Characterization of a Putative Receptor Binding Surface on Skint-1, a Critical Determinant of Dendritic Epidermal T Cell Selection.

Dendritic epidermal T cells (DETC) form a skin-resident γδ T cell population that makes key contributions to cutaneous immune stress surveillance, including non-redundant contributions to protection from cutaneous carcinogens. How DETC become uniquely associated with the epidermis was in large part solved by the identification of Skint-1, the prototypic member of a novel B7-related multigene family. Expressed only by thymic epithelial cells and epidermal keratinocytes, Skint-1 drives specifically the development of DETC progenitors, making it the first clear candidate for a selecting ligand for non-MHC/CD1-restricted T cells. However, the molecular mechanisms underpinning Skint-1 activity are unresolved. Here, we provide evidence that DETC selection requires Skint-1 expression on the surface of thymic epithelial cells, and depends upon specific residues on the CDR3-like loop within the membrane-distal variable domain of Skint-1 (Skint-1 DV). Nuclear magnetic resonance of Skint-1 DV revealed a core tertiary structure conserved across the Skint family, but a highly distinct surface charge distribution, possibly explaining its unique function. Crucially, the CDR3-like loop formed an electrostatically distinct surface, featuring key charged and hydrophobic solvent-exposed residues, at the membrane-distal tip of DV. These results provide the first structural insights into the Skint family, identifying a putative receptor binding surface that directly implicates Skint-1 in receptor-ligand interactions crucial for DETC selection.

树突状表皮T细胞（DETC）形成一种皮肤驻留的γδ T细胞群，对皮肤免疫应激监测起着关键作用，包括在抵御皮肤致癌物方面具有不可替代的作用。Skint - 1这一新型B7相关多基因家族的原型成员的发现，在很大程度上解决了DETC如何与表皮独特关联的问题。Skint - 1仅由胸腺上皮细胞和表皮角质形成细胞表达，它特异性地驱动DETC祖细胞的发育，使其成为非MHC/CD1限制性T细胞选择配体的首个明确候选者。然而，支撑Skint - 1活性的分子机制尚未明确。在此，我们提供的证据表明，DETC的选择需要Skint - 1在胸腺上皮细胞表面表达，并且取决于Skint - 1膜远端可变结构域（Skint - 1 DV）内CDR3样环上的特定残基。Skint - 1 DV的核磁共振显示出Skint家族中保守的核心三级结构，但表面电荷分布差异很大，这可能解释了其独特的功能。关键的是，CDR3样环在DV的膜远端尖端形成了一个静电特性不同的表面，具有关键的带电和疏水的溶剂暴露残基。这些结果为Skint家族提供了首个结构方面的见解，确定了一个假定的受体结合表面，该表面表明Skint - 1直接参与对DETC选择至关重要的受体 - 配体相互作用。