Paradoxical Immunological Activity of Flagellin: A Novel Method to Reduce GvHD In Allogeneic HSCT Recipients.

Graft-vs-host disease (GvHD) is a major complication in allogeneic Hematopoietic Stem Cell Transplant (HSCT) recipients. Using flagellin, a bacterial protein that agonistically binds with TLR5, we previously reported that two doses of flagellin (before irradiation and after allogeneic HSCT with 5×10⁶ splenocytes) in H-2^K à H-2^b model significantly reduced GvHD and had 67% long-term survival by 132 days post-transplant whereas control recipients had severe acute GvHD and 100% early mortality within 15 days post transplant. Here, we report the mechanism by which flagellin reduces GvHD using the same H-2^K à H-2^b HSCT murine model.

3×10⁶ splenocytes and 5 ×10⁶ T cell depleted bone marrow (BM) cells harvested from the congeneic H-2^K donors were transplanted into lethally irradiated (11Gy) C57BL/6 (H-2^b) recipients. 50-μg HPLC purified LPS-free flagellin diluted in ice-cold PBS were administered i.p 3 hours before irradiation and 24 hours after transplant. Control recipients were treated with PBS. Recipients (6/group) were sacrificed on day 4 and 10 after post transplant. Serum was collected to determine cytokines by ELISA and splenocytes were analyzed by FACS to determine immune cells phenotypes.

Although both Fla and PBS-treated recipients showed identical weight-loss within day 10 post transplant, surprisingly significantly lower numbers of cells/spleen were determined in the spleens of Fla recipients compared to control recipients [Fla 0.9 ± 0.1 (x10⁶) vs PBS 1.7 ± 0.4(x10⁶), p=0.002] on day 4 post transplant but not on day 10 [Fla 186.1 ± 35.1 (x10⁶) vs PBS 151.2 ± 40.5(x10⁶), p=0.43] post-transplant. We investigated the paradoxical immune response of flagellin on donor T cells on day 4-post transplant. First, we determined the numbers of donor spleen-derived Thy1.2+ T cells per spleen. The numbers of donor spleen-derived T cells per spleen were significantly lower in Fla recipients compared to PBS-treated recipients [Fla 0.005 ± 0.002 (x10³) vs PBS 0.04 ± 0.03(x10³), p=0.02]. Accordingly, donor spleen-derived both CD4 and CD8 T cells per spleen of Fla recipients were also found significantly lower compared to PBS-treated recipients (CD4, p=0.04; CD8, p= 0.003). The CD62L, a naïve and also markers for allo-reactive T cells that cause GvHD were found significantly lower in both CD4 and CD8 T cells (CD4, p= 0.03; CD8, p=0.003) and the inducible co-stimulatory molecule 1 (ICOS-1), another prominent T cells activation marker were also found significantly lower (CD4, p= 0.04; CD8, p=0.007) in Fla recipients compared to PBS-treated recipients. These lower immune phenotypes of donor T cells in Fla recipients may reduce the initiation of GvHD at the early time points of transplant. However, flagellin-induced reduction of donor T cells activity were not suppressed considerably as the numbers of donor spleen-derived CD4 T cells expressing activation markers such as CD25 {Fla 0.5 ± 0.2 (x10³) vs PBS 4.5 ± 0.5 (x10³), p=0.07} and CD69 {Fla 0.5 ± 0.2 (x10³) vs PBS 6.5 ± 5.0 (x10³), p=0.08} per spleen were not found significantly different. On the other hand, the numbers of CD8 T cells those expressed CD25 {Fla 0.2 ± 0.04 (x10³) vs PBS 1.3 ± 0.7 (x10³), p=0.01} and CD69 {Fla 0.4 ± 0.1 (x10³) vs PBS 2.4 ± 1.7 (x10³), p=0.03} per spleen were found significantly lower in Fla recipients compared to PBS-treated recipients. Although over 90% of donor spleen CD4 T cells of both Fla and PBS-treated recipients expressed PD-1, 24% and 32% respectively, expressed IFN-γ after vitro PMA stimulation. Similar results were found in case of CD8 T cells. The over expression of PD-1 in HSCT recipients, thus did not make the donor T cells exhausted as they expanded over 100 times within day 10 post transplant with the expression of PD-1. Although similar level of serum IFN-γ was determined between the Fla and PBS-treated recipients on day 10 post-transplant, IFN-γ level was found below detection limit on day 4 post-transplant. The serum levels of IL-1β and IL-10 were undetectable on both days 4 and 10 post-transplant. Serum LPS were found identical in both Fla and PBS-treated groups determined on day 4 and 10 days post transplant.

Flagellin protected allogeneic HSCT recipients from lethal GvHD by reducing donor CD62L+ and ICOS-1+ T cells expansion and activation within 4 days post-transplant without compromising their normal immune responses.

No relevant conflicts of interest to declare.