Elicit: IL-12/IL-23 Inhibition and T Cell Differentiation

IL-12/IL-23 Inhibition and T Cell Differentiation

How does IL-12/IL-23 inhibition affect Th1 and Th17 differentiation in humans?

The answer depends critically on whether the antibody hits the shared p40 subunit (ustekinumab, briakinumab — blocks both IL-12 and IL-23) or the p19 subunit (guselkumab, risankizumab, tildrakizumab — IL-23 only). These produce qualitatively different effects on Th1 vs Th17 in humans.

p40 inhibition (IL-12 + IL-23): hits both axes, but in vivo Th1 suppression is weaker than the mechanism predicts

Ustekinumab cleanly neutralizes both IL-12–driven Th1 responses (IFN-γ) and IL-23–driven Th17 responses (IL-17) in human in vitro bioassays, since p40 is shared (Benson et al. 2008). In a single-dose study in psoriasis patients, anti-p40 reduced lesional IFN-γ and other type-1 cytokines/chemokines at two weeks (Toichi et al. 2006). The cleanest in vivo demonstration of dual suppression comes from an RCT in allogeneic HCT: ustekinumab significantly reduced both IL-17 and IFN-α production by host-reactive donor T cells, with a compensatory rise in IL-4 (Pidala et al. 2017).

In Crohn's disease, ustekinumab decreased peripheral-blood Th17 frequency and downregulated the Th17 differentiation pathway in colonic mucosa, while anti-TNF did not (Ihara et al. 2021).

The counterintuitive finding worth flagging: a small psoriasis study found that ustekinumab improved skin disease without measurably altering memory CD4+ cytokine production, Th1/Th17 differentiation from naive cells, or TCR repertoire (Tsuda et al. 2012). So at least in peripheral blood T cells of treated psoriasis patients, the expected Th1/Th17 collapse isn't always seen — improvement may reflect blocking IL-12/23 signaling on innate cells (dendritic cells, myeloid cells) rather than wholesale reprogramming of circulating T-cell populations.

Selective IL-23p19 inhibition: stronger and more selective Th17 effects

p19-selective blockade leaves IL-12 → Th1 signaling intact and targets the IL-23 → Th17 maintenance loop specifically. In psoriatic arthritis, guselkumab produced a significantly greater reduction in serum IL-17A and IL-17F than ustekinumab, normalizing them to healthy-control levels by week 16 (Siebert et al. 2019). A separate PsA cohort showed early (week 4) and sustained reductions in IL-17A, IL-17F, and IL-22 with guselkumab in both biologic-naive and TNFi-IR patients. Risankizumab in Crohn's selectively downregulated an IL-23–dependent mucosal pathogenic Th17 subset (CCR6+CXCR3−CCR5+ pTh17) in both lamina propria and intraepithelial layers, while ustekinumab in the same study reduced Th1/17 mucosal subsets but not pTh17 — a possible mechanistic basis for risankizumab's edge over ustekinumab on endoscopic response in SEQUENCE (Noviello et al. 2024).

The most granular human data come from single-cell skin profiling in psoriasis: IL-23 inhibition preferentially eliminates IL-23R-expressing T17 subsets (IL17A+IFNG+ and IL17F+IL10−), while an IL-23R-negative IL-17A+IL-17F+ subset actually expands as a proportion after IL-23 blockade (Kim et al. 2025). So "Th17 suppression" is really subset-selective pruning, not uniform depletion.

Mechanism for why IL-23 matters specifically to Th17

IL-23 is required for Th17 stability and effector function — it acts through IL-23R/STAT3, and blocking that signal (with antibodies or with the naturally occurring soluble Δ9 IL-23Rα antagonist) inhibits IL-23–induced STAT3 phosphorylation, IL-17A/IL-17F production, and Th17 maturation in vitro (Yu & Gallagher 2010). IL-12 plays the analogous role for Th1 via STAT4/IFN-γ, which is why p40 blockade (but not p19) reaches Th1.

Bottom-line picture

• p40 (ustekinumab): biochemically dual, but in treated patients the Th17 signal is more consistently reduced than the Th1 signal, and effects can be uneven across tissue vs blood.
• p19 (guselkumab, risankizumab): selectively and more deeply suppresses Th17 effector cytokines and pathogenic IL-23R+ Th17 subsets, leaving Th1 essentially untouched.
• The clinical implication — selective p19 blockade gives better Th17 readouts in head-to-head PsA, psoriasis, and Crohn's comparisons — is concordant across these studies.

Most of this evidence is from psoriasis, psoriatic arthritis, and Crohn's; data are thinner for non-IL-23-driven indications and for direct comparisons of Th1 outcomes between p40 and p19 blockade in the same patients.