Ioannis Tassiulas, MD, PhD

Cytokines and the Jak-STAT signaling pathway.

Cytokines are secreted proteins that mediate communication between cells and are critical for the development and regulation of the immune response. Cytokines act by binding with high affinity and specificity to cell surface receptors, triggering signal transduction pathways that ultimately lead to gene activation cascades that regulate cellular activation, differentiation, proliferation and survival. Deregulation of cytokine production or cytokine networks have been implicated in the pathogenesis of a number of human autoimmune/ inflammatory diseases including SLE.
Cytokines relay their intracellular signals by utilizing the Jak-Stat signal transduction pathway. The binding of the cytokines to their receptors that lack intrinsic kinase activity, activates Janus kinases (Jaks), protein tyrosine kinases that are associated with the receptor. Typically each cytokine activates a distinct pair of of two of the four known Jaks. Jak kinases are required for tyrosine phosphorylation and activation of latent cytoplasmic transcription factors termed signal transducers and activators of transcription (STATs). After cytokine stimulation, STATs are rapidly tyrosine phosphorylated, and subsequently dimerize and translocate to the nucleus, where they can activate gene transcription. Tyrosine phosphorylation of STATs is essential for activation, dimerization and DNA binding.
We have shown that IFN signaling is regulated by crosstalk with the ITAM-Syk pathway, a signal transduction pathway that is used specifically by immune cells and is key in cell activation. Crosstalk between IFN and Syk-mediated pathways is not static but depends on the activation state of macrophages and is induced by priming with IFN (Fig. 2). Thus, macrophage responses to IFN are determined by integration of signals from two main pathways that regulate immune responses (1).
Thus, the nature of cytokine responses can be altered or reprogrammed by the nature of a particular stimulus or the microenvironment that the cells are exposed.

Immune cell receptors

A growing number of receptors including the Toll-like receptors (TLRs) and receptors that transmit signals via the ITAM containing adaptor molecules FcR and DAP12 play important roles in the activation status of the cells of the immune system. Activation of these receptors by their ligands seems to be very important for effective development and resolution of an immune response. Signaling pathways activated downstream of these receptors include the NF-kB, MAPKs, PI3K and Ca++-induced pathways that play a central role in the activation, differentiation and survival of the cells. There is experimental evidence of regulation and crosstalk between cytokine and immune receptor signaling in cells of the immune system. A paradigm is the inhibition of interleukin-10 (IL-10) signaling in macrophages after ligation of the FcgRs by immune complexes (2). In this case macrophages become refractory to the anti-inflammatory actions of IL-10, a mechanism that may play important role in the perpetuation of inflammation in diseases characterized by the presence of immune complexes such as inflammatory arthritis and systemic lupus erythematosus.

Tpl2 kinase

Tumor progression locus 2 (Tpl2) encodes a serine/threonine protein kinase that is activated by provirus integration in Moloney murine leukemia virus-induced T cell lymphomas and mouse mammary tumor virus-induced mammary adenocarcinomas. Tpl2 transduces Toll-like and death receptor signals in a variety of cell types and plays an important role in innate immunity and inflammation. Tpl2 knockout (Tpl2-/-) mice develop normally and have no obvious phenotypic defects. However, Tpl2-/- mice are resistant to LPS-induced endotoxin-shock, as well as to TNF-induced inflammatory bowel disease. Tpl2 plays an obligatory role in the transduction of extracellular signal-regulated kinase (ERK) activation signals induced by LPS and CD40L inmacrophages and B cells and by TNF in macrophages. The failure of ERK activation by LPS in Tpl2-/- macrophages gives rise to defects in the expression of cytokines, chemokines and other molecules involved in the regulation of innate and adaptive immunity.
We have shown that Tpl2 kinase is activated downstream of the FcgRs and regulate phagocytosis and cytokine and chemokine production. Moreover in an animal model of immune thrombocytopenia pharmacological targeting of Tpl2 prevented the development of thrombocytopenia. These data identify Tpl2 kinase as a potential therapeutic target for autoimmune diseases.