Experimental infection with Trypanosoma cruzi increases the population of CD8(+), but not CD4(+), immunoglobulin G Fc receptor-positive T lymphocytes

Deletion of Fibrinogen-like Protein 2 (FGL-2), a Novel CD4+ CD25+ Treg Effector Molecule, Leads to Improved Control of Echinococcus multilocularis Infection in Mice

Background

The growth potential of the tumor-like Echinococcus multilocularis metacestode (causing alveolar echinococcosis, AE) is directly linked to the nature/function of the periparasitic host immune-mediated processes. We previously showed that Fibrinogen-like-protein 2 (FGL2), a novel CD4⁺CD25⁺ Treg effector molecule, was over-expressed in the liver of mice experimentally infected with E. multilocularis. However, little is known about its contribution to the control of this chronic helminth infection.

Methods/Findings

Key parameters for infection outcome in E. multilocularis-infected fgl2^-/- (AE-fgl2^-/-) and wild type (AE-WT) mice at 1 and 4 month(s) post-infection were (i) parasite load (i. e. wet weight of parasitic metacestode tissue), and (ii) parasite cell proliferation as assessed by determining E. multilocularis 14-3-3 gene expression levels. Serum FGL2 levels were measured by ELISA. Spleen cells cultured with ConA for 48h or with E. multilocularis Vesicle Fluid (VF) for 96h were analyzed ex-vivo and in-vitro. In addition, spleen cells from non-infected WT mice were cultured with rFGL2/anti-FGL2 or rIL-17A/anti-IL-17A for further functional studies. For Treg-immune-suppression-assays, purified CD4⁺CD25⁺ Treg suspensions were incubated with CD4⁺ effector T cells in the presence of ConA and irradiated spleen cells as APCs. Flow cytometry and qRT-PCR were used to assess Treg, Th17-, Th1-, Th2-type immune responses and maturation of dendritic cells. We showed that AE-fgl2^-/- mice exhibited (as compared to AE-WT-animals) (a) a significantly lower parasite load with reduced proliferation activity, (b) an increased T cell proliferative response to ConA, (c) reduced Treg numbers and function, and (d) a persistent capacity of Th1 polarization and DC maturation.

Conclusions

FGL2 appears as one of the key players in immune regulatory processes favoring metacestode survival by promoting Treg cell activity and IL-17A production that contributes to FGL2-regulation. Prospectively, targeting FGL2 could be an option to develop an immunotherapy against AE and other chronic parasitic diseases.

In larval E. multilocularis infection causing alveolar echinococcosis (AE) in humans as well as mice, immune tolerance and/or down-regulation of protective immunity is a marked characteristic of this chronic disease. Our study provides a comprehensive evidence for a major involvement of the recently identified CD4⁺ CD25⁺ Regulatory T Cell Effector Molecule FGL2 to the outcome of AE. Our major findings are as follows: 1) FGL2 is mostly secreted by Tregs and partly contributes to their functions; 2) FGL2 can down-regulate the maturation of DCs, suppress Th1 and Th17 immune responses, and support Th2 and Treg immune responses, and finally 3) IL-17A contributes to FGL2 secretion. Based on the present findings in mice, we will investigate FGL2 as a potential marker of progression of AE in human patients, or as a potential immunotherapeutical target. Early prediction of parasite regression (currently not yet possible) would allow clinicians to plan for withdrawing benzimidazole treatment, which is currently administered for life. Then, FGL2 should be investigated as a target for an anticipated immunomodulatory treatment of patients with progressive AE, especially of those who are non- or low-responders to benzimidazole treatment, or who suffer from side-effects due to chemotherapy.

Adoptive transfer of allogeneic liver sinusoidal endothelial cells specifically inhibits T-cell responses to cognate stimuli

Although it is well known that liver allografts are often accepted by recipients, leading to donor-specific tolerance of further organ transplants, the underlying mechanisms remain unclear. We had previously used an in vitro model and showed that mouse liver sinusoidal endothelial cells (LSECs) selectively suppress allospecific T-cells across major histocompatibility complex (MHC) barriers. In the present study, we established an in vivo model for evaluating the immunomodulatory effects of allogeneic LSECs on corresponding T-cells. Allogeneic BALB/cA LSECs were injected intraportally into recombination activating gene 2 γ-chain double-knockout (RAG2/gc-KO, H-2(b)) mice lacking T, B, and natural killer (NK) cells. In order to facilitate LSEC engraftment, the RAG2/gc-KO mice were injected intraperitoneally with monocrotaline 2 days before the adoptive transfer of LSECs; this impaired the host LSECs, conferring a proliferative advantage to the transplanted LSECs. After orthotopic allogeneic LSEC engraftment, the RAG2/gc-KO mice were immune reconstituted intravenously with C57BL/6 splenocytes. After immune reconstitution, mixed lymphocyte reaction (MLR) assay using splenocytes from the recipients revealed that specific inhibition of host CD4(+) and CD8(+) T-cell proliferation was greater in response to allostimulation with irradiated BALB/cA splenocytes rather than to stimulation with irradiated third party SJL/jorllco splenocytes. This inhibitory effect was attenuated by administering anti-programmed death ligand 1 (PD-L1) monoclonal antibody during immune reconstitution in the above-mentioned mice, but not in RAG2/gc-KO mice engrafted with Fas ligand (FasL)-deficient BALB/cA LSECs. Furthermore, engraftment of allogeneic BALB/cA LSECs significantly prolonged the survival of subsequently grafted cognate allogeneic BALB/cA hearts in RAG2/gc-KO mice immune reconstituted with bone marrow transplantation from C57BL/6 mice. In conclusion, murine LSECs have been proven capable of suppressing T-cells with cognate specificity for LSECs in an in vivo model. The programmed death 1/PD-L1 pathway is likely involved in these suppressive effects.

