CD5

Summary

This marker is expressed by normal T cells and a subset of normal B cells, however it main diagnostic role is in diagnosis of certain B cell disorders:

• In mature B cell neoplasms CD5 is used to indicate likely CLL or MCL (both have consistent expression of CD5)

• In acute lymphoblastic leukaemia CD5 is useful to indicate T lineage, with (relatively) low levels of aberrant expression

• It has low frequency of expression on other lymphoma types.

Normal expression and function

CD5 binds to the B-cell ligand CD72, and has a negative effect on T and B cell receptor signalling raising the threshold for response to antigen (possibly limiting responses to self-antigens). It is a pan T cell antigen (present on thymocytes and mature T cells) that is also expressed on a subset of B cells (B1 subset comprising around 10% of adult B cells, but with a wide range according to age and disease state). CD5 is expressed at higher level on T cells than B cells, and expression is increased on activation.

Diagnostic role

• CD5 is expressed by almost all cases of CLL/SLL and the majority of cases of mantle cell lymphoma (90%)

• However, CD5 can be expressed by marginal zone lymphoma (15%); lymphoplasmacytoid lymphoma (5-10%); and occasionally diffuse large B cell lymphomas (10%) (where it may be associated with poorer prognosis and Activated B Cell (ABC) subtype).

• CD5 is expressed by most mature T cell neoplasms (with the exception of hepatosplenic T Cell lymphoma and T cell large granular lymphocytic leukaemia).

• CD5 is useful in the diagnosis and classification of T cell acute lymphoblastic leukaemia (T-ALL) since it is positive in 90% of T-ALL, although expression may be dim or absent in early precursor T ALL.

Other relevant information:

Aberrant expression of CD5 by B-ALL (rare) or AML (around 10%) is described, but in both cases CD5 expression appears less common compared with other “ambiguous markers”.

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SUMMARY TABLES

Expression in primitive cell types
AML	CMML	B ALL	BL	T ALL	ETP ALL	MPAL	Hgn
5-20%	CMML	<5%	<5%	40-80%	40-80% DIM	20-40%	20-40%

Notes: CD5 is a useful marker of T-lineage in acute leukaemia although not absolutely specific. Dim expression in ETP-ALL is useful dagnostically. There may be weak expression on AML and CMML in some cases. It is also expressed on late stage haematogones

Expression in mature B cell neoplasms
CLL	MCL	FL	MZL	HCL	DLBCL	LPL	PCL
80-100%	80-100%	<5%	5-20%	5-20%	5-20%	<5%	<5%

Notes: Expression of CD5 in matureB cells is strongly suggestive of either CLL or MCL, however it is not exclusive to those disorders so care ful examination of overall marker profiles is essential.

Expression in mature T cell neoplasms*
ATLL	CTCL/Sezary	T-PLL	T-LGL	NK-LGL
80-100%	80-100%	80-100%	80-100%	<5%

Notes: CD5 is consitently expressed in mature T cell malignancies but not in NK lineage.

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