Ustekinumab's Impact on IL-12/IL-23 Pathways

What cellular pathways does ustekinumab inhibit via IL-12/IL-23 (p40) blockade?

I'll pull up the key literature on ustekinumab's mechanism of action via IL-12/IL-23 p40 subunit blockade.

Mechanism of Action

Ustekinumab binds the p40 subunit shared by IL-12 and IL-23 and blocks both from engaging the IL-12Rβ1 receptor chain — the common component of both receptor complexes. This single point of blockade cuts off two distinct downstream signaling cascades simultaneously. Elliott et al. 2009 +1

The IL-12 → Th1 axis

IL-12 (p35/p40 heterodimer) normally engages a receptor complex of IL-12Rβ1 and IL-12Rβ2, triggering Tyk2 and JAK2 activation, which phosphorylates STAT4. Activated STAT4 drives differentiation of naive CD4+ T cells into Th1 cells and induces IFN-γ transcription via the T-bet transcription factor. IFN-γ in turn activates macrophages and amplifies the inflammatory loop. Ustekinumab prevents this entire cascade by blocking IL-12 from reaching its receptor. Watford et al. 2004 +2

The IL-23 → Th17 axis

IL-23 (p19/p40 heterodimer) engages a receptor composed of IL-12Rβ1 and IL-23R. Binding activates JAK2 and Tyk2, phosphorylating STAT3 (the dominant STAT here, distinct from the STAT4 bias of IL-12). STAT3 then drives expression of RORγt, the master transcription factor of Th17 cells. RORγt promotes IL-17A, IL-17F, IL-22, IL-21, and CCL20 production, as well as maintaining Th17 lineage commitment. Very recent phosphoproteomic work (Yang et al., 2025, Journal of Immunology) shows IL-23R also modulates chromatin remodeling proteins to reinforce RORγt expression and suppresses a nuclear hormone receptor that otherwise sustains a homeostatic Th17 state. Sanz 2015 +1. Separately, a 2020 PLoS Biology phosphoproteomics study found that IL-23 activates JAK2 → ROCK (Rho-associated kinase) signaling to promote Th17/Tγδ17 cell migration to inflamed tissue, an effect also cut off by upstream p40 blockade. Pastor-Fernández et al. 2020

Secondary and Downstream Effects

Because IL-12 and IL-23 are upstream regulators, blocking them affects a broad set of downstream mediators: TNF-α, IL-6, IL-8, MMP3, and the chemokine CCL20 all diminish in treated patients. Fiechter et al. 2021. A 2021 Frontiers in Immunology synovial biopsy study in psoriatic arthritis found that ustekinumab significantly downregulates MAPK-ERK and PI3K-Akt signaling in inflamed synovium, while the Wnt pathway was upregulated — a pattern suggesting the drug's in vivo effects extend well beyond the primary JAK-STAT axes, possibly through reduced macrophage and stromal cell activation secondary to decreased Th1/Th17 cytokines. Fiechter et al. 2021 +1

One nuance worth flagging: in psoriatic skin, p40 expression is dominated by the IL-23p19 subunit being upregulated (not IL-12p35), suggesting IL-23 is the more pathogenically relevant of the two targets. This informed the subsequent development of selective IL-23p19 blockers (risankizumab, guselkumab), which appear to have at least comparable efficacy in psoriasis with a more restricted mechanism.

This is from an initial search — a deeper dive into specific tissue compartments or comparison with selective IL-23 inhibitors could surface additional mechanistic detail.