Mycoplasma bovis-Induced Inhibition of Bovine Peripheral Blood Mononuclear Cell Proliferation Is Ameliorated after Blocking the Immune-Inhibitory Programmed Death 1 Receptor

Abstract

<i>Mycoplasma bovis</i>-induced immune suppression is a major obstacle faced by the host for controlling infections. <i>M. bovis</i> impairment of antigen-specific T-cell responses is achieved through inhibiting the proliferation of peripheral blood mononuclear cells (PBMCs). This impairment may contribute to the persistence of <i>M. bovis</i> infection in various sites, including lungs, and its systemic spread to various organs such as joints, with the underlying mechanisms remaining elusive. Here, we elucidated the role of the immune-inhibitory receptor programmed death 1 (PD-1) and its ligand (PD-L1) in <i>M. bovis</i> infection. Flow cytometry (FCM) analyses revealed an upregulation of PD-L1 expression on tracheal and lung epithelial cell lines after <i>M. bovis</i> infection. In addition, we found increased PD-L1 expression on purified lung lavage macrophages following <i>M. bovis</i> infection by FCM and determined its localization by immunofluorescence analysis comparing infected and control lung tissue sections. Moreover, <i>M. bovis</i> infection increased the expression of the PD-1 receptor on total PBMCs and in gated CD4⁺ and CD8⁺ T-cell subpopulations. We demonstrated that <i>M. bovis</i> infection induced a significant decrease in CD4⁺ PD-1^INT and CD8⁺ PD-1^INT subsets with intermediate PD-1 expression, which functioned as progenitor pools giving rise to CD4⁺ PD-1^HIGH and CD8⁺ PD-1^HIGH subsets with high PD-1 expression levels. We blocked PD-1 receptors on PBMCs using anti-PD-1 antibody at the beginning of infection, leading to a significant restoration of the proliferation of PBMCs. Taken together, our data indicate a significant involvement of the PD-1/PD-L1 inhibitory pathway during <i>M. bovis</i> infection and its associated immune exhaustion, culminating in impaired host immune responses.