Adoptive cell therapy of malignant diseases takes advantage of the cellular immune system to recognize and destroy cancer cells. This is impressively demonstrated by redirecting T cells with a chimeric antigen receptor (CAR) towards CD19, inducing complete and lasting remission of leukemia in more than two-thirds of patients in early phase trials. We outline how the CAR strategy is highly specific in redirecting T cells towards pre-defined target cells, however, reaches its limits when targeting solid tumors with a tremendous phenotypic heterogeneity. After initial tumor reduction by CAR T cells, antigen-negative cancer cells not recognized by CAR may give rise to tumor relapse. The situation may be overcome by CAR-mediated activation of T cells in the tumor, releasing inducible IL-12 which augments T-cell activation and attracts and activates innate immune cells to eliminate antigen-negative cancer cells in the targeted lesion. CAR T cells with a transgenic 'payload', so-called TRUCK T cells or the 'fourth-generation' CAR T cells, are worthwhile to explore to shape the tumor environment by the inducible release of transgenic immune modifiers. Such TRUCK T cells are moreover envisioned to be applied in fields beyond cancer therapy including the therapy of virus infections, auto-immune diseases or metabolic disorders.

Adoptive therapy with chimeric antigen receptor (CAR) redirected T cells recently showed remarkable anti-tumor efficacy in early phase clinical trials; self-repression of the immune response by T-c...

The overriding goal of adoptive cell therapy with chimeric antigen receptor (CAR) redirected T cells in oncology is to eliminate cancer cells from infiltrated tissues. Clinical trials document that this form of immunotherapy can induce lasting remissions of hematologic malignancies; however, the successes could not yet be transferred to the treatment of solid tumors. In this situation, modulating the immune regulation within the solid tumor tissue is thought to be key. In order to induce a pro‐inflammatory milieu CAR T cells were additionally engineered to release a transgenic cytokine upon CAR signaling in the targeted tumor tissue. Such TRUCKs (“T cells redirected for antigen‐unrestricted cytokine‐initiated killing”), also called “4^th generation” CAR T cells, combine the direct anti‐tumor attack of the CAR T cell with the immune modulating capacities of the delivered cytokine. Through CAR induced release, the cytokine is ideally deposited in the targeted tissue alleviating systemic side effects. The TRUCK concept is currently explored using a panel of cytokines, including IL‐7, IL‐12, IL‐15, IL‐18, IL‐23 and combinations thereof, and is entering early phase trials. Future developments will expand the application to a broader panel of released proteins converting CAR T cells to “living factories” of therapeutically active, locally deposited products with the potential to eliminate some clinical deficits of the currently used CAR T cells in the field of solid tumors.

Adoptive T cell therapy has recently shown powerful in initiating a lasting anti-tumor response with spectacular therapeutic success in some cases. Specific T cell therapy, however, is limited since a number of cancer cells are not recognized by T cells due to various mechanisms including the limited availability of tumor-specific T cells and deficiencies in antigen processing or major histocompatibility complex (MHC) expression of cancer cells. To make adoptive cell therapy applicable for the broad variety of cancer entities, patient's T cells are engineered ex vivo with pre-defined specificity by a recombinant chimeric antigen receptor (CAR) which consists in the extracellular part of an antibody-derived domain for binding with a tumor-associated antigen and in the intracellular part of a TCR-derived signaling moiety for T cell activation. The specificity of CAR mediated T cell recognition is defined by the antibody domain, is independent of MHC presentation and can be extended to any target for which an antibody is available. We discuss the advantages and limitations of MHC-independent T cell targeting by an engineered CAR and review most significant progress recently made in early stage clinical trials to treat cancer.

Adoptive T-cell transfer showed promising efficacy in recent trials raising interest in T cells with redirected specificity against tumors. T cells were engineered with a chimeric antigen receptor (CAR) with predefined binding and CD3ζ signaling to initiate T-cell activation. CD28 costimulation provided by a CD28-CD3ζ signaling CAR moreover improved T cell activation and persistence; however, it failed to meet the expectations with respect to mounting attacks against solid tumors infiltrated with regulatory T (Treg) cells. We revealed that a CD28 CAR–redirected T-cell attack is accompanied by higher numbers of Treg cells infiltrating the tumor and is less efficient against cancer cells in presence of Treg cells than a CD3ζ CAR T-cell attack. Deletion of the lck binding moiety in the CD28 CAR endodomain, however, improved redirected anti-tumor activity in presence of Treg cells without impairing interferon-γ (IFN-γ) secretion, proliferation, and cytolysis. CD28 modification abrogated interleukin-2 (IL-2) induction upon CAR engagement which in turn is no longer available to sustain Treg cell persistence. CARs with the modified CD28 endodomain thereby expedite the implementation of adoptive T-cell therapy in patients with a variety of cancer types that are heavily infiltrated by Treg cells.