Expression of PSGL-1 on leukocyte subsets. (A) Human PBMC were isolated by Ficoll density gradient centrifugation and stained for two-color flow cytometry as described in Materials and Methods. Electronic scatter gating was used to select specific subpopulations for detailed analysis. All NK cells (CD16⁺), monocytes (CD14⁺), and T cells (CD3⁺) were positive for PSGL-1 as measured by KPL1, whereas B cells (CD19⁺) expressed low levels of the KPL1 epitope. (B) All CD4, CD8, α/β (WT31 positive), or γ/δ (anti-TCR-γ/δ-1 positive) cells express uniform levels PSGL-1 as measured by KPL1. Horizontal and vertical lines delineating positive and negative staining were set with appropriate negative control MoAb.