Flagellin, a TLR5 Agonist, Reduces GvHD in Allogeneic HSCT Recipients While Enhancing Anti-Viral Immunity: A Novel Therapeutic Approach
Authors: Hossain Mohammad S , Jaye David L , Pollack Brian P , Farr Alton B , Roback John , Gewirtz Andrew T , Waller Edmund K 
Published: October 2019

Abstract
Abstract 144
In MHC-mismatched allogeneic hematopoietic stem cell transplantation (allo-HSCT), host antigen specific donor T cells mediate acute and chronic graft-versus-host disease (GvHD). Based upon the radio-protective effects of flagellin, a TLR5 agonist protein (∼50 kDa) extracted from bacterial flagella, we reasoned that flagellin might modulate donor T cells immune responses toward host antigens, reduce GvHD, and improve immune responses to CMV infection in experimental models of allogeneic HSCT.
Two 50mg/mouse i.p doses of highly purified flagellin were administered 3 hrs before irradiation and 24 hrs after allo-HSCT in H-2b ^ CB6F1 and H-2k ^ B6 models. GvHD scores were obtained with weekly clinical examination and with histological scoring of intestine, colon, liver and skin at necropsy. Flagellin treatment successfully protected allo-HSCT recipients from acute and chronic GvHDs after transplantation of 5×106 splenocytes and 5×106 T cell depleted (TCD) BM, and significantly increased survival compared to PBS-treated control recipients. Reduced acute GvHD was associated with significant reduction of a) early post-transplant proliferation of donor CD4+ and CD8+ T cells measured by Ki67 and CFSE staining, b) fewer CD62L+, CD69+, CD25+, ICOS-1+ and PD-1+ donor CD4+ and CD8+ T cells compared with the PBS-treated control recipients. Decreased numbers of activated and proliferating donor T cells were associated with significantly reduced pro-inflammatory serum IFN-g, TNF-a, and IL-6 on days 4–10 post transplant in flagellin-treated recipients compared with the PBS-treated recipients. Interestingly, both flagellin-treated recipients and PBS-treated recipients had over 99% donor T cell chimerism at 2 months post transplant. Moreover, MCMV infection on 100+ days post-transplant flagellin-treated mice significantly enhanced anti-viral immunity, including more donor MCMV-peptide-tetramer+ CD8+ T cells in the blood (p<0.05), and less MCMV in the liver on day 10 post infection (p<0.02) compared with the PBS-treated control recipients. Overall immune reconstitution after flagellin-treatment was robust and associated with larger numbers of CD4+CD25+foxp3+ regulatory T cells in the thymus.
To further define the role of flagellin-TLR5 agonistic interactions in the reduction of GvHD, we next generated B6 ^ TLR5 KO (KO) and KOB^6 radiation chimeras by transplanting 10 × 106 BM cells from wild-type (WT) B6 or TLR5 KO donors into the congenic CD45.1+ B6 or KO recipients conditioned with 11Gy (5.5Gyx2) TBI. The radiation chimeras were irradiated again with 9.0Gy (4.5Gy × 2) on 60 days after the first transplant and transplanted with 3 × 106 splenocytes and 5 × 106 TCD BM from H-2K congenic donors. Two 50mg doses of flagellin were administered 3 hrs before irradiation and 24 hrs after HSCT. All flagellin-treated B6 ^ B6 radiation chimeras survived with only 12% weight-loss by 80 days post transplant compared with 50% survival among recipients of flagellin-treated B6 ^ KO and 40% survival among KO ^ B6 radiation chimeras. All flagellin-treated KO^ KO and PBS-treated radiation chimeras died within 65 days post transplant. These data suggested that interaction of flagellin with the TLR5 expressing host gut epithelium and donor hematopoietic cells are both required for the maximum protective effect of this TLR5 agonist on GvHD in allogeneic HSCT recipients.
Together our data demonstrate that peritransplant administration of flagellin effectively controls acute and chronic GvHD while preserving enhanced post-transplant donor anti-opportunistic immunity. Since flagellin has been found to be safe for use in humans as vaccine adjuvant in a number of clinical trials, the clinical use of flagellin in the setting of allogeneic HSCT is of interest.

Disclosures:
No relevant conflicts of interest to declare.