However, it is becoming clear that in a range of inflammatory contexts, ectopic or tertiary lymphoid tissues can develop inappropriately under pathological stress. Here we summarize the role of stromal cells in the development of homeostatic lymphoid tissue, and assess emerging evidence that suggests a critical role for stromal

AZD0530 chemical structure involvement in the tertiary lymphoid tissue development associated with chronic infections and inflammation. Secondary lymphoid organs (SLOs) function to increase the efficiency of interactions between rare, antigen-specific lymphocytes and antigen-presenting cells, concentrating antigen and lymphocytes in a supportive environment that facilitates the initiation of an adaptive immune response. Homeostatic lymphoid tissue organogenesis proceeds via exquisitely controlled spatiotemporal interactions between haematopoietic lymphoid tissue inducer populations and multiple subsets of non-haematopoietic

stromal cells. However, it is becoming clear that in a range of inflammatory contexts, ectopic or tertiary lymphoid organs (TLOs) can develop inappropriately under pathological BMS-777607 stress. Here we summarize the role of stromal cells in the development of homeostatic lymphoid tissue, and assess emerging evidence that suggests a critical role for stromal involvement in the TLO development associated with chronic infections and inflammation. Peripheral lymphoid tissue generation occurs sequentially in the developing mouse embryo from embryonic days E11 to E16.[1, 2] Lymph node (LN) development is thought to be initiated by the production of retinoic acid, which acts on mesenchymal stromal cells at predetermined anatomical sites to induce expression

of the chemokine CXCL13[3] (Fig. 1). It has been proposed that outgrowing nerves are responsible for the production of retinoic acid in development, as they express RALDH2, an enzyme required for the conversion of retinal to retinoic acid.[3] A CXCL13 gradient attracts CXCR5+ haematopoietic cells to the LN anlagen; the first cells to arrive are lymphoid tissue-inducer cells (LTis),[4] derived from fetal liver progenitor cells that can also give rise to B cells, T cells, natural killer cells and dendritic cells.[5] Depsipeptide order The LTis express lymphotoxin (LT) α1β2 (LTα1β2), a cytokine that is the major determinant of SLO development.[6-8] LTα1β2 is a heterotrimeric complex, comprising membrane-bound LTβ and soluble LTα. Together these bind to the lymphotoxin-β receptor (LTβR) that is predominantly expressed by mesenchymal stromal cells. Interestingly, the first CXCR5+ LTis recruited to the site of LN formation express receptor activator of nuclear factor-κB ligand (RANKL), rather than LTα1β2.[9, 10] Indeed the initial clustering of LTis can occur without LTα1β2 expression by LTis[9] or LTβR expression on mesenchymal stromal cells.