Pathogen invasion of the central nervous system (CNS) are an important cause of infection-related mortality worldwide. Further, survivors could suffer permanent neurological disability. Crossing of the blood-brain barrier (BBB) is the fundamental step by which pathogens establish brain infection. While a number of cell culture and animal models exist, they are limited either by their inability to recapitulate the 3D architecture of the BBB or by their genetic and cellular complexity which prevents systematic, more powerful approaches.

In our recent work, we propose an innovative model of brain infection using Drosophila larvae. This model displays architectural intricacy while allowing experimental simplicity and takes full advantage of the swiss army knife that is Drosophila genetics. 

Drosophila as a straightforward platform for screening pathogen neurotropism and deciphering host-pathogen interactions

We first developed an ex vivo screening protocol using the whole fly CNS. We showed that a number of pathogens known to trigger CNS infection in mammals are able to cross the Drosophila BBB, including the strictly human Neisseria meningitidis, what reveals a likely conservation of core BBB-pathogen interactions. This set-up can serve as a fast and powerful screening platform to investigate molecular and cellular mechanisms contributing to the crossing of the BBB.

To assess pathogenic virulence and impact in the whole organism, we then established an in vivo brain infection protocol through direct pathogen microinjection into the fly circulatory system. In particular, this in vivo set-up evaluates the contribution of circulating immune cells to the BBB crossing and pathogenicity.

The combination of these two approaches thus provides the community with a new biological playground in which novel factors and mechanisms contributing to CNS infection could be quickly investigated and further precisely dissected through Drosophila genetics.

Identifying a new mechanism of brain invasion by GBS exploiting host barrier machinery

As a proof of principle, we focused on Group B Streptococcus (GBS), a leading cause of meningitis in newborn. Screening GBS mutants for surface components and using Drosophila genetics to specifically target putative host-pathogen interaction candidates, we identified (1) GBS surface lipoproteins, more precisely Blr lipoprotein, as a new virulence factor crucial for BBB crossing and (2) Drosophila lipoprotein receptor LpR2 (for Lipophorin Receptor 2) as a host receptor for Blr, leading to GBS internalisation in the BBB and subsequent brain invasion.

Moreover, we showed that Blr is a conserved virulence factor required for BBB crossing and pathogenicity in a murine model of streptococcal brain infection. Interestingly, Drosophila LpR2 is the orthologue of the mammalian LDLR and VLDLR proteins, which are present on brain blood vessels and which could represent interesting pharmacological targets.

Ultimately, understanding the exact cellular and molecular pathways employed by pathogens to hijack and corrupt the physiological role of host receptors will provide new insight into therapeutic approaches to strengthen the BBB and restrict pathogen entry during infection as well as bypassing this barrier to target therapeutics molecules.