Different stages of B cell differentiation in non-T acute lymphoblastic leukemia.

Immunoglobulin heavy chain gene rearrangement was evaluated in 19 cases of acute lymphoblastic leukemia (ALL) and correlated with the immunological phenotypic expression on primary or phorbol diester (12-O-tetradecanoylphorbol-13-acetate [TPA])-induced cells. One case of common ALL (cALL), one case of T-ALL, and one undifferentiated acute leukemia that responded to anti-myeloid drugs after unsuccessful anti-lymphoid induction therapy, had germ line heavy chain genes. Rearranged immunoglobulin genes were instead found in 15 of the 16 cALL cases studied and in a case of non-T, non-B, non-common ("null") ALL, which suggested the B cell origin of the neoplastic cells. All cases bearing a heavy chain gene rearrangement were HLA-DR positive. However, the unique cALL case with a germ line configuration was also HLA-DR positive, which confirmed that both the cALL antigen and HLA-DR antigen were not per se expression of B cell commitment. On the other hand, a complete search for B cell-related markers (BA-1 and B1 monoclonal antibodies, as well as cytoplasmic immunoglobulins [CyIg]) in the cALL cases showed that at least one B cell marker could be detected either on primary or on TPA-induced cells in all cases in which a gene rearrangement had occurred. Incubation with TPA allowed the detection of one B cell marker in a case in which the primary cells were negative, and increased the expression of B cell markers in all but one of the cALLs tested. The only cALL case that was not rearranged expressed no B cell markers either on primary or on TPA-induced cells. The non-T, non-B, non-common ("null") case that was rearranged also showed no phenotypic evidence of B cell markers on primary and induced cells. These findings indicate that: (a) practically all cases of cALL appear to be of B cell origin as shown by gene rearrangement analysis; (b) DNA studies are relevant for a more precise characterization of individual cases of undifferentiated acute leukemia; (c) a complete survey for B cell markers may establish the B cell origin of the cALL blasts, as long as the analysis on primary cells is complemented by differentiation induction assessment; and (d) most cases of non-T ALL appear to be characterized by the expansion of neoplastic cells "frozen" at different levels along the B cell differentiation pathway, the first detectable marker being heavy chain gene rearrangement, followed by BA-1, B1, and CyIg expression.

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