CD57 expression marks perforin-expressing T cells. (A-D) Freshly isolated, resting PBMCs from 12 healthy adult volunteers were surface stained with fluorochrome-conjugated antibodies to CD3, CD4, CD8, CD14, CD19, CD27, CD28, CD45RA, CD45RO, CD56, CD57, and CD62L, followed by fixation, permeabilization, and intracellular staining with isotype control antibodies or antibodies to perforin and granzyme B, and then analyzed by flow cytometry. Lymphocytes were gated on forward scatter/side scatter plots. (A-B) Concatenated data for 30 000 randomly selected CD3⁺CD14⁻CD19⁻CD4⁻CD8⁺ living lymphocytes (CD8⁺ T cells) from each of 12 healthy adult donors. (A) Probability state model of CD8⁺ T cells. The parameters were added to the model in the following order: granzyme B, perforin, CD28, CD57, CD27, and finally CD62L, which was branched because of the observed heterogeneity relative to the other parameters. CD45RA and CD45RO were not included as parameters in the model, but are displayed in the final analysis. (B) Plots showing the distribution of differentiation marker expression on individual cells, as indicated, according to the probability state model. (C) Plots show CD57 versus isotype or perforin-staining on T-cell and NK cell subsets, as indicated, from one representative donor. (D) Plots show CD57 versus CD27 staining on T-cell subsets, as indicated, on data concatenated from 7 healthy donors. Histograms display the expression of perforin, granzyme B, CD28, CD45RA, CD45RO, and CD62L in the CD27⁺CD57⁺, CD27⁺CD57⁻, CD27⁻CD57⁺, CD27⁻CD57⁻ T-cell subsets, as indicated.