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 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 or CD33 isoforms, unmodified acute myeloid leukemia (AML) cell lines and primary AML specimens representing different genotypes for the CD33 splicing single nucleotide polymorphism. CD33 was recognized on cells overexpressing CD33 and unmodified AML cell lines; however, minimal/no cell surface detection of CD33 was observed in primary AML specimens. Both isoforms were detected intracellularly using novel antibodies. Minimal cell surface expression of CD33 on primary AML/progenitor cells warrants further studies on anti-CD33 immunotherapeutics.