Bronchus-associated lymphoid tissue (BALT) is occasionally found in the lungs of mice and humans; however, its role in respiratory immunity is unknown. Here we show that mice lacking spleen, lymph nodes and Peyer's patches generate unexpectedly robust primary B- and T-cell responses to influenza, which seem to be initiated at sites of induced BALT (iBALT). Areas of iBALT have distinct B-cell follicles and T-cell areas, and support T and B-cell proliferation. The homeostatic chemokines CXCL13 and CCL21 are expressed independently of TNFalpha and lymphotoxin at sites of iBALT formation. In addition, mice with iBALT, but lacking peripheral lymphoid organs, clear influenza infection and survive higher doses of virus than do normal mice, indicating that immune responses generated in iBALT are not only protective, but potentially less pathologic, than systemic immune responses. Thus, iBALT functions as an inducible secondary lymphoid tissue for respiratory immune responses.

Ectopic or tertiary lymphoid tissues, such as inducible bronchus-associated lymphoid tissue (iBALT), form in non-lymphoid organs after local infection or inflammation. However, the initial events that promote this process remain enigmatic. Here we show that iBALT formed in murine lungs as a consequence of pulmonary inflammation during the neonatal period. Although CD4+CD3− lymphoid tissue inducer (LTi) cells were found in neonatal lungs, particularly after inflammation, iBALT was formed in mice lacking LTi cells. Instead, we found that interleukin 17 (IL-17) produced by CD4+ T cells was essential for iBALT formation. IL-17 acted by promoting the lymphotoxin-α-independent expression of CXCL13, which was important for follicle formation. These results suggest that IL-17-producing T cells are critical for the development of ectopic lymphoid tissues.

Cyclic ADP-ribose is believed to be an important calcium-mobilizing second messenger in invertebrate, mammalian and plant cells. CD38, the best-characterized mammalian ADP-ribosyl cyclase, is postulated to be an important source of cyclic ADP-ribose in vivo. Using CD38-deficient mice, we demonstrate that the loss of CD38 renders mice susceptible to bacterial infections due to an inability of CD38-deficient neutrophils to directionally migrate to the site of infection. Furthermore, we show that cyclic ADP-ribose can directly induce intracellular Ca++ release in neutrophils and is required for sustained extracellular Ca++ influx in neutrophils that have been stimulated by the bacterial chemoattractant, formyl-methionyl-leucyl-phenylalanine (fMLP). Finally, we demonstrate that neutrophil chemotaxis to fMLP is dependent on Ca++ mobilization mediated by cyclic ADP-ribose. Thus, CD38 controls neutrophil chemotaxis to bacterial chemoattractants through its production of cyclic ADP-ribose, and acts as a critical regulator of inflammation and innate immune responses.

Summary The omentum is a site of B1 lymphopoiesis and immune responsiveness to T-independent antigens. However, it is unknown whether it supports immune responses independently of conventional lymphoid organs. We show that the omentum collects antigens and cells from the peritoneal cavity and supports T-dependent B cell responses, including isotype switching, somatic hypermutation and limited affinity maturation, despite the lack of identifiable follicular dendritic cells. The omentum also supports CD4 and CD8 responses to peritoneal antigens and recruits effector T cells primed in other locations. Unlike conventional lymphoid organs, milky spots in the omentum develop in the absence of lymphoid tissue inducer cells, but require CXCL13. Although the lymphoid architecture of milky spots is disrupted in lymphotoxin-deficient mice, normal architecture is restored by reconstitution with lymphotoxin-sufficient hematopoietic cells. These results indicate that the milky spots of the omentum function as unique secondary lymphoid organs that promote immunity to peritoneal antigens.

Location of embryonic lymph node development is determined by the initial clustering of lymphoid tissue inducer cells. We demonstrate that both CXCL13 and CCL21 attracted E12.5–E14.5 lymphoid tissue inducer cells and that initial clustering exclusively depended on CXCL13. Retinoic acid induced early CXCL13 expression in stromal organizer cells independent of lymphotoxin signaling. Notably, neurons adjacent to the lymph node anlagen expressed enzymes essential for retinoic acid synthesis. Furthermore, stimulation of parasymphathetic neural output in adults led to a retinoic acid receptor-dependent induction of CXCL13 in the gut. Therefore, our data show that initiation of lymph node development is controlled by retinoic acid-mediated expression of CXCL13 and suggest that retinoic acid may be provided by adjacent neurons.

Secondary lymphoid organs (SLOs) promote primary immune responses by recruiting naive lymphocytes and activated APCs. However, their role in the persistence or responsiveness of memory lymphocytes is unclear. We tested whether memory cells were maintained and could respond to challenge in the absence of SLOs. We found that influenza-specific CD8 cells in the lung acquired a memory phenotype, underwent homeostatic proliferation, recirculated through nonlymphoid tissues, and responded to and cleared a challenge infection in the complete absence of SLOs. Similarly, influenza-specific virus-neutralizing antibody was generated and maintained in the absence of SLOs. Inducible bronchus-associated lymphoid tissue (iBALT) was also formed in the lungs of previously infected mice and may provide a niche for the maintenance of memory cells at the local level. These data show that SLOs are dispensable for the maintenance of immunologic memory and directly demonstrate the utility of local tissues, such as iBALT, in secondary immune responses.

Summary Immunity to the intestinal parasite, Heligomosomoides polygyrus, is dependent on the successful generation of Th2 memory cells. We show that B cells contribute to immunity against H. polygyrus by producing antibody and by promoting expansion and differentiation of primary and memory Th2 cells. We also demonstrate that cytokine-producing “effector” B cells are essential for effective immunity to H. polygyrus. TNFα production by B cells is necessary for sustained Ab production, while IL-2 production by B cells is necessary for Th2 expansion and differentiation. These results show that B cells mediate protection to pathogens not only by presenting antigen and secreting antibody but also by producing cytokines that regulate the quality and magnitude of humoral and cellular immune responses.

Mice lacking CD38, an ectoenzyme that generates the calcium-mobilizing metabolite cADPR, make reduced T cell-dependent antibody responses. Despite the predicted role for CD38 in B cell activation, we find that CD38 regulates the migration of dendritic cell (DC) precursors from the blood to peripheral sites and controls the migration of mature DCs from sites of inflammation to lymph nodes. Thus, T cells are inefficiently primed in Cd38(-/-) mice, leading to poor humoral immune responses. We also show that CD38 and cADPR modulate calcium mobilization in chemokine-stimulated DCs and are required for the chemotaxis of immature and mature DCs to CCL2, CCL19, CCL21, and CXCL12. Therefore, CD38 regulates adaptive immunity by controlling chemokine receptor signaling in DCs.