Interleukin
16(IL-16) has been characterized as a chemoattractant for a variety of CD4+
immune cells: lymphocytes, monocytes, eosinophils and dendritic cells. Following
binding to the CD4 receptor on lymphocytes, IL-16 stimulation results in
activation and autophosphorylation of p56lck, a Src family protein tyrosine
kinase(PTK), that is associated with the cytoplasmaic tail of CD4. Hck, which
is a member of the Src family, has been shown to be a critical component in the
signal transduction pathway of several monocyte/macrophage-activating factors.
Since monocytes lack the p56lck, Hck activation, by association with the
intracytoplasmic tail of CD4, could be involved in the mediation of monocyte
responses to IL-16. The activation of the MAPK family of kinases(ERK,JNK and
p38) is often initiated by proximal activation of PTKs.
In view of these
findings, we decided to investigate the involvement of Hck and MAPK pathways in
the signaling cascade initiated by IL-16 in monocyte/macrophages and to test
the effector functions induced by IL-16 stimulation.
We have shown that
IL-16 stimulation of PMA-MΦ (differentiated THP-1 human monocytes)
involves the activation and the phosphorylation of Hck and the MAPK enzymes
ERK, JNK and p38. CD4 molecule is required to elicit IL-16-induced cell
signaling in PMA-MΦ, and the resulting MAPK phosphorylation is dependent
on Hck activation. IL-16 stimulation of PMA-MΦ results also in selective
secretion of certain chemokines( MCP-1, RANTES and MIP-3α) and the
cytokine IL-10. The secretion of MCP-1, RANTES and IL-10 is associated with Hck
and MAPK activation whereas MIP-3α secretion might be associated with some
other signal transduction pathways induced by IL-16. Our
findings suggest that IL-16, except of being a direct chemoattractant on CD4+
inflammatory cells, may augment its chemotactic activity by the induction of
the secretion of chemokines, which affect also CD4-negative inflammatory
cells.
