Novel CD33 antibodies unravel localization, biology and therapeutic implications of CD33 isoforms

The aim of this study was to establish the therapeutic relevance of the CD33^D2 isoform by developing novel antibodies targeting the IgC domain of CD33. Two novel IgC-targeting antibodies, HL2541 and 5C11-2, were developed, and CD33 isoforms were assessed using multiple assays in cells overexpressing either CD33^FL or CD33^D2 isoforms, unmodified acute myeloid leukemia (AML) cell lines and primary AML specimens representing different genotypes for the CD33 splicing single nucleotide polymorphism. CD33^D2 was recognized on cells overexpressing CD33^D2 and unmodified AML cell lines; however, minimal/no cell surface detection of CD33^D2 was observed in primary AML specimens. Both isoforms were detected intracellularly using novel antibodies. Minimal cell surface expression of CD33^D2 on primary AML/progenitor cells warrants further studies on anti-CD33^D2 immunotherapeutics.