Effector-mediated subversion of proteasome activator (PA)28αβ enhances lysosomal targeting of Legionella pneumophila within cytokine-activated macrophages

Legionella pneumophila is a natural pathogen of amoebae that causes Legionnaires Disease in immunocompromised individuals via replication within macrophages. L. pneumophila virulence and intracellular replication hinges on hundreds of Dot/Icm-translocated effector proteins, which are essential for biogenesis of the replication permissive Legionella-containing vacuole (LCV). However, effector activity can also enhance mammalian host defense via effector-triggered immunity. The L. pneumophila effector LegC4 is important for virulence in amoebae but enhances host defense against L. pneumophila in the mouse lung and, uniquely, within macrophages activated with either tumor necrosis factor (TNF) or interferon (IFN)-{gamma}. The mechanism by which LegC4 potentiates cytokine-mediated host defense in macrophages is unknown. Here, we found that LegC4 enhances cytokine-mediated phagolysosomal fusion with Legionella-containing vacuole (LCV) and binds host proteasome activator (PA)28, which forms a heterooligomer with PA28{beta} to facilitate ubiquitin-independent proteasomal degradation of oxidant-damaged (carbonylated) proteins. We found that oxidative stress was sustained in the presence of LegC4 and that the LegC4 restriction phenotype was relieved in PA28{beta}-deficient macrophages and in the lungs of mice in vivo. Our data also show that oxidative stress is sufficient for LegC4-mediated restriction in macrophages producing PA28{beta}. PA28{beta} has been traditionally associated with antigen presentation; however, our data support a novel mechanism whereby effector-mediated subversion of PA28{beta} enhances cell-autonomous host defense against L. pneumophila under inflammatory and oxidative stress conditions. This work provides a solid foundation to evaluate induced proteasome regulators as mediators of innate immunity. Author SummaryPro-inflammatory cytokines induce antimicrobial host defense pathways within macrophages to control intracellular pathogens. We discovered that the Legionella pneumophila effector protein LegC4 potentiates pathogen clearance within cytokine-activated macrophages. Here, we show a central role for host proteasome activator (PA)28{beta} in LegC4 restriction. PA28{beta} is upregulated by cytokine signaling and under oxidative stress conditions to facilitate proteasomal degradation of oxidant-damaged proteins. We found that LegC4 binds PA28{beta} and that LegC4 restriction was lost in PA28{beta}-deficient (Psme1/2-/-) macrophages and a mouse model of Legionnaires Disease in vivo. Furthermore, oxidative stress was sustained in the presence of LegC4 and was sufficient for LegC4 restriction in PA28{beta}-producing cells macrophages. Finally, we found that L. pneumophila replication was attenuated within PA28{beta}-deficient macrophages irrespective of LegC4. These findings support a model whereby subversion of host proteostasis machinery triggers cell-autonomous host defense within macrophages under inflammatory and oxidative stress conditions.