© 2003 by The Society for Integrative and Comparative Biology
Cellular Receptors and Signal Transduction in Molluscan Hemocytes: Connections with the Innate Immune System of Vertebrates1
1 Department of Pathobiological Sciences, School of Vet Medicine, University of Wisconsin-Madison, 2115 Observatory Drive (Biotron), Madison, Wisconsin 53706-1087
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The involvement of circulating hemocytes as the principal cellular effector mediating molluscan immune responses is well established. They participate in a variety of internal defense-related activities including microbial phagocytosis, multicellular encapsulation, and cell-mediated cytotoxicity reactions that are presumed to be initiated through foreign ligand binding to hemocyte receptors and subsequent transduction of the binding signal through the cell resulting in appropriate (or in some cases, inappropriate) hemocyte responses. At present, however, although functional evidence abounds as to the existence of hemocyte "recognition" receptors, few have been characterized at the molecular level. Similarly, signal transduction systems associated with various receptor-mediated hemocyte functions in molluscs are only beginning to be investigated and understood. This review examines what is currently known about the molluscan hemocyte receptors and the putative signal transduction pathways involved in regulating their cellular behaviors/activities. The cumulative data implies the presence of various hemocyte-associated receptors capable of binding specific carbohydrates, extracellular matrix proteins, growth factors, hormones, and cytokines. Moreover, receptor-ligand interactions appear to involve signaling molecules similar to those already recognized in vertebrate immunocyte signal transduction pathways, such as protein kinases A and C, focal adhesion kinase, Src, Ca2+ and mitogen-activated protein kinase. Overall, the experimental evidence suggests that molluscan immune responses rely on molecules that share homology with those of vertebrate signaling systems. As more information regarding the molecular nature of hemocyte recognition receptors and their associated signaling molecules is accumulated, a clearer picture of how hemocyte immune responses to invading organisms are regulated will begin to emerge.
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INTRODUCTION |
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SYNOPSISINTRODUCTIONGASTROPOD HEMOCYTES REPRESENT A...HEMOCYTE RECEPTORSHEMOCYTE LECTINS AND THEIR...HEMOCYTE INTEGRIN-RELATED...HEMOCYTE GROWTH...CONCLUSIONSReferences |
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A common theme that integrates the vast field of immunology is the critical ability of cells comprising the immune system of each organism to communicate with both their internal and external environments. "Communication," in this context, is taken to mean being able to receive a chemical signal(s), usually via surface membrane receptors, transducing that signal across the membrane and through the cytoplasm, eventually to target effector molecules that, in turn, trigger appropriate responses (Heldin and Purton, 1996
). In cells associated with the vertebrate immune system, receptor-ligand interactions resulting in the regulation (i.e., activation or deactivation) of numerous cellular functions have been shown to involve a complex network of intracellular signaling molecules. For example, cellular adhesion and motility, cytotoxic responses, endocytosis, induction of cell death, and the like, have been linked to signal transduction pathways involving a diversity of cellular proteins including, but not restricted to, various small guanine nucleotide binding proteins (GTPases) such as Rho and Ras, phosphoinositide 3- and Src-kinases, protein kinases C or A (PKC/PKA) and members of the mitogen-activated protein kinase (MAPK) family including erks and p38 (Dong et al., 2001; Greenberg, 2001; Torgersen et al., 2002).
In contrast to the extensive data on immunocyte signaling systems found in higher vertebrates, relatively little is known about such systems in invertebrates, especially what molecules may be involved in signal transduction pathways and whether different cellular functions are regulated through the same or different pathways. For molluscs the mechanisms of signal transduction, in general, have been derived mainly from neurobiological studies (Crow et al., 1998; Nakhost et al., 1998; Tensen et al., 1998; Tierney, 2001) that have begun to establish the presence of intracellular signaling pathways, many of which appear to be homologous with those of mammals. In this review we will focus on the current knowledge of the identity and role of signaling molecules and pathways involved in molluscan immune responses, with emphasis on the gastropods. The discussion of cell signaling will be presented according to the type of cellular receptors believed to initiate such signals in the primary immune effector cells, the hemocytes.
