Monoclonal antibody anti-MPO is useful in recognizing minimally differentiated acute myeloid leukaemia.

The enzyme myeloperoxidase (MPO) is the most specific marker of myeloid lineage. The recognition of acute myeloid leukaemia (AML) with minimally differentiation (AML-M0) is established with methods that include myeloid markers CD13/CD33 and detection of MPO in blast cells by immunological techniques or electron microscopy cytochemistry (EM). We have analysed the presence of MPO in leukaemic blast cells by conventional cytochemistry and immunological methods using a monoclonal antibody anti-MPO (CLB-MPO1) in 121 cases of acute leukaemia. The aim of the study was to investigate the sensitivity of this McAb to identify AML-M0, as CD13/CD33 can be expressed in some cases of acute lymphoblastic leukaemia (ALL) and EM cytochemistry is not always available in many laboratories. Anti-MPO was positive in all cases of AML (M1-M5) which were positive by Sudan Black B reaction in similar or higher percentage ratio for each case, although in some of them did not label with CD13/CD33 tested by IF and IPc techniques. Based on the anti-MPO positivity, 5 out of 10 cases called undifferentiated leukaemia (AUL) were reclassified as AML-M0, though 4 cases were CD13/CD33 negative. Furthermore, after analysing the anti-MPO expression among 32 cases of ALL, we had to reclassify four of them as acute biphenotypic leukaemia. We conclude that anti-MPO is a very sensitive and reliable tool in AML diagnosis and has an important role in distinguishing minimally differentiated AML and biphenotypic acute leukaemia from AUL and ALL.

Transcription of unrearranged Ig H chain genes in human B cell malignancies. Biased expression of genes encoded within the first duplication unit of the Ig H chain locus.

Analysis of the H chain class switch recombination events on the productive and nonproductive alleles of several human B cell lines has demonstrated that, in the majority of cases, the two loci had rearranged different CH genes. The absence of a strict correlation between the two rearrangement events has been interpreted in favor of a stochastic switching mechanism in human B cells. Based on the accessibility model for the H chain class switch, we have considered an alternative explanation, namely, that transcriptional activation of more than one CH gene could render more than one isotype accessible for recombination. In order to test this possibility, we have stimulated a number of IgM-bearing B cell tumors and analyzed the transcriptional response of their unrearranged C gamma and C alpha H chain genes at the Ig class level by Northern hybridization and at the subclass level by RNA-RNA solution hybridization. Our data show that human clonal B cell populations can simultaneously express germ-line transcripts of both gamma and alpha genes. Our data also demonstrate a bias in the expression of the different isotypes in the B cell chronic lymphocytic leukemias analyzed, with the genes present with the first duplication unit of the H chain locus (gamma 3, gamma 1, and alpha 1) being expressed almost exclusively. These findings indicate that transcriptional activation of large chromosomal regions containing more than one CH gene could render all the genes available for recombination and, therefore, provide an explanation for the asymmetry in the rearrangement events that have previously been described in human B cell tumors. The possible relevance of our findings to the molecular mechanism of the H chain class switch is discussed.