Graft-Derived IL-6 Amplifies Proliferation and Survival of Effector T Cells That Drive Alloimmune-Mediated Vascular Rejection.

BACKGROUND: IL-6 is an inflammatory cytokine that controls effector T cell responses but the mechanisms by which it controls allogeneic immune responses and vascular rejection that leads to transplant arteriosclerosis (TA) are poorly understood. METHODS: We have examined the mechanism by which IL-6 contributes to the pathogenesis of vascular rejection and TA using a murine aortic interposition model of vascular rejection. RESULTS: The absence of IL-6 production from artery graft cells reduced the development of vascular rejection and arteriosclerotic thickening. There was no apparent effect of donor-derived IL-6 on endothelial cell integrity or on the intimal accumulation of smooth muscle cells, macrophages, and anti-donor antibodies. However, reduced vascular pathology in IL-6 artery grafts was accompanied by a significant reduction in the accumulation of CD4 and CD8 T cells. Further, the absence of graft-derived IL-6 resulted in a significant decrease in the activation and proliferation of alloreactive CD4 and CD8 T cells after transplantation as well as in a marked increase in cell death of effector T cells. Alloreactive effector T cells that expanded in the absence of IL-6 were also more susceptible to Fas-mediated activation-induced cell death in vitro. Finally, systemic neutralization of IL-6R did not reduce arteriosclerotic thickening but reduced endothelial integrity in allograft arteries, indicating differential effects of specific elimination of IL-6 in graft cells and systemic IL-6 neutralization. CONCLUSIONS: Donor-derived IL-6 amplifies the expansion of allogeneic T cell responses that cause vascular rejection and TA by increasing T cell proliferation and preventing Fas-mediated T cell death.

Mouse model of alloimmune-induced vascular rejection and transplant arteriosclerosis.

Vascular rejection that leads to transplant arteriosclerosis (TA) is the leading representation of chronic heart transplant failure. In TA, the immune system of the recipient causes damage of the arterial wall and dysfunction of endothelial cells and smooth muscle cells. This triggers a pathological repair response that is characterized by intimal thickening and luminal occlusion. Understanding the mechanisms by which the immune system causes vasculature rejection and TA may inform the development of novel ways to manage graft failure. Here, we describe a mouse aortic interposition model that can be used to study the pathogenic mechanisms of vascular rejection and TA. The model involves grafting of an aortic segment from a donor animal into an allogeneic recipient. Rejection of the artery segment involves alloimmune reactions and results in arterial changes that resemble vascular rejection. The basic technical approach we describe can be used with different mouse strains and targeted interventions to answer specific questions related to vascular rejection and TA.

Bim regulates alloimmune-mediated vascular injury through effects on T-cell activation and death.

OBJECTIVE: Bim is a proapoptotic Bcl-2 protein known to downregulate immune responses and to also be required for antigen-induced T-cell activation. However, it is not known how the effect of Bim on these offsetting processes determines the outcome of allogeneic immune responses. We have defined the role of Bim in regulating alloantigen-driven T-cell responses in a model of vascular rejection. APPROACH AND RESULTS: Bim was required for proliferation of CD4 and CD8 T cells, and for interleukin-2 production, in T cells stimulated with alloantigen in vitro. Moreover, a partial reduction in Bim expression was sufficient to attenuate T-cell activation, whereas a complete elimination of Bim was required to prevent CD4 T-cell death in response to cytokine withdrawl. When alloimmune-mediated vascular rejection was examined using an aortic interposition model, there was significantly less intimal thickening in Bim(+/-), but not Bim(-/-), graft recipients. T-cell proliferation in response to allograft arteries was significantly reduced in both Bim(+/-) and Bim(-/-) mice, but cell death was attenuated only in Bim(-/-) animals. CONCLUSIONS: Bim controls both T-cell activation and death in response to alloantigen stimulation. These processes act cooperatively to determine the outcome of immune responses in allograft arteries.

Immune-mediated vascular injury and dysfunction in transplant arteriosclerosis.

