BACKGROUND
listeria monocytogenes  is a gram-positive bacterial rod which may cause severe infections, particularly in immunocompromised patients as well as in fetuses, neonates, and the elderly  <cit> .
congenital infection is the most severe form, usually presenting as granulomatosis infantiseptica, a generalized disease involving the central nervous system , the gastrointestinal tract, the respiratory system, the lymphatic system, and the kidney.
mortality from neonatal listeriosis is high and virtually 100% if left untreated  <cit> .
l. monocytogenes was recognized as a foodborne pathogen since the 1980s when several outbreaks of listeriosis were identified in europe and north america  <cit> .
selenium  is of fundamental importance to human health.
it is an essential component of several major metabolic pathways, including thyroid hormone metabolism and antioxidant enzyme defense systems.
se is incorporated as selenocysteine at the active site of a wide range of selenoproteins  <cit> .
under physiological conditions, the se in selenocysteine is an extremely efficient biological catalyst.
among the selenoproteins with identified biological functions are the antioxidant enzymes glutathione peroxidase  and thioredoxin reductase .
dietary se is essential for a healthy immune system  <cit>  and se influences both the innate and the adaptive immune responses  <cit> .
the effects of se deficiency include reduced t-cell numbers and impaired lymphocyte proliferation and function  <cit> .
innate immunity that includes nk cells, nkt cells, dendritic cells, and neutrophils are all critical in controlling the early primary infection  <cit> .
inflammatory cytokines produced by innate immune cells including tnf-α, ifn-γ, il- <dig> and il- <dig> are also important for curtailment of infection during the first week before onset of adaptive immunity  <cit> .
the mechanisms of host response to infections under conditions of selenium deficiency need to be understood to enable better management of epidemic outbreaks and to reduce the risk of this disease.
in particular, very little is known about the specific host-pathogen interactions of intracellular bacteria with a se-deficient host.
in this study, se-deficient c57bl/ <dig> mice become susceptible to infection of l. monocytogenes due to impaired systemic innate immunity that is plausibly triggered by altered immune responsiveness to l. monocytogenes under the se-deficient environment.
RESULTS
se-deficient mice had lower amounts of selenium and gpx
se deficiency was performed by feeding  <dig> days pregnant female mice with se deficient chow.
measurements of se levels and of gpx activity in the group se deficient mothers confirmed the deficiency , while control mothers and control offspring had mean levels of  <dig> ng and  <dig> ng se/ml,  <dig>  u and  <dig>  u gpx/ml, in se deficient mothers and offspring the values were  <dig> ng and  <dig> ng se/ml,  <dig>  u and  <dig>  u gpx/ml , respectively.
se-deficient diets administered during mating, pregnancy and lactation led to a reduction of  <dig> -and  <dig> -times, respectively of se and gpx in the mothers.
the blood of all mice was collected and then the plasma was separated to measure the concentration of se, mda, activity of sod, gsh-px and cat.
there were extremely significant difference between the two groups  in se level and the activity of sod, gsh-px and cat; but significant difference  between the two groups in the concentration of mda .
se-deficient mice exhibit increased bacterial burden in organs
we performed oral inoculation of se-adequate and se-deficient c57bl/ <dig> mice with l. monocytogenes.
after infection via the oral route, and followed bacterial invasion in the small intestine , translocation to mesenteric lymph nodes  and spread to the liver , spleen  and brain  of c57bl/ <dig> mice over a  <dig> h period.
we observed, at  <dig> h post infection of  <dig> ×  <dig> cfu bacteria, a difference in the bacterial persistence of the se-deficient mice in the intestine and mesenteric lymph nodes compared to the se-adequate mice .
at  <dig> to  <dig> h post infection, multiplication of bacteria in the intestine, lymph node and the liver of se-adequate mice were impaired .
bacterial counts of the se-adequate and se-deficient mice were similar in the spleen at  <dig> h post infection .
liver and brain of the se-deficient mice were infected consistently at  <dig> to  <dig> h post infection compared to se-adequate .
se-deficient mice exhibit a decreased number of innate immune cell types in spleen
to address the mechanism responsible for the decrease of host resistance to l. monocytogenes infection under the conditions of se deficiency, we evaluated the phenotype and numbers of the various immune cell types in the spleen .
four groups of mice were included in the study: group i, non-infected se-adequate; group ii, non-infected se-deficient; group iii, infected se-adequate; and group iv, infected se-deficient.
most striking was the significant increase in nk <dig> + and dx5+ cells  in infected se-adequate mice , indicative of a rapid onset of nk cell response to infection which was lacking in infected se-deficient mice  .
both subsets of dx5+cd94+  and dx5+ ly49d+  were lower in infected se-deficient mice  .
l. monocytogenes infection of se-deficient mice  induced decreased percentages of many innate immune cell types defined by the expression of f <dig> , mac- <dig>  cd11c , and gr- <dig>  .
in comparison to non-infected se-adequate  mice, group ii mice in the absence of any infection exhibited significantly reduced percentages of cd4+ and cd8+ t cells  .
the overall numbers  of nk cells and their subsets in the spleen were also markedly decreased in infected se-deficient  mice, reiterating the lack of effective nk response against l. monocytogenes infection under the conditions of se deficiency.
