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 ß-
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 (3- galactoside-α-2,6- sialyltransferase