Macrophage Inflammatory State Influences Susceptibility to Lysosomal Damage

Macrophages possess mechanisms for reinforcing the integrity of their endolysosomal membranes against damage. This property, termed inducible renitence, was previously reported for macrophages stimulated with LPS, peptidoglycan, IFN-γ, or TNF-α. Here, we expanded the macrophage subtypes examined to include populations with well-defined functional roles : classically activated macrophages (CA-Mφ), alternatively activated macrophages (AA-Mφ), and regulatory macrophages (Reg-Mφ). We determined that renitence is a property of CA-Mφ and Reg-Mφ, but not of AA-Mφ. Furthermore, LPS-activated macrophages possess features of both CA-Mφ and Reg-Mφ, based on their cytokine secretion profiles. As the generation of these three classes of renitent macrophages required exposure to LPS, a Toll-like receptor (TLR) ligand, we assessed whether TLR stimulation generally induced renitence. Stimulation of TLRs 2/1, 3, and 4 induced renitence, whereas stimulation of TLRs 7/8 and 9 induced modest levels of lysosomal damage protection. Renitence induced by TLR stimulation required the signaling adaptors MyD88 and TRIF. Surprisingly, the specific signaling adaptor usage requirements for some TLRs differed from those established for canonical TLR signaling. Of note, renitence induced by LPS, a TLR4 ligand, required signaling through TRIF but not MyD88. Consistent with this pattern, the type I IFN response, which is triggered by LPS stimulation through a TRIF-dependent, MyD88-independent pathway, contributed to renitence. A biologically relevant type I IFN trigger in macrophages, murine norovirus-1 (MNV-1) infection, also induced renitence. This work establishes the concept that susceptibility to lysosomal damage within macrophages varies according to inflammatory state and depends on the type I IFN response.