the number of dx5+ly49+, dx5+cd94+ and dx5+ cells in the spleen was significantly decreased in infected se-deficient  mice in comparison to infected se-adequate   .
infected se-deficient  mice also showed significant reductions in the percentages of dendritic cells, neutrophils, and tcr-γδ+ t cells as evident from the decreased splenic expression of cd11c, gr- <dig>  and tcr-γδ, respectively .
in total, the overall numbers of these innate immune cell subsets also supported their decrease in the spleen.
infected se-deficient  mice exhibited lower percentages of mac-1+ cells .
thus, whether macrophage numbers are modulated in infected se-deficient mice  is unclear.
the percentages of t and b cells defined by the expression of cd <dig> , cd <dig> , cd <dig> , and b <dig> were also lower in infected se-deficient  mice in comparison to non-infected se-adequate  .
however, as t cells showed a constitutive reduction in se-deficient mice even in the absence of infection, it was deduced that l. monocytogenes did not cause any further reduction of adaptive immune cell types.
furthermore, effects of l. monocytogenes infection were studied at  <dig> hour post infection, an early time point when adaptive immunity may be expected to play little role.
in summary, it appeared that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was severely compromised under the conditions of se deficiency.
se-deficient mice exhibit reduced splenic nk cytotoxicity
nk cells are the first line of defense against many infections, and are capable of mounting a cytotoxic response against infected target cells.
se-adequate and se-deficient mice were challenged with l. monocytogenes, and the ability of splenic effectors to kill nk sensitive targets was evaluated at  <dig> hour post infection.
splenic effectors from infected se-adequate mice killed yac- <dig> targets effectively, and exhibited negligible killing on nk-insensitive p <dig> target cells .
in contrast, splenic effectors from infected se-deficient mice exhibited weak killing of yac- <dig> target cells .
in total, the level of nk cytolytic activity on  <dig> hour of l. monocytogenes infection in infected se-deficient mice was significantly lower than that of infected se-adequate mice.
so the difference in levels of killing could be due to reduced nk numbers in the spleens or a lower cytotoxic ability of the nk cells.
levels of cytokine in the serum samples of se-deficient mice
in comparison to non-infected se-adequate mice, l. monocytogenes infection evoked increased production of serum il- <dig> at  <dig> h post infection in infected se-adequate mice.
however, this increase in serum il- <dig> did not occur in infected se-deficient mice  .
in contrast, serum il- <dig> levels were elevated in infected se-deficient mice  in comparison to infected se-adequate  or se-adequate controls   .
thus, l. monocytogenes infected se-deficient mice exhibited a skewed peripheral inflammatory response characterized by overt production of il- <dig> and decreased production of il- <dig>  il- <dig> and il- <dig> levels demonstrated insignificant differences between se-adequate and se-deficient mice .
in contrast, serum levels of il- <dig>  il- <dig>  ifn-γ, il- <dig> and il-1β were similar in both infected se-adequate and se-deficient mice at  <dig> h post infection, however tnf was not detected in the serum of any of the groups .
cytokine expression in the livers of se-deficient mice could be modulated by l. monocytogenes infection
as l. monocytogenes infection was causing a dramatic increase in the number of bacteria in the organs of se-deficient mice, we sought to ascertain the relative levels of inflammatory  and anti-inflammatory cytokines  in the livers.
this was achieved by performing quantitative rt-pcr on samples from se-deficient and se-adequate mice and normalizing values to expression levels of β-actin.
livers from l. monocytogenes infected se-adequate mice showed a significant increase in the expression of cytokines il- <dig>  tnf-α, il- <dig>  and il- <dig> relative to expression levels in the liver of se-deficient mice  .
ifn-γ expression levels also showed an increased trend in the livers of se-adequate mice .
in contrast, the expression levels of il-12p <dig> and tgf-β were similar in liver of the two groups.
discussion and
CONCLUSIONS
dietary selenium  is an essential micronutrient that affects various aspects of human health, including optimal immune responses.
however, in some regions with lower soil selenium contents, the increasing number of some diseases in humans was observed in association with selenium deficiency  <cit> .
se deficiency is thought to contribute to disease by making the body more susceptible to nutritional and biochemical stresses as well as infectious diseases.
selenium deficiency cause directly or indirectly keshan disease, kashin-beck disease, myxedematous endemic cretinism, and influenza  <cit> .
in this study, we measured selenium levels, cytokine expression levels, antioxidant enzyme activity, and bacterial burden in organs after se deficient c57bl/ <dig> mice were infected with l. monocytogenes.
we found the decrease of c57bl/ <dig> mice resistance to infection is associated with the reduced antioxidant enzyme activity and decreased innate immune responses.
the fundamental importance of se to optimal immune function has been widely demonstrated.
an adequate se intake is essential for an appropriate immune response to various infectious diseases.
se supplementation results in more rapid poliovirus clearance  <cit>  as well as lowered hospitalization rates among human immunodeficiency virus -infected patients  <cit> .
se deficiency is associated with keshan disease, a disease correlated with coxsackievirus infection of the heart muscle  <cit> .
