The major clinical components of cancer immunotherapy (modulating cell-mediated immune mechanisms)

Introduction

Among the clinical successes of cancer immunotherapy, the strategy of immune checkpoint blockade has been adopted most broadly. Immune checkpoints refers to the molecular pathways responsible for blunting the antitumor immune response. These include mechanisms that limit the creation of cytotoxic T cells and that result in exhaustion of tumor-reactive immune cells. Though a range of approaches directed toward unique vulnerabilities of a specific cancer hold promise, the targeted manipulation of common molecular pathways that suppress the effector arm of the immune system has demonstrated efficacy across many malignancies. This benefit has been realized through clinical development of monoclonal antibodies that disinhibit the immune system, thereby fueling excitement for pharmacologic manipulation of additional immune checkpoints and for use of checkpoint blockade in combination with other cancer therapeutics.

Background

Attempts to generate an immune response against cancer have focused on creation of tumor-reactive T cells and sustained activation of these cytotoxic effector cells. Early efforts were aimed at identification of tumor-associated antigens with the goal of immunizing a host against these peptides. Related work sought to expand the number of cytotoxic T cells capable of recognizing and destroying cancer cells. Through this work, several mechanisms were identified as key mediators by which tumors evade the immune system. Among these, cytotoxic T-lymphocyte antigen 4 (CTLA-4, CD152), an inhibitory signal in the immune synapse, proved to be important. Blockade of the CTLA-4 receptor was shown to induce tumor regression in preclinical models. Furthermore, clinical use of anti-CTLA-4 therapy provided benefit to patients with advanced cancer, thereby leading to U.S. Food and Drug Administration (FDA) approval in 2011. Likewise, development of pharmacologic agents blocking additional immune checkpoints have been successful. Most notable, inhibition of the programmed death 1 (PD-1, CD279) receptor and its primary ligand programmed death ligand-1 (PD-L1, CD274, B7-H1) has shown even greater efficacy across a wide range of malignancies. The success of immune checkpoint blockade was recognized by awarding the 2018 Nobel Prize in Physiology or Medicine to James Allison for work on CTLA-4 and Tasuku Honjo for work on PD-1. Their discoveries have revolutionized the field of immunotherapy, leading to research exploring other immune checkpoint pathways and ways to combine checkpoint blockade with other cancer treatments.

Cytotoxic T-lymphocyte antigen 4

As outlined in Chapter 7 , tumor-reactive T cells are generated when naïve T cells receive two signals from antigen-presenting cells. There must be interaction between the T-cell receptor and its cognate peptide–major histocompatibility complex (MHC; signal 1) and engagement of the costimulatory receptor CD28 with one of its ligands CD80 or CD86 (signal 2), as depicted in Fig. 10.1 . Though early efforts in the development of cancer immunotherapies often focused on signal 1, interest in targeting signal 2 was bolstered by the clinical success of inhibiting the negative costimulatory receptor CTLA-4. Compared with CD28, CTLA-4 has a greater affinity for CD80 and CD86 (see Fig. 10.1 ) and can outcompete CD28 for these costimulatory ligands, thereby impairing T-cell activation. Furthermore, CTLA-4 has the potential to dampen antitumor immune responses, because CTLA-4 is absent on resting memory T cells but becomes rapidly expressed after T-cell activation. This feedback inhibition likely facilitates immune tolerance but also allows tumors to evade the immune system.

The seminal observation that CTLA-4 blockade can potentiate the antitumor immune response came from experiments in which administration of anti-CTLA-4 antibodies led to rejection of transplanted murine colon carcinoma in immunocompetent mice. This preclinical success was followed by clinical development of monoclonal antibodies targeting CTLA-4. Ipilimumab (Yervoy, Bristol-Myers Squibb) was approved by the FDA in 2011 for the treatment of advanced melanoma based on results of two phase III clinical trials. ^, Further studies have shown efficacy in subsets of patients with a range of cancers including surgically resected melanoma, renal cell carcinoma, colorectal cancer, hepatocellular carcinoma, and non-small cell lung cancer. Though the potential effect of targeting CTLA-4 was clear from these trials, the magnitude of clinical benefit derived from ipilimumab monotherapy was modest. Clinical data pooled from studies of ipilimumab-treated patients with advanced melanoma reported a median overall survival of approximately 1 year (11.4 months) and fewer than 20% of patients had long-term survival. Another CTLA-4-blocking monoclonal antibody tremelimumab (Pfizer, MedImmune, Astra Zeneca) was evaluated in patients with advanced melanoma and demonstrated a similar median overall survival of approximately 1 year (12.6 months); this was compared with standard-of-care chemotherapy, which had a median overall survival of 10.7 months, and the small improvement in survival was not statistically significant (hazard ratio = 0.88, P = .13). Interestingly, the duration of response was substantially longer in patients receiving tremelimumab compared with standard-of-care chemotherapy (35.8 vs 13.7 months, P = .001).

