J Immunol. 2026 Nov 2;215(11):vkag313. doi: 10.1093/jimmun/vkag313.
ABSTRACT
Recently, primary natural killer (NK) cells have been engineered to express a transgenic T-cell receptor (NK:TCR), enabling recognition of intracellular and extracellular tumor antigens via HLA-restricted presentation. While this strategy broadens antigen specificity, NK cells lack canonical co-stimulatory receptors required for optimal T-cell activation. In T cells, CD28 engagement with CD80 and CD86 provides the essential “signal 2” that drives proliferation, differentiation, and cytokine production. We therefore examined whether expression of wild-type CD28 could facilitate co-stimulatory function to NK:TCR cells. Wild-type CD28 was introduced into NK-92 NFAT reporter cells and primary NK cells to assess its activity in the absence of TCR signaling. In parallel, CD28 was co-expressed with either relevant or irrelevant TCRs to evaluate its role in TCR-mediated responses. Our results show that CD28 signaling in NK cells differs from its role in T cells. CD28 functioned as a primary activating receptor, inducing activation upon interaction with CD80 or CD86 even in the absence of TCR engagement, resulting in the lysis of CD80- and/or CD86-positive cells, including antigen-presenting cells. This effect required the intracellular signaling domain of CD28. Blocking experiments demonstrated that both CD80 and CD86 are sufficient to drive activation, with no evidence for additional ligands. Notably, the presence of an irrelevant TCR did not restrain CD28-driven activation. These findings reveal fundamental differences in CD28-mediated signaling between NK and T cells and highlight a potential safety concern for NK-based therapies, as full-length CD28 expression may promote TCR-independent activation and cytotoxicity toward CD80/CD86-expressing cells.
PMID:43024831 | DOI:10.1093/jimmun/vkag313