Solid organ transplantation is the only treatment for end-stage organ failure but this life-saving procedure is limited by immune-mediated rejection of most grafts. Blood vessels within transplanted organs are targeted by the immune system and the resultant vascular damage is a main contributor to acute and chronic graft failure. The vasculature is a unique tissue with specific immunological properties. This review discusses the interactions of the immune system with blood vessels in transplanted organs and how these interactions lead to the development of transplant arteriosclerosis, a leading cause of heart transplant failure.

Inflammatory cytokines determine the susceptibility of human CD8 T cells to Fas-mediated activation-induced cell death through modulation of FasL and c-FLIP(S) expression.

The nature of inflammatory signals determines the outcome of T cell responses. However, little is known about how inflammatory cytokines provided to human CD8 T cells during activation affects their susceptibility to post-activation cell death. We have examined and compared the effects of the inflammatory cytokine IL-12, as well as the combination of IL-1, IL-6, and IL-23 (IL-1/6/23) on the susceptibility of primary human CD8 T cells to post-activation cell death. Human CD8 T cells activated in the presence of IL-1/6/23 underwent significantly less cell death after activation as compared with those activated in IL-12. This was due to reduced susceptibility to Fas-mediated activation-induced cell death (AICD). Mechanistically, the reduced level of cell death in CD8 T cells activated in IL-1/6/23 was a result of a low level of FasL expression and high level of c-FLIP(S) expression. When the effect of IL-1, IL-6, and IL-23 individually was examined, IL-1 or IL-6 alone was sufficient to inhibit CD8 T cell death that occurs after activation in IL-12. IL-1, but not IL-6, inhibited expression of FasL, whereas IL-6, but not IL-1, increased c-FLIP(S) expression. Our findings show that the presence of IL-1 and/or IL-6 during activation of human CD8 T cells attenuates Fas-mediated AICD, whereas IL-12 increases the susceptibility of activated CD8 T cells to this form of cell death.

CD155 on human vascular endothelial cells attenuates the acquisition of effector functions in CD8 T cells.

OBJECTIVE: CD155 is a cell surface protein that has recently been described to exert immune regulatory functions. We have characterized the expression of CD155 on human vascular endothelial cells (ECs) and examined its role in the regulation of T-cell activation. METHODS AND RESULTS: CD155 was expressed on resting human vascular ECs and was upregulated in an interferon-γ (IFNγ)-dependent manner. When the function of CD155 in regulating T-cell activation was examined, antibody-mediated neutralization of CD155 did not affect CD8 T-cell proliferation in response to stimulation with ECs. However, neutralization of CD155 activity or small interfering RNA-mediated inhibition of CD155 expression in ECs increased expression of IFNγ and cytotoxic effector function in activated CD8 T cells. CONCLUSIONS: CD155 is an IFNγ-inducible immune regulatory protein on the surface of human ECs that attenuates the acquisition of effector functions in CD8 T cells.

TLR agonists that induce IFN-beta abrogate resident macrophage suppression of T cells.

Resident tissue macrophages (Mφs) continually survey the microenvironment, ingesting Ags and presenting them on their surface for recognition by T cells. Because these Ags can be either host cell- or pathogen-derived, Mφs must be able to distinguish whether a particular Ag should provoke an immune response or be tolerated. However, the mechanisms that determine whether Mφs promote or inhibit T cell activation are not well understood. To investigate this, we first determined the mechanism by which murine resident peritoneal Mφs suppress in vitro T cell proliferation in the absence of pathogens and then explored the effects of different pathogen-derived molecules on Mφ immunosuppression. Our results suggest that, in response to IFN-γ, which is secreted by TCR-activated T cells, resident peritoneal Mφs acquire immunosuppressive properties that are mediated by NO. However, pretreatment of Mφs with LPS or dsRNA, but not CpG or peptidoglycan, eliminates their suppressive properties, in part via the induction of autocrine-acting IFN-ß. These results suggest TLR agonists that activate TRIF, and consequently induce IFN-ß, but not those that exclusively signal through MyD88, abrogate the immunosuppressive properties of Mφs, and thus promote T cell expansion and elimination of invading microorganisms.