MPAL

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

The recognition of blast cells requires them to have features of primitive phenotype, markers can help support this, although morphology may also be required

The expression of some markers is typically associated with primitive cells and can help recognise the primitive nature of blast cells. These markers may also have some lineage specificity.

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MARKERS USEFUL TO CONFIRM PRIMITIVE NATURE OF BLAST CELLS IN AML
These markers are useful to recognise the primitive nature of cells rather than specific lineage 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). It is important to realise however also not all AML cases will fit this pattern of low-CD45/low-SSc - particularly APL and monocytic AML. (2) Additional markers that are often expressed on myeloid cells can help confirm the primitive nature of the cells: in AML this is most often CD34, although other markers may contribute.
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.
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 that may be associated with primitive nature in AML
In the context of a proven AML diagnosis a number of markers may be expressed that are considered to predominantly 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 "primitivity". These include: CD38, HLA-DR, TDT, CD7.

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SECTION 2: Assigning blast cells as myeloid lineage

Essentially, leukaemic cells can be assigned solely to myeloid lineage under one of two circumstances:

1: A myeloid lineage-defining marker is expressed and no B or T-lineage defining markers are expressed

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MYELOID-LINEAGE-DEFINING MARKERS
The present WHO classification recognises only one marker as lineage-defining in AML Expression of MPO is lineage defining and should indicate AML (or MPAL if lymphoid lineage also confirmed)
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)

2: At least two myeloid-associated markers are expressed, and lineage cannot be assigned as B cell or T cell by lineage-specific or lineage-associated lymphoid markers

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MYELOID-LINEAGE ASSOCIATED MARKERS (general)
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.

The markers already decribed 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.

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MYELOID LINEAGE MARKER ASSOCIATED WITH SPECIFIC DIFFERENTIATION
While most cases of AML will be diagnosed using MPO, CD117, CD13 and CD33, some cases may be more difficult, particularly those significant differentation. In those cases supplementary antibodies may be helpful to make a diagnosis or confirm a pattern of differentation. 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.
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	One of tne of the first lineage-markers acquired in AML with good relative specificity
Monocytic lineage markers
CD11c	One of tne of the first lineage-markers acquired in AML with good relative specificity
CD14	A good marker for AML that is often less strongly expressed in monocytic forms
CD64	A good lineage marker for AML aquired a little later, higher than CD13 in monocytic forms
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
CD42b	Platelet gycoprotein Ib
CD36	Relatively non-specific (seen in erythroid and monocytic leukaemias) but often strongly expressed

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OTHER MARKERS OF MYELOID LINEAGE
While most cases of AML will be diagnosed using MPO, CD117, CD13 and CD33, some cases may be more difficult, particularly those significant differentation. In those cases supplementary antibodies may be helpful to make a diagnosis or confirm a pattern of differentation. 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.
Granulocytic lineage markers
CD10	One of tne of the first lineage-markers acquired in AML with good relative specificity
CD11b	One of tne of the first lineage-markers acquired in AML with good relative specificity
Monocytic lineage markers
CD11c	One of tne of the first lineage-markers acquired in AML with good relative specificity
CD14	A good marker for AML that is often less strongly expressed in monocytic forms
CD64	A good lineage marker for AML aquired a little later, higher than CD13 in monocytic forms
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
CD42b	Platelet gycoprotein Ib
CD36	Relatively non-specific (seen in erythroid and monocytic leukaemias) but often strongly expressed

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WHEN SHOULD ALTERNATIVE OR AMBIGUOUS LINEAGE BE ASSIGNED?

This occurs if:

• Any myeloid lineage-defining markers are present and 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

OR

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

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Sections

• Markers of "primitive nature": go to section
• Core markers of most cases: go to section
• Aberrant markers often seen: go to section
• Notes 1: specific subtypes: go to section
• Notes 2: ambiguous lineage go to section

(note: for further information on any marker or section click the blue highlighted text)

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OTHER MARKERS OF MYELOID LINEAGE
While most cases of AML will be diagnosed using MPO, CD117, CD13 and CD33, some cases may be more difficult, particularly those significant differentation. In those cases supplementary antibodies may be helpful to make a diagnosis or confirm a pattern of differentation. 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.
Granulocytic lineage markers
CD10	One of tne of the first lineage-markers acquired in AML with good relative specificity
CD11b	One of tne of the first lineage-markers acquired in AML with good relative specificity
Monocytic lineage markers
CD11c	One of tne of the first lineage-markers acquired in AML with good relative specificity
CD14	A good marker for AML that is often less strongly expressed in monocytic forms
CD64	A good lineage marker for AML aquired a little later, higher than CD13 in monocytic forms
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
CD42b	Platelet gycoprotein Ib
CD36	Relatively non-specific (seen in erythroid and monocytic leukaemias) but often strongly expressed

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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

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