The power of combining adoptive cell therapy (ACT) and pathogen-boosted vaccination to treat solid tumors

ABSTRACT

Recent advancements in adoptive cell therapy (ACT) are opening up new frontiers for cancer immunotherapy. CAR T cells targeting CD19 have emerged as a remarkable T cell-based therapy for the successful treatment of certain types of leukemia and lymphomas. Despite these clinical successes, as well as significant breakthroughs in T cell engineering, the treatment of solid tumors with ACT remains a relentless challenge. Thus, the current consensus of the field is that an urgent need exists for the design of innovative approaches that can improve the efficacy of ACT in treating solid cancers while maintaining a high degree of reliability and safety.

KEYWORDS:

• engineered adoptive cell therapy
• pathogen-boosted vaccination

Conventional ACT therapy using tumor-infiltrating lymphocytes

The safety and efficacy of therapeutically infusing non-genetically modified tumor infiltrating lymphocytes (TILs) into patients with melanoma has been validated over the last 20 y. ¹ Although several investigations have demonstrated the feasibility of this approach and have identified that TILs can target a wide array of tumor neo-antigens, there remain several outstanding issues that ultimately limit the usage of TILs in cancer therapy. First, TILs are dependent on the endogenous T cell receptor (TCR) for recognition of tumor antigens, which from a therapeutic standpoint, requires that MHC matching is performed for each patient. Additionally, the affinity of the TCR for neo-antigens is relatively low (compared with that of viral or bacterial antigens) and consequently can impede the activation, differentiation, and cytolytic activity of tumor-specific T cells. Finally, the procurement of TILs can be both technically and logistically challenging. Thus, there is a desperate need for alternative ACT approaches.

Engineering T cells to treat cancer

The recent emergence of engineered T cells has transformed the field of cancer immunotherapy and has demonstrated the potential to overcome some of the limitations of using conventional TILs for ACT. The advent of chimeric antigen receptors (CARs) and use of genetically modified TCRs comprise the 2 major T cell engineering platforms that aim to refine T cell recognition of tumor antigens and subsequently direct enduring T cell responses against cancerous cells. CARs are synthetic molecules that contain an intracellular signaling domain and an extracellular antibody domain that allows for user-specified MHC-independent recognition of antigens. ^2,3 CAR therapy targeting CD19 has shown extreme promise in treating certain B cell malignancies. For instance, one clinical trial reported a 90% complete response rate among 30 adult and pediatric patients with relapsed/refractory ALL. ⁴ Similar success rates among other B-cell leukemia and lymphoma cohorts have been achieved using various CD19 CAR designs. ^5,6 These clinical studies clearly demonstrate the value and therapeutic potential of CAR T cell therapy. However, despite these successes, the efficacy of CAR therapy has thus far been limited to hematological malignancies, and the field of immuno-oncology awaits a clear demonstration of clinical efficacy against solid-tumors. The reasons for this low clinical efficacy against solid tumors remain unclear, but may include the suboptimal expansion and persistence of adoptively transferred T cells, poor trafficking to and retention within a suppressive tumor microenvironment (TME), and the shear heterogeneity of cancerous cells that can bear a wide array of tumor-associated antigens (TAAs). An additional limitation of CAR T cell therapy is that it requires surface expression of tumor antigens on cancerous cells.

To address some of these issues, Kershaw et al. developed an elegant strategy that involves modification of ACT to generate T cells with dual-specificity. To do this, the authors used T cells with an endogenous TCR specific for a strong immunogen and genetically engineered these cells to co-express a chimeric antigen receptor specific for a TAA. ⁷ Using both in vitro and in vivo models, it was demonstrated that these dual-specific T cells displayed robust expansion and anti-tumor reactivity upon allogeneic immunization. Subsequently, Kershaw and colleagues reported results from a phase I clinical study demonstrating that dual-specific alloreactive CAR T cells could be applied safely to ovarian cancer patients. ⁸ However, T cell persistence and localization to the TME could not be detected long-term, nor was any appreciable clinical benefit observed. Thus, ACT-mediated inhibition of large solid tumors remains a major challenge to the field, and the development of innovative new approaches may be necessary for future clinical success.

