José L. Cohen

Natural regulatory T cells control the development of atherosclerosis in mice

Atherosclerosis is an immunoinflammatory disease elicited by accumulation of lipids in the artery wall and leads to myocardial infarction and stroke. Here, we show that naturally arising CD4+CD25+ regulatory T cells, which actively maintain immunological tolerance to self and nonself antigens, are powerful inhibitors of atherosclerosis in several mouse models. These results provide new insights into the immunopathogenesis of atherosclerosis and could lead to new therapeutic approaches that involve immune modulation using regulatory T cells.

CD4+CD25+ Immunoregulatory T Cells New Therapeutics for Graft-Versus-Host Disease

CD4+CD25+ immunoregulatory T cells play a pivotal role in preventing organ-specific autoimmune diseases and in tolerance induction to allogeneic organ transplants. We investigated whether these cells could also control graft-versus-host disease (GVHD), the main complication after allogeneic hematopoietic stem cell transplantation (HSCT). Here, we show that the few CD4+CD25+ T cells naturally present in the transplant regulate GVHD because their removal from the graft dramatically accelerates this disease. Furthermore, the addition of freshly isolated CD4+CD25+ T cells at time of grafting significantly delays or even prevents GVHD. Ex vivo–expanded CD4+CD25+ regulatory T cells obtained after stimulation by allogeneic recipient-type antigen-presenting cells can also modulate GVHD. Thus, CD4+CD25+ regulatory T cells represent a new therapeutic tool for controlling GVHD in allogeneic HSCT. More generally, these results outline the tremendous potential of regulatory T cells as therapeutics.

Recipient-type specific CD4 + CD25 + regulatory T cells favor immune reconstitution and control graft-versus-host disease while maintaining graft-versus-leukemia

CD4+CD25+ regulatory T cells (Tregs) play a pivotal role in preventing organ-specific autoimmune diseases and in inducing tolerance to allogeneic organ transplants. We and others recently demonstrated that high numbers of Tregs can also modulate graft-versus-host disease (GVHD) if administered in conjunction with allogeneic hematopoietic stem cell transplantation in mice. In a clinical setting, it would be impossible to obtain enough freshly purified Tregs from a single donor to have a therapeutic effect. Thus, we performed regulatory T cell expansion ex vivo by stimulation with allogeneic APCs, which has the additional effect of producing alloantigen-specific regulatory T cells. Here we show that regulatory T cells specific for recipient-type alloantigens control GVHD while favoring immune reconstitution. Irrelevant regulatory T cells only mediate a partial protection from GVHD. Preferential survival of specific regulatory T cells, but not of irrelevant regulatory T cells, was observed in grafted animals. Additionally, the use of specific regulatory T cells was compatible with some form of graft-versus-tumor activity. These data suggest that recipient-type specific Tregs could be preferentially used in the control of GVHD in future clinical trials.

Bone Marrow Mesenchymal Stem Cells Suppress Lymphocyte Proliferation In Vitro but Fail to Prevent Graft-versus-Host Disease in Mice

Several reports have suggested that mesenchymal stem cells (MSCs) could exert a potent immunosuppressive effect in vitro, and thus may have a therapeutic potential for T cell-dependent pathologies. We aimed to establish whether MSCs could be used to control graft-vs-host disease (GVHD), a major cause of morbidity and mortality after allogeneic hemopoietic stem cell transplantation. From C57BL/6 and BALB/c mouse bone marrow cells, we purified and expanded MSCs characterized by the lack of expression of CD45 and CD11b molecules, their typical spindle-shaped morphology, together with their ability to differentiate into osteogenic, chondrogenic, and adipogenic cells. These MSCs suppressed alloantigen-induced T cell proliferation in vitro in a dose-dependent manner, independently of their MHC haplotype. However, when MSCs were added to a bone marrow transplant at a MSCs:T cells ratio that provided a strong inhibition of the allogeneic responses in vitro, they yielded no clinical benefit on the incidence or severity of GVHD. The absence of clinical effect was not due to MSC rejection because they still could be detected in grafted animals, but rather to an absence of suppressive effect on donor T cell division in vivo. Thus, in these murine models, experimental data do not support a significant immunosuppressive effect of MSCs in vivo for the treatment of GVHD.

