Leirião P, Del Fresno C, Ardavín C.
Monocytes have the capacity to differentiate into macrophages or
dendritic cells (DCs) after extravasation into lymphoid and non-lymphoid
tissues. They have thus been consequently considered as precursors, but
not effector cells, recirculating exclusively through the blood. In
this report, we demonstrate for the first time that, after subcutaneous
injection, activated monocytes migrated through the lymphatics from the
dermis into the draining lymph nodes, by a CCR7-dependent mechanism.
LPS-activated monocytes were less efficient than DCs in stimulating CD4+ T cells, but unexpectedly, they were highly efficient in inducing antigen-specific CD8+ T-cell proliferation by cross-presentation, both in vitro and in vivo. Interestingly, CD8+
T cells stimulated in vivo by activated monocytes expressed a high
level of CD62L suggesting that they had undergone an unconventional
activation process.
In conclusion, our data strongly support the concept
that monocytes can behave not only as precursors cells for macrophages
and dendritic cells, but also as effectors cells with the capacity to
migrate from the periphery to the lymph nodes through the lymph and to
cross-present antigens to CD8+ T cells. These results suggest that monocytes can play an important role in the induction and regulation of CD4+ and CD8+ T-cell responses.