Relevance of Monoclonal Antibodies in the Diagnosis of Unusual T-Cell Acute Lymphoblastic Leukaemia

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Abstract
Three patients with adult acute lymphoblastic leukemia (ALL), the T-cell nature of which was identified only using a panel of monoclonal antibodies (MoAb), are described. All cases were E[erythrocyte]-rosette negative, surface Ig (SmIg) negative, common ALL (CALLA) antigen negative, terminal deoxynucleotidyl transferase (TdT) positive and acid phosphatase positive. The T-cell origin of the blasts was demonstrated by the positivity with RFA-1, a MoAb which detects an antigen of MW 65-69,000 present on the membrane of thymocytes and mature T-lymphocytes. Of the 3 cases, 2 were positive with OKT6, which recognizes cortical thymocytes. MoAb directed against more mature T lineage cells (OKT3, OKT4, OKT8, OKT11A) were consistently negative (.ltoreq. 12%). Use of a combination of MoAb is important in detecting individual cases of T-ALL, which otherwise might be classified as undifferentiated acute leukemia or null-ALL. MoAb detecting a T-cell antigenic determinant of MW 65-69,000 (e.g., RFA-1, OKT1, Leu1) appear to be the most specific reagents for T-ALL.