Flow Cytometry:B-lymphoid lineage assignment in MPAL

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Requirements to assign B-lymphoid lineage 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)
Marker option 2
Required Weak expression of CD19 Required also TWO of either: CD10, CD22, or CD79a (surface or cytoplasmic)
Marker option 3
CD19 is not expressed In this rare occurrence then requires THREE of either: CD10, CD22, or CD79a (surface or cytoplasmic)

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Notes on interpretation of B-lineage
CD19 is considered by WHO to have high but not complete specificity for B-lineage. Its’ use requires the intensity of expression to be considered: “strong” CD19 expression should exceed 50% of the level seen on normal B progenitors in at least some of the leukaemic cells, while weaker CD19 expression has lower specificity and requires stronger additional evidence of B-lineage commitment (see Table above). Note also: (1) CD79a has frequent expression in T-LL/LBL, so should not be used to suggest B-lineage if that disorder is considered. (2) PAX5 identified by immunohistochemistry has a low evidence base in MPAL diagnosis so should be interpreted with caustion. (3) In the absence of CD19 expression, B-lineage may still be assigned if three other B antigens are expressed.

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