CD28–CD3ζ signaling complex (CD28ζ)

The CD28–CD3ζ signaling complex is a synthetic intracellular signaling architecture primarily utilized in second-generation chimeric antigen receptors (CARs) to redirect T-cell activity against specific targets, such as cancer cells [1, 2]. It consists of the cytoplasmic signaling domains of the CD28 costimulatory molecule fused in tandem with the CD3ζ (zeta) chain of the T-cell receptor (TCR) complex [4, 6]. This fusion allows a single receptor to provide both the primary activation signal and the necessary costimulatory signal upon binding to a target antigen, bypassing the need for major histocompatibility complex (MHC) presentation [1, 12]. Biologically, this complex triggers rapid T-cell proliferation, robust production of cytokines like IL-2 and IFN-γ, and potent cytotoxic activity [8, 14]. In clinical practice, the CD28–CD3ζ complex is the functional engine of several FDA-approved CAR-T therapies, including axicabtagene ciloleucel, used to treat refractory B-cell malignancies [2, 7]. However, the intense and rapid signaling kinetics associated with this complex are linked to a higher risk of severe toxicities, such as cytokine release syndrome (CRS) and neurotoxicity, compared to other costimulatory domains like 4-1BB [7, 11]. Additionally, CD28-based CARs are often associated with faster T-cell exhaustion, which can limit the long-term persistence of the therapeutic cells in some patients [2, 3].

Other names

CD28-CD3zetaCD28/CD3zeta28zSecond-generation CAR signaling domainCD28-CD3ζ signalosomeCD28-CD247 signaling complex

The complex initiates T-cell activation through a dual-signaling mechanism: the CD3ζ (zeta) domain provides the primary activation signal (Signal 1) via phosphorylation of its immunoreceptor tyrosine-based activation motifs (ITAMs) and recruitment of ZAP-70, while the CD28 domain provides the costimulatory signal (Signal 2) by recruiting PI3K, Grb2, and PKCθ, collectively driving robust effector functions and proliferation [1, 14].