Genetic absence of PD-L1 does not restore CD8+ T cell function during respiratory virus infection and delays virus clearance.

A key mediator of T cell impairment during respiratory virus infection is the inhibitory receptor PD-1. PD-1 is induced on T cells following antigen exposure, whereas proinflammatory cytokines upregulate the ligands PD-L1 and PD-L2. Respiratory virus infection leads to upregulation of PD-L1 on airway epithelial cells, dendritic cells, and alveolar macrophages. However, the role of PD-L1 on different cell types in acute respiratory virus infections is not known. We sought to determine the role of PD-L1 on different cell types in CD8+ T cell impairment. We found that PD-L1-/- mice challenged with human metapneumovirus or influenza showed a similar level of CD8+ T cell impairment compared to wild-type (WT) mice. Moreover, virus clearance was delayed in PD-L1-/- mice compared to WT. CD8+ T cells from PD-L1-deficient mice expressed higher levels of inhibitory receptors both at baseline and after respiratory virus infection. The antibody blockade of PD-L2 failed to restore function to the impaired cells. While reciprocal bone marrow chimeras between WT and PD-L1-/- mice did not restore CD8+ T cell function after the respiratory virus challenge, mice that received the PD-L1-/- bone marrow had higher inhibitory receptor expression on CD8+ cells. This discrepancy in the inhibitory receptor expression suggests that cells of the hematopoietic compartment contribute to T cell impairment on CD8+ T cells.IMPORTANCEThe phenomenon of pulmonary CD8+ T cell impairment with diminished antiviral function occurs during acute respiratory virus infection mediated by Programmed Cell Death-1 (PD-1) signaling. Moreover, PD-1 blockade enhances T cell function to hasten viral clearance. The ligand PD-L1 is expressed in many cell types, but which cells drive lung T cell impairment is not known. We used genetic approaches to determine the contribution of PD-L1 on lung T cell impairment. We found that PD-L2 cannot compensate for the loss of PD-L1, and PD-L1-deficient mice exhibit increased expression of other inhibitory receptors. Bone marrow chimeras between PD-L1-deficient and wild-type mice indicated that hematopoietic PD-L1 expression is associated with inhibitory receptor upregulation and impairment.

Influenza Suppresses Neutrophil Recruitment to the Lung and Exacerbates Secondary Invasive Pulmonary Aspergillosis.

Aspergillus fumigatus is an environmental fungus that can cause invasive pulmonary aspergillosis when spores are inhaled into the respiratory tract and invade airway or lung tissue. Influenza is a common respiratory virus that can cause severe respiratory disease, and postinfluenza invasive pulmonary aspergillosis, which is becoming a well-recognized clinical problem, typically occurs in critically ill patients. Mice challenged with influenza A PR/8/34 H1N1 and subsequently challenged with A. fumigatus had increased fungal burden, viral burden, inflammation, and mortality compared with single infected mice. Neutrophil recruitment in the lung of superinfected mice was decreased; however, mice were not neutropenic, and there was no difference in absolute blood neutrophils between groups. Additionally, CXCL1 and CXCL2 were decreased in lungs of superinfected mice compared with controls. IFN levels were increased in mice that received influenza, and deletion of STAT1 resulted in decreased fungal burden, increased airway and lung neutrophils, and increased CXCL1 compared with wild-type mice, whereas deletion of STAT2 did not change fungal burden or airway neutrophilia compared with wild-type mice. These data demonstrate a mechanism by which influenza A-induced STAT1 signaling inhibits neutrophil recruitment and increases susceptibility to postinfluenza invasive pulmonary aspergillosis.

CD4+ Regulatory T Cells Exert Differential Functions during Early and Late Stages of the Immune Response to Respiratory Viruses.

Acute respiratory virus infection (ARI) induces CD8+ T cells with diminished cytokine production and functional impairment. The role of cellular mediators of immune impairment, specifically CD4+ regulatory T cells (Tregs), is incompletely understood in ARI. Tregs are known suppressors of effector T cell function, but whether they are detrimental or beneficial in ARI remains controversial. We show in this paper that Treg depletion leads to increased CD8+ T cell function and lower virus titer in mice infected with human metapneumovirus. We further demonstrate that Tregs play a temporal role in the immune response to human metapneumovirus and influenza: Treg depletion before infection pathologically reduces virus-specific CD8+ T cell numbers and delays virus clearance, whereas depletion 2 d postinoculation enhances CD8+ T cell functionality without reducing virus-specific CD8+ T cell numbers. Mechanistically, Treg depletion during immune priming led to impaired dendritic cell and CD8+ T cell migration. Further, early Treg depletion was associated with immune skewing toward a type 2 phenotype characterized by increased type 2 innate lymphoid cells and TH2 CD4+ T cells, which was not observed when Treg depletion was delayed until after inoculation. These results indicate that the presence of Tregs at inoculation is critical for efficient priming of the CD8+ T cell response to ARI, whereas later in infection, Tregs are dispensable for virus clearance.