Macrophages play an important role in host defense against invading micro-organisms and they are also key players in initiating and maintaining an immune response. However, macrophages can also play negative roles, such as in chronic inflammatory disease. Also, tumor-associated macrophages (TAMs), which are present in large numbers in many tumors, appear to play an important role in promoting the progression of solid tumors to an invasive, metastatic phenotype. Macrophages are therefore a prime target for therapies, but it is important to elucidate the mechanisms by which they are recruited to and activated in tissues.
Studying the molecular mechanisms of phagocytosis
Phagocytosis via receptors for the Fc portion of IgG (FcgRs) or for complement (CR3) requires actin assembly, pseudopod extension, and phagosome closure. Actin polymerization in response to particle binding requires the activation of members of the Rho GTPases, either Rac or Cdc42 for FcgR-mediated phagocytosis or Rho in the case of CR3-mediated phagocytosis. Both Rac and Cdc42 regulate the cytoskeleton in part through the activation of the Wiskott Aldrich Syndrome/ WASP verprolin-homologous (WASP/WAVE) family of proteins. Several human pathogens, such as Legionella pneumophila and Mycobacterium tuberculosis, are internalized by macrophages, escape destruction and grow and divide inside the macrophage. The phagocytic processes by which these organisms are internalized are currently unknown. We are currently dissecting the downstream signaling pathways that mediate these processes during phagocytosis.
Studying the molecular mechanisms of chemotaxis
The directed movement of cells in response to chemoattractants involves several complex, interrelated processes, including directional or chemotactic sensing, polarity, and motility. These processes are mediated by complex, interacting signaling pathways that appear to have many similarities but yet have distinct characteristics depending on the chemoattractant and receptor. We are currently dissecting the signaling pathways required for macrophage chemotaxis towards;
- CSF-1, a growth factor for macrophage survival and differentiation produced by many tumors and found in high concentrations in arthritic joints.
- Chemokines that direct monocyte recruitment to different tissues
- Legionella pneumophila, mediated by an unknown factor
Mechanisms of monocyte/macrophage recruitment to tissues
Understanding the specific signaling components required for migration towards each of these factors will allow us to test the importance of these signaling components in vivo. Macrophages will be manipulated ex vivo,to produce macrophages with specific migratory defects, and then reintroduced back into a host. The ability of these altered macrophages to migrate into the tissue space will be tested in animal models of peritoneal infection, pneumonia (Legionella introduced into the lung), and breast cancer.
The role of macrophages in the tumor microenvironment
It is now increasingly recognized that the tissue microenvironment plays a critical role in tumor progression, but most studies on tumor metastasis involve the use of endpoint assays or in vitro studies on cell lines. It appears that macrophages and tumor cells participate in a paracrine interaction, with the tumor cells secreting CSF-1 and macrophages secreting EGF, but the precise roles of this paracrine interaction in tumor metastasis are unknown. In vitro we will use a reconstituted paracrine assay between macrophage and carcinoma cells and in vivo interactions of macrophages and tumor cells in the tumor microenvironment will be examined by intravital imaging using multiphoton microscopy.