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Proc Natl Acad Sci U S A. 2014 Sep 16;111(37):13463-8. doi: 10.1073/pnas.1410980111. Epub 2014 Aug 28.

Translocalized IgA mediates neutralization and stimulates innate immunity inside infected cells.

Author information

1
Medical Research Council Laboratory of Molecular Biology, Division of Protein and Nucleic Acid Chemistry, Cambridge CB2 0QH, United Kingdom.
2
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.

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.

PMID:
25169018
PMCID:
PMC4169910
DOI:
10.1073/pnas.1410980111
[Indexed for MEDLINE]
Free PMC Article

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