Circumventing tolerance at the T cell or the antigen-presenting cell surface: Antibodies that ligate CD40 and OX40 have different effects

Effector and central memory T helper 2 cells respond differently to peptide immunotherapy

Peptide immunotherapy (PIT) offers realistic prospects for the treatment of allergic diseases, including allergic asthma. Much is understood of the behavior of naive T cells in response to PIT. However, treatment of patients with ongoing allergic disease requires detailed understanding of the responses of allergen-experienced T cells. CD62L expression by allergen-experienced T cells corresponds to effector/effector memory (CD62L(lo)) and central memory (CD62L(hi)) subsets, which vary with allergen exposure (e.g., during, or out with, pollen season). The efficacy of PIT on different T helper 2 (Th2) cell memory populations is unknown. We developed a murine model of PIT in allergic airway inflammation (AAI) driven by adoptively transferred, traceable ovalbumin-experienced Th2 cells. PIT effectively suppressed AAI driven by unfractionated Th2 cells. Selective transfer of CD62L(hi) and CD62L(lo) Th2 cells revealed that these two populations behaved differently from one another and from previously characterized (early deletional) responses of naive CD4(+) T cells to PIT. Most notably, allergen-reactive CD62L(lo) Th2 cells were long-lived within the lung after PIT, before allergen challenge, in contrast to CD62L(hi) Th2 cells. Despite this, PIT was most potent against CD62L(lo) Th2 cells in protecting from AAI, impairing their ability to produce Th2 cytokines, whereas this capacity was heightened in PIT-treated CD62L(hi) Th2 cells. We conclude that Th2 cells do not undergo an early deletional form of tolerance after PIT. Moreover, memory Th2 subsets respond differently to PIT. These findings have implications for the clinical translation of PIT in different allergic scenarios.

Combination peptide immunotherapy based on T-cell epitope mapping reduces allergen-specific IgE and eosinophilia in allergic airway inflammation

Peptide immunotherapy using soluble peptides containing allergen-derived immunodominant T-cell epitopes holds therapeutic promise for allergic asthma. Previous studies in BALB/c mice using the immunodominant peptide epitope of chicken ovalbumin (p323-339) have been unable to demonstrate therapeutic effects in ovalbumin-induced allergic airway inflammation. We have previously shown that intravenous application of p323-339 can effectively tolerise p323-339-reactive T cells in a non-allergic model in C57BL/6 mice. This study aimed to assess the effects of using p323-339 immunotherapy in a C57BL/6 model of ovalbumin-induced allergic airway inflammation, identify any additional epitopes recognized by the ovalbumin-responsive T-cell repertoire in C57BL/6 mice and assess the effects of combination peptide immunotherapy in this model. Ovalbumin-reactive T-cell lines were generated from ovalbumin-immunized C57BL/6 mice and proliferative responses to a panel of overlapping peptides covering the ovalbumin sequence were assessed. Soluble peptides (singly or combined) were administered intravenously to C57BL/6 mice before the induction of ovalbumin-induced allergic airway inflammation. Peptide immunotherapy using the 323-339 peptide alone did not reduce the severity of allergic airway inflammation. An additional immunodominant T-cell epitope in ovalbumin was identified within the 263-278 sequence. Combination peptide immunotherapy, using the 323-339 and 263-278 peptides together, reduced eosinophilia in the bronchoalveolar lavage and ovalbumin-specific IgE, with apparent reductions in interleukin-5 and interleukin-13. Characterization of the T-cell response to a model allergen has allowed the development of combination peptide immunotherapy with improved efficacy in allergic airway inflammation. This model holds important potential for future mechanistic studies using peptide immunotherapy in allergy.

