Classic psychedelics do not affect T cell and monocyte immune responses

Psychedelics are psychoactive substances that alter cognition and perception principally via agonism at serotonin (5-HT) receptors, with 5-HT2A activation considered the primary psychedelic trigger. The so-called classic psychedelics include tryptamines (for example N,N-dimethyltryptamine, DMT, and psilocybin/psilocin), lysergamides (such as lysergic acid diethylamide, LSD) and phenethylamines (such as mescaline). Earlier research has reported therapeutic potential of these compounds across several psychiatric and neuropsychiatric indications, and some in vivo and ex vivo studies have suggested immune-modulatory actions linked to 5-HT2A and 5-HT1A receptor activation. However, reports are heterogeneous and direct effects on human immune cells in vitro have been sparsely explored, raising safety questions because T cells and monocytes play central roles in host defence and inflammation. Aday and colleagues set out to test whether four classic psychedelics—LSD, psilocin, DMT and mescaline—directly affect primary human T lymphocyte function and monocyte NF-κB signalling in vitro. The study specifically examined T cell proliferation, early activation marker expression and stimulated cytokine release from CD4+ and CD8+ T cells, and LPS-triggered NF-κB induction in a monocytic reporter cell line, to clarify whether these compounds exert direct immunomodulatory effects that might be relevant to clinical safety in assisted psychotherapy settings.

Peripheral blood mononuclear cells (PBMCs) were isolated from anonymised adult donor blood obtained via hospital blood donation services; the extraction states no sample ID numbers were available. Cells were cultured in RPMI 1640 medium with 10% fetal calf serum, L-glutamine and antibiotics, and stimulated with anti-human CD3 and CD28 monoclonal antibodies (100 ng/ml) to activate T cells. Cell proliferation was assessed using CFSE (carboxyfluorescein diacetate succinimidyl ester) labelling. PBMCs were labelled with CFSE and cultured with test compounds for 72 h; cyclosporine A (CsA, 5 µg/ml) served as a positive control for proliferation inhibition. Intracellular cytokine production and activation marker expression were measured after 48 h of drug treatment: cells were re-stimulated with phorbol-12-myristate-13-acetate (PMA, 50 ng/ml) and ionomycin (500 ng/ml) in the presence of protein transport inhibitor for 4 h, fixed with paraformaldehyde and stained with fluorochrome-conjugated antibodies for flow cytometric analysis. Monocytic NF-κB activity was evaluated using a THP-1 NF-κB–eGFP reporter cell line; cells were stimulated with lipopolysaccharide (LPS, 100 ng/ml) and incubated for 24 h with medium, control substances or psychedelics. Dexamethasone was included as a positive control for NF-κB inhibition. The psychedelic concentration range tested was 1–30 µM. The authors note that these concentrations are suprapharmacological for most compounds when compared to reported maximal plasma concentrations from human studies (for example: LSD ~12.1 ± 1.2 nM, psilocin ~97.9 ± 26.4 nM, DMT ~236.4 ± 67.5 nM after the regimens cited; mescaline plasma levels after a 200 mg oral dose were reported around 4 µM). Flow cytometry analyses were performed across independent experiments: three replicates for PBMC proliferation and cytokine assays and four replicates for the THP-1 NF-κB assays. Statistical comparisons used one-way ANOVA (Brown–Forsythe and Welch tests) followed by Dunnett’s T3 multiple comparisons versus stimulated control cells; p < 0.05 was the threshold for statistical significance.