Taken in sum, data from these clinical trials exploring the activity of anti-CTLA-4 therapy led to two fundamental conclusions. First, blockade of an immune checkpoint like CTLA-4 has potential value to patients with a spectrum of human malignancies. Whereas many treatment regimens are only effective against one tumor type, targeting a molecular immune pathway allows broader utility across a range of cancers. Second, despite low response rates with anti-CTLA monotherapy, patients who benefit from this treatment often have favorable long-term outcomes not seen with conventional chemotherapy. This difference seen at the tail of the survival curve is exciting to patients, clinicians, and scientists alike. Therefore it is not surprising that the early experience with CTLA-4 blockade fueled efforts to target other immune checkpoint pathways, to identify predictive biomarkers of response, and to combine CTLA-4 blockade with other cancer therapies.

Programmed death 1

Subsequent to the promising results from clinical trials using anti-CTLA-4 antibodies, there was rapid development of agents targeting the PD-1 pathway. The PD-1 receptor is similar to CTLA-4 in that both are expressed on activated T cells and can transduce inhibitory signals when engaged with their ligands. However, these immune checkpoint pathways have important differences. For example, the known ligands for PD-1 are PD-L1 (CD274, B7-H1) and PD-L2 (CD273, B7-DC) (see Fig. 10.1 ). Unlike the CTLA-4 ligands that are expressed on antigen-presenting cells, the PD-1 ligands are found on tumors and other cells within the tumor microenvironment. It is believed that PD-L1 is the dominant ligand present on solid tumors, whereas PD-L2 is found at higher levels on hematologic malignancies. Either ligand can inhibit activated T cells that express PD-1. Whereas the CTLA-4 pathway is generally felt to mediate feedback inhibition between antigen presenting cells and effector T cells in secondary lymphoid tissues, the PD-1 pathway is believed to block tumor-reactive T cells within the tumor microenvironment. Regardless of the precise mechanism(s), PD-1 seems to promote T-cell exhaustion and dampen the antitumor immune response.

Much of our insight into the PD-1 pathway stems from preclinical testing. Notably, PD-L1 expression on tumor cells was shown to confer resistance to host immunity. Furthermore, blockade of the PD-1 ligand inhibited tumor growth in this mouse model of myeloma. Likewise, administration of a monoclonal antibody to PD-L1 in combination with adoptive transfer of tumor-reactive T cells resulted in long-term survival of squamous cell carcinoma-bearing mice. Beyond its promise of clinical benefit, mouse models also suggested that targeting the PD-1 pathway may have less toxicity than targeting other immune checkpoints. Whereas CTLA-4 knockout mice develop uncontrolled lymphoproliferation and have early lethality, PD-1 knockout mice have milder and delayed-onset autoimmunity. Based on these encouraging findings, therapies targeting the PD-1 pathway were advanced into human trials.

Early clinical success was seen with the anti-PD-1 monoclonal antibody nivolumab (Opdivo, Bristol-Myers Squibb). In a Phase I dose escalation study of 39 patients, there were objective responses seen in 3 patients with melanoma, renal cell carcinoma, and colorectal cancer. Subsequent studies with nivolumab have shown durable benefit in patients with non-small cell lung cancer, renal cell carcinoma, and melanoma, as well as efficacy in a number of other cancers; therefore nivolumab received FDA approval in 2014. Of note, pembrolizumab (Keytruda, Merck) was the first monoclonal antibody targeting PD-1 approved by the FDA. This followed impressive results from a large early-phase study in which clinical responses were seen in 52 of 135 patients with advanced melanoma who received the anti-PD-1 antibody (originally named lambrolizumab), including patients previously treated with the anti-CTLA-4 monoclonal antibody ipilimumab.

The efficacy of pembrolizumab was not limited to melanoma, because its use has benefit in the treatment of groups of patients with bladder cancer, breast cancer, cervical cancer, colorectal cancer, gastroesophageal cancers, head and neck squamous cell carcinoma, hepatocellular carcinoma, lymphoma, Merkel cell carcinoma, non-small cell lung cancer, renal cell carcinoma, and tumors with high mutational burden or deficiencies in mismatch repair. Building on the success of nivolumab and pembrolizumab, several other agents targeting the PD-1/PD-L1 pathway have been approved. This includes cemiplimab (Libtayo, Sanofi/Regeneron), which has shown efficacy in the treatment of advanced nonmelanoma skin cancers and PD-L1-expressing non-small cell lung cancer. There has also been clinical success with anti-PD-L1 therapies, including FDA approval of avelumab (Bavencio, Pfizer), atezolizumab (Tecentriq, Genentech), and durvalumab (Imfinzi, Astra Zeneca). With many more trials underway and additional agents in clinical development, blockade of the PD-1 immune checkpoint pathway continues to be a key target in cancer therapy.

