This is a parallel group, double-blinded, placebo-controlled study. Participants with MDD (n=90) and HC (n=90) will be randomly assigned (2:1) to receive either lipopolysaccharide (LPS) (0.8ng/kg of body weight; E. coli group O:113) or placebo (same volume of 0.9% saline) administered as an intravenous bolus. This will yield the following groups: MDD-LPS (n=60), MDD-Placebo (n=30), HC-LPS (n=60), HC-placebo (n=30). Individuals who are eligible for the study will be randomly assigned through the use of a block randomization scheme to ensure the inclusion of equal numbers of HC and MDD participants in each group. MDD participants will complete up to 10 study visits (V1-V10) while the HCs will complete up to four visits (V1-V4). At visit 1 (V1), each subject will be consented and will complete the following: self-report and clinician-administered scales (including the Columbia Suicide Severity Rating Scale), a physical exam that includes an EKG and vital signs (temperature, pulse, blood pressure), safety labs (CBC, CMP, TSH, HbA1C, HIV, HCV), a urine drug screen, an alcohol breathalyzer, and a baselineMRI session comprising the following scans: clinical, anatomical, visceral interoceptive attention (VIA) task, monetary incentive delay task (MID) and resting state fMRI. Vital signs will be assessed in a supine position with a completely automated device. Blood pressure and heart-rate measurements will be preceded by at least five minutes of rest in a quiet setting without distractions. Blood will be drawn via venipuncture using sterile techniques by a trained phlebotomist or nurse.

Subjects that pass the safety screen return two days later for the experimental session (V2). Prior to infusion of LPS or saline, the Columbia, EKG, urine drug screen, and breathalyzer are repeated, vital signs are taken, and baseline (T0) rating scales and blood draws are completed. A nurse will insert a catheter with a heparin lock into the non-dominant forearm for blood draws and a continuous saline flush, and one into the dominant forearm for LPS or saline administration. Prior to the LPS/saline infusion, participants will be hydrated with 500cc of saline. The LPS to be used in the study will be obtained from the NIH Clinical Center. Vital signs will be assessed every 30 min post infusion of LPS/saline for the first 2 hours (and thereafter every hour). Blood samples will be taken at baseline (T0), at T1 and then at T1.5 and T3.5 (\~ peak response) and prior to discharge at T6. Participants also will complete hourly measures of sickness symptoms (e.g., fatigue, pain), anxiety, depression, and anhedonia. Symptoms will be recorded and graded according to FDA criteria. Two hours post-infusion at the approximate peak of the inflammatory response, the participants will complete the same MRI scans as at baseline. Subject to the judgment of a physician, the participants will be discharged 6 hours post injection so that the total experimental procedure will take \~8 hours to complete. Participants return the following day (T24) and one week later (V4) for mood ratings, blood draws, and identical MRI sessions. MDD participants will also be followed longitudinally for 6 months (phase 2), in the process completing remote or in-person psychological assessments at months 1 through 6 post LPS-infusion.

Participants will be recruited from the Tulsa metropolitan area through internet, radio, print advertisements and will be evaluated at LIBR. We may also advertise the study to students on Tulsa-area campuses using flyers and potentially also recruitment tables on campus, and discussions with organizations, classes, and faculty where permission from the appropriate university officials is obtained. A total of 200 human participants (180 completers of phase I) will be involved in the proposed research. Half the participants will be males or females between the ages of 18 and 65 with MDD and current symptoms of depression (mild to severe) and half the participants will be HC with no personal history of psychiatric illness. A DSM-V diagnosis of MDD will be made with the MINI International Neuropsychiatric Interview and current symptoms of depression will be measured with the clinician-administered Montgomery Asberg Depression Rating Scale (MADRS), the self-report Patient Health Questionnaire (PHQ-9), and the Quick Inventory of Depressive Symptoms (QIDS-SR). Depressed participants will be required to be in good general health (as evaluated during the screening visit, including physical exam, safety labs and EKG) and to be 18-65 years of age. MDD participants will be required to have symptoms of depression, that is, a MADRS score of ≥7 or a PHQ-9 score ≥10. MDD participants are also required to be unmedicated for at least 4 weeks (8 weeks for fluoxetine) or currently receiving treatment with only one anti-depressant medication. HC participants will be required to be in good general health and to be 18-65 years of age.

There are three main aims: to identify immune pathways and neural circuits that respond differently to LPS in MDD vs. HC subjects; (2) to test whether the strength of inflammatory changes induced by LPS is associated with degree of change in anhedonic symptoms and neural circuits in the MDD group, and (3) to identify a biotype of MDD that shows a differential immunological and neurophysiological response to LPS. The main outcome variables are symptoms of anhedonia measured with the Snaith-Hamilton Pleasure Scale (SHAPS), cytokines (Il-6, IL-8, IL-10, and TNF), and BOLD signal change in the neural circuitry mediating interoceptive processing (the insula and cingulate cortex), and BOLD signal change in the neural circuitry mediating reward (ventral striatum). The exploratory aim is to determine whether the acute inflammatory response to LPS can predict the clinical course of depression over a period of six months. The main outcome of this component of the study is self-reported depressive symptoms assessed with the QIDS-SR.