Host F-box protein 22 enhances the uptake ofBrucellaby macrophages and drives a sustained release of pro-inflammatory cytokines through degradation of the anti-inflammatory effector proteins ofBrucella
Authors: Mazumdar Varadendra , Joshi Kiranmai , Nandi Binita Roy , Namani Swapna , Gupta Vivek Kumar , Radhakrishnan Girish 
Published: March 2021

Brucellaspecies are intracellular bacterial pathogens, causing the world-wide zoonotic disease, brucellosis.Brucellainvade professional and non-professional phagocytic cells, followed by resisting intracellular killing and establishing a replication permissive niche.Brucellaalso modulate the innate and adaptive immune responses of the host for their chronic persistence. The complex intracellular cycle ofBrucellamajorly depends on multiple host factors but limited information is available on host and bacterial proteins that play essential role in the invasion, intracellular replication and modulation of host immune responses. By employing an siRNA screening, we identified a role for the host protein, FBXO22 inBrucella-macrophage interaction. FBXO22 is the key element in the SCF E3 ubiquitination complex where it determines the substrate specificity for ubiquitination and degradation of various host proteins. Downregulation of FBXO22 by siRNA or CRISPR-Cas9 system, resulted diminished uptake ofBrucellainto macrophages, which was dependent on NF-κB-mediated regulation of phagocytic receptors. FBXO22 expression was upregulated inBrucella-infected macrophages that resulted induction of phagocytic receptors and enhanced production of pro-inflammatory cytokines through NF-κB. Furthermore, we found that FBXO22 recruits the effector proteins ofBrucella, including the anti-inflammatory proteins, TcpB and OMP25 for degradation through the SCF complex. We did not observe any role for another F-box containing protein of SCF complex, β-TrCP inBrucella-macrophage interaction. Our findings unravel novel functions of FBXO22 in host-pathogen interaction and its contribution to pathogenesis of infectious diseases.

Author Summary

Brucellosis is a major zoonotic disease world-wide that poses a serious veterinary and public health problem in various countries, impacting their economic development. Brucellosis is caused by the species of intracellular bacterial pathogen,Brucellathat replicates in professional and non-professional phagocytic cells.Brucellais considered as a stealthy pathogen as it invades/suppresses host defense responses using various virulence strategies.Brucellahijacks many cellular processes for gaining entry into the target cells, followed by establishing a replication permissive niche. However, host proteins that are involved inBrucella-macrophage interaction remains obscure. Here, we identified the host protein, FBXO22 that recruits target proteins to SCF E3 ubiquitination complex for their ubiquitination and degradation. We found that down-regulation and upregulation of FBXO22 decreased and enhanced the uptake ofBrucellaby macrophages, respectively. Our subsequent studies revealed thatBrucellainduces the expression of FBXO22 that resulted activation of NF-κB and the concomitant upregulation of phagocytic receptors that might have contributed to the enhanced uptake ofBrucella. TheBrucella-induced expression of FBXO22 resulted enhanced production of pro-inflammatory cytokines. We have also found that FBXO22 targetsBrucellaeffectors, including the anti-inflammatory effector proteins for degradation through the SCF complex. Our experimental data reveals that FBXO22 plays an important role in the uptake of microbial pathogens by macrophages and pathogenesis of infectious diseases that is resulting from overt inflammatory responses.