The balance between antiviral and antibacterial responses duringM. tuberculosisinfection is regulated by the ubiquitin ligase CBL

Abstract

As a first line of immune defense, macrophages must be able to appropriately sense and respond to diverse types of pathogens. In addition, many pathogens activate multiple sensors and generate complex signals in a responding cell. How macrophages integrate multiple pathogen-associated signals into a coherent physiologic response is unclear. Upon entering a macrophage,M. tuberculosis (Mtb)induces proinflammatory cytokines that activate antibacterial responses. Surprisingly, it also triggers antiviral responses that actually hinder the host response toMtb. The ubiquitin ligase CBL suppresses these antiviral responses and shifts macrophages toward a more antibacterial state duringMtbinfection. However, the mechanisms by which CBL regulates immune signaling are unknown. We found that CBL broadly suppresses the expression of antiviral effector genes and then used quantitative mass-spectrometry to identify potential CBL substrates. We identified over 46,000 ubiquitylated sites inMtb-infected macrophages, and subsequent analysis of CBL-deficient cells identified roughly 400 CBL-dependent ubiquitylation events, highlighting potential substrates. Using genetic interaction analysis of CBL and its putative substrates we identified the Fas associated factor 2 (FAF2) adapter protein as a key signaling molecule protein downstream of CBL. Together, these analyses identify thousands of new ubiquitin-mediated signaling events during the innate immune response and reveal an important new regulatory hub in this response.