PD-1 /PD-L1 checkpoint in hematological malignancies

Highlights

• PD-1/PD-L1 axis and its role in immune surveillance in hematological malignances.

• Targeting the PD-1/PD-L1 pathway to activate anti-tumor immunity.

• Safety, activity, and efficacy of anti–PD-1 antibody in hematological diseases.

• Anti-PD-1 antibody therapy enhances eradication of refractory Hodgkin Lymphoma.

• Anti-PD-1 and hypomethylating agents a new combination for leukemia patients.

Abstract

Programmed cell death protein 1 (PD-1), is a cell surface receptor with an important role in down-regulating the immune system and promoting self-tolerance by suppressing T cell inflammatory activity. PD-1/PDL1 axis represents a checkpoint to control immune responses and it is often used as a mechanism of immune escaping by cancers and infectious diseases. Many data demonstrate its important role in solid tumors and report emerging evidences in lymphoproliferative disorders. In this review, we summarized the available data on the role of PD-1/PD-L1 checkpoint in lymphoproliferative diseases and the therapeutics use of monoclonal blocking antibodies.

Introduction

Programmed death protein 1 (PD-1), also known as CD279, represents a checkpoint that normally controls some immune responses. Recently, its role together with PD-1 Ligand (PD-L1) have been largely investigated because the PD-1/PD-L1 axis has been recognized as an important mechanism for neoplastic cells to escape immune surveillance. Considering the promising results achieved targeting and blocking PD-1 pathway in solid tumors, in the last few years new clinical and preclinical trials have been conducted in hematological malignancies, too.

Aim of this review is to revise the medical literature regarding the role of PD-1 checkpoint in principal hematologic neoplastic disorders and the future therapeutic perspective offered by its blockade.

The PD-1/PD-L1 axis is one of the major mechanisms of immune escaping exerted by several cancer types in which upregulation of PD-L1 is observed [[1], [2], [3]]. PD-L1 interacts with its inhibitory receptor PD-1 expressed on activated T cells with the function of promoting self-antigen tolerance and balances the immune regulation to avoid antigen persistence and immune-mediated pathologies [[4], [5]] The PD-L1 immune induced expression by tumor is considered to be an adaptive resistance mechanism for tumor cells in response to immune challenge [[3], [6]]. Several studies have been focusing on PD-L1 upregulation and maintenance in tumor cells, and the main important factor is the IFNγ released by infiltrating effector T cells in the tumor microenvironment. However, beside the IFNγ contribution other cytokines such as GM-CSF and IL-4 have shown potential contribution [[7], [8]].

PD-L1 is the main ligand of PD-1 and is expressed on several immune effector cells such as T cells, B cells and NK cells. PD-1/PD-L1 interaction negatively by upregulates effector cells function towards different mechanisms such as exhaustions, anergy and apoptosis [[8], [9]]. Considering that PD-1 expression on cells per se an be associated to different cell functionality, the balance among activation, cytotoxic function and exhaustion is fine-tuned and depends on the level of expression of PD-L1 in target cells [10]. In the case of tumor cells where PD-L1 is upregulated, PD-1+ cells goes under exhaustion which is characterized by reduction of functional capacity, proliferation and cytotoxic activity [6].

Briefly, following recognition by Immune system of mutated or upregulated antigens expressed in the tumor cells, effector cells are activated. After activation, effector cells start releasing several cytokines and cytolytic molecules, among which IFNγ promotes PD-L1 upregulation in tumor cells [10]. PD-1-PD-L1 interaction subsequently impairs the capability of the infiltrated effector cells to both release cytokines and exerts cytolytic functions therefore promoting cancer progression (Fig. 1). Together with effector cells inactivation, PD-1-PD-L1 axis also promote Th1 cells conversion into Regulatory T cells with detrimental effect for tumor-specific immune responses [11]. These general mechanisms have been studied in different solid tumors but may also be applied to several hematological neoplastic diseases with some differences in the upregulation mechanisms [12].

In this review we summarize the available data on PD-1/PL1 checkpoint in these diseases.

