CD83L signaling enhances the expansion of antigen-specific CD8⁺ T cells. HLA-A2⁺ CD8⁺ T cells were stimulated by MART1 (A) or Flu (B) peptide-pulsed APCs (APC/A2, APC/A2/CD80, APC/A2/CD83, or APC/A2/CD80/CD83). After 3 rounds of antigen stimulation and IL-2/IL-15 addition in between the stimulations, the cultures were stained with the relevant tetramer to determine percentage of peptide-specific T cells. The number of peptide-specific T cells was determined by calculating the product of the total number of T cells and the percentage of tetramer-staining cells. Representative results from 4 different donors are presented. (A) Compared with APC/A2, large increases in the percentage and number of antigen-specific CD8⁺ T cells were observed when T cells were stimulated by MART1-pulsed APC/A2/CD80 (percentage, P = .017; number, P = .002) and APC/A2/CD80/CD83 (percentage, P = .009; number, P < .001), but not by APC/A2/CD83 (percentage, P = .48; number, P = .30). When compared with APC/A2/CD80, APC/A2/CD80/CD83 generates a significant increase in the percentage and number of MART1-specific T cells (percentage, P = .012; number, P = .003). indicates A2; □, A2/CD80; , A2/CD83; ▪, A2/CD80/CD83. (B) Likewise, when APCs are pulsed with Flu peptide, the percentage and total number of antigen-specific T cells was increased when T cells were stimulated by APC/A2/CD80 (percentage, P = .009; number, P = .01) and APC/A2/CD80/CD83 (percentage, P = .009; number, P = .023), but not by or APC/A2/CD83 (percentage, P = .32; number, P = .32). When compared with APC/A2/CD80, APC/A2/CD80/CD83 generates a significant increase in the percentage and number of Flu-specific T cells (percentage, P = .01; number, P = .035). indicates A2; □, A2/CD80; , A2/CD83; ▪, A2/CD80/CD83.