This study provides evidence that a soluble, secreted, mannose-specific CTL mediates symbiont acquisition from the environment by L. oneistus.
The significance of the CTL protein sequence polymorphism remains unclear. Based on homology modeling, all Mermaid isoforms are expected to share the same sugar specificity. Nevertheless, we cannot exclude that different isoforms, either alone or in combinations, display different affinities for the same sugar residues.
Although the predicted MW of secreted Mermaid is 16.4 kDa, this CTL migrated at ~28 kDa in Western blots of worm protein extracts separated under reducing conditions, probably due to dimerization. Alternatively, the CTL might form a stable, SDS- and dithiothreitol-resistant complex with large carbohydrates on the symbiont surface.
Mermaid is significantly similar to the Japanese eel AJL2, a divalent CTL that agglutinated E. coli
and suppressed its growth. The agglutination of pathogens in the skin mucus by AJL2 might hinder their penetration into the eel body, allowing eels to wash them off by shedding new mucus (36
). Although some parasitic nematode CTLs also have strong similarities with mammalian lectins (11
), this has not been observed for free-living nematode CTLs other than Mermaid (4
The posterior GSO-specific Mermaid expression pattern, resulting from either posterior specific gene transcription or anterior specific degradation of mermaid
mRNA, contradicts a direct role of this CTL in L. oneistus
's response to pathogens. This contrasts with what has been observed with Caenorhabditis elegans
, where increases in CTL expression in the gut have been observed upon infection (14
). An indirect role of Mermaid in immune defense, however, cannot be excluded: a functional symbiotic coat appears to protect stilbonematids from being fouled by deleterious microbes.
Since we could not amplify mermaid gene fragments from symbiont genomic DNA by PCR with specific primers, symbiont staining by anti-peptide antibody cannot be attributed to bacterial endogenous expression (data not shown). We suggest, therefore, that once secreted onto the worm surface, CTL polymers bind mannose residues on the bacterial surface and are thereby retained on the worm rather than diffusing into the environment. This implies that, before symbiosis establishment, mucus must be viscous enough to prevent the diffusion of the CTL polymers out of the mucus layer.
The presence of Mermaid in the mucus overlying the posterior region of the nematode could facilitate the recruitment of specific symbionts from the environment by mediating their aggregation. In the association between the squid Euprymna scolopes
and Vibrio fischeri
, both gram-positive and gram-negative bacteria can induce shedding of host mucus during light organ development. Nevertheless, only gram-negative bacteria form aggregates therein, and only V. fischeri
ultimately colonizes the mature light organ (23
Recombinant Mermaid competed the symbionts off L. oneistus
in vivo, indicating that bacteria are immobilized on the cuticle by CTL-containing mucus. The preferential adherence of microbes to host mucus is crucial in intestinal symbioses (35
) and in the E. scolopes
association, where V. fischeri
must establish its dominance in the developing light organ. Although N
-acetylneuraminic acid and N
-acetylgalactosamine are present in squid exudates (24
) and V. fischeri
chemotaxis was shown toward N
-acetylneuraminic acid (3
), it remains unknown how these or other as-yet-unidentified molecules might mediate V. fischeri
adhesion to the host mucus.
Although the recombinant CTL induced symbiont loss in all L. oneistus individuals, dissociation was not complete on any worm. Therefore, additional molecules might help establish this highly specific association. It also remains unclear whether d-mannose binds to Mermaid as an homopolymer or forms a specific motif with other sugar residues. The fact that ConA (a d-mannose-binding CTL) does not induce symbiont dissociation and that His-Mermaid induces it much faster than free d-mannose suggests that the latter is only part of the lock-and-key mechanism.
A better characterization of Mermaid sugar specificity, as well as of the LPS composition of both L. oneistus
and S. majum
symbionts, will help elucidate the role played by this CTL
expression in the latter stilbonematid. Although S. majum
symbionts belong to at least five different phylotypes of cocci only distantly related to L. oneistus
symbionts, His-Mermaid induced their agglutination (K. Vanura and S. Bulgheresi, unpublished data). Therefore, we currently cannot exclude a CTL-mediated acquisition of bacteria other than L. oneistus
-specific symbionts. In this case, the high sulfide concentration in L. oneistus
's microenvironment could still select S. majum
symbionts, along with other “unwanted” microbes, off the surface of L. oneistus
. In fact, symbiotic L. oneistus
tolerates much higher concentrations of thiols than does S. majum
In the shallow-water nematode L. oneistus, Mermaid is exclusively secreted onto the bacterium-associated moiety of the worm cuticle. Since this mannose-specific lectin aggregates and attaches the symbionts on the host surface, our results provide a missing link between host-secreted mucus and beneficial microbes in a marine environment.
The surprising ability of Mermaid to compete with human DC-SIGN in binding microbial antigens further confirms how blurred the border between symbiosis and pathogenesis may be. Since pathogens such as Mycobacterium tuberculosis
and human immunodeficiency virus type 1 may subvert several functions of dendritic cells by interacting with DC-SIGN (6
), it will be very exciting to test the ability of His-Mermaid to block these interactions.