Clonal deletion: A mechanism of tolerance in mixed bone marrow chimeras

The mechanism of antigen-specific immunologic unresponsiveness which results from lethal irradiation and mixed (syngeneic-allogeneic) bone marrow cell (BMC) reconstitution is unknown. To determine whether clonal deletion is the mechanism of tolerance in this model, monoclonal antibody (Mab) RR-4-4, specific for a T-cell receptor (Vβ6) reactive against the minor alloantigen Mls^a, was employed. Six-week-old B10 mice (H-2^b, Mls^b, Thy1.2) were tolerized to AKR antigens (H-2^k, Mls^a, Thy1.1) by whole body irradiation (950 R) and iv infusion of T-cell-depleted (TCD) B10 BMC + non-TCD AKR BMC. Chimerism and antigen-specific tolerance were documented by flow microfluorometry (FMF), skin grafting, mixed lymphocyte reaction, and cell-mediated lympholysis. When tolerant B10 mice (n = 15) had accepted AKR skin grafts for >100 days, these animals were studied for the presence of host Vβ6+ T cells using Mab RR-4-4. FMF revealed that 0-5% of host (B10) lymph node and spleen cells from chimeras were Vβ6+ while 15-20% of lymph node and spleen cells from control B10 mice expressed Vβ6. These data demonstrate that clonal deletion occurs in the lethal irradiation-mixed reconstitution model as evidenced by the near total elimination of Mls^a-reactive Vβ6+ T cells and suggest that it maybe a mechanism responsible for tolerance in adult mice.