# Characterization of T-cell immune responses against Staphylococcus chromogenes naturally-induced subclinical mastitis in dairy cows

## TL;DR

This study explores how T-cells respond to a bacterial infection causing silent inflammation in dairy cows' udders, revealing a key role for γδ T cells in immune defense.

## Contribution

The study is the first to show the specific activation and functional role of γδ T cells in subclinical mastitis caused by Staphylococcus chromogenes in dairy cows.

## Key findings

- S. chromogenes infection increases γδ T cells and decreases CD4+CD8+ T cells in cow milk.
- γδ T cells in infected cows show elevated CD44 expression and produce higher levels of immune-related genes like IFN-γ and TNF-α.
- These γδ T cells release PRF1, which kills bacteria-infected cells more effectively in subclinical mastitis.

## Abstract

Subclinical mastitis in dairy cows is an asymptomatic inflammatory condition of the mammary gland during lactation. It is frequently associated with reduced milk yield, impaired milk quality, and an elevated risk of progression to clinical mastitis. The dysregulation of immune homeostasis characterizing this subclinical state represents a significant yet often overlooked concern in dairy herds.

We characterized T cell subsets in cows with Staphylococcus chromogenes-induced subclinical mastitis and healthy controls.

We found that S. chromogenes infection induced an increase in the percentage of γδ T cells and a decrease in CD4+CD8+ double-positive (DP) T cells within milk cells, while significantly elevating the percentage of γδ T cells expressing CD44. Cell sorting combined with quantitative RT-PCR has indicated that infection by S. chromogenes may activate γδ T cells. Specifically, in subclinical mastitis milk, the expression levels of five cytokines—IFN-γ, IL-2, TNF-α, IL-17, and GZMB—were significantly higher in WC1+ γδ T cells compared to healthy cows. Furthermore, these cells released elevated levels of PRF1, which directly killed target cells infected with bacteria compared to the levels observed in healthy cows.

Our results reveal, for the first time, the significant role of γδ T cells in S. chromogenes -induced subclinical mastitis, providing new insights into the immune protective mechanisms against subclinical mastitis in cows.