natural killer t ( nkt ) cells are recently discovered innate - like subset of lymphocytes expressing both nk and t cell markers . nkt cells are a phenotypically and functionally diverse subset of t cells that recognize self- and microbial lipids [ 1 , 2 ] . most nkt cells are restricted by mhc - i like molecule cd1 , which can further distributed into two major subsets : type i and type ii nkt cells ( table 1 ) . type i nkt cells are also called invariant nkt ( inkt ) , expressing exclusively limited t cell receptor  ( tcr ) and tcr receptors , that is , v14-j18 predominately paired with v8.2 , v7 , or v2 in mice and v24-j18 paired with v11 in human [ 2 , 3 ] . inkt cells secrete a wide array of cytokines and chemokines immediately upon activation through tcr engagement by glycolipids . inkt cells exert their regulatory and cytotoxic activities also through direct contact , granzyme b , or fasl - induced mechanisms [ 46 ] . hence , inkt cells exert both regulator and effector cell function and bridge the innate and adaptive immune responses . recently , inkt cells have been further classified into nkt1 , nkt2 , and nkt17 lineages based on their cytokine profiles and distinct transcription factors , t - bet , gata-3 , and rort , as conventional t helper 1 ( th1 ) , th2 , and th17 cells [ 8 , 9 ] . on the other hand , type ii nkt cells , which are also called non - inkt or variant nkt ( vnkt ) cells , express more diverse tcr and tcr receptors . there still exists a minor group of cd1 nonrestricted nkt cells , referred to as nkt - like cells [ 11 , 12 ] . a hallmark of inkt cells is their capacity to rapidly produce copious amounts of cytokines and chemokines upon tcr stimulation , which endows these cells with potent immunomodulatory activities for a wide variety of immune responses and diseases ( figure 1 ) . inkt cells exhibit potent effector functions and play critical roles in antimicrobial defense , cancer immunosurveillance , and modulation of immune - mediated disorders [ 1316 ] . as inkt cells recognize glycolipids that are enriched in microbes and parasites , they are believed to play important roles in the infections caused by these pathogens . hygiene hypothesis ,  which tries to explain the declined infections with rising autoimmune and atopic diseases in the recent two to three decades [ 17 , 18 ] . in this review , we summarize the roles of inkt cells in parasitic infections , with particular emphasis on the involvement of inkt cells in the hygiene hypothesis and underlying mechanisms . albeit being widely studied during viral and bacterial infections , the role of inkt cells during parasite infections remains largely unexplored . as helminth infections can usually induce th2-dominated immune responses and inkt cells can rapidly produce copious amounts of various cytokines including interleukin-4 ( il-4 ) , these cells might be important players in the initial steps leading to th2 responses during helminthiasis . recent reports have indicated that nkt cells are involved in the pathogenesis of several parasite infections in animal models and patients , playing , in most cases , protective or regulatory roles towards hosts . it is caused by digenetic blood trematodes , of which there are three main species : schistosoma mansoni , s. japonicum , and s. haematobium . previous studies by others and us indicated that egg deposit in the liver was a determining factor to drive th2 response in s. mansoni and s. japonicum infections in mice [ 20 , 21 ] . nkt cells may contribute to regulating the cytokine secretion profiles during infections . in mice infected with s. mansoni , inkt cells exhibited an activated phenotype . hepatic inkt cells produced both interferon- ( ifn- ) and il-4 following schistosome egg deposited in the liver . further studies revealed that s. mansoni activated both inkt and non - inkt cells in vivo . inkt cells contributed to th1 cell differentiation , whereas non - inkt cells might be mostly implicated in th2 cell differentiation in response to this parasite . luo and colleagues reported that nk and nkt cells were activated and expanded from draining mesenteric lymph node ( mln ) in mice 57  wk after infection with s. japonicum . . however , the kinetics of nkt cells and their precise roles in immunomodulation during schistosome infections remain unclear . this is also true to nematode infections , where nkt cells remains ill - defined in their pathogenesis . a pioneer work from balmer and colleagues found an expansion of nkt ( cd3nk1.1 ) as early as 24 hours following the infection with brugia pahangi . however , depletion of nk1.1-expressing cell had no effect on the th2 development during the gastrointestinal nematode trichuris muris infection . inkt cells have been reported playing crucial roles in the pathogenesis of protozoan infections . in malaria , early interactions between blood - stage plasmodium parasites and cells of the innate immune system , including innate - like nkt cells , are important in the timely control of parasite replication and in the subsequent elimination and resolution of the infection . the lipid extracts from murine malaria parasites could actually be loaded onto cd1 molecules to stimulate inkt cell by the use of artificial antigen - presenting beads . the level of protective antimalaria immunity was greatly enhanced by coadministration of -galactosylceramide ( -galcer ) with suboptimal doses of irradiated sporozoites or recombinant viruses expressing a malaria antigen in mice . inkt cells were increased in numbers and played critical roles through the secretion of ifn- in reducing liver - stage burden to a secondary infection by murine malaria plasmodium yoelii . -c - galcer displayed a superior inhibitory activity against the liver stages of the rodent malaria parasite p. yoelii compared to its parental glycolipid , -galcer . this adjuvant effect depended on nkt cell activation , which was able to boost ifn- production by nk cells and memory cd8 t cells . visceral leishmaniasis ( kala - azar ) is a deadly disease caused by the parasitic protozoa leishmania donovani . inkt cells are involved in the pathogenesis of leishmaniasis . in patients with visceral leishmaniasis , bone - marrow - derived non - inkt cells dominantly produced ifn- in response to l. donovani antigen in vitro . post - kala - azar dermal leishmaniasis is a chronic dermal complication that occurs usually after recovery from visceral leishmaniasis . there was a raised proportion of circulating nkt cells in these patients compared to health controls . karmakar and colleagues isolated a natural ligand of nkt cells , -(14)-galactose terminal glycosphingophospholipid ( gspl ) from this parasite to treat infected balb / c mice . this immunotherapy with gspl induced ifn- through the cooperative action of tlr4 and nkt cells , which contributed to the effective control of acute parasite burden in the infected animals . by use of inkt cell - deficient ( j18 ) c57bl/6 mice , another study demonstrated that inkt cells played a role in early and sustained proinflammatory cytokine response warranting efficient organization of hepatic granulomas and parasite clearance of l. donovani . nkt cell activation by -galcer during intradermal dnap36 priming was highly protective against murine cutaneous leishmaniasis , resulting in the heightened activation and development of cd4 and cd8 effector and memory t cells . conversely , activation of inkt cells exacerbated , rather than ameliorated , experimental visceral leishmaniasis by l. donovani , which was correlated with a bias towards increased il-4 production by inkt . this may illustrate the double - edged sword of nkt cell - based therapy in leishmaniasis . inkt cells protect mice against toxoplasma gondii infection . by oral infection of mildly virulent strain me49 t. gondii cysts , most cd1d - deficient c57bl/6 mice died within 2  wk of infection compared to no death in wt mice . after activation with t. gondii , nkt cells were important mediators of the immune response via a robust ifn--mediated effect that limited parasite replication and allowed for parasite clearance . nevertheless , this strong th1 response , when uncontrolled , can mediate the lethal ileitis . treatment of mice with a single injection of -galcer one day before infection activated intestinal nkt and led to a shift in cytokine secretion towards a th2 profile and a dramatic increase in treg cells in mlns , which alleviated intestinal lesions and increased survival of mice . on the other hand , inkt cells may negatively regulate the immune response against t. gondii infection possibly by producing il-4 and suppressing the induction of heat shock protein 65 . the latter is induced in host macrophages by t cells and plays an essential role in protective immunity in this infection . nkt cells are involved in the pathogenesis of some other protozoan infections , providing protection against infections in most cases . cd8 nkt cells were able to activate macrophages to kill trypanosoma congolense through the production of nitrogen oxides , whereas treg cells prevented the activation of the cd8 nkt cells . however , another report indicated that loss