Abstract IA22: Advanced T cell engineering for cancer immunotherapy.
Authors: Jensen Michael 
Published: August 2014

Targeted eradication of cancer by adoptive transfer of antigen specific T cells is the subject of intensive clinical exploration. While many of the key conceptual and technical barriers that have previously limited translation to human clinical trials have been overcome, as reflected by the growing number of FDA IND-supported studies in progress, significant refinement is needed to evolve this therapeutic modality for reproducibly effective and safe applications in the oncology clinic. The approach of genetically engineering T cells for cancer adoptive therapy is now at the forefront of translational research activities in this field. Our group, as well as others, has advanced technologies to target tumor cells using T cells modified to express chimeric antigen receptors (CARs). These engineering strategies, when applied to the genetic modification of central memory T cells (TCM) endowed with the capacity to engraft and repopulate functional memory T niches, is the basis of an unprecedented opportunity to advance cancer immunotherapy applications beyond CML and melanoma. Accordingly, we have combined our development of third generation self-inactivating (SIN) lentiviral vectors that direct the expression of tumor targeting CARs with cGMP compliant methodologies to isolate, transduce, and expand TCM-derived effector cells (TE(CM)) for the purpose of conducting clinical trials, initially for patients with CD19+ B cell malignancies (leukemias and lymphomas).

Recent CD19-specific CAR T cell trials involving patients with CLL and ALL have revealed unprecedented anti-tumor potency, however, tumor regressions have been accompanied by significant life-threatening toxicities and subsequent ongoing B cell aplasia. The anti-tumor potency of current iterations of second generation CD19-specific CAR T cells therefore needs to be matched with strategies to 1.) regulate the functional outputs mediated by CAR signaling, and, 2.) effect the timely ablation of CAR+ T cells to allow for B cell reconstitution. To date, CAR T cell clinical trials have involved vector systems that utilize constitutive promoters. The field clearly needs regulated CAR expression systems capable of responding in real time to clinician-prescribed small molecule inputs to better manage the complex interplay of clinical parameters related to the tempo/extent of tumor regression and side effects, including on-target collateral ablation of healthy cells, for example non-neoplastic B cells, and toxicities resulting from T cell cytokine production (cytokine storm) and tissue inflammation. The Jensen laboratory is focused on developing clinically useful small drug regulated transgene transcriptional regulatory systems and/or drug regulated riboswitches that modulate the stability of transgene mRNA to affect changes in transgene expression levels. The current status of our development of these technologies will be presented.

The development of T cell ablation mechanisms that are both highly effective and triggered by commercially available FDA-licensed drugs will be increasingly important as long term engraftment becomes the normative outcome following adoptive transfer of CAR redirected T cells. Ablation of CAR expressing T cells is a significant issue given the observed long-term persistence of CD19-specific T cells, their ongoing stimulation by nascent B cell precursors from bone marrow precursors, and their capacity to mediate ongoing B cell aplasia. Routine clinical use of suicide technologies has been hampered by the immunogenicity of xenogeneic suicide constructs such as HSV-TK, and the lack of commercially available pharmaceutical grade triggering drugs such as those that trigger dimerization based caspase suicide transgenes. Our group has therefore endeavored to design alternate suicide strategies based on the engineering of human EGFR to derive an inert truncated type I transmembrane-extracellular segment of human EGFR, designated EGFRt, that retains the binding epitope of cetuximab. Our studies support the utility of EGFRt when co-expressed with CAR's as a selection, tacking, and suicide construct. Current IND supported clinical trials are employing co-expressed EGFRt and will examine the suicide utility of this construct in patients.

Citation Format: Michael Jensen. Advanced T cell engineering for cancer immunotherapy. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr IA22.