Functional tight junction barrier localizes in the second layer of the stratum granulosum of human epidermis

Background: Mammalian epidermis has two diffusion barriers, the stratum corneum (SC) and tight junctions (TJs). We reported previously that a single living cell layer exists between the SC and TJ-forming keratinocytes in mice; however, the exact location of the TJ barrier in human epidermis has not been defined. Objective: To investigate the precise distribution of epidermal TJs in relation to various cell-cell junction proteins and the SC and to clarify the barrier function of TJs against macromolecules in human skin. Methods: The localization of various junctional proteins was investigated in human skin sections and in the roofs of bullae formed by ex vivo exfoliative toxin (ET) treatment in three dimensions. ET and single-chain variable fragments (scFv) against desmoglein 1 were used as large diffusion probes. Results: Human stratum granulosum (SG) cells have a distinct distribution of TJ, adherens junction, and desmosome proteins in the uppermost three layers (SG1-SG3 from the surface inward). Ex vivo injection of ET or scFv demonstrated that only SG2-SG2 junctions function as a TJ barrier, limiting the inside-out diffusion of these proteins. The roofs of bullae formed by ex vivo ET treatment consisted of SC, SG1 cells, and TJ-forming SG2 cells, probably mimicking bulla formation in bullous impetigo. Conclusion: Human epidermis has three SG cell layers with distinct properties just beneath the SC, of which only SG2 cells have functional TJs. Our results suggest that human epidermal TJs between SG2 cells form a paracellular diffusion barrier against soluble proteins, including immunoglobulins and bacterial toxins.