Proliferation: Across three independent experiments, none of the tested psychedelics (LSD, psilocin, DMT, mescaline) altered proliferation of stimulated primary human T cells at concentrations up to 30 µM. The positive control CsA produced strong inhibition of T cell proliferation. The addition of 10 µM of any psychedelic to CsA further accentuated proliferation inhibition relative to CsA alone. CD8+ T cell activation and mediator release: Measurement of CD69 as an early activation marker showed no reduction after exposure to any psychedelic. Interleukin-2 (IL-2) release from stimulated CD8+ T cells was not significantly changed by psychedelics, whereas CsA significantly reduced IL-2; this CsA-mediated reduction was more pronounced when 10 µM of any psychedelic was added. Release of CC-chemokine ligand 4 (CCL4/MIP-1β) and tumour necrosis factor-alpha (TNF-α) from CD8+ T cells was not significantly affected by psychedelics; CsA reduced both CCL4 and TNF-α, with no consistent further accentuation by concurrent psychedelic treatment. CD4+ T cell activation and mediator release: Similar to CD8+ results, CD69 expression on CD4+ T cells was not reduced by any psychedelic. IL-2 and several other cytokines—including IL-21, IFN-γ and IL-17—were generally unchanged by psychedelics at concentrations up to 10 µM. CsA reduced IL-2, IL-21, IFN-γ and IL-17 as expected. Notably, at 30 µM each psychedelic reduced TNF-α release from stimulated CD4+ T cells to below 50% of control; the magnitude of this reduction was similar to the combination of CsA with 10 µM psychedelic. The authors emphasise that the 30 µM concentration is suprapharmacological for most tested compounds. Monocytic NF-κB induction: In THP-1 NF-κB–eGFP reporter assays (four independent experiments), none of the psychedelics reduced LPS-triggered NF-κB signalling as measured by GFP expression. Dexamethasone, included as a positive control, significantly reduced the NF-κB signal. Overall, the study found no evidence that LSD, psilocin, DMT or mescaline directly modulate the proliferation, activation marker expression or stimulated cytokine release of primary human T lymphocytes, nor that they inhibit NF-κB induction in a monocytic reporter line. Statistical testing used one-way ANOVA with Dunnett’s T3 comparisons and p < 0.05 as significance cutoff.

Aday and colleagues interpret their findings as evidence that LSD, psilocin, DMT and mescaline do not directly modulate primary human T lymphocyte proliferation or stimulated cytokine production, nor do they inhibit LPS-triggered NF-κB signalling in a monocytic reporter cell line. The authors frame this as reassuring for the safety of classic psychedelics when used in assisted psychotherapy for patients with life-threatening diseases, where direct immunosuppression of lymphocytes or monocytes would be undesirable. They acknowledge that prior in vitro and animal studies have reported anti-inflammatory effects linked to 5-HT2A or 5-HT1A receptor activation; however, those studies often used much higher concentrations than clinical plasma levels or examined downstream responses to externally applied inflammatory mediators, limiting direct comparability. The authors discuss an in vivo human study in which DMT caused transient decreases in CD3+ and CD4+ cell percentages shortly after dosing; this earlier work suggested an indirect mechanism mediated by neuroendocrine changes, particularly cortisol release, rather than direct drug–immune cell interactions. The current authors note that psychedelics, as well as stimulant compounds like d-amphetamine and MDMA, can acutely raise cortisol and activate the sympathetic nervous system—responses known to modulate lymphocyte distributions and functions—and propose that such indirect stress-related mechanisms could underlie immune changes observed in vivo. Several limitations acknowledged by the authors are methodological and contextual: the in vitro assays required 24–72 h incubation to measure cytokine secretion and proliferation reliably, whereas some in vivo immune shifts peak within hours; consequently, transient in vivo effects tied to rapid neuroendocrine changes may not be captured ex vivo. They also note that downstream consequences of altered TNF-α signalling were not investigated here, and that some previously reported immunomodulatory agents (for example the selective 5-HT1A agonist DPAT) may differ from classic psychedelics in potency or receptor selectivity, which could explain discrepancies between studies. The authors conclude that while classic psychedelics do not appear to exert direct immunosuppressive effects on human T cells or monocytes in vitro, indirect anti-inflammatory effects observed in vivo are plausibly mediated via cortisol or sympathetic activation rather than direct action on immune cells.

The study concludes that LSD, psilocin, DMT and mescaline do not directly stimulate or suppress proliferation or cytokine secretion of primary human T lymphocytes nor do they inhibit NF-κB induction in monocytes in vitro. The authors add that classic psychedelics may nevertheless produce indirect anti-inflammatory effects in humans or animal models by inducing cortisol release—potentially via 5-HT2A receptor activation—but these effects are not attributable to direct stimulation of T lymphocytes or monocytes. The findings are presented as relevant to the safe use of classic psychedelics in assisted psychotherapy for patients with life-threatening illnesses.