Hodgkin’s Lymphoma (HL) and in particular the classical type (cHL), is a lymphoid neoplasm characterized by the presence of a small number of malignant Reed-Sternberg cells (RS) (Fig. 2) within an extensive but ineffective inflammatory and immune-cell infiltrate [[13], [14]]. Patients with newly-diagnosed cHL are usually treated with combination chemotherapy regimes achieving a 5 years PFS ranging from 70% to 98%, depending on the stage at diagnosis [[15], [16]]. Currently, the standard treatment for patients with relapsed/refractory cHL is high-dose chemotherapy followed by autologous stem cell transplantation (ASCT), that provides a 5-years PFS of 40–60% [[17], [18], [19]]. In patients who relapse after ASCT, the prognosis is poor with a median OS of 1–2 years[20]. An important innovation in the treatment of these patients has been the introduction of brentuximab vedotin (BV), a monoclonal anti-CD30 antibody, able to induce an overall response rate (ORR) of 75% with a median Progression free Survival (PFS) of 9.3 months [[21], [22]]. Unfortunately, for patients with progression after ASCT and BV, no standard treatment options are so far available. However, advances in understanding cHL pathogenesis, its interaction with the microenviroment and immune-escape strategies are leading to the identification of novel therapeutic agents [23].

The typical cHL microenviroment is characterized by a vast reactive milieu of immune cells such as lymphocytes, macrophages, histiocytes, plasma cells and fibroblasts. Among this variegated population, there is a variable quote of T-cells defined as “exhausted”, due to their interaction with inhibitory molecules on tumor cells [24]. Recent studies have demonstrated how the programmed death protein-1 (PD-1) pathway has a central role in immune system escape by cHL cells.

The PD-1 pathway normally works as a checkpoint to limit certain T-cell mediated immune responses [25]. PD-1 ligands 1 and 2 (PD-L1 and PD-L2) are normally expressed on several antigen- presenting cells, like dendritic cells and macrophages, and they both engage PD-1 receptors that are expressed on activated T cells [[25], [26]]. Their binding with PD-1 receptor results in a “T- cell exhaustion”, a reversible temporary inhibition of T-cell activation and proliferation. Recently, it has been demonstrated that viruses and tumors, including cHL, have developed mechanisms that exploit the PD-1 pathway to escape immune detection [27]. Overexpression of PD-L1 on RS cells and of PD-1 on tumor- infiltrating lymphocytes creates a potent inhibitory signal that contributes to maintain the immunosuppressive microenviroment typical of cHL, allowing the evasion of immune surveillance by tumor cells [28].

Different mechanisms have been investigated as responsible for the upregulation of PD-1 pathway in HL. Genomic-based studies have demonstrated that the genes encoding PD-L1 and PD-L2 are located on chromosome 9, which, interestingly, is the target of recurrent genetic abnormalities seen in cHL [[23], [24], [25], [26], [27], [28], [29]]. Amplifications and other genetic aberrations including chromosome 9p24.1, lead to PD-L1 overexpression in RS cells [29]. In addition, chromosome 9p24.1 also incorporate the gene encoding for Janus Kinase 2 (JAK2) that, when overexpressed, further increase PD-L1 transcriptional levels [[27], [29], [30]] Furthermore, it has been demonstrated that even Epstein-Barr virus (EBV) can increase the expression of PD-L1 in EBV-positive HL [31].

Considering these evidences, it becomes very important and useful to understand more about the frequency and diagnostic and prognostic importance of PD-L1 expression in HL.

Several studies were performed to investigate and quantify the PD-1/PD-L1 expression levels in HL. In a study by Chen et al. [31], a large set of primary tumors (N = 237), including HL samples (N = 53), were analyzed for PD-L1 expression by immunohistochemistry. The results showed a strong PD-L1 expression in HL samples compared with other aggressive types of Non-Hodgkin Lymphomas, with the higher levels in nodular sclerosis and mixed cellularity cHL (82% of cases had at least 90% of RS cells positive for PD-L1).

Another recent study by Menter et al. [32], examined a large cohort of HL (n = 280) and B-cell lymphomas (n = 619), for PD-L1 expression. Even this work confirmed that PD-L1 is overexpressed in RS cells in most cases (65% in the nodular sclerosis subtype, 67% in the lymphocyte-deplete subtype, 81% in the mixed cellularity subtype and 90% in lymphocyte-rich subtype), and showed that PD-L1 assessment can be helpful in the differential diagnosis of cHL and other B-cell lymphomas.

In addition, a recent work by Roemer et al. [33], evaluated 108 patients with newly diagnosed cHL: 97% of them presented concordant alteration of PD-L1 and PD-L2 loci and the incidence of 9p24.1 amplification was significantly increased in patients with advanced stage, who also presented a shorter PFS, demonstrating that this aberration has also a prognostic relevance.

