a rational design of new approaches aiming to control protozoan 
 infections depends not only on advances in our knowledge of 
 virulent factors , molecular pathogenesis , and immune responses involved 
 in the host defence , but also on our understanding of finely tuned 
 co - stimulatory signallings that play a key role in immunity 
 against infections . cd40-cd40l interaction has emerged in the last 
 decade as an essential system that regulates host immune defence 
 against infectious and noninfectious diseases . cd40 , a surface 
 glycoprotein receptor belonging to tnf receptor family is 
 expressed by a number of cells including immunocompetent cells 
 such as professional apcs , b lymphocytes , activated cd4 + and 
 cd8 t lymphocytes as well as nonhematopoetic cells 
 [ 15 ] . its ligand , cd40l , is a co - stimultory molecule 
 that can be expressed on various cells such as cd4 + t lymphocytes , 
 b lymphocytes , natural killer ( nk ) cells , dendritic cells ( dcs ) , 
 monocytes and macrophages [ 69 ] . cd40-cd40l interactions 
 play an important role in the regulation of   thymus - dependant 
 humoral immune responses through   cognate   interaction 
 of b and t cells which   promotes b cell proliferation , ig   class 
 switching , and generation of b cell memory [ 10 , 11 ] . on the 
 other hand , engagement of cd40 present on apcs with cd40l on 
 t cells is crucial for the priming and expansion of 
 antigen - specific cd4 + t cells and for the induction of 
 costimulatory molecules on apcs . it is well known that triggering of cd40 on the surface of apcs leads to the production 
 of cytokines , particularly il-12 which plays a central role in the 
 activation of t cells to produce ifn- , thereby directing 
 cell - mediated immunity towards th1 subset that is required for 
 effective immunity against intracellular pathogens 
 [ 1315 ] . naturally occurring mutation in human cd40l gene 
 results in a defect in cd40 signalling and leads to hyper igm 
 syndrome . theses patients not only are defective in humoral 
 immunity but also exhibit impaired t cell - mediated immunity and 
 therefore are susceptible to infections [ 16 , 17 ] . in this 
 review , we focus on insights provided by different studies arguing 
 that cell - mediated immunity against intracellular parasites 
 depends upon cd40-cd40l interactions . major insights gained into paradigms of th - subset came from 
 studies on immunity to leishmania infection . resistance 
 to leishmania which multiplies as an amastigote within 
 macrophages depends on polarization of the immune response towards 
 th1 type in which il-12 and ifn- play a pivotal role , 
 while disease progression is linked to development of th2 type 
 response . since cd40-cd40l interaction is crucial for promoting th1 immune response , various studies were focused on the 
 impact of such interaction on the outcome of experimental 
 infection . the first evidence for a direct role of these molecule 
 pairs in promoting immune responses against protozoan infections 
 came from studies showing that cd40- and cd40l - deficient mice are 
 susceptible to leishmania infections [ 1921 ] . t cells from these mice fail to produce ifn- suggesting a defect in th1 response . these deficient mice were unable to mount an effective immunity against parasite 
 infection due to a defect in t cell mediated activation of 
 macrophages . beside their role in th1 immune response , cd40-cd40l 
 interactions were shown also to stimulate macrophages to produce 
 number of cytokines and inflammatory mediators among which nitric 
 oxide ( no ) plays a key role in parasite killing . theses basic studies on leishmania infections clearly pointed towards a major role of cd40-cd40l interactions in skewing 
 immune response to th1 type that is required for antiparasite host 
 defence . t. cruzi is an obligate intracellular parasite that 
 invades several types of cells in vertebrate hosts . development of 
 a th1-like immune response was sown to be associated with the 
 control of infection in mice . in particular , il-12 , ifn- , and tnf play a crucial role in the development of cell - mediated immunity against the parasite 
 . indeed , treatment of t. cruzi - infected mice with anti - il-12 mab increases parasitemia and mortality while an 
 exogenous supply of il-12 confers protection against infection 
 [ 25 , 26 ] . in view of the important role of il-12 , it seemed likely that cd40 ligation is important for induction of effector phases of immune response . the potent effect of the cd40-cd40l 
 pathway in t. cruzi infection was first assessed by using 
 cd40l - transfected 3t3 fibroblasts to monitor parasitological and 
 immunological parameters in infected mice . this study indicated that supernatants of murine spleen cells stimulated with 
 cd40l - transfected cells prevent infection of macrophages in vitro 
 and this phenomenon depends on de novo production of nitric oxide 
 ( no ) . anti - il-12 , anti - ifn- , and anti - tnf mab neutralize the effect of supernatants suggesting the importance of these cytokines in the prevention of macrophage 
 infection . this in vitro data were further supported by in vivo 
 experiments showing that coinoculation of cd40l - transfected 3t3 
 fibroblasts and t. cruzi into mice leads to reduced 
 parasitemia and mortality and this effect is abolished by 
 injection of anti - il-12 mab . recently , we examined further the 
 role of cd40 ligation in t. cruzi infection by using a 
 new approach based on generation of cd40l - transfected parasite 
 strain . mice inoculated with this recombinant strain exhibit a very low parasitemia and no mortality associated with 
 preserved production of ifn- by spleen cells compared to 
 wild - type strain . these findings highlight the potent role of 
 cd40-cd40l interaction in the stimulation of an effective immunity 
 against t. cruzi . tachyzoites of t. gondii can disseminate in the host 
 because of its ability to infect many nucleated cells . since 
 ifn- is the major cytokine required for the activation of 
 a cell - mediated immunity against t. gondii , and giving the importance of il-12 in the stimulation of early 
 ifn- synthesis , one may suspect a critical role of cd40-cd40l interaction in the control of toxoplasma infections . studies performed in human with hyper igm syndrome due 
 to a natural cd40l mutation revealed that these patients exhibited 
 a defect in ifn- secretion in response to t. 
