Abstract 3117: CD80 can be the marker of chronic myeloid leukemia stem cells and regulated by BCR-ABL signaling

Chronic myeloid leukemia (CML) is a hematological malignancy caused by an abnormal tyrosine kinase, BCR-ABL1. Leukemic stem cells (LSCs) in this disease proliferate indefinitely. Also, the LSCs normally differentiate into mature neutrophils. Although tyrosine kinase inhibitors (TKIs) that target the kinase activity of BCR-ABL1 improved prognosis of CML patients, many patients still have residual diseases and need life-long treatment with the TKIs. This is due to a small number of CML LSCs that cannot be completely eradicated by the TKIs. It has been hypothesized that not only chemotherapy alone, but also immune surveillance systems of patients are needed to completely eradicate cancerous cells from their body. We hypothesized that residual CML cells might escape from the immune surveillance. Surfaces molecules associated with immune surveillance are examined on CML cells. At first, we confirmed expression levels of CD80 and PD-L1 by flowcytometry using CML cell lines. Our data showed that CD80 was expressed on surfaces of CML derived cell, K562, not PD-L1. Moreover, an engineered K562 subline is generated with the constitutive active JAK2V617F gene and became dependent on JAK-STAT signaling, but not BCR-ABL. This subline did not show CD80 on the surface. The TKIs inhibited expression of CD80 on the parental K562. Meanwhile, an ERK inhibitor decreased CD80 expression in K562. These results suggested that CD80 was downstream of BCR-ABL-ERK signaling pathway, not JAK-STAT pathway. When K562 cells were differentiated into mature neutrophils by ATRA, levels of CD80 decreased. Cytotoxicity assays using K562 and NK cells from normal individuals showed that the parental K562 cells and K562 cells with decreased levels of CD80 were similarly killed. These data suggested that CD80 on K562 played a minimum role in NK cell immune surveillance. We also found that only a small fraction of CD11b positive monocytes in normal peripheral blood showed expression of CD80. Also, blastic cells of CML blastic phase showed CD80 expression. These results suggested that only blastic cells of CML and a fraction of monocytes expressed CD80. These cells were distinguished by expression of CD11b as a marker. Although it is currently impossible to distinguish normal stem cells from CML LSCs, our study suggested that CD80/CD11b might be helpful to distinguish the CML LSCs in patients9 bone marrow. It may be possible to collect a very small number of CML LSCs in CML patients under the TKI treatment using these antibodies and characterize the residual LSCs. Citation Format: Huixin Li, Shunichiro Yasuda, Satoru Aoyama, Chenyang Zhang, Yohei Kawano, Miyuki Azuma, Norihiko Kawamata. CD80 can be the marker of chronic myeloid leukemia stem cells and regulated by BCR-ABL signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3117.