Lymph-Borne Chemokines and Other Low Molecular Weight Molecules Reach High Endothelial Venules via Specialized Conduits While a Functional Barrier Limits Access to the Lymphocyte Microenvironments in Lymph Node Cortex

Abstract

Lymph-borne, soluble factors (e.g., chemokines and others) influence lymphocyte recirculation and endothelial phenotype at high endothelial venules (HEVs) in lymph node cortex. Yet the route lymph-borne soluble molecules travel from the subcapsular sinus to the HEVs is unclear. Therefore, we injected subcutaneously into mice and rats a wide variety of fluorophore-labeled, soluble molecules and examined their distribution in the draining lymph nodes. Rather than percolating throughout the draining lymph node, all molecules, including microbial lipopolysaccharide, were very visible in the subcapsular and medullary sinuses but were largely excluded from the cortical lymphocyte microenvironments. Exclusion prevailed even during the acute lymph node enlargement accompanying viral infection. However, low molecular mass (MW) molecules, including chemokines, did gain entry into the cortex, but in a very defined manner. Low MW, fluorophore-labeled molecules highlighted the subcapsular sinus, the reticular fibers, and the abluminal and luminal surfaces of the associated HEVs. These low MW molecules were in the fibers of the reticular network, a meshwork of collagen fibers ensheathed by fibroblastic reticular cells that connects the subcapsular sinus floor and the HEVs by intertwining with their basement membranes. Thus, low MW, lymph-borne molecules, including chemokines, traveled rapidly from the subcapsular sinus to the HEVs using the reticular network as a conduit.

Distinct molecular composition of blood and lymphatic vascular endothelial cell junctions establishes specific functional barriers within the peripheral lymph node

“…In contrast, APC and soluble antigens arrive via afferent lymphatic vessels and enter the lymph node via the subcapsular sinus. Functional studies using fluorescently labeled tracers have previously demonstrated that soluble antigens do not freely diffuse into the lymph node parenchyma containing the lymphocytes, but are rather contained within the subcapsular sinus [3]. Most of the lymph is channeled around the lymphocyte compartment towards the medullary sinuses, a labyrinth of lymphatic vessels and sinuses at the hilus of the lymph node.…”

“…Two distinct distribution patterns of soluble lymph-borne molecules in the lymph node have been reported. Large molecules are retained in subcapsular and medullary sinuses, whereas small lymph-borne molecules with a molecular weight below 70 kDa or a diameter smaller than 4 nm can gain access to the cortex of lymph nodes [3]. Their transport is directed by a network of collagen type I -containing reticular fibers ensheathed by a single layer of fibroblastic reticular cells (FRC) [4].…”

“…The reticular network connects the subcapsular sinus and HEV by interweaving its collagen fibers with those of the extracellular matrix of both compartments [5]. The extracellular matrix inside the FRC network has been termed conduit [6] due to its function as transporting system for small soluble antigens from the afferent lymph directly to the HEV, without percolating through the lymphocyte compartment [3]. Besides its critical function in directing the transport of soluble antigens, the FRC network orchestrates the directed movement of dendritic cells (DC), T and B cells within the lymph node to enhance the frequency of their encounters [4,7].…”

The Lymph Node Revisited: Development, Morphology, Functioning, and Role in Triggering Primary Immune Responses

“…It is not clear how this could be concluded since Kelly had not carried out a tridimensional analysis of the presumed cords, a necessary task given their reportedly complex geometry and the acknowledged difficulty of distinguishing them in normal lymph nodes (Kelly, 1975;Gretz et al, 1997). Moreover, it is disconcerting that the ''paracortical cords'' were not mentioned 1 year earlier in a proposal on how ''paracortex'' morphology facilitates the triggering of a cellular response (Gretz et al, 1996) or a few years later in an article examining APC-lymphocyte interaction (Gretz et al, 2000). In our opinion, the existence of these cords is unsubstantiated.…”

“…Injecting instead a foreign albumin, Sainte-Marie and Peng (1986) observed its transport by fibers running through the thickness of the subcapsular sinus, subsinus layer, extrafollicular zone, deep cortex periphery, and adjoining HEV basement membranes; no endothelial permeation was detected. Recently, Gretz et al (2000) stated that drained fluorophore-labeled low MW dextrans and proteins diffused from peri-HEV fibers in between HEV cells and into an HEV's lumen. However, in the absence of counterstaining with a standard technique, it is questionable whether the small pale structures in their supporting Fig.…”