Pathogen-based cancer immunotherapy

The anti-tumor effect of bacterial infections has long been noted throughout history. The Egyptian physician Imhotep was the first to document this association in 2600 BC. ⁹ However, the clinical implementation of this concept first occurred in 1868 when W. Busch inoculated a cancer patient with streptococcus pyrogenes and subsequently noticed a decrease in tumor malignancy. ¹⁰ A few decades later, in 1891, William Coley (now known as the “Father of Immunotherapy”) performed the first systematic study of bacteria-based immunotherapy for the treatment of malignant bone and soft-tissue sarcomas. ¹¹ Since this time, several studies have demonstrated the potent adjuvant-like properties of using bacteria and other pathogens to boost the immune system to help combat cancer. A key mechanism by which pathogens may enhance anti-tumor immunity is through the induction of a highly inflammatory environment, which is often lacking in the endogenous immune response against TAAs. Indeed, as TAAs are usually endogenous antigens bearing strong homology to self-proteins, the antigen presentation process of TAA often occurs without the co-stimulatory signals necessary for robust T cell activation and expansion, thereby facilitating the development of immune tolerance. ¹² By contrast, infectious pathogens are naturally capable of inducing vigorous innate and adaptive immune responses in a manner that is largely dependent on host sensing of pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors (PRRs). ¹³ Thus, the strong immunogenic properties of pathogens constitute them as a powerful platform to revert the immunosuppressive TME and “awaken” or initiate T cell responses against malignant cells. The therapeutic potential of bacteria-based cancer vaccines (particularly those using Listeria monocytogenes) has clearly been demonstrated in multiple preclinical models. ^14-19 These promising findings have led to the development of several clinical vaccines, including CRS-207 and ADXS11, which are currently being evaluated in clinical trials for the treatment of multiple types of solid cancers. ²⁰ However, despite the success of these preclinical bacteria-based vaccines in limiting tumor growth and prolonging host survival, rarely did complete remission or tumor eradication occur. Thus, alternative therapeutic approaches with a higher degree of clinical efficacy will likely be necessary to achieve durable and complete remission of solid tumors.

Two is better than one: Combining pathogen-boosted immunotherapy with engineered ACT

Recently, our laboratory and the Kershaw laboratory have independently refined and modified the dual-specific T cell ACT approach to encompass a pathogen-based cancer vaccine. Our strategy, termed Reenergized ACT (ReACT) combines the strength of ACT with the robust immunogenic properties conferred by bacterial vaccination. ²¹ To do this, we genetically engineered tumor-specific CD8 T cells to express a second TCR specific for a bacterial antigen present on recombinant Listeria monocytogenes (Lm). Using Lm as a pathogen-based vaccine, we then evaluated this methodology in a pre-clinical murine B16-F10 melanoma model. Strikingly, ReACT therapy led to the vigorous expansion and localization of dual-specific T cells to the sites of tumor, ultimately resulting in primary tumor eradication and long-term protection against recurrence. Mechanistically, we identified that this combinatorial approach effectively recruits cytotoxic CD8 T cells to the tumor site and reverts the immunosuppressive nature of the TME, thereby resulting in durable anti-tumor immunity.

Similarly, Kershaw and colleagues recently generated dual-specific T cells that co-express a CAR specific for the Her2 neo-antigen and a TCR specific for the gp100 antigen of the melanocyte protein. ²² The authors evaluated this ACT method while incorporating vaccination (ACTIV) with a recombinant vaccinia virus that expresses gp100. Importantly, ACTIV therapy resulted in the durable and complete remission of a variety of Her2⁺ tumors without evidence of off-target effects or toxicity toward normal Her2-expressing tissues. Furthermore, mice that received ACTIV therapy were resistant to tumor rechallenge, indicating the formation of potent immunological memory. Collectively, these 2 recent studies using pathogen-based vaccines in combination with genetically modified ACT demonstrate the potential to achieve a high degree of efficacy in treating solid tumors, a feat that has thus far proven immensely difficult to accomplish. A future identification of the precise cellular and molecular mechanisms by which tumor eradication is achieved and immunological memory develops in response to these respective therapies will undoubtedly have important clinical implications. Additionally, it will be extremely important to determine whether combinatorial ACT and pathogen-boosted immunotherapy can be translated into clinical practice to provide robust and enduring anti-tumor immunity against solid cancers. As listeria-based vaccines have already displayed promising anti-tumor effects and safety in ongoing clinical trials (https://clinicaltrials.gov/ct2/home), we remain cautiously optimistic that combinatorial pathogen-boosted vaccination and modified ACT will synergistically enhance anti-tumor immunity against a broad range of malignancies.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Funding

This work is supported by NIH grants AI125741 (W. C.), 5T32HL007209 (R. Z.).