Flexible Antigen-Specific Redirection of Human Regulatory T Cells Via a Novel Universal Chimeric Antigen Receptor System

Based on compelling evidence from a vast number of in vitro and in vivostudies, Tregs have become an attractive cell population to treat or even prevent auto- and alloimmunity including Graft-versus-Host disease (GvHD). However, several safety concerns still exist as for example the risk of global immunosuppression using polyclonal Tregs. In fact, experiments in mice showed that adoptive transfer or induction of antigen-specific Tregs is more potent regarding suppression of pathogenic immune responses when compared to polyclonal Treg populations. Unfortunately, the isolation and expansion of naturally occurring antigen-specific Tregs is technically difficult, labour-intensive, and time-consuming. An attractive way to overcome these limitations and to endow polyclonal Treg populations with a desired antigen-specificity is their engraftment with chimeric antigen receptors (CARs). In this context, CAR-modification represents a promising approach to redirect polyclonal Tregs in an antigen-specific manner to suppress ongoing self-destructive immune responses at the site of inflammation.

Nevertheless, until now redirection of CAR-engineered T cells is limited to a single target antigen, restricting this approach to an unflexible monospecific therapy. Therefore, we developed a more flexible universal CAR (UCAR) platform that allows redirection of T cells to an in principal unrestricted number of surface antigens. T cells are engrafted with UCARs that bind to a small peptide epitope derived from a human nuclear protein. Cross-linkage to target cells is mediated by independent target modules that provide antigen-specificity and comprise the peptide epitope recognized by the UCAR. In order to target different tissue antigens, the target modules can easily be exchanged. Thereby, once established, the treatment strategy can easily be applied to various auto- and alloimmune diseases.

At present, the CD45RA⁺ population is the Treg subset of choice for a clinical application as these cells have the highest capacity to maintain phenotypic and functional Treg properties upon prolonged ex vivo expansion. Here we show that highly pure, sorted CD4⁺CD25⁺CD127^lowCD45RA⁺ Tregs can be genetically manipulated using lentiviral gene transfer, resulting in approximately 70 % of UCAR-expressing Treg cells. The transduction procedure itself did not affect the phenotype of UCAR-engineered Tregs as it was similar to non-transduced wildtype cells. Both Treg populations presevered FOXP3 expression even after prolonged in vitro cultivation (> 95 % FOXP3⁺). Upon incubation with antigen-positive target cells and a respective target module UCAR-engineered Tregs upregulate the activation markers CD69 and LAP demonstrating that the cells can be restimulated antigen-specifically. Most importantly, UCAR-engrafted Tregs were functionally activated upon antigen encounter, demonstrated by suppression of proliferation and expansion of cocultured autologous T effector cells.

Taken together, our results pave the way towards an application of UCAR technology for a site-specific recruitment of CAR-modified Tregs into inflamed tissues aiming at re-establishing immune homeostasis. Due to its high flexibility UCAR-engrafted Tregs can easily and universally be used for treatment of various autoimmune diseases or GvHD just by exchanging the tissue-specific target modules.