To the Editor<scp>TIGIT</scp> versus <scp>CD</scp>226: Hegemony or coexistence? (2013)

In a recent paper published in the European Journal of Immunology, Stanietsky et al. demonstrated that TIGIT inhibits the cytotoxicity of murine NK cells upon interaction with its counter-receptor CD155 expressed by the target cells 1. Interestingly, a subpopulation of the cells not only expresses TIGIT but also CD226, another counter-receptor of CD155. CD226 represents an activating receptor on cytotoxic T cells and NK cells 2. Stanietsky et al. 1 convincingly demonstrated by surface plasmon resonance that in mouse very much like in human, TIGIT binds to CD155 with higher affinity than CD226. As discussed by the authors, these findings are highly reminiscent of other well-studied antagonistic receptor/ligand pairings and a hypothesis was put forward by several groups placing the CD155/CD226/TIGIT triad in functional kinship to ligand switch systems such as CD28/CTLA-4 that share the same receptors of the B7 family 3-5. Here, CTLA-4-mediated inhibition of T-cell activation becomes dominant soon after initial stimulation via CD28 because the activator is out-competed for binding by increasing amounts of an inhibitor that displays higher affinity toward the receptor. In line with such reasoning, the authors show in a series of in vitro experiments that the inhibitory influence of TIGIT in NK-cell-mediated killing appears dominant over the activating potential of CD226. However, the view of the dominance model where the inhibitor TIGIT displaces its activating competitor from binding in the NK-cell/target-synapse neglects the fact that CD226 on cytotoxic T and NK cells is permanently associated in cis with LFA-1. This facilitates a functionally relevant tyrosine phosphorylation of CD226 via Fyn activated by the integrin 6. Therefore, it is difficult to compare the affinities obtained in vitro by using soluble purified components with the more relevant CD226/CD155 avidity effective on the surfaces of apposing cells that may be modulated by the associated integrin. Moreover, the dominance model disregards the possibility that in case of abundant CD155 expression on target cells, both TIGIT and CD226 may bind to the contacting surface in a noncompetitive manner and thus exert effects simultaneously. These assumptions are supported by several remarkable observations. Tumor cells overexpressing CD155 were shown to provoke a prominent downregulation of CD226 on locally present NK cells in vivo. This resulted in diminished NK-cell cytotoxicity via coupling onto CD155 when compared with that of NK cells that had not downregulated their CD226 levels 7. Similarly, cytotoxic T cells were shown to kill less efficiently HIV-infected targets that accomplished downregulation of CD226 on cytotoxic T cells 8. These findings illustrate an important contribution of CD226-driven activities that appear independent of a putative inhibition via TIGIT. As a key mechanism to prevent killing, inhibitory NK-cell receptors interfere with the formation of an F-actin ring underneath the periphery of the synapse underscoring the importance of LFA-1/F-actin for efficient elimination of target cells 9. Clustering of CD226 at the NK-cell/target interface could counteract inhibitor-driven synapse retraction by keeping integrin at the synapse's periphery. In addition, cross-ligation of CD226 on the surface was shown to trigger its activation in cytotoxic T and NK cells 2, a process that could establish an alternative link of the CD226/LFA-1 complex to theactin cytoskeleton via recruitment of adaptors such as 4.1G to the cytoplasmic tail of CD226. Another aspect relates to the complex society of activating and inhibitory NK-cell receptors that are unevenly distributed among cells of the NK-cell pool, thus creating a great deal of heterogeneity in the NK-cell population. With respect to CD226/TIGIT, we found that splenic NK cells from C57BL/6 mice can be subdivided into three subpopulations expressing either TIGIT or CD226 or both (Fig. 1A). Thus there is a prominent subpopulation of NK cells that is out of reach for a potential TIGIT-mediated dominance. We had already shown earlier that expression of CD226 on NK cells segregates with that of other NK-cell receptors 10 that we had suggested might play a role in NK-cell target recognition. In light of this, the killing efficiencies observed by the authors may reflect the sum of positive and negative influences rather than an inhibition that knocks out activation. We also note that Stanietsky et al. postulate the existence of hitherto unidentified ligands for TIGIT and CD226 on PBMCs since they observed binding of recombinant TIGIT as well as CD226 onto PBMCs that lacked detectable expression of CD155 (and nectin-2); however, this contrasts our own findings that peripheral blood cells express significant levels of CD155 (Fig. 1B). Although we do not know the reason for these discrepancies at present, our observations would explain the binding of the recombinant TIGIT and CD226 proteins to PBMCs. Taken together, it is conceivable that TIGIT dominance over CD226 may be restricted to a limited set of scenarios such as initial T-cell activation inducing TIGIT expression, whereas coexistence may prevail in many other cases, as for example in NK-cell-mediated destruction of target cells. The authors declare no financial or commercial conflict of interest. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.