CD55 organizes lipid raft-LCK signaling to potentiate NK-cell antitumor immunity.

Natural killer (NK) cells are central to antitumor immunity but rapidly lose function in the tumor microenvironment (TME). Here, we identify CD55, previously recognized as a complement regulatory protein, as an inducible membrane organizer that coordinates activating receptor signaling to potentiate NK cell-mediated antitumor responses. Upon initial tumor encounter, NK cells upregulate CD55 via an NKG2D-p65 transcriptional axis. Unlike in T-cells, where it has a known co-stimulatory role, CD55 on NK cells directly engages tumor-expressed CD97 in trans to trigger lipid raft aggregation and LCK activation, acting as a self-sufficient primary signal initiator that drives cytotoxicity. However, upon prolonged tumor exposure, CD55 expression on NK cells progressively declines, coinciding with the well‑recognized downregulation of NKG2D upon chronic exposure. Within the TME, this loss of CD55 causally impairs NK-cell function. In cancer patients, low CD55 expression in tumor-infiltrating NK cells correlates with poor clinical outcomes. Restoring CD55 expression in both conventional and chimeric antigen receptor-engineered NK cells augments LCK signaling, enhances effector function and persistence, and improves antitumor efficacy in vivo. Thus, NK cells deploy a CD55-dependent autonomous activation mechanism upon tumor encounter, whereas chronic exposure drives CD55 loss and functional dysfunction, a state that can be therapeutically reversed by CD55 restoration.