Lymphocyte activation gene 3

Subsequent to the clinical success of PD-1 and CTLA-4 pathway blockade, the LAG-3 (CD223) immune checkpoint has become another target of interest in cancer immunotherapy. LAG-3 is a type I transmembrane protein and member of the immunoglobulin superfamily, which is expressed on lymphocytes and binds MHC class II. Early interest in LAG-3 stemmed from its ability to enhance the function of antigen-presenting cells and thereby create a more robust antitumor immune response. Tumor cells engineered to express LAG-3 had impaired growth compared with native tumor cells when implanted into immunocompetent mice, and these mice were protected from future challenge with native tumor cells. This strategy of enhancing host immunity against cancer led to the development of soluble LAG-3 as a vaccine adjuvant. The first agent tested in clinical trials was eftilagimod alpha (IMP321, Immutep), a recombinant protein combining the LAG extracellular domains with the Fc domain of human immunoglobulin (Ig) G1. Patients with metastatic renal cell carcinoma were treated with escalating doses of the compound, which led to activation of CD8 T cells and reduced tumor growth in some patients. Subsequent clinical trials combined eftilagimod alpha and cytotoxic chemotherapy, suggesting modest clinical benefit in patients with metastatic breast cancer. Studies evaluating soluble LAG-3 with cancer vaccines or in combination with PD-1 blockade are ongoing.

In addition to its ability to bind MHC class II, LAG-3 can interact with fibrinogen-like protein 1 (FGL1) (see Fig. 10.1 ). This distinct function is believed to trigger T-cell exhaustion and blunt T-cell proliferation within the tumor microenvironment, mimicking the interaction between PD-1 and its ligands. Interestingly, LAG-3 and PD-1 are both overexpressed by immune cells in models of chronic infection and LAG-3 expression often parallels PD-1 expression in tumor-reactive T cells. Furthermore, mice deficient in both LAG-3 and PD-1 develop extensive lymphocytic infiltration that results in early death. Based on the clinical success of anti-PD-1 therapy and the potential synergy between the two pathways, preclinical evaluation of LAG-3 blockade has focused on combinatorial approaches with anti-PD-1 agents. Encouragingly, combined anti-PD-1 and anti-LAG-3 treatment has been effective in mouse cancer models. The anti-LAG-3 monoclonal antibody relatlimab (BMS-986016, Bristol-Myers Squibb), which was approved by the FDA in 2022, has been tested in a large phase I/II trial with or without nivolumab in the treatment of patients with advanced cancer. There were encouraging results from this study (NCT01968109) in patients with melanoma, including those previously treated with anti-PD-1/anti-PD-L1 therapy. A subsequent randomized phase II/II trial of nivolumab with or without relatlimab demonstrated greater progression-free survival from dual checkpoint inhibition in patients with previously untreated melanoma. With additional studies ongoing and novel agents in development, it seems that therapies targeting LAG-3 will have a promising role in the treatment of cancer.

Other immune checkpoint pathways

As the understanding of other immune checkpoints pathways grows, there is interest in clinical development of new agents to expand the armamentarium of cancer therapy. Though a comprehensive summary is beyond the scope of this work, a general overview of additional promising targets is provided in the following sections.

Conclusions

Immune checkpoint blockade has rapidly become one of the most exciting cancer treatments. Building on the pioneering work of Nobel laureates James Allison and Tasuku Honjo, scientists continue to develop new agents that can augment the antitumor immune response. Whether through promoting generation of tumor-reactive immune cells or preventing exhaustion of cytotoxic T cells, targeting these molecular pathways has the potential to treat a broad range of malignancies. The clinical success of anti-CTLA-4 therapy provided a glimpse of this promise. However, the broad efficacy of targeting the PD-1 pathway has been transformational. As such, clinicians continue to evaluate these new therapies and explore rational combinations to maximize the clinical benefit for patients with cancer.

Case vignette

A 17-year-old male presents in the clinic with subjective fever and easy bruising for the past week. After appropriate workup a diagnosis of t(9;22)/BCR-ABL1 negative pre-B cell acute lymphoblastic leukemia (ALL) is made. Induction therapy with vincristine, anthracycline, asparaginase, and steroids fails to result in complete remission. Additional options for achieving remission to enable hematopoietic cell transplantation (HCT) are discussed with the patient and family, and treatment with tisagenlecleucel, a chimeric antigen receptor (CAR) T-cell product, is scheduled. The patient’s lymphocytes are collected, the autologous CAR T cell product is prepared, and infusion is performed after administration of preparative lymphodepleting chemotherapy. On the fourth day postinfusion the patient develops fever (38.9°C), hypotension (BP 73/35), and hypoxia (Sa o ₂ 87%), concerning for cytokine release syndrome (CRS). Intravenous fluids, oxygen, vasopressors, and tocilizumab (interleukin [IL]-6 antagonist) are given, with rapid improvement in the patient’s vital signs. Subsequent disease monitoring of the bone marrow and spinal fluid is significant for a lack of detectable disease. This case demonstrates the induction of remission for a poor prognosis malignancy after adoptive transfer of CAR T cells, a revolution in the field of immunotherapy.