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Supplement

Zinc Deficiency Impairs Immune Responses against Parasitic Nematode Infections at Intestinal and Systemic Sites^{1 ,2}

Institute of Parasitology, School of Dietetics and Human Nutrition, McGill University, Macdonald Campus, Ste-Anne de Bellevue, Quebec H9X 3V9, Canada

³To whom correspondence should be addressed.

	ABSTRACT

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
Research on the complex interactions among host nutritional status, parasitic infection and immune responsiveness has focused on the detrimental consequences of parasitic infections on host nutritional status and on mechanisms by which malnutrition impairs immunocompetence. Curiously, relatively few studies have examined the effects of malnutrition on the immune response in the parasite-infected host, and even fewer have considered the events occurring at the intestinal level, where absorption of nutrients occurs, intestinal parasites reside, and the gastrointestinal-associated lymphoid tissues play a role in directing both the local and the more systemic immune responses. Our work using a zinc-deficient nematode-infected mouse model reveals that parasites are better able to survive in the zinc-deficient hosts than in well-nourished hosts; that the production of interleukin-4 in the spleen of zinc-deficient mice is depressed, leading to depressed levels of IgE, IgG₁ and eosinophils; and that the function of T cells and antigen-presenting cells is impaired by zinc deficiency as well as by energy restriction. Given the paramount role of the gastrointestinal-associated lymphoid tissues in inducing and regulating immune responses to intestinal parasites and in orchestrating responses in the spleen and peripheral circulation, we conclude that zinc deficiency (in association with energy restriction) exerts profound effects on the gut mucosal immune system, leading to changes in systemically disseminated immune responses and, importantly, to prolonged parasite survival.

KEY WORDS: • zinc deficiency • immunity • nematode infection • T helper cells • energy restriction • gut-associated lymphoid tissue

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
The gastrointestinal (GI)⁴ tract is not only an organ for digestion, absorption and excretion but also home to many parasitic organisms. It is one of the largest immunological organs of the body, and it serves as the first line of defense against orally administered antigens (e.g., food proteins) and intestinal pathogens (e.g., bacteria, parasites). Gut-associated lymphoid tissues (GALT) make up ~25% by weight of the gut mucosa and submucosa and thus constitute the largest extrathymic site of lymphocytes in humans (McBurney, 1994). Cells in GALT respond to intestinal pathogens by processing antigens for recognition by lymphocytes, by initiating a cascade of specialized immune responses to the antigens, by regulating the migration of immune mediators from the periphery to the infected gut and by participating directly in cytotoxic activities that limit parasite establishment and survival. In addition to these specific immunological responses, the GI tract performs nonspecific barrier functions (Van der Hulst et al. 1998 , Welsh et al. 1998 ). Mucus secretion and formation of tight cell junctions prevent the entry of bacteria and other pathogenic antigens, and rapid mucosal turnover enables the repair of epithelial or lymphoid cells damaged by parasitic infections.

Many nutritional deficiencies have been shown to cause atrophy of lymphoid tissues, leading to lower numbers of immune cells and functional defects during antigen-specific responses. Specifically, zinc deficiency causes atrophy of lymphoid tissues (Keen and Gershwin 1990 , Shi et al. 1994 ) and depressed cutaneous delayed-type hypersensitivity reactions (Sempertegui et al. 1996 , Shi et al. 1994 ). In animals, experimental zinc deficiency reduces antibody production, T cell proliferation and cytokine production in response to mitogens and specific antigens (reviewed by Fraker 2000 , Harbige 1996 , Scrimshaw and San Giovanni 1997 ). These findings, obtained from analyses of thymus or spleen, reflect the essential requirement for zinc by CD4⁺ helper T and B cells in systemic lymphoid organs and presumably apply to lymphoid tissues in the intestine, although information on the effects of zinc deficiency on the barrier function and on specific immune responses in the GI tract is severely lacking. The gut mucosa has one of the highest protein turnover rates of all tissues in the body (McBurney 1993 , Nakshabendi et al. 1995 ), and therefore the GALT is likely very sensitive to changes in nutritional status (Van der Hulst et al. 1998 , Wykes et al. 1996 ). Zinc deficiency has been associated with gross abnormalities in epithelial structure (Moran and Lewis 1985 ) and thus may weaken the epithelial barrier, allowing greater penetration and establishment of the larval stages of GI nematodes. It may interfere with the recruitment and localization of immune cells to the intestinal epithelium, thus impairing the host’s ability to effectively combat parasitic infection.