All these evidences about the importance of PD-1/PD-L1 checkpoint in the pathogenesis and prognosis of cHL, together with the lack of therapeutic opportunities for patients with relapsed/refractory cHL after ASCT and BV, suggest how important could be the pharmacological blockade of this pathway as treatment approach for this set of patients [[34], [35]].

At the moment, two different fully human IgG4 subtype monoclonal antibodies anti-PD-1 are available for treating HL: nivolumab and pembrolizumab, both already approved for treatment of patients with relapsed/refractory melanoma and patients with metastatic non-small-cells lung cancer who failed prior therapies [[36], [37], [38], [39]].

Nivolumab has been investigated in cHL by Ansell et al. in a trial published in 2015 [29]. In this study they enrolled 23 patients with relapsed/refractory cHL: 78% of them had received previous treatment with BV and 78% relapsed after ASCT. All patients were treated with nivolumab 3 mg/Kg every 2 week until disease progression or CR or for a maximum of 2 years. The ORR was 87% (of whom 17% achieved CR and 70% a PR), while the remaining 13% had a stable disease. Drug-related adverse events of grade 2–3 occurred in 78% of patients and the most common were rash and thrombocytopenia. Grade 3–4 toxicity was observed in 52% of patients. An update of the study was presented at the 2015 ASH Annual Meeting and, after a follow-up of 101 weeks, the median PFS had yet to be reached, and the Overall Survival (OS) was 91% at 1 year and 83% at 1.5 years [40]. Another study from Younes et al. [41], enrolled 80 patients with cHL who failed both ASCT and therapy with BV. Among these patients, the ORR at a median follow up of 8.9 months was 66.3% and the safety profile was acceptable, confirming that nivolumab might be a new treatment option for this difficult to treat patients’ population.

A very recent study demonstrated the efficacy and tolerability of Nivolumab in patients with relapsed/refractory HL after allogenic hematopoietic cell transplantation (allo-HCT), 20 patients were treated with single agent Nivolumab until disease progression or intolerable toxicity. ORR was 95%, in particular, the 1 year PFS was 58.2% and OS was 78.7%. About graft-versus-host disease (GVHD) after allo-HCT was present in 6/20 patients (30%), demonstrating an acceptable safety profile [42].

In a recent multicenter retrospective analysis on 31 lymphoma patients (29 with cHL) receiving anti-PD-1 antibody (28 nivolumab and 3 pembrolizumab) for relapse post-allo-HCT, ORR was 77% (15 CR and 8 PR). However, 17 (55%) patients developed treatment-emergent GVHD [43].

Another similar study reported the positive outcome of three patients with relapsed cHL following allo-HCT and treated with the anti-PD-1 antibody, nivolumab. All patients were free from GVHD [44].

In conclusion, PD-1 blockade in relapsed cHL allo-HCT patients appears to be efficacious but frequently complicated by the rapid onset of severe and treatment-refractory GVHD.

An ongoing trial on the use of pembrolizumab in hematologic malignancies has included a separate cohort of 31 patients with relapsed/refractory cHL [45]. These patients were treated with single agent pembrolizumab every 2 week until disease progression or for a maximum of 2 years or unacceptable toxicity. The ORR at 12 weeks was 65% (16% CR and 48% PR). ORR was significantly higher in patients who previously underwent ASCT then in the group that was ineligible to ASCT or refused it (73% vs 44%). The treatment was, generally well tolerated and the most common adverse effects were hypothyroidism, diarrhea and pneumonitis. Considering these results, this study demonstrates that, like nivolumab, pembrolizumab has significant activity in patients with relapsed/refractory cHL, offering a new treatment opportunity to these patients.

Recently a phase II KEYNOTE-087 trial, conducted by Chen et al. [46] has shown that the programmed cell death protein 1-inhibitor pembrolizumab is highly active in patients with relapsed or refractory classical Hodgkin lymphoma. They administered pembrolizumab every 3 weeks in 3 cohort of patients the first after autologous stem cell transplantation (ASCT) and subsequent brentuximab vedotin (BV); the second after salvage chemotherapy and BV, but, ineligible for ASCT because of chemoresistant disease; and the third one after ASCT, but without BV after transplantation. 210 patients were enrolled and treated. The ORR was 69.0%, in particular, ORRs were 73.9% for cohort 1, 64.2% for cohort 2, and 70.0% for cohort 3. The most common adverse events were hypothyroidism and pyrexia.