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GASTROPOD HEMOCYTES REPRESENT A MOLECULARLY HETEROGENEOUS POPULATION |
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SYNOPSISINTRODUCTIONGASTROPOD HEMOCYTES REPRESENT A...HEMOCYTE RECEPTORSHEMOCYTE LECTINS AND THEIR...HEMOCYTE INTEGRIN-RELATED...HEMOCYTE GROWTH...CONCLUSIONSReferences |
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Circulating blood cells known as hemocytes, represent the main cellular component of the molluscan immune system. Several studies examining the composition of hemocyte populations of gastropods suggest that hemocytes are composed of a mixture of different subpopulations of cells. Flow cytometric analyses of hemocytes from the freshwater snails Biomphalaria glabrata (Johnston and Yoshino, 2001
) and Planorbarius corneus (Franceschi et al., 1991) demonstrate that circulating hemocyte populations could be divided into two main subtypes differing in their granularity and size. In both molluscan species, the larger granular cell subpopulation also has been shown to differ functionally from the smaller agranular cell type. For example, only the small round hemocyte (RH) of P. corneus are able to lyse a human cell line in a natural killer (NK) cytotoxicity test indicating an early sharing of functional characteristics between RH and NK-related cells (Franceschi et al., 1991). Large granular hemocytes of B. glabrata, characterized by the absence of the monoclonal antibody BGH1 surface marker (Yoshino and Granath, 1985; Johnston and Yoshino, 2001), is highly phagocytic compared to the nonphagocytic BGH1+ agranular cell subset. A similar distinction of hemocyte subpopulations based on expression of a unique surface epitope also has been made in Lymnaea stagnalis; namely hemocytes expressing the monoclonal antibody LS1 surface marker are thought to represent undifferentiated cells (LS1+ subset), whereas the LS1– cell subset is comprised of mature hemocytes (Dikkeboom et al., 1985). It is presumed that hemocyte subpopulations that differ both chemically and functionally are regulated in their activities or behaviors through specific receptors and the signals conveyed by their interaction with appropriate ligands. Unfortunately, in comparison with immunocytes of vertebrates, little is known about the receptors mediating immune-related activities or their associated pathways of intracellular signaling in molluscan hemocytes. |
HEMOCYTE RECEPTORS |
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SYNOPSISINTRODUCTIONGASTROPOD HEMOCYTES REPRESENT A...HEMOCYTE RECEPTORSHEMOCYTE LECTINS AND THEIR...HEMOCYTE INTEGRIN-RELATED...HEMOCYTE GROWTH...CONCLUSIONSReferences |
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The first step in executing an immune response is the detection by hemocytes of foreign invaders and/or non-self cells/tissues, presumably through receptors associated with the surface membrane. Signals generated by ligand binding are then transduced across the membrane, usually in conjunction with receptor phosphorylation, triggering a cascade of downstream chemical reactions, ultimately directing these signals to target organelles (e.g., nucleus, cytoskeleton) resulting in the induction of appropriate cellular responses (Heldin and Purton, 1996
). A number of receptors from hemocytes of gastropod molluscs have been identified based on crossreactivity with various anti-receptor antibodies or through functional assays, although relatively few have been characterized at the molecular level. For the purpose of discussion here, hemocyte receptors are grouped into several broad categories: Lectins (or lectin-like receptors), integrin-related proteins, and growth factor/hormone/cytokine-like receptors. Many of these receptors have been described or identified in cells from B. glabrata, the intermediate host of the human blood fluke Schistosoma mansoni, and therefore, will be the focus of this review. However, this is a rapidly evolving field of study and one should not ignore an important body of literature covering aspects of molecular reception and signaling in other nonhemocytic molluscan systems, especially those involving neuronal control of digestion/feeding (e.g., Canesi et al., 2000; Furukawa et al., 2001) and reproductive behavior (Kendel, 2001; Tierney, 2001). |
HEMOCYTE LECTINS AND THEIR SIGNALING NETWORKS |
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It has been established that numerous carbohydrate (CHO)-binding proteins are present in cell-free hemolymph or plasma of B. glabrata (Monroy et al., 1992
; Mansour et al., 1995). The finding that some of these proteins are able to bind to parasite larval stages has prompted speculation that they may be functionally-associated with host immunity. Recently, a unique family of proteins with CHO-reactivity, referred to as fibrinogen-related proteins or Freps (Adema et al., 1997; Leonard et al., 2001), has been shown to be induced in snails in response to infection with the trematode, Echinostoma paraensei, and specifically precipitate excretory-secretory proteins released by larvae. These lectin-like molecules are produced in hemocytes and appear to mediate their CHO-reactivity through a calcium-dependent carbohydrate recognition domain (CRD) located in the fibrinogen-like C-terminal segment of the protein (Adema et al., 1997). Complementary DNA encoding another B. glabrata-derived polypeptide with homology to a mammalian selectin C-type CRD also has been found (Duclermortier et al., 1999). Selectins are a family of mammalian CHO-reactive membrane proteins found on endothelial cells, leukocytes and/or platelets that serve as adhesion receptors in a variety of roles including leukocyte extravascular trafficking and inflammation (Kilpatrick, 2002; Patel et al., 2002). Like the Freps, the B. glabrata selectin-like polypeptide is synthesized in hemocytes and in cells of the B. glabrata embryonic (Bge) cell line, and occurs primarily as a secreted (soluble) protein. At present, however, a functional role, if any, of these soluble lectins in mediating specific cell-cell adhesive functions or anti-parasite activity is unknown.