Much of the recent work in our laboratory has concentrated on the effects of zinc deficiency on the host immune response to GI nematode infections; therefore, this review focuses on the impact of zinc deficiency on infection with a GI nematode parasite and the underlying host immune responses that control parasite growth, survival and reproduction. We will argue the following: (1) the efficiency of immunological responses against gastrointestinal nematodes depends on the zinc nutriture of the host; (2) initial immunological events occurring shortly after infection at the local intestinal level orchestrate subsequent events occurring both at the gut and at more distant systemic sites; (3) zinc deficiency impairs these processes, both because of its limited concentration and because of the limited energy intake that occurs concurrent with the ingestion of a zinc-deficient diet; (4) zinc deficiency impairs not only Th1-type responses, as hypothesized in the recent literature, but also Th2-type responses; and (5) because the immune response elicited to infectious agents normally includes many redundancies, the ultimate consequence of zinc deficiency in controlling infection needs to be established in an infected host.

	GALT in intestinal infections

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
The gut mucosal immune system consists of two anatomically and functionally distinct compartments: (1) the specialized local inductive sites (Peyer’s patches, isolated lymphoid follicles, mesenteric lymph nodes), where intestinal antigens are first recognized (Bogen et al. 1991 , Laissue et al. 1993 , Regoli et al. 1994 ); and (2) diffuse effector sites (intraepithelium and lamina propria), where the outcome of an effective immune response is elimination of the infectious agent (Beagley and Elson 1992 ). Luminal antigens are transported across epithelial barriers either by specialized epithelial M cells or by intraepithelial lymphocytes (mostly T cells) to the organized lymphoid tissues within the mucosa (e.g., Peyer’s patches). After epithelial transport, antigens are processed and presented by antigen-presenting cells (APC) such as dendritic cells, B cells, macrophages and other intestinal epithelial cells. Naive T lymphocytes first interact with antigen-primed APC in aggregated Peyer’s patches and single lymphoid follicles and then further differentiate in the germinal centers of the lymphoid follicles. Thereafter, the antigen-specific T and B cells leave the epithelial barrier to collect in the mesenteric lymph nodes (MLN), which drain the mucosa and supply the peripheral bloodstream with gut-derived or locally activated immune cells, or both. From the blood, the lymphocytes migrate to systemic lymphoid tissues such as the spleen and peripheral lymph nodes, where the lymphocytes proliferate and mature either into effector lymphocytes, which secrete cytokines and mediate T cell–dependent humoral immunity, or into memory cells that can respond rapidly to the infection on secondary encounter (Salmi and Jalkanen 1991 ). Peripheral lymphocytes can preferentially leave the blood vessels and move into the intestinal lamina propria and intraepithelium (Kagnoff 1996 , Shanahan 1994 ) by expressing adhesion receptors that are recognized by specific endothelial molecules lining the gut mucosal lymphoid tissues (Smith and Weis 1996 ). As a result, most of the antigen-committed and differentiated lymphocytes that enter the effector sites of GALT are likely to have had prior contact with, and specific activation by, parasite antigens located in the gut mucosa.