 gondii . the lack of ifn- production was linked to impaired il-12 secretion , indicating that cd40-cd40l signalling was required for an optimal t cell activation and production of 
 ifn-. on the other hand , toxoplasma infection 
 was also investigated in cd40l - deficient mice . this study showed that these mice produced less il-12 than wild type 
 when infected with t. gondii . moreover , cd40l - deficient 
 mice succumbed to toxoplasmic encephalitis indicating 
 that these mice were not able to control parasite replication in 
 the brain and suggesting an important role of the cd40-cd40l 
 interaction in this process . furthermore , cd40 signalling was 
 shown to regulate ifn--independent host protection 
 against toxoplasma infection through tnf--dependant induction of macrophage antimicrobial activity 
 . susceptibility of both patients with hyper igm syndrome and cd40l - deficient mice to toxoplasma 
 infection argues for the requirement of cd40/cd40l signalling for 
 resistance to parasite infection . as reported above , cd40-cd40l interaction is crucial for the 
 outcome of infection in a number of intracellular parasite models . stimulation of cd40 on apcs has proved to be useful for 
 amplification of th1-type response in which il-12 and ifn- 
 play a cardinal role ( figure 1 ) . the first approach 
 used to modulate cd40 signalling was based on agonistic anti - cd40 
 ab . injection of these ab in mice infected with leishmania stimulates il-12 and ifn- production 
 and induces killing of the parasites within macrophages 
 [ 34 , 35 ] . similarly , administration of anti - cd40l mab in mice 
 infected with t. cruzi results in a stimulation of 
 ifn--activated macrophages to produce no and tocontrol parasite infection . although cd40l exists in 
 nature predominantly as a membrane - anchored molecule , the 
 molecular characterization of cd40l molecule indicated that the 
 extra - cellular carboxy - terminal region can be soluble and 
 biologically active [ 6 ,   36 ] . therefore , a variety of reports were focused on this agonistic molecule with the aim to assess its stimulatory role in the control of parasite infection as it is the 
 case for other infectious diseases [ 14 , 37 ] . studies in patients with hyper igm syndrome that exhibit deficient secretion of il-12 showed that in vitro incubation of peripheral blood 
 mononuclear cells ( pbmc ) with a soluble cd40l resulted in enhanced 
 il-12-dependant production of ifn- in response to 
 toxoplasma gondii . soluble 
 cd40l was also shown to activate murine macrophages in vitro to 
 control the replication of t. gondii . as for many adjuvant proteins , the major obstacles related to 
 stability and issues of in vivo delivery of cd40l had to be faced . 