in our present study, we observed a difference in the bacterial persistence of the se-deficient mice in the intestine and mesenteric lymph nodes compared to the se-adequate mice at  <dig> to  <dig> h post infection of  <dig> ×  <dig> cfu bacteria.
in addition, liver and brain of the se-deficient mice were infected consistently at  <dig> to  <dig> h post infection compared to se-adequate.
these results suggest that se deficiency is implicated probably in playing roles both in the crossing of the intestinal barrier and in the early or later stages of the l. monocytogenes infection process.
comparative analysis was conducted between the se-deficient and se-adequate mice following the infection of l. monocytogenes .
overall results showed that the activity of antioxidant  in se-adequate mice was significantly higher than that in se-deficient mice following the infection of l. monocytogenes.
it is suggested that the relativity might exist between se deficiency and infection of l. monocytogenes.
ribeiro et al.
<cit>  suggested a decrease in gsh levels in host might render them susceptible to schistosomula.
it may explain the susceptibility of host to infection of l. monocytogenes due to partly a decrease in gsh levels of se deficient mice in this study.
in general, the roles of selenoenzymes, gpx, in the intracellular and extracellular antioxidant defense systems have been well characterized.
gpx isoenzymes use gsh as a donor of reducing equivalents to detoxify h2o <dig>  peroxi-derivatives of various organic compounds   <cit> , and peroxynitrite  <cit> .
thus, gpx is essential for maintenance of the cellular redox status and for the regulation of the redox-sensitive signaling and transcription mechanisms.
notably, although se deficiency has been shown to suppress gpx expression and decrease the corresponding enzymatic activities, a similar phenomenon was also observed in this study.
selenium deficiency could lead to enhanced lipid peroxidation through loss of selenium-dependent glutathione peroxidase activity  <cit> .
therefore, effects of se deficiency on immune function may not be solely explained by the change in gsh levels.
in order to evaluate the effect of se on immune cell types, another experiment was conducted to analyze the phenotype and numbers of the various immune cell types in the spleen of l. monocytogenes infection under the conditions of se deficiency.
most striking was the significant increase in nk <dig> + and dx5+ cells, indicative of a rapid onset of nk cell response to infection which was lacking in infected se-deficient mice.
both subsets of dx5+cd94+and dx5+ly49d+ were also relatively lower in infected se-deficient mice.
l. monocytogenes infection of se-adequate mice induced increased percentages of many innate immune cell types defined by the expression of f <dig>  mac- <dig>  cd11c, and gr- <dig>  compared to infected se-deficient mice.
in comparison to se-adequate mice, se-deficient mice in the absence of any infection exhibited significantly reduced percentages of cd4+ and cd8+ t cells, whereas differences in other cell types were insignificant.
the overall numbers of nk cells and their subsets in the spleen was also markedly decreased in infected se-deficient mice, reiterating the lack of effective nk response against l. monocytogenes infection under the conditions of se deficiency.
in total, the overall numbers of these innate immune cell subsets also supported their decrease in the spleen.
the percentages of t and b cells defined by the expression of cd <dig>  cd <dig>  cd <dig>  and b <dig> were also lower in infected se-deficient mice in comparison to non-infected se-adequate mice.
however, as t cells showed a constitutive reduction in se-deficient mice even in the absence of infection, it was deduced that l. monocytogenes did not cause any further reduction of adaptive immune cell types.
furthermore, effects of l. monocytogenes infection were studied at  <dig> hour post infection, an early time point when adaptive immunity may be expected to play little role.
in summary, it appeared that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was to certain extent compromised under the conditions of se deficiency.
it has been proved that se is involved in regulating oxidative stress, redox, and other crucial cellular processes in nearly all tissues and cell types, including those involved in innate and adaptive immune responses  <cit> .
a number of studies in agricultural animals have provided insight into the effects of se deficiency or se-supplementation on immune responses  <cit> .
in most cases, these studies have demonstrated an enhancement of both cell-mediated and humoral immune responses by increasing levels of se intake.
in experimental animal studies, se-deficiency has been shown to result in less robust immune responses to viruses, tumors, and allergens, compared to se-adequate controls  <cit> .
however, the results are less clear regarding the benefits of se-supplementation above adequate levels in conferring additional immunological protection.
limited data from studies in humans suggest that se-supplementation may enhance immunity, including both humoral and cell-mediated responses  <cit> .
however, it remains unclear whether all types of immune responses are reduced by decreasing se intake.
se may influence cytokine production of innate immune.
we measured cytokine levels in the serum of each group mice.
il- <dig> production is important for influencing the function of many cell types including dendritic cells, nk cells, and t cells.
in comparison to non-infected se-adequate mice, l. monocytogenes infection evoked increased production of serum il- <dig> at  <dig> h post infection in infected se-adequate mice.
however, this increase in serum il- <dig> did not occur in infected se-deficient mice.
in contrast, serum il- <dig> levels were elevated in infected se-deficient mice in comparison to infected se-adequate or se-adequate controls.
l. monocytogenes infected se-deficient mice exhibited a skewed peripheral inflammatory response characterized by overt production of il- <dig> and decreased production of il- <dig>  il- <dig> and il- <dig> levels demonstrated insignificant differences between se-adequate and se-deficient mice.
