# An early burst of cytokine production before the first cell division influences CD8 T cell differentiation

## TL;DR

CD8 T cells produce cytokines before dividing, and this early burst influences their development into effector or memory cells.

## Contribution

The study reveals an early, intrinsic cytokine burst in CD8 T cells that shapes their differentiation before cell division.

## Key findings

- CD8 T cells show divergent IL-2 and IFN-γ production within 24 hours of activation, before dividing.
- Early IL-2 production reduces STAT5 signaling in the same cell, without affecting IL-2 receptor expression.
- Cytokine reporter systems show that early functional differences determine T cell fate biases and memory formation.

## Abstract

The differentiation of CD8 T cells into effector and memory populations is guided by a combination of antigenic, costimulatory, and cytokine signals. Here we show that, within 24 h of activating naïve CD8 T cells, populations emerge with divergent patterns of interleukin (IL)-2 and interferon (IFN)-γ synthesis. This rapid, dynamic, and heterogeneous burst of cytokine production manifests with every CD8 T cell specificity analyzed, is apparent in vivo and in vitro, and occurs prior to the first cell division. Nevertheless, how the intrinsic manufacture of distinct cytokines forecasts and influences the properties and fates of the producer cell itself are not well defined. We demonstrate that the initial cell intrinsic synthesis of IL-2 attenuates IL-2-dependent STAT5 signaling, but that this is not due to differences in the surface expression of the IL-2 receptor complex. The functionally discrete subsets are transcriptionally distinct and display differences in the expression of hallmark effector and memory associated genes. Using cytokine reporter systems, we reveal that these early functional differences are consequential for establishing fate biases and directing the gain of effector and memory T cell properties. The bifurcation between the abilities of IL-2-producing and non-producing subsets to elaborate STAT5 signaling is consistent with a model in which non-IL-2-producing CD8 T cells are more receptive to extrinsic IL-2 signals and preferentially contribute to the early surge of effector formation. Despite this, both IL-2-producing and non-producing CD8 T cells can go on to acquire memory traits, indicating that there is developmental diversity within each cytokine producing subset.