Treatment of Participants With Primary or Secondary Progressive Multiple Sclerosis

The goal of this clinical trial is to treat male and female participants with two types of Multiple Sclerosis (MS) called primary progressive or secondary progressive MS. The main questions the trial aims to answer are the following: * Is TRX319 safe when administered to patients with progressive forms of MS? * At what dose does TRX319 work the best to treat participants with primary and or secondary progressive MS? * Is pre-conditioning (with Bendamustine) needed to allow TRX319 to better treat participants with primary and/or secondary progressive MS? Participants will be asked to be on study for up 1 year and may receive up to 3 total administrations of TRX319. While on study, participants will have blood tests and other assessments (MRI scans and lumbar punctures) done to understand the safety of TRX319 and how it may benefit their multiple sclerosis.