T Lymphocyte-Endothelial Interactions: Emerging Understanding of Trafficking and Antigen-Specific Immunity

Abstract

Antigen-specific immunity requires regulated trafficking of T cells in and out of diverse tissues in order to orchestrate lymphocyte development, immune surveillance, responses, and memory. The endothelium serves as a unique barrier, as well as a sentinel, between the blood and the tissues, and as such it plays an essential locally tuned role in regulating T cell migration and information exchange. While it is well established that chemoattractants and adhesion molecules are major determinants of T cell trafficking, emerging studies have now enumerated a large number of molecular players as well as a range of discrete cellular remodeling activities (e.g., transmigratory cups and invadosome-like protrusions) that participate in directed migration and pathfinding by T cells. In addition to providing trafficking cues, intimate cell-cell interaction between lymphocytes and endothelial cells provide instruction to T cells that influence their activation and differentiation states. Perhaps the most intriguing and underappreciated of these "sentinel" roles is the ability of the endothelium to act as a non-hematopoietic "semiprofessional" antigen-presenting cell. Close contacts between circulating T cells and antigen-presenting endothelium may play unique non-redundant roles in shaping adaptive immune responses within the periphery. A better understanding of the mechanisms directing T cell trafficking and the antigen-presenting role of the endothelium may not only increase our knowledge of the adaptive immune response but also empower the utility of emerging immunomodulatory therapeutics.

Keywords: MHC; TCR; Trafficking; antigen presentation; endothelium; immunomodulation; lymphocyte; tolerance.

Figure 1

Blood–lymph circulatory system and lymphocyte trafficking. Upper panel: schematic shows the contiguous blood–lymph circulatory system. Arterial, oxygen-rich, blood (pink) flows away from the heart and into the microvasculature (arterioles, capillaries, and venules). Oxygen-depleted blood (blue) flows from the microvasculature back to the heart. Lymph (green) collected from the tissues (yellow) is taken up by the lymphatic capillaries to flow through the afferent lymphatic vessels, lymph nodes (LN), efferent vessels, and back into the blood circulation via the lymphatic duct. Local microvasculature of the LN (i.e., high endothelial venules; HEV) serves as a location for lymphocytes to enter the LN. Dark blue and dark green lines indicate the vascular and lymphatic endothelial barriers, respectively. Boxed region (lower panel) shows a segment of a postcapillary venule during the process of lymphocyte extravasation. This process evolves in stages: (1) transient rolling interactions mediated predominantly by selectins; (2) subsequent chemokine-dependent activation; (3) firm arrest, which is mediated by the binding of lymphocyte integrins (e.g., LFA-1, Mac-1, and VLA-4) to endothelial cell-adhesion molecules (e.g., ICAM-1, ICAM-2, and VCAM-1); (4) lymphocyte lateral migration on the surface of the endothelium, probing for a site to penetrate through it (tenertaxis); and (5) Lymphocytes diapedesis across the endothelial barrier to enter the interstitium either following the paracellular route (by opening a gap between two adjacent endothelial cells) or transcellular route (by migrating directly through the body of a single endothelial cell).

Figure 2

Dynamic remodeling of lymphocytes and endothelium during diapedesis (A–C). Schematic shows lymphocyte (green) and EC (blue) dynamics during T cell lateral migration over, and transcellular diapedesis across, the endothelium. (A–C) Successive time points at intervals of ~30–60 s. Dynamic insertion (~0.2–1 μm in depth) and retraction of multiple actin-rich lymphocyte invadosome-like protrusions (ILPs) into the apical surface of the endothelium occurs during lateral migration (A–C). Once a location of sufficiently low endothelial resistance has been identified (tenertaxis), an ILP progressively extends several micrometers in depth, ultimately breaching the endothelium transcellularly (C). Also shown is the “transmigratory cup” structure (asterisks), which consists of vertical endothelial microvilli-like projections (rich in F-actin; red, ICAM-1, VCAM-1, PECAM-1, and JAM-1) that surround the periphery of adherent lymphocytes (B–D). Electron micrograph of a T cell (green) extending multiple ILP (red asterisks) into the surface of two endothelial cells (EC1, EC2; blue) near an intact junction in order to probe for a site to initiate diapedesis (i.e., breach the endothelial barrier) (60).

Figure 3

Endothelial cells as “semiprofessional” non-hematopoietic APCs. Schematic comparison of the Ag presentation, costimulatory, coinhibitory, and adhesion molecules expressed by dendritic cells (gray) and endothelial cells (blue). Note that endothelial cells, unlike most other non-hematopoietic cells, express most of the critical molecules found in DC express. Important exceptions include CD80 and CD86 that are critical for the activation of naive T cells, as well as the costimulatory/adhesion molecules DC-SIGN. Blue, black, and orange “X”s indicate the possible sites of action for several emerging T cell/APC-directed immunomodulatory therapeutics.

Figure 4

Imaging the T cell–endothelial immunological synapse (podo-synapse). (A) Schematic (upper panel) represents formation of stabilized arrays of lymphocyte ILPs protruding into the endothelial surface (note that the labeling of the plasma membrane in green and the cytosol in red corresponds the live-cell imaging experiment, below) following antigen recognition. Images show light microscopy of T cells [by differential interference contrast (DIC) interacting with mem-YFP (green) and cytosol DsRed (ref) transfected ECs]. Podo-prints on endothelium are evidenced as rings of plasma membrane (mem-YFP) where cytosol is excluded (black areas in the “cytosol” image at 15 min). This imaging approach readily reveals the dynamics and discrete three-dimensional architecture of individual ILPs as well as the collective “podo-synapse” ILP array during Ag recognition (112). (B) Lymphocytes were incubated with activated, Ag-pulsed endothelium for 5 min, fixed, and stained as indicated and imaged by confocal microscopy. ILPs are enriched in ICAM-1 (green), LFA-1 (red), and Actin (blue), as well as many other immunological signaling molecules (e.g., TCR, MHC-II, PKC-Φ, phyosphotyrosine, and HS1 not shown) (112, 151).

Figure 5

Model for ILPs function in Ag recognition and response to Ag presented by the endothelium. (A) Schematic shows side views of a memory/effector T cell (green) interacting with the endothelium (blue) presenting cognate Ag. During lateral migration (step 1), lymphocytes dynamically drive ILPs against the opposing cell (step 2, inset 2a). Close interactions between T and APC/target cells, which are partially opposed by the cell glycocalyces (inset 2b), form at ILP tips, facilitating TCR/MHC interactions (inset 2c) in these zones that allows initiation of a response (step 3, calcium flux). (B) Avid calcium signaling (Fura-2, rainbow range indicator) response for effector/memory T cells migrating on endothelial presenting cognate antigen (lower panel) but not antigen-negative control endothelial (upper panel) (112). (C) T cells were labeled with Fura-2 and imaged live migrating on Ag-pulsed, mem-DsRed transfected endothelium. Upper panels show mem-DsRed. Arrows indicate initial ILP formation (see rings of fluorescence; see also Figure 4A). Middle panels indicate calcium flux values on a rainbow scale. Lower panels show the DIC image of the T cell–EC interaction. Note that as in (A), initial ILPs (read out hear by visualization of the cognate podo-prints) preceded the initiation of calcium flux which follows shortly after and accumulation of stabilized ILPs (i.e., formation of a podo-synapse) occurs commensurate with peak calcium flux (112).