Prolongation of allograft survival in mice by administration of anti-Thy 1 serum. I. Mediation by in vivo activation of regulatory T cells.

A single 0.5-ml dose of anti-Thy 1.2 serum raised in AKR (H-2k, Thy 1.1) mice by immunization with C3H (H-2k, Thy 1.2) thymocytes prolonged mean graft survival (MGS) of C57BL/L (H-2b, Thy 1.2) skin on BALB/c (H-2d, Thy, 1.2) mice from 9.2 to 13.0 days (p less than 0.01). Absorption of anti-Thy 1.2 serum with C3H but not AKR brain removed this activity. MGS was prolonged by anti-Thy 1.2 serum given as long as 2 weeks before grafting. Four separately raised anti-Thy 1.2 preparations prolonged MGS, one to a dilution of 1:128. Anti-Thy 1.1 serum prolonged MGS on AKR mice but not on BALB/c mice. Thymectomy 3 weeks or more before graft placement or treatment of BALB/c mice with anti-lymphocyte serum (ALS) diminished the effectiveness of anti-Thy 1.2 serum. Furthermore, infusion of 25 times 10(6) spleen cells from BALB/c mice given anti-Thy 1.2 serum prolonged MGS on syngeneic recipients to 15.6 days whereas spleen cells from mice given no treatment or ALS had no effect. Thymocytes from anti-Thy 1.2 serum treated donors were ineffective in prolonging MGS on transfer. Transfer or serum from anti-Thy 1.2 serum-treated mice also had no effect. The transfer of graft prolongation with lymphoid cells from anti-Thy 1 serum treated mice suggests anti-Thy 1 serum may prolong MGS by activating suppressor T cells.