Early induction of PD-1 on CD8 T cells responding to self-antigen. (A, B) Unsorted Thy1.1-marked HA-specific CD8 T cells from clone 4 TCR transgenic mice were CFSE-labeled and adoptively transferred into animals that express HA as a self-antigen in different amounts (C3-HA^low and C3-HA^high) or to WT mice infected with an attenuated L monocytogenes strain that expresses HA (LM-HA). Plots shown are gated on CD8⁺ Thy1.1⁺ lymphocytes (clone 4 donor cells). (A) PD-1 expression on clone 4 CD8 T cells as a function of division (CFSE dilution). Horizontal line represents isotype control. (B) PD-1 expression on clone 4 T cells transferred to mice that express high levels (C3-HA^high, inverted triangles) or low levels (C3-HA^low, squares) of HA. Mean ± SEM shown. (C) Unlabeled clone 4 T cells transferred as above, total cell number in spleen is shown. For a-c, 5 animals per group, data representative at least 2 experiments. (D) Adoptive transfer into C3-HA^low mice was performed in the presence of 100 μg of the indicated blocking antibody (administered intraperitoneally on day 0). Total number of clone 4 cells per spleen on day 4 is shown. n = 5.