Hemagglutinins and bacterial agglutinins of earthworms

The biological roles of invertebrate agglutinins have been and remain an unresolved subject of controversy. Classical studies on agglutinins, beginning with the pioneer work of Noguchi (1903) on Limulus polyphemus and Homarus americanus have emphasized their hemagglutinating properties, an approach that has been criticized for its lack of biological relevance. While erythrocyte agglutination has proven useful for determining various properties of invertebrate agglutinins, it does not address the question of their natural function. More recently, invertebrate agglutinins have been investigated for their ability to interact with pathogenic agents such as bacteria (for review, see Pistole, 1982), yeast (Van der Knapp et al., 1982; Renwrantz and Stahmer, 1983) and parasitic protozoans (Ingram et al., 1984). In addition, the possible relationship of agglutinins to defense mechanisms of both vertebrates and invertebrates has been indicated by the observation that limulin, the major agglutinin of Limulus polyphemus, bears a number of similarities to vertebrate C-reactive proteins (Robey and Liu, 1981). In annelids, there have been no studies on bacterial agglutinins prior to our work with Lumbricus (Stein et al., 1985; Stein et al., submitted). Earthworms are particularly appropriate for studying bacterial agglutinins since their coelomic fluid contains constant low levels of bacteria and fungal spores, and their agglutinins are both naturally occurring and inducible. Although our initial studies on Lumbricus agglutinins were directed toward their hemagglutinating properties, our recent observations using bacteria have allowed us to reach the following conclusions: 1) Lumbricus coelomic fluid normally contains agglutinins against both erythrocytes and bacteria. After injecting worms with either erythrocytes or bacteria, agglutinin titers increase in coelomic fluid. This increase appears to be due to both an increase in numbers of agglutinins as well as levels of specific agglutinins. 2) Absorption studies, temperature effects and sugar inhibition analyses suggest that agglutinins which bind to erythrocytes are identical to bacterial agglutinins, but there are additional agglutinins capable of reacting only with bacteria. 3) The inducibility and bacterial binding properties of Lumbricus agglutinins suggest that they serve an immune function by participating in the earthworm's defense against bacterial infection. In this sense, the agglutinins serve as a humoral surveillance system that entraps and prevents the multiplication of pathogenic bacteria.