Casein-induced murine amyloidosis. Amyloidogenesis in vitro by monolayer spleen explants of casein-injected mice

Monolayer explants were established in tissue culture from the spleens of normal and casein-injected mice. Most of the round-shaped adherent cells were considered to be macrophages; they phagocytosed red blood cells and latex particles, formed rosettes with sheep red blood cells coated with IgG antibody (IgGEA) but not with sheep red blood cells (E) or E sensitized with IgM antisheep red blood cell antibody and complement (IgMEAC), and they contained acid phosphatase and nonspecific esterase. The adherent cells from primary explants were metabolically active and incorporated ³H-tryptophane into cytoplasmic proteins. However, they failed to divide, as evidenced by their inability to incorporate ³H-thymidine into nucleic acids. Adherent cells from secondary explants were thought to be fibroblasts; they were spindle-shaped on light microscopic examination, did not phagocytose red blood cells or latex particles, failed to form rosettes with E, IgGEA, or IgMEAC and did not contain acid phosphatase or nonspecific esterase. These cells were also metabolically active and in addition were capable of division. A large proportion of the monolayer spleen explant cells established from amyloidotic mice contained amyloid fibrils, as assessed by polarization and fluorescent microscopy, but this was considered to be a consequence of phagocytosis. However, a soluble component antigenically related to amyloid was produced in vitro. Its presence was assessed by the ability of culture supernatants to inhibit a ¹²⁵I mouse amyloid rabbit antimouse amyloid radioimmunoassay. Synthesis in vitro was shown by a progressive increase in the amount of this soluble component with time and by the incorporation of ³H-tryptophane into intracellular and secreted (or shed) amyloid. This in vitro system may prove useful in studying the cell of origin and the cellular events involved in amyloid production.