Immunopathology of schistosomiasis

Waterborne parasitic diseases plague tropical regions of the world with the development of water resources often increasing transmission. Skin-penetrating cercariae (infectious stages of schistosome parasites) mature within their mammalian host, form sexual pairs and produce several hundred eggs per day. Many eggs are swept within the circulation and in the case of Schistosoma mansoni and S. japonicum, become lodged within hepatic sinusoids, invoking a fibrotic granulomatous response. Animal studies have identified a moderate type 1 helper (Th1) response to parasite antigens; however, a robust Th2 response to egg-derived antigens dominates and propagates fibrogenesis within the liver. Elegant T helper cell polarization studies have highlighted that critical control of Th1, Th2 and interleukin (IL)-17-secreting lymphocytes is necessary to prevent severe liver pathology. Alternatively activated macrophages develop in the Th2 milieu and upregulate Fizz1, Ym-1 and Arg-1. The possible contribution of macrophages to fibrogenesis and their role in immune regulation are discussed. Within the liver, natural (CD4(+)CD25(+) Forkhead box protein 3 (Foxp3)(+)) and inducible (CD4(+)Foxp3(-)) Treg's are recruited, providing an essential regulatory arm to stabilize the immune response and limit immunopathology. This review ties together current thinking of how the granulomatous response develops, causing much of the associated immunopathology, with extensive discussions on how regulatory cells and cytokine decoy receptors serve to limit the extent of immune-mediated pathology during schistosomiasis.     

血吸虫病肝纤维化形成的作用机制

血吸虫病(schistosomiasis)是世界上严重危害人类健康的人兽共患寄生虫病之一。在中国主要流行的是日本血吸虫病,其致病机制是由于血吸虫卵沉积于宿主肝内,形成虫卵肉芽肿,导致肝纤维化。血吸虫病肝纤维化免疫过程中,肝星状细胞、T细胞、B细胞及多种细胞因子的调节起着重要作用。现就血吸虫病肝纤维化形成机制的相关研究进展进行了综述。     

The molecular basis of granuloma formation in schistosomiasis. III. In vivo effects of a T cell-derived suppressor effector factor and IL-2 on granuloma formation

Granuloma formation in schistosomiasis is characterized by the formation of a large lesion in acutely infected animals which subsequently decreases in size as disease progresses into the chronic phase. These in vivo studies confirm and extend previous in vitro observations on the regulation of granulomatous hypersensitivity by a T cell-derived suppressor effector factor (TseF). TseF regulation of granuloma formation in vivo and DTH are shown to be both antigenically and genetically restricted. This suppression is accompanied by a suppression of the ability of cells derived from TseF recipients to function in an in vitro assay of granuloma formation. Antigenic recognition, defined by cellular proliferation in response to antigenic stimulation, is uneffected by TseF administration. Administration of IL-2 reduces TseF function in acutely infected mice and results in increased liver granuloma size. However, the ability of cells derived from these animals to form granulomas in vitro is uneffected. Cells obtained from chronically infected IL-2 recipients do not produce TseF in vitro and granuloma size is increased in these animals. Animals receiving both IL-2 and TseF continue to demonstrate decreased granuloma formation, indicating that IL-2 does not effect the ability of preformed TseF to function. These observations suggest that TseF modulates granuloma formation in vivo and may interact with IL-2 in a dynamic process which determines the intensity of the granulomatous response.     

Elevated serum levels of TNF-alpha, sTNF-RI and sTNF-RII in murine schistosomiasis correlate with schistosome oviposition and circumoval granuloma formation

Levels of circulating tumour necrosis factor (TNF-alpha) and its soluble receptors are elevated in chronic human schistosomiasis. However, the kinetics of TNF-alpha production and release of its soluble receptors have not been studied in humans or animals. Here we report on increased levels of TNF-alpha and its soluble receptors in murine schistosomiasis, beginning with schistosome oviposition and circumoval granuloma formation. TNF-alpha, sTNF-RI and sTNF-RII were measured in sera of mice infected with Schistosoma mansoni each week for 10 weeks postinfection. TNF-alpha levels increased gradually in all mice during the first 3 weeks. From 6th week postinfection, TNF-alpha levels in infected mice increased steadily, whereas those of uninfected mice remained essentially unchanged. sTNF-RI levels fluctuated in all mice during the first 3 weeks, and increased in infected mice during the following 5 weeks. sTNF-RII levels were similar in all mice for the first 4 weeks but increased in infected mice throughout the remainder of the experimental period. These data may be helpful in understanding pathogenesis in schistosomiasis as TNF-alpha plays a crucial role in circumoval granuloma formation and adversely affects schistosome fecundity.     

Angiogenesis and schistosomal granuloma formation

An increasing amount of evidences points to angiogenesis as playing a paramount role in fibrosis development. However granulomas in general, and periovular schistosomal granulomas in particular, are considered avascular structures, although they usually result in dense areas of focal fibrosis. In order to clarify this apparent paradox, the presence of blood vessels was systematically searched in hepatic schistosomal granulomas of mice, during different stages of the infection, and at different stages of granuloma evolution, by means of vascular injections of colored masses, demonstration of laminin in vascular basement membranes and by ultra structural analysis. Vascular proliferation appeared evident at the early stages of granuloma formation, gradually decreasing thereafter, older granulomas becoming almost avascular structures, sometimes delimited at the periphery by a rich vascular network.     

