Demonstration of specific receptors for fluoresceinated casein on human neutrophils and monocytes using flow cytometry

Casein is chemotactic for human neutrophils (PMNs) and monocytes. The binding of fluorescein (FITC)-conjugated casein (mixture of alpha, beta, and kappa-casein) and purified alpha-casein to PMNs, monocytes, and lymphocytes was analyzed using flow cytometry. These studies demonstrate that 75-95% of PMNs and 46-85% of monocytes have membrane receptors for casein while lymphocytes lack these receptors. The binding of FITC-casein and FITC-alpha-casein was specific and was blocked only by unlabeled casein and alpha-casein, but not by ovalbumin, bovine or human serum albumin, beta-casomorphin, C5a, or formyl-methionyl-leucylphenylalanine (fMLP). The binding of FITC-casein was reversible when PMNs were stained with this fluorescent agent and subsequently incubated with unlabeled casein. Double-labeling studies of mononuclear cells using FITC-casein and the OKM1 monoclonal antibody in conjunction with a rhodamine conjugated anti-Ig second antibody demonstrate that mononuclear cells binding FITC-casein also stain with the OKM1 monoclonal antibody, indicating a specificity for monocytes.