According to the 'missing-self' hypothesis, natural killer (NK)-cell tolerance is maintained by the interaction of inhibitory NK-cell receptors with MHC class I molecules expressed at the surface of autologous cells. However, NK cells from MHC-class-I-deficient humans maintain self-tolerance — so how does this occur? Megan McNerney and colleagues now show that there is another system for controlling NK-cell tolerance, in which engagement of 2B4 (also known as CD244) at the surface of mouse NK cells by CD48 at the surface of autologous cells provides a 'stop' signal for NK-cell cytotoxicity.

Previous studies have shown that engagement of 2B4 can inhibit NK-cell responses to tumour cells. In this study, the authors first asked whether both Ly49 molecules and 2B4 (which are inhibitory NK-cell receptors in mice) are required for the inhibition of NK cells or whether either system alone is sufficient. In NK-cell cytotoxicity assays, maximal cytotoxicity was detected in the absence of both inhibitory systems, showing that they are non-redundant for protection against NK-cell cytotoxicity.

So, is 2B4 responsible for maintaining NK-cell tolerance in the absence of MHC class I expression: for example, in β2-microglobulin (β2m)-deficient mice? In cytotoxicity assays carried out in the absence of 2B4–CD48-mediated inhibition, β2m−/− NK cells had a higher lytic capacity, indicating that the 2B4–CD48 system normally operates in the absence of MHC class I expression. Similar results were obtained in the reverse situation, using NK cells from C57BL/6 mice that lack all known MHC-class-I-engaging inhibitory receptors but retain expression of 2B4 (and express MHC class I molecules).

The authors then investigated the role of 2B4 in an in vivo situation, by looking at the effect of 2B4 on the elimination of syngeneic cells in bone-marrow transplants. Wild-type and 2b4−/− mice were injected with mixed bone marrow containing labelled β2m-deficient bone-marrow cells from C57BL/6 mice, as well as wild-type C57BL/6 bone-marrow cells labelled with a different dye. Spleens were isolated 2 days later, and the remaining donor cells were counted. Compared with wild-type mice, 2b4−/− mice showed greater rejection of transplanted β2m−/− cells.

These results show that 2B4–CD48 interactions are an additional system for controlling NK-cell tolerance.