Isolation of cardiac mast cells in experimental Trypanosoma cruzi infection

Mast cells (MC) secrete diverse pre-stored chemical mediators that are pivotal in inflammatory and fibrotic etiologies, such as Trypanosoma cruzi-induced myocardiopathy. However, due to reduced number of cardiac MC, in situ and in vitro identification, and difficult tissue isolation, these cells are rarely addressed. In this work we optimized the identification of cardiac and peritoneal MC and developed an enzymatic method for MC isolation using control and T. cruzi-infected mice. MC were identified by: toluidine blue (TB); alcian blue (AB)/safranin (S); AB or a mixed solution composed by AB/S/TB. Previous evaluations of cardiac MC in T. cruzi infection were based on TB staining and our results using AB/S/TB solution showed an increase in, at least, five times the detection of MC. This mixed solution may improve the identification of MC populations also from skin, mucosa and tissues that are infected by other pathogens or under the influence of chronic inflammation, leading to more precise results. Furthermore, the appropriate combination of samples (frozen/unfixed/thick slices) and staining protocols can assure the best evaluation of MC. We have also isolated cardiac MC using collagenase and developed a highly efficient 60%/70% Percoll-graded protocol that enriched in, at least, 95% the population of cardiac MC.

Electrocardiographic findings in acutely and chronically T. cruzi-infected mice treated by a phenyl-substituted analogue of furamidine DB569

Aromatic diamidines have been successfully used to combat a wide range of parasites that cause important human infections. Recently we reported that a N-phenyl-substituted analogue of furamidine (DB569) exerts a micromolar trypanocidal activity against Trypanosoma cruzi in vitro. Since DB569 also reduces the cardiac parasitism and increases the survival rates of T. cruzi-infected mice, our present aim was to analyze the potential protection of DB569 in the development of altered cardiac electrical conduction system during acute and chronic T. cruzi infection. In our experimental model of acute infection (Swiss mice inoculated with Y strain of T. cruzi), the prevailing disorder observed in electrocardiogram (ECG) analyses was sinus bradycardia. This ECG alteration was reverted in acutely infected mice treated with DB569. Interestingly, the DB569 treatment reduced significantly the numbers of CD8(+) T cells in the cardiac infiltration. In addition, the noticed protection of DB569 in the ECG findings of acutely-infected animals was further extended to the chronic infection. Our data suggest that the reversion to and further maintenance of normal ECG profile in the DB569-treated infected animals may be associated with the reduced cardiac CD8(+) lymphocyte infiltration and parasitism that might be ultimately contributing to their increased survival rates.

Differential impact of metacyclic and blood trypomastigotes on parasitological, serological and phenotypic features triggered during acute Trypanosoma cruzi infection in dogs

A detailed follow-up investigation of the major parasitological, serological and phenotypic features in dogs experimentally infected with metacyclic (MT) and blood (BT) trypomastigotes of Trypanosoma cruzi strain Berenice-78, typifying vectorial and transfusional transmission of human Chagas disease, has been conducted. Although there were no changes with respect to the window of patent-parasitaemia, significant differences between MT- and BT-infected dogs in both the prepatent period (days 23 and 19, respectively) and the day of maximum parasitaemia (days 26 and 22, respectively) were recorded. A progressive enhancement in the level of T. cruzi-specific antibodies accompanied infection by both MT and BT forms, although higher IgG titres developed on days 14 and 21 following infection with MT forms. Higher Thy-1(+)/CD21(+) and lower CD4(+)/CD8(+) cell ratios, occasioned by increased levels of Thy-1(+) and CD8(+) T-cells and reduced frequencies of CD4(+) T-cells and CD21(+) B-lymphocytes, were observed in both MT- and BT-infected animals. The reduced frequency of CD14(+) leukocytes was revealed as the most relevant phenotypic feature intrinsic to T. cruzi infection independent of inoculum source. BT-specific phenotypic features included an early reduction in the percentage of circulating CD21(+) and CD14(+) leukocytes, together with a higher Thy-1(+)/CD21(+) cell ratio on day 42. On the other hand, higher levels of CD8(+) T-cells, together with a lower CD4(+)/CD8(+) cell ratio on day 28, were characteristic of MT infection. These findings emphasise the importance of inoculum source and suggest that vectorial or transfusional routes of T. cruzi infection may trigger distinct parasite-host interactions during acute Chagas disease.