title: Translocalized IgA mediates neutralization and stimulates innate immunity inside infected cells.
abstract: IgA is the most prevalent antibody type on mucosal surfaces and the second most prevalent antibody in circulation, yet its role in immune defense is not fully understood. Here we show that IgA is carried inside cells during virus infection, where it activates intracellular virus neutralization and innate immune signaling. Cytosolic IgA-virion complexes colocalize with the high-affinity antibody receptor tripartite motif-containing protein 21 (TRIM21) and are positive for lysine-48 ubiquitin chains. IgA neutralizes adenovirus infection in a TRIM21- and proteasome-dependent manner in both human and mouse cells. Translocated IgA also potently activates NF-κB signaling pathways in cells expressing TRIM21, whereas viral infection in the absence of antibody or TRIM21 is undetected. TRIM21 recognizes an epitope in IgG Fc that is not conserved in IgA; however, fluorescence anisotropy experiments demonstrate that direct binding to IgA is maintained. We use molecular modeling to show that TRIM21 forms a nonspecific hydrophobic seal around a β-loop structure that is present in IgG, IgM, and IgA, explaining how TRIM21 achieves such remarkable broad antibody specificity. The findings demonstrate that the antiviral protection afforded by IgA extends to the intracellular cytosolic environment.
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom.
authors.forename: Susanna R
authors.initials: SR
authors.lastname: Bidgood
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom.
authors.forename: Jerry C H
authors.initials: JC
authors.lastname: Tam
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom.
authors.forename: William A
authors.initials: WA
authors.lastname: McEwan
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom.
authors.forename: Donna L
authors.initials: DL
authors.lastname: Mallery
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom lcj@mrc-lmb.cam.ac.uk.
authors.forename: Leo C
authors.initials: LC
authors.lastname: James
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000257:Adenoviridae Infections / Q000276:immunology*; D000818:Animals; D057134:Antibodies, Neutralizing / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000378:metabolism; D016207:Cytokines / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007113:Immunity, Innate; D007070:Immunoglobulin A / Q000097:blood / Q000378:metabolism*; D007075:Immunoglobulin M / Q000378:metabolism; D042541:Intracellular Space / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D016328:NF-kappa B / Q000378:metabolism; D009500:Neutralization Tests; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011485:Protein Binding; D012261:Ribonucleoproteins / Q000737:chemistry / Q000378:metabolism; D000074405:Valosin Containing Protein; D014779:Virus Replication; D000099207:SS-A Antigen
chemical_list: D000251:Adenosine Triphosphatases; D057134:Antibodies, Neutralizing; D018797:Cell Cycle Proteins; D016207:Cytokines; D007070:Immunoglobulin A; D007075:Immunoglobulin M; D016328:NF-kappa B; D046988:Proteasome Endopeptidase Complex; D012261:Ribonucleoproteins; D000074405:Valosin Containing Protein; D000099207:SS-A Antigen