Cell adhesion and the immune system: a case study using earthworms.

In the earthworm's immune system, cell adhesion, which occurs by putative receptors on leukocytes, is essential after recognition of self vs. non-self. Confrontation with foreign antigens is a normal event in the environment, replete with microbial pathogens that pose a threat to survival. To better understand what happens when an effector cell first recognizes a foreign target followed by its adhesion to it, isolated leukocytes, in sufficient quantities to be subjected to various analyses, have been extremely beneficial. In vitro approaches when accompanied by biochemical, immunological, and molecular technologies, have opened up new vistas concerning the immune response of earthworms and other invertebrates. The most recent discovery includes the preliminary identification of cell differentiation (CD) markers that play vital roles in recognitive and adhesive events. Certain leukocyte effectors show characteristics of natural killer (NK) cells that may act differently depending upon their source, whether autogeneic, allogeneic, xenogeneic, or expressed under normal or varying environmental conditions including exposure to xenobiotics. At the level of earthworm evolution, there is apparently a dissociation of phagocytosis from the process of killing by NK-like effectors. There are at least three future challenges. First, it is essential to determine the precise nature of the CD markers with respect to their molecular structure. Second, once their molecular and biochemical characteristics have been defined, the role of these markers in cellular and humoral mechanisms must be clarified in order to define effector cell products and resulting immune responses. Third, there is a need to differentiate between the several lytic factors that have been found in earthworms with respect to molecular structure, and biochemical and functional characterization.