Abstract
Multiple sclerosis (MS) is a chronic neurological autoimmune disorder characterized by neuroinflammation, demyelination, and axonal degeneration leading to permanent central nervous system damage. Current disease modifying therapies (DMTs) are effective in reducing disease activity and relapse frequency, but do not halt progression leaving patients with advanced secondary progressive or primary progressive disease without therapeutic options. Additionally, DMTs administered long-term lose efficacy due to generation of neutralizing antibodies and have major side effects. Autologous hematopoietic stem cell transplantation (HSCT) has been shown in clinical trials as effective in achieving long-term remission and neurological improvement in patients with highly active, relapsing-remitting MS and is recommended as a treatment in some countries. However, the mechanisms underlying the long-term clinical remission post-HSCT remain unknown. Cyclophosphamide (Cy) has been successfully used following allogeneic HSCTs to promote tolerance and reduce GVHD.
Here, studies were performed in C57BL/6 mice to determine the optimal time and conditioning to administer a syngeneic HSCT (syn-HSCT) after induction of EAE. Mice received total body irradiation (TBI) and a syn-HSCT at D.8 or D.16 post-induction, with or without T cell addition. Animals treated at D.8 had significantly delayed disease onset compared to untreated mice (Fig. 1). Importantly, T cell replete HSCT did not alter disease onset or incidence. Higher TBI (11.5 vs. 9.5 Gy) resulted in a greater delay (Fig. 2). Mature donor T cells and T cells derived from transplanted marrow-derived stem cells were detected in the spinal cord 2 months post-HSCT. Recipient CD4 T cells that survived the conditioning and HSCT were also present in the cord and lymphoid tissues post-HSCT. We hypothesize that EAE development post-HSCT was due to surviving recipient anti-MOG specific Teff cells generated prior to syn-HSCT. To our knowledge, this is the first report demonstrating the ‘source’ of the disease mediating Teff cells post-HSCT.
On this basis, we propose that more stringent removal/suppression of Teff should further improve outcomes. Thus, we employed Cy, post-syn-HSCT (PTCy) which can delete and functionally suppress Teff in a dose dependent manner. Application of 50 mg/kg PTCy on D.11 & 12 (= Days 3 and 4 post-syn-HSCT) resulted in marked delay and suppression of EAE clinical signs (Fig. 3). In fact, 57% of these recipients did not exhibit disease up to 65 days post-induction while all untreated mice were sick by D.25. This demonstrates that PTCy can be combined with syn-HSCT to treat an autoimmune disorder. Our long-term objective is to combine HSCT + PTCy deletion/inhibition of Teff cells with augmented immune regulation by expanding Tregs in vivo following additional low dose Cy treatment to develop new treatment modalities for MS.