Natural Killer Cell Tolerance Persists Despite Significant Reduction of Self MHC Class I on Normal Target Cells in Mice

Abstract

BackgroundA major group of murine inhibitory receptors on Natural Killer (NK) cells belong to the Ly49 receptor family and recognize MHC class I molecules. Infected or transformed target cells frequently downmodulate MHC class I molecules and can thus avoid CD8+ T cell attack, but may at the same time develop NK cell sensitivity, due to failure to express inhibitory ligands for Ly49 receptors. The extent of MHC class I downregulation needed on normal cells to trigger NK cell effector functions is not known.Methodology/Principal FindingsIn this study, we show that cells expressing MHC class I to levels well below half of the host level are tolerated in an in vivo assay in mice. Hemizygous expression (expression from only one allele) of MHC class I was sufficient to induce Ly49 receptor downmodulation on NK cells to a similar degree as homozygous expression, despite a strongly reduced cell surface level of MHC class I. Co-expression of weaker MHC class I ligands in the host did not have any further effect on the degree of Ly49 downmodulation. Furthermore, a single MHC class I allele could downmodulate up to three Ly49 receptors on individual NK cells. Only when NK cells simultaneously expressed several Ly49 receptors and hemizygous MHC class I levels, a putative threshold for Ly49 downmodulation was reached.ConclusionCollectively, our findings suggest that in interactions between NK cells and normal untransformed cells, MHC class I molecules are in most cases expressed in excess compared to what is functionally needed to ensure self tolerance and to induce maximal Ly49 downmodulation. We speculate that the reason for this is to maintain a safety margin for otherwise normal, autologous cells over a range of MHC class I expression levels, in order to ensure robustness in NK cell tolerance.

Simulations of the NK Cell Immune Synapse Reveal that Activation Thresholds Can Be Established by Inhibitory Receptors Acting Locally

“…When the signal inhibition radius is small, the inhibition is more influenced by the precise locations of bound molecules on the grids, but when the radius is very large, inhibition spreads over almost the whole grid, and the signal's decrease with HLA numbers becomes faster than linear for all molecule numbers. A similar saturation effect was observed in measurements of NK cell cytotoxicity as a function of HLA molecule numbers (73,81). The strong experimentally observed saturation effect was more similar to the one observed here under the distance-based model than to the results from the quantity-based model.…”

“…Unlike in the quantity-based model, the effect of decreasing MICA numbers is not seen, but there is a very clear saturation effect as a function of HLA numbers: at some point, increasing HLA numbers does not lead to more inhibition, as most KIRs are already bound to their ligands. Similar effects were observed by Brodin et al (81). This latter saturation effect is similar to the experimental observations by Almeida et al (73,81), where specific thresholds in the level of target cell HLA-C had to be exceeded to inhibit cytotoxicity and to cause segregation of HLA-C from ICAM-1 at the synapse; beyond that HLA-C level, however, inhibition always occurred to the same extent.…”

“…Similar effects were observed by Brodin et al (81). This latter saturation effect is similar to the experimental observations by Almeida et al (73,81), where specific thresholds in the level of target cell HLA-C had to be exceeded to inhibit cytotoxicity and to cause segregation of HLA-C from ICAM-1 at the synapse; beyond that HLA-C level, however, inhibition always occurred to the same extent. In contrast to the quantity-based .…”

Naive Donor NK Cell Repertoires Associated with Less Leukemia Relapse after Allogeneic Hematopoietic Stem Cell Transplantation

“…Therefore, the contribution of self-KIR + adaptive NK cells to relapse protection in this context depends on mechanisms that override inhibitory KIR or, alternatively, on a complete loss of HLA class I on leukemic blasts. Studies in mice showed that 50% MHC class I expression is sufficient to maintain tolerance (48). Although some studies suggested that the expression of HLA class I is low in myeloid malignancies (49), this was relative to the much higher levels on lymphoid cells (autologous T cells).…”

NK cells control breast cancer and related cancer stem cell hematological spread

“…Each group was further divided into three subgroups in which the NK cell compartment was manipulated by either depleting NK cells with a neutralizing antibody (TMb1) 28,29 or by activating NK cell cytotoxicity with an interferon type I inducer (tilorone). 30 The third group of mice was treated with PBS as a control. Twenty-one days after cell injection, mice were sacrificed and lung metastases were counted and analyzed for size after H&E staining.…”