Revisiting Interleukin-12 as a Cancer Immunotherapy Agent

In this issue of Clinical Cancer Research, Hu and colleagues report on the plasmid gene transfer of an IL12 gene expression cassette to transplantable mouse tumors and to xenografted human tumors in NSG mice (1). IL12 local expression is attained by injecting plasmid DNA and performing in vivo electroporation inserting a needle-shaped electrode (1). This strategy is currently under investigation in clinical trials as a single agent (NCT01579318, NCT00323206, NCT01502293, and NCT02345330) and in combination with pembrolizumab (NCT02493361 and NCT03132675). In these mice, IL12 is combined with doxorubicin, a chemotherapy agent with multiple effects on the tumor tissue microenvironment that can help the antitumor immune response, such as immunogenic cell death of a fraction of tumor cells (2) and reduction of regulatory T cells and myeloid-derived suppressor cells (3, 4). Indeed, under treatment, tumors accumulated infiltrating cytotoxic T cells (CD8⁺ NKG2D⁺). If exogenous T cells recognizing tumor antigens are infused, these adoptively transferred cells extravasate and infiltrate the tumor much more efficiently.

Adoptive T-cell therapy is revolutionizing cancer therapy mainly for hematologic malignancies. Chimeric antigen receptor–transduced T cells, tumor-infiltrating lymphocytes (TIL), and T-cell receptor–transduced cells are in the limelight of clinical development. Early observations suggested that IL12 local gene therapy can be synergistically combined with adoptive T-cell transfer (5). Moreover, attempts to engineer T cells to produce IL12 in a controlled fashion have been shown to be remarkably efficacious in preclinical models, although toxic due to leaky expression of the retroviral construct (6). Therefore, safer modes of conferring T cells the ability to produce IL12 in an autocrine fashion are needed.

A remarkable function of IL12 is its ability to induce IFNγ release from natural killer (NK) cells as well as CD4⁺ and CD8⁺ T cells. In fact, IL12 signaling via STAT-4 is critical for Th1 differentiation and acquisition of cytolytic functions by CD8⁺ T cells (7). IFNγ in turn strongly modifies the tumor microenvironment. The best studied beneficial mechanisms (Fig. 1) are as follows:

• i. Enhancing MHC I antigen presentation in tumor cells.

• ii. Inducing the expression of CXCL9, 10, and 11 chemokines to attract NK, Th1, and CD8⁺ T cells.

• iii. Transforming M2 macrophages into activated antitumor M1 macrophages.

• iv. Acting on endothelial cells to mediate antiangiogenesis in a CXCR3-dependent fashion while enhancing the expression of homing receptors for T-cell recruitment.

Unfortunately, IFNγ is also the main mediator of the toxic effects of IL12 and over time, turns on immunoregulatory mechanisms, such as PD-L1 and IDO-1 expression, which mediate adaptive resistance to immunotherapy.

In the era of checkpoint inhibitors and adoptive T-cell therapy, we must revisit IL12 as an antitumor agent. On the one hand, IL12 can be synergistic with PD-1/PD-L1 blockade (8), and on the other, it might help adoptive T-cell therapy in several ways, chiefly by including attraction and homing to the tumor tissue, as reported by Hu and colleagues in mouse models (1). The anticipated importance of these mechanisms is that IL12 might be a key tool to translate the efficacy of adoptive T-cell therapy to a wider spectrum of tumors aside from B-cell malignancies, melanoma, and synovial sarcoma.

IL12 is neglected to some extent in clinical development with some exceptions. Merck Serono is testing a fusion protein encompassing single-chain IL12 coupled to an antibody that binds extracellular dsDNA (9). There are also attempts by Moderna Therapeutics/MedImmune to transfer mRNA encoding IL12 as well as the intratumoral gene-electroporation strategies by OncoSec Medical (NCT03132675) already commented on.

It must be said that IL12 is a powerful wild horse difficult to harness (10). Systemic treatment has a narrow therapeutic window due to circulating IFNγ levels that can be fatal. Permanent retroviral gene transfer of IL12 into T cells has serious safety problems (11). The route to clinical success of IL12-based immunotherapy must contemplate three key concepts:

• i. Transient exposure or expression.

• ii. Combination with other agents, chiefly including adoptive T-cell therapy and checkpoint inhibitors.

• iii. If possible, targeting the cytokine or its function or its expression to the tumor microenvironment.

The potential of IL12 as a partner in combination immunotherapy strategies is promising and in need of improvement based on biotechnology, gene therapy, and cell therapy. The promotion of T-cell infiltration into tumors by IL12 is certainly an exciting feature. IL12 is “back to the future.”

I. Melero reports receiving commercial research grants from Alligator, Bristol-Myers Squibb, and Roche, speakers bureau honoraria from MSD, and is a consultant/advisory board member for Bayer, Bioncotech, Bristol-Myers Squibb, F/Star, Medimmune, Merck Serono, and Tusk. No potential conflicts of interest were disclosed by the other authors.

Conception and design: P. Berraondo, I. Melero

Writing, review, and/or revision of the manuscript: P. Berraondo, I. Etxeberria, M. Ponz-Sarvise, I. Melero

Study supervision: I. Melero

P. Berraondo is supported by a Miguel Servet II contract (CPII15/00004) and grant PI16/00668 from Instituto de Salud Carlos III co-financed by European FEDER funds. I. Melero is supported by MINECO (SAF2014-52361-R and SAF2017-83267-C2-1R), European Commission VII Framework and Horizon 2020 programs (IACT and PROCROP), Cancer Research Institute (CRI) CLIP Grant 2017, Fundación de la Asociación Española Contra el Cáncer (AECC), and Fundación BBVA.