CD81 expressed on human thymocytes mediates integrin activation and interleukin 2-dependent proliferation.

Lymphocyte recognition of antigen by the antigen-specific T cell receptor (TCR) and coreceptor complexes rapidly alters the cell's adhesive properties facilitating high avidity cell-ligand interactions necessary for lymphocyte development and function. Here, we report the expression of CD81 (target of antiproliferative antigen [TAPA]-1) on human thymocytes and the physical association of CD81 with CD4 and CD8 T cell coreceptors. Antibody ligation of CD81 on thymocytes promotes the rapid induction of integrin-mediated cell-cell adhesion via lymphocyte function-associated molecule-1 (LFA-1). Cross-linking CD81 is also shown to be costimulatory with signaling through the TCR/CD3 complex inducing interleukin 2-dependent thymocyte proliferation. These data suggest that a CD81-mediated pathway in thymocytes is involved in the regulation of both cell adhesion and activation.

Characterization of a 70-kDa, EBV gp350/220-binding protein on HSB-2 T cells.

EBV binds and infects HSB-2 T cells via a receptor distinct from CD21. To further study this novel EBV receptor, we expressed the first 470 amino acids of the EBV-gp350/220 using the baculovirus expression system. The recombinant gp350/220(1-470) has a m.w. of 95 kDa, reacts with anti-gp350/220 Abs, and binds CD21 in ELISA. Radiolabeled gp350/220(1-470) binds both HSB-2 and Raji cells. The gp350/220(1-470) protein also inhibits EBV binding to both HSB-2 and Raji, detected by flow cytometry. Lysates of HSB-2 cells compete with CD21 for binding to gp350/220(1-470), suggesting that the two receptors bind related epitopes on the recombinant protein. Scatchard analysis reveals that gp350/220(1-470) binds to 34,000 high affinity sites/HSB-2 cell (Kd = 0.92 x 10(-8) M) compared with the 97,000 high affinity sites bound/Raji cell (Kd = 1.78 x 10(-8) M). Utilizing a gp350/220(1-470)-affinity matrix, we identify a 70-kDa (55-kDa nonreduced) protein on the surfaces of 125I-labeled HSB-2 cells. Binding of this protein to the matrix is inhibited by anti-gp350/220 Ab 72A1. In summary, we characterize a novel EBV-binding molecule on HSB-2 cells, compare its reactivity with gp350/220 to that of CD21, and provide evidence of a gp350/220-reactive, 70-kDa protein on the surfaces of HSB-2 cells. In view of previous evidence of HSB-2 infectivity by EBV, we propose that the 70 kDa protein represents the novel EBV receptor.

Effects of SP500263, a novel selective estrogen receptor modulator, on bone, uterus, and serum cholesterol in the ovariectomized rat.

We describe here the activity of a novel selective estrogen receptor modulator, SP500263. When given to adult ovariectomized (OVX) rats for 28 days at doses of 0.3, 1, or 3 mg/kg/day, we found that SP500263 partially protected against OVX-induced loss of bone mineral content in the distal ends of femurs and in the whole bone. SP500263 also antagonized the OVX-induced increase in body weight. However, unlike 17beta-estradiol, SP500263 at efficacious doses did not prevent the OVX-induced loss in uterine wet weight. A small but significant effect on uterine wet weight was noted with raloxifene dosed at 1 mg/kg. As expected, SP500263 but not raloxifene acted as an estrogen antagonist on the uterus in adult rats when administered for 7 days at 30 mg/kg/day. Finally, SP500263 had no statistically significant effects on total serum cholesterol and serum triglycerides in OVX rats treated for 28 days. Raloxifene had no significant effects on body weight, bone mineral content, and serum cholesterol or triglycerides in the OVX-rat model. In summary, SP500263 is a new orally active SERM that acts in rats as an estrogen agonist on bone without causing uterine stimulatory effects.