levels of il- <dig> and il- <dig> in se-deficient mice were similar compared to se-adequate mice at  <dig> h post infection.
serum levels of ifn-γ, il- <dig> and il-1β were shown also similar in both infected se-adequate and se-deficient mice at  <dig> h post infection, however tnf was not detected in the serum of any of the groups.
a study in mice involving se-deficiency and the gastrointestinal parasite, heligmosomoides polygyrus, showed that low dietary se resulted in decreased resistance to the nematodes upon secondary infection  <cit> .
the lowered resistance did not appear to involve il- <dig>  as se-deficiency had no effect on circulating levels of this key cytokine involved in host protection.
maintenance of non-infected se-deficient mice is associated with liver secretion of anti-inflammatory cytokines il- <dig>  <cit>  and tgf-β.
as l. monocytogenes infection was causing a dramatic increase in the number of bacteria in the organs of se-deficient mice, we measured the relative levels of inflammatory  and anti-inflammatory cytokines  in the livers by using quantitative rt-pcr on samples from each se-deficient mice and normalizing values to expression levels of β-actin.
livers from infected se-deficient mice showed a significant increase in the expression of cytokines il- <dig>  tnf-α, il- <dig>  and il- <dig> relative to expression levels in the liver of non-infected se-deficient mice.
ifn-γ expression levels also showed an increased trend in livers of infected se-deficient mice.
in contrast, the expression levels of il-12p <dig> and tgf-β were similar in liver of the two groups.
liver cytokine expression also suggested overt production of il- <dig> in se-adequate hosts.
infection with l. monocytogenes induces a strong t-helper type  <dig>  response, resulting in expansion of l. monocytogenes specific cd8+ t cells, which play a significant role in bacterial clearance.
however, the se-deficient mice had a more t helper type  <dig> -like pattern of cytokine expression.
il- <dig> stimulates t-cell activation and expansion  <cit> , especially the development of th <dig> cells by stabilizing the accessibility of the il- <dig> gene  <cit> .
in vivo, il- <dig> neutralization inhibits il- <dig> production  <cit> .
gsh levels in the immune cells may play a pivotal role in determining the th1/th <dig> balance.
gsh depletion in antigen presenting cells  shifts the immune response toward a th <dig> response both in vitro and in vivo  <cit> .
the intracellular gsh levels have also been correlated with the th <dig> cytokine production versus the th <dig> cytokine production by macrophages and cd4+t cells  <cit> .
the effect of gsh levels on th1/th <dig> balance is partly mediated by il- <dig> and il- <dig>  <cit> .
our data support these studies by showing a higher level of il- <dig> expression in se-adequate mice.
although a difference in il- <dig> expression between se-adequate and se-deficient mice was not observed in our study, the diminished th <dig> response in se-deficient mice may be explained by the changes in production of other chemokines and cytokines by apcs and macrophages, such as il- <dig>
while higher levels of se have generally been shown to provide protection against viruses, the relationship between host se status and immunity to bacterial pathogens is not so one-sided  <cit> .
most of the available data suggest the influence of se status on resistance to infections with bacteria, parasites and fungi, indicates different results.
for example, an early study involving candida albicans infection in mice showed that se-deficiency resulted in higher susceptibility to this pathogen  <cit> .
the authors demonstrated an impaired ability of se-deficient, glycogenelicited peritoneal neutrophils to kill c. albicans in vitro, which was repeated in other experiments and shown to be due to reduced superoxide production, oxygen consumption and glucose utilization  <cit> .
in contrast, killing of salmonella typhimurium and staphylococcus aureus by neutrophils was unaffected by se-deficiency  <cit> .
se-deficient rats exhibited increased mortality after i.p.
injection of s. aureus, although injection of lower doses of organisms did not result in mortality differences when comparing se-deficient and -adequate rats.
in contrast, se-deficiency was shown to increase survival of rats infected with pathogens such as s. typhimurium, plasmodium bergeii, or l. monocytogenes  <cit> .
se-deficient mice infected with trypanosoma musculi, a protozoan parasite specific to mouse, cleared the pathogen whereas mice fed se-adequate or -supplemented chow exhibited sustained parasitemia  <cit> .
a possible explanation was offered by the observation that trypanosomes lack sufficient capacity to reduce oxidative stress because the parasites do not have the classical glutathione peroxidase /gr enzymatic system  <cit> , thus making them reliant on se-sufficient host antioxidants for survival.
moreover, studies in mice infected with t. cruzi demonstrated that se-deficiency in the murine host increased the severity of t. cruzi-induced chronic inflammatory myopathy  <cit> .
in our study, despite the increasing burden in intestine, liver and brain in infected se-deficient mice, mice were not observed dead throughout the experiments.
overall, it is explained that the increased the number of bacteria in tissues found in this study were due to inadequate immune responses or increased oxidative stress and inflammation in these tissues.
in summary, se-deficient mice had an altered innate immune response to l. monocytogenes infection.
our results suggest that the outcome of l. monocytogenes infection of se-deficient mice is associated with se status of host.
further studies will focus on mechanistic studies that involve specific selenoproteins and the roles they play during inflammation and immune responses not only because many selenoproteins play critical roles in reducing oxidative stress and balancing redox in a wide variety of tissues and cell types, including those involved in innate and adaptive immune responses but also because the functions for many selenoproteins remain unknown.