of inkt cells did not affect the susceptibility or resistance in cd1d c57bl/6 mice to the infections with virulent african trypanosomes , t. congolense or t. bruce . lotter and colleagues identified a lipopeptidophosphoglycan from entamoeba histolytica membranes ( ehlppg ) as a possible inkt natural ligand . ehlppg treatment , similar to -galcer application , induced protective ifn- but not il-4 production from inkt cells and significantly reduced the severity of amebic liver abscess in mice infected with e. histolytica . by the use of cd1d ko mice , it was found that inkt cells contributed to resistance against this protozoan and to the control of inflammation in the colitis induced by the infection . inkt cells play important roles in the pathogenesis of some other parasitic diseases , as well as of a wide range of microbe infections , as seen in recent nice reviews [ 10 , 46 , 47 ] . nkt cells play protective role against a wide range of parasite infections as discussed above , whereas the underlying molecular mechanisms are not fully elucidated . shifting of host 's cytokine secretion profiles may account for the protective or , in some cases , pathogenic effects of nkt cells on parasitic infections . activated inkt cells can also transactivate many other immune cells or attract these cells to the sites of infection to exert their regulatory roles . like nk cells , activated nkt cells can also mediate cytotoxic activity , possibly involving both perforin / granzyme and fas / fasl pathways [ 5 , 6 , 48 ] . parasites are enriched in lipid , which may contain natural ligands for nkt cells as discussed in the next section . therefore , it is not surprising that inkt cells participate in the pathogenesis of a range of different parasitic infections . the role of inkt cells in some parasitic infections and possible effect mechanisms are summarized in table 2 . the  hygiene hypothesis  was proposed in 1989 by strachan   to explain the dramatic increase in the prevalence of autoimmune and allergic diseases over the past two to three decades . according to this hypothesis , reduced exposure to microorganisms and parasites in childhood is the main cause for the increased incidence of both th1-mediated autoimmune diseases and th2-mediated allergic diseases . currently , the hypothesis is becoming more accepted with accumulating epidemiological and clinical evidences to support [ 5052 ] . although the exact scientific underpinnings for the hygiene hypothesis and the underlying mechanisms by which infections affect the immune system to prevent diseases have remained a puzzle over the years for both scientists and clinicians , there is increasing recognition that exposure to infectious agents evokes fundamental effects on the development and behavior of the immune system [ 18 , 53 ] . the core of this hypothesis consists in the notion that the microbial environment interfaces with the innate immune system and modulates its ability to impart instructions to adaptive immune responses , particularly when such interactions occur in utero and/or in early life . many chronic infections , both microbial and parasitic , induce forms of immune suppression or downmodulation . recent studies have indicated that infectious agents stimulate a large variety of regulatory t cells , such as th2 , treg , tr1 , and nkt cells , which secrete immunosuppressive cytokines / chemokines , such as il-10 and tgf- to alter the th1/th2 balance . infections may induce the generation of regulatory macrophages , dendritic cells , innate lymphoid cells ( ilc ) , nk , and b cells [ 5658 ] . tlr - myd88 pathway is believed to be involved in the induction of different subsets of regulatory t cells , such as treg and nkt cells . the ability of infectious agents to regulate the immune system of their host is an increasingly fascinating topic . helminths , as long - lived parasites , are remarkable for their ability to manipulate host immunity , protecting themselves from elimination and minimizing severe pathology in the host [ 53 , 56 , 59 , 60 ] . immunomodulation by parasitic helminths is a general phenomenon that is conserved across species , classes , and even phyla . therefore , parasitic infections are a major theme in the hygiene hypothesis . allergies and autoimmune diseases are less prevalent in countries with higher burdens of helminths and other parasitic organisms . there are strong epidemiological evidences to support the premise that the dramatic increase in atopic disease in the developed world is a direct consequence of the eradication of helminth infections . at least