The continuous migration of lymphocytes from intestinal lymphoid tissues to the bloodstream and back enables the GALT to carry out two important roles in the defense against intestinal parasites. First, it allows delivery of the parasite antigen to peripheral sites, initiating a widely disseminated immune response, and second, it promotes trafficking of gut-derived lymphocytes from the blood to effector sites within the intestinal epithelium. Gut-associated lymphocytes further contribute to host defense against GI parasites by secreting cytokines that regulate the appropriateness, magnitude and phenotypic expression of immune responses (Finkelman et al. 1997 , Mosmann and Sad 1996 ). Depending on the type of antigenic stimulus, undifferentiated T helper (Th) cells transform into either Th1 or Th2 cells (Fig. 1 ). Bacterial, viral and protozoan infections usually stimulate a Th1 response, characterized by elevated levels of Th1 cytokines [i.e., interleukin (IL)-2, IL-12, interferon (IFN)-] and effectors such as macrophages, natural killer cells and neutrophils. In such Th1 responses, cell-mediated immunity involving phagocytosis is responsible for the functional immunity. In contrast, the immune response to intestinal nematode parasites depends on the production of Th2 cytokines (e.g., IL-4, IL-5, IL-10), which mediate antibody-dependent effector responses. These Th2 cytokines released in the GALT attract progenitors of B cells, mucosal mast cells (MMC) and eosinophils by chemotaxis to the mucosal epithelium, where they proliferate and mature in response to the stimulatory signals of cytokines and parasite antigens. Nematode infections of the GI tract induce an up-regulation of gene expression for Th2 cytokines in MLN and Peyer’s patches before changes in splenic mRNA levels (Svetic et al. 1993 ). Local cytokines in GALT also promote mucosal mastocytosis, proliferation of IgE-secreting B cells in situ, transport of systemic IgE into the intestinal lumen of nematode-infected mice and IgE-mediated MMC degranulation and release of cytotoxic inflammatory substances (Haig et al. 1984 , Ramaswamy et al. 1996 , Wahid et al. 1994 , Wastling et al. 1997 ). Moreover, recent evidence suggests that the cytokine milieu in GALT regulates peripheral responsiveness in spleen and other peripheral organs during intestinal parasitic infection (Shi et al. 1998b ). Taken together, the gut-associated immune system stimulates host resistance to intestinal parasites through its role in antigen presentation, cell activation and trafficking, proliferation of immune effectors that secrete regulatory cytokines and/or mediate protection and induction of systemic immune responses.

View larger version (26K): [in this window] [in a new window]   Figure 1. Schematic representation of differentiation and cross-regulation of T helper (Th) cells in relation to the cytokine milieu and infectious agents (adapted from Mosmann and Sad, 1996 ). TGF, transforming growth factor.

	Evidence that zinc deficiency impairs intestinal immunity

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
Epidemiological evidence to support a role for zinc in maintenance of the gut epithelial barrier and intestinal immunity is limited and difficult to interpret. In a double-blind, randomized, controlled trial of children with acute diarrhea, daily supplementation with elemental zinc reduced the risk of continued diarrhea by 21%, after controlling for stunting, age and baseline plasma zinc concentration, and also reduced the severity of the diarrhea, particularly in stunted children (Sazawal et al. 1995 ). The improvements in the growth of and reduction in diarrheal episodes in zinc-supplemented children were most striking in children who were moderately or severely malnourished (Tomkins et al. 1993 ) although not necessarily zinc deficient (Sazawal et al. 1995 ). Viral, bacterial and protozoan infections that cause diarrhea are likely to elicit a Th1 response, and therefore the finding of reduced duration and severity of acute diarrhea in association with zinc supplementation is consistent with the hypothesis that zinc is required for a Th1-dependent immune response. However, we note that this and related studies have been conducted without knowledge of the specific etiological agent responsible for the diarrhea, with the high probability that several infections are concurrent, including nematode infections that cause diarrhea (and stimulate a Th2 response), and with the likelihood that multiple nutritional deficiencies are also present in the children. In addition, we note that the authors did not provide evidence of improved immunological functions, in response to zinc supplementation.