Ex Vivo-Expanded CD4+CD25+ Immunoregulatory T Cells Prevent Graft-versus-Host-Disease by Inhibiting Activation/Differentiation of Pathogenic T Cells

CD4+CD25+ immunoregulatory T cells (Tregs) can be administered to inhibit graft-vs-host disease (GVHD) while preserving graft-vs-leukemia activity after allogeneic bone marrow transplantation in mice. Preclinical studies suggest that it is necessary to infuse as many Tregs as conventional donor T cells to achieve a clinical effect on GVHD. Thus, it would be necessary to expand Tregs ex vivo before transplantation. Two strategies have been proposed: expansion of Tregs stimulated by anti-CD3/CD28-coated microbeads for polyclonal activation or by host-type allogeneic APCs for selecting Tregs specific for host Ags. In this study, we describe the mechanisms by which ex vivo-expanded Tregs act on donor T cells to prevent GVHD in mice. We demonstrate that expanded Tregs strongly inhibited the division, expansion, and differentiation of donor T cells, with a more pronounced effect with Tregs specific for host Ags. These latter cells permit the efficient and durable control of GVHD and favor immune reconstitution.

CD4+CD25+ Regulatory T Cell Depletion Improves the Graft-Versus-Tumor Effect of Donor Lymphocytes After Allogeneic Hematopoietic Stem Cell Transplantation

Immunological effects of donor lymphocyte infusion for treatment of recurrent malignancy after allogeneic hematopoietic cell transplantation can be enhanced by depleting T regulatory cells in the infused cells and in the recipient. Regulatory T Cells Interfere with Graft-Versus-Tumor Effects Patients search for organ donors who are close genetic matches to avoid immune reactions. But sometimes a little immune activation is a good thing. Hematopoietic stem cells in the form of bone marrow are often used to treat blood cancers, and the donated cells not only engraft in bone to provide a source of healthy blood cells but also contain T cells that attack and destroy any remaining cancerous cells. This graft-versus-tumor effect is also harnessed when such patients suffer a relapse and donor T cells are infused into the patient. These infusions often fail to quell the malignancy, however. The reason for failure, Maury et al. have now shown, is that the donor cells can often include regulatory T cells (Tregs), a class of T cells that dampens the immune response. Removing these cells before the infusion markedly improved the graft-versus-tumor effect and the patients’ survival. The beneficial graft-versus-tumor effect of transplantation can be accompanied by the not-so-desirable graft-versus-host disease. Like the transplanted T cells that perceive cancer cells as foreign, T cells can also attack the host’s skin, liver, intestinal lining, and other internal organs—a condition that is serious but can be treated. The authors of this study used the presence of graft-versus-host disease as a sign that there were active, functioning T cells that also provided graft-versus-tumor effects. They treated 17 patients with relapsed blood cancer who had previously received an infusion of lymphocytes and had neither clinical manifestations of graft-versus-host disease nor control of their malignancy. After receiving a new infusion of lymphocytes from which the Tregs had been removed, two of the patients developed graft-versus-host disease for the first time in their transplant history. Hypothesizing that this low rate of response was a result of Treg cells present in the recipient, they treated four of the patients who needed more infusions with the same Treg-depleted cells but now infused them immediately after recipient Tregs were eliminated with lymphopdepletive chemotherapy. These four patients, all of whom had Hodgkin’s lymphoma, reacted to the infused cells by developing graft-versus-host disease, a sign that the infused cells were likely attacking the tumor cells as well. When the whole group was assessed 1 year after treatment, the patients who had experienced a graft-versus-host reaction after cell infusion were found to have survived longer, likely a result of successful immune control of the cancer cells by the infused Treg-depleted lymphocytes. This preliminary study shows that depleting donor lymphocytes of inhibitory regulatory T cells can be a safe and effective way to free active T cells from inhibition so that they can fight cancer cells in the recipient. Further studies are needed, but this seemingly inappropriate encouragement of immune reactions in transplant recipients may prove a boon to patients with blood cancers. Donor T cells play a pivotal role in the graft-versus-tumor effect after allogeneic hematopoietic stem cell transplantation. Regulatory T cells (Tregs) may reduce alloreactivity, the major component of the graft-versus-tumor effect. In the setting of donor lymphocyte infusion after hematopoietic stem cell transplantation, we postulated that Treg depletion could improve alloreactivity and likewise the graft-versus-tumor effect of donor T cells. The safety and efficacy of Treg-depleted donor lymphocyte infusion was studied in 17 adult patients with malignancy relapse after hematopoietic stem cell transplantation. All but one had previously failed to respond to at least one standard donor lymphocyte infusion, and none had experienced graft-versus-host disease. Two of the 17 patients developed graft-versus-host disease after their first Treg-depleted donor lymphocyte infusion and experienced a long-term remission of their malignancy. Four of the 15 patients who did not respond after a first Treg-depleted donor lymphocyte infusion received a second Treg-depleted donor lymphocyte infusion combined with lymphodepleting chemotherapy aimed to also eliminate recipient Tregs. All four developed acute-like graft-versus-host disease that was associated with a partial or complete and durable remission. In the whole cohort, graft-versus-host disease induction through Treg depletion was associated with improved survival. These results suggest that Treg-depleted donor lymphocyte infusion is a safe, feasible approach that induces graft-versus-host or graft-versus-tumor effects in alloreactivity-resistant patients. In patients not responding to this approach, the combination of chemotherapy-induced lymphodepletion of the recipient synergizes with the effect of Treg-depleted donor lymphocyte infusion. These findings offer a rational therapeutic approach for cancer cellular immunotherapy.

