REGIONS OF THE CD8 MOLECULE INVOLVED IN THE REGULATION OF CD2-MEDIATED ACTIVATION

The CD8 molecule regulates T cell activation mediated via the CD3 T ce ll receptor and the adhesion molecule CD2. CD8 mAbs have been found to inhibit early (Ca2+ rise) as well as late events (cytotoxicity, proli feration, and lymphokine secretion) mediated via the CD2 pathway. A pa nel of eight anti-human CD8 mAbs was tested for inhibition of CD2-medi ated Ca2+ rise in a cytotoxic T cell clone. The inhibition ranged from 5 to 53% independently of mAb isotype and affinity measured by half s aturation binding. We then characterized these mAbs for their reactivi ty toward three mutants of the human CD8 alpha carrying amino acid seq uence changes in the surface exposed loops homologous to the immunoglo bulin CDR1, 2, and 3. The mutations included replacement of the human CD8 alpha CDR1- and CDR2-like loops by the homologous mouse sequences and the insertion of a glycine in the middle of the CDR3-like loop. Th us, five mAbs were found to be affected by the mutation in the CDR2-li ke loop but not by alterations in the other CDR-like loops. Conversely , the other two mAbs (8E1.7 and B9.8) were affected only by mutations in the CDR1- and CDR3-like loops, respectively. Cross-inhibition exper iments were essentially in agreement with these results. interestingly , all the mAbs directed against the CDR2-like loop were potent inhibit ors of CD2-mediated Ca2+ rise, with one exception probably due to poor affinity. Thus, in addition to being a site of interaction with major histocompatibility complex Class I as recent data have indicated, thi s region of the CD8 alpha subunit may play a role in regulating T cell activation. (C) 1994 Academic Press, Inc.