In addition to hemocyte-derived soluble lectin-like proteins, the presence of CHO-reactive receptors at the surface of gastropod and bivalve hemocytes also has been demonstrated through biochemical and functional analyses (Vasta et al., 1982, 1984), direct binding studies using glycosylated bovine serum albumin (BSA) (Hahn et al., 2000), or through CHO-specific inhibition of hemocyte functions such as phagocytosis (Richards and Renwrantz, 1991; Fryer et al., 1989), cellular adherence to parasites (Castillo and Yoshino, 2002) or binding to larval excretory-secretory glycoproteins (Johnston and Yoshino, 2001). However, despite a considerable body of functional evidence for the existence of membrane-associated hemocyte lectin-like receptors in molluscs, to date such receptors have yet to be isolated and molecularly characterized. Consequently, very few studies of specific signaling pathways associated with lectin-like receptors in these cellular systems have been undertaken.
Phagocytosis, or the adherence and internalization of small foreign particles (e.g., bacteria) is normally initiated by the binding of particles to specific cell receptors. Such adhesive interactions may be the result of particles binding directly to recognition receptors at the cell membrane, or indirectly, through the reactivity of phagocyte receptors with particle-bound soluble recognition molecules (opsonins). The signaling pathways regulating phagocytosis in mammalian cell systems are well delineated, especially those associated with Fc, complement, Toll-like and lectin receptors (Aderem and Underhill, 1999; Kwiatkowska and Sobota, 1999; Hebert, 2000; Greenberg and Grinstein, 2002; Termeer et al., 2002 and others). Given that endocytosis is an "ancient" function shared by cells of all protozoan and metazoan phyla, it may be assumed that a high degree of conservation also exists in the general signaling pathways mediating this common cellular activity. However, with the exception of recent studies in insects, very little is known regarding phagocytosis signaling pathways in most invertebrate groups, including molluscs. For example, studies by Foukas et al. (1998) have demonstrated that phagocytosis of E. coli by Drosophila hemocytes appears to require Ras and mitogen-activated protein kinase (MAPK) activation for bacterial attachment and the involvement of a ß-integrin for internalization. Focal adhesion kinase (FAK) phosphorylation and Src-kinase also appear to be important in bacterial phagocytosis (Metheniti et al., 2001). Recently identified transmembrane peptidoglycan recognition proteins, PGRP-SA (Michel et al., 2001) and PGRP-LC (Gottar et al., 2002; Ramet et al., 2002), exhibiting binding specificities for gram-positive and gram-negative bacterial peptidoglycans, respectively, appear to confer resistance to bacterial infection, in part through a protective phagocytic response. Interestingly, PGRP-SA utilizes the Toll signaling network, whereas PGRP-LC receptor involves the IMD-Relish intracellular signaling pathway. Although the above studies did not address whether or not bacterial phagocytosis might involve possible lectin-like receptors, the fact that lipopolysaccharides or peptidoglycans represent major binding ligands mediating both the phagocytic and signaling events leaves open this possibility.
