards a global vaccination approach against multi-resistant staphylococcus aureus targeting the cellular, humoral and innate arms of the immune system – MRSA-VAC

Context. Methicillin-resistant Staphylococcus aureus (MRSA) is at present the most commonly identified antibiotic-resistant pathogen in many parts of the world. MRSA infections represent a major Public Heath issue with important impacting socio-economical consequences. Several prophylactic and therapeutic approaches have been tested and clinically developed in the past decades, but none of them has yet been successful. The pharmaceutical industry actually develops two immunological approaches against MRSA as an alternative to antibiotherapy. The first one, conventional « active » vaccination, aims to provide long-term immunity but suffers from the fact that the risk of MRSA infection is too low to provide grounds for large-scale preventive vaccination and remains unpredictable in a number of cases (trauma, burns for example). The second one, passive vaccination, consists in administrating human polyclonal antibodies (Abs) or humanized monoclonal Abs targeting different Staphylococcus aureus (SA) antigens. It is quick and effective but offers only transient protection, is rather costly and is associated with side effects such as allergic reactions. Finally, the humoral response is not sufficient to protect from all types of MRSA infections inasmuch as SA is also a facultative intracellular pathogen.
Objectives and experimental approach. The aims of the present project are twofold.
First, we will develop a novel anti-SA vaccination strategy susceptible to confer a quick and efficient humoral protection against MRSA, that would combine the advantages of active and passive immunization with little side effects. Most SA candidate vaccines are protein Ag that trigger a thymus-dependent (TD) Ab response that takes weeks and one or two booster immunizations to build up protective humoral immunity. Here we propose to target defined protein vaccine candidates to the thymus-independent (TI) arm of the humoral immune system. As opposed to the TD pathway, TI responses induce a quick rise of protective serum Ab titers after a single immunization. Moreover, as opposed to TD Abs which are highly selected in the germinal center and react only with a few non-promiscuous epitopes, the TI response promotes activation of a lower-affinity but more diverse B cell repertoire covering a larger range of antigenic epitopes. We aim to bring the proof of concept in an animal preclinical model that this approach allows to generate in a matter of days an anti-SA humoral protection susceptible to confer a broad range protection against different SA subtypes. To reach this goal we will generate “particulate conjugates” in which the Ag of choice (a hemolysin or Clumping factor A) is conjugated to polystyrene beads together with a synthetic TLR9 ligand. Such particulate Ag allow strong cross-linking of the B cell Ag receptors and targeting of the TLR9 co-stimulatory signal to Ag-specific B cells.
Second, we will explore the immunogenicity of the intracellular form of SA and its recognition by different effectors of innate and adaptive cellular immunity. This part of the project is an exploratory phase in which we plan to determine which cellular effectors (NK, NKT, CD8 T cells, ?d T cells) and which effector mechanisms are required to efficiently control intracellular SA infection and give rise to full vaccinal protection. It is intended to give clues as to how the cellular immune response should be manipulated in order to induce protective host immunity. We will evaluate the susceptibility of SA-infected cells to cytotoxicity mediated by innate lymphocytes in vitro. We will analyze CD8 T cell immunity against SA antigens and how it is affected by SA super Ags. We will analyze which type of cellular immunity is triggered by intracellular SA in vivo. Finally, we will determine whether “particulate SA Ag” can elicit both rapid humoral immunity and strong CD8 T cell responses to efficiently combat all forms of MRSA infections.