Characterization of a conduit system containing laminin-5 in the human thymus: a potential transport system for small molecules

Abstract

T cells develop in the thymus in a highly specialized cellular and extracellular microenvironment. The basement membrane molecule, laminin-5 (LN-5), is predominantly found in the medulla of the human thymic lobules. Using high-resolution light microscopy, we show here that LN-5 is localized in a bi-membranous conduit-like structure, together with other typical basement membrane components including collagen type IV, nidogen and perlecan. Other interstitial matrix components, such as fibrillin-1 or -2, tenascin-C or fibrillar collagen types, were also associated with these structures. Three-dimensional (3D) confocal microscopy suggested a tubular structure, whereas immunoelectron and transmission electron microscopy showed that the core of these tubes contained fibrillar collagens enwrapped by the LN-5-containing membrane. These medullary conduits are surrounded by thymic epithelial cells, which in vitro were found to bind LN-5, but also fibrillin and tenascin-C. Dendritic cells were also detected in close vicinity to the conduits. Both of these stromal cell types express major histocompatibility complex (MHC) class II molecules capable of antigen presentation. The conduits are connected to blood vessels but, with an average diameter of 2 μm, they are too small to transport cells. However, evidence is provided that smaller molecules such as a 10 kDa dextran, but not large molecules (>500 kDa), can be transported in the conduits. These results clearly demonstrate that a conduit system, which is also known from secondary lymphatic organs such as lymph nodes and spleen, is present in the medulla of the human thymus, and that it might serve to transport small blood-borne molecules or chemokines to defined locations within the medulla.

Neural Crest–Derived Pericytes Promote Egress of Mature Thymocytes at the Corticomedullary Junction

“…Postcapillary venules at the corticomedullary junction have large (10–50 μm) diameters (4) and are thin walled, with only a single layer of ensheathing α-SMA + pericytes (35), perhaps indicating specialization to support thymocyte egress. Our findings exclude a major role for lymphatic vessels in thymocyte egress in the young adult mouse, and the lack of LYVE1 + lymphatics in the human thymic medulla suggests a similar conclusion for human thymocyte egress (39). However, it remains possible that lymphatics contribute to egress from thymi of aged or diseased animals, where they are sometimes more prevalent (11, 40).…”

Stromal Cells in Chronic Inflammation and Tertiary Lymphoid Organ Formation

“…The close association of FRC-like cells with ECM components such as collagen fibers, laminin, and fibronectin, within areas that costain for CCL21 and contain DCs, is indicative of the presence of a conduit system similar to that described in human thymus and lymph nodes, which carry blood-borne molecules and chemokines (163,164).…”

Tenascin-C at a glance

“…However, tenascin-C has also been shown to inhibit cell migration, especially in the case of growth cones (Faissner, 1997). Tenascin-C located in the reticular fibers of the thymus controls lymphocyte influx and appears to define places of leukocyte maturation (Drumea-Mirancea et al, 2006). Similar structures identified as poorly characterized bundles of matrix (matrix tracks) may exist throughout tumors (Kaariainen et al, 2006;Spenlé et al, 2015a) where they could have a role similar to that of reticular fibers in the thymus or other lymphoid tissues ) (see poster).…”