some helminthes seem to have antiallergic or anti - inflammatory effects in humans . experimental evidences have also shown the significant suppression for the development of airway hyperresponsiveness ( ahr ) in mice infected with numerous helminths , including blood fluke schistosoma japonicum , filaria litomosoides sigmodontis , nematode heligmosomoides polygyrus , and nippostrongylus brasiliensis . these mice show attenuated airway inflammation with reduced infiltration of eosinophilia in the bal and lung and allergen - specific ige in sera . experimental studies have also shown protective effects of helminth infections in animal models of autoimmunity . surprisingly , helminths have been shown to suppress various types of autoimmune disease , such as collagen - induced arthritis , experimental autoimmune encephalomyelitis , and type 1 diabetes in murine models as reviewed recently . helminth infections might be beneficial to the induction of multiple regulatory mechanisms , including various regulatory cell populations , inhibitory receptors , blocking antibodies , and two prominent cytokines : il-10 and tgf- [ 61 , 66 ] . thus , it is not surprising that helminths can modulate immunopathology , whether in the context of allergic inflammation or autoimmune disease , either directly or indirectly . a study shows that serum antibodies to toxoplasma gondii tend to be negatively associated with allergic sensitization to food and aeroallergens in children from different geographical areas in greece , netherlands , china , india , and russia . a negative association also exists between t. gondii infection and the presence of multiple sclerosis . the literature is limited regarding the involvement of inkt cells in this hypothesis . on the other hand , many microorganisms and parasites contain various lipids in their structures . the bacterial and parasitic cell wall is a cellular structure highly enriched in a variety of glycolipids and lipoproteins . along with -galcer , growing evidences suggest that some microorganisms , including mycobacteria , sphingomonas , borrelia , helicobacter pylori , streptococcus pneumoniae , and group b streptococcus , can produce cd1d - restricted ligands capable of activating a proportion of inkt cells . hence , inkt cells can respond directly by recognizing the glycolipid antigens expressed by these bacteria . inkt cells can also respond indirectly to many other bacteria such as salmonella enterica and staphylococcus aureus . of note , some protozoan and helminthic parasites also contain natural ligands of nkt cells and several candidates have been successfully isolated . the excretory and secretory ( es ) products , which are often glycosylated , are found in the bloodstream of infected hosts and dictate particular functional immune responses that allow persistence of the parasite , typically by inducing th2-associated cytokines and expansion of various regulatory cell subsets , including nkt cells [ 22 , 56 , 69 ] . the adult worms of schistosoma mansoni express a range of glycoconjugates , such as galactosylceramide and glucosylceramide , which may contain natural ligands of nkt cells . infection with s. mansoni or exposure to eggs from this helminth inhibited the development of type 1 diabetes in nod mice . in addition , soluble extracts from worms or eggs of this schistosome possess the similar ability as infection to prevent the onset of diabetes if injection is given at early age ( 4  wk old ) . soluble adult worm antigen or soluble egg antigen may expand the inkt cell population in nod mice , although the lipids binding to inkt cells have not been targeted in this study . lipid extracts from murine malaria parasites can actually be loaded onto cd1 molecules to expand inkt cells . lotter and colleagues have identified a lipopeptidophosphoglycan from e. histolytica membranes as a possible inkt natural ligand , which can stimulate inkt cells to produce ifn- to exert protective role against this infection . karmakar and colleagues isolated a natural ligand of nkt cells , -(14)-galactose terminal glycosphingophospholipid ( gspl ) , from l. donovani to treat infected balb / c mice . these investigations pave the way to identify natural ligands of inkt cells for the development of novel therapeutic agents . inkt cells may exert their immunomodulatory effects in hygiene hypothesis through the interaction with treg , th17 , and other immune cells . upon activation with microbial and parasitic lipid antigens , inkt cells rapidly produce a wide range of cytokines and chemokines , transactivating many immune