Tumor emergence is sensed by self-specific CD44hi memory Tregs that create a dominant tolerogenic environment for tumors in mice.

Early responses of Tregs and effector T cells (Teffs) to their first encounter with tumor cells have been poorly characterized. Here we have shown, in both implanted and in situ-induced mouse tumor models, that the appearance of tumor cells is immediately sensed by CD44hi memory Tregs that are specific for self antigens. The rapid response of these Tregs preceded and prevented activation of naive antitumor Teffs. The relative speed of the Treg versus the Teff response within the first 2-4 days determined the outcome of the antitumor immune response: tolerance or rejection. If antitumor memory Teffs were present at the time of tumor emergence, both Tregs and Teffs were recruited and activated with memory kinetics; however, the Tregs were unable to control the Teffs, which eradicated the tumor cells. This balance between effector and regulatory responses did not depend on the number of Tregs and Teffs, but rather on their memory status. Thus, in the natural setting, dominant tolerogenic immunosurveillance by self-specific memory Tregs protects tumors, just as it protects normal tissues. More generally, our results reveal that the timing of Treg and Teff engagement, determined by their memory status, is an important mode of regulation of immune responses.

Regulatory and Effector T Cell Activation Levels Are Prime Determinants of In Vivo Immune Regulation

Little is known about the in vivo conditions in which CD4+CD25+ regulatory T cells (Treg) exert their suppressive effect in nonlymphopenic mice. To this end, we analyzed Treg-mediated suppression of expansion and cytokine production at different levels of Ag-specific CD4+CD25− T cell activation. Using Ab-mediated depletion of endogenous Treg, we show that basal immunosuppression is dependent on effector T cell activation. These polyclonal Treg, which were poorly activated in our immunization conditions, were effective in weak but not high T cell activation context. In contrast, the same immunization conditions led to proliferation of cotransferred Ag-specific Treg. Those efficiently inhibited T cell proliferation and cytokine production even in strong T cell activation context. Interestingly, Treg selectively suppressed expansion or cytokine production depending on the experimental approach. The importance of the immune context for efficient suppression is further supported by the observation that Treg depletion exacerbated diabetes of NOD mice only at the early stage of the disease. Overall, our study suggests that Treg-mediated suppression depends on the relative activation of Treg and effector T cells in vivo. This balance may be a critical factor in the regulation of immune responses.

Therapeutic potential of self‐antigen‐specific CD4+CD25+ regulatory T cells selected in vitro from a polyclonal repertoire

CD4+CD25+ regulatory T cells (Treg) play a major role in the prevention of autoimmune diseases. Converging evidence indicates that Treg specific for self‐antigens expressed by target tissues have a greater therapeutic potential than polyclonal Treg. Therefore, the selective expansion of rare self‐antigen‐specific Treg naturally present in a polyclonal repertoire of Treg is of major importance. In this work, we investigated the potential of different dendritic cell (DC) subsets to expand antigen‐specific Treg in mice. The in vitro selective expansion of rare islet‐specific Treg from polyclonal Treg could only be achieved efficiently by stimulation with CD8+ splenic DC presenting islet antigens. These islet‐specific Treg exerted potent bystander suppression on diabetogenic T cells and prevented type 1 diabetes. This approach opens new perspectives for cell therapy of autoimmune diseases.

Immune reconstitution is preserved in hematopoietic stem cell transplantation coadministered with regulatory T cells for GVHD prevention

Recipient-specific regulatory T cells (rsTreg) can prevent graft-versus-host disease (GVHD) by inhibiting donor T-cell expansion after hematopoietic stem cell transplantation (HSCT) in mice. Importantly, in adult humans, because of thymus involution, immune reconstitution during the first months after HSCT relies on the peripheral expansion of donor T cells initially present in the graft. Therefore, we developed a mouse model of HSCT that excludes thymic output to study the effect of rsTreg on immune reconstitution derived from postthymic mature T cells present within the graft. We showed that GVHD prevention with rsTreg was associated with improvement of the limited immune reconstitution compared with GVHD mice in terms of cell numbers, activation phenotype, and cytokine production. We further demonstrated a preserved in vivo immune function using vaccinia infection and third-party skin-graft rejection models, suggesting that rsTreg immunosuppression was relatively specific of GVHD. Finally, we showed that rsTreg extensively proliferated during the first 2 weeks and then declined. In turn, donor Treg proliferated from day 15 on. Taken together, these results suggest that rsTreg GVHD prevention is associated with improved early immune reconstitution in a model that more closely approximates the biology of allogeneic HSCT in human adults.