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Acute leukaemia types

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SECTION 1: Establishing the primitive nature of the blast cells in AML

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Blast cells are recognised by morphology using features such as primitive nuclear or basophilic cytoplasmic appearances; similar principles are applied in flow cytometry where particular markers suggest a more primitive nature.


Table: Markers primarily used to confirm primitive nature in AML.  
The expression of some markers is typically associated with primitive cells and can help recognise the primitive nature of blast cells.

Note that these markers may also have some lineage specificity but are not generally used to assign lineage

CD45 A marker expressed by all leukocytes and their precursors. In AML expression is characteristically "weak" i.e. significantly less intense than normal lymphocytes or monocytes. In monocytic AML expression may be stronger, particular in mare mature monocytic forms where expression may resemble normal monocytes.
CD34 Frequently expressed by AML blast cells (40-80% of cases) - most often in less differentiated forms of AML. Expression is also seen frequently (and often more strongly) in ALL
Other markers: In the context of a proven AML diagnosis a number of markers may be expressed that reflect the primitive nature of the cells. Some of these are more frequently associated with lymphoid disorders, but providing other criteria for AML are met they should simply be considered to show "primitivness".

These include: CD38, HLA-DR, TDT, CD7.



SECTION 2: Assigning blast cells as myeloid lineage i.e. Diagnosing AML

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In morphology other features are used to indicate myeloid lineage, some are highly specificsuch as Auer rods, others may suggest myeloid lineage but be less specific. Again similar principles apply in flow cytometry


There are two ways to make a diagnosis of AML

Requirements to diagnose AML by flow cytometry
Pattern 1

A myeloid lineage-defining marker pattern is present
AND

no lineage-defining markers of T or B cells are present

See Table 1 for lineage-defining marker patterns in AML
Pattern 2

At least two myeloid lineage-associated markers are present
AND

there are no lineage defining marrkers of T or B eclls

AND

no more than one lymphoid-associated marker is present

See Table 2A and 2B for lineage-associated markers in AML


Table 1: Myeloid-lineage defining markers in AML
Only one marker is presently used to assign myeloid lineage in AML (note restrictions)

Note that these markers may also have some lineage specificity but are not generally used to assign lineage

MPO A lineage-defining marker in AML when expressed (around 80% of cases). More frequent in cases with granulocytic maturation. When detected by flow cytometry is diagnostic of myeloid differentiation is established (either AML or MPAL)
Table 2A: Myeloid lineage-associated markers (Main Set)
Each of these markers is frequently expressed in AML (80% of cases). They support a diagnosis of myeloid lineage, but are not sufficiently specific to be AML-defining when expressed alone


These markers may be considered the most frequent markers expressed by AML. Having said that each may also be expressed by cells of ALL. General concepts:

  • In the absence of markers suggesting alternative lineage expression of two of these markers may be used to support a diagnosis of AML
  • One or more of CD117, CD33, CD13 may be expressed by otherwise typical ALL is allowed and does not change lineage assignment


CD117 An early marker of myeloid lineage, seen in up to 80% of AML and vauable in recognising more primitive differentaiion forms (note that aberrant expression is seen in up to 20% of ALL cases)
CD33 A good marker for AML, particularly for those cases with granulocytic maturation, CD33 is often less strongly expressed in AML with monocytic dfferentiation and strongly expressed in APL.
CD13 A good lineage marker for AML that is acquired a little later in differentation than CD117 or CD33; expression of CD13 is often higher than CD33 in AML with monocytic differentiation.
Table 2B: Myeloid lineage-associated markers (Specific differentiation)
The markers already described above are sufficient to make a diagnosis of typical AML in most cases. Typically the markers in this table are associated with specific features of differentiation so will not be present in all cases, but may be helpful as "lineage-assocated" markers in difficult cases. Care should be taken when doing so, since in some cases specificity may be lower than for typical myeloid markers.
Granulocytic lineage markers
CD10 Most frequently a marker expressed in ALL subtypes, but expression of CD10 may occur some more differentiated cases of AML (particularly APL and AML with monocytic maturation
CD11b A marker of both granulocytic and monocytic maturation, this marker has previously been associated with less good outcome in a number of studies
Monocytic lineage markers
CD11c This marker is most associated with monocytic maturation in AML being fairly well corellated with CD11B, but overall probably less specific for monocytic differentiation that CD14
CD14 Primarily a marker of monocytic maturation in AML, seen most often in more differentiated forms, when present CD14 be considered a strong indicator of monocytic phenotype.
CD64 A good lineage marker for monocytic differentiation in AML, expressed in both monoblastic and monocytic forms, not fully sepicific when expressed at lower levels.
Features associated with erythroid differentiation
Most often CD34, CD45 and HLA-DR are weak or negative, although CD117 and CD36 are generally expressed. There may be some expression of platelet markers in some cases.
CD71 Frequently expressed though not fully lineage specific
CD235 A good marker of erythroid differentiation but acquired late and therefore may not be expressed
Features associated with megakaryocytic differentiation
Most often CD34, CD45 and HLA-DR are weak or negative, although CD13 and CD33 may be expressed
CD41 Platelet glycoprotein IIbIIIa
CD61 Platelet glycoprotein IIIa
CD36 Relatively non-specific (seen in erythroid and monocytic leukaemias) but often strongly expressed



