Extra

Summary

CD33 a useful marker contributing to the diagnosis of AML, but is less specific than myeloperoxidase and may be less sensitive than CD117.

The molecule is expressed by normal maturing myeloid cells, and there is recognised aberrant expression by primitive lymphoid cells (10-20%).

Normal expression and function

CD33 a transmembrane receptor that is thought to function as an inhibitory protein supressing signals of the innate immune system. It is strongly expressed on multi-lineage hematopoietic progenitors and also on myelomonocytic precursors (although generally less strongly than CD13). Expression is absent from the more primitive pluripotent hematopoietic stem cells and CD33 is down-regulated on mature granulocytes, human mast cells, and blood basophils.

Diagnostic role

• Overall around 80-90% of AML cases express CD33; although the marker tends not to be expressed the least mmature forms, as well as those with erythroid or megakaryocytic differentiation. High CD33 expression may correlate with NPM1 mutation
• Approximately 10% of B or T lymphoblastic leukaemias and lymphomas may aberrantly express CD33 as well as up to 25% of myeloma cases
• CD33 not expressed by mature cells of erythroid, platelet, B-cells T-cells or NK-cell lineage
• CD33 is expressed throughout myeloid maturation and is found on mature myeloid cells, though less strongly on cells of monocytic lineage

Other relevant information:

CD33 has received particular attention for its role as the molecular target of antibody-based treatment strategies in the therapy of AML.

---

SUMMARY TABLES

Note The colour and % refer to the proportion of cases expressing the marker. Additional comments on strength, or site of expression (if relevant) are given below the table. For full details of key click here. For further information about a particular disease state click <I> adjacent to the condition name

Expression in primitive cell types
AML	CMML	B ALL	Burkitt	T ALL	ETP ALL <I>	MPAL <I>	Hgns <I>
80-100%*	80-100%	20-40%**	5-20%	5-20%	20-40%**	20-40%**	80-100%***

Notes: * most cases of AML express this marker although it can be absent (see notes above) ** CD33 is commonly expressed in ALL (particularly B-ALL), but in the correct context can also contribute to detection of MPAL or ETP-ALL, *** Haematogones frequently express CD33

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
<5%	<5%	<5%	<5%	<5%	<5%	<5%	5-20%

Notes: CD33 is occasionally expressed in mature lymphoid malignancies more frequently in b cell types