Poly(ethylene glycol): protein-repulsive or albumin-compatible?

In the literature, many papers deal with the behavior of proteins in aqueous media in the presence of poly(ethylene glycol) (PEG) molecules or poly(ethylene oxide) (PEO) segments, physically adsorbed onto, or covalently attached to, macromolecules or to solid surfaces. In particular, it is well known that PEO segments make foreign materials stealthy, i.e. they are much less detected by the immune system either through humoral reactions or, at the cell level, through opsonins. Revisiting the literature led us to challenge the largely accepted opinion that the decreased recognition of PEO segment-bearing foreign macromolecules and particles by the mononuclear phagocyte system is primarily the consequence of the repulsion of all blood proteins by PEG segments through the excluded volume effect. This challenge is based on the finding that albumin and PEG are compatible in phosphate-buffered saline at room temperature and at concentrations comparable to those measured by others on the surface of PEO segment-bearing species, whereas fibrinogen and PEG phase-separated and were incompatible despite the much lower concentration of the latter protein. According to literature and to these observations, it is proposed that the stealth effect induced by PEO segments is primarily due to the compatibility between PEO segments of intermediate molar mass and albumin, thus rendering PEO-bearing macromolecules or surfaces to look like native albumin. Under such conditions, the hospitality offered by PEG macromolecules or PEO segments to albumin, the dominant plasma protein, results in a 'chameleon' effect that prevents the activation of other PEG-compatible or -incompatible plasma proteins or cells involved in foreign body recognition and elimination. PEG with molar masses > or = 8000 did not accommodate albumin in agreement with the excluded volume phenomenon.