Until recently, studies on the interaction between zinc deficiency and intestinal nematode infections have been limited in number. In humans, plasma zinc levels were shown to be significantly negatively correlated with numbers of Trichuris trichiura in Jamaican children (Bundy and Golden 1987 ), although it was not clear whether the low plasma zinc levels caused higher infection levels, whether the higher infection levels caused a depression in plasma zinc concentration or whether the association was related to some undefined third factor. The only human study designed to test cause and effect was undertaken in Guatemalan children from an urban primary school. After drug treatment for intestinal worms, reinfection rates were monitored in children receiving daily zinc supplements over several months compared with placebo control subjects. No reduction in reinfection rate was detected in the zinc-supplemented children (Grazioso et al. 1993 ). Zinc supplementation did improve the plasma zinc concentration but did not alter the hair zinc content. These two studies highlight the difficulties associated with designing and interpreting human trials. In the first study, virtually 100% of the children from a place-of-safety in Kingston, Jamaica, were infected with hundreds to thousands of T. trichiura, and close to 50% of the children were zinc deficient. In contrast, the Guatemalan study involved primary school children in an urban setting, where the prevalence of infection was only 28% for T. trichiura, and 21% for Ascaris lumbricoides, where the authors did not report the intensity of infection and where only 7% of the children were considered to be zinc deficient at the beginning of the study. Results from laboratory studies have been more illuminating.

The first set of laboratory studies on the effects of zinc deficiency on intestinal nematodes examined whether parasites survived for longer periods of time in animals fed a zinc-deficient diet. Not surprisingly, the results varied depending on the level of dietary zinc. Fenwick et al. (1990a) reported delayed expulsion and higher burdens of Trichinella spiralis in rats fed a zinc-deficient diet (3 mg zinc/kg diet) compared with control animals, Fenwick et al. (1990b) showed a similar pattern in rats infected with Strongyloides ratti and Boulay et al. (1998) reported prolonged survival of Heligmosomoides polygyrus in mice also fed a 3 mg zinc/kg diet. In contrast, a dietary zinc intake of 5 mg/kg did not increase the intensity of H. polygyrus infection in mice (Minkus et al. 1992 ). The results were also dependent on the parasite under investigation and the type of infection protocol. Although 3 mg zinc/kg diet prolonged the survival of T. spiralis and S. ratti in rats and of H. polygyus in mice, it had no effect on the number or size of Nippostrongylus brasiliensis in rats (El-Hag et al. 1989 ). Also, the effect of dietary zinc deficiency (3 mg zinc/kg diet) on H. polygyrus was evident in naive mice responding to the infection for the first time (a "primary" infection) but not in previously immunized mice fed a "challenge" infection (Boulay et al. 1998 ). When the level of dietary zinc was reduced to 0.75 mg/kg, H. polygyrus survival was enhanced in both the primary and the challenge infection protocols. From these studies, we conclude that dietary zinc deficiency can improve the survival of intestinal nematode parasites in animal models under controlled experimental conditions but that the effects are dependent on both the severity of the deficiency and the nature of the infection.

It is important to stress that the ultimate indicators of the impact of zinc status on host immune function are the intensity, duration and severity of parasitic infection and not the number or function of immune cells from uninfected animals or humans. Helminth infections elicit strong and diversified immune responses, which are now thought to include many redundancies. If our goal is to understand the effect of zinc deficiency (or other nutritional deficiencies) on functional immunity against parasitic infections, our experimental model must stimulate the full spectrum of immune responses, so we can determine whether the impact of the zinc deficiency exceeds the host’s capacity for a multidimensional response. Otherwise, we may observe many specific immunological defects during zinc deficiency yet never understand whether they make a difference in the host’s ultimate ability to resist or control infection.

	Experimental model: H. polygyrus in mice

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
To understand how zinc deficiency promotes parasite survival, we used the H. polygyrus mouse model (Shi et al. 1994 , 1995 , 1997 , 1998a ). Zinc-sufficient mice were fed a 60 mg zinc/kg diet, and zinc-deficient mice were fed a 0.75 mg zinc/kg diet. We controlled for the reduced food intake that normally accompanies dietary zinc deficiency (Hambidge et al. 1986 ) by using a pair-fed group of mice fed 80% of the intake of the zinc-sufficient mice. This additional control group not only serves to highlight those effects that are directly attributable to zinc deficiency but also provides insight into the importance of energy restriction, per se, on the host–parasite interaction and on the host immune responses.

