Killer inhibitory receptor mismatching and liver transplantation outcome

The Predictive Value of Soluble Major Histocompatibility Complex Class I Chain-Related Molecule A (MICA) Levels on Heart Allograft Rejection

BACKGROUND

Recently the presence of a soluble form of major histocompatibility complex class I chain-related molecule A (sMICA) has been detected in the sera of patients with tumors. Shedding of sMICA by tumor cells downregulates NKG2D-mediated antitumor immunity. The aim of this investigation was to study the possible involvement of sMICA in the allograft acceptance after heart transplantation (HTX).

METHODS

We monitored the levels of sMICA by specific enzyme-linked immunosorbent assay (ELISA) in a total of 146 serum samples obtained from 34 heart transplantation patients followed up during the first year post-HTX.

RESULTS

The persistence of sMICA expression was correlated with the clinical evolution of these patients. sMICA was detected in the serum of 21 of 34 patients (61.70%) between 15 and 20 days after implantation and was practically absent in pretransplant serum samples. Twenty of these 21 patients (95.24%) with sMICA did not experience episodes of severe rejection during this period (P = 0.0001), whereas sMICA was practically absent in patients with manifestations of severe acute rejection. The longitudinal study of these patients revealed that the presence of sMICA was consistently maintained in 75% of the patients with good graft status during the period of observation.

CONCLUSION

This has led us to believe that the presence of levels of sMICA during the first year post-HTX may contribute to allograft acceptance. Additionally, functional studies indicate that sMICA downregulates NKG2D surface expression, which may lead to a functional impairment of cell-mediated cytolysis. These data suggest a significant correlation between the presence of sMICA and a lower incidence of rejection.

Activation of human microvascular endothelial cells with TNF-alpha and hypoxia/reoxygenation enhances NK-cell adhesion, but not NK-Cytotoxicity

BACKGROUND

Ischemia/reperfusion injury (I/R) and cellular rejection in solid organ transplantation are characterized by adhesion molecule up-regulation on the graft endothelium, a prerequisite for leukocyte recruitment. The contribution of NK cells to I/R and allograft rejection is not well understood. The aim of the present study was to investigate allogeneic interactions between human NK cells and microvascular endothelial cells (MVEC) with special regard to the differential impact of TNF-alpha and hypoxia/reoxygenation in an in vitro model of I/R.

METHODS

MVEC were stimulated in vitro for 8 h with TNF-alpha, exposed to hypoxia (1% O2), hypoxia/reoxygenation, and combinations thereof in a hypoxia chamber. Cell surface expression of adhesion molecules on MVEC was analyzed by flow cytometry, and adhesion molecule shedding by ELISA. NK cell adhesion on MVEC was determined under shear stress, and NK cytotoxicity using Cr-release assays.

RESULTS

Surface expression of ICAM-1, VCAM-1, and E-/P-selectin on MVEC was up-regulated by TNF-alpha but unaffected by hypoxia/reoxygenation in the absence of TNF-alpha. ICAM-1 expression was further increased by a combination of TNF-alpha and hypoxia/reoxygenation, whereas TNF-alpha-induced E-/P-selectin expression was strongly reversed by hypoxia/reoxygenation. NK cell adhesion increased after exposing MVEC to TNF-alpha and hypoxia/reoxygenation. Susceptibility of MVEC to NK cytotoxicity was enhanced by TNF-alpha and slighty reduced by hypoxia/reoxygenation.

CONCLUSIONS

Endothelial activation with TNF-alpha, but not hypoxia/reoxygenation, induced NK cytotoxicity whereas the combination thereof induced the strongest NK cell adhesion. Our findings suggesting a role for NK cells in allograft responses support the development of anti-inflammatory treatment strategies to prevent I/R.

The changing role of natural killer cells in solid organ rejection and tolerance

Natural killer (NK) cells have emerged as a particular focus of interest in transplantation due to their ability to distinguish allogeneic major histocompatibility complex (MHC) antigens and their potent cytolytic effector mechanisms. Once relegated to the field of bone marrow transplantation, NK cells have recently been shown to participate in the immune response against solid organ allo- and xenografts. These new findings suggest that the role of NK cells in solid organ rejection and tolerance needs to be reexamined.

Role of natural killer cell subsets in cardiac allograft rejection

To achieve donor-specific immune tolerance to allogeneic organ transplants, it is imperative to understand the cell types involved in acute allograft rejection. In wild-type mice, CD4(+) T cells are necessary and sufficient for acute rejection of cardiac allografts. However, when T-cell responses are suboptimal, such as in mice treated with costimulation-targeting agents or in CD28-deficient mice, and perhaps in transplanted patients taking immunosuppressive drugs, the participation of other lymphocytes such as CD8(+) T cells and NK1.1(+) cells becomes apparent. We found that host NK but not NKT cells were required for cardiac rejection. Ly49G2(+) NK cells suppressed rejection, whereas a subset of NK cells lacking inhibitory Ly49 receptors for donor MHC class I molecules was sufficient to promote rejection. Notably, rejection was independent of the activating receptors Ly49D and NKG2D. Finally, our experiments supported a mechanism by which NK cells promote expansion and effector function of alloreactive T cells. Thus, therapies aimed at specific subsets of NK cells may facilitate transplantation tolerance in settings of impaired T-cell function.

Immunobiology of Natural Killer Lymphocytes in Transplantation

Natural killer (NK) lymphocytes are powerful effector cells of the peripheral immune system. NK cell functions are controlled by the expression of a variety of cell surface receptors with either inhibitory or activating roles. The genetic and functional diversity of this repertoire of receptors and the role of human leukocyte antigen class I histocompatibility molecules as a major group of NK receptor ligands endows NK cells with an innate alloreactive capacity. Early studies of experimental bone marrow transplantation revealed an important role for NK cells in the rejection of allogeneic grafts and contributed significantly to our understanding of NK cell behavior. Both animal models and in vitro studies have since implicated NK cells as contributors to the pathology of clinical transplantation. However, recent clinical studies have demonstrated the potential benefits of exploiting NK cell alloreactivity in mismatched hematopoietic stem cell transplantation for particular types of acute leukemia. Future investigations of NK cell alloreactive functions will undoubtedly reveal additional roles and potential therapeutic applications of this fundamental cell type in clinical transplantation.

Natural killer receptor repertoires in transplantation

Natural killer (NK) lymphocytes are potent effector cells that are controlled by the expression of a variety of cell surface receptors with either inhibitory or activating functions. The genetic and functional diversity of this receptor repertoire and the role of HLA class I molecules as a major group of NK receptor ligands create an innate alloreactive capacity in this cell type. Both animal models and in vitro studies have implicated NK cells as contributors to the pathology of clinical transplantation. However, recent clinical studies have indicated the potential benefit of exploiting NK cell alloreactivity in mismatched haematopoietic stem cell transplantation. Further investigations of NK cell alloreactivity will undoubtedly reveal additional applications of this fundamental cell type in clinical transplantation.