 listeria monocytogenes is a gram-positive bacterial rod which may cause severe infections, particularly in immunocompromised patients as well as in fetuses, neonates, and the elderly. congenital infection is the most severe form, usually presenting as granulomatosis infantiseptica, a generalized disease involving the central nervous system, the gastrointestinal tract, the respiratory system, the lymphatic system, and the kidney. mortality from neonatal listeriosis is high and virtually 100% if left untreated. l. monocytogenes was recognized as a foodborne pathogen since the 1980s when several outbreaks of listeriosis were identified in europe and north america. selenium is of fundamental importance to human health. it is an essential component of several major metabolic pathways, including thyroid hormone metabolism and antioxidant enzyme defense systems. se is incorporated as selenocysteine at the active site of a wide range of selenoproteins. under physiological conditions, the se in selenocysteine is an extremely efficient biological catalyst. among the selenoproteins with identified biological functions are the antioxidant enzymes glutathione peroxidase and thioredoxin reductase. dietary se is essential for a healthy immune system and se influences both the innate and the adaptive immune responses. the effects of se deficiency include reduced t-cell numbers and impaired lymphocyte proliferation and function. innate immunity that includes nk cells, nkt cells, dendritic cells, and neutrophils are all critical in controlling the early primary infection. inflammatory cytokines produced by innate immune cells including tnf-α, ifn-γ, il- and il- are also important for curtailment of infection during the first week before onset of adaptive immunity. the mechanisms of host response to infections under conditions of selenium deficiency need to be understood to enable better management of epidemic outbreaks and to reduce the risk of this disease. in particular, very little is known about the specific host-pathogen interactions of intracellular bacteria with a se-deficient host. in this study, se-deficient c57bl/ mice become susceptible to infection of l. monocytogenes due to impaired systemic innate immunity that is plausibly triggered by altered immune responsiveness to l. monocytogenes under the se-deficient environment.  se-deficient mice had lower amounts of selenium and gpx se deficiency was performed by feeding days pregnant female mice with se deficient chow. measurements of se levels and of gpx activity in the group se deficient mothers confirmed the deficiency, while control mothers and control offspring had mean levels of ng and ng se/ml, u and u gpx/ml, in se deficient mothers and offspring the values were ng and ng se/ml, u and u gpx/ml, respectively. se-deficient diets administered during mating, pregnancy and lactation led to a reduction of -and -times, respectively of se and gpx in the mothers. the blood of all mice was collected and then the plasma was separated to measure the concentration of se, mda, activity of sod, gsh-px and cat. there were extremely significant difference between the two groups in se level and the activity of sod, gsh-px and cat; but significant difference between the two groups in the concentration of mda. se-deficient mice exhibit increased bacterial burden in organs we performed oral inoculation of se-adequate and se-deficient c57bl/ mice with l. monocytogenes. after infection via the oral route, and followed bacterial invasion in the small intestine, translocation to mesenteric lymph nodes and spread to the liver, spleen and brain of c57bl/ mice over a h period. we observed, at h post infection of × cfu bacteria, a difference in the bacterial persistence of the se-deficient mice in the intestine and mesenteric lymph nodes compared to the se-adequate mice. at to h post infection, multiplication of bacteria in the intestine, lymph node and the liver of se-adequate mice were impaired. bacterial counts of the se-adequate and se-deficient mice were similar in the spleen at h post infection. liver and brain of the se-deficient mice were infected consistently at to h post infection compared to se-adequate. se-deficient mice exhibit a decreased number of innate immune cell types in spleen to address the mechanism responsible for the decrease of host resistance to l. monocytogenes infection under the conditions of se deficiency, we evaluated the phenotype and numbers of the various immune cell types in the spleen. four groups of mice were included in the study: group i, non-infected se-adequate; group ii, non-infected se-deficient; group iii, infected se-adequate; and group iv, infected se-deficient. most striking was the significant increase in nk + and dx5+ cells in infected se-adequate mice, indicative of a rapid onset of nk cell response to infection which was lacking in infected se-deficient mice. both subsets of dx5+cd94+ and dx5+ ly49d+ were lower in infected se-deficient mice. l. monocytogenes infection of se-deficient mice induced decreased percentages of many innate immune cell types defined by the expression of f, mac- cd11c, and gr-. in comparison to non-infected se-adequate mice, group ii mice in the absence of any infection exhibited significantly reduced percentages of cd4+ and cd8+ t cells. the overall numbers of nk cells and their subsets in the spleen were also markedly decreased in infected se-deficient mice, reiterating the lack of effective nk response against l. monocytogenes infection under the conditions of se deficiency. the number of dx5+ly49+, dx5+cd94+ and dx5+ cells in the spleen was significantly decreased in infected se-deficient mice in comparison to infected se-adequate. infected se-deficient mice also showed significant reductions in the percentages of dendritic cells, neutrophils, and tcr-γδ+ t cells as evident from the decreased splenic expression of cd11c, gr- and tcr-γδ, respectively. in total, the overall numbers of these innate immune cell subsets also supported their decrease in the spleen. infected se-deficient mice exhibited lower percentages of mac-1+ cells. thus, whether macrophage numbers are modulated in infected se-deficient mice is unclear. the percentages of t and b cells defined by the expression of cd, cd, cd, and b were also lower in infected se-deficient mice in comparison