Cytological Evidence of Telocyte Involvement in Skin Immune Regulation Following Jet Needle-Free Injection of an Inactivated Porcine Circovirus Vaccine

A recently identified mesenchymal cell population, telocytes (TCs), has been found in many tissues of different animal species. Jet needle-free injection (JNFI) is a promising non-invasive drug-delivery method that can trigger effective immune responses in the skin. In this preliminary morphological study, an inactivated porcine circovirus vaccine was delivered into pig neck skin by JNFI, and untreated normal neck skin served as the control. Histopathology, immunohistochemistry (IHC), immunofluorescence (IF), and transmission electron microscopy (TEM) were used to evaluate TC distribution, ultrastructure, and selected quantitative TEM parameters 24 h after injection. TCs were widely distributed in porcine skin, located between collagen fibers and around blood vessels, adipocytes, and sweat glands. They were also observed in contact with mast cells. TCs around sweat glands were CD34+, Vimentin+, and α-SMA+, whereas TCs at other sites were CD34+, Vimentin+, and α-SMA−. After JNFI, inflammatory cell infiltration into the skin was observed; TCs were in contact with these cells, and TCs surrounding adipocytes redistributed into the adjacent loose connective tissue. Quantitative TEM analysis showed that TC profile density and visible telopod length did not differ significantly between normal skin and JNFI 24 h skin (p ≥ 0.05). In contrast, vesicle profiles per TC increased significantly in both perivascular and adipose-associated compartments (p < 0.05). These findings provide morphological evidence suggesting that TCs may participate in early cutaneous responses after JNFI vaccine delivery.