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.

The therapeutic success of adoptive therapy with chimeric antigen receptor (CAR) engineered T cells depends on the appropriate costimulation of CD3ζ to induce full T cell activation. Costimulatory endodomains of the CD28 family are therefore fused with CD3ζ in a dual signalling CAR. Serious adverse events in two most recent trials; however, highlight the need to analyse in more detail the impact of each costimulatory endodomain on individual effector functions of redirected T cells. We therefore performed a thoroughly controlled side‐by‐side comparison of the most frequently used endodomains with respect to their impact on CD4⁺ and CD8⁺ T cell effector functions. CD28 reinforced T cell proliferation and is mandatory to induce IL‐2. In the absence of added IL‐2, CD28 and OX40 (CD137) but not 4‐1BB (CD134) enhanced specific cytolysis. While CD28, 4‐1BB and OX40 similarly improved pro‐inflammatory cytokine secretion, OX40 most efficiently prevented activation induced cell death of CD62L⁻ effector memory T cells. CD28 was superior to initiate the T cell response, OX40 and 4‐1BB sustained the response in long term with OX40 being most effective. We consequently combined the beneficial functions in a 3rd generation CD28‐OX40 CAR which substantially improved the antitumor response without loosing specificity.

The treatment of leukemia/lymphoma by chimeric antigen receptor (CAR) redirected T cells with specificity for CD19 induced complete remissions in the majority of patients with a realistic hope for cure. However, recent follow-up data revealed a substantial risk of relapse through leukemic cells which lack the CAR targeted antigen. In this situation a bispecific CAR with binding domains for CD19 and CD20 is aimed at recognizing also leukemic cells with only one cognate antigen. The anti-CD20-CD19 bispecific CAR induced a full T cell response upon engagement of CD19 or CD20 on target cells showing a true “OR” gate recognition in redirecting T cell activation. T cells with the anti-CD20-CD19 CAR efficiently killed patient’s chronic lymphocytic leukemia (CLL) cells in vitro. The bispecific CAR T cells cleared pediatric acute lymphocytic leukemia (ALL) with a mixed CD19+CD20+/CD20- phenotype from blood and bone marrow of transplanted mice while anti-CD20 CAR T cells left CD20- leukemic cells behind without cure from disease. Data indicate the superior anti-leukemic activity in the control of leukemia implying that the anti-CD20-CD19 bispecific CAR T cells may reduce the risk of relapse through antigen-loss leukemic cells in the long-term.

Chimeric antigen receptor (CAR) redirected T cells are efficacious in the treatment of leukemia/lymphoma, however, showed less capacities in eliminating solid tumors which is thought to be partly due to the lack of cytokine support in the tumor lesion. In order to deliver supportive cytokines, we took advantage of the inherent ability of mesenchymal stem cells (MSCs) to actively migrate to tumor sites and engineered MSCs to release both IL7 and IL12 to promote homeostatic expansion and Th1 polarization. There is a mutual interaction between engineered MSCs and CAR T cells; in presence of CAR T cell released IFN-γ and TNF-α, chronic inflammatory Th2 MSCs shifted towards a Th17/Th1 pattern with IL2 and IL15 release that mutually activated CAR T cells with extended persistence, amplification, killing and protection from activation induced cell death. MSCs releasing IL7 and IL12 were superior over non-modified MSCs in supporting the CAR T cell response and improved the anti-tumor attack in a transplant tumor model. Data demonstrate the first use of genetically modified MSCs as vehicles to deliver immuno-modulatory proteins to the tumor tissue in order to improve the efficacy of CAR T cells in the treatment of solid malignancies.