to non-infected se-adequate. however, as t cells showed a constitutive reduction in se-deficient mice even in the absence of infection, it was deduced that l. monocytogenes did not cause any further reduction of adaptive immune cell types. furthermore, effects of l. monocytogenes infection were studied at hour post infection, an early time point when adaptive immunity may be expected to play little role. in summary, it appeared that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was severely compromised under the conditions of se deficiency. se-deficient mice exhibit reduced splenic nk cytotoxicity nk cells are the first line of defense against many infections, and are capable of mounting a cytotoxic response against infected target cells. se-adequate and se-deficient mice were challenged with l. monocytogenes, and the ability of splenic effectors to kill nk sensitive targets was evaluated at hour post infection. splenic effectors from infected se-adequate mice killed yac- targets effectively, and exhibited negligible killing on nk-insensitive p target cells. in contrast, splenic effectors from infected se-deficient mice exhibited weak killing of yac- target cells. in total, the level of nk cytolytic activity on hour of l. monocytogenes infection in infected se-deficient mice was significantly lower than that of infected se-adequate mice. so the difference in levels of killing could be due to reduced nk numbers in the spleens or a lower cytotoxic ability of the nk cells. levels of cytokine in the serum samples of se-deficient mice in comparison to non-infected se-adequate mice, l. monocytogenes infection evoked increased production of serum il- at h post infection in infected se-adequate mice. however, this increase in serum il- did not occur in infected se-deficient mice. in contrast, serum il- levels were elevated in infected se-deficient mice in comparison to infected se-adequate or se-adequate controls. thus, l. monocytogenes infected se-deficient mice exhibited a skewed peripheral inflammatory response characterized by overt production of il- and decreased production of il- il- and il- levels demonstrated insignificant differences between se-adequate and se-deficient mice. in contrast, serum levels of il- il- ifn-γ, il- and il-1β were similar in both infected se-adequate and se-deficient mice at h post infection, however tnf was not detected in the serum of any of the groups. cytokine expression in the livers of se-deficient mice could be modulated by l. monocytogenes infection as l. monocytogenes infection was causing a dramatic increase in the number of bacteria in the organs of se-deficient mice, we sought to ascertain the relative levels of inflammatory and anti-inflammatory cytokines in the livers. this was achieved by performing quantitative rt-pcr on samples from se-deficient and se-adequate mice and normalizing values to expression levels of β-actin. livers from l. monocytogenes infected se-adequate mice showed a significant increase in the expression of cytokines il- tnf-α, il- and il- relative to expression levels in the liver of se-deficient mice. ifn-γ expression levels also showed an increased trend in the livers of se-adequate mice. in contrast, the expression levels of il-12p and tgf-β were similar in liver of the two groups. discussion and  dietary selenium is an essential micronutrient that affects various aspects of human health, including optimal immune responses. however, in some regions with lower soil selenium contents, the increasing number of some diseases in humans was observed in association with selenium deficiency. se deficiency is thought to contribute to disease by making the body more susceptible to nutritional and biochemical stresses as well as infectious diseases. selenium deficiency cause directly or indirectly keshan disease, kashin-beck disease, myxedematous endemic cretinism, and influenza. in this study, we measured selenium levels, cytokine expression levels, antioxidant enzyme activity, and bacterial burden in organs after se deficient c57bl/ mice were infected with l. monocytogenes. we found the decrease of c57bl/ mice resistance to infection is associated with the reduced antioxidant enzyme activity and decreased innate immune responses. the fundamental importance of se to optimal immune function has been widely demonstrated. an adequate se intake is essential for an appropriate immune response to various infectious diseases. se supplementation results in more rapid poliovirus clearance as well as lowered hospitalization rates among human immunodeficiency virus -infected patients. se deficiency is associated with keshan disease, a disease correlated with coxsackievirus infection of the heart muscle. in our present study, we observed a difference in the bacterial persistence of the se-deficient mice in the intestine and mesenteric lymph nodes compared to the se-adequate mice at to h post infection of × cfu bacteria. in addition, liver and brain of the se-deficient mice were infected consistently at to h post infection compared to se-adequate. these results suggest that se deficiency is implicated probably in playing roles both in the crossing of the intestinal barrier and in the early or later stages of the l. monocytogenes infection process. comparative analysis was conducted between the se-deficient and se-adequate mice following the infection of l. monocytogenes. overall results showed that the activity of antioxidant in se-adequate mice was significantly higher than that in se-deficient mice following the infection of l. monocytogenes. it is suggested that the relativity might exist between se deficiency and infection of l. monocytogenes. ribeiro et al. suggested a decrease in gsh levels in host might render them susceptible to schistosomula. it may explain the susceptibility of host to infection of l. monocytogenes due to partly a decrease in gsh levels of se deficient mice in this study. in general, the roles of selenoenzymes, gpx, in the intracellular and extracellular antioxidant defense systems have been well characterized. gpx isoenzymes use gsh as a donor of reducing equivalents to detoxify h2o peroxi-derivatives of various organic compounds, and peroxynitrite. thus, gpx is essential for maintenance of the cellular redox status and for the regulation of the redox-sensitive signaling