New Horizons for Immune Gene Therapy

The synergy of gene therapy and immunotherapy has allowed great progress in the treatment of hematological malignancies. In recent years, T cells engineered to express chimeric antigen receptors (CARs) directed against CD19 have led to unprecedented rates of complete responses in B-cell acute lymphoblastic leukemia, chronic lymphoblastic leukemia, and diffuse large B-cell lymphoma. These remarkable results prompted the Food and Drug Administration approval of Tisagenlecleucel (marketed as Kymriah) and Axicabtagene ciloleucel (marketed as Yescarta), two anti-CD19 CAR-T therapies. However, many challenges remain. Treatment is often associated with cytokine release syndrome and sometimes further leads to neurotoxicity. In addition, many patients may still relapse with a CD19-negative blast or a CD19-positive blast following CAR-T exhaustion and depletion. Furthermore, T-cell responses represent only one of many potentially therapeutic responses, and the genetic manipulation of other immune cells is at a much earlier stage of development. Most importantly, the great potential of immune gene therapy is yet to be manifested in the treatment of solid tumors and autoimmune diseases or in organ transplantations. This special issue of Human Gene Therapy features selected publications trying to tackle the many remaining hurdles using diverse cutting-edge technologies.

CD19 targeting leads to transient and well tolerated B-cell depletion. It has proven more challenging to find antigens associated with solid tumors that could be similarly targeted with minimal on-target off-tumor toxicities. Epithelial cell adhesion molecule (EpCAM) has emerged as one potential such target, as it is overexpressed to varying degrees in most carcinomas and further serves as a biomarker for cancer stem cells and circulating cancer cells. In this issue, Zhang et al. (402–412) demonstrate the safety and efficacy of EpCAM-targeting CAR-T cells in the setting of human colon cancer xenotransplants. CD133 is also expressed on cancer stem cells originating from various epithelial tissues, and anti-CD133 CARs have shown safety in recent clinical trials. However, efficacy may be hindered by evasion mechanisms used by cancer cells, including immune checkpoint inhibition. Indeed, monoclonal antibodies against immune checkpoint proteins have revolutionized the treatment of melanoma and other malignancies. CAR-T cells that have had their programmed cell death protein 1 (PD-1) checkpoint receptor deleted were shown to have increased potency. Here, Hu et al. (446–458) demonstrate how PD-1 deletion and expression of anti-CD133 CARs can be achieved in T cells using plasmid delivery of CRISPR/Cas9 and the sleeping beauty transposon, leading to tumor growth inhibition in an orthotopic glioma model. Chicaybam et al. (511–522) develop a complimentary technology for the expansion and stimulation of transposon-engineered CAR-T cells with an Epstein–Barr virus–transformed lymphoblastoid cell line facilitating increased survival in a mouse model of leukemia.

An alternative approach to address the scarcity of tumor specific antigens is to induce antigen expression artificially in cancer cells. Wang et al. (471–484) design a novel method for cancer-specific expression of artificial neoantigens from a promoter that is responsive to nuclear factor kappa B, which is activated in most tumor types. Yingxi Xu et al. (497–510) in turn show how the overexpression of surface antigens can be induced to increase the efficiency of targeting by CAR-T cells. In particular, they use a histone deacetylase inhibitor to boost expression of CD20 on malignant B cells, leading to increased CAR-T cell cytotoxicity. Targeting CAR genes into hematopoietic stem cells (HSCs) may facilitate a long-lasting multilineage immune effect but may also be associated with an increased risk of sustained toxicities. Kao et al. (413–428) introduce a suicide system for HSC-derived CAR cells based on co-expression of a truncated epidermal growth factor receptor allowing ablation of engineered cells in engrafted mice following cetuximab treatment.

Natural killer (NK) cells bear the promise of becoming an “off the shelf” immunotherapy, as their effect is dependent on neither pre-stimulation nor human leukocyte antigen matching. In addition, compared to modified T cells, NK administration is associated with a reduced risk for graft versus host disease. In this issue, Kloess et al. (381–401) show that while CAR cells derived from the NK-92 cell line may have superior antileukemic efficacy compared to donor-derived NK cells, the former are also more prone to off-target toxicities. Chen Xu et al. (459–470) explore an alternative source for NK cells, which express membrane bound interleukin-21 and are derived from umbilical cord blood. Importantly, they show that these eUCB-NK cells can inhibit growth of HT29 colon adenocarcinoma xenografts. While NK and T cells may be engineered to perform effector functions, dendritic cells (DCs) can be modified and activated as an immunization strategy to express antigens to be presented in a major histocompatibility complex (MHC) context. Here, Tsitoura et al. (429–445) compare different virus-based vectors for their effect on splenic DCs in mice following systemic transduction. Importantly, they demonstrate that the intensity of a T cell–mediated immune response is strongly correlated with the induction level of DC maturation by the different vectors. An alternative immunization approach entails in vivo muscle electroporation (EP) with plasmid DNA (pDNA) coding the immunogen. Schommer et al. (523–533) demonstrate that local administration of chondroitinase ABC allows modification of the extracellular matrix structure and subsequent increase of gene distribution and expression upon intramuscular pDNA EP. Finally, Figueiredo et al. (485–496) develop a novel transplant genetic modification technology to reduce risk of organ rejection. In particular, they use ex vivo lentivector perfusion to deliver MHC-targeting short-hairpin RNAs in porcine lungs, achieving high silencing rates without reducing viability or tissue integrity.

The diverse papers in this issue thus exemplify the remarkable potential of immune gene therapy still waiting to be realized. If you share this sentiment of need and opportunity, we invite you to join the second international conference on lymphocyte engineering, ICLE 2019, set to be held in London on September 13–15, 2019. Together with prominent speakers including Nicolas Restifo, Stanley Riddel, Carl June, and Malcolm Brenner, we will address the still-standing challenges on the road to a profound and lasting impact on global health. On behalf of the scientific board, it is our pleasure to invite you to the ICLE 2019. See you in London!