H. polygyrus–infective larvae are ingested by the mouse, and they penetrate into the submucosal musculature of the small intestine (for a general review of the biology of this parasite, see Scott and Tanguay, 1994 ). If the mouse has never been previously infected with this parasite, the resulting fourth stage larvae reside in the musculature for ~1 wk, and they then migrate back into the intestinal lumen, where they mature as adult male and female worms that live for several months in most strains of mouse. This type of infection is considered a "primary" infection, because the host "sees" the parasite for the first time. The fourth stage larvae are highly antigenic, and they stimulate the host to initiate an immune response. However, before this response becomes fully effective, the larvae have moved into the lumen of the intestine, where they mature into adult worms that release an immunosuppressive factor that protects them against host immune responses (Monroy et al. 1989 ). Thus, in the race between parasite development and host immunity, the parasite wins in a primary infection and is able to establish a long, chronic infection. The situation changes dramatically, however, when the parasite enters a previously infected host that has been treated with drugs to remove the first infection. The host is able to mount a more rapid, specific Th2 response that attacks the larvae while they remain in the submucosa. Larval development is delayed, and those that do migrate back into the intestinal lumen are rapidly expelled, so that within 3–4 wk, the infection is cleared (Monroy & Enriquez 1992 ). This infection protocol is referred to as a "challenge" infection. Thus, depending on whether we use a primary or challenge infection protocol, we can establish a chronic infection without protective immunity or a more acute infection with strong, effective immune response.

Figure 2 represents the course of immunological events known to occur during H. polygyrus infection in well-nourished mice. During the first 6–12 h after primary infection, Th2 cytokine mRNA is detected in Peyer’s patches, leading to the activation of Th cells and differentiation into the Th2 phenotype (Svetic et al. 1993 ). Within 1–2 d, Th2 cytokine mRNA is detectable in the MLN, and the respective cytokines are detectable in the MLN at 2–4 d postinfection. Peripheral responses have first been reported at 6–8 d postinfection, as evidenced by elevated levels of antibodies (Slater and Keymer 1988 ). These responses persist during the primary infection, although the levels never increase to the levels reached after a challenge infection. In the challenge infection, the exact timing of initial mRNA or cytokine production in the intestinal cells is unknown, but mastocytosis in the intestinal epithelium occurs very rapidly (Dehlawi and Wakelin 1988 ). As noted earlier, there is considerable evidence to support the hypothesis of redundancy in this immune response. Urban et al. (1991) demonstrated that the host remains able to control H. polygyrus infections if IL-5 activity is blocked by neutralizing antibodies and in the absence of blood eosinophilia. In addition, host protection against H. polygyrus does not appear to require IgG1 responses (Katona et al. 1991 ); that is, blockage of a single effector (i.e., blood eosinophils or IgG₁) or cytokine (i.e., IL-5) does not prevent the ability of the host to resist nematode parasites. It is clear, however, that IL-4 and IgE must be present for an effective immune response against H. polygyrus (Finkelman et al. 1997 , Urban et al. 1991 ).

View larger version (25K): [in this window] [in a new window]   Figure 2. Schematic representation of the cascade of immunological events known to occur in response to an H. polygyrus infection. APC, antigen presenting cells; MHC, major histocompatibility complex; MMC, mucosal mast cells; Th, T helper cells.

	Effect of zinc deficiency on immune response to H. polygyrus

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

View larger version (26K): [in this window] [in a new window]   Figure 3. Both zinc deficiency (Zn-) and energy restriction (ER) promote development of Heligmosomoides polygyrus from larvae to adults. Panel A: Percentage of adult worms 9 d after a primary infection. Panel B: Total number of worms (larvae and adults) 9 d after a primary infection. Within each graph, histograms with different letters are significantly different (P < 0.05). (Reprinted with permission from Shi et al. 1995 ).

View larger version (29K): [in this window] [in a new window]   Figure 4. Both zinc deficiency (Zn-) and energy restriction (ER) prolong survival of H. polygyrus during a primary infection (A) and during a challenge infection (B). Within each graph, histograms with different letters are significantly different (P < 0.05). (Reprinted with permission from Shi et al. 1995 .)

