Differentiation Linked Expression of p28,33 (Ia-like) Structures on Human Leukaemic Cells

S ummary . A differentiation linked expression of 'Ia-like' (B cell associated) structures was detected by heterologous antisera against p28,33 antigen on human leukaemic cells in immunofluorescence test. A double fluorochrome immunofluorescence test was used to distinguish Ia⁺ ( p28,33 , ⁺) leukaemic cells from B lymphocytes. Leukaemic cells with varying intensities of anti-p28,33 staining were separated using the Fluorescence Activated Cell Sorter. The results indicate that p28,33 is very strongly expressed on undifferentiated blast cells from Philadelphia chromosome (Ph¹) positive 'lymphoid' blast crisis and non-T,non-B acute lymphoid leukaemia. In Ph¹ positive myeloid blast crisis and in acute myeloid leukaemia the p28,33 antigen was only moderately strongly expressed on myeloblasts and virtually absent on promyelocytes. The p28,33 antigen was also absent on the relatively more mature cells during the chronic phase of chronic myeloid leukaemia. Non-leukaemic samples such as bone marrow samples from fetus, and from patients with severe megaloblastic anaemia or myeloid hyperplasia (due to hypersplenism in Felty syndrome) were also found to contain p28,33 positive (and membrane immunoglobulin negative) immature blast cells.
On the basis of these observations the hypothesis is proposed that 'Ia-like' antigens on leukaemic cells reflect the origin of these leukaemias from haemopoietic stem cells which are themselves Ia positive. The corollary of this view is that 'Ia-like' membrane structures may play an important role not only in immunological responses of B cell, but also in the regulation of early haemopoietic cell differentiation.