Homing and migration
MC progenitors circulating in the blood, in humans defined as
CD34+, KIT+,
FcεRI+ cells 68, enter a specific
tissue where they complete their maturation under the influence of
locally produced factors. Surprisingly, the homing mechanisms for MC
progenitors are still mostly unknown, except that it is a tissue
specific process 69-71. In addition to their basal
homing, MC numbers clearly increase at sites of inflammation, such as in
allergic rhinitis and asthma. This increase in MC numbers is likely a
consequence of both increased proliferation and survival, as well as
maturation of their progenitors recruited in response to
chemoattractants 72,73. The fact that some chemokine
receptors, such as CCR1, CXCR2 and CXCR4, are expressed on both
progenitors and mature MCs suggests that MC accumulation at sites of
inflammation may also involve relocalization of mature MCs within a
specific tissue 71-76.