Immunophenotypic analysis of T-acute lymphoblastic leukemia. A CD5-based ETP-ALL perspective of non-ETP T-ALL

T‐cell antigens [CD5,CD1a,CD8] define early T‐cell precursor acute lymphoblastic leukemia (ETP‐ALL). To understand immature T‐ALL of which ETP‐ALL is part, we used these antigens to subcategorize non‐ETP T‐ALL for examining expression of myeloid/stem cell antigens (M/S) and clinical features. Using CD5 (+/−) to start categorization, we studied 69 routinely immunophenotyped patients with T‐ALL. CD5− was a homogenous (CD8,CD1a)− M/S+ ETP‐ALL group (n = 9). CD5+ cases were (CD8,CD1a)− pre‐T‐ALL (n = 22) or (CD8,CD1a)+ (n = 38) thymic/cortical T‐ALL; M/S+ 20/22 (90.91%) in former and 22/38 (57.89%) in latter (P = 0.007). ETP‐ and pre‐T‐ALL together (CD1a−,CD5−/+ immature T‐ALL group) were nearly always M/S+ (29/31; 93.55%). In multivariate analysis, only ETP‐ALL predicted poor overall survival (P = 0.02). We conclude (i) CD5 negativity in T‐ALL almost always means ETP‐ALL. CD1a and CD8 negativity, as much as CD5, marks immaturity in T‐ALL, and the CD5+/−/CD1a−/CD8− immature T‐ALL group needs further study to understand the biology of the T‐ALL–myeloid interface. (ii) ETP‐ALL patients may be pre‐T‐ALL if CD2+; CD2+, conversely, CD5−/CD1a−/CD8− pre‐T ALL patients are ETP‐ALL. (iii) Immunophenotypic workup of T‐ALL must not omit CD1a, CD5, CD8 and CD2, and positivity of antigens should preferably be defined as recommended for ETP‐ALL, so that this entity can be better evaluated in future studies of immature T‐ALL, a group to which ETP‐ALL belongs. (iv) ETP‐ALL has poor prognosis.