# CD4+ T‐Cell‐Intrinsic IL‐6 Is Critical for Th17 Differentiation and Dampened Responsiveness to CD8+ T Cell‐Mediated Suppression

## TL;DR

This study shows that IL-6 produced by CD4+ T-cells is essential for Th17 cell development and resistance to suppression, and that IL-21 can replace IL-6 in this process.

## Contribution

Demonstrates that T-cell-intrinsic IL-6 is critical for Th17 differentiation and effector resistance, and that IL-21 can bypass this requirement.

## Key findings

- T-cell-produced IL-6 is crucial for optimal Th17 cell development and resistance to suppression.
- IL-21 can bypass the need for IL-6 in Th17 differentiation and effector resistance.
- Loss of IL-6 leads to reduced IL6R stability and STAT3 activation, impairing Th17 function.

## Abstract

This study investigates the role of T‐cell‐intrinsic IL‐6 in Th17 differentiation and effector resistance. Using CRISPR‐Cas9‐mediated Il6 knockdown in primary human CD4+ T‐cells, we demonstrate that T‐cell‐produced IL‐6 is crucial for the optimal development of Th17 cells. Lack of T‐cell‐endogenous Il6 resulted in impaired IL‐17A production as well as significantly increased responsiveness to immune suppression. Importantly, these effects could not be reversed by the addition of exogenous IL‐6, likely due to interference with Il6R expression in the absence of endogenous Il6, with consequent reduction in STAT3 phosphorylation. Blockade of IL‐6 receptor replicated the functional effects of Il6 knockdown and revealed a feedback loop between Il6 and Il6R expression. Of note, IL‐21 was able to bypass the need for IL‐6 in both Th17 differentiation and the development of resistance. Our findings emphasize T‐cell‐endogenous IL‐6 as a critical cytokine in Th17 cell biology and highlight IL‐6 and IL‐21 as potential immunotherapeutic targets.

1. Normal state: T‐cell‐endogenous IL‐6 stabilizes IL6R and promotes STAT3 activation, functional Th17 differentiation with resistance to immune suppression. 2. Il6‐KO: Loss of T‐cell endogenous IL‐6 destabilizes IL6R expression, leading to suboptimal Th17 and greater susceptibility to suppression, despite availability of exogenous IL‐6. 3. IL‐21 bypasses the requirement for endogenous IL‐6 by restoring STAT3 signaling and fully rescuing Th17 differentiation and effector resistance.