Type I interferons (15 and one subtypes) contribute to immediate defence against pathogens, development of an adaptive immunity and protective antitumor responses. Several IFN subtypes participate vigorously in the cytokine network that regulates differentiation, function and homeostasis of a variety of cell lineages. IFN are pathogenic factors in a number of systemic and organ-specific autoimmune and inflammatory diseases but they are also among the most widely used therapeutic agents. An important aspect of our research centers on the molecular mechanisms of type I IFN action, with particular attention to receptor and Jak tyrosine kinases functioning, differential activities of IFN2 and IFN, engagement of non-Stat signaling cascades and mechanisms leading to apoptosis and cellular desensitization.

I-Among the many IFN subtypes IFN is the most potent in exerting growth inhibitory activity. We have been interested in uncovering the molecular basis of this / differential. We found that IFN 2 and IFN activate to the same extent the Jak/Stat pathway and are equipotent in gene induction and cell cycle arrest. However, IFN is more potent in inducing apoptosis, an important arm of the antiproliferative activity. We have studied to what extent the control of mRNA translation contributes to this 2/ differential. Our data show that IFN induces more robust activation of p70 S6K, a key regulator of translation and higher accumulation of some IFN-stimulated gene protein products. Thus, translational regulation occurs and may well contribute to other subtype-specific activities. We plan to elucidate further the mechanism and identify key proteins and mRNA elements responsible for IFN-stimulated translational control.

II- The type I IFN system needs to be rigorously regulated both at the levels of production and cell response. Indeed, its perturbation can lead to autoimmune manifestations. Cells that have responded to type I IFN enter into a desensitized or refractory state. In collaboration with Gilles Uzé (CRNS, Montpellier) we found that this long-lasting desensitization concerns all subtypes, but not IFN . Moreover, differential desensitization can be induced by type III IFN, another family of innate antiviral cytokines acting via a distinct receptor on epithelial cells. We are presently deciphering the mechanistic aspects of this novel type I/type III IFN crosstalk that involves USP18, an IFN-induced isopeptidase. These studies may have relevance in the set up of clinical protocols, especially for the treatment of HCV-infected patients resistant to conventional IFN 2-based therapy.

III- Tyk2 is one of the four members of the Janus family of protein tyrosine kinases. Tyk2 is activated by type I and type III IFNs, as well as by immune cytokines (eg IL-10, IL-12, IL-23) that contribute to a balanced immune response. Thus, Tyk2 appears critical in the integration of complex threshold signals and may well represent an ideal target for therapeutical intervention in autoimmune diseases. Genome wide association studies have linked Tyk2 haplotypes or individual SNPs to systemic and organ-specific autoimmune diseases (multiple sclerosis, systemic lupus erythematosus, Crohn’s disease). Thus, Tyk2 variants may underlie differential susceptibilities to complex immune diseases. We are exploring the biochemical and functional behaviour of some nonsynonymous Tyk2 variants with the aim to assess to what extent they affect immune cytokine signaling and primary T cell responses.

IV- The outcome of a naïve T cell activated by an antigen-presenting cell depends on the integration of signals emanating from the T cell antigen receptor (TCR), costimulatory receptors and cytokine environment. One part of our research is dedicated to the analysis of the molecular mechanisms that regulate the early biochemical TCR signaling. Our data indicate that the strength and duration of TCR stimulation are critical in the establishment of positive and negative regulatory loops that modulate T cell signaling and activation. We have been recently interested in the debated question regarding the mechanism by which TCR stimulation triggers signal initiation. We have shown that a substantial pool of the Src protein tyrosine kinases, Lck and Fyn, is unexpectedly basally activated in human naïve CD4⁺T cells. Moreover, we have proposed that the transmembrane adaptor, Linker for activation of T cells (LAT), contributes to maintain or stabilize active Src kinases close to their substrates. Remarkably, this new control exerted by LAT on signal initiation is efficient at low strength of TCR engagement. These studies have also allowed us to identify some intracellular signaling events that do not or weakly depend on LAT. Another aspect of our research aims at understanding how TCR signaling and cytokines influence T cell differentiation and function. We focus on a crosstalk between TCR and the innate cytokines type I interferons that influence the human CD4⁺T cell response. Analyses are in progress to define the underlying mechanism at the cellular and molecular levels. These studies should provide new insights into the action of type I IFN and contribute to understand why IFN treatment is beneficial only in some multiple sclerosis patients.

Throughout these studies we hope to gain new insights into the molecular mechanisms of IFN action, understand how individual IFN subtypes act differentially and unravel the nature of the cooperative crosstalk between IFN and stimulation of the TCR.

Keywords: signaling, tyrosine kinase, innate immune response, T cell receptor signaling, T cell differentiation, cytokines, inflammatory diseases

Acuto, O., V. Di Bartolo,and F. Michel. 2008. Tailoring T-cell receptor signals by proximal negative feedback mechanisms. Nature Rev. Immunol. 8:699-712.PMID: 18728635

Libri V., D. Schulte, A. van Stijn, J. Ragimbeau, L. Rogge, and S. Pellegrini. 2008. Jakmip1 is expressed upon T cell differentiation and has an inhibitory function in cytotoxic T lymphocytes. J. Immunol. 181:5847-5856.PMID: 18941173

Moraga, M., D. Harari, G. Schreiber, G. Uzé, and S. Pellegrini. 2009. Receptor density is key to the interferon alpha2/beta differential activities. Mol. Cell. Biol. 29:4778-4787.PMID: 19564411

Payelle-Brogard, B. and S. Pellegrini. 2010. Biochemical monitoring of the early endocytic traffic of the type I interferon receptor. J. IFN and Cytok. Res. 30:89-98.PMID: 20028207

Dong S., B. Corre, K. Nika, S. Pellegrini, and F. Michel. 2010. T cell receptor signal initiation induced by low-grade stimulation requires the cooperation of LAT in human T cells. PLoS One. 2010. 5(11):e15114.PMID: 21152094