Differential expression of MHC class II antigens in chronic B-cell disorders

Cells from the peripheral blood of 22 patients with chronic B-cell disorders were examined for the expression of surface MHC class II antigens. We made use of well characterized monoclonal antibodies (McAbs) specific for HLA-DP, -DQ and -DR molecules (B7/21, Tu22, RFDR1 and RFDR2) and of another McAb, RFD1, associated with the class II system. By using indirect immunofluorescence and flow cytometry we found both non-coordination and heterogeneity in expression of MHC class II antigens but generally with a hierarchy of positivity: DR > DQ > DP. This suggests a sequence of gradual acquisition of HLA-D antigens and indicates distinct differences in maturation arrest of the individual cases. However, after treatment with phorbol ester TPA and calcium ionophore A23187, all cases expressed the previously absent molecules indicating that the structural genes for these products remained intact. TPA and A23187 increased both the number of positive cells in most cases and the fluorescence staining intensities of all class II markers including RFD1. Thus, leukaemic cells may express different combinations of class II antigens reflecting: (i) a predetermined order of gradual acquisition of class II molecules; (ii) differences in the stages of maturation arrest; and (iii) in the cases of disordered expression a desynchronized regulation of these markers.