The Leukocyte Postal Code

1. The Leukocyte Postal Code MelisAnahtar MSc in Integrated Immunology, melis.anahtar@oriel.ox.ac.uk Introduction • The human body has an amazing system to ensure that leukocytes are able to get to the right tissue at the right time. This system is conceptually similar to the postal code used for sorting mail. A person wishing to send her medical records to her doctor might address the package with OX3 9DU. The post office would recognize: • “OX” as the city of Oxford • “3” as North East Oxford and Headington • “9DU” as the John Radcliffe Hospital. • Thus, her package would be delivered to this precise address out of millions of possible mailboxes spread throughout the 250,000 km2 comprising the United Kingdom. • Instead of using letters and numbers, leukocytes express unique combinations of selectins, integrins, and chemokine receptors that enable a similar level of specificity. For example, a naïve T cell is able to find and enter the lymph node by expressing L-selectin, αLβ2 integrin, and CCR7. Another example is that a T cell that was activated in a Peyer’s patch expresses PSGL‑1, α4β7 integrin, and CCR9 in order to return to the small intestine after circulating in the blood. Five critical steps Mature T cell homing to skin DC homing to lymph node Leukocytes travel very quickly in the bloodstream and must be able to first recognize their target tissue via patterns of signaling molecules expressed on endothelial cells and then overcome high fluid pressures to adhere to these cells and squeeze out of the blood vessels. To accomplish this amazing feat, leukocytes utilize a five step process: Recognize and weakly tether to endothelial cells via selectins Slow down and sample the endothelium by rollingalong the vessel wall while Activatinghigh-affinity integrins on the leukocyte surface Utilize these strong hook-like integrins to arrest on the vessel wall, and Get into the target tissue via extravasation. Each step in the cascade ensures that the leukocytes are recruited to the right tissue at the right time. • Effector memory T cells are recruited to the skin for two distinct purposes: • - immune surveillance (steady-state conditions) • - responding to a pathogen (inflamm. conditions) • The exact mechanisms of T cell homing for these two situations are not completely known, and there has been controversy in the past few years over the importance of various cytokine receptors. • Dendritic cells (DC) that have been activated in the periphery must make their way into the nearest lymph node to initiate a T cell-dependent immune response or tolerance. • An activated DC must change its membrane expression in order to enter the lymph node and then effectively activate T cells once inside. • A lymph node-bound DC down regulates CCR1, CCR2, and CCR5, which are associated with type 1 responses, and upregulatesCCR7 and CXCR4, which enable entry into the lymph node. Thus, DCs and lymphocytes use similar “addresses” to gain entry into the lymph node. • DCs are also important in priming lymphocytes. Neutrophil homing • Neutrophils are rapidly recruited to the site of infection by tissue macrophages and mast cells in order to trigger inflammation. • Neutrophils adhere using the same mechanisms as lymphocytes. • Tethering and rolling are mediated by PSGL-1 on the neutrophil’ssurface binding to P- and E-selectin on the endothelium. • CXCR1 and CXCR5 are used to interact with inflammatory cytokines expressed by endothelial cells. • Activation of the β2 integrins LFA-1 and αMβ2 on the neutrophils enable rolling and arrest using ICAM-1. Mature lymphocyte homing to mucosal tissues • The mucosal system is made up of gut, bronchial, and nasal-associated lymphoid tissues. • T cells that have been activated in the mucosal-associated lymphoid tissue (MALT) inductive sites—Peyer’s patches, mesenteric lymph nodes, tonsils, etc.—acquire the mucosal-homing receptors required to return to the MALT effector sites from the bloodstream after draining through the mesenteric and cervical lymph nodes. • The “address” of mucosal homing lymphocytes must include CCR9 and active α4β7integrin. • Note: neither CLA nor L-selectin are expressed on these cells, which ensures that gut-activated lymphocytes return to the gut. Naïve lymphocyte circulation and entry into lymph nodes Conclusion • The selective recruitment of leukocytes is fundamentally important for maintaining a functional immune system. • It is a very complex and dynamic process involving many cell types, receptors, and ligands. • The level of detail known about this “postal code” is increasing by the day, as new genetic and cellular techniques allow scientists to create transgenic mice and compare the gene expression of different cellular subsets. • Greater understanding of this “postal code” may lead to therapies for diseases such as ulcerative colitis, Crohn’s disease, and cancer. • A naïve lymphocyte must continuously recirculate and sample the secondary lymph nodes to find its cognate antigen. • A naïve lymphocyte must enter and exit the secondary lymphoid tissues hundreds or thousands of times. • It carries a very general “address” comprised of L-selectin, αLβ2, and CCR7 in order to do so. Selected References • Zarbock A, Abram CL, Hundt M, Altman A, Lowell CA, Ley K. J Exp Med. 2008 Sep 29;205(10):2339-47. • Rot A, von Andrian UH. Annu Rev Immunol. 2004;22:891-928. • Forster R, Schubel A, Breitfeld D, Kremmer E, Renner-Muller I, Wolf E, et al. Cell. 1999 Oct 1;99(1):23-33. • Zabel BA, Agace WW, Campbell JJ, Heath HM, Parent D, Roberts AI, et al. J Exp Med. 1999 Nov 1;190(9):1241-56. • Rott LS, Briskin MJ, Andrew DP, Berg EL, Butcher EC. J Immunol. 1996 May 15;156(10):3727-36. • Sallusto F, Schaerli P, Loetscher P, Schaniel C, Lenig D, Mackay CR, et al. Eur J Immunol. 1998 Sep;28(9):2760-9. 36. • Iyer CV, Evans RJ, Lou Q, Lin D, Wang J, Kohn W, et al. Exp Hematol. 2008 Sep;36(9):1098-109.