Summary: A pilot randomized controlled clinical trial provides the first targeted clinical evidence that immunotherapy may offer a new treatment avenue for people with treatment-resistant depression. The study tested whether tocilizumab, an anti-inflammatory drug used for autoimmune conditions such as rheumatoid arthritis, can reduce depressive symptoms by blocking the interleukin‑6 (IL‑6) inflammatory pathway.
Researchers screened a 30-person cohort to identify those with persistent, low-grade systemic inflammation, then treated and followed participants to determine whether reducing peripheral inflammation could produce meaningful improvement in mood and function, bypassing traditional neurotransmitter-focused mechanisms.
Key Facts
- The serotonin bottleneck: Standard antidepressants target brain monoamines such as serotonin, norepinephrine, and dopamine. Yet about one in three people with depression do not respond adequately to these treatments.
- Inflammation in depression: Research indicates roughly one-third of people with depression show peripheral inflammatory markers, suggesting an overactive immune system may contribute to psychiatric symptoms for a subgroup of patients.
- IL‑6 as a causal target: Genetic and longitudinal studies (including Mendelian randomization) implicate interleukin‑6 (IL‑6) as a driver of depressive illness rather than a simple bystander, supporting IL‑6 inhibition as a rational treatment target.
- Encouraging remission rates: In this small proof‑of‑concept trial, 54% of participants treated with tocilizumab achieved remission, versus 31% in the placebo group.
- Number needed to treat (NNT): The observed NNT for tocilizumab in this sample was 5, which compares favorably to typical first‑line selective serotonin reuptake inhibitors (SSRIs), for which the NNT is usually around 7.
- Precision medicine approach: This trial is the first to target the IL‑6 receptor (IL‑6R) in depression and to use an engineered biological screening method to select patients most likely to benefit from immune‑modulating therapy.
Source: University of Bristol
Immunotherapy as a promising option for difficult-to-treat depression
A University of Bristol–led pilot randomized controlled trial, published in JAMA Psychiatry, explores whether systemic inhibition of IL‑6 with the monoclonal antibody tocilizumab can reduce depressive symptoms in people who have not improved with standard antidepressants. The trial provides early clinical evidence that targeting inflammation can improve mood, fatigue, anxiety, and quality of life for a biologically defined subgroup of patients.
The pilot trial enrolled 30 adults with moderate‑to‑severe depression who had demonstrated poor response to antidepressants and who showed low‑grade systemic inflammation on two separate blood tests. Participants were randomized to a single intravenous infusion of tocilizumab (n = 14) or normal saline placebo (n = 16) and were assessed at baseline and 7, 14, and 28 days post‑infusion.
As expected in a small proof‑of‑concept study, most differences did not reach conventional statistical significance. However, the pattern of change favored tocilizumab: stepwise improvements over time were seen in somatic depressive symptoms, overall depression severity, fatigue, state anxiety, and quality of life, with the largest effects at day 28. At final follow‑up, remission occurred in 53.9% of the tocilizumab group compared with 31.3% of the placebo group, producing an estimated NNT of 5. Response rates also favored tocilizumab (46.2% vs 18.8%; NNT = 4).
Baseline high‑sensitivity C‑reactive protein (hs‑CRP) levels, rather than circulating IL‑6 levels, tracked clinical improvement, suggesting that hs‑CRP may be a practical biomarker for identifying patients most likely to benefit from anti‑IL‑6 immunotherapy. Tocilizumab was generally well tolerated in this small sample, with no serious adverse events or study withdrawals attributed to the drug.
Golam Khandakar, Professor of Psychiatry and Immunology and the study’s senior author, described the trial as an important milestone toward new treatments for difficult‑to‑treat depression, emphasizing that this is among the first randomized trials to evaluate immunotherapy in psychiatry, the first to target IL‑6R in depression, and the first to apply a biologically targeted selection strategy.
Lead author Dr Éimear Foley noted that depression affects a large proportion of the population and that many patients do not derive sufficient benefit from existing treatments. The study’s targeted approach—treating patients who both failed standard antidepressants and showed consistent low‑grade inflammation—provides a roadmap for precision psychiatry and supports conducting a larger phase III trial to establish efficacy and safety in a more definitive sample.
