PAX5 as a diagnostic tool in unconventional acute leukemia: Establishing immunohistochemical evidence of B-cell lineage.

Acute leukemia lineage assignment is critical for clinical management. When CD19 expression is dim by flow cytometry, it becomes challenging to meet criteria for B-lymphoblastic leukemia/lymphoma (B-ALL) or B-lineage in mixed phenotype. While many B-lineage specific markers are available by immunohistochemistry (IHC), they have not been systematically validated. This study highlights the utility of PAX5, BOB.1 and OCT2 in lineage assignment when CD19 is dim, allowing for consideration as additional diagnostic criteria. The databases of two academic institutions were searched for cases: acute undifferentiated leukemia (AUL), acute myeloid leukemia (AML) with minimal differentiation, mixed phenotype acute leukemia (MPAL), and all acute leukemias with weak CD19-expression. Cases were stained with PAX5, BOB.1 and OCT2. The IHCs were scored by intensity and percentage of positive blasts. A total of 55 cases had weak CD19 expression: 18 B-ALL and 37 acute leukemias with weak CD19-expression (14 MPAL). For negative controls, we selected 21 AML with minimal differentiation, 5 T/myeloid cases, 5 near early T-cell precursor (ETP) T-ALLs and 3 ETP T-ALLs. By PAX5 IHC, 18/18 (100 %) B-ALL cases were positive; all minimal differentiation myeloid and T/myeloid leukemias were negative (0/26), and 36/37 (98 %) acute leukemias with weak CD19 were positive. Our multicenter data support PAX5 IHC as a sensitive marker of B-lineage assignment when CD19 is dim to negative, especially in acute leukemias with ambiguous lineage or MPAL. BOB.1 and OCT2 can also be helpful but these demonstrated lower sensitivity. In cases where CD19 is dim, with weak CD79a and/or CD22, PAX5 IHC can "rescue" the lineage assignment.