Résumé : Inflammatory bowel diseases (IBD), including Crohn’s disease (CD) and ulcerative colitis (UC), are chronic inflammatory immune-mediated diseases of the gastrointestinal tract. Two-thirds of IBD patients will develop severe disease, with complications that will require frequent surgeries and hospital admissions, and will seriously impair their quality of life. The ultimate clinical challenge of precision medicine in IBD is to find predictive markers to anticipate the development of severe disease and to monitor treatment in these patients.In the first part of my PhD thesis, we have carried out several studies monitoring the biologics used in IBD patients with severe disease. We have evaluated the pharmacokinetics of the following biologics used in IBD patients: infliximab, vedolizumab, and ustekinumab. We have focused on measuring trough levels (TLs) (defined as the serum drug level measured just before the next drug administration) early on after initiating biologic treatment to predict patient outcomes, including long- term responses in patients treated with infliximab and vedolizumab. In addition, we are currently conducting a prospective multicentric study that aims to design a pharmacokinetic model of infliximab at induction in IBD patients (EudraCT: CT 2015- 004618-10) (End of study expected by December 2019 but interim analysis available in the present work). Moreover, we have reported on the efficacy of ustekinumab in a large national cohort of highly refractory CD patients and have also examined the benefit of early measurement of ustekinumab TLs in these patients. Finally, we have reported novel findings on the impact of different wash-out periods (defined as the time frame between the discontinuation of one biologic and the initiation of a second biologic on the pharmacokinetics of the second-line biologic). Altogether, over the past 3 years, our data suggest the importance of measuring TLs early on during induction to predict long-term response to biologics during maintenance therapyIn the second part of my PhD thesis, we have analysed the inter-variability of the immune response in healthy subjects. Inflammation is the obvious key driver and underlying mechanism of disease severity in IBD. Therefore, the magnitude of inflammation must help define the phenotype of mild to severe disease. Delineating the inter-variability of the immune response in a healthy cohort constitutes a fundamental step to uncovering the genetic factors underlying this variability. We have performed whole blood cell cultures in a highly selected population of more than 400 healthy subjects stimulated with several Toll-like receptor (TLR) agonists and a T-cell receptor (TCR) antagonist. We found that the magnitude of the immune response (the high- or low-cytokine producer phenotype) was independent of the cytokine measured and the TLR agonists used. Thus, a donor exhibits a specific immune (cytokine) response or “immunotype” across cytokines released and TLR stimulation. Importantly, the high- or low-cytokine producer phenotype was different and did not overlap between the TLR and TCR stimulation conditions. In other words, a donor who is ahigh-cytokine producer following TLR stimulation will not be a high-cytokine producer following TCR stimulation (or the inverse). Therefore, we have defined TLR- or TCR- related Immunotypes (IT) as “a grading classification of the magnitude of the cytokine immune response” with IT1, IT2, and IT3 as low, intermediate, and high immunotypes. This suggests that two independent TLR and TCR ITs (TLR IT1 and TCR IT3) can co-exist in the same subject. We are now currently evaluating the genetic markers underlying these ITs before validating them in large cohort of IBD patients with mild-to-moderate and severe disease.This PhD thesis provides some data suggesting that the assessment of the pharmacokinetics of biologics early on at the initiation of treatment could help predict how the patient will respond in the long run. In parallel, this PhD thesis provides some advances in the understanding of the inter-variability of the immune response, a fundamental step before the identification of potential genetic markers underlying the inter-variability of inflammation and, hence, the severity of disease in IBD.