cell types , such as th1 , th2 , and treg . in eae model , inkt cells th17 cells in the mlns are greatly reduced in cd1 mice or j281 mice , which lack inkt cells . inkt cells induce the conversion of nave diabetogenic bdc2.5 t cells into foxp3(+ ) treg cells in the pancreatic lymph nodes accumulating in the pancreatic islets . in addition , inkt cells can suppress both antigen - induced acute arthritis and collagen - induced chronic arthritis , likely via inhibition of arthritogenic th1 cells . in toxoplasma gondii infection , activation of inkt cells by -galcer can lead to a shift to th2 cytokine profile and a significant increase in treg cells in mlns , which exerts protective role and increases survival of mice . the recently defined innate lymphoid cells ( ilc )   share some features with inkt cells . upon activation without the need of prior sensitization , both cells can release copious amounts of th1 , th2 , and/or th17 cytokines that shape subsequent innate and adaptive immune responses . although sparse up to now , there exists experimental evidence for direct interactions of ilcs and nkt cells possibly via their effector cytokines . for example , nkt cells , as well as alveolar macrophages , secrete endogenous il-33 that enhance il-5 production from ilc2 in lungs during influenza virus infection . the interaction of inkt cells with other t cell subsets and underlying mechanisms remain to be elucidated . taken together , microbe and parasite infections , especially at early lifetime , parasitic worms are able to survive in their mammalian host for many years due to their ability to manipulate the immune response . the underlying mechanisms regarding how infections affect the immunity of hosts remain to be clarified . upregulation of regulatory t cell subsets , such as treg , and induction of inhibitory cytokines and/or chemokines are the common findings . given the fact that many microbes and parasites are enriched in lipid antigens and nkt cells are unique t cell subset that can recognize lipid antigens , it is reasonable to speculate that nkt cells play key roles in this hypothesis ( figure 2 ) . inkt cells are unique innate - like t cell subset that bridge between innate and acquired immunity systems . inkt cells exert both effector and regulatory functions through direct contact or quick secretion of copious amounts of cytokines , chemokines , and other mediators upon their tcr engagement by glycolipid antigens . these cytokines and chemokines critically regulate the downstream differentiation of th1 , th2 , th17 , and other cells . therefore , inkt cells have been postulated to have an important proximal immunoregulatory role and influence both innate and acquired immune systems . they participate in the host 's immunity and immunopathogenesis of a wide range of parasite infections as discussed above . more thorough investigation is clearly necessary to better define their mode of activation and their regulatory functions in parasitic infections . although the involvement of inkt cells in the hygiene hypothesis and the contribution to autoimmunity and allergic inflammation remains to be fully elucidated , exploiting inkt cells in helminth immunomodulatory mechanisms may lead to opening a new avenue to develop novel safer therapeutic agents for these diseases based on the manipulation of inkt cell function .
invariant natural killer t ( inkt ) cells are unique subset of innate - like t cells recognizing glycolipids . 
 inkt cells can rapidly produce copious amounts of cytokines upon antigen stimulation and exert potent immunomodulatory activities for a wide variety of immune responses and diseases . 
 we have revealed the regulatory effect of inkt cells on autoimmunity with a serial of publications . on the other hand , the role of inkt cells in parasitic infections , especially in recently attractive topic  hygiene hypothesis , 
 has not been clearly defined yet . 
 bacterial and parasitic cell wall is a cellular structure highly enriched in a variety of glycolipids and lipoproteins , some of which may serve as natural ligands of inkt cells . in this review 
 , we mainly summarized the recent findings on the roles and underlying mechanisms of inkt cells in parasite infections and their cross - talk with th1 , th2 , th17 , treg , and innate lymphoid cells . in most cases , 
 inkt cells exert regulatory or direct cytotoxic roles to protect hosts against parasite infections . 
 we put particular emphasis as well on the identification of the natural ligands from parasites and the involvement of inkt cells in the hygiene hypothesis .