and transcription mechanisms. notably, although se deficiency has been shown to suppress gpx expression and decrease the corresponding enzymatic activities, a similar phenomenon was also observed in this study. selenium deficiency could lead to enhanced lipid peroxidation through loss of selenium-dependent glutathione peroxidase activity. therefore, effects of se deficiency on immune function may not be solely explained by the change in gsh levels. in order to evaluate the effect of se on immune cell types, another experiment was conducted to analyze the phenotype and numbers of the various immune cell types in the spleen of l. monocytogenes infection under the conditions of se deficiency. most striking was the significant increase in nk + and dx5+ cells, indicative of a rapid onset of nk cell response to infection which was lacking in infected se-deficient mice. both subsets of dx5+cd94+and dx5+ly49d+ were also relatively lower in infected se-deficient mice. l. monocytogenes infection of se-adequate mice induced increased percentages of many innate immune cell types defined by the expression of f mac- cd11c, and gr- compared to infected se-deficient mice. in comparison to se-adequate mice, se-deficient mice in the absence of any infection exhibited significantly reduced percentages of cd4+ and cd8+ t cells, whereas differences in other cell types were insignificant. the overall numbers of nk cells and their subsets in the spleen was also markedly decreased in infected se-deficient mice, reiterating the lack of effective nk response against l. monocytogenes infection under the conditions of se deficiency. in total, the overall numbers of these innate immune cell subsets also supported their decrease in the spleen. the percentages of t and b cells defined by the expression of cd cd cd and b were also lower in infected se-deficient mice in comparison to non-infected se-adequate mice. however, as t cells showed a constitutive reduction in se-deficient mice even in the absence of infection, it was deduced that l. monocytogenes did not cause any further reduction of adaptive immune cell types. furthermore, effects of l. monocytogenes infection were studied at hour post infection, an early time point when adaptive immunity may be expected to play little role. in summary, it appeared that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was to certain extent compromised under the conditions of se deficiency. it has been proved that se is involved in regulating oxidative stress, redox, and other crucial cellular processes in nearly all tissues and cell types, including those involved in innate and adaptive immune responses. a number of studies in agricultural animals have provided insight into the effects of se deficiency or se-supplementation on immune responses. in most cases, these studies have demonstrated an enhancement of both cell-mediated and humoral immune responses by increasing levels of se intake. in experimental animal studies, se-deficiency has been shown to result in less robust immune responses to viruses, tumors, and allergens, compared to se-adequate controls. however, the results are less clear regarding the benefits of se-supplementation above adequate levels in conferring additional immunological protection. limited data from studies in humans suggest that se-supplementation may enhance immunity, including both humoral and cell-mediated responses. however, it remains unclear whether all types of immune responses are reduced by decreasing se intake. se may influence cytokine production of innate immune. we measured cytokine levels in the serum of each group mice. il- production is important for influencing the function of many cell types including dendritic cells, nk cells, and t cells. in comparison to non-infected se-adequate mice, l. monocytogenes infection evoked increased production of serum il- at h post infection in infected se-adequate mice. however, this increase in serum il- did not occur in infected se-deficient mice. in contrast, serum il- levels were elevated in infected se-deficient mice in comparison to infected se-adequate or se-adequate controls. l. monocytogenes infected se-deficient mice exhibited a skewed peripheral inflammatory response characterized by overt production of il- and decreased production of il- il- and il- levels demonstrated insignificant differences between se-adequate and se-deficient mice. levels of il- and il- in se-deficient mice were similar compared to se-adequate mice at h post infection. serum levels of ifn-γ, il- and il-1β were shown also similar in both infected se-adequate and se-deficient mice at h post infection, however tnf was not detected in the serum of any of the groups. a study in mice involving se-deficiency and the gastrointestinal parasite, heligmosomoides polygyrus, showed that low dietary se resulted in decreased resistance to the nematodes upon secondary infection. the lowered resistance did not appear to involve il- as se-deficiency had no effect on circulating levels of this key cytokine involved in host protection. maintenance of non-infected se-deficient mice is associated with liver secretion of anti-inflammatory cytokines il- and tgf-β. as l. monocytogenes infection was causing a dramatic increase in the number of bacteria in the organs of se-deficient mice, we measured the relative levels of inflammatory and anti-inflammatory cytokines in the livers by using quantitative rt-pcr on samples from each se-deficient mice and normalizing values to expression levels of β-actin. livers from infected se-deficient mice showed a significant increase in the expression of cytokines il- tnf-α, il- and il- relative to expression levels in the liver of non-infected se-deficient mice. ifn-γ expression levels also showed an increased trend in livers of infected se-deficient mice. in contrast, the expression levels of il-12p and tgf-β were similar in liver of the two groups. liver cytokine expression also suggested overt production of il- in se-adequate hosts. infection with l. monocytogenes induces a strong t-helper type response, resulting in expansion of l. monocytogenes specific cd8+ t cells, which play a significant role in bacterial clearance. however, the se-deficient mice had a more t helper type -like pattern of cytokine expression. il- stimulates t-cell activation and expansion, especially