Idiopathic Pancytopenia Caused By Immune Checkpoint Inhibitor
Authors: Lee Yongjae , Choi Yoon Seok , Lee Jeong Seok , Kwon Minsuk 
Published: December 2023

Immune checkpoint inhibitors (ICI) have demonstrated clinical utility in various cancers, but they also induce immune-related adverse events (irAEs) such as thyroiditis, pneumonitis, or colitis. Immune-related(IR) hematologic toxicities, including anemia, neutropenia, thrombocytopenia, or pancytopenia are rarely reported but can be fatal. However, the underlying immunobiology of IR hematologic toxicities by ICIs has been very poorly investigated. We report a case of IR pancytopenia by ICIs including an in-depth analysis of single-cell RNA sequencing for the bone marrow (BM). A 57-year-old man with metastatic clear cell renal cell carcinoma was treated with nivolumab (anti-PD-1) and ipilimumab (anti-CTLA-4) and showed rapid tumor reduction. After 7 months of ICI treatment, he showed fever and pancytopenia with hemoglobin - 8.3 g/dl, platelet count - 64,000 /㎕, WBC count - 2500 cells/㎕, absolute neutrophil count - 745 cells/㎕, MCH - 25.9pg, MCV - 82.0fl, MCHC - 31.5g/dl, RDW - 18.2% and reticulocyte count - 1.65%. Direct and indirect antiglobulin test was negative. On BM aspiration, the number of erythroid and megakaryocytic series was slightly increased and that of myeloid series was in the normal range. In BM biopsy, cellularity was 70-80% and three hematopoietic series showed mild hyperplasia. There were no abnormal findings in morphology and chromosomal analysis. BM diagnosis was idiopathic cytopenia of undetermined significance. Although he was treated with methylprednisolone (1mg/kg), hematologic improvement was minimal. Cyclosporine demonstrated the hematologic response in the case. After 5 months of immune suppressive therapy (6-month-discontinuation of nivolumab), he maintained anti-tumor responses by ICIs. We cautiously re-challenge nivolumab as maintenance therapy, but the pancytopenia reappeared. ICI was permanently terminated, and therapy was changed to axitinib (VEGF tyrosine kinase inhibitor). To gain comprehensive insights, we performed single-cell analysis on BM aspirate. Our approach aimed to retain the integrity of both mononuclear cells and granulocytes without cryopreservation by analyzing the sample promptly after collection. Following this, we segregated the sample into distinct fractions, namely bone marrow mononuclear cells (BMMC) and whole blood (BM), and subjected both fractions to in-depth single-cell analysis. This method allowed us to explore the cellular composition and heterogeneity within the BM aspirate, contributing valuable insights to our research. The analysis detected 19,227 cells and identified unique cellular subsets, including terminally differentiated CD8+ T cells (CD8 T EF) and T Helper 17 cells (T H17), which are not commonly found in BM. CD8 T EF cells expressed activation and exhaustion markers, HLA-DRA and KLRG1. Additionally, T H17 cells demonstrated elevated expression of typical T H17 markers RORC and CCR6, as well as polyfunctional markers, including KLRB1, TNFRSF4, and TNFRSF18. Interestingly, previous studies on IR-myocarditis have shown an increased presence of CD8 T EF in the peripheral blood. Moreover, the studies have associated aplastic anemia with the T H17 cell population. These findings offer significant insights into the molecular and cellular mechanisms, as well as the BM microenvironment, contributing to the IR hematologic toxicities by ICIs.