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.

Antibody-antigen kinetics constrain intracellular humoral immunity

“…The binding of TRIM21 itself to antibody occurs with high affinity and only a few antibodies are necessary to provoke a neutralization response11. TRIM21 has highest affinity for IgG but is also capable of binding IgM and IgA, albeit with reduced affinity (17 μM and 50 μM respectively to monomeric PRYSPRY domain)12319. Given that full-length dimeric TRIM21 increases affinity to IgG by ~375-fold, the functional affinity to IgA and IgM is likely to be submicromolar.…”

TRIM21: a cytosolic Fc receptor with broad antibody isotype specificity

“…While Fc receptors are highly selective in regard to isotype and subtype binding, TRIM21 not only binds IgG but also IgM and IgA (32,35). Interestingly, the binding site for TRIM21 on IgA and IgM overlaps with that of Fc a receptor I (FcaRI), Fca/l receptor, pIgR, as well as several pathogen-produced Fc receptors (61)(62)(63)(64)(65)(66)(67)(68).…”

Intracellular Antiviral Immunity

“…In contrast, the binding of TRIM21 is pH independent and does not require protonation. TRIM21 is capable of binding all IgG subclasses with nM affinity (Keeble et al, 2008) and has also been shown to bind IgM (Mallery et al, 2010) and IgA (Bidgood et al, 2014), however with lower affinities of 17 and 50 μM, respectively.…”