Dendritic cell trafficking within the lymph node

Lymph node architecture

Lymph vessels from different peripheral drainage areas often congregate at one LN. The LN is composed of (i) the outer subcapsular sinus (entry side for afferent lymphatic vessels), (ii) the B cell follicles (zone of B cell activation beneath the sinus), (iii) the inner T cell paracortex (zone of T cell activation), and (iiii) the medullary sinus (exit site for leukocytes) 244 _{(Fig. 11).}

Figure 11: Lymph node architecture. Lymph fluid and dendritic cells arrive via afferent lymphatic vessels. Dendritic cells pass the sinus and area between B cell follicles (B) before entering the T cell cortex (T). Arrows indicate the route of soluble antigen along fibroblastic reticular cell (FRC) conduits. The detailed view indicates FRC conduits bridging the sinus with the HEV (scheme taken from von Andrian and Mempel 69).

The subcapsular sinus is populated by different resident cell types. Stromal retothelial cells build a 3D sponge-like structure resting on a floor consisting of sinus-lining cells and a discontinuous BM. Subcapsular macrophages settle between the retothelial cells and contribute to the extremely cell-rich composition of the sinus encasing the LN parenchyma 245_.

Both follicles and paracortex comprise immune cell types that reside in a stromal cell network. While follicles bear B cells in the follicular dendritic cell network, the paracortex is densely packed with T cells and few DCs in the sponge-like scaffold of fibroblastic reticular cells (FRCs) 246. FRCs reside on and continuously enwrap highly ordered ECM tubes termed conduits, according to their function in channelling small molecules through the cortex (Fig. 11). The conduit, varying in diameter from 200 nm to 3 µm, consists of an inner core of

Introduction

fibronectin and typical BM membrane proteins such as laminins, collagen IV and nidogens 247, 248_{. Although more than 95% of the conduit is covered with FRCs, gaps remain that are} covered by hematopoietic cells 249, 250. Mainly resident DCs localize close to FRCs enabling them to rapidly uptake (within minutes) subcutaneously injected small molecules that percolate exclusively in the LN conduits 248. The FRC fiber network is qualitatively homogenous throughout the T cell paracortex, but is described as more densely packed in a zone called “cortical ridge”. Here, in close vicinity to the B cell follicles, a high density of HEVs ensures entry of leukocytes from the blood stream into the LN 251. Of note, FRC networks and conduits are not exclusive to the LN T cell paracortex, but are found in almost similar composition in the splenic white pulp and the thymic medulla 252, 253.

Migration into and within the lymph node

The mechanisms of DC migration through the cell-rich subcapsular sinus have not been investigated so far. Locomotion kinetics obtained from intravital microscopy studies suggested that the sinus might not represent a major anatomical barrier for DCs 254. This is in line with observations on T cells that effortlessly glide into the subcapsular sinus, when exposed to an artificial chemokine gradient 255.

Unlike in the sinus, FRCs of the T cell paracortex abundantly express CCL19 and CCL21 suggesting a chemokine gradient along the sinus-cortex border. When both chemokines are absent in the cortex (as observed in plt/plt mice), DCs do not enter the LN parenchyma, arguing for a dominant role of CCR7 in LN entry 189, 190. Our knowledge about paracortical DC migration is mainly based on multiphoton microscopy studies and is still descriptive. Bone-marrow derived DCs entered the T cell paracortex and congregated close to HEVs 18 to 20 h after subcutaneous injection, before at later time points DCs were more evenly distributed 256, 257. One day after injection, DCs showed remarkable motility with average speeds of 6-7 µm/min 257, 258, before velocities decreased again and DCs integrated into a dense network of sessile DCs 259. Interestingly, endogenous dDCs also travelled close to HEVs 24 h after ear inflammation, while epidermal LCs required 72 h to localize to deeper cortex areas 203. The underlying mechanisms of this phenomenon are not clear, but differential expression of chemokine receptors such as CXCR5 might play a role 260.

How DC motility in the lymph node is sustained and how DCs generate traction for locomotion are open questions, but studies on T cell migration revealed some intriguing principles that might also hold true for DCs. Early intravital microscopy work suggested that

Introduction

T cells perform a dynamic random walk in the LN and encounters with DCs were regarded as random collisions 69, 261. However, when simultaneously visualising the stromal FRC network, T cell paths led preferentially along the reticular fiber network and turned out to be deterministic and not random 262, 263. Similar to DCs, naïve T cells express CCR7 and it appeared appealing that its ligands CCL19 and CCL21 are expressed along FRCs and might contribute to deterministic T cell migration 106, 240. Indeed, interference with CCR7 signalling on T cells decreased the basal motility almost by half 255, 264, 265. Hence, FRCs appear to be a chemokinetic and adhesive surface. Surprisingly, simultaneous functional blockade of the major integrins α4β1, α4β7 and LFA-1 on T cells did not influence T cell motility 106. This contrasts with studies on NK cells, another leukocyte subset, which required integrin α2β1- mediated close contact to FRCs for effective migration in the LN 266.

In summary, T cell motility in the paracortex requires signalling through CCR7 ligands expressed on FRCs. Although T cells migrate along the FRC network, current data argues against adhesion-mediated traction as force-generating principle for T cell movement in the LN. However, it is unknown how an alternative migration mode might mechanistically work and if adhesion-independent migration is general for all leukocytes.