View larger version (30K): [in this window] [in a new window]   Figure 5. Serum concentrations of IgE (A, B) and parasite-specific IgG1 (C, D) and eosinophilia (E, F) are modified by zinc deficiency (Zn-) or energy restriction (ER), or both, during a primary infection (A, C, E) and during a challenge infection (B, D, F). Different letters represent significant differences throughout the infection (A, B, C, D) or at specific time-points (E, F) (P < 0.05). (Reprinted from Shi et al. 1997 .)

View larger version (24K): [in this window] [in a new window]   Figure 6. Zinc deficiency (Zn-) and energy restriction (ER) impair ability of antigen-presenting cells (APC) (A) and T cells (B) to stimulate in vitro proliferation of splenic T cells. Within each graph, histograms with different letters are significantly different (P < 0.05). (Reprinted from Shi et al. 1998a .)

View larger version (30K): [in this window] [in a new window]   Figure 7. Zinc deficiency (Zn-) or energy restriction (ER), or both, impairs function of antigen-presenting cells (APC) (A, C, E) and T cells (B, D, F), as measured by in vitro production of interleukin (IL)-4 (A, B), IL-5 (C, D) and interferon (IFN)- (E, F) by spleen cells. Within each graph, histograms with different letters are significantly different (P < 0.05). (Reprinted with permission from Shi et al. 1998a .)

From these experiments, a series of important observations arises. First, the clearest, direct effect of zinc deficiency is that it interferes with the ability of T cells to produce IL-4. IL-4 is the most important, pivotal cytokine necessary to drive an effective Th2 response against H. polygyrus (Urban et al. 1991 ). Without sufficient IL-4, production of IgE and IgG₁ is impaired, mucosal mast cell proliferation and degranulation are prevented and IL-5 production is reduced. We noted that zinc deficiency, possibly together with energy restriction, also reduced levels of IL-5. It is possible that this effect is secondary to the impaired IL-4 production seen in zinc-deficient T cells, in which case we suggest that energy restriction exerts an additional, direct effect on the ability of T cells to produce IL-5.

Second, energy restriction has important immunosuppressive effects in this system. Well-designed experiments on zinc deficiency should include controls for the concurrent reduced food intake, as a confirmation that any observed effects are due to the lower zinc intake, not the reduced energy intake. We have shown clear effects of the reduced energy on APC function, on the ability of T cells to secrete IFN- and, most importantly, on the actual rate of clearance of infection. Given how common energy restriction is, with or without zinc deficiency, we believe that this result has potentially important implications that need to be pursued.

Third, our work raises interesting questions about how the phenotype of the immune response (Th1 or Th2) alters the consequences of zinc deficiency and the interpretation of the data. Although it has been implied, if not directly suggested, that zinc deficiency impairs Th1 responses but not Th2 responses (Sprietsma 1997 ), our data very clearly demonstrate that the response to infections that normally stimulate a Th2 response is diminished by zinc deficiency. Zinc-deficient mice showed reduced proliferation of spleen cells, their ability to produce IL-4 was markedly impaired and their levels of IgE, IgG₁ and eosinophils were reduced. Clearly, zinc deficiency also influences Th2 responsiveness. This is of particular importance in areas in which intestinal helminths are common and coincident with dietary zinc deficiencies and other parasites.

Fourth, impaired IFN- production was observed as a result of energy restriction on both APC and T cell function, and not because of zinc deficiency. We suggest that some of the effects on Th1 responses attributed to zinc deficiency may in fact be caused by the concurrent energy restriction.

	Suggestions for future research

TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 
As mentioned previously, there is a remarkable paucity of information about the specific effects of zinc deficiency on the GALT. We know that the parasite lives in the intestine, that the intestine is an important immunological organ and that cells primed in the GALT migrate through the mesenteric lymph nodes to the systemic circulation and then home back to the intestine. Our data clearly indicate that zinc deficiency exerts dramatic effects on the systemic response (impaired IgE and IgG₁ antibody production and eosinophila, function of splenic T cells and APC) and that worms are better able to survive in zinc-deficient mice, which suggests that the systemic immunosuppression caused by zinc deficiency translates back to the GALT. If anything, we expect the effects of zinc deficiency to be even more dramatic at the local intestinal level.

