# Interferon-β induction heterogeneity during KSHV infection is correlated to levels and activation of the transcription factors ATF2 and RelA, and not IRF3

## TL;DR

This study shows that during KSHV infection, IFN-β production varies between cells due to differences in AP-1 and NF-κB activity, not IRF3.

## Contribution

The study identifies ATF2 and RelA, not IRF3, as key regulators of IFN-β induction heterogeneity during KSHV infection.

## Key findings

- IFN-β induction during KSHV infection correlates with ATF2 and RelA activation, not IRF3.
- Viral gene expression does not explain IFN-β heterogeneity among infected cells.
- AP-1 and NF-κB, rather than IRF3, determine which infected cells produce IFN-β.

## Abstract

Careful regulation of type I interferons (IFN) like IFN-β is vital for balancing tissue damage and protection against infections. Heterogeneity in type I IFN expression among virally infected cells is a common phenomenon that may help limit IFN responses, but the source of this heterogeneity is poorly understood. We previously found that during Kaposi’s sarcoma-associated herpesvirus replication, type I IFN induction was limited to a small percentage of infected cells. This heterogeneity was not explained by viral gene expression. Here, we used a fluorescent reporter and fluorescence activated cell sorting to investigate the source of the heterogeneity. Surprisingly, the canonical IFN induction pathway culminating in the activation of the IRF3 transcription factor was similarly activated between cells that made high vs. low/no IFN-β. In contrast, the activation or expression of the two other IFN transcription factors, the NF-κB subunit RelA and the AP-1 subunit ATF2, correlated with IFN-β induction. Our results suggest that during viral infection, activation of IRF3 does not automatically result in IFN responses at the level of individual cells, but that other factors, such as NF-κB and AP-1, are limiting for type I IFN induction.

The ability of mammalian cells to react to viral infections is a crucial step in the induction of immune responses. The first course of action for the cell is to express and release type I interferons like interferon-β (IFN-β), secreted molecules that warn surrounding cells. Single-cell level examination of gene expression has revealed that surprisingly, during many viral infections, only a small fraction of infected cells make IFN-β. This is likely a mechanism to prevent immune system overreactions. However, it remains unclear why only some cells respond. Here, we find that during infection with Kaposi’s sarcoma-associated herpesvirus, an oncogenic virus that affects immunocompromised individuals, the transcription factors AP-1 and NF-κB, rather than the more commonly studied IRF3, may decide which cells go on to make IFN-β. Our findings contribute to a better understanding of complex gene regulation and shed light on a process that fights an oncogenic virus.