POLLEN GRAINS BIND TO LUNG ALVEOLAR TYPE-II CELLS (A549) VIA LUNG SURFACTANT PROTEIN-A (SP-A)

Lung surfactant protein A (SP-A) is the most abundant surfactant-assoc iated protein present in the lung. A receptor for SP-A has been shown to be present on A549 alveolar type II cells and on other cell types, including alveolar macrophage. The SP-A receptor on A549 cells has bee n identified as the collectin receptor, or C1q receptor, which binds s everal structurally-related ligands. SP-A contains C-type lectin domai ns, but the role of carbohydrate binding by SP-A in physiological and pathological phenomena is not yet established. In this paper we report the binding of SP-A to pollen from Populus nigra italica (Lombardy Po plar), Poa pratensis (Kentucky blue grass), Secale cerale (cultivated rye) and Ambrosia elatior (short ragweed). Saturable and concentration dependent binding of SP-A to pollen grains was observed. Interaction of SP-A with pollen grains takes place through water-extractable compo nents, in which the major species present, in Lombardy poplar pollen, are 57 kD and 7 kD (glyco)proteins. The binding of SP-A to pollen grai ns and their aqueous extracts was calcium ion dependent and was inhibi ted by mannose, and is therefore mediated by the lectin domain. Bindin g of SP-A to pollen grains was found to mediate adhesion of pollen gra ins to A549 cells. The results suggest that pollen grains or other car bohydrate-bearing particles (e.g. microorganisms) could potentially in teract with different cell types via the collectin receptor (C1q Recep tor) in the presence of SP-A.