Preliminary studies in molluscs using B. glabrata hemocytes and cells of the B. glabrata embryonic (Bge) cell line have shown that these cells also are able to phagocytose heat killed E. coli (Fig. 1), and that such activity is inhibited in a dose-dependent fashion by calphostin C, a specific inhibitor of diacylglycerol (DAG)-dependent PKC (Fig. 2). These results suggest that phagocytosis of E. coli by snail hemocytes may occur through activation of a PKC-associated signaling pathway. However, Bge cells appeared to be more sensitive to calphostin C than hemocytes, being significantly inhibited in their phagocytic activity at 1 nM calphostin C compared to controls, and to a level of phagocytosis comparable to hemocytes at 10 nM of drug (Fig. 2). This result may be due in part to a higher variability in in vitro phagocytosis rates in hemocytes, or possibly to a more complex network of signaling pathways associated with hemocyte receptors mediating bacterial phagocytosis. Alternative pathways appear to be used in other molluscan species. For example, phagocytosis by oyster hemocytes is altered by noradrenaline through a ß-adrenergic receptor-coupled cAMP/PKA pathway, similar to that found in vertebrate systems (Lacoste et al., 2001). Clearly further study of signaling molecules regulating phagocytosis, especially those mediated by membrane-associated lectins (Fryer et al., 1989; Richards and Renwrantz, 1991), within and across molluscan classes is warranted.
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Hemocytic encapsulation responses to metazoan parasites, in contrast to bacterial phagocytosis, require the coordinated interactions of hemocytes, not only with the parasite and its secreted products, but also with each other, to form multicellular capsules that effectively eliminate the foreign intruder (Yoshino et al., 2001
). For example, hemocytes from a S. mansoni-resistant strain of B. glabrata would be required to migrate to the vicinity of invading larvae (sporocyst stage), initiate binding to the sporocyst tegument (recognition), produce a multicellular capsule around the parasite through cell-cell interactions, and finally effect a "killing" event involving cytotoxic activation. The specific receptors mediating different phases of this antiparasitic response have not yet been identified, although recent studies have implicated the involvement of different complex carbohydrates in the initial binding of B. glabrata hemocytes and Bge cells to live sporocysts (Castillo and Yoshino, 2002), representing the "recognition" phase of the parasite-host interaction, and in the triggering of hemocyte cytotoxic reactions (Hahn et al., 2001). In this latter case, exposure of B. glabrata hemocytes to specific BSA-conjugates of mannose, galactose or fucose stimulated cellular production of reactive oxygen species (ROS) such as superoxide or hydroperoxides, thus implicating hemocyte lectin-type receptors in signaling a potential sporocyst killing response (Hahn et al., 2000). Previous findings that oyster hemocyte ROS are inhibited by noradrenalin, and can be modulated by ß-adrenoreceptor agonists and antagonists, suggest that ß-adrenergic-type receptors also may be involved in regulating ROS production in some molluscan species (Lacoste et al., 2001). Likewise, exposure of bivalve hemocytes to phorbol myristate acetate (PMA), a diacylglycerol analogue, resulted in an increase in ROS (Pipe, 1992; Anderson, 1994) suggesting the involvement of a PKC-type signal transduction mechanism in the generation of an oxidative burst. To date, however, we still know very little regarding the specific signaling pathways triggered by receptor-CHO binding or those involved in regulating ROS production in molluscan hemocytes.
Excretory-secretory glycoproteins (ESP), released by early developing larval trematodes previously have been reported to affect snail hemocyte behaviors such as motility and cell spreading (Lodes and Yoshino, 1990; Noda and Loker, 1989a), phagocytosis (Noda and Loker, 1989b; Connors and Yoshino, 1990; Nunez et al., 1997), ROS production (Connors and Yoshino, 1990) and encapsulation responses (Loker et al., 1992), suggesting that hemocyte receptor-ESP interactions were providing intracellular signals critical to hemocyte immune function. Moreover, binding of ESP to B. glabrata hemocytes appears to be through lectin-like CHO binding receptors (Hertel et al., 2000; Johnston and Yoshino, 2001), indicating the involvement of lectin-like receptors in transducing these behavior-modulating cellular signals. In support of this notion, Hertel et al. (2000) have shown that in response to E. paraensei ESP, B. glabrata hemocytes display cell rounding accompanied by intracellular calcium transients or fluxes. Interestingly, exposure of cells to a mannose-BSA neoglycoconjugate produced a nearly identical pattern of calcium transients and rounding as echinostome ESP treatment, inferring that echinostome ESP-mediated signaling may be initiated through lectin-like receptors and involve Ca+2 mobilization as a second messenger system. Important questions still remain to be answered when considering the complex molecular interactions between hemocytes and their larval trematodes: Can the binding of different CHO ligands to hemocytes result in either up- or down-regulation of specific responses, and if so, is this the result of receptor-ligand triggering of distinct intracellular signal transduction pathways? Have some larval trematodes "discovered" (and are exploiting) signaling pathways that lead to cellular deactivation, thus circumventing or ward off potentially damaging immune reactions? These represent some of the very interesting, but challenging, questions to be addressed in future studies.
