Multicenter Randomized Double Blind Controlled-study to Assess the Potential of Methotrexate Versus Placebo to Improve and Maintain Response to Anti TNF- Alpha Agents in Adult Patients With Moderate to Severe Psoriasis

Inclusion Criteria: * Male or female patients aged 18 years or older * Patients with moderate to severe chronic plaque psoriasis with or without psoriatic arthritis AND who had started any first line of anti TNF alpha according to the labeling of these drugs BEFORE the study (i. e. the study will be restricted to anti TNF alpha naïve patients (first course). Patients who have been previously treated with any other non anti TNFA alpha biopharmaceutical (ustekinumab or anti IL17- secukinumab, ixekizumab, brodalumab) as a first line of biotherapy for psoriasis could be enrolled) after a wash out period of at least 5 half life time of the drug i.e. 16 weeks before inclusion * No significant anomalies from a blood sampling performed within 15 days before patient selection that could lead to MTX contraindication * Patients with an EARLY start of anti TNF alpha, i. e. within the 7 days preceding the first study drug (methotrexate or placebo) administration * Male or female patients agreeing to use a reliable method of birth control during the study * Male patients agreeing to use a reliable method of birth control during the study i. e. preservative and for at least 6 months following the last dose of investigational product, the patient's partner treated by methotrexate must be notified of the teratogenic risk of methotrexate and should be under effective contraception throughout the study * Female patients: Are women of childbearing potential who are negatively tested for pregnancy and agree to use a reliable method of birth control (every month) or remain abstinent during the study and for at least 6 months following the last dose of investigational product, whichever is longer. Methods of contraception considered acceptable include oral contraceptives, contraceptive patch, intrauterine device, vaginal ring. And Negative serum b-Human Chorionic Gonadotrophin (B-HCG) test at screening Or Women of non-childbearing potential, defined as: Women who have had a surgical sterilization (hysterectomy, bilateral oophorectomy, or tubal ligation) Or Women \> = 60 years of age Or Women \>=40 and \< 60 years who have had a cessation of menses for \>=12 months and a follicule stimulating hormone (FSH) test confirming non - childbearing potential * Patients with previous failure or intolerance but no absolute contraindication to previous methotrexate medication for psoriasis can be enrolled, on the condition that methotrexate (whatever the dose) has been stopped at least two months before the inclusion. * For patients who have never been previously treated with MTX, taking a test dose of MTX (2.5 mg to 5 mg) with normality of the laboratory tests conducted one week to remove a reaction idiosynchrasie before inclusion in the protocol * Patients should be affiliated to the French Social Security system * Patients who have given written consent for the study Exclusion Criteria: * Patients with isolated pustular, erythrodermic and or guttate forms of psoriasis * Patients with prior use of any anti TNF alpha * Patients who have known active liver disease (with the exception of a simple liver steatosis, transaminases and/or alkaline phosphatases \> 2 ULM ) or history of liver disease in the past 2 years, whatever the related diagnosis but which could interfere with MTX safety and according to the summary of the SmPC * Intake of restricted medications (cf section VIII.5.) or other drugs considered likely to interfere with the safe conduct of the study, as assessed by the investigator and according to the Summary of the Product Characteristics (SmPC), including any drug intakes that could interfere with methotrexate metabolism or that could enhance liver and /or hematologic toxicity and according to the SmPC * Patient with evidence or positive test for HIV, Hepatitis C virus, Hepatitis B virus (patients who are negative for hepatitis B surface antigen but positive for anti-hepatitis B anti body (HBsAb+ and HBcAb+) and negative for serum HBV DNA may participate in the study * High alcohol intake defined as more than 60 g of daily intake (approx daily intake of 0.5 l of wine or equivalent), * Patients who have a known allergy or hypersensitivity to MTX * Patients who have a known serious adverse event to MTX prior to the trial leading to MTX discontinuation in the past * Presence of significant hematologic or renal disorder or abnormal laboratory values at screening that, in the opinion of the investigator is associated with an unacceptable risk to the patient to participate in the study * Clinical laboratory test results at screening that are outside a normal reference rating for the population and are considered clinically significant, or/and have any of the following specific abnormalities: * Total white blood cell count \<3G/L * Neutrophil count \< 1.5 G/l * Lymphocytes count \< 0.5G/l * . Platelet count \< 100 G/l * Aspartate aminotransferase (AST) or alanine aminotransferase (ALT)\>3 times the upper limit of normal (ULM) * Hemoglobin \<8.5g/dL (85.0 g/L) * Creatinine clearance \<40ml/min (Cockcroft formula) * For women: pregnant or breast feeding * Patients who have an active or serious infection or history of infections (bacterial, viral, fungal or mycobacteria), requiring hospitalization or intra venous anti-infectives infusion within 4 weeks prior to the baseline, * Patients who have primary or secondary active immunodeficiency * Patients who had live vaccine administration within 4 weeks prior to baseline * Patients who have any current or active cancer (with the exception of patient with successfully treated basal cell carcinoma or in situ cervix carcinoma) * Patients who had history of malignancy within 5 years prior to the trial that could contraindicate the use of an immunosuppressant * Patients who will not be available for protocol which require study visits or procedures * Patients who is not affiliated to the French Social Security system * Patients unable to give informed consent and/or comply with all required study procedures