Protection induced by a Francisella tularensis subunit vaccine delivered by glucan particles

Abstract

Francisella tularensis is an intracellular pathogen causing the disease tularemia, and an organism of concern to biodefence. There is no licensed vaccine available. Subunit approaches have failed to induce protection, which requires both humoral and cellular immune memory responses, and have been hampered by a lack of understanding as to which antigens are immunoprotective. We undertook a preliminary in silico analysis to identify candidate protein antigens. These antigens were then recombinantly expressed and encapsulated into glucan particles (GPs), purified Saccharomyces cerevisiae cell walls composed primarily of β-1,3-glucans. Immunological profiling in the mouse was used to down-selection to seven lead antigens: FTT1043 (Mip), IglC, FTT0814, FTT0438, FTT0071 (GltA), FTT0289, FTT0890 (PilA) prior to transitioning their evaluation to a Fischer 344 rat model for efficacy evaluation. F344 rats were vaccinated with the GP protein antigens co-delivered with GP-loaded with Francisella LPS. Measurement of cell mediated immune responses and computational epitope analysis allowed down-selection to three promising candidates: FTT0438, FTT1043 and FTT0814. Of these, a GP vaccine delivering Francisella LPS and the FTT0814 protein was able to induce protection in rats against an aerosol challenge of F. tularensis SchuS4, and reduced organ colonisation and clinical signs below that which immunisation with a GP-LPS alone vaccine provided. This is the first report of a protein supplementing protection induced by LPS in a Francisella vaccine. This paves the way for developing an effective, safe subunit vaccine for the prevention of inhalational tularemia, and validates the GP platform for vaccine delivery where complex immune responses are required for prevention of infections by intracellular pathogens.

A Francisella novicida Mutant, Lacking the Soluble Lytic Transglycosylase Slt, Exhibits Defects in Both Growth and Virulence

“…Several subunit and live attenuated vaccines have been tested. Whelan et al (2018) recently showed that a subunit vaccine, comprising Francisella LPS and the FTT_0814 protein delivered within glucan particles, was able to induce protection against Ft Schu S4 challenge in the Fischer 344 rat model. However, other subunit vaccines based on LPS, carbohydrates, and other surface antigens were unable to provide complete protection, and it is thought that both humoral and cell-mediated immune responses are required to achieve protective immunity (Pechous et al, 2009).…”

In silico vaccine design against Chlamydia trachomatis infection

“…Studies confirm the release of CD4+ T cells and IFN γ cells to protect from the infections. Thus, suggesting the importance of identification of T-cell epitopes epitope-based vaccine approach has been reported for Francisella tularensis (Whelan et al 2018), Haemophilus influenzae (Zahroh et al 2016), Dengue virus (Chakraborty et al 2010), Human coronaviruses (Oany et al 2014), Chikungunya virus (Islam et al 2012), and Ebola (Srivastava et al 2016).…”

A novel vaccine platform using glucan particles for induction of protective responses againstFrancisella tularensisand other pathogens

“…Subunit vaccines with GPs encasing soluble alkaline extracts of Cryptococcus neoformans acapsular strain (cap59) protected mice challenged with lethal doses of highly virulent C. neoformans (60% survival) by inducing an antigen‐specific CD4 + T cell response (positive for IFN‐γ, IL‐17a) that reduced the fungal colony‐forming units (CFU) more than 100‐fold from the initial challenge dose . A similar strategy of vaccinating mice with GP encapsulating antigens proved efficacious against Histoplasma capsulatum , F. tularensis , Blastomyces dermatitidis and C. posadasii . The versatility of using GPs vaccines against microbial pathogens is summarized in Table .…”

“…We hypothesized that subunit vaccines are most attractive due to their defined nature and thus good safety profiles, but that we needed to deliver promising candidate antigens in a manner that induced both humoral and cellular immune responses to achieve protection, as only a balanced humoral and cellular immune memory response, supported by innate immune mechanisms, protects against tularaemia (reviewed by Roberts et al and Krokova et al ). We therefore decided to employ the GP vaccine delivery platform described above to address this challenge . This work is summarized below.…”