# Crustacean Mab21 proteins drive tissue-specific antiviral immunity by activating IKKε outside the canonical nucleic-acid sensing paradigm

## TL;DR

Shrimp Mab21 proteins activate antiviral immunity by enhancing kinase activity, not by sensing nucleic acids, and function in a tissue-specific manner.

## Contribution

Identifies a non-canonical immune mechanism in shrimp where Mab21 proteins activate IKKε without nucleic acid sensing.

## Key findings

- Shrimp Mab21 proteins do not bind DNA/RNA or produce cGAMP but activate LvIKKε to trigger antiviral signaling.
- Mab21 proteins function in a tissue-specific manner, with distinct roles in hemocytes, gills, hepatopancreas, and intestine.
- Silencing Mab21 proteins increases WSSV viral load and reduces shrimp survival.

## Abstract

The Mab21/cGAS protein family has diversified across metazoans to regulate development and innate immunity. In vertebrates, cGAS detects cytosolic DNA and synthesizes 2′3′-cGAMP to activate STING–TBK1–IRF signaling, while invertebrate cGAS-like receptors (cGLRs) recognize RNA or DNA and generate non-canonical cyclic dinucleotides. However, whether shrimp Mab21 proteins function as canonical nucleic acid sensors remains unresolved. Here, we identified three Mab21 proteins from Litopenaeus vannamei—LvMab21–1, LvMab21–2, and LvMab21–3. Although they are phylogenetically related to cGAS-like proteins, none bound dsDNA or dsRNA or synthesized cGAMP in response to ISD or poly(I:C). Instead, all three interacted directly with the TBK1 homolog LvIKKε, promoted its phosphorylation at serine 175, and thereby activated the downstream IRF–Vago4 signaling axis. This mechanism defines a non-canonical nucleic acid sensing paradigm, whereby Mab21 proteins act as protein-based enhancers of kinase activation rather than as nucleic acid–dependent CDN synthases. We further show that these proteins display tissue-specific antiviral functions: all three act in hemocytes, LvMab21–1 predominates in hepatopancreas, LvMab21–2 and LvMab21–3 are most critical in gills, and LvMab21–1 and LvMab21–3 cooperate in intestine. Silencing any Mab21 paralog reduced survival and increased white spot syndrome virus (WSSV) burden, underscoring their physiological relevance. Together, our findings expand the known repertoire of innate immune strategies within the Mab21 family, highlight a previously unrecognized non-canonical mechanism of interferon-like activation, and reveal tissue-specific specialization that tailors antiviral responses across shrimp organs. These insights provide both evolutionary context and candidate targets for breeding disease-resistant shrimp.

Shrimp aquaculture is threatened by viral diseases such as white spot syndrome virus (WSSV), yet the mechanisms of shrimp antiviral immunity remain poorly understood. In mammals and insects, members of the Mab21/cGAS protein family detect viral DNA or RNA and produce small messenger molecules that trigger interferon responses. We investigated whether shrimp possess similar sensors. We identified three Mab21 proteins in the Pacific white shrimp (Litopenaeus vannamei) and found that, unlike their mammalian and insect counterparts, they do not recognize DNA or RNA and do not produce canonical messenger molecules. Instead, these proteins interact with the kinase IKKε and promote its activation, which in turn drives the interferon-like pathway and expression of antiviral effectors. Remarkably, different Mab21 proteins act in different tissues—hemocytes, gills, hepatopancreas, and intestine—allowing shrimp to mount tissue-specific antiviral responses. Silencing these proteins increased viral loads and reduced survival during WSSV infection. Our findings uncover a non-canonical immune strategy in crustaceans and highlight Mab21 proteins as potential targets for breeding disease-resistant shrimp.