Lactoferrin binding to human peripheral blood cells: An interaction with a B-enriched population of lymphocytes and a subpopulation of adherent mononuclear cells

Lactoferrin binding to human peripheral blood cells has been investigated. Purified human lactoferrin (LF) was radiolabeled with ¹²⁵I, and its binding to platelets, neutrophils, adherent mononuclear cells (ADMC), null cells, rosetting cells (E⁺), and nonrosetting cells (E^-) was assessed. Significant levels of binding, suggestive of a specific receptor mechanism, were observed with ADMC and E^- cells. Neutrophils exhibited some LF binding, but this appeared to be largely nonspecific. A complete dose-response curve of cells in suspension could not be obtained because of cell aggregation beyond a LF concentration of about 250 μg/ml. An attempt to overcome this problem was made by using ADMC coated in plastic vials; this procedure yielded a figure of 2.0 x 10⁸ LF receptors per ADMC and a mean binding affinity of 3.75 x 10⁵ liter/mol. The binding to cells was calcium dependent but was not influenced by metabolic inhibitors (sodium azide, sodium fluoride, ouabain, or N-ethylmaleimide) or by incubation at 4°C. The addition of mannose (20 mg/ml) to the ¹²⁵I LF prior to incubation with cells resulted in a reduction of LF binding of 1 order of magnitude. There was no steric hindrance of LF binding by monomeric IgG, aggregated IgG, transferrin, anti-Ia antiserum, or normal human serum depleted of LF. A subpopulation of ADMC with LF receptors (LRF⁺) was demonstrated after enrichment of LFR⁺ cells by rosetting with LF-coated red cells; this subpopulation of ADMC exhibited a 60-fold increase in LF binding. Metabolic stimulation of ADMC by incubation with E. coli endotoxin resulted in a 48-fold increase in LF binding. The apparently very high number of LF receptors per cell is probably an artefact caused by the fact that LF usually exists as a polymer; it is hypothesized that the LF receptor may have a unique structure capable of interacting with many LF polymers.