<i>Ehrlichia chaffeensis</i> Co-Opts Phagocytic Hemocytes for Systemic Dissemination in the Lone Star Tick, <i>Amblyomma americanum</i>

Abstract

<b><i>Introduction:</i></b> Hematophagous arthropods can acquire and transmit several pathogens of medical importance. In ticks, the innate immune system is crucial in the outcome between vector-pathogen interaction and overall vector competence. However, the specific immune response(s) elicited by the immune cells known as hemocytes remains largely undefined in <i>Ehrlichi</i>a <i>chaffeensis</i> and its competent tick vector, <i>Amblyomma americanum</i>. <b><i>Methods:</i></b> We utilized injection of clodronate liposome to deplete tick granulocytes combined with infection with <i>E. chaffeensis</i> to demonstrate their essential role in microbial infection. <b><i>Results:</i></b> Here, we show that granulocytes, professional phagocytic cells, are integral in eliciting immune responses against commensal and pathogen infection. The chemical depletion of granulocytes led to decreased phagocytic efficiency of tissue-associated hemocytes. We demonstrate that <i>E. chaffeensis</i> can infect circulating hemocytes, and both cell-free plasma and hemocytes from <i>E. chaffeensis-</i>infected ticks can establish <i>Ehrlichia</i> infection in recipient ticks. Lastly, we provide evidence to show that granulocytes play a dual role in <i>E. chaffeensis</i> infection. Depleting granulocytic hemocytes increased <i>Ehrlichia</i> load in the salivary gland and midgut tissues. In contrast, granulocyte depletion led to a reduced systemic load of <i>Ehrlichia</i>. <b><i>Conclusion:</i></b> This study has identified multiple roles for granulocytic hemocytes in the control and systemic dissemination of <i>E. chaffeensis</i> infection.