Unidirectional signaling triggered through 2B4 (CD244), not CD48, in murine NK cells

Engagement of 2B4 (CD244) with CD48 results in activation, costimulation, or inhibition of NK cell activities, depending on the cell types and the stage of differentiation. In vivo, 2B4⁺ NK cells can interact with CD48⁺ NK cells and also with surrounding CD48⁺ hematopoietic cells. Similarly, CD48⁺ NK cells may be triggered by adjacent 2B4⁺ NK cells or other hematopoietic cells expressing 2B4, e.g., monocytes, basophils, γδ T cells, etc. As CD48 was also shown to function as an activating receptor, 2B4/CD48 binding in the settings of NK-to-NK or NK-to-non-NK cell interactions may generate bidirectional signals. To address this question, we examined the consequence of CD48 or 2B4 ligation using two experimental settings: one with target (syngeneic EL4 and allogeneic P815) cells, ectopically expressing surface 2B4 or CD48, and the other with direct cross-linking with plate-bound mAb. Here, we report that ligation of CD48 with 2B4⁺ EL4 or 2B4⁺ P815 targets, in the absence of other receptor engagement, did not alter NK cell cytotoxicity or proliferation significantly. Similarly, cross-linking of NK cells with plate-bound anti-CD48 mAb in the absence or presence of a suboptimal dose of IL-2 did not modulate NK proliferation, cytotoxicity, or cytokine production. Nonetheless, 2B4 cross-linking promoted NK cell proliferation and effector functions consistently in both settings. Therefore, our results demonstrate unequivocally that CD48 on surrounding NK or non-NK cells serves primarily as a ligand to stimulate 2B4 on the adjacent NK cells in mice.