Furthermore, differential stromal subset expression of oxysterol determines B-cell positioning within lymphoid tissue,[40] adding a further level of complexity to the regulation of lymphocyte localization by stromal cells within SLOs. During inflammation or infection, SLO stromal networks have a degree of plasticity. For example

T-cell and B-cell networks grow and remodel[41, 42] accompanied by changes to homeostatic chemokine expression[43] and lymphatics,[44-46] enabling lymphocyte motility. Data have revealed a key role for IL-7-expressing stromal cells in the infection-induced remodelling of murine LN, PLX4032 with lymphatic endothelial cells found to be the major producers of IL-7 using an in vivo IL-7 fate-mapping system and the staining of human LN sections.[35] Importantly, the in vivo ablation of IL-7-expressing stromal cells abolished infection-driven changes in LN architecture, highlighting the crucial role that these cells play in both the development and subsequent remodelling of the LN. Interestingly FRCs are capable of directly modifying LN endothelial cell growth and expansion,[45] suggesting that both stromal–stromal and stromal–leucocyte interactions regulate the processes OSI-906 purchase underlying

the formation and remodelling of lymphoid tissues. In addition to the developmentally imprinted homeostatic tissues discussed above, ‘intermediate’ lymphoid tissues exist that can be considered as somewhere between predetermined and inflammatory lymphoid tissues. Isolated lymphoid follicles (ILFs) Etofibrate are primarily B-cell follicle-containing lymphoid structures that form at predetermined sites along the length of the mesenteric wall of the small intestine.[47] The

ILFs develop from cryptopatches, clusters of LTi cells seen in both mouse[48] and human[49] intestine. As with the LN, LTi–stromal interactions are vital in ILF formation[50] mediated via LTβR signalling,[47, 51] which is aided by the recruitment of naive LTα1β2-expressing B cells.[52] Recent work has also revealed that the cytokine IL-22 may also be involved in the maintenance of ILFs during bacterial-induced inflammation.[53] Mice kept in a specific-pathogen-free environment develop few and small ILFs,[51] whereas infection with Salmonella enterica greatly enlarges individual ILFs, but importantly does not increase their overall number.[54] The ILFs therefore represent a partially programmed lymphoid tissue lying between ectopic and predetermined. Their anatomical location is predetermined and their developmental processes show many similarities to LN expansion, yet their formation is dependent upon environmental signals, namely microbial stimulation.[54, 55] Truly distinct from developmentally encoded lymphoid tissue are ectopic or TLOs, also known as tertiary lymphoid tissue.