CD13 is a cell membrane bound glycoprotein expressed by myeloid cells at all stages of the differentiation pathway. Sequence data has revealed homology with aminopeptidase-N, an enzyme that cleaves peptides preferentially from the N-terminus. A number of antibodies are available and commonly used as part of leukaemia immunophenotyping protocols. The expression and functional characteristics of nine CD13 antibodies have been examined using leukaemic cells of the myeloid linage. Using flow cytometric and spectrophotometric assays these nine antibodies have been subclustered into three groups: those that inhibited MY7 binding, those that inhibited aminopeptidase-N enzyme activity and those that did neither. In addition comparison of cytoplasmic CD13 expression at presentation with surface CD13 expression following induction by phorbol ester has revealed the potential of AML blast cells to produce CD13 molecules de novo or from existing cytoplasmic stores. Comparison of FAB class with aminopeptidase enzyme activity suggests FAB-M4 have higher activity than FAB-M1 cells. These findings may be a further aid for the clinical diagnosis of AML.

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