Determination of myeloid antigen expression on childhood acute lymphoblastic leukaemia cells: Discrepancies using different monoclonal antibody clones

Prospective clinical studies including large numbers of patients have led t o the conclusion that co-expression of myeloid antigens in childhood acute lymphoblastic leukaemia (My+ ALL) does not have prognostic significance. Ho wever, reports of the frequency of My+ ALL in children vary widely across l aboratories using different mAb clones and staining and analysing procedure s. Taking two commonly accepted thresholds of positivity for myeloid antige ns (20 and 30%), we analysed the immunoreactivity of the most widely employ ed mAb clones against CD13 (SJ1D1, L138 and My7) and CD33 (My9, P67.6 and D 3HL60) and compared the proportions of My+ ALL detected by these clones in childhood ALL. The correlation between myeloid antigen expression and the p resence of the t(12;21) translocation was analysed concomitantly in the sam e samples. The percentage of ALL cases positive for myeloid markers varied significant ly depending on the mAb clone and the positive threshold. Among patients wi th B-ALL, the proportion of CD13+ ALL was significantly lower using SJ1D1 t han using L138 or My7, while the proportion of CD33+ ALL was significantly higher for My9 than for P67.6 or D3HL60. Analysis of the co-expression of C D13 and CD33 on B-ALL cells using combinations of mAb clones showed that th is frequency was either underestimated by the SJ1D1/D3HL60 or overestimated by the L138/P67.6 and My7/My9 combinations. A correlation between CD13/CD3 3 positivity and the t(12;21) translocation was uniformly observed in B-ALL patients for a positive threshold of 30%, whereas SJ1D1/D3HL60 detected no correlation between t(12;21) and CD13/CD33 positivity when the threshold w as lowered to 20%. These data show that the mAb clones commonly used to det ect the CD13 and CD33 surface antigens have variable immunoreactivity again st childhood ALL cells, which may partly explain the conflicting reports co ncerning the prognostic significance of myeloid antigen expression in paedi atric ALL and its association with different translocations. The present fi ndings may also be of clinical importance for therapeutic choices.