Dendritic cells are professional APCs that are critical for effective T-cell stimulation. Both DCs and T cells possess an array of stimulatory molecules that are necessary for sustained and durable immune response, making them potential therapeutic targets.
Dendritic Cell–Based Immunostimulatory Agents
Toll-like receptors (TLRs) are a group of 13 receptors present in DCs that have distinct ligands, and the receptor-ligand interaction leads to the activation of DCs, inducing the expression of type I interferons, cytokines (eg, IL-12), and costimulatory molecules (eg, CD80, CD86, and CD40) that are critical for T-cell activation (20,21). Clinical trials with TLR agonists have shown promise, particularly in combination with other therapeutic modalities. The imidazoquinolone Imiquimod ligates TLR7; in several clinical trials, topical application resulted in an 80% to 90% clearance rate for superficial basal cell carcinoma (22,23). Systemic administration of TLR7 and TLR9 agonist as monotherapy in melanoma and renal cell carcinoma had a strong immune response but failed to demonstrate objective clinical response (24,25). Although they failed as monotherapy, their capacity to boost the immune system makes them a suitable adjuvant therapy. Both CpG and imiquimod induced increased levels of tumor antigen-specific T cells in patients with melanoma and prostate cancer vaccinated with recombinant protein tumor antigen NY-ESO1 showing promise in combination therapy (26,27).
Another attractive target for enhancing DC function is CD40, which is present on APCs, including DCs, and ligation of CD40 with its ligand CD40L present on T cells is critical for T-cell priming (28). CD40 targeting agents as monotherapy showed modest clinical benefit in non-Hodgkins lymphoma and melanoma (29). To increase its efficacy, a novel approach utilized electroporation to introduce messenger RNA encoding CD40 ligand, constitutively active TLR4, CD70, and multiple melanoma tumor antigens into autologous DCs (TriMix-DC). Tumor regressions were observed in 6 of 17 patients who had received interferon-α-2b in combination with TriMix-DC (30). Further, combination of anti-CD40 antibody and the chemotherapy agent gemcitabine showed tumor regression in both humans and mice (31).
T-Cell Based Immunostimulatory Agents
Engagement of the TCR on T cells with MHC serves as a first signal for T-cell activation; the second signal is mediated by the binding of costimulatory molecules on the T-cell surface to B7 proteins (such as CD80 or CD86) on APCs. Both signals are critical for effective T-cell activation, proliferation, and migration, making costimulatory molecules an interesting therapeutic target for sustained immune response (32,33).
Greater attention has been focused on 41BB (CD137) and OX40 (CD134). Preclinical studies with antimouse anti-CD137 have shown promising activity in combination with antitumor antibody. Most recently, it was shown that targeting anti-CD137 increases the efficacy of cetuximab (anti–epidermal growth factor receptor monoclonal antibody [mAb]) in murine xenograft models, making it another suitable target for combination therapy (34). Clinical studies with PF-05082566 (Pfizer, anti-CD-137) in combination with rituximab in B-cell Lymphoma and MK-3475 (anti–programmed cell death 1 [PD-1]) in solid tumor are ongoing. Further, a phase I clinical trial using a mouse mAb that agonizes human OX40 signaling in patients with advanced cancer showed that patients treated with one course of the anti-OX40 mAb (9B12) had an acceptable toxicity profile and regression of at least one metastatic lesion in 12 of 30 patients (35).
Immune Checkpoint Blockade
T-cell activation is tightly controlled by immune-suppressive cells and cytokines as well as by the coinhibitory molecules present in T cells, such as CTLA-4 or PD-1 (see Fig. 48-2) (36). The CTLA-4 is expressed by activated CD4 and CD8 T cells, and it competes with costimulator CD28 for binding to its ligands (B7 proteins). Binding of CTLA-4 to B7 proteins interrupts CD28 costimulatory signals and serves as a negative regulator of T-cell responses.
T-cell activation begins with interaction of the T-cell receptor (TCR) on a T cell with major histocompatibility complex (MHC) bound to antigen on an antigen-presenting cell (APC) (signal 1); activation of the T cell requires additional signals that are provided by the interaction between CD28 and other costimulatory molecules (signal 2). T-cell activation is inhibited by coinhibitory molecules such as CTLA4, PD-1.
For many years, it was widely accepted that T-cell responses can be turned “on” via T-cell receptor and CD28 costimulation, but the concept of turning “off” T-cell responses did not exist until the discovery of CTLA-4. It was a paradigm shift in cancer immunotherapy to move away from vaccine strategies aimed at turning on T-cell responses to immunotherapy strategies aimed at turning off T-cell inhibitory pathways (37). Preclinical studies with anti-CTLA-4 antibodies demonstrated rejection of syngeneic transplanted tumors in mice.
