# Host Cells Upregulate Phosphate Transporter PIT1 to Inhibit Ehrlichia chaffeensis Intracellular Growth

## TL;DR

Host cells fight Ehrlichia chaffeensis infection by boosting a phosphate transporter called PIT1, which limits bacterial growth.

## Contribution

Host cells use PIT1 to transport phosphate from bacterial compartments, inhibiting Ehrlichia chaffeensis growth via the MyD88-NF-κB pathway.

## Key findings

- Host cells upregulate PIT1 upon Ehrlichia chaffeensis infection.
- PIT1 transports phosphate from ECVs to the cytosol, inhibiting bacterial growth.
- Ech_1067 induces PIT1 upregulation through the MyD88-NF-κB pathway.

## Abstract

Ehrlichia chaffeensis infects and proliferates inside monocytes or macrophages and causes human monocytic ehrlichiosis (HME), an emerging life-threatening tick-borne zoonosis. After internalization, E. chaffeensis resides in specialized membrane-bound inclusions, E. chaffeensis-containing vesicles (ECVs), to evade from host cell innate immune responses and obtain nutrients. However, mechanisms exploited by host cells to inhibit E. chaffeensis growth in ECVs are still largely unknown. Here we demonstrate that host cells recognize E. chaffeensis Ech_1067, a penicillin-binding protein, and then upregulate the expression of PIT1, which is a phosphate transporter and transports phosphate from ECVs to the cytosol to inhibit bacterial growth. We found that host cells upregulate the PIT1 expression upon E. chaffeensis infection using transcriptome sequencing, qRT-PCR and Western blotting, and PIT1 is localized on the ECV membrane in infected THP-1 cells using confocal microscopy. Silence of PIT1 using shRNA enhances E. chaffeensis intracellular growth. Finally, we found that E. chaffeensis Ech_1067 induces the upregulation of PIT1 expression through the MyD88-NF-κB pathway using recombinant protein for stimulation and siRNA for silence. Our findings deepen the understanding of the innate immune responses of host cells to inhibit bacterial intracellular growth and facilitate the development of new therapeutics for HME.