Murine allogeneic CAR T cells integrated before or early after posttransplant cyclophosphamide exert antitumor effects.

Relapse limits the therapeutic efficacy both of chimeric antigen receptor (CAR) T cells and allogeneic hematopoietic cell transplantation (allo-HCT). Patients may undergo these therapies sequentially to prevent or treat relapsed malignancy. However, direct integration of the 2 therapies has been avoided over concerns for potential induction of graft-versus-host disease (GVHD) by allogeneic CAR T cells. We have shown in murine T-cell-replete MHC-haploidentical allo-HCT that suppressive mechanisms induced immediately after posttransplant cyclophosphamide (PTCy), given on days +3/+4, prevent GVHD induction by alloreactive T cells infused as early as day +5. Therefore, we hypothesized that allogeneic CAR T cells given in a similarly integrated manner in our murine MHC-haploidentical allo-HCT model may safely exert antitumor effects. Indeed, allogeneic anti-CD19 CAR T cells given early after (day +5) PTCy or even prior to (day 0) PTCy cleared leukemia without exacerbating the cytokine release syndrome occurring from the MHC-haploidentical allo-HCT or interfering with PTCy-mediated GVHD prevention. Meanwhile, CAR T-cell treatment on day +9 or day +14 was safe but less effective, suggesting a limited therapeutic window. CAR T cells infused before PTCy were not eliminated, but surviving CAR T cells continued to proliferate highly and expand despite PTCy. In comparison with infusion on day +5, CAR T-cell infusion on day 0 demonstrated superior clinical efficacy associated with earlier CAR T-cell expansion, higher phenotypic CAR T-cell activation, less CD4+CD25+Foxp3+ CAR T-cell recovery, and transcriptional changes suggesting increased activation of CD4+ CAR T cells and more cytotoxic CD8+ CAR T cells. This study provides mechanistic insight into PTCy's impact on graft-versus-tumor immunity and describes novel approaches to integrate CAR T cells and allo-HCT that may compensate for deficiencies of each individual approach.

Engineered regulatory T cells expressing myelin-specific chimeric antigen receptors suppress EAE progression.

The expansion of polyclonal T regulatory cells (Tregs) offers great promise for the treatment of immune-mediated diseases, such as multiple sclerosis (MS). However, polyclonal Tregs can be non-specifically immunosuppressive. Based on the advancements with chimeric antigen receptor (CAR) therapy in leukemia, we previously engineered Tregs to express a T-cell receptor (TCR) specific for a myelin basic protein (MBP) peptide. These TCR-engineered specific Tregs suppressed the proliferation of MBP-reactive T effector cells and ameliorated myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). Herein, we extend this approach by creating human regulatory T cells expressing functional single-chain chimeric antigen receptors (scFv CAR), targeting either MBP or MOG. These scFv CAR-transduced Tregs retained FoxP3 and Helios, characteristic of Treg cells, after long-term expansion in vitro. Importantly, these engineered CNS targeting CAR-Tregs were able to suppress autoimmune pathology in EAE, demonstrating that these Tregs have the potential to be used as a cellular therapy for MS patients.

Posttransplantation cyclophosphamide expands functional myeloid-derived suppressor cells and indirectly influences Tregs.

Posttransplantation cyclophosphamide (PTCy), given on days +3 and +4, reduces graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (HCT), but its immunologic underpinnings are not fully understood. In a T-cell-replete, major histocompatibility complex-haploidentical murine HCT model (B6C3F1→B6D2F1), we previously showed that PTCy rapidly induces suppressive mechanisms sufficient to prevent GVHD induction by non-PTCy-exposed donor splenocytes infused on day +5. Here, in PTCy-treated mice, we found that depleting Foxp3+ regulatory T cells (Tregs) in the initial graft but not the day +5 splenocytes did not worsen GVHD, yet depleting Tregs in both cellular compartments led to fatal GVHD induced by the day +5 splenocytes. Hence, Tregs were necessary to control GVHD induced by new donor cells, but PTCy's impact on Tregs appeared to be indirect. Therefore, we hypothesized that myeloid-derived suppressor cells (MDSCs) play a complementary role. Functionally suppressive granulocytic and monocytic MDSCs were increased in percentages in PTCy-treated mice, and MDSC percentages were increased after administering PTCy to patients undergoing HLA-haploidentical HCT. PTCy increased colony-stimulating factors critical for MDSC development and rapidly promoted the generation of MDSCs from bone marrow precursors. MDSC reduction via anti-Gr1 treatment in murine HCT did not worsen histopathologic GVHD but resulted in decreased Tregs and inferior survival. The clinical implications of these findings, including the potential impact of expanded MDSCs after PTCy on engraftment and cytokine release syndrome, remain to be elucidated. Moreover, the indirect effect that PTCy has on Tregs, which in turn play a necessary role in GVHD prevention by initially transplanted or subsequently infused T cells, requires further investigation.