β₂-microglobulin modified with advanced glycation end products delays monocyte apoptosis

Background. A local inflammatory reaction to β₂-microglobulin (β₂m) amyloid deposits by monocytes/macrophages is a characteristic histologic feature of dialysis-related amyloidosis (DRA). Since β₂m modified with advanced glycation end products (AGE-β₂m) is a major constituent of amyloid in DRA, we tested the hypothesis that AGE-β₂m affects apoptosis and phenotype of human monocytes. Methods. Human peripheral blood monocytes were incubated with or without in vitro-derived AGE-β₂m, and their viability, extent of apoptosis, morphology, and function examined over the subsequent four days. Results. AGE-modified but not unmodified β₂m significantly delayed spontaneous apoptosis of human peripheral blood monocytes in adherent and nonadherent cultures. The effect of AGE-β₂m on monocytes apoptosis was time- and dose-dependent and was attenuated by a blocking antibody directed against the human AGE receptor (RAGE). There was no difference in effect between AGE-β₂m and that of AGE-modified human serum albumin. Culture of monocytes with AGE-β₂m did not alter membrane expression of Fas or Fas ligand. Monocytes cultured with AGE-β₂m underwent substantial changes in morphology similar to those observed when monocytes differentiate into macrophages. The cultured cells increased in size and vacuolization, and their content of β-glucuronidase and acid phosphatase increased by 5- to 10-fold at day 4. Expression of the monocyte-macrophage membrane antigens HLA-DR, CD11b, and CD11c also increased at day 4. Although exhibiting phenotypic characteristics of macrophages, monocytes cultured with AGE-β₂m functioned differently than macrophages cultured with serum. Superoxide production in response to phorbol myristic acetate was maintained in monocytes cultured with AGE-β₂m, but declined with time in cells cultured with serum. Constitutive synthesis of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and prostaglandin E₂ (PGE₂) increased in monocytes cultured for four to six days with AGE-β₂m. Conclusions. These findings support a novel role for AGE modified proteins such as AGE-β₂m that may contribute to the development of a local inflammatory response, with predominant accumulation of monocytes/macrophages, in DRA.