Cell-bound albumin is the 70-kDa peptidoglycan-, lipopolysaccharide-, and lipoteichoic acid-binding protein on lymphocytes and macrophages.

The 70-kDa protein that is present on the surface of lymphocytes and macrophages and that binds peptidoglycan, lipopolysaccharide, lipoteichoic acid, heparin, and sulfated heparinoids was identified as cell-bound albumin. It originated from the tissue culture medium or from the serum in vivo and was not produced by the cells. The following results supported this conclusion: (a) mouse and human cell lines grown in serum-free and albumin-free medium did not have this 70-kDa protein and did not bind peptidoglycan and lipopolysaccharide in photoaffinity cross-linking assay; (b) the appearance of the 70-kDa protein in these cells and the ligand binding were restored by 30-min incubation with serum or purified albumin; (c) soluble albumin bound peptidoglycan and lipopolysaccharide, and this binding was inhibited by the same competitive inhibitors that inhibited the binding to the 70-kDa cellular protein; (d) albumin co-migrated with the 70-kDa cellular protein in two-dimensional polyacrylamide gel electrophoresis; (e) peptide maps of albumin and the 70-kDa cellular protein digested with four proteases were identical; (f) the only protein recognized by anti-albumin monoclonal and polyclonal antibodies on Western immunoblots was the 70-kDa protein that exactly matched with the cellular peptidoglycan/lipopolysaccharide-binding protein; (g) anti-albumin monoclonal antibodies immunoprecipitated the 70-kDa cellular protein; and (h) species-specific anti-bovine, anti-human, and anti-mouse albumin antibodies recognized the 70-kDa protein on mouse and human cells according to the species of albumin that was present in the culture medium or in the serum in vivo, but not according to the species of the cells. The cell-bound albumin was not required for cell activation, because macrophage cell lines that were grown in albumin-free medium and did not have the cell-bound albumin (the 70-kDa protein) fully responded to the stimulation by peptidoglycan and lipopolysaccharide with production of tumor necrosis factor-alpha.