Patients with psoriasis show elevated inflammasome activity in blood — increased plasma IL-1β and IL-18, constitutive overexpression of the inflammasome sensors NLRP3, NLRP1, and AIM2 in peripheral blood cells, and heightened caspase-1 reactivity in myeloid subsets — and the cytokine TNF-α can activate the NLRP3 inflammasome directly, without a priming signal, through a TNFR–caspase-8–caspase-1 pathway that is independent of pyroptosis. Anti-TNF therapy, but not methotrexate, normalizes these inflammasome readouts, suggesting a mechanism for the cardiovascular risk–reducing effect attributed to TNF blockade. However, all of this evidence is ex vivo or cross-sectional; whether TNF-α–mediated NLRP3 activation causally drives psoriasis-associated systemic inflammation and the development of atherosclerosis and major adverse cardiovascular events in vivo remains unestablished. The problem is to settle this causality — positively or negatively — including the mechanistic attribution of the observed readouts to caspase-1 versus other inflammatory proteases, the activation requirements of inflammasomes in the CD4+ lymphoid compartment, and replication in adequately powered clinical cohorts with cardiovascular endpoints.

Background

Psoriasis is a chronic immune-mediated skin disease that is not confined to the skin: it is associated with systemic inflammation and elevated risk of cardiovascular comorbidities, particularly atherosclerosis (plaque buildup in arteries) and major adverse cardiovascular events (MACE, a composite of myocardial infarction, stroke, and cardiovascular death). A candidate molecular bridge between skin disease and vascular disease is the inflammasome — a multiprotein complex assembled by innate immune cells around sensor proteins such as NLRP3, NLRP1, and AIM2. Upon activation, the inflammasome activates caspase-1, a protease that processes the inactive precursors of the cytokines interleukin-1β (IL-1β) and IL-18 into secreted proinflammatory mediators, and typically also triggers pyroptosis, a lytic inflammatory cell death. IL-1β in particular is an established driver of atherosclerosis: the CANTOS trial showed that blocking IL-1β with the monoclonal antibody canakinumab reduces recurrent cardiovascular events in patients with stable coronary artery disease.

Verma, Fekri, Sigurdardottir, Bivik Eding, Sandin, and Enerbäck (Journal of Investigative Dermatology, 2021; epub 2020) examined inflammasome activity directly in the blood of psoriasis patients. They found elevated plasma IL-1β and IL-18 that did not correlate with skin lesion severity; increased constitutive expression of NLRP3, NLRP1, and AIM2 in peripheral blood cells (including patients with mild disease); and increased reactivity of caspase-1 in myeloid blood subsets, measured with FLICA (fluorochrome-labeled inhibitor of caspases — a cell-permeable probe, in this case YVAD-FMK–based, that binds active caspase-1 and is read by flow cytometry). Mechanistically, they showed that TNF-α (tumor necrosis factor-α), a key psoriasis cytokine, selectively activated the NLRP3 inflammasome in human whole blood without requiring a priming signal, acting through a TNFR–caspase-8–caspase-1 'alternative' inflammasome pathway that proceeds independently of pyroptosis. Clinically, patients on anti-TNF therapy — but not patients on methotrexate — had normalized plasma IL-1β/IL-18 and normalized caspase-1 reactivity. The authors proposed that this TNF-α–mediated inflammasome activation may contribute to the systemic inflammation of psoriasis and that its normalization by TNF blockade could explain the cardiovascular risk reduction observed in some epidemiological studies of anti-TNF-treated patients.

Several issues were left explicitly unresolved. All findings came from unmanipulated whole blood under short-term ex vivo stimulation, so the in vivo causal significance — for systemic inflammation, for atherosclerosis and MACE, and for non-blood tissue sources — was untested. The FLICA-based caspase-1 readout and IL-1β/IL-18 production were not formally attributed to caspase-1 alone: other inflammatory caspases or parallel proteolytic pathways (for example caspase-8 itself) could contribute. The CD4+ T-cell compartment showed increased sensor expression without short-term caspase-1 reactivity, leaving the activation requirements and functional consequences of inflammasomes in lymphoid cells undefined. And the clinical observations (e.g., reduced plasma IL-1β/IL-18 under anti-TNF) did not reach statistical significance at the available sample sizes, so replication in larger cohorts with standardized endpoints was needed.

Problem Statement

Establish, or refute, the causal contribution of TNF-α–mediated NLRP3 inflammasome activation to psoriasis-associated systemic inflammation and cardiovascular comorbidity in vivo. Verma et al. (2021) showed that TNF-α selectively activates the NLRP3 inflammasome in psoriasis patient blood ex vivo — via a TNFR–caspase-8–caspase-1 pathway independent of pyroptosis — and that anti-TNF therapy normalizes plasma IL-1β/IL-18 and cellular caspase-1 reactivity; the unresolved question is whether this axis actually drives disease in the living organism. A complete answer requires, as jointly supporting parts of one objective: (i) in vivo causal evidence — for example, in animal models combining psoriasiform skin inflammation with an atherosclerosis-prone background using cell-type-specific deletion or inhibition of NLRP3/caspase-1, or in longitudinal human studies with inflammatory and vascular endpoints — determining whether TNF-α–driven NLRP3 activation causally elevates systemic IL-1β/IL-18, promotes atherosclerosis progression, and contributes to major adverse cardiovascular events, or is instead an epiphenomenon of systemic inflammation; (ii) attribution of the observed caspase-1 reactivity (FLICA signal) and IL-1β/IL-18 production in patient blood specifically to caspase-1 rather than other inflammatory caspases or parallel proteolytic pathways; (iii) definition of the activation requirements and functional consequences of inflammasome sensors in the CD4+ lymphoid compartment, where sensor expression is increased but short-term caspase-1 reactivity is absent; and (iv) replication, in adequately powered patient cohorts with standardized clinical endpoints and longitudinal follow-up, of whether anti-TNF–induced normalization of inflammasome readouts predicts clinically meaningful reductions in cardiovascular risk — including reconciliation with the prior randomized PET/CT trials that found no reduction in vascular inflammation under TNF blockade. The answer may adjudicate these parts from the existing in vivo and clinical literature and must identify the observations or experiments that would decisively settle each; it is not required to report new primary data.

References

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