CD22

Summary

CD22 expression is largely restricted to B-lymphocytes

The major value of CD22 lies in the identification of ALL based on high lineage-specificity and very early acquisition in cytoplasm followed by cell surface expression

Normal expression and function

CD22 is a membrane adhesion receptor whose expression is mostly restricted to B lymphocytes where it is believed to be a negative regulator of activation. Expression is developmentally related - first appearing in cytoplasm of pro-B cells then on the surface in pre-B cells. CD22 is fully expressed by mature-B cells, but is lost during plasma cell differentiation.

Diagnostic role

• Cytoplasmic expression of CD22 gives particular value in the detection of pro-B ALL where diagnosis requires expression at least two of three markers: CD22 (membrane or cytoplasm), CD19 or CD79a. CD22 may not be uniformly expressed across a cell population with expression often being seen on only 60-90 of cells
• in mature B cell neoplasms CD22 has relatively low expression in CLL, MCL and FL, but has high expression in HCL

Other relevant information:

Basophils express CD22

---

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	BL	T ALL	ETP ALL	MPAL	Hgn
<5%	<5%	80-100%	80-100%	<5%	<5%	20-40%	80-100%

Notes: Expression of CD22 is a feature of B-lineage commitment that is not essential for diagnosis, but strongly suggests B lineage. Aberrant expression is uncomon and should suggest the possibility of MPAL

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
80-100%	80-100%	80-100%	80-100%	80-100%	80-100%	80-100%	<5%

Notes: The level of expression may vary both between and within cases of mature B cell neoplasm, but expression tends to be lost by mature plasma cells

Expression in mature T cell neoplasms*
ATLL	CTCL/Sezary	T-PLL	T-LGL	NK-LGL
<5%	<5%	<5%	<5%	<5%

Notes: Expression of CD22 is uncommon in mature T cell disorders

---