Xanthoma-inducing beta-lipoprotein immune complexes

Untreated rabbits and rabbits sensitized with human beta-lipoproteins were intracutaneously injected with either human or rabbit beta-lipoproteins (2 mg as protein). Forty-eight hours later, at the human beta-lipoprotein injection site, dense cell infiltrations stood out sharply demarcated from the surrounding region. The infiltrating cells were composed of polymorphonuclear cells, nuclear dust and histiocytes. Foam cell nests were observed in sensitized rabbit dermis one week after human beta-lipoprotein injection. Direct immunofluorescence employing fluorescein isothiocyanate labeled antihuman beta-lipoprotein antiserum showed that the injected human beta-lipoproteins were incorporated into the infiltrating cells of sensitized rabbit dermis up to 48 hours after injection. Specific fluorescence was greatly reduced but lipid autofluorescence remained in mature foam cells. During the 3-week examination period, no trace of foam cell nests was found at the site of human beta-lipoprotein injection on non-sensitized rabbits. Intracutaneous rabbit beta-lipoprotein injection revealed no histiocytic infiltrations in either sensitized or non-sensitized rabbits. These findings indicated that intracutaneously injected human beta-lipoporteins were incorporated into rabbit dermal histiocytes more easily in aggregated immune complex form than in their natural state. These histiocytes were easily transformed to foam cells. Apoproteins of the incorporated lipoproteins were degraded, and lipid residues accumulated in mature foam cells.