It has taken a while, but the Journal of Innate Immunity has now published an editorial [1] that discusses the Stolzenberg et al. paper [2].

As Labrie and Brundin say in the final paragraph: "... it will be important to understand what mechanisms are responsible for modulating pathogen-induced alpha-synuclein increases in the gut" i.e. how the alpha-syn is normally cleared (and what stops that from happening in PD).

https://www.karger.com/Article/FullText/479653

[1] Alpha-Synuclein to the Rescue - Immune Cell Recruitment by Alpha-Synuclein during Gastrointestinal Infection, Viviane Labrie & Patrik Brundin, J Innate Immun 2017;9:437–440.

[2] A role for neuronal alpha-synuclein in gastrointestinal immunity, Stolzenberg et al., J Innate Immun 2017; 9:456–463.

Two years have now passed, and some progress has been made. A recent open-access paper by Michael Zasloff et al. brings us up to date (including information on a couple of clinical trials).

Gastrointestinal Immunity and Alpha-Synuclein (September 2019):
Gastrointestinal Immunity and Alpha-Synuclein - IOS Press