Clinical implications of aberrant PD-1 expression for acute leukemia prognosis

Acute leukemia (AL) is a malignant clonal disease originating from hematopoietic stem cells. Abnormal blast cells and immature cells (leukemia cells) in the bone marrow proliferate in large numbers and inhibit normal hematopoiesis. It is mainly divided into acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) [1]. Chemotherapy is currently the main treatment option for patients with AML and ALL but is limited due to the severe side effects and drug resistance [2]. Cancer immunotherapy has recently been developed to improve the specificity and strength of the immune system against cancer. In recent years we have also witnessed a breakthrough in the field of oncology that is represented by the development of novel agents: the immune checkpoint inhibitors, which "release the brakes" of the immune system. Immune checkpoint inhibitors, which is currently a hot area of research and may have important therapeutic value [3]. Consistently, PD-1 (Programmed Cell Death Protein 1) and its ligand (PD-L1) inhibitors have been approved by FDA and have been shown to be quite effective in several neoplasms, including leukemia [4].

Almost 30 years ago, Jenkins et al. had shown that effective activation of naive T cells requires the participation of TCR (T cell receptor) and B7/CD28 signals [5]. PD-1 (Programmed death protein 1, CD279) is an inhibitor receptor that belongs to the B7/CD28 family. As early as 1992, PD-1 was identified on T cells undergoing apoptosis by Ishida Y et al. [6]. PD-1 expressed on a broad variety of cells including activated T cells, B cells, monocytes, dendritic cells, and NK cells, except for naive lymphocytes prior to activation [7, 8]. Of note, PD-1 is highly expressed on tumor-specific T cells [9]. PD-1 plays an important role in inhibiting immune responses and promoting self-tolerance through modulating the activity of T-cells, activating apoptosis of antigen-specific T cells and inhibiting apoptosis of regulatory T cells [10‐12].

Recent research reported that PD-1 expression is related with poor prognosis of cancers. PD-1/PD-L1-targeted inhibitors play an important role in cancers such as breast cancer, lung cancer, colorectal cancer, gastric cancer, bladder cancer, pancreatic cancer, prostate cancer, DLBCL and so on [10, 13‐15]. In this study, we evaluated the expression of PD-1 on CD4+ and CD8+ T lymphocytes in Peripheral Blood and summarized the role of PD-1 which is a crucial factor affecting the prognosis of AML and ALL patients.

Traditional therapies combined with immune checkpoint inhibitors including anti-PD-1 antibody have shown better therapeutic efficacy in a variety of cancer types, including acute leukemia [16‐18]. It is known that the occurrence and development of acute leukemia is closely related to the reduction of the body’s immune level, especially the abnormality of cellular immunity. T lymphocytes perform cellular immune function, and patients with leukemia are often accompanied by changes in the number of T cell subsets and functional impairment. CD3+ cells are usually used to define the total number of T lymphocytes. CD4+ cells are the main response cells in the immune response. CD8+ cells can produce cell-mediated cytotoxicity on target cells, and at the same time have a regulatory immunosuppressive effect on CD4+ cells [19‐21]. In the present study, we used flow cytometry to evaluate PD-1 expression on the surface of CD4+ and CD8+ T lymphocytes in the peripheral circulation of AML and ALL patients and its clinical significance. Our results showed that the CD4/CD8 ratio in the peripheral circulation of AML patients and newly diagnosed ALL patients was increased to varying degrees compared to healthy controls. The proportion of CD4+ and CD8+ T lymphocytes and ratio of CD4/CD8 are important indicators for evaluating immune function. Generally, the normal range of CD4/CD8 ratio is about 1.4–2.0. Abnormal CD4/CD8 ratio was associated with immune dysfunction in patients, the unstable CD4+ /CD8+ ratio was not conducive to the balance of cellular immune responses.

Recent studies have found that the PD-1 expression is up-regulated in lung cancer, gastric cancer, hepatocellular carcinoma, multiple myeloma, breast cancer, renal cell carcinoma and melanoma [22‐28]. The high expression of PD-1 continuously activates the PD-1/PD-L1 signaling pathway, thereby inhibiting various signaling pathways. In addition, some studies have shown that high expression of PD-1 may be a poor prognostic factor in some malignant tumors such as lymphoma [29, 30], lung cancer [31] and breast cancer [32]. At present, PD-1/PD-L1 monoclonal antibodies have made breakthroughs in clinical trials for the treatment of non-small cell lung cancer, which further indicates that PD-1 and its ligands play an important role in anti-cancer therapy. It also provides a new targeted therapy idea for the first and second-line treatment of malignant tumors.

In hematological malignancies, PD-1 expression has been reported increased in patients with Hodgkin’s lymphoma, diffuse large B-cell lymphoma and chronic lymphocytic leukemia [33‐35]. Furthermore, PD-1+ T cells have also been proven in follicular lymphoma as an independent prognostic factor of overall survival [36, 37]. As far as the field of leukemia is concerned, current studies have reported that PD-1 is highly expressed in chronic lymphocytic leukemia and AML patients, and the expression level correlates with prognosis. However, clinical Implications of aberrant PD-1 expression in peripheral CD4+ and CD8+ T lymphocytes of AML and ALL patients in assessing the prognosis of diseases, remains inconclusive. The differential expression of PD-1 on different types of T cells in the peripheral circulation of AL patients and its relationship with the clinical characteristics and prognosis are still worthy of further investigation.

Our results indicated that expression levels of PD-1 on CD4+ and CD8+ T lymphocytes were significantly increased in newly diagnosed and non-remission patients compared to healthy controls both in AML and ALL patients. According to our analysis, the relationship between high PD1 expression on different T cell types and prognosis is different. To further investigate the clinical significance of PD-1 in AL, this study first analyzed the relationship between the expression of PD-1 and its clinical characteristics. Except for the increased CD8+ PD1+ levels in M2 AML patients, there was no significant association between the most common clinical indications and PD-1 expression in ALL and AML patients, indicating that PD-1 expression is not affected by general clinical manifestations and common hematologic indicators. The high level of CD4+ PD1+ and CD8+ PD1 + T lymphocytes were respectively poor prognostic indicators of AML patients and ALL patients but had no significant correlation with most common clinical risks. We found that the AML patients with the higher percentage of CD4+ PD1+ in peripheral blood had shorter OS than those with the lower percentage. ALL patients with low levels of CD8+ PD1+ in the peripheral blood have a significant survival advantage over patients with high levels of CD8+ PD1+ in the peripheral circulation. Together, these results were indicated that the high level of CD4+ PD1+ T lymphocytes and CD8+ PD1+ were respectively poor prognostic indicators of AML patients and ALL patients. High expression of PD-1 in the peripheral blood of AML and ALL patients was related to their poor prognosis.

To conclude, our evidence may be limited though, but preliminarily reveals the expression level of PD-1 in peripheral circulation of newly diagnosed or non-remission AML and ALL patients was significantly higher than healthy persons. The high level of CD4+ PD1+ and CD8+ PD1+ T lymphocytes were respectively poor prognostic indicators of AML patients and ALL patients but had no significant correlation with most common clinical risks. Aberrant PD-1 expression correlates with the prognosis of AL patient and may thus serve as poor prognostic indicators. Immunotherapy using PD-1 inhibitors may be a promising strategy for AML and ALL patients with peripheral circulating CD4+ PD1+ and CD8+ PD1+ T lymphocytes positively expressed, respectively.