The CD28 costimulation at TCR signaling plays a pivotal role in the regulation of the T cell response. To elucidate the role of T cells in periodontal disease, a system of cell transfer with TCR/CD28-dependent Th1 or Th2 clones was developed in rats. Gingival injection of specific Ag, Actinobacillus actinomycetemcomitans 29-kDa outer membrane protein, and LPS could induce local bone resorption 10 days after the transfer of Ag-specific Th1 clone cells, but not after transfer of Th2 clone cells. Interestingly, the presence of LPS was required not only for the induction of bone resorption but also for Ag-specific IgG2a production. LPS injection elicited the induction of expression of both B7-1 and B7-2 expression on gingival macrophages, which otherwise expressed only MHC class II when animals were injected with Ag alone. The expression of B7 molecules was observed for up to 3 days, which corresponded to the duration of retention of T clone cells in gingival tissues. Either local or systemic administration of CTLA4Ig, a functional antagonist of CD28 binding to B7, could abrogate the bone resorption induced by Th1 clone cells combined with gingival challenge with both Ag and LPS. These results suggest that local Ag-specific activation of Th1-type T cells by B7 costimulation appeared to trigger inflammatory bone resorption, whereas inhibition of B7 expression by CTLA4Ig might be a therapeutic approach for intervention with inflammatory bone resorption.

Erythropoietin (EPO) promotes viability, proliferation and differentiation of mammalian erythroid progenitor cells via its specific cell surface receptor (EPO-R). We have previously shown that truncated EPO-Rs containing 267 amino acids or less were defective in internalization of 125 I-EPO, whereas internalization via a receptor derivative containing 276 amino acids was unaffected, thus directing focus to the nine amino acid residues FEGLFTTHK at positions 268^276 [Levin, Cohen, Supino, Yoshimura, Watowich, Neumann, FEBS Lett. 427 (1998) 164^170]. Here, a panel of EPO-R mutants was generated to determine the role of these residues in EPO endocytosis, down regulation of cell surface receptors and EPOmediated signaling. While linking amino acid residues 268^276 to a truncated EPO-R (v v+9 EPO-R) conferred both ligand uptake and ligand-independent down regulation of the respective receptor from the cell surface, Phe 272 was crucial for EPO endocytosis but not for ligand-independent down regulation. Additional receptor motifs probably play a role in EPO endocytosis and receptor down-regulation, as these processes were not adversely impaired in v v268^276 EPO-R. A central role of residues 268^276, in particular Phe, was demonstrated by the inability of v v268^276 and F268,272A EPO-Rs to support EPOmediated signal transduction. ß 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.