MIMM Trevor Owens Mon Oct 4 - Lymphoid tissue - organization and development Wed Oct 6 - Lymphocyte traffic and adhesion signaling Fri Oct 8 - B cell tolerance - deletion versus anergy Mon Oct 18 - Transgenic models of B cell tolerance Cyster JG. Lymphoid organ development and cell migration. Immunol Rev 2003;195:5-14.

Secondary lymphoid tissue Lymph nodes (LN) Axillary, inguinal, brachial, popliteal - peripheral LN Cervical - brain associated Mesenteric - gut associated Peyer’s Patches (PP) - gut associated (embedded) Spleen Naïve lymphocytes tend to go to secondary lymphoid tissue Memory cells favour migration to tertiary sites (skin, lamina propria, joints, lung) Migratory behaviour/properties Naïve - homogeneous Memory - heterogeneous (eg. cytokine subsets, tissue specificity)

Mucosa-associated lymphoid tissues Antigens enter across the mucosal epithelium. Mucosal epithelial cells immediately overlying the lymphoid tissue are specialized for uptake of antigen from the lumen of the mucosa. M cells overlying PP in the gastrointestinal tract have been shown to transport particles as large as intact microorganisms

Spleen Largest single lymphoid organ in mammals Contains up to 25% of the body’s mature lymphocytes White pulp - organized lymphoid compartment in which regulated activation and maturation of antigen-dependent B and T cells occur. Unlike the LN, the spleen has a single vascular supply - immune cells and antigen enter tissue via the splenic artery. In humans, approximately 5% of the total cardiac output is directed through the spleen PALS (peri-arteriolar lymphoid sheath) - contains interdigitating dendritic cells (antigen-presenting cells). The central arteriole, after it penetrates the PALS, forms a marginal sinus, lined with a mucosal addressin cell adhesion molecule-1 (MAdCAM-1)–expressing endothelium

Lymph nodes - ‘...aggregates of lymphoid cells …. interspersed throughout the lymphatic system of mammals.’ ‘.. effective filters of microbial and particulate material which has gained access to the lymph…sites of immunoglobulin synthesis, and cell-mediated immunity.’ Lymphocytes = 3X weight of lymph node leave every hour in efferent lymph 90% of cells in efferent lymph recently left the blood to enter the LN - may recirculate 1-2 times/day Therefore, most cells are ‘in transit’ Meeting place for constituents of immunological reactions

Paracortex - medullary cords, populated prominently by macrophages and plasma cells - lead to the medullary sinus. Paracortical cord - functional unit of the paracortex - stretches from the base of a B cell follicle to an underlying medullary cord. Approximately 100–1000  m in diameter - thought to provide a space in which antigen-presenting cells (primarily DC) can encounter rare antigen-specific T lymphocytes Each segment of paracortex composed of hundreds of paracortical cords. In the area of the primary B cell follicles, the tissue is penetrated by only a few fibers. This suggests that the requirements for cell trafficking change dra- matically when cells leave the paracortex and enter the follicle structure.

Mediators of secondary lymphoid tissue development Lymphotoxin (LT1  ß2)-binds to LTßR FDC’s, follicles, T zone stromal cells B cell (+ some T cell)-derived LT critical for spleen FDC’s May be other cell sources for lymph node development, since GC’s continue to form in LN in LT-deficient -mice B LTß KO mice - splenic CXCL13 reduced to about 50%, T zone chemokines unaffected TNFalpha Integrin  4ß7-Binds to MAdCAM (spleen, Peyer’s patch) CXCL13 (BLC) binding to CXCR5 IL-7 binding to IL-7R  TRANCE/TRANCEL CCR7 ligands - CCL19, CCL21

LT and TNF are required for the development and function of B and T zone stromal cells that make the chemokines which are necessary for lymphocyte compartmentalization in the spleen. BLC expression by follicular stromal cells is defective in TNF-, TNF receptor 1 (TNFR1)-, LT  - and LTß- deficient mice. Expression of the T cell attractant, secondary lymphoid tissue chemokine (SLC), and the SLC-related chemokine ELC, by T zone stromal cells are markedly depressed in LT  -, and LTß-deficient mice. Treatment of adult mice with antagonists of LTß also leads to decreased BLC expression.

Spleen development Development of lymphoid areas (white pulp) is later than the spleen itself Spleen development requires Hox11, wt-1, Bapx1 NKX2.3 critical for lymphoid organisation - plays a role in MAdCAM induction, but this is not the only way that NKX2.3 operates

PP inducer cells Migrate from fetal liver to intestine  4ß7+ IL-7R  + CD45+ Signal via LT  1ß2 to mesenchymal ‘organizer’ cells - induced by LTßR signaling to express cytokines, chemokines and adhesion molecules IL-7 may feedback on inducer cells to drive more LT, also TRANCE CXCL13 feeds back to recruit more CXCR5+ inducer cells, as well as to recruit TNF-expressing B cells CXCL13 also promotes further LT expression by B cells Inducer cells may also operate in LN development May have other origins besides fetal liver Some CXCL23, CCL19 and CCL21 chemokine expression is LT and TNF-independent - alternate CCL21 gene Low level expression of CXCL13 may precede inducer cell migration

Chemokines in development of secondary lymphoid organs SLC - secondary lymphoid tissue chemokine, or, 6CkineCCL21 ELC - EBV-induced molecule 1 ligand chemokine/MIP-3ßCCL19 BLC - B lymphocyte chemoattractant, or, B cell attracting CXCL13 chemokine-1 (BCA-1)

SLCExpressed by HEV’s, stromal cells in T-dependent zones Binds to CCR7 ELCExpressed by macrophages, dendritic cells, and some non- hematopoietic cells Binds to CCR7 SLC promotes T cell extravasation via HEV’s, then ELC+SLC promote CCR7 + T cell migration into the T cell area BLC Expressed by FDC’s in B cell follicles Binds to CXCR5 (BLR1) on B cells - may also attract activated T cells, to promote T:B interaction Antigen-responsive B cells may express CCR7, to promote T:B interaction - move into paracortex

(from Cyster JG, 1999.Science 286:2098)