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HEMOCYTE INTEGRIN-RELATED RECEPTORS AND THEIR SIGNALING NETWORKS |
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As integrin receptors have been reported to regulate vertebrate cell spreading and motility (Hynes, 1992
; Holly et al., 2000), the possibility that homologues of this same family of receptors might play a similar role in molluscan cell behaviors was investigated. Initial studies demonstrated that the tetrapeptide, arg-gly-asp-ser (RGDS), could inhibit the spreading activity of both B. glabrata hemocytes and cells of the closely-related Bge cell line, suggesting the presence of an integrin-like receptor(s) and their involvement in cellular adhesion/spreading behavior (Davids and Yoshino, 1998). A ß-integrin receptor subunit cDNA subsequently was cloned from Bge cells and a near identical partial sequence also was identified in B. glabrata hemocytes (Davids et al., 1999; Yoshino et al., 1999).
To examine which signaling molecules might be involved downstream of receptor activation, a panel of specific inhibitory drugs targeted to various signaling molecules were used in in vitro cell spreading assays. The results of drug inhibition studies demonstrated that proteins with pharmacological similarities to PKC, Ras and MAPK kinase (Mek) may be regulating both Bge cell and hemocyte spreading (Humphries et al., 2001; J.E.H., unpublished observations). However, compared to Bge cells, a lower sensitivity of B. glabrata hemocytes to specific PKC/Ras/MAPK inhibitors (unpublished observations), suggests that RGDS-mediated spreading in these cells may be regulated by a "network" of signaling pathways possibly involving multiple receptors, rather than a single, predominant one. At the molecular level, evidence in B. glabrata for specific homologues of mammalian signaling molecules is scant. An intracellular receptor for activated protein kinase C (RACK) has been identified from whole snail cDNA, and was shown to be capable of binding activated mammalian PKC (Lardans et al., 1998). This finding provides indirect evidence for the presence of a PKC-like protein(s) in this snail species. In addition, we have cloned from Bge cells a partial cDNA sequence with significant homology to p38 (Accession no. AF486290
[GenBank]
), a MAPK family member associated with stress (inflammatory) signaling (Huang and Erikson, 1996). As more information is gained regarding the types of signaling molecules present in snail cells, a clearer picture of the potentially complex networks involved in simple adhesive/spreading behaviors of hemocytes should begin to emerge.
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HEMOCYTE GROWTH FACTOR/HORMONE/CYTOKINE RECEPTORS AND INTRACELLULAR SIGNALING |
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Because cytokines, growth factors and hormones are known to play prominent roles in regulating cellular activity in vertebrate immunity, it has been of interest, from a comparative and phylogenetic perspective, to investigate whether or not such molecules elicit responses in molluscan hemocytes, presumably through binding interactions with appropriate receptors. Several studies have revealed the ability of circulating hemocytes to respond to a variety of soluble vertebrate factors, indirectly suggesting the presence of receptors to a variety of growth, hormonal and cytokine-like factors. For example, receptors for platelet-derived growth factor (PDGF-alpha/ß) and transforming growth factor-ß (TGF-ß) have been reported in hemocytes of mussels Mytilus galloprovincialis, and when bound to their respective ligands, appear to modulate various cellular functions such as phagocytosis and cell motility (Ottaviani et al., 1997a
; Kletsas et al., 1998). Exposure of Haliotis (abalone) hemocytes to epidermal growth factor (EGF) or porcine insulin stimulates protein and DNA synthesis (Lebel et al., 1996), again suggesting the presence of their respective receptors and the ability to transduce ligand-mediated intracellular signaling. An insulin-like receptors cDNA has been cloned from the Bge cells line (Lardans et al., 2001) and adrenocorticotrophic hormone (ACTH)-like receptor mRNA was localized in Mytilus hemocytes (Ottaviani et al., 1998), thus supporting the prediction of the presence of growth factor/hormone receptors in molluscan hemocytes. Similarly, peptide derivatives of proopiomelanocortin (POMC), which include ß-endorphin, ACTH and alpha-melanotropin (melanocyte-stimulating hormone; alpha-MSH), have been shown to affect hemocyte behavior in a variety of molluscan species, implying the presence of their hemocyte-associated peptide receptors (Stefano et al., 1989; Duvaux-Miret et al., 1992; Sassi et al., 1998). The presence of cytokine-like receptors on molluscan hemocytes, likewise, has been inferred by the induction of various hemocyte functions by exposure to vertebrate cytokines. For example exposure of B. glabrata hemocytes to human recombinant interleukin (IL)-1 results in the priming for superoxide production and increased phagocytosis (Granath et al., 1994; Connors et al., 1995), while IL-8 induces in mussel hemocytes increased phagocytic activity and a chemotactic response (Ottaviani et al., 2000).
