Surface of human sperm bears three differently charged CD52 forms, two of which remain stably bound to sperm after capacitation

Gp20 is a sialoglycoprotein of the human sperm surface with a core peptide homologous to the leukocyte antigen CD52, a GPI-anchored glycosylated prote in which is described by the monoclonal antibody CAMPATH-1. Comparative ana lyses, by means of CAMPATH and anti-gp20, indicated that they describe it i n morphologically and functionally different ways, suggesting that the resp ective epitopes are different but also casting doubt on the immunological i dentity of the antigen. In the present study, we used immunodepletion to de monstrate that CAMPATH and anti-gp20 interact with the same antigen, but th at anti-gp20 has a much higher avidity for the antigen than CAMPATH. Anion exchange fractionation analysis of the antigen revealed three differently c harged gp20-CD52 forms, the least charged of which, was largely without a G PI-anchor. All three forms were associated with freshly ejaculated sperm, w hereas capacitated sperm only contained the two GPI-anchored, more charged forms, which were also the ones found in the prostasome fraction of seminal plasma and in leukocytes. The two charged, GPI-anchored forms were describ ed as homogeneous by anti-gp20, since they ran as a singlet; the third form ran as a doublet. When tested for insertion into Jurkat T cells, the mediu m charged form inserted the most readily and the less charged one could not be inserted at all.