Isolated lymphoid follicle maturation induces the development of follicular dendritic cells

Isolated lymphoid follicles (ILFs) are recently identified lymphoid structures in the small intestine with features similar to Peyer's patches (PPs). Using immunohistochemistry we characterized the composition of ILFs in the small intestines of immunocompetent mice and of mice that lacked PPs as a result of either genetic deficiency of lymphotoxin or temporary in utero lymphotoxin-beta receptor-signalling blockade. We showed that although both immature and mature ILFs were present in the intestines of immunocompetent mice, PP-deficiency induced a significantly greater number of mature ILFs. We found that in addition to B-lymphocyte-containing germinal centres, mature ILFs also possessed large networks of follicular dendritic cells (FDCs). These features were not detected within immature ILFs. Indeed, the presence of FDCs could be used to reliably distinguish ILF maturity. Further analysis revealed that the area occupied by the FDCs within mature ILFs was substantial. The total area occupied by FDCs in all the mature ILFs in mice lacking PPs was equivalent to the total area occupied by FDCs in all the PPs and the few mature ILFs in immunocompetent mice. Based on these data we reasoned that in the absence of PPs, mature ILFs are important inductive sites for intestinal immune responses. Indeed, in mice that lacked PPs, ILF maturation coincided with a restoration of faecal immunoglobulin A levels to values that were comparable to those found in immunocompetent mice. Taken together, these data imply that the induction of germinal centres and FDC networks within mature ILFs in response to PP deficiency provides an important compensatory mechanism.