The CD4+ T-cell response is central for control of Anaplasma marginale infection in cattle. However, this infection induces a functional exhaustion of antigen-specific CD4+ T cells in cattle immunized with A. marginale outer membrane proteins or purified outer membranes (OM), which presumably facilitates persistence of this rickettsia. In the present study, we hypothesize that T-cell exhaustion following infection is induced by the upregulation of immunoinhibitory receptors on T cells, such as programmed death-1 (PD-1) and lymphocyte activation gene-3 (LAG-3). Therefore, OM-specific T-cell responses and the kinetics of PD-1+LAG-3+ exhausted T cells were monitored in A. marginale-challenged cattle previously immunized with OM.
Consistent with earlier studies, OM-specific T-cell proliferation and interferon-γ (IFN-γ) production were significantly suppressed in the challenged animals by five weeks post-infection (wpi). In addition, clinical signs, such as bacteremia and anemia, also peaked in these animals at 5 wpi. Flow cytometric analysis revealed that the percentages of PD-1+LAG-3+ exhausted T cells in the CD4+, CD8+, and γδ T-cell populations were gradually increased in the peripheral blood of the animals. Importantly, the highest percentages of these exhausted T-cell subsets were observed at 5 wpi, concurrent with the peak of functional exhaustion of OM-specific T cells and clinical signs. The immunoinhibitory ligand PD-L1 was also significantly upregulated on CD14+ antigen presenting cells in the peripheral blood of the challenged animals, following the same kinetics of the expression. Finally, in vitro blockade of the PD-1 and LAG-3 pathways by specific antibodies partially restored OM-specific T-cell proliferation and IFN-γ production at 5 wpi. Taken together, these results indicate that co-expression of PD-1 and LAG-3 on T cells contributes to the rapid exhaustion of A. marginale-specific CD4+ T cells following infection and that these immunoinhibitory receptors regulate T-cell responses during bovine anaplasmosis.