Phagolysosomenfusion

Phagolysosome fusion, also known as phagosome–lysosome fusion, is a key step in phagocytosis in which a mature phagosome merges with a lysosome to form a phagolysosome. In this compartment, ingested material is exposed to lysosomal hydrolases and an acidic environment, enabling degradation and antimicrobial activity.

The process follows phagosome maturation. Early phagosomes carry markers such as Rab5 and phosphatidylinositol 3-phosphate, and mature into late phagosomes with Rab7. Tethering factors, along with SNARE proteins, mediate the fusion of the phagosome with lysosomes. The lysosome contributes enzymes, proton pumps, and membranes that seal the phagosome, creating the phagolysosome. V-ATPases acidify the lumen to a pH around 4.5–5.0, which activates proteases, lipases, nucleases, and other hydrolases that digest the contents.

Functionally, phagolysosome fusion is essential for killing pathogens and clearing cellular debris. It also plays a role in antigen processing, supplying peptides for presentation on MHC class II molecules and aiding the activation of CD4+ T cells in professional antigen-presenting cells.

Pathogens can interfere with this process by blocking phagosome maturation or fusion, allowing survival within host cells. Examples include certain strategies observed in Mycobacterium tuberculosis, Legionella, and Salmonella species.

Dysfunction of phagolysosome formation can compromise microbial killing and immune processing, highlighting its importance in innate and adaptive immunity.