Different types of response to foreign antigens by leech leukocytes M. de Eguileor, 1 A. Grimaldi, 1 G. Tettamanti, 1 R. Valvassori, 1 E. L. Cooper, 2 G. Lanzavecchia 1 Abstract. We used morphological and immunocytochemical approaches to characterize and to show the behavior of cells involved in leech inflammatory responses. Leeches were injected with bacterial lipopolysaccha- ride, fluoresceinated yeasts, sulfate spheres and ciliates (Protozoa). Shortly after injection, migrating cells appeared in the area of injection. The response of the cells occurred in relation to the injected micro or macro anti- gens. Each injection first provoked a migration of cells towards the non-self material. Afterwards, different responses (degranulation, phagocytosis, encapsulation, melanization) occurred. The migrating cells involved in these series of processes have a similar behavior and are characterized by CD markers of macrophages, NK cells and granulocytes, which are typical of many invertebrates and vertebrates. © 2000 Harcourt Publishers Ltd Keywords: leeches, LPS, phagocytosis, degranulation, encapsulation, melanization Introduction Animal immune systems can recognize foreign antigens and can respond with a wide repertoire of reactions. One response is phagocytosis, which is defined as the capacity of cells to engulf foreign material. This non-specific event, linked to specific immune activity, has the prime function of freeing organisms from unwanted, potentially pathogenic agents (Cooper, 1981; Bilej, 1994; Porchet-Henneré, 1990; Cooper et al., 1992; Dales & Kalaç, 1992; Porchet-Henneré & Vernet, 1992; Porchet-Henneré et al., 1992; Cooper et al., 1996; Blanco et al., 1997; Ottaviani & Franceschi, 1997). Recently, phagocytosis has been more clearly defined and associated with ‘an essential component of the innate, natural, non-specific, non-anticipatory, non-clonal immune system, largely confined to invertebrates but essential for interaction with the more advanced and presumably less ancient adaptive, induced, specific, anticipatory and clonal immune system of all vertebrates (Stein et al., 1977; Cooper et al., 1996; Fearon & Locksley, 1996; Hughes & Yeager, 1997; Medzhitov & Janeway, 1997). The issue of ancient immune systems in invertebrates is indisputable, but the less ancient is controversial, largely due to a dearth of studies and the failure of certain studies to address the hallmarks of specificity and memory. Another type of response against very large non-self material is encapsulation. The event occurs when the foreign antigen is too cumbersome to phagocytose, and it is found in all invertebrates from the most primitive to the most evoluted (Parrinello & Rindone, 1981; Ratcliffe et al., 1985; Gotz & Boman, 1986; Porchet-Henneré, 1990; Soderhall et al., 1996; De Leo et al., 1997). In many inverte- brates like sipunculids, a closely related group to annelids (Blanco et al., 1997), the migrating cells involved in defence processes have a similar behavior and show CD markers of macrophages of vertebrates. Despite an abundance of studies on phagocytosis, encap- sulation, melanization in invertebrates, and CD markers, there is little information on Hirudinea (Mann, 1962; Sawyer, 1986). We consequently have begun to analyse the inflamma- tory and immune activity of leeches and present herein results that characterize the complex responses to living and non-living foreign bodies. 1 DBSF, University of Insubria, 21100 Varese, Italy 2 Laboratory of Comparative Immunology, Department of Neurobiology, UCLA School of Medicine, Los Angeles, CA 90095–1763, USA Received 11 January 1999 Accepted 12 November 1999 Correspondence to: Magda de Eguileor, DBSF, Università dell’Insubria, Via Dunant 3, 21100 Varese, Italy. Tel.: +39 332 421310; Fax: +39 332 421330; E-mail: magda@mailserver.unimi.it 40 Tissue & Cell Tissue & Cell, 2000 32 (1) 40–48 © 2000 Harcourt Publishers Ltd DOI: 10.1054/tice.1999.0085, available online at http://www.idealibrary.com