Abstract 342: Differential CD45 isoform expression and function in AML versus healthy leukocytes

Abstract CD45 (the leukocyte common antigen) is a phosphatase present on all malignant and non-malignant leukocytes and hematopoietic progenitors and is the most abundant glycoprotein displayed by these cells. The human CD45 gene is alternatively spliced by reassortment of just three exons, exon 4 (“A”), exon 5 (“B”), and exon 6 (“C"), to create five distinct “R” isoforms (RO (no splice exons), RABC, RAB, RBC, and RB), with inclusion of A, B, or C exons enriching for expression of glycans (carbohydrates). Despite decades of investigation, the expression and phosphatase function of these isoforms on human AML cells remains unknown. To address this question, flow cytometry and western blot analysis was performed on AML primary cells (n=3) and KG1a and U937 cell lines versus healthy monocytes (n=3) utilizing exon-specific CD45-targetting mAbs. We then compared the contribution of CD45 phosphatase activity in KG1a and U937 cells versus healthy monocytes by employing a highly specific CD45 phosphatase inhibitor in cell culture for 24hrs, assessing induction of cell death after exposure to the inhibitor (n=3). Our results confirm reports of others indicating that "CD45RA- positivity" is a characteristic phenotype of AML cells, yet, distinctly, our data indicate that the most prevalent isoform identified on AML cells as "CD45RA-positive" is, actually, CD45RABC, the largest and most glycosylated isoform of CD45. This finding is in striking contrast to healthy monocytes, which do not express CD45RABC. Additionally, culturing of human AML cells with a CD45 phosphatase inhibitor induces rapid cell death within 24 hours but does not affect healthy monocytes. Thus, the conspicuous expression of the CD45 isoform CD45RABC among AML cells is associated with a survival advantage. Current studies in AML are directed to elaborating the prevalence of CD45RABC expression and uncovering the downstream effectors of its enzymatic activity to elucidate how such pathways sustain AML growth/survival. Citation Format: Evan Ales, Robert Sackstein. Differential CD45 isoform expression and function in AML versus healthy leukocytes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 342.