CD14^hi monocytes selectively costimulate CD40L expression. (A) Elutriated monocytes (Total) were initially fractionated into CD16⁺ and CD16⁻ populations by magnetic bead sorting, and then CD14^hi cells were further selected from the CD16⁻ population by positive selection on magnetic beads. (B) Purified CD4⁺ T cells were stimulated with anti-TCR mAb for 6 hours without monocytes (None) or after reconstitution with elutriated monocytes (Total), CD14^hiCD16⁻, or CD14^loCD16⁺ purified monocyte subsets. Percentage CD40L⁺CD4⁺ T cells are expressed relative to the cultures reconstituted with total monocytes. Data are mean ± SE of 3 donors.