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Evaluation of two sources of soluble HL‐A antigens: platelets and serum

Platelets and human serum have been evaluated as sources for extraction of soluble HL‐A antigens. 3 M KCl was found to efficiently solubilize HL‐A antigens from platelets with a recovery ranging between 50 and 100 %. However, because of the low density of HL‐A determinants on platelets, the yield of soluble antigens is low, as only 1 mg of protein can be recovered from 1 × 109 platelets. Thus, while it is difficult to solubilize sufficient antigens from platelets for chemical characterization, it is possible to use these materials for biological applications such as pretreatment of kidney recipients. Soluble HL‐A antigen is present in human serum, since following extensive ultracentrifugation, it can still effectively inhibit the cytotoxic activity of HL‐A alloantisera. The same HL‐A specificities were found to be present in varying amounts among different donors. Partial purification of such antigens can be achieved by ion‐exchange chromatography of serum on QAE‐Sephadex.