This research group is developing a therapeutic Crohn's vaccine:
https://crohnsmapvaccine.com/faq/
Their idea is that Crohn's is caused by the bacterium mycobacterium avium subspecies paratuberculosis (or MAP for short). MAP has been proven to cause Johne's disease in cattle so the idea is that Crohn's is the same thing in people.
They've started safety trials in patients with Crohn's. The next step is a wider trial to demonstrate efficacy.