Test

Background

CLL is a neoplasm of B-lymphocytes whose distinctive biology often (but not always) allows confident diagnosis: CLL cells have an "anergic" penotype i.e. they have down-regulated their B-cell receptor (BCR) and its associated molecules - so immunoglobulin light chains, CD19, CD20 and particularly CD79b are all weakly expresssed. CLL cells generally express CD5 and CD23 (which are normally expressed on only a small proporton of normal B-cells) which gives them a relatively unique immunophenotypic profile. Consequently typical CLL can be diagnosed with some confidence using flow cytometry. However, atypical cases are regularly encoutered and can cause diagnosic difficulty.

file CLL image file CD45CLL.jpg left 250px

Transfused red cells
Probably the most common cause encountered in the laboratory. Transfused blood cells co-exist with diseased cells. The transfused cells have a normocytic normochromic appearance, while the abnormal erythrocytes differ (having the characteristics of the underlying disorder e.g. microcytic hypochromic cells).

This image shows a typical CLL cells and a dot plot of CD45/SSc. Note that although the CLL cells (shown in blue) generally lie close to the position of normal lymphocytes, consistent with their anergic natures they will generally have lower expression of CD45 than normal typical T cells (shown in red) or NK cells (shown in green). They also have wider range of CD45 expression causing them to have a "sausage like" shape. This is a fairly typical appearance of CLL though not in itself diagnostic.

Immunophenotype:

Markers of B-lineage

• Immunoglobulin light chains: expect either light chain restriction with low intensity, or undetectable expression
• CD19 should be detected, but intensity is generally weak in CLL
• CD20 has expected expression in CLL, but intensity is generally weak
• CD79b expression is characteristically weak in CLL

Markers allowing diagnostic distinction

• FMC7 will have low or absent expression, high expression is particularly may suggest alternative disorders such as MCL or MZL
• CD5 should be expressed at moderate intensity in CLL (note also on expressed on mcl and occasionally on other lpd)
• CD23 has expected expression in CLL often at low intensity, generally absent in mcl but variably present on other lpd
• CD200 expressed on most CLL cases as well as other b-lpd but is most frequently absent in mcl optional markers extended panel for atypical cases
• CD43 a pan b-cell marker with expected expression in CLL; frequently but not always absent in mcl

Pitfalls