Assay of SLPI binding to monocytes. Radioiodinated SLPI was added to monocytes (12 × 10⁶) in binding buffer (DMEM, 5 mmol/L HEPES, and 0.1% BSA). Cells were then pelleted and washed twice with PBS, and cell pellets were assayed for ¹²⁵I emission. Nonspecific binding of ¹²⁵I-SLPI (binding in the presence of a 200-fold excess of unlabeled SLPI) was subtracted from the indicated results. (A) 2 nmol/L ¹²⁵I-SLPI was added to monocytes at 37°C (▵) or 4°C (•) and incubated for the designated times before pelleting cells. (B) Increasing amounts of ¹²⁵I-SLPI were added to monocytes and incubated overnight at 4°C. (C) 3 nmol/L ¹²⁵I-SLPI was added to monocytes for 1 hour at 4°C in binding buffer adjusted to designated pH values. Values are the mean ± SD; n = 3.