Immunp Evolution lectin.These results coupled with more detailed character- absent.In earthworms (Eisenia fetida).it was demon ization point out the importance of these molecules in strated that their coelomocytes (leucocytes)can kill the eltineK562whcncoetnrndwmhLoTR cel sing citich echinoderms.Isolated from starfish leucocytes,these IL-1- been identified:(a)small (8-11um)electron-dense cells hch as SC)that were stained by themo layed-typ ivation of macrophages for cytostasis or suppression diendnmmtmeepons for B2-microglobulin and Thy-1:and(b)large (12-15 um) of T-dependent but not th markers.Ba-m LPS with Both e meer CD profilin.a small actin-binding protein involved in signal antigens in all of the animal models and MHC class I and transduction. Comparison of deduced amino acid se- class II markers in earthworms.In vertebrates these y0 h clotting factors ociSc1ectim,poteaseinihoA.scnepole istics.Rather precise characteristics appear to be asso First,there appeared to b clastas and plasmi ting the ent resp I proteins and as a peti aps and body plar esengok2ertpa invertebrates exhibit reactions that are function ally stiesoftheaorcienractinScendreend Cr,the spo h cn as rejectio with them reacti certain cellular differentiation(CD)antigens have beer cific inflammatory responses.Thus, identified and associated with particular the debris that cannot be sequestered by phagocytosi S IS with the formation of granulomas. the orn-po S:gran ed in antigens is enormous.Specific identification has allowed sequence information on the putative perforin molecule to targets hei and eff a nal antibo to delineate particular cells have also been used to define Vertebrate Immune Systems ifcantibodies oteins are oad discipline. oted nh merely phenotypic markers but are themselves involved in invertebrates to antibody synthesis even in the most several lymphocyte responses.For example,in mammals primitive species of the present-day vertebrates. the most helper Tlymph cytes are CD ontext ol comp CD? The two most fre nt functions ibuted to various cd inherited durine evolution.in order ogether this antigens are:(1)to promote cell-cell interactions and enormous mosaic of responses.Advanced invertebrate adhesion,and (2)to transduce signals that lead to ines diverged at the level of the coelenterates.The chordate ates t ng, consists certain cd markers have bee revealed in earthworms 101 leeches sir nculid worms and snails.It isimportant to note thatcertainother markersare separate openings that become the mouth and anus. 6 ENCYCLOPEDIA OF LIFE SCIENCES/e2001 Na shing Group/ ww.els.netlectin. These results coupled with more detailed character￾ization point out the importance of these molecules in defence against invading bacteria or altered cells and a fundamental role as cell adhesion molecules involved in cell–cell and cell–matrix interaction. Other interesting molecules play a role in the humoral immune response of echinoderms. Isolated from starfish leucocytes, these IL-1- like proteins exhibit a wide range of biological activities, such as inflammatory reactions identical to delayed-type hypersensitivity, inhibition of macrophage migration, activation of macrophages for cytostasis or suppression of T-dependent but not T-independent immune responses. Later results have revealed that sea urchin coelomocytes activated with lipopolysaccharide (LPS) respond to immune challenge from LPS with significant elevation of profilin, a small actin-binding protein involved in signal transduction. Comparison of deduced amino acid se￾quences with known protein sequences revealed significant matches with diverse proteins such as an invertebrate homologue of a vertebrate complement component, clotting factors, complement receptor or regulatory proteins, C-type lectin, protease inibitors, serine protease with similarities to thrombin, elastase and plasmin, proteins involved in signalling, lysosomal proteins and cytoskeletal proteins. Despite their simpler organization and body plan, invertebrates exhibit reactions that are functionally equivalent to vertebrate cellular responses such as rejection of foreign transplants and the killing of various tumour cell targets cocultured with them in vitro. In earthworms, certain cellular differentiation (CD) antigens have been identified and associated with particular functions that seem to be mediated primarily by leucocytes, although the intervention of and cooperation with humoral components cannot be excluded. As mentioned earlier, the value of CD antigens is enormous. Specific identification has allowed immunologists to functionally classify the cells participat￾ing in various immune responses, to isolate them and to individually analyse their specificities, response patterns and effector mechanisms. The monoclonal antibodies used to delineate particular cells have also been used to define specific alterations in particular lymphocyte subsets that might occur in various diseases. Further investigations of the effects of monoclonal antibodies on lymphocyte function have shown that these surface proteins are not merely phenotypic markers but are themselves involved in several lymphocyte responses. For example, in mammals most helper T lymphocytes are CD3 1 CD4 1 CD8 2 and most cytotoxic lymphocytes are CD3 1 CD4 2 CD8 1 . The two most frequent functions attributed to various CD antigens are: (1) to promote cell–cell interactions and adhesion, and (2) to transduce signals that lead to lymphocyte activation. Using this approach, certain CD markers have been revealed in earthworms, leeches sipunculid worms and snails. It is important to note that certain other markers are absent. In earthworms (Eisenia fetida), it was demon￾strated that their coelomocytes (leucocytes) can kill the human tumour cell line K562 when cocultured with it. To clarify which cells are responsible, and using cytofluori￾metric analysis, microscopy and mouse anti-human monoclonal antibodies, two types of coelomocytes have been identified: (a) small (8–11 mm) electron-dense cells (SC) that were stained by the monoclonal antibodies for epitopes present on human cell adhesion molecules (CD11a, CD45RA, CD45RO, CDw49b, CD54) and those for b2-microglobulin and Thy-1; and (b) large (12–15 mm) electron-lucent cells (LC) that were negative for these same markers. b2-microglobulin, Thy-1 and Lyt-2 (CD8a) have been previously demonstrated by serological methods. Both cell types were negative for numerous other CD antigens in all of the animal models and MHC class I and class II markers in earthworms. In vertebrates these markers play important roles during the intricate mechan￾isms of cytotoxic reactions and immunoglobulin synthesis. The mechanisms in earthworms have several character￾istics. Rather precise characteristics appear to be asso￾ciated with the killing response. First, there appeared to be a division of labour reflecting the ancient response as well as a perhaps more recent response. SC were active during recognition, rapidly binding to K562 targets and probably effecting the killing. In contrast, LC were phagocytic, vestiges of the ancient reaction. Second, release of 51Cr, the assay system for cytotoxicity, revealed rapid, significant and equal levels of killing at 4, 20 and 378C. Third, after cytotoxicity is complete in vitro, there are reactions associated with nonspecific inflammatory responses. Thus, the debris that cannot be sequestered by phagocytosis is encapsulated, with the formation of granulomas. Finally, perforin-positive granules are present in the cytoplasm of coelomocytes. Since perforin is involved in cell lysis, sequence information on the putative perforin molecule is needed to confirm that it may be partially responsible for lysis of tumour cell targets. Vertebrate Immune Systems Immunology is now a broad discipline, ranging in subject matter from the oft-quoted phagocytosis in unicellular invertebrates to antibody synthesis even in the most primitive species of the present-day vertebrates, the cyclostomes. In the context of comparative immunology, it is essential now to encompass major themes, perhaps inherited during evolution, in order to piece together this enormous mosaic of responses. Advanced invertebrate lines diverged at the level of the coelenterates. The chordate line, to which vertebrates belong, consists of animals characterized by the deuterostomate pattern of embry￾ogenesis. In such a pattern, the blastula possesses two separate openings that become the mouth and anus. Immune Response: Evolution 6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net