Abstract
The poor clinical response of 11q23-positive [also known as mixed lineage leukemia (MLL)] acute leukemia (AL) to chemotherapy-containing regimens has stimulated interest in developing alternative therapeutic strategies. Among them is immunotherapy. Since no leukemia-specific antigen has been identified in 11q23-positive AL, we are developing an antibody-based immunotherapeutic strategy which targets the HMW-MAA. This antigen, which is a membrane bound proteoglycan, represents an attractive target because of its high expression on the surface of 11q23-positive AL blasts and its restricted distribution in normal tissues. Taking advantage of a unique panel of monoclonal antibodies (mAb) recognizing 7 distinct and spatially distant antigenic determinants, we have analyzed the antigenic profile of HMW-MAA by flow cytometry in samples from 15 adult and 14 pediatric patients with 11q23-positive AL. Our results demonstrate a differential expression of the HMW-MAA antigenic determinants and that their expression pattern correlates with cytogenetic subgroups. Specifically, all the determinants were expressed on 6 adult samples [3 t(11;19)(q23;p13), 2 t(4;11)(q21;q23), and 1 t(10;11)(p12;q23)]. In contrast only 3 determinants were detected on 8 adult samples [3 t(9;11)(p22;q23), and 1 each with t(6;11)(q27;q23), t(11;12)(q23;q13), t(11;14)(q23;p11.2), inv(11)(q21q23.2) and add(11)(q23)]. No antigenic determinant was detected on leukemic cells from the adult patient with t(2;11)(p21;q23). Interestingly, the antigenic profile of HMW-MAA expressed on leukemic cells from pediatric patients was different, since all the determinants were expressed on leukemic cells from 6 t(4;11), 1 t(9;11), 1 t(11;1)(1;13;9)(q23;q25p34;q14.3;p13), 2 t(11;19) and 1 del(11)(q14q23). On the other hand no determinant was detectable on the leukemic cells from 3 children [1 with both t(1;11)(p32;q23) and t(4;11), 1 inv(11)(p15q23) and 1 add(11)(q23)]. Whether the difference in the antigenic profile of HMW-MAA expressed by adult and pediatric 11q23-positive AL cells reflects the different pathogenesis of AL in adults and children remains to be determined. Our data show that the differential expression of antigenic determinants of HMW-MAA on 11q23-positive AL cells does not reflect structural differences in the HMW-MAA expressed by various types of 11q23-positive AL as indicated by the results of Western blotting analysis. Further, the differential expression does not correlate with MLL gene rearrangement, since fluorescent in-situ hybridization (FISH) performed on 15 adult samples detected MLL gene rearrangement in 4 of the 6 samples that express all the determinants and in 6 of the 8 samples that express only 3 determinants. In addition, the pediatric sample with inv(11) that does not express any determinant, has MLL gene rearrangement by FISH. Finally, the differential expression does not correlate with the presence of MLL partial tandem duplication, since it was detected in 1 sample that expresses all the antigenic determinants and in 2 samples that express only 3 determinants. These findings emphasize the need to use more than one HMW-MAA-specific mAb to phenotype 11q23-positive AL and to select patients to be treated with HMW-MAA-specific antibody-based immunotherapy.
MLL AT-hook sequence is strongly conserved in infant acute leukemia with or without MLL gene rearrangement
