Immunologically Defined Subclasses of Acute Lymphoblastic Leukaemia in Children: their Relationship to Presentation Features and Prognosis

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Abstract
Leukemic cells from 542 patients under 21 yr of age with acute lymphoblastic leukemia (ALL) were typed with immunological cell surface markers; 379 of these patients were followed for > 1 yr. They were divided into 4 subgroups: common (c) ALL, T (thymic) ALL, null (or unclassified) ALL and a rare lymphoma/leukemia type B-ALL. A T cell phenotype was found more frequently in boys and was usually associated with a high white cell [WBC] count at presentation. A mediastinal thymic mass was present in 53% of T-ALL patients but was not observed in any unequivocal non-T ALL. Clinical prognosis differed substantially between the 3 major phenotypic classes, remission induction rate and remission duration being lowest in T-ALL, better in null ALL and highest in cALL. There was a much higher incidence of CNS involvement in the T-ALL group than in the cALL group or null ALL group and although this was strongly correlated with WBC count it was also significantly associated with T-ALL independent of WBC count. There is a strong correlation between high WBC count and poor clinical response (remission induction and duration). When the 3 major immunological subclasses are adjusted for WBC count, the prognostic correlation of antigenic phenotype is reduced and statistically insignificant. Immunological (and enzymatic) phenotypic of ALL subclasses may not be in independent correlate of prognosis but nevertheless is linked to other cell differentiation features, especially growth rate and sites of clonal expansion (e.g., marrow vs thymus), which critically influence the size of the clonogenic leukemic population and its associated evolutionary status with respect to drug resistant mutants at diagnosis and introduction of therapy.