Purification and chemical characterisation of membrane glycoproteins from rat thymocytes and brain, recognised by monoclonal antibody MRC OX 2

The MRC OX 2 monoclonal antibody recognises antigens present on rat thymocytes, brain, follicular dendritic cells in lymphoid organs, vascular endothelium, some smooth muscle and B-lymphocytes. The OX 2 antigens recognised by this antibody were purified from brain and thymus, by solubilisation with sodium deoxycholate, affinity chromatography with MRC OX 2 antibody and gel filtration. The purified brain and thymocyte OX 2 antigens were glycoproteins with apparent Mr 41000 and 47000 respectively as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. Rabbit antisera raised against the purified antigens were analysed by radioimmunoassay and immunoperoxidase-staining of tissue sections. The brain and thymocyte OX 2 antigens were antigenically very similar to those on the other tissues. This indicates that the unusual pattern of distribution was not the result of fortuitous cross-reaction of the MRC OX 2 antibody, as the rabbit sera would be expected to recognise more determinants on the antigen than that recognised by the monoclonal antibody. The amino acid compositions of brain and thymus OX 2 antigens were very similar but with no distinguishing features. Carbohydrate compositions showed that the OX 2 antigens were highly glycosylated, with brain OX 2 antigen containing 24% and thymocyte OX 2 antigen 33% by weight of carbohydrate. Both OX 2 antigens contained carbohydrate residues typical of structures N-linked to asparagine but lacked galactosamine, indicating the absence of O-linked structures. Thymocyte OX 2 contained higher levels of galactose and sialic acid but less fucose than brain OX 2. Similar differences had been observed for brain and thymocyte Thy-1 antigens and were also observed in pooled glycoproteins purified by lentil affinity chromatography from these tissues, reflecting overall differences in the patterns of glycosylation in the two tissues. The OX 2 antigens showed many similarities to Thy-1 antigens in their odd patterns of distribution, characteristic migration on polyacrylamide gels in sodium dodecyl sulphate, and carbohydrate compositions. It is possible that OX 2 antigens, like Thy-1 antigens, have homologies with immunoglobulin domains. A possible role for OX 2 antigens in cell interactions necessary for tissue organisation is discussed.