THE DOWN-MODULATION OF CD4 INDUCED BY THE GM1 GANGLIOSIDE IS REGULATED BY PHOSPHATASES AND KINASES - EVIDENCE FROM ENZYME-INHIBITORS AND ANTI-CD45 ANTIBODIES

The involvement of protein kinases and phosphatases in the down-modula tion of expression of CD4 molecules on peripheral blood lymphocytes (P BL) by gangliosides was studied. Exposure of PBL either to genistein o r to H7 practically abolished the downmodulation of CD4 induced by GM1 and diminished their susceptibility to CD4(+) down-modulation by expo sure to GD1a. Staurosporine had no effect on the down-modulation of CD 4 by either GM1 or GD1a. Orthovanadate treatment drastically inhibited the down-regulation of CD4 induced by GM1 but had no effect on down-m odulation of CD4 induced by GD1a. Exposure to monoclonal antibodies (m Abs) against CD45 and CD45RA but not against CD45RO abrogated the down -modulation of CD4 by GM1. The down-modulation of CD4 elicited by GD1a , GD1b, or GT1b was not inhibited by anti-CD45RA and anti-CD45RO mAbs. MAbs against CD3, CD2, or HLA-DR had no effect on the GM1-induced dow n-modulation of CD4. In view of the differences obtained between GM1 a nd GD1a it was of interest to check whether these gangliosides compete d for cellular binding sites. When PBL were first treated with anti-CD 45RA and GM1 or with orthovanadate and GM1, which had a negligible eff ect on CD4 expression, and subsequently treated with GD1a the expressi on of CD4 was down-modulated. This demonstrated that GD1a binds to sit es on the cell membrane to which GM1 does not bind. The present study indicates that the capacity of GM1 to downmodulate CD4 depended on the CD45 and particularly CD45RA molecules, while other gangliosides may utilize different cell surface structures to down-modulate the express ion of CD4. (C) 1998 Academic Press.