Rethinking TYK2 P1104A: a flawed evolutionary trade-off in tuberculosis?

Area of Science:

• Immunology
• Evolutionary genetics
• Human genetics

Background:

• The TYK2 P1104A variant has been considered an evolutionary trade-off, conferring protection against autoimmunity while increasing tuberculosis (TB) susceptibility.
• This variant's frequency decline in European populations since the Bronze Age has been attributed to this supposed compromise.
• Recent evidence challenges the simplicity of this evolutionary trade-off narrative.

Purpose of the Study:

• To re-evaluate the mechanistic impact of the TYK2 P1104A variant on immune signaling pathways.
• To investigate the variant's role in host defense against Mycobacterium tuberculosis and other pathogens.
• To explore the implications of the variant's effects on immune regulation and evolutionary pressures.

Main Methods:

• Analysis of TYK2 catalytic activity and downstream cytokine signaling (IL-12, IL-23, type I interferon).
• Assessment of immune cell responses, including interferon-gamma production and Th17 cell differentiation.
• Review of clinical data on IL-23 and IL-17 inhibitor use and TB reactivation rates.

Main Results:

• The P1104A variant reduces TYK2 activity but preserves IL-12-dependent interferon-γ responses crucial for antimycobacterial immunity.
• The variant selectively impairs IL-23 signaling, attenuating Th17 responses, and dampens type I interferon signaling.
• Clinical data show no significant increase in TB reactivation with IL-23/IL-17 inhibition, questioning the essential role of these pathways in TB defense.

Conclusions:

• The TYK2 P1104A variant may fine-tune immune responses by uncoupling protective Th1 immunity from detrimental inflammation, rather than simply increasing TB susceptibility.
• The variant's effects could influence TB outcomes by modulating pathology and immune suppression.
• Multiple infectious pressures, not solely TB, may have driven the evolutionary trajectory of the TYK2 P1104A variant.