SECTION 3: When to consider alternative lineage assignment

The "aberrent" expression of lymphoid markers is frequently encountered in AML, and in some diagnoses their presence may be expected. The interpretation of these markers will very much depennd on the nature of the marker and/or the number of aberent markers detected. A broad guide to interpretation is given in Table 3 below.


NOTES on abberancy
Definition and usage
What can be considered "aberrant"
What should not be considered aberrant

B cell markers frequently expressed in AML: changes lineage assignment, may change assignment, does not change assignment T cell markers frequently expressed in AML

MPAL

  • Any myeloid lineage-defining marker is present but B-Lymphoid and/or T-lymphoid defining markers are also present

or

  • At least two myeloid lineage-associated markers are present but also 'two markers of B-lymphoid and/or T-lymphoid lineage are present

OR

  • Cells have no more than one lineage-associated marker of any lineage

ETP-ALL PCDCN





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NON MYELOID MARKERS FREQUENTLY "ABERENTLY" EXPRESSED IN AML
Markers of lymphoid lineage indicating primitive dfferentiation in AML


Some additional markers are regarded as indicating primitive nature when expressed by AML blasts; these markers may be more usually assocated with T-lineage or B-lineage ALL, but when expressed in AML they are considered to indicate less differentiated forms.

TDT Expressed in some cases of AML (5-20%) particularly in less differentiated blast cells, often on a sub-population of cells. More typically associated with primitive ALL blast cells.
CD7 Predominantly seen as a T-cell antigen, CD7 tends may be expressed by AML blasts (20-40%) where it most often indicates a more primitive forms or MPAL. CD7 is most consistently a marker of T-lineage (including T-ALL and more mature froms).
Markers of lymphoid differentation abererantly expressed in AML


Some additional markers are regarded as indicating primitive nature when expressed by AML blasts; these markers may be more usually assocated with T-lineage or B-lineage ALL, but when expressed in AML they are considered to indicate less differentiated forms.

TDT Expressed in some cases of AML (5-20%) particularly in less differentiated blast cells, often on a sub-population of cells. More typically associated with primitive ALL blast cells.
CD7 Predominantly seen as a T-cell antigen, CD7 tends may be expressed by AML blasts (20-40%) where it most often indicates a more primitive forms or MPAL. CD7 is most consistently a marker of T-lineage (including T-ALL and more mature froms).

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IMPORTANT ADDITIONAL POINTS 1
AML subtypes with distinctive phenotype


The first step in AML diagnosis to establish the primitive nature of the abnormal cells:

(1) Typically AML blasts have low CD45 expression and cause low side scatter. This means that they form a relatively uniform and distinctive population that is clearly separate from that of lymphocytes on CD45/SSc plots (for further details see this section) also not all AML forms may fit this pattern - particularly APL and monocytic AML.
(2) Additional markers often expressed on myeloid cells may help confirm the primitive nature of the cells: in AML this is most often CD34, although other markers may contribute.

Mixed Phenotype Acute leukaemia (MPAL)
Formerly known as leukaemia of ambiguous lineage
Early T-precursor acute lymphoblastic leukaemia (ETP-ALL)
A (relatively new entity)

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IMPORTANT ADDITIONAL POINTS 2
Acute leukaemias with ambiguous lineage features


The first step in AML diagnosis to establish the primitive nature of the abnormal cells:

(1) Typically AML blasts have low CD45 expression and cause low side scatter. This means that they form a relatively uniform and distinctive population that is clearly separate from that of lymphocytes on CD45/SSc plots (for further details see this section) also not all AML forms may fit this pattern - particularly APL and monocytic AML.
(2) Additional markers often expressed on myeloid cells may help confirm the primitive nature of the cells: in AML this is most often CD34, although other markers may contribute.

Mixed Phenotype Acute leukaemia (MPAL)
Formerly known as leukaemia of ambiguous lineage: Myeloid MPO or monocytic defined by at least two markers (CD14, CD11c, CD64, NSE, lysozyme. T lineage CD3 (s or m), STRONG Cd19, with strong CD79a cCD22 or CD10 OR wk CD19 also with obove
Early T-precursor acute lymphoblastic leukaemia (ETP-ALL)
A (relatively new entity)
Undifferentiated acute leukaeemia
A (relatively new entity)
Non-haematopoietic tumours
need to know problems