Human Regulatory T Cells Do Not Suppress the Antitumor Immunity in the Bone Marrow: A Role for Bone Marrow Stromal Cells in Neutralizing Regulatory T Cells

Abstract

Purpose: Regulatory T cells (Tregs) are potent tools to prevent graft-versus-host disease (GVHD) induced after allogeneic stem cell transplantation or donor lymphocyte infusions. Toward clinical application of Tregs for GVHD treatment, we investigated the impact of Tregs on the therapeutic graft-versus-tumor (GVT) effect against human multiple myeloma tumors with various immunogenicities, progression rates, and localizations in a humanized murine model.Experimental Design: Immunodeficient Rag2 À/À gc À/À mice, bearing various human multiple myeloma tumors, were treated with human peripheral blood mononuclear cell (PBMC) alone or together with autologous ex vivo cultured Tregs. Mice were analyzed for the in vivo engraftment, homing of T-cell subsets, development of GVHD and GVT. In additional in vitro assays, Tregs that were cultured together with bone marrow stromal cells were analyzed for phenotype and functions.Results: Treatment with PBMC alone induced variable degrees of antitumor response, depending on the immunogenicity and the growth rate of the tumor. Coinfusion of Tregs did not impair the antitumor response against tumors residing within the bone marrow, irrespective of their immunogenicity or growth rates. In contrast, Tregs readily inhibited the antitumor effect against tumors growing outside the bone marrow. Exploring this remarkable phenomenon, we discovered that bone marrow stroma neutralizes the suppressive activity of Tregs in part via production of interleukin (IL)-1b/IL-6. We furthermore found in vitro and in vivo evidence of conversion of Tregs into IL-17-producing T cells in the bone marrow environment.Conclusions: These results provide new insights into the Treg immunobiology and indicate the conditional benefits of future Treg-based therapies.

Risk model incorporating donor IL6 and IFNG genotype and gastrointestinal GVHD can discriminate patients at high risk of steroid refractory acute GVHD

“…34,35 In addition, G allele in a IL6 SNP has been shown to correlate with higher IL6 levels, 36 and is associated with increased risk of grade II-IV aGVHD. 37 IL6 can neutralize suppressive activity of regulatory T cells (T-regs), 38 hence enhancing GVHD. Blockade of IL6 signaling has been shown to increase absolute number of T-regs by augmentation of T-reg production 39 thereby decreasing the severity of GVHD.…”

Cellular immunotherapy in multiple myeloma: Lessons from preclinical models

“…Adoptive transfer of Tregs from 5T2MM-bearing immunocompetent C57/KalwRij mice into syngeneic mice led, in contrast to Treg infusions in the previously described xenogeneic transplantation model [85], to increased progression of MM disease in recipient mice [87]. These results suggest that cultured Tregs in xenogeneic murine transplantation models are susceptible to bone marrow stroma-mediated neutralization of their suppressive activity, while Tregs isolated from myelomabearing syngeneic mice may be less susceptible to this neutralization [117] or present a more suppressive phenotype.…”

“…Highly suppressive Tregs expand and accumulate in the bone marrow after allo-SCT [62], and these cells might be able to convert into myeloma-promoting Th17 cells [25,85]. Murine studies suggest that combining allo-SCT with Id-or DC vaccination provides a possible solution to these issues [81,82].…”

“…In order to study the effect of Treg infusions on the GvM and GvHD effect, RAG2 −/− γc −/− immunodeficient mice bearing various human MM tumors were infused with human PBMCs alone or together with cultured autologous Tregs [85]. Human PBMC infusions alone induced variable GvM responses, depending on the degree of immunogenicity and growth rate of the myeloma cells.…”

“…In view of these results, the authors showed that the bone marrow stroma is able to neutralize the suppressive activity of Tregs through the production of IL-1β and IL-6. In addition, Tregs could convert into Th17 cells in the bone marrow microenvironment of the recipient mice [85].…”