 in this regard , host cells transfected with cd40l gene were 
 developed as a way of delivery in different parasitic models 
 [ 27 , 38 ] . interestingly , chen et al . described a strategy based 
 on directing cd40l to macrophages by coexpressing the molecule on 
 the surface of a cell line along with gp63 recombinant 
 leishmania antigen and showed that mice treated with 
 these cotransfected cells produce higher amounts of il-12 and 
 control the disease progression . delivery of cd40l expressed by transfected 3t3 fibroblasts was shown to reduce 
 parasitemia in t. cruzi - infected mice . recently , we developed a new concept based on the use of the 
 pathogenic organism as a vehicle for cd40l delivery . following transfection of t. cruzi with cd40l gene , the 
 encoded molecule was found properly processed and secreted across 
 the parasite membrane . notably , cd40l recombinant strain exhibited 
 lower virulence and induced higher inf- production when 
 injected into mice . moreover , surviving mice resisted a challenge 
 infection with wild - type strain , thereby confirming the vaccine 
 adjuvant capacity of cd40l . the molecular characterization of cd40l and the analysis of its 
 binding domains were determinant steps towards the manipulation of 
 cd40 signalling . as for tnf receptors family , the signalling through cd40 depends upon the formation of a cd40l 
 trimer complex that can each bind three cd40 molecules . incorporation of an isoleucine zipper motif that improves 
 trimerization of the cd40l was shown to enhance its 
 biological activity . advances in the molecular 
 structures of cd40l binding domains allowed a conception of small 
 cd40l mimetic molecules that could compete with the binding of 
 cd40l homotrimers and induce il-12 secretion by dcs . interestingly , recent findings indicate that when these mini - cd40l 
 synthetic peptides were coinjected with t. cruzi into 
 mice , a low parasitemia associated with enhanced cd8 t cells 
 producing ifn- was observed . these recent reports further support the importance of cd40l delivery as 
 an adjuvant that can be used to drive type 1 immune response 
 against intracellular parasite infection 
 ( figure 1 ) . protozoan parasites have a remarkable ability to adapt to 
 different host microenvironments . to make the host  as safe as 
 possible for them , they have evolved many devises among which 
 the immunosuppression is considered as a powerful mechanism to 
 subvert the host 's immune response . interfering with cd40 presentation or signalling can be one of the powerful arms 
 used by the parasite . indeed , cd40 molecules were found to be 
 reduced in the surface of macrophages and dcs following their 
 infection by t. cruzi [ 45 , 46 ] . recently , the cd40 
 was defined as a central molecule through which counteractive 
 immune responses can be triggered . based on 
 the cross - linking experiments with anti - cd40 ab on 
 macrophages , this study indicated that the strength of cd40 signalling 
 activates p38-mitogen activated protein kinase ( p38mapk ) or 
 extracellular stress - related kinase 1/2 ( erk-1/2 ) signalling 
 molecules leading to a differential expression of il-12 or il-10 . cd40 cross - linking at lower doses induces activation of erk-1/2 
 and production of il-10 , a cytokine which promotes 
 immunosuppression , whereas at higher doses it induces activation 
 of p38mapk and secretion of il-12 ( figure 2(a ) ) . how 
 such strength of cd40 signalling is operating through interaction 
 of the immunocompetent cells and whether it can influence the th1 
 and th2 immunoregulatory processes are still unclear . interestingly , leishmania - infected macrophages treated 
 with an intermediate dose of anti - cd40 ab produced more il-10 and 
 less il-12 than uninfected macrophages ( figure 2(b ) ) , 
 suggesting that leishmania infection promotes il-10 
 production that would favour disease progression 
 . this study is in line with a previous report 
 indicating that p38mapk - dependant cd40 signalling is impaired in 
 leishmania - infected macrophages . overall , the involvement of cd40 signalling in the host - parasite 
 interaction further exemplifies the refined nature of the 
 host - pathogen cross - talk . increasing evidence points towards the crucial role of cd40-cd40l 
 for the development of a cellular host protective immune response 
 against intracellular parasites . although the role of cd40-cd40l 
 signalling in b cell maturation and isotype switching is well 
 documented , little is known about the potent stimulation of cd40 
 signalling that can promote humoral immune response against 
 extracellular parasites . a study on african trypanoma 
 infection in a model of scid mice reconstituted with a bovine 
 immune system indicated that administration of an agonistic 
 antibody against cd40 enhanced mice survival to infection with 
 trypanosoma congolense , and was associated with increased 
 production of specific igg . further studies aiming to depict accurately the cd40 signalling - dependant protective th2 
 immunity against protozoan parasites are yet to be investigated . 
 the stimulation of cd40 signalling by cd40l and its derivatives 
 can be considered as a useful adjunct in a vaccine strategy 
 against protozoan infections . current knowledge in host - parasite 
 interaction includes a breakthrough in the modulation of cd40 
 signalling brought about by using soluble cd40l , cd40l - transgenic 
 microorganisms or small peptide mimetics of the cd40l . however , 
 recent findings outlined the possible regulation of cd40 
 signalling by the parasite and therefore stressed the need of a 
 further understanding of the host - pathogen crosstalk that could 
 lead to novel approaches for disease control .
activation of the immune system against protozoan infections relies 
 particularly on two specific signals provided by cognate interaction of t 
 cells with antigen presenting cells ( apcs ) . 
 the first signal is attributed 
 to binding of the t - cell receptor ( tcr ) to peptide / mhc complexes on the 
 surface of apcs , whereas the second signal is triggered through binding of 
 several costimulatory molecules on the surface of apcs with their 
 corresponding receptors on t cells . among these costimulatory signallings 
 , 
 cd40/cd40l interactions have been particularly investigated in protozoan 
 infection models with regard to their potential to amplify cell - mediated 
 immunity against intracellular parasites . 
 this article reviews current 
 studies of the potential role of cd40/cd40l interaction in the modulation of 
 immune responses against some protozoan parasites and highlights recent 
 developments regarding manipulation of this interaction for promoting 
 control of parasite infections .