Synthesis of angiotensins by cultured granuloma macrophages in murine schistosomiasis mansoni

Components of the angiotensin system are present in granulomas of murine schistosomiasis mansoni. Angiotensins may have immunoregulatory function. Granuloma macrophages cultured for up to 3 days generated substantial angiotensin I (AI) and angiotensin II (AII) which appeared in the culture supernatants. Macrophage monolayers were incubated with 3H-labeled amino acids, and culture supernatants were extracted with acetone and analyzed by HPLC. Radiolabeled products eluted at times corresponding to those of authentic angiotensins. Immunoadsorption of angiotensins with angiotensin antisera removed reputed radiolabeled angiotensins from the supernatants. Treatment of the elution fraction corresponding to that of authentic AI with angiotensin-converting enzyme resulted in the generation of radiolabeled polypeptides which coeluted with authentic AII and His-Leu. Similar experiments conducted with nonadherent granuloma cells devoid of macrophages failed to demonstrate angiotensin production. These results suggest that granuloma macrophages can synthesize angiotensins.     

Dynamics of fibrosis production and resorption in intestinal schistosomiasis of mice.

A histological, morphometric and immunocytochemical study of schistosomal periovular granulomas in the liver and intestines of mice revealed that intestinal granulomas are smaller and contain less collagen than those in the liver. After curative treatment intestinal granulomas undergo a relatively more rapid resorption, although the general pattern of collagen degradation apparently does not differ from that observed in the liver. Tendency to form scattered, usually isolated granulomas that are only mildly fibrogenic, coupled with a well-balanced process of resorption appear as the explanation why intestinal fibrosis is not an outstanding feature of schistosomiasis as it is in the liver.     

Evidence for nitric oxide action on in vitro granuloma formation through pivotal changes in MIP-1alpha and IL-10 release in human schistosomiasis.

Nitric oxide (NO) has been implicated, both and paradoxically, as a pro- and anti-inflammatory agent in a wide range of circumstances. It is of common concern that NO can be either up- or downregulated by different inflammatory cytokines. Attempting to assess the contribution of NO to the granulomatous response, we used the in vitro granuloma (IVG) model which consists on a reaction of mononuclear cells around polyacrylamide beads conjugated to antigens. Our assays employed Schistosoma mansoni antigens and human peripheral blood mononuclear cells (PBMC) from schistosomiasis patients. Recently, we have described evidence for a regulatory role of NO, with the aid of an inhibitor of NO synthesis, L-NAME. The addition of L-NAME to IVG cultures elicited an increase on the granuloma formation index. Based on these data we decided to investigate the mechanisms involved in the effects of L-NAME-enhanced granuloma formation. Cytokines and chemokines are involved in inflammatory responses by, particularly the latter, inducing migration and adhesion of leukocytes, which led us on this search for their interactions with NO on granulomatous reaction. We evaluated the cytokine/chemokine-secreting profile of PBMC (treated and not treated with L-NAME) on the IVG reaction in order to investigate how NO could interfere on the release of these soluble mediators. Comparison of cell culture releasing amounts of IL-2, IL-10, TNFalpha, IFNgamma, MIP-1alpha, MCP-1, and RANTES demonstrated that MIP-1alpha had increased levels when NO production was blocked with L-NAME, whereas IL-10 secretion decreased in presence of L-NAME. The other tested cytokines (IL-2, TNFalpha, and IFNgamma) and chemokines (MCP-1 and RANTES) showed no significant differences between the presence or absence of L-NAME. Results obtained in this work suggest that inhibition of NO production could upregulate the IVG reaction on human schistosomiasis through changes in the cytokine/chemokine profile released by PBMC. The mechanisms involved may lead to a MIP-1alpha-increased and IL-10-decreased secretion under our experimental conditions, which could partly account for the previously ascribed IVG-exacerbating action of NO inhibition.     

Effects of granuloma modulation induced by regulatory-T-lymphocyte activity on angiotensin II/III production by granuloma macrophages in murine schistosomiasis mansoni

Angiotensins are produced by granuloma macrophages in murine Schistosoma mansoni. During the course of infection, granuloma undergo a T-cell-dependent process called modulation in which their maximal size decreases. This study was undertaken to establish whether angiotensin production by granuloma macrophages is altered by immunoregulatory lymphocytes. Granuloma macrophages from modulated lesions released and contained more angiotensin II/III (AII/III) and less angiotensin I (AI) than those from the acute infection. Captopril, a specific angiotensin-converting-enzyme (ACE) inhibitor, appreciably decreased AII/III produced by macrophages from modulated granulomas. Adoptive transfer of splenic T lymphocytes from chronically infected donors into acutely infected recipients altered angiotensin production by the granuloma macrophages in a manner similar to that seen in modulated lesions. However, no difference was detected in the capacity of granuloma macrophages from acutely or chronically infected mice to metabolize 125I-AI or -AII added to cell cultures. Similarly, captopril did not alter the metabolism of exogenously administrated angiotensins. These findings suggest that regulatory T lymphocytes influence the metabolism by granuloma macrophages of endogenously produced angiotensins at least in part by induction of macrophage ACE activity. However, the degradation of extracellular AI and AII may result from the activity of enzymes other than ACE which are not inducible by modulation.     

