Significance of Phi bodies in acute leukaemia

Material from 39 patients with acute leukaemia was investigated with the peroxidase cytochemical reaction using 3,3'diaminobenzidine (DAB) and other substrates in order to test their sensitivity in detecting myeloid differentiation. The proportion of positive blasts and of cases with Auer rods in acute myeloid leukaemia (AML) was significantly greater with DAB than with benzidine. In addition, Phi bodies were demonstrated in AML blasts only when DAB was used; Phi bodies were also observed in two out of seven cases of chronic granulocytic leukaemia in "myeloid" blast crisis but were not seen in any case of acute lymphoblastic leukaemia. Phi bodies were more numerous when the reaction was carried out at pH 9.7, and their number was significantly reduced in the presence of 3-amino 1,2,4-triazole. Both findings suggest that the Phi bodies derive from catalase-containing granules (microperoxisomes) and are distinct from Auer rods, which derive from peroxidase-containing (primary) granules. Like Auer rods, Phi bodies appear to be characteristics of immature myeloid cells in leukaemia but are seen with a higher frequency than Auer rods in acute myeloid leukemia.