Lymph Node Fibroblastic Reticular Cells Attenuate Immune Responses Through Induction of Tolerogenic Macrophages at Early Stage of Transplantation

Abstract

Background. Fibroblastic reticular cells (FRCs) are a type of stromal cells located in the T zone in secondary lymphoid organs. Previous studies showed that FRCs possess the potential to promote myeloid differentiation. We aim to investigate whether FRCs in lymph nodes (LNs) could induce tolerogenic macrophage generation and further influence T-cell immunity at an early stage of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Methods. LNs were assayed to confirm the existence of proliferating macrophages after allo-HSCT. Ex vivo—expanded FRCs and bone marrow cells were cocultured to verify the generation of macrophages. Real-time quantitative PCR and ELISA assays were performed to observe the cytokines expressed by FRC. Transcriptome sequencing was performed to compare the difference between FRC-induced macrophages (FMs) and conventional macrophages. Mixed lymphocyte reaction and the utilization of FMs in acute graft-versus-host disease (aGVHD) mice were used to test the inhibitory function of FMs in T-cell immunity in vitro and in vivo. Results. We found a large number of proliferating macrophages near FRCs in LNs with tolerogenic phenotype under allo-HSCT conditions. Neutralizing anti–macrophage colony-stimulating factor receptor antibody abolished FMs generation in vitro. Phenotypic analysis and transcriptome sequencing suggested FMs possessed immunoinhibitory function. Mixed lymphocyte reaction proved that FMs could inhibit T-cell activation and differentiation toward Th1/Tc1 cells. Injection of FMs in aGVHD mice effectively attenuated aGVHD severity and mortality. Conclusions. This study has revealed a novel mechanism of immune regulation through the generation of FRC-induced tolerogenic macrophages in LNs at an early stage of allo-HSCT.

The Potential Clinical Application of Induced Tolerogenic Macrophages

“…These findings are consistent with previous studies conducted to define ex vivo-generated regulatory macrophages. 6 Liu et al 5 This study has revealed a novel mechanism underpinning the role of the microenvironment in secondary lymphoid organs, specifically highlighting the interactions between FRCs and macrophages for immune tolerogenic regulation. Various aspects of lymph node FRCs and their role in inducing regulatory macrophages to treat or prevent GvHD remains to be fully elucidated.…”

“…The current study by Lui et al 5 demonstrated that FRC macrophages induced ex vivo from BMC possesses the potential for immunosuppressive capacity and M-CSF was the critical cytokine for FRC to induce macrophages. 5 M-CSF was found to be highly expressed in FRC, but further study needs to define the accurate source of M-CSF at local stromal environment in lymph nodes including endothelial cells, follicular dendritic cells, marginal reticular cells, and fibroblastic FRCs. The ex vivo generation of tolerogenic macrophages may help ensure sufficient cells for the use of this therapy in the clinic.…”

“…Tolerogenic macrophages actively participate in T-cell immune regulation. 4 The publication by Liu et al 5 in this issue of Transplantation sought to define the relationship between FRCs and lymph node macrophages for their tolerogenic roles using in vitro culture system and in vivo mouse models. Their data provide evidence suggesting that lymph nodes FRC-induce regulatory macrophages at an early stage post-allo-HSCT.…”

“…10 The feasibility of translating FRC-induced macrophages to a clinical cell therapy product for allogeneic solid organ transplantation has been underpinned by several clinical pilot studies. 8 Taken together, the study by Liu et al 5 provides important preclinical insight into the role FRCs play in promoting regulatory macrophages and their role in alleviating the risk of GvHD in allogeneic HSC transplantation.…”

“…These findings are consistent with previous studies conducted to define ex vivo–generated regulatory macrophages. 6 Liu et al 5 further elucidated the role of macrophage colony-stimulating factor (M-CSF) as a critical cytokine for the induction of regulatory macrophages. FRC-induced macrophages inhibited T-cell activation and differentiation toward Th1/Tc1 cells ex vivo.…”