We suggest that studies be conducted to investigate, in more depth, mechanisms by which zinc deficiency impairs immunity (Fig. 8 ). Considering the gut as an inductive site, we suggest that zinc deficiency may impair antigen presentation and production of cytokines, as it does in the spleen. We suggest that it may impair the priming of Th2 cells and that its effects on gross epithelial structure and function (Moran and Lewis 1985 ) may also impair the more nonspecific barrier function of the intestine. Changes in epithelial barrier integrity, mucus secretion and intestinal smooth muscle contractility may facilitate early worm migration and interfere with mechanical processes involved in parasite expulsion from the intestine. Impaired DNA synthesis for cell growth and replication may result in reduced renewal of damaged epithelial cells and diminished clonal expansion and proliferation of T cells. In addition, zinc is a cofactor for protein kinase C, which is critical for proliferation of T cells (Csermely et al. 1988 ). Zinc is known to play an important regulatory role in signal transduction pathways (MacDonald 2000 ) that influence gene expression and in turn direct cell function. This may be partially responsible for the observed disruptions in T cell and APC function. Impaired selective trafficking of immune cells may lead to reduced recruitment of immune progenitors or memory cells from the periphery into the GALT, lower circulation of activated lymphocytes bearing parasite antigens to systemic lymphoid organs and decreased localization of gut-derived lymphocytes to the lamina propria and intraepithelial spaces where effector cells mediate their antiparasitic activities. Finally, zinc deficiency may prevent the degranulation of eosinophils and mast cells, thus blocking their effector function. Any combination of these mechanisms may explain the lack of systemic responsiveness and increased survival of H. polygyrus observed during zinc deficiency and energy restriction.

View larger version (39K): [in this window] [in a new window]   Figure 8. Schematic representation of the associations between zinc deficiency (Zn-) and energy restriction (ER) on local and systemic immuneresponses. APC, antigen-presenting cells; MMC, mucosal mast cells; Th, T helper cells.

	ACKNOWLEDGMENTS

 
Much of the work reported in this review was performed by graduate students in our laboratory (T. Minkus, M. Boulay, H. N. Shi and R. Ing), and we appreciate their many contributions to the development of our understanding of this complex area.

	FOOTNOTES

 
¹ Presented at the international workshop "Zinc and Health: Current Status and Future Directions," held at the National Institutes of Health in Bethesda, MD, on November 4–5, 1998. This workshop was organized by the Office of Dietary Supplements, NIH and cosponsored with the American Dietetic Association, the American Society for Clinical Nutrition, the Centers for Disease Control and Prevention, Department of Defense, Food and Drug Administration/Center for Food Safety and Applied Nutrition and seven Institutes, Centers and Offices of the NIH (Fogarty International Center, National Institute on Aging, National Institute of Dental and Craniofacial Research, National Institute of Diabetes and Digestive and Kidney Diseases, National Institute on Drug Abuse, National Institute of General Medical Sciences and the Office of Research on Women’s Health). Published as a supplement to The Journal of Nutrition. Guest editors for this publication were Michael Hambidge, University of Colorado Health Sciences Center, Denver; Robert Cousins, University of Florida, Gainesville; Rebecca Costello, Office of Dietary Supplements, NIH, Bethesda, MD; and session chair, Craig McClain, University of Kentucky, Lexington.

² Supported by grants from the Natural Sciences and Engineering Research Council of Canada and by the Fonds FCAR pour l’aide et le soutien à la recherche.

⁴ Abbreviations used: APC, antigen-presenting cell; ER, energy restriction; GI, gastrointestinal; GALT, gut-associated lymphoid tissues; IFN, interferon; IL, interleukin; MLN, mesenteric lymph nodes; MMC, mucosal mast cells; Th, T helper cell.

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TOP ABSTRACT INTRODUCTION GALT in intestinal infections Evidence that zinc deficiency... Experimental model: H. polygyrus... Effect of zinc deficiency... Suggestions for future research REFERENCES

 

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