One study participant reflected positively on contributing to research, underscoring the importance of clinical trials to advance treatment options.
Funding: The double‑blind proof‑of‑concept randomized controlled trial recruited 30 participants via the University of Cambridge and the Cambridgeshire and Peterborough NHS Foundation Trust and followed them for four weeks after infusion. The research was funded by Wellcome, with additional support from NIHR Biomedical Research Centres (Bristol and Cambridge) and the BMA Foundation J Moulton grant.
Key Questions Answered
A: Emerging evidence shows that a substantial subgroup of people with depression have systemic inflammation. Inflammatory proteins such as interleukin‑6 can influence brain function and neurotransmission. By blocking the IL‑6 pathway with tocilizumab, peripheral inflammation is reduced, which may in turn relieve inflammatory signals to the brain and alleviate depressive symptoms for patients whose illness is driven in part by immune activation.
A: Small pilot trials are designed as proof‑of‑concept studies to estimate effect sizes and identify measurable outcomes, not to provide definitive evidence. The trial produced consistent directional improvements across several clinically meaningful measures and a sizable difference in remission rates, supporting the biological hypothesis and justifying a larger, adequately powered phase III trial to determine efficacy definitively.
A: The trial used a precision medicine approach: only patients who both failed standard antidepressant therapy and showed persistent low‑grade inflammation on repeat blood tests were enrolled. This targeted selection increases the likelihood of detecting a treatment effect by matching the biological mechanism of the drug to the patients most likely to benefit.
Editorial Notes
- Edited by a Neuroscience News editor.
- Journal paper reviewed in full by the editorial team.
- Additional context provided by staff for clarity and completeness.
About this research news
Author: Joanne Fryer (University of Bristol)
Source: University of Bristol
Contact: Joanne Fryer – University of Bristol
Image: Image credited to Neuroscience News
Original Research: Open access. “Interleukin 6 as a treatment target for depression: a proof‑of‑concept randomized clinical trial” by Éimear M. Foley et al., JAMA Psychiatry. DOI: 10.1001/jamapsychiatry.2026.1053
Abstract
Interleukin 6 as a treatment target for depression: a proof‑of‑concept randomized clinical trial
Importance
Interleukin‑6 (IL‑6) is a central inflammatory cytokine implicated in depression. Randomized clinical trials testing IL‑6 inhibition for depression have been limited.
Objective
To identify likely treatment‑sensitive outcomes and estimate effect sizes for systemic IL‑6 inhibition in people with difficult‑to‑treat depression.
Design, Setting, and Participants
This 4‑week, double‑blind, parallel‑arm, placebo‑controlled randomized trial recruited adults with moderate‑to‑severe ICD‑10 depression, poor antidepressant response, low‑grade systemic inflammation (hs‑CRP ≥ 0.3 mg/dL on two tests), and prominent somatic depressive symptoms. Participants were randomized and assessed at baseline and 7, 14, and 28 days after a single infusion. Data analysis occurred from 2023 to 2025.
Intervention
Single intravenous infusion of the IL‑6 receptor antagonist tocilizumab (8 mg/kg, max 800 mg) or normal saline placebo.
Main Outcomes and Measures
Primary outcome: somatic depressive symptoms at day 14. Secondary outcome: total depression severity. Exploratory outcomes included fatigue, anxiety, anhedonia, quality of life, and cognition, all measured with validated instruments and interpreted against clinically meaningful thresholds.
Results
Thirty participants (mean age 41.1 years; 80% female) were randomized (14 to tocilizumab, 16 to placebo); 29 completed follow‑up. No primary or secondary outcome reached statistical significance in this small sample. Nonetheless, tocilizumab showed greater stepwise improvement over time across several outcomes, with the largest differences at day 28. Remission and response rates favored tocilizumab (remission: 53.9% vs 31.3%, NNT = 5; response: 46.2% vs 18.8%, NNT = 4). Baseline hs‑CRP, but not IL‑6 level, tracked clinical improvement. Tocilizumab was well tolerated.
Conclusions and Relevance
These results identify candidate outcomes, effect sizes, and patient selection strategies for testing systemic IL‑6 inhibition in difficult‑to‑treat depression and support the need for a large‑scale efficacy trial.
Trial Registration
ISRCTN Identifier: ISRCTN16942542