Induction of granuloma modulation in murine schistosomiasis mansoni by enteric exposure to schistosome eggs

Enteric administration of antigen can induce systemic tolerance. In murine schistosomiasis mansoni, blood flukes produce eggs which enter the intestine. An immunologic phenomenon associated with this disease is a spontaneous diminution in the intensity of the granulomatous response in the liver, lungs, and colonic mucosa with chronic infection, which is termed modulation. It was determined whether modulation of liver granulomas could be induced by enteric immunization with schistosome eggs. Mice infected for 4 wk were immunized by injection of 25,000 eggs into cecal pouches. This induced modulation of liver granulomas by the eighth week of infection. Neither cecal injection of normal saline nor i.p. or subcutaneous injection of eggs could induce the modulatory process. Modulation could be adoptively transferred from enterically immunized donors by injection of spleen cells into infected recipients or into uninfected recipients with synchronous liver granulomas induced by the hepatic embolization of schistosome eggs. Spleen cells treated with anti-Thy-1.2 or anti-Lyt-1.1 and complement could no longer adoptively transfer modulation. These data show that enteric immunization with schistosome eggs can induce modulation of the liver granuloma by a cellular mechanism similar to that described for the natural infection.     

The molecular basis of granuloma formation in schistosomiasis. II. Analogies of a T cell-derived suppressor effector factor to the T cell receptor

During Schistosoma mansoni infection, Ts cells regulate granulomatous modulation via antigenically and genetically restricted suppressor inducer and suppressor effector factors. The T suppressor effector factor (TseF) directly suppresses granuloma formation both in vitro and in vivo. In this study, we probe the molecular basis of these TseF properties. Using techniques of heterodimeric chain reduction with DTT and in vitro functional complementation, chimeric molecules were constructed. By analyzing genetic restrictions, antigenic specificities, and phenotypic markers, the contributions of the component chains to 72 kDa TseF reactivity were determined. One chain bore an Ag receptor and imparted antigenic specificity. The other chain bore an IJ determinant, a TCR beta-chain allotypic determinant, a suppressor effector phenotypic determinant, and imparted functional genetic restriction. Functional activity required covalent, probably sulfhydryl mediated, linkage as succinylation prevented the separated component chains from reconstituting functional activity. Additional studies demonstrated that anti-serum directed against either the T cell receptor or the T3 epsilon-chain could abrogate functional activity. However, TseF bore no T3 epsilon-chain phenotypic marker per se suggesting that TseF effects T lymphocytes via transmembrane signal transduction. These studies suggest that a regulatory network is operative in granuloma modulation. This regulatory network is mediated by a soluble TseF that bears significant structural homologies to the classic TCR.     

The role of chemokines in Schistosoma mansoni granuloma formation

The eradication of parasitic diseases, such as schistosomiasis, has been the focus of investigations worldwide for many decades. However, attempts to control their continual spread have, at best, been met with limited success. In the face of these results, it is important to attempt to understand and thus to control the pathology of these widespread diseases. In this review, Nicholas Lukacs, Steven Kunkel, Robert Strieter and Stephen Chensue focus on a family of cytokines that play a pertinent role for leukocyte recruitment in chronic inflammation.     

Experimental murine schistosomiasis in the absence of B7 costimulatory molecules: reversal of elicited T cell cytokine profile and partial inhibition of egg granuloma formation

The granulomatous inflammation in infection with the helminth Schistosoma mansoni represents a cellular hypersensitivity reaction mediated by, and dependent upon, MHC class II-restricted CD4+ Th cells sensitized to parasite egg Ags. The current work examines the role and significance of the B7:CD28/CTLA-4 pathway in providing the costimulation necessary for the activation of these pathogenic T cells. In vitro T cell responses in B7-1-/- mice, 7-8 wk postinfection, were no different from wild-type controls, but the absence of B7-2 molecules resulted in a decrease in egg Ag-induced proliferation with increased IFN-gamma production. Both B7-1-/- and B7-2-/- mice exhibited intact granuloma formation. In contrast, CD4+ Th cells from B7-1/2 double-deficient mice displayed a dramatic loss of proliferative capacity upon stimulation with egg Ag. Most strikingly, these T cells secreted only IFN-gamma, but not IL-4 and IL-10, a pattern entirely opposite to that displayed by wild-type controls. Despite these major differences in T cell reactivity, B7-1/2-/- mice had only a limited reduction of granuloma size and fibrosis, without appreciable difference in cellular composition. These results show that substantial granuloma formation can occur under conditions of limited T cell expansion and restricted Th1-type cytokine production. They also support the notion that the combined effect of B7 signaling is not as critical for Th1 cell activation as it is for the development of the Th2 dominant environment characteristic of the evolving schistosome infection in H-2b mice.