The direct "cause and effect" relationship illustrated above between ligand treatment and hemocyte reactivity to ligand exposure suggests not only the presence of appropriate receptors, but also the ability to communicate or transduce the ligand-initiated signal into the cell to produce an effector response. These results, of course, raise the question: what is the nature of the signaling pathways being used in these systems? Based on pharmacological studies, as observed with integrin-like receptors, by employing specific inhibitor drugs it appears that ACTH-receptor signaling induces Mytilus hemocyte shape changes though an adenylate cyclase/cAMP/ protein kinase A (PKA) pathway (Sassi et al., 1998). In contrast, corticotropin-releasing hormone (CRH) appears to induce cellular shape changes, not only through activation of PKA and PKC, but also requires extracellular calcium and the synergistic action of cAMP and inositol 1,4,5 triphosphate (Malagoli et al., 2000). The cell shape changes induced by PDGF-alpha/ß and TGF-ß in Mytilus hemocytes appear to be transduced through a PKA/PKC phosphoinositide-phospholipase C-type pathway similar to the CRH-receptor (Ottaviani et al., 1999). However, PDGF signaling, unlike that of TGF, is extracellular calcium-independent (Ottaviani et al., 1997b). Little is known regarding cytokine receptor-mediated signaling in molluscan hemocytes. Pharmacological studies on mussel hemocytes indicate that IL-8-induced changes in cell spreading may be regulated through PKC and PKA, and is associated with reorganization of actin filaments. The involvement of the common signaling elements PKC and PKA by a variety of receptor types (growth factors, hormones, cytokines, integrins) suggest a conservation of signaling pathways within molluscan hemocytes. However, more research in this area is needed, especially at the molecular level, to begin to more precisely define all of the molecules involved in linking receptors to cell function.
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As described in this review, molluscs possess a very effective, multifaceted immune/defense system, incorporating both cellular and humoral components. By its nature, an immune system depends on the ability of cells to recognize the presence of foreign antigens (by pattern recognition) and elicit appropriate responses. The entire process of "recognition-to-response" depends on complex intracellular signal transduction systems that we are only beginning to understand in the context of molluscan immunity. Although research of this area in molluscs is still in its infancy, recent discoveries to date imply that molluscan immune-mediated signal transduction is well developed and most likely utilizes a variety of molecular pathways shared in common with vertebrates and the more intensely studied invertebrates such as Drosophila. In general terms, hemocyte-mediated immune responses of molluscs reflect those innate cellular responses of vertebrates; they involve surface membrane receptors including integrin- and lectin-like proteins, as well as receptors capable of binding mammalian cytokines, growth factors and peptide hormones. Similarly, there is evidence to suggest that protein kinases such as PKA, PKC, FAK and MAPK, small G proteins such as Ras, and second messengers like cAMP and [Ca+2]i, are participants in regulating molluscan hemocyte functions such as cell spreading, phagocytosis or ROS production. However, much of the available data on which this review was based, had been acquired through functional and/or inhibition assays, as well as immunocytochemistry utilizing specific, but crossreactive, antibody probes. Clearly their exists a need for more research at the molecular level in order to fully characterize homologous receptors, signaling molecules and pathways involved in hemocyte immune responses in molluscs.
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ACKNOWLEDGMENTS |
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The authors gratefully acknowledge the assistance of Courtney Copeland, supported by the University of Wisconsin SRP-Biology Undergraduate program, and Michelle Browne in generating some of the preliminary data on B. glabrata phagocytosis presented in this paper.
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FOOTNOTES |
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1 From the Symposium Comparative Immunology presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 2–6 January 2002, at Anaheim, California.
2 Corresponding author; E-mail: yoshinot{at}svm.vetmed.wisc.edu
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References |
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