Induction of B-cell immune tolerance by antigen-modified cytotoxic T lymphocytes

Siglecs induce tolerance to cell surface antigens by BIM-dependent deletion of the antigen-reactive B cells

Infusion of blood cells from a donor can induce humoral tolerance in a recipient and increase the probability of successful organ transplant, a clinical method defined as donor-specific transfusion (DST). Despite the clinical success of DST, the immunological mechanisms by which blood cells displaying a foreign Ag induce tolerance remain poorly understood. Based on recent findings showing that the B cell siglecs, CD22 and Siglec-G, can promote tolerance to Ags presented on the same surface as their ligands, we speculated that the B cell siglecs are key players in tolerance induced by DST. Using a variety of chemical and genetic approaches, we show that the B cell siglecs mediate tolerance to cell surface Ags by initiating an inhibitory signal that culminates in elimination of the Ag-reactive B cell. CD22 and Siglec-G are recruited to the immunological synapse by sialic acid ligands on the Ag-bearing cells, producing a tolerogenic signal involving Lyn and the proapoptotic factor BIM that promotes deletion of the B cell and failure of mice to develop Abs to the Ag upon subsequent challenge. We speculate that this tolerogenic mechanism is a contributing factor in DST and a mechanism of peripheral B cell tolerance to cell surface autoantigens.

Modulation of CD8+ T cell responses to AAV vectors with IgG-derived MHC class II epitopes

Immune responses directed against viral capsid proteins constitute a main safety concern in the use of adeno-associated virus (AAV) as gene transfer vectors in humans. Pharmacological immunosuppression has been proposed as a solution to the problem; however, the approach suffers from several potential limitations. Using MHC class II epitopes initially identified within human IgG, named Tregitopes, we showed that it is possible to modulate CD8⁺ T cell responses to several viral antigens in vitro. We showed that incubation of peripheral blood mononuclear cells with these epitopes triggers proliferation of CD4⁺CD25⁺FoxP3⁺ T cells that suppress killing of target cells loaded with MHC class I antigens in an antigen-specific fashion, through a mechanism that seems to require cell-to-cell contact. Expression of a construct encoding for the AAV capsid structural protein fused to Tregitopes resulted in reduction of CD8⁺ T cell reactivity against the AAV capsid following immunization with an adenoviral vector expressing capsid. This was accompanied by an increase in frequency of CD4⁺CD25⁺FoxP3⁺ T cells in spleens and lower levels of inflammatory infiltrates in injected tissues. This proof-of-concept study demonstrates modulation of CD8⁺ T cell reactivity to an antigen using regulatory T cell epitopes is possible.

The use of donor-derived veto cells in hematopoietic stem cell transplantation

The induction of immune tolerance by specific agents, as opposed to general immune suppression, is a most desirable goal in transplantation biology. One approach to attain this goal is afforded by the use of donor-derived cells endowed with veto activity, which is the ability of a cell to specifically suppress only T cells directed against its antigens. A megadose of purified veto CD34(+) hematopoietic stem cells is already used in patients to allow hematopoietic stem cells transplantation (HSCT) across major genetic barriers, while avoiding severe graft versus host disease (GVHD). However, allowing engraftment of such T cell-depleted HSCT under safer reduced intensity conditioning (RIC) protocols still remains a challenge. Therefore, combining megadose of CD34(+) HSCT with other GVHD-depleted veto cells could enable facilitation of engraftment of HSCT under RIC without the adverse complication of GVHD. This approach might provide a safer modality for enabling engraftment of HSCT, enabling its application in elderly patients who cannot tolerate intensive protocols and to a variety of patients with non-malignant disorders, associated with longer life expectancy, in whom the use of a high risk conditioning cannot be considered.

Altered differentiation, diminished pathogenicity, and regulatory activity of myelin-specific T cells expressing an enhanced affinity TCR

Whereas increased affinity enhances T cell competitiveness after immunization, the role of affinity in modulating the pathogenicity of self-reactive T cells is less established. To assess this, we generated two myelin-specific, class II MHC-restricted TCR that differ only in a buried hydroxymethyl that forms a common TCR β-chain V region variant. The variation, predicted to increase TCR stability, resulted in a ~3log(10) difference in TCR sensitivity with preserved fine specificity. The high-affinity TCR markedly diminished T cell pathogenicity. T cells were not deleted, did not upregulate Foxp3, and barring disease induction were predominantly naive. However, high-affinity CD4(+) T cells showed an altered cytokine profile characterized by the production of protective cytokines prior to experimental allergic encephalomyelitis induction and decreased effector cytokines after. Further, the high-affinity TCR promoted the development of CD4(-)CD8(-) and CD8(+) T cells that possessed low intrinsic pathogenicity, were protective even in small numbers when transferred into wild-type mice and in mixed chimeras, and outcompete CD4(+) T cells during disease development. Therefore, TCR affinities exceeding an upper affinity threshold may impede the development of autoimmunity through altered development and functional maturation of T cells, including diminished intrinsic CD4(+) T cell pathogenicity and the development of CD4(-)Foxp3(-) regulatory populations.