The suppression of protective Type 2 immunity is a principal factor driving the chronicity of helminth infections, and has been attributed to a range of Th2 cell-extrinsic immune-regulators. However, the intrinsic fate of parasite-specific Th2 cells within a chronic immune down-regulatory environment, and the resultant impact such fate changes may have on host resistance is unknown. We used IL-4gfp reporter mice to demonstrate that during chronic helminth infection with the filarial nematode Litomosoides sigmodontis, CD4+ Th2 cells are conditioned towards an intrinsically hypo-responsive phenotype, characterised by a loss of functional ability to proliferate and produce the cytokines IL-4, IL-5 and IL-2. Th2 cell hypo-responsiveness was a key element determining susceptibility to L. sigmodontis infection, and could be reversed in vivo by blockade of PD-1 resulting in long-term recovery of Th2 cell functional quality and enhanced resistance. Contrasting with T cell dysfunction in Type 1 settings, the control of Th2 cell hypo-responsiveness by PD-1 was mediated through PD-L2, and not PD-L1. Thus, intrinsic changes in Th2 cell quality leading to a functionally hypo-responsive phenotype play a key role in determining susceptibility to filarial infection, and the therapeutic manipulation of Th2 cell-intrinsic quality provides a potential avenue for promoting resistance to helminths.

Author Summary

Helminth parasites mount chronic infections in over 1 billion people worldwide, of which filarial nematode infections account for 120 million. A major barrier to the development of protective Th2 immunity lies in the dominant down-regulatory immune responses invoked during infection. Although this immune suppression is linked with a range of Th2 cell-extrinsic immune regulators, the fate of CD4+ Th2 cells during chronic infection, and the role of Th2 cell-intrinsic regulation in defining protective immunity to infection is largely unknown. In this study, we use a murine model of filarial nematode infection to show that as infection progresses the Th2 effector cells responsible for killing helminths become functionally hypo-responsive, developing a phenotype similar to adaptive tolerance or exhaustion, and their ability to clear infection becomes impaired. We further demonstrate that we can therapeutically manipulate the intrinsic functional quality of hypo-responsive Th2 cells via the PD-1/PD-L2 co-inhibitory pathway to reawaken them and enhance resistance to infection. Thus, our data provide the first demonstration that Th2 cell-intrinsic hypo-responsiveness plays a key role in determining susceptibility to helminth infection.

The immune suppression that characterizes human helminth infections can hinder the development of protective immunity or help reduce pathogenic inflammation. Signaling through the T cell co-stimulator GITR counteracts immune down-regulation by augmenting effector T cell responses and abrogating suppression by Foxp3+ regulatory T cells. Thus, super-physiological antibody-mediated GITR co-stimulation represents a novel therapy for promoting protective immunity towards parasitic helminths, whilst blocking physiological GITR-GITRL interactions may provide a mechanism for dampening pathogenic Th2 inflammation. We investigated the super-physiological and physiological role of the GITR-GITRL pathway in the development of protective and pathogenic Th2 responses in murine infection models of filariasis (Litomosoides sigmodontis) and schistosomiasis (Schistosoma mansoni). Providing super-physiological GITR co-stimulation using an agonistic anti-GITR mAb over the first 12 days of L. sigmodontis infection initially increased the quantity of Th2 cells as well as their ability to produce Th2 cytokines. However, as infection progressed the Th2 responses reverted to normal infection levels and parasite killing remained unaffected. Despite the Th2 promoting role of super-physiological GITR co-stimulation, antibody-mediated blockade of the GITR-GITRL pathway did not affect Th2 cell priming or maintenance during L. sigmodontis infection. Blockade of GITR-GITRL interactions during the acute egg-phase of S. mansoni infection resulted in reduced Th2 responses, but this effect was confined to the spleen and did not lead to changes in liver pathology. Thus, although super-physiological GITR co-stimulation can therapeutically enhance Th2 responses, physiological GITR-GITRL interactions are not required for the development of Th2-mediated resistance or pathology in murine models of filariasis and schistosomiasis.

Background: As one of the natural perturbants, infection with cytomegalovirus (CMV) is believed to play a role in the development of Type I diabetes. Using the DP-BB rat model for autoimmune diabetes, we here report about possible mechanisms responsible for R(at)CMV-induced accelerated onset of diabetes.

Methods: Rats were i.p. infected with 2 × 106 plaque forming units (pfu) RCMV and followed for diabetes development. Presence of RCMV antigens and DNA was analyzed by immunohistochemistry and PCR on pancreatic tissue and isolated islets. The effect of viral infection on peritoneal macrophages (pMΦ) and diabetes development was studied by analyzing numbers of pMΦ, virus permissiveness and by depletion of this subset by peritoneal lavage.

Results: RCMV accelerated onset of diabetes without infecting pancreatic islets. Immunohistochemistry and PCR on pancreas and isolated islets indicated that islets are non-permissive for RCMV. Infection results in an influx of pMΦ 1 day p.i. of which ~0.05% showed signs of reproductive infection. Depletion of pMΦ on days 1-3 p.i. completely counteracted the accelerating effect of RCMV.

Interpretation: RCMV accelerates onset of diabetes without infecting pancreatic islets. pMΦ might function as an carriage to disseminate virus to the pancreas where they enhance activation of autoreactive T cells resulting in accelerated onset of diabetes.

Background: Viral infections are thought to play a role in the development of autoimmune diseases like type 1 diabetes. In this study we investigated the effect of Rat Cytomegalovirus (RCMV) infection on cellular immunity in a well-defined animal model for diabetes, the Biobreeding (BB) rat.

Methods: Diabetes prone (DP)- and Diabetes resistant (DR)-BB rats were infected with 2 × 106 plaque forming units (pfu) RCMV. Diabetes development was monitored by frequent blood-glucose analysis. Effects of RCMV on CD4+, CD8+ and Vβ-TCR+ T-cell subsets were measured in vivo, and in vitro after restimulation with RCMV-infected fibroblasts. Proliferative capacity was determined by 3H-Thymidine incorporation.

Results: RCMV-infection resulted in a significant acceleration of diabetes onset in DP-BB rats ( p=0.003). Percentages CD4+ and CD8+ T-cells were not affected in vivo. In vitro, RCMV-restimulation resulted in a decreased CD4+/CD8+ blastoid T-cell ratio compared to ConA ( p=0.00028). Furthermore, RCMV-restimulation resulted in a strong RCMV-specific proliferation, which comprises about 50% of the response triggered by ConA. Vβ-TCR percentages did not change upon RCMV-infection or RCMV-restimulation.

Interpretation: RCMV-restimulation of splenic T-cells in vitro resulted in a strong RCMV-specific proliferation, probably also including autoreactive T-cells. In vivo, this polyclonal response might be involved in the observed accelerated diabetes development in DP-BB rats upon RCMV-infection.