Opposing roles of endosomal innate immunity proteins IFITM3 and TLR7 in human metapneumovirus infection

Abstract

Human metapneumovirus (hMPV) utilizes a bifurcated cellular entry strategy, fusing either with the plasma membrane or, after endocytosis, with the endosome membrane. Whether cellular factors restrict or enhance either entry pathway is largely unknown. We found that the interferon-induced transmembrane protein 3 (IFITM3) inhibits hMPV infection to an extent similar to endocytosis-inhibiting drugs, and an IFITM3 variant that accumulates at the plasma membrane in addition to its endosome localization provided increased virus restriction. Mechanistically, IFITM3 blocks hMPV F protein-mediated membrane fusion, and inhibition of infection was reversed by the membrane destabilizing drug amphotericin B. Conversely, we unexpectedly found that infection by some hMPV strains is enhanced by Toll-like receptor 7 (TLR7), an endosomal protein, suggesting that cellular entry via endocytosis may be particularly advantageous for hMPV despite eventual restriction of this pathway upon induction of IFITM3. Overall, our results identify IFITM3 and TLR7 as endosomal factors differentially regulating hMPV infection.

Antiviral Protection by IFITM3 In Vivo

“…Mice treated with amphotericin B experience more severe influenza virus infections, similar to IFITM3 KO mice [ 69 ]. We recently determined that IFITM3 is the first known restriction factor for human metapneumovirus infection, and we confirmed that amphotericin B completely neutralizes the antiviral activity of IFITM3 against both influenza virus and human metapneumovirus [ 22 ]. Thus, it will be interesting and important to determine whether clinical use of amphotericin B has the unintended consequence of increasing susceptibility to or pathogenicity of these and other virus infections due to its neutralizing effects on IFITM3.…”

“…Y20 phosphorylation is mediated by the Src family kinase Fyn and results in accumulation of IFITM3 at the plasma membrane and decreased antiviral activity against viruses, such as influenza virus, that enter cells through endocytosis and fuse with endosomes [ 24 , 25 ]. While phosphorylation may decrease IFITM3 activity against influenza virus, the altered localization enhances its ability to restrict certain viruses that fuse at the plasma membrane as mentioned above [ 18 , 21 , 22 ]. Phosphorylation of IFITM3 is dynamic, involving currently unidentified phosphatases [ 24 , 25 ].…”

“…Mutation of the IFITM3 ubiquitination sites or depletion of NEDD4 from cells results in increased stability of IFITM3 [ 11 , 28 ]. As such, NEDD4 KO cells accumulate high levels of basal IFITM3 even in the absence of IFN stimulation and are thus resistant to infection with IFITM3-sensitive viruses [ 22 , 28 ]. It is unknown whether IFITM3 is deubiquitinated in cells, and like methylation, it is also unclear whether the negative regulation of IFITM3 via ubiquitination is beneficial to cells given that these modifications limit the antiviral function of IFITM3.…”

“…Indeed, among the long list of viruses that have now been shown to be restricted by IFITM3, a commonality has emerged in that their membrane fusion reactions generally occur in endosomes where IFITM3 is most abundant [ 19 , 20 ]. IFITM variants that show altered localization may restrict membrane fusion of a distinct subset of viruses, such as those that fuse at the plasma membrane [ 18 , 21 , 22 ].…”