MPAL

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Important Note

MPAL is uncommon (2-3% of acute leukaemia). Flow cytometry provides initial evidence of MPAL. However conclusions may subsequently be modified (e.g. by results of immunohistochemistry, cytogenetics, or genetics). It is important that the provisional nature of flow cytometry assignment of MPAL is acknowledged.

Frequency of MPAL types Not all forms of MPAL occur with the same frequency, the following types are regognised.
MPAL with features of Myeloid and B-lineage	This is the most common MPAL (more than 50% of MPAL cases).
MPAL with features of Myeloid and T-lineage	The second most frequent form (over one third of MPAL cases).
MPAL with combinations: T- and B- +/-myeloid-lineage	These forms are infrequent (around 10% of MPAL cases)
MPAL with defining molecular features	Following further analysis cases may be assigned additionally as: MPAL with t(9;22) (q34.1;q11.2); BCR-ABL1; MPAL with t(v;11q23.3); with KMT2A re-arrangement

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Lineage Assignment
Assignment to a lineage requires that cases meet specific criteria. These are given below (see also the associted notes).

1. Assignment to myeloid lineage This is based on definite evidence of granulocytic maturation (MPO expression) or sufficient evidence to assign monocytic lineage. For full details go to Table: Myeloid lineage assignment (table)

2. Assignment to B-lymphoid lineage This is based on definite evidence of CD19 together with sufficient supporting evidence of B-lymphoid maturation. For full details go to Table: Lymphoid lineage assignment (table)

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Table 1

MYELOID LINEAGE ASSIGNMENT ‎.

Requirements to assign myeloid lineage in MPAL The assignment of myeloid lineage recognises MPO expression to be a defining marker of myeloid lineage. However MPO is expressed only by around 80% of AML cases. The assignment of myeloid lineage can therefore also be made if monocytic lineage can be established, requiring at least 2 of 5 possible lineage markers to be detected (although only 3 of these can be established by flow cytometry).
Marker option 1
Demonstrate expression of Myeloperoxidase (MPO)	Myeloperoxidase expression alone may be sufficient to establish myeloid lineage, but be aware of the limitations: (1) intensity should be at least half of that of mature neutrophils in at least of proportion of cells measured by the same method; (2) there are circumstances where judgement is required (see notes).
Marker option 2
Demonstrate clear evidence of monocytic lineage	If MPO is not demonstrated then myeloid lineage may still be assigned through demonstration of monocytic features. This can be assigned by the detection of at least two of the following features: By flow cytometry: CD11c, CD14, CD64; by other approaches: lysozyme or non-specific esterase in malignant cells (enzyme cytochemistry)

Notes on interpretation of MPO positivity in MPAL
The WHO classification advises that MPO expression is assessed by its intensity of expression and includes some flexibility in interpretation. The is based on several considerations:
(1) The techniques used to detect MPO do not have the same sensitivity: immunohistochemistry > flow cytometry > enzyme-cytochemistry. This means diagnosis of MPAL may be method dependent – expressing MPO as intensity allows expression to be compared with that of normal cells detected using the same method.
(2) MPO is not fully specific for myeloid lineage in all cases: this is particularly the case when expression is uniform and dim, so an element of subjectivity is present when interpreting (particularly when detected by more sensitive techniques such as immunocytochemistry).
(3) Context of MPO may be important: Applying a threshold of >10% cells being positive for MPO may improve specificity. Detecting variability of expression level of MPO by blast cells may suggest partial maturation and support myeloid lineage origin (often together with variable light scatter). The presence of other myeloid markers may provide greater confidence that MPO is lineage specific. Similarly, be aware of common patterns of aberrancy that are associated with specific alternative diagnoses: Dim (weak) expression of MPO may be a feature of otherwise typical B-LL/LBL; Burkitt-like entities may have strong MPO staining however other investigations will confirm their nature and other myeloid markers will be absent.

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Table 2

LYMPHOID LINEAGE ASSIGNMENT ‎‎.

Requirements to assign B-lymphoid lineage in MPAL For B-lymphoid lineage assignment the key lineage-marker is CD19. However, this marker has recognised expression in many cases of otherwise typical AML cases so additional criteria of expression intensity it is required that other markers must be expressed in addition to allow B-lineage assignment. Rarely CD19 is absent in which case a laregr number o other B-cell markers is required.
Marker option 1
Required Strong expression of CD19 Required also ONE of either: CD10, CD22, or CD79a (surface or cytoplasmic)	To meet the definition of "strong" the expression, the intensity of CD19 must exceed that of 50% of normal B lymphocytes
Marker option 2
Required Weak expression of CD19 Required also TWO of either: CD10, CD22, or CD79a (surface or cytoplasmic)	To meet the definition of "weak" the expression intensity of CD19 must be less than that of 50% of normal B lymphocytes
Marker option 3
CD19 is not expressed In this rare occurrence then requires THREE of either: CD10, CD22, or CD79a (surface or cytoplasmic)	This is a rare occurrence and should be carefully considered

Notes on interpretation of MPO positivity in MPAL
The WHO classification advises that MPO expression is assessed by its intensity of expression and includes some flexibility in interpretation. The is based on several considerations:

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Requirements to assign T-lymphoid lineage in MPAL For T-lymphoid lineage assignment the key lineage-marker is CD3. Provided that this marker meets the intensity criteria then it is T-lineage defining in MPAL. This expression can be determined either by CD3 detection by flow cytometry OR by immunocytochemistry on trephine.
Marker requirement
Either CD3 surface or cytoplasm (strong)	To meet the definition of "strong" the expression intensity must exceed that of 50% of normal T lymphocytes
Or CD3 by immunocytochemistry (strong)	For immunohistochemistry it is important a non-zeta chain reagent is used