These preclinical studies led to the eventual development of an antibody to block human CTLA-4 (ipilimumab), which was subsequently shown in a phase III randomized, controlled trial in patients with metastatic melanoma to improve the median overall survival (38). Importantly, additional studies have shown that a subset of patients treated with anti-CTLA-4 had durable clinical responses lasting 10 or more years (39). This trial led to the approval of ipilimumab by the FDA in March 2011 for the treatment of patients with metastatic melanoma. A number of factors have been proposed to potentially serve as biomarkers for response to ipilimumab therapy. Recent studies suggest that an increase in TILs correlates with clinical response of anti-CTLA-4 therapy (40). In addition, sustained Inducible T-cell COStimulator (ICOS) expression on CD4 T cells has also been observed to correlate with survival of patients with melanoma treated with anti-CTLA-4 therapy (41).
Consistent with this observation, increased frequency of ICOS+ CD4 T cells can also serve as a pharmacodynamic biomarker for anti-CTLA-4 therapy (42). ICOS is one of the costimulatory receptors of T cells. A preclincial study showed that engagement of the ICOS pathway markedly enhanced efficacy of CTLA-4 blockade in cancer immunotherapy (43). This finding provides a potential mechanism for future clinical studies with agonistic signaling through ICOS in combination with blockade of CTLA-4.
Programmed cell death 1 is another negative regulator of T-cell response that is mainly expressed by activated CD4 and CD8 T cells as well as APCs (44). In preclinical studies, blocking antibodies against PD-1 resulted in reduction of tumor metastasis and growth in a number of experimental tumor models (45,46). These preclinical results led to many clinical trials. In a phase I clinical trial, a fully human immunoglobulin (Ig) G4 anti-PD-1 mAb (nivolumab) was evaluated in patients with relapsed or refractory Hodgkin’s lymphoma and demonstrated objective responses in 20 patients (87%), including 17% with a complete response (47).
A randomized, controlled, phase III clinical study compared nivolumab versus dacarbazine (chemotherapy) in patients with previously untreated melanoma (without the BRAF mutation), and the overall response rate favored nivolumab (40% vs 14%) (48). Another phase III clinical trial compared nivolumab versus chemotherapy (dacarbazine or carboplatin plus paclitaxel) in patients with ipilimumab-refractory advanced melanoma induced an overall response rate of 32% versus 11%, respectively, leading to the accelerated FDA approval of nivolumab for patients with unresectable or metastatic melanoma no longer responding to other drugs (49).
Pembrolizumab is another humanized IgG4 mAb targeting PD-1 that demonstrated an overall response rate of 26% in patients with ipilimumab-refractory advanced melanoma in a phase I clinical trial, which prompted its accelerated FDA approval (50). Nivolumab also had an overall response rate of 87% in patients with Hodgkin’s lymphoma who had failed brentuximab vedotin (47). These studies show that nivolumab and pembrolizumab have significant clinical activity in a variety of heavily pretreated patients with solid tumor malignancies as well as patients with hematological malignancies.
Programmed cell death 1 has two ligands, PD-L1 and PD-L2, with distinct expression profiles. The PD-L1 ligand is expressed not only on APCs but also on T cells, B cells, and nonhematopoietic cells, including tumor cells. Expression of PD-L2 is largely restricted to APCs, including macrophages and myeloid DCs, as well as mast cells. Promising clinical results were observed with drugs targeting PD-L1. A phase I trial with anti-PD-L1 (human IgG4 mAb; BMS-936559) demonstrated an objective response rate of 6% to 17% in patients with advanced non–small cell lung cancer (NSCLC), melanoma, and Renal Cell Carcinoma (RCC) (51). Another agent targeting PD-L1, MPDL3280A (human IgG1), was engineered with a modification in the Fc domain that eradicates antibody-dependent cellular cytotoxicity. MPDL3280A showed objective response rates of 13% to 26% in multiple solid tumor malignancies, including NSCLC, melanoma, RCC, colorectal cancer, gastric cancer, and head and neck squamous cell carcinoma (52). Remarkably, MPDL3280A also had a 26% objective response rate in bladder cancer (53).
Because anti-CTLA-4 and anti-PD-1 target distinct inhibitory pathways in T cells, preclinical studies have shown that concurrent targeting of CTLA-4 and PD-1 significantly improves therapeutic efficacy when compared to the monotherapies (54). A phase I clinical trial evaluated the concurrent treatment of advanced melanoma with ipilimumab plus nivolumab using various doses of both drugs (four cohorts). The objective response rate was 40% when all four cohorts were included and 53% in the cohort representing the maximum dose that was associated with an acceptable level of toxicities. This latter cohort was also associated with unprecedented 1- and 2-year overall survival rates of 94% and 88%, respectively (55). These results show that drugs targeting the immune checkpoints, CTLA-4, PD-1, and PD-L1 as monotherapy or in combinations, are likely to become the standard of care in various solid as well as hematologic malignancies.