A microarray approach to the analysis of peripheral deletion of CD8⁺ T cells. (A) Three forms of CD8⁺ T-cell fate were chosen for microarray comparison of gene expression profiles: deletional tolerance, immunity, and lymphopenia-induced proliferation. In deletional tolerance (left panels), naive T cells (top, round cell) proliferate in response to antigen presented by DCs (top, irregular cell). However, the responding T cells fail to acquire effector functions, and all eventually die by apoptosis (ie, the population contracts). In immunity (middle panels), naive T cells proliferate in response to antigen presented by DCs. In contrast to deletional tolerance, these cells acquire effector functions and, although most cells contract/die at the completion of the CTL immune response, some persist as memory cells. During lymphopenia-induced proliferation (right panels), T cells proliferate in response to homeostatic cytokines and low-avidity TCR/self-major histocompatibility complex interactions with DCs and the responding cells acquire a “memory” phenotype. There is no net loss of cells during this response and hence no contraction and relatively little cell death. (B) Presort and postsort profiles. A total of 2 × 10⁶ CFSE-labeled Ly5.1⁺ OT-I cells were injected intravenously into antigen-free C57BL/6 (B6), RIP-OVA^hi, OCS/LPS-treated C57BL/6 (OCS/LPS), or Rag^−/− mice. The cells were then stained for CD8 and Ly5.1, and the viable, CD8⁺Ly5.1⁺ cells that had either not divided (B6) or had undergone 2 or more divisions (RIP-OVA^hi, OCS/LPS, and Rag^−/−) were sorted by FACS. (Top panels) The CFSE profiles of viable, CD8⁺Ly5.1⁺ cells presort. (Bottom panels) The same cells postsort. The dotted rectangle represents the sort gate. Representative profiles from 1 of 2 sorts are shown.