the development of th cells by stabilizing the accessibility of the il- gene. in vivo, il- neutralization inhibits il- production. gsh levels in the immune cells may play a pivotal role in determining the th1/th balance. gsh depletion in antigen presenting cells shifts the immune response toward a th response both in vitro and in vivo. the intracellular gsh levels have also been correlated with the th cytokine production versus the th cytokine production by macrophages and cd4+t cells. the effect of gsh levels on th1/th balance is partly mediated by il- and il-. our data support these studies by showing a higher level of il- expression in se-adequate mice. although a difference in il- expression between se-adequate and se-deficient mice was not observed in our study, the diminished th response in se-deficient mice may be explained by the changes in production of other chemokines and cytokines by apcs and macrophages, such as il- while higher levels of se have generally been shown to provide protection against viruses, the relationship between host se status and immunity to bacterial pathogens is not so one-sided. most of the available data suggest the influence of se status on resistance to infections with bacteria, parasites and fungi, indicates different results. for example, an early study involving candida albicans infection in mice showed that se-deficiency resulted in higher susceptibility to this pathogen. the authors demonstrated an impaired ability of se-deficient, glycogenelicited peritoneal neutrophils to kill c. albicans in vitro, which was repeated in other experiments and shown to be due to reduced superoxide production, oxygen consumption and glucose utilization. in contrast, killing of salmonella typhimurium and staphylococcus aureus by neutrophils was unaffected by se-deficiency. se-deficient rats exhibited increased mortality after i.p. injection of s. aureus, although injection of lower doses of organisms did not result in mortality differences when comparing se-deficient and -adequate rats. in contrast, se-deficiency was shown to increase survival of rats infected with pathogens such as s. typhimurium, plasmodium bergeii, or l. monocytogenes. se-deficient mice infected with trypanosoma musculi, a protozoan parasite specific to mouse, cleared the pathogen whereas mice fed se-adequate or -supplemented chow exhibited sustained parasitemia. a possible explanation was offered by the observation that trypanosomes lack sufficient capacity to reduce oxidative stress because the parasites do not have the classical glutathione peroxidase /gr enzymatic system, thus making them reliant on se-sufficient host antioxidants for survival. moreover, studies in mice infected with t. cruzi demonstrated that se-deficiency in the murine host increased the severity of t. cruzi-induced chronic inflammatory myopathy. in our study, despite the increasing burden in intestine, liver and brain in infected se-deficient mice, mice were not observed dead throughout the experiments. overall, it is explained that the increased the number of bacteria in tissues found in this study were due to inadequate immune responses or increased oxidative stress and inflammation in these tissues. in summary, se-deficient mice had an altered innate immune response to l. monocytogenes infection. our results suggest that the outcome of l. monocytogenes infection of se-deficient mice is associated with se status of host. further studies will focus on mechanistic studies that involve specific selenoproteins and the roles they play during inflammation and immune responses not only because many selenoproteins play critical roles in reducing oxidative stress and balancing redox in a wide variety of tissues and cell types, including those involved in innate and adaptive immune responses but also because the functions for many selenoproteins remain unknown. 
se-deficient mouse models of listeriosis were used to study the innate immune response during the course of l. monocytogenes infection. the analysis of cell types of spleen from se-deficient mice revealed that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was to a certain extent compromised under conditions of se deficiency. the cytokine levels in the serum and cytokine expression levels in the livers from se-deficient mice revealed that the innate immune response of se-deficient mice was impaired throughout the course of infection. these results suggest that innate immune response is altered by se deficiency after infection with l. monocytogenes. in conclusion, induced susceptibility of host resistance is associated with an impaired innate immune response following infection with l. monocytogenes in c57bl/ se-deficient mice. the concentration of mda, sod, gpx and cat in blood has revealed that lower se level exist in se-deficient mice. blood samples from mouse models were used for se status. intestine, mesenteric lymph node, liver, spleen and brain from each mouse were to study the bacterial burden in organs.
 susceptibility or resistance to infection with listeria monocytogenes correlates with selenium deficiency in response to infection.  se-deficient mouse models of listeriosis were used to study the innate immune response during the course of l. monocytogenes infection. blood samples from mouse models were used for se status. the concentration of mda, sod, gpx and cat in blood has revealed that lower se level exist in se-deficient mice. intestine, mesenteric lymph node, liver, spleen and brain from each mouse were to study the bacterial burden in organs. the analysis of cell types of spleen from se-deficient mice revealed that the ability of the host to elicit a rapid recruitment and activation of systemic innate immune response to infection was to a certain extent compromised under conditions of se deficiency. the cytokine levels in the serum and cytokine expression levels in the livers from se-deficient mice revealed that the innate immune response of se-deficient mice was impaired throughout the course of infection. these results suggest that innate immune response is altered by se deficiency after infection with l. monocytogenes.  in conclusion, induced susceptibility of host resistance is associated with an impaired innate immune response following infection with l. monocytogenes in c57bl/ se-deficient mice. 