Inclusion Criteria:
For MDD participants:
* Written informed consent;
* Ability to comply with the requirements of the study as determined by the PI;
* Ages 18-70 years;
* Any gender;
* DSM-5 diagnosis of MDD in a current Major Depressive Episode;
* Immune enrichment criterion: elevated monocyte count ≥ 500 cells/μL at screening;
* If patient is on antidepressant medication, they must be on a stable dose for ≥4 weeks prior to treatment;
* SHAPS score ≥20
* If female of childbearing potential, must agree to use of a medically accepted form of contraception, or else agree to abstinence until 6 months after the last dose of study drug;
* Male patients, if heterosexually active with a partner who is female of childbearing potential, pregnant, or breastfeeding, must agree to barrier contraception for the treatment period and for at least 6 months after the last dose of the study drug. Female partners of male participants must use at least one form of highly effective contraception starting at least one cycle prior to male patient study drug initiation until 6 months after the last dose of study drug.
* Meet all MRI safety criteria.
For Healthy Volunteers:
* Written informed consent;
* Ability to comply with the requirements of the study as determined by the PI;
* Ages 18-70
* Any gender;
* No current or past DSM-5 psychiatric disorder;
* Meet all MRI safety criteria.
Exclusion Criteria:
For MDD Participants
* A primary DSM-5 psychiatric diagnosis other than MDD, with the exception of comorbid anxiety disorders (including agoraphobia, generalized anxiety disorder, social anxiety disorder, panic disorder) and post-traumatic stress disorder, which are permitted.
* History of schizophrenia, schizoaffective disorder, other psychotic disorder, MDD with psychotic features, or bipolar I or II disorder.
* Diagnosis of a major neurocognitive disorder.
* Moderate or severe substance use disorder within the past 6 months (excluding nicotine use disorder).
* Positive urine toxicology screen for illicit substances at screening.
* Serious or imminent risk of self-harm or violence, as determined by the PI, including:
* Suicide attempt within the past 2 years, or
* C-SSRS ideation score \>2 within the past month.
* Any contraindication to MRI, including claustrophobia, retained metallic foreign bodies, magnetic implants or pacemakers, or inability to tolerate MRI procedures.
* Clinically significant abnormalities on physical examination or laboratory testing.
* Unstable or clinically significant medical illness, including but not limited to hepatic, renal, gastrointestinal, respiratory, cardiovascular (including ischemic heart disease), endocrine, neurologic (including history of severe head injury), immunologic, or hematologic conditions.
* Evidence of active or untreated infection, including
* Active tuberculosis (TB) or untreated latent TB
* Positive QuantiFERON-TB Gold test at screening
* Known HIV infection
* Active Hepatitis B or Hepatitis C infection
* Current or recent (within an appropriate washout period) use of biologic therapies or other immunosuppressive agents (PRN NSAIDs permitted).
* Known hypersensitivity to tocilizumab or its excipients.
* Receipt of a live or live-attenuated vaccine within 30 days prior to first dose, or planned receipt during the study period.
* Pregnancy, breastfeeding, or unwillingness to use effective contraception during the study and for 6 months after the last dose.
* Any condition that, in the opinion of the PI, would compromise participant safety or data integrity.
For Healthy Volunteers
* Any current or unstable medical illness, including hepatic, renal, gastrointestinal, respiratory, cardiovascular (including ischemic heart disease), endocrine, neurologic (including history of severe head injury), immunologic, or hematologic disease.
* Use of biologic therapies or immunosuppressive agents (PRN NSAIDs permitted).
* Positive urine toxicology screen for illicit substances at screening.
* Pregnancy at the time of baseline assessments (e.g., MRI).
