Synthetic TAG epitope (Alpha-fetoprotein-derived) (TAG)

The Synthetic TAG epitope is a specialized protein domain that serves as the recognition site for Antigen Receptor Complex T cells (ARC-T cells) within the ARC-SparX platform developed by Arcellx [1][2]. Derived from the third domain of human alpha-fetoprotein (AFP), this epitope is engineered to be "cryptic," meaning it is normally occluded in the native AFP molecule to prevent off-target binding to endogenous proteins [2][5]. In the ARC-SparX system, the TAG epitope is incorporated into a soluble adapter protein called SparX (Soluble Protein Antigen-Receptor X-linker), which also contains a binding moiety for a specific tumor antigen, such as BCMA or CD123 [3][4]. The primary biological function of the TAG epitope is to facilitate the formation of a tri-molecular complex consisting of the ARC-T cell, the SparX adapter, and the target tumor cell [1][5]. The ARC-T cell expresses a synthetic receptor (the ARC) that specifically binds to the TAG epitope with high affinity [2]. This interaction redirects the T cell's cytotoxic activity toward any cell labeled with the SparX adapter, allowing for a controllable and modular approach to immunotherapy [4]. By adjusting the dose of the SparX protein or using different SparX adapters, clinicians can potentially manage toxicities like cytokine release syndrome and overcome tumor antigen heterogeneity or escape [2][3].

Other names

TAG domainAFP-derived TAGD-domain epitopeSparX TAGAFP domain III-derived epitope

Tri-molecular complex formation between ARC-T cells, SparX adapter proteins, and tumor-associated antigens, leading to T-cell receptor signaling and cytotoxic activity.