TLineage ALL Subtypes

The incidence of T-lineage ALL is lower in children (about 15 percent) than in adults (20 to 25 percent). Nowadays, patients with T-cell ALL fare better than those with B-cel! ALL.31,317,322,336-340 The use of cytosine arabinoside and cyclophosphamide may account for improved treatment results in T-ALL, owing to the the ability of T lymphoblasts to accumulate high levels of cytosine arabi-noside triphosphate.341 Subgroups of T-lineage ALL are commonly defined based on the number of T markers present (maturational stage) and the selective expression of antigens such as CD1 and CD10 (Table 15-4). Significant associations between cytogenetic abnormalities and immunophenotypic T-lineage characteristics or outcome are less pronounced in T-lineage than in B-lineage ALL.334,340,342,343

Pro-T, Pro-Thymocyte T, Pre-T, or stage T1A ALL represents the most immature stage of T-lineage ALL and is characterized by the absence of CD2. The diagnosis of T-lineage ALL derives from the presence of intracytoplasmic CD3 (cCD3) and surface expression of CD7, with or without CD5. This subgroup has shorter disease-free and overall survival when compared with patients with more mature T-lineage ALL.281'282'317'339'340 CD2-negative T-ALL may be associated with a mediastinal mass and lym-phadenopathy in adults,281 although unfavorable presenting features were not seen at an increased frequency in children.340 However, after adjusting for known risk criteria, CD2 negativity still remained a prognostic factor for event-free survival in T-lineage ALL.282

The Immature, Early Thymocyte, Early T ALL, stage T1B ALL, is characterized by the expression of CD2 in addition to CD5 and CD7. The CD2+CD10- T-ALL subtype correlates with disruption of the TAL-1, also termed SCL, gene, which occurs in a large percentage of patients with T-ALL.344>345

The Cortical T, Common Thymocyte, or stage TuALL demonstrates expression of the thymocyte marker CD1. With respect to membrane CD3, CD4, and CD8 expression, there is large variability within this subtype. CD1 positivity carried prognostic significance for survival in some clinical trials.31,280 CD10 can be frequently found on the surface of lymphoblasts from this T-ALL subtype and confers a favorable prognosis in chil-dren30 as well as adults.31,346 Although the biologic basis for this observation is unclear, it is of interest that in normal lymphopoiesis CD10 marks T lymphocytes destined for apopto-sis32,33 Expression of CD34 in T-lineage ALL, which occurs much less frequently than in B-lineage ALL, correlates with CD10 negativity and CNS disease at presentation140,347 and with delayed achievement of complete remission.347 HLA-DR, another stem-cell antigen, can be detected in fewer than 20 percent of T-ALL cases. There exist conflicting conclusions regarding its prognostic significance.31,346

Mature T, Late T, Medullary Thymocyte, T or stage TlnALL may lack TdT expression. Typically, CD3 and TCR proteins are present in the cell membrane, whereas in earlier stages TCR proteins may be found in the cytoplasm. The bulk of membrane CD3-positive T-ALLs are positive for either CD4 or CD8 and express the classical TCR-a/p, reflecting the maturation stage of the majority of circulating T cells and mature thymocytes. TCRyS+ T-cell ALL, which may or may not be doubly negative for CD4 and CD8, occurs in approximately 10 percent of T-lineage ALL or about half of CD3,TCR+ T-ALL cases.276,277,348-350 Thus, in TALL expressing TCR proteins, the TCRyS is used much more frequently than by thymocytes or circulating T cells. TCRy8+ T-ALL appears as a distinct subgroup in children, with better event-free survival when compared with TCRaP+ disease.276

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