| Summary: | Using monoclonal antibodies (mAbs) characterised at the Fourth International Leucocyte Typing Workshop, the expression and distribution of the cell-surface antigen CDw75 on normal peripheral blood lymphocytes and on cells from patients with B-cell chronic lymphocytic leukaemia (B-CLL) was studied and further defined. In addition, expression of the antigen on solid lymphoid and non-lymphoid tissues was studied, and a comparison was made between expression on normal tissues and on tumour tissues. The structure of the epitopes recognised by the different CDw75 mAbs was also analysed, and several attempts were made to isolate and characterise the antigen biochemically. The CDw75 antigen had been reported to be a cell-surface β-galactoside-α-2,6-sialyltransferase enzyme, the amino acid sequence of which had already been determined. From this amino acid sequence, synthetic peptides were constructed and mAbs were raised against sialyltransferase. These mAbs were used to isolate the enzyme and characterise it biochemically. The results of this study show that CDw75 is expressed primarily on the surface of mature B-cells and on leukaemias and lymphomas. It is also expressed on T-cell subsets, and different epitopes are expressed on T-cells at different activation states. CDw75 is also expressed in B-cell areas of lymphoid tissues as well as on non-lymphoid tissues such as liver, breast, pancreas and intestine. Expression levels are altered in tumour tissues. The CDw75 mAbs do not recognise a single epitope. Some recognise a sialylated epitope, whereas one epitope is masked by sialic acid. The α-2,6-sialyltransferase enzyme is expressed in a perinuclear area of the cell, likely to be the Golgi Apparatus, and is not expressed on the surface of B-cells. The CDw75 antigen is not a cell-surface sialyltransferase enzyme, but is a cell-surface carbohydrate epitope whose expression is regulated by the activity of the β-galactoside-α-2,6-sialyltransferase enzyme.
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