Effector-mediated subversion of proteasome activator (PA)28αβ enhances host defense againstLegionella pneumophilaunder inflammatory and oxidative stress conditions
Authors: Ngwaga Tshegofatso , Chauhan Deepika , Salberg Abigail G. , Shames Stephanie R. 
Published: December 2021

Legionella pneumophilais a natural pathogen of amoebae that causes Legionnaires’ Disease in immunocompromised individuals via replication within macrophages.L. pneumophilavirulence and intracellular replication hinges on hundreds of Dot/Icm-translocated effector proteins, which are essential for biogenesis of the replication permissiveLegionella-containing vacuole (LCV). However, effector activity can also enhance mammalian host defense via effector-triggered immunity. TheL. pneumophilaeffector LegC4 is important for virulence in amoebae but enhances host defense againstL. pneumophilain the mouse lung and, uniquely, within macrophages activated with either tumor necrosis factor (TNF) or interferon (IFN)-γ. The mechanism by which LegC4 potentiates cytokine-mediated host defense in macrophages is unknown. Here, we found that LegC4 enhances cytokine-mediated phagolysosomal fusion withLegionella-containing vacuole (LCV) and binds host proteasome activator (PA)28α, which forms a heterooligomer with PA28β 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 PA28αβ-deficient macrophages and in the lungs of micein vivo. Our data also show that oxidative stress is sufficient for LegC4-mediated restriction in macrophages producing PA28αβ. PA28αβ has been traditionally associated with antigen presentation; however, our data support a novel mechanism whereby effector-mediated subversion of PA28αβ enhances cell-autonomous host defense againstL. pneumophilaunder inflammatory and oxidative stress conditions. This work provides a solid foundation to evaluate induced proteasome regulators as mediators of innate immunity.

Author Summary

Pro-inflammatory cytokines induce antimicrobial host defense pathways within macrophages to control intracellular pathogens. We discovered that theLegionella pneumophilaeffector protein LegC4 potentiates pathogen clearance within cytokine-activated macrophages. Here, we show a central role for host proteasome activator (PA)28αβ in LegC4 restriction. PA28αβ is upregulated by cytokine signaling and under oxidative stress conditions to facilitate proteasomal degradation of oxidant-damaged proteins. We found that LegC4 binds PA28αβ and that LegC4 restriction was lost in PA28αβ-deficient (Psme1/2-/-) macrophages and a mouse model of Legionnaires’ Diseasein vivo. Furthermore, oxidative stress was sustained in the presence of LegC4 and was sufficient for LegC4 restriction in PA28αβ-producing cells macrophages. Finally, we found thatL. pneumophilareplication was attenuated within PA28αβ-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.