PD-1 signalling in CD4(+) T cells restrains their clonal expansion to an immunogenic stimulus, but is not critically required for peptide-induced tolerance

The ultimate outcome of T-cell recognition of peptide-major histocompatibility complex (MHC) complexes is determined by the molecular context in which antigen presentation is provided. The paradigm is that, after exposure to peptides presented by steady-state dendritic cells (DCs), inhibitory signals dominate, leading to the deletion and/or functional inactivation of antigen-reactive T cells. This has been utilized in a variety of models providing peptide antigen in soluble form in the absence of adjuvant. A co-inhibitory molecule of considerable current interest is PD-1. Here we show that there is the opportunity for the PD-1/PD-L1 interaction to function in inhibiting the T-cell response during tolerance induction. Using traceable CD4(+) T-cell receptor (TCR) transgenic cells, together with a blocking antibody to disrupt PD-1 signalling, we explored the roles of PD-1 in the induction of tolerance versus a productive immune response. Intact PD-1 signalling played a role in limiting the extent of CD4(+) T-cell accumulation in response to an immunogenic stimulus. However, PD-1 signalling was not required for either the induction, or the maintenance, of peptide-induced tolerance; a conclusion underlined by successful tolerance induction in TCR transgenic cells genetically deficient for PD-1. These observations contrast with the reported requirement for PD-1 signals in CD8(+) T-cell tolerance.

Dendritic cell expression of OX40 ligand acts as a costimulatory, not polarizing, signal for optimal Th2 priming and memory induction in vivo

Costimulatory cross-talk can occur at multiple cellular levels to potentiate expansion and polarization of Th responses. Although OX40L ligand (OX40L) is thought to play a key role in Th2 development, the critical cellular source of this molecule has yet to be identified. In this study, we demonstrate that OX40L expression by the initiating dendritic cell (DC) is a fundamental requirement for optimal induction of primary and memory Th2 responses in vivo. Analysis of the kinetics of the residual Th2 response primed by OX40L-deficient DC suggested a failure to stimulate appropriate expansion and/or survival of T cells, rather than an inability to polarize per se. The dependence upon OX40L was predominantly due to the provision of signaling through OX40 rather than retrograde signaling to the DC. Mechanistically, impaired Th2 priming in the absence of OX40L was not due to exaggerated regulation because there was no evidence of increased expansion or function of regulatory cell populations, suppression through IL-10 production, or hyporesponsiveness to secondary challenge. These data define a critical role for DC-derived OX40L in the induction and development of Th2 responses in vivo.

The net effect of costimulatory blockers is dependent on the subset and activation status of the autoreactive T cells

In this study, we investigated whether CD4 and CD8 autoreactive T cells have different costimulatory requirements for their activation in vitro by testing the effect of a panel of Abs specific for various costimulatory molecules. Our results showed that CD8 interphotoreceptor retinoid-binding protein-specific T cells are more dependent on costimulatory molecules for activation than their CD4 counterparts. Interphotoreceptor retinoid-binding protein-specific T cells are less dependent on costimulatory molecules in the secondary response than the primary response. We also showed that blockade of costimulatory molecules can either promote or inhibit the proliferation of autoreactive T cells, depending on the degree of activation of the cells. Our results show that anti-costimulatory molecule treatment can have diverse actions on autoreactive T cell subsets, the net effect being determined by the subset of immune cells affected and the type and dose of treatment used.

Role and mechanisms of CD4+CD25+ regulatory T cells in the induction and maintenance of transplantation tolerance

To gain transplantation tolerance between donor organs and hosts is the ultimate goal of all sorts of organ transplantations. Induction of regulatory T cells has been demonstrated to lead to transplantation tolerance. This paper will review subsets of regulatory T cells, the role and mechanisms of CD4(+)CD25(+) regulatory T cells (Tregs) in graft rejection and tolerance, pathway used by Tregs to recognized alloantigens, pathways of Tregs homing into the graft and effects of immunosuppression on Tregs. It was well known that Tregs play a pivotal role in transplantation tolerance. The mechanisms by which Tregs exert their regulatory effect in the induction and maintenance of transplantation tolerance, anthropogenically, consist of physical cell-to-cell contact with potential target cells, autocrine and paracrine properties. ICAM-1, TGF-beta, CTLA-4, GITR and OX40 (CD134), etc. are involved in the regulatory function of Tregs through cell-to-cell contact mechanism. IL-10 and TGF-beta are two important soluble mediators involved in the autocrine mechanism by which Tregs exert their regulatory function. Paracrine properties refer to re-educate potentially destructive alloresponsive T cells to gain regulatory function. All that discussed above could illustrate, at least partially, how naturally occurring Tregs exert their regulatory function in vivo as they constitute only 5-10% of peripheral CD4(+) T cells.