Host-reactive cells show massive apoptosis involving Fas/FasL interactions. (A) Proportion of apoptotic (Annexin-V–positive) T cells in F1 recipients injected with B6.SJL vs. lpr cells. The mean numbers of apoptotic CD4⁺/CD8⁺ T cells was 4.5%/3% in B6.SJL controls and 5.8%/3.4% in lpr controls (n = 3; data not shown). Results are expressed as the mean ± SD, n = 3. *P < .05, **P < .01, ***P < .005 relative to B6.SJL donors. (B) Histograms showing results for a representative F1 recipient studied on day 16 after injection of B6.SJL or lpr cells (shaded diagram). The clear histogram represents a negative control (untreated B6.SJL or lpr mouse).