New observations on the trafficking and diapedesis of monocytes.
OBJECTIVE
Monocytes play multiple roles in immune system functions and inflammatory diseases such as atherosclerosis. These roles are coupled to diverse trafficking and cellular migration behaviors. Here, we review recent advances in our understanding of such behaviors with emphasis on broad scale trafficking patterns and the cellular and molecular mechanisms regulating diapedesis, a central aspect of trafficking.


RESULTS
Monocytes consist of 'inflammatory' and 'resident' subsets, which exhibit differential functions and trafficking properties. Notably, the spleen has recently been identified as a reservoir of inflammatory monocytes, which are readily recruited to injured myocardium and possibly other tissues. Resident monocytes have been shown to undergo long-range crawling within the lumen of the microvasculature, which facilitates immune surveillance and rapid response to infection. Monocyte diapedesis has been demonstrated to utilize both para and transcellular migration routes facilitated by endothelial 'transmigratory cups'. A significant number of new adhesion molecules and signaling pathways have recently been uncovered as functional mediators and modulators of these processes.


CONCLUSIONS
Our improving understanding of monocyte trafficking and migration mechanisms has begun to shed light on the functions of these often enigmatic cells. Continued progress in this area will be critical for elucidating roles of monocytes in disease and for developing therapeutics that target monocytes.
Kamei
Masataka
M
Center for Vascular Biology Research, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA.
Carman
Christopher V
CV
D000818:Animals; D002448:Cell Adhesion; D002465:Cell Movement; D006801:Humans; D009000:Monocytes / Q000166:cytology* / Q000276:immunology; D019408:Platelet Endothelial Cell Adhesion Molecule-1 / Q000502:physiology; D015398:Signal Transduction; D013154:Spleen / Q000166:cytology / Q000276:immunology
D019408:Platelet Endothelial Cell Adhesion Molecule-1