Secretion of lysosomal hydrolases by stimulated and nonstimulated macrophages

Area of Science:

• Immunology
• Cell Biology
• Biochemistry

Background:

• Macrophages are crucial immune cells involved in host defense and tissue homeostasis.
• Understanding macrophage secretory functions, particularly lysosomal hydrolase release, is key to deciphering their roles in inflammation and disease.
• Previous studies have indicated differential activation states of macrophages, but their comparative secretory profiles require further elucidation.

Purpose of the Study:

• To compare the biochemical and secretory properties of different types of mouse peritoneal macrophages in long-term culture.
• To investigate the mechanisms and regulation of lysosomal hydrolase secretion by macrophages.
• To determine if lysosomal hydrolase secretion is an in vivo or in vitro phenomenon.

Main Methods:

• Peritoneal macrophages were isolated from untreated mice and mice treated with thioglycollate medium (TA), proteose peptone medium (PP), or Streptococcus A cell wall material (SA).
• Cells were cultured for up to 2 weeks to assess biochemical and secretory properties.
• Quantification of protein, lactate dehydrogenase, lysosomal hydrolases (e.g., beta-glucuronidase, acid phosphatase), and plasminogen activator was performed.
• The effect of cycloheximide on enzyme secretion was evaluated to assess dependence on protein synthesis.

Main Results:

• TA-elicited macrophages exhibited higher levels of protein, lactate dehydrogenase, lysosomal hydrolases, and significantly more plasminogen activator compared to PP-elicited, SA-elicited, and non-elicited macrophages.
• All macrophage types secreted considerable amounts of lysosomal hydrolases into the medium, independent of phagocytosis, indicating true secretion.
• Lysosomal enzyme secretion was dependent on protein synthesis, as evidenced by inhibition with cycloheximide, and was not enhanced by serum.
• TA-elicited macrophages showed the highest secretion rates of lysosomal hydrolases, approximately four times greater than other elicited or non-elicited macrophages.
• Plasminogen activator secretion was markedly higher (20-50 times) in TA-elicited cells compared to others.
• Acid glycosidases were detected in peritoneal lavage media, suggesting in vivo secretion.

Conclusions:

• Thioglycollate-elicitation induces a distinct macrophage phenotype with enhanced secretory capacity, particularly for lysosomal hydrolases and plasminogen activator.
• Macrophage lysosomal hydrolase secretion is a true secretory process, dependent on ongoing protein synthesis and occurring independently of phagocytosis.
• The findings suggest that active secretion of lysosomal hydrolases is an in vivo characteristic of macrophages.