MSCs-ILC2s-Tecs Axis Promotes the Recovery of Thymic Function in aGVHD

Background: Previous studies found that mesenchymal stem cells (MSCs) administrated at the acute graft versus host disease (aGVHD) phase effectively decreased the incidence and severity of chronic GVHD (cGVHD) by boosting thymic regeneration. However, the mechanism for MSCs repairing the damaged thymus caused by aGVHD is unclear. It has been reported that innate lymphoid cells (ILCs) are also important for the structural remodeling of the thymus in recipients after allogeneic bone marrow transplantation (allo-BMT). Here, we explored the mechanisms on how MSCs promote thymus recovery post allo-BMT through ILCs.

Methods: We carried out studies in a aGVHD mice model of fully MHC-mismatched myeloablative bone marrow transplantation (C57BL/6 to BALB/c). MSCs or ILC2s were generated and administrated intravenously at day 7 post-BMT to the treated groups to compare therapeutic effects of GVHD with the untreated group. Clinical scores and survival were recorded once every days. Flow cytometery was employed for analyzing the number and proportion of ILCs and their subtypes (ILCs Lin-CD45+iCD3-CD127+, type 1 ILCs (ILC1s) Tbet+, type 2 ILCs (ILC2s) GATA3+, type 3 ILCs (ILC3s) RORγt+) in the thymus to assess the effect of MSCs on ILCs in the thymus at day 14 post-BMT. We analyzed the thymic T cells subsets and thymic epithelial cells (TECs) substes among these two groups to evaluate the repair effect of ILC2s for thymus. Thymic ILC2s were cultured alone or co-cultured with murine MSCs in vitro to investigate the effect and underlying mechanism of MSCs on ILC2s.

Results: We found that ILC2s in the thymus are sensitive to conditioning therapy and and further impaired by aGVHD. But infusion of MSCs was effective in reducing the lethality of aGVHD and promote expansion of thymic ILC2s in aGVHD mice compared with the control group. The absolute number of ILC2s in the thymus had a significant negative correlation with GVHD scores in aGVHD mice. Then we found that ILC2s infusion significant increases in thymus size and weight, as well as the number of total thymocytes and the more organized cortical medullary structure compared to the untreated groups. The number of TECs and its proportion of thymus stroma were significantly improved, including cortical TEC and medullary TECs. As for thymocyte, ILC2s infusion significantly increased the number and proportion of CD4+CD8+T cells. In vitro study showed co-cultured ILC2s had a increased proliferation compared to ILC2s cultured alone, and anti-IL33 antibody inhibited the pro-proliferative effect of MSCs on ILC2. Then we perform in vivo validation by injecting IL33 knocked down MSCs and found that the effect of MSC in repairing the damaged thymus was reduced. And the number and proportion of thymic ILC2s were also significantly decreased in the thymus. Furthermore, treatment with rIL33 on day 7-11 also ameliorated aGVHD and promoted the proliferation of thymic ILC2s in aGVHD mice as well as MSCs infusion.

Conclusion: These findings suggest that MSCs participate in thymus regeneration from aGVHD damage through ILC2s-mediated tissue recovery. The study outlined here provide a new insight into the mechanism of MSC therapy for aGVHD.

No relevant conflicts of interest to declare.