IMMUNOLOCALIZATION OF SERUM AMYLOID-A AND AA-AMYLOID IN LYSOSOMES IN MURINE MONOCYTOID CELLS - CONFOCAL AND IMMUNOGOLD ELECTRON-MICROSCOPICSTUDIES

Murine AA amyloid (AA) protein represents the amino-terminal two-third portion of SAA(2), one of the isoforms of serum amyloid A. Whether pl asma membrane-bound or lysosomal enzymes in activated murine monocytoi d cells degrade SAA(2) to generate amyloidogenic AA-like peptides is n ot clearly understood, although AA has been localized in the lysosomes . Here we show, using confocal and immunogold microscopy (IEM), that b oth SAA and AA localize in lysosomes of activated monocytoid cells fro m amyloidotic mice. Rabbit anti-mouse AA IgG (RAA) and two monoclonal antibodies against murine lysosome-associated membrane proteins (LAMP- 1 and LAMP-2) were used to immunolocalize SAA/AA and lysosomes, respec tively. Confocal analysis co-localized both anti-RAA and anti-LAMP-1/L AMP-2 reactivities in the perikaryal organelles which by IEM proved to be electron-dense lysosomes. LAMP-1/LAMP-2-specific gold particles we re also localized on lysosomal and perikaryal AA. The results suggest sequestration of SAA into the lysosomes. Since monocytoid cells are no t known to phagocytose native amyloid fibrils, our results implicate l ysosomes in AA formation.