3-phosphoinositide-dependent kinase 1 deficiency perturbs Toll-like receptor signaling events and actin cytoskeleton dynamics in dendritic cells.

The adaptive immune response depends on dendritic cell (DC) activation by microbial products that signal via pattern recognition receptors and activate mitogen-activated protein kinases, NFkappaB and PI3K. The contribution of the AGC kinase family, including protein kinase B, protein kinase C, p90kDa ribosomal S6 kinase, and S6 kinase, has been little investigated because the probable redundancy among their isoforms makes their study difficult. We took advantage of the fact that all these kinases are regulated by the upstream master kinase 3-phosphoinositide-dependent kinase 1 (PDK1). Here we analyze various properties of DC from mice expressing approximately 10% of normal PDK1 (PDK1(fl/-)). DC populations in lymphoid and nonlymphoid tissues appeared normal in PDK1(fl/-) mice, and some in vitro responses to lipopolysaccharide (LPS) such as cytokine production were normal in cultured bone marrow DC. However, LPS-induced expression of class II major histocompatibility complex and CD86 were elevated in PDK1(fl/-) BMDC and PDK1(fl/-) spleen DC produced more interleukin-10 and -12, implying an attenuating role for PDK1. Unexpectedly, PDK1(fl/-) DC had a significantly reduced capacity for LPS-stimulated macropinocytosis and phagocytosis that correlated with a lowered F-actin/G-actin ratio, apparently because of increased actin depolymerization. Several PDK1-regulated kinases, some of which feed into actin regulators, showed reduced activation in PDK1(fl/-) DC. Reintroduction of PDK1 restored S6 kinase activity, increased levels of F-actin, and boosted macropinocytosis thus linking PDK1 and its downstream effectors to the unusual phenotype of PDK1(fl/-) DC.