Enhanced responsiveness to subsequent bacterial infection in macrophages stimulated with conserved protein patterns

Abstract Immune modulation via exposure to a primary stimulus can modify innate immune cell responses to a subsequent unrelated infection. Previously, we found that conserved mammalian proteins which display a pattern of repeating protein subunits (RPSP) enhance clearance of a secondary respiratory bacterial infection in a TLR2/1-dependent manner. RPSPs like filamentous actin or ferritin are expressed intracellularly in a steady-state, thus their detection by extracellular pattern recognition receptors should elicit danger-like signals. Unlike classical DAMPs, which prompt an inflammatory response, TLR2/1-based recognition of RPSPs did not result in proinflammatory cytokine production or neutrophil lung infiltration. Yet, upon challenge with Staphylococcus aureus, RPSP inoculated mice exhibited a more robust inflammatory response and better bacterial clearance than control mice. This TLR2/1-based RPSP recognition was not limited to mammalian proteins. Enhanced S. aureus clearance was achieved with a synthetic peptide (Q11; QQKFQFQFEQQ) that organizes into RPSP fibers and is non-immunogenic. Further, we found that RPSP enhanced S. aureus clearance was dependent on non-classical Type I IFN signaling whereby IFNβ production was required, but absence of the IFNAR1 subunit of the Type I IFN receptor did not abrogate S. aureus clearance. In vivo depletion and bacterial clearance assays showed that macrophages were essential for this subsequent improved response, which persisted after resting cells for days post RPSP stimulation. Our results indicate that RPSP stimulation of macrophages enhances these cell’s abilities to respond to a secondary infection in an IFNβ dependent manner and has features suggestive of trained immunity.