- Email: [email protected]
One Niche to Rule Both Maintenance and Loss of Stemness in HSCs
Immunity
Previews One Niche to Rule Both Maintenance and Loss of Stemness in HSCs Motonari Kondo1,* 1Department of Molecular Immunology, Toho University School of Medicine, Tokyo 143-8540, Japan *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.003
Hemato-lymphopoiesis initiated by hematopoietic stem cells (HSCs) is tightly regulated by factors present in the bone marrow (BM) niche. Using genetically modified mice, Gomes et al. (2016) show that IL-7 is produced by HSC niche-forming cells and find that the same niche that controls HSC self-renewal can also support differentiation. Hematopoietic stem cells (HSCs) continuously give rise to all classes of blood cells, divided into lymphoid and myeloid cell lineages, through multiple lineage restriction steps in the bone marrow (BM). HSCs reside in specialized microenvironments called HSC niches that are composed of BM stromal cells, such as CXCL12-abundant reticular (CAR) cells, mesenchymal stem/stromal cells (MSCs), and endothelial cells (Morrison and Scadden, 2014). It has been suggested that other cells types, such as osteoblasts in the endosteum region of the BM, are also involved in the formation of HSC niches, although this concept is controversial (Morrison and Scadden, 2014). The conditional deletion of key factors necessary for the maintenance of HSCs in the BM, including stem cell factor (SCF) and CXCL12, has uncovered ‘‘true’’ HSC niche cells in the BM (Morrison and Scadden, 2014). Upon the initiation of HSC maturation, HSCs gradually lose their self-renewal potential and become multipotent progenitors (MPPs). The upregulation of Flt3 (or Flk-2) occurs during this maturation process (Lai and Kondo, 2008). However, it is unclear how and where HSCs begin maturation in the BM. If HSCs migrate away from HSC niches, they cannot maintain stemness. Therefore, it is possible that the exit of HSCs from HSC niches is a differentiation cue. Another question is whether a lineage-specific microenvironment exists in the BM, such as a lymphoid niche that can support HSC maturation and differentiation into the lymphoid lineage. In this issue of Immunity, Gomes et al. (2016) sought to determine the location, HSC matura-
tion, and differentiation in the BM. They unexpectedly find that the same HSC niches that control HSC self-renewal also regulate HSC differentiation into MPPs. Interleukin 7 (IL-7) is an indispensable cytokine for the development of T cells (in both humans and mice) and B cells (in mice only). The chemokine CXCL12 has also been suggested to be necessary for the proliferation and maintenance of B cell precursors. However, the precise relationship between IL-7and CXCL12-producing BM stromal cells is unclear. To dissociate the role of CXCL12 in maintenance of HSCs and their maturation, Gomes et al. (2016) conditionally deleted the gene encoding CXCL12 receptors, Cxcr4, using Flk2-Cre mice crossed with floxed Cxcr4 mice. Using BM cells from these Cxcr4 conditional knockout mice and wild-type mice to examine hematopoietic cell development in adult BM microenvironments, the authors developed BM chimeras and found that numbers of MPPs and common lymphoid progenitors (CLPs) were markedly reduced in the absence of CXCR4. In accordance with this observation, the production of mature T, B, and natural killer (NK) cells was also significantly reduced. Cxcr4-deficient MPPs and CLPs were found in the spleen, suggesting that the retention of these cells in the BM is impaired. In addition, both Cxcr4-sufficient and -deficient CLPs could normally give rise to B and NK cells during in vitro culture. Therefore, the authors concluded that the CXCL12-CXCR4 axis is necessary for the maintenance and retention of MPPs and CLPs in the BM.
Interestingly, Gomes et al. (2016) demonstrated that Ly6D+ CLPs are located in the proximity of IL-7+ BM stromal cells in a CXCL12-CXCR4dependent manner. In the presence of the CXCR4 antagonist AMD3100, this proximity of Ly6D+ CLPs to IL-7+ BM stromal cells disappears. In addition, the phosphorylation of a signal molecule, STAT5, in Ly6D+ CLPs is dependent on the proximity of Ly6D+ CLPs to IL-7+ BM stromal cells. Gomes et al. (2016) more directly clarified this using Il7-GFP-Cxcl12-DsRed double-reporter mice, in which approximately 60% of CXCL12+ BM stromal cells express IL-7. These results seem to contradict the results obtained by Tokoyoda et al. (2004), who showed that IL-7 and CXCL12 are produced by distinct BM stromal cells. Cells that produce high levels of CXCL12 were defined as CAR cells, which are components of HSC niches (Omatsu et al., 2010). The difference in these studies may be attributed to differences in the experimental approach taken by the two research teams. Tokoyoda et al. (2004) detected IL-7 using anti-IL-7 antibodies by immunofluorescence microscopy, whereas Gomes et al. (2016) used Il7-ECFP or GFP reporter mice and visualized GFP and/or CFP with specific antibodies or by flow cytometry, which could enhance the signal. Therefore, Gomes et al. (2016) might be able to detect lower levels of IL-7 than the levels that can be detected using the methods of Tokoyoda et al (2004). More importantly, the conditional deletion of IL-7 in CAR cells, which share functional similarities with MSCs that express leptin receptor (LEPR) (Zhou et al., 2014), results in a
Immunity 45, December 20, 2016 ª 2016 Elsevier Inc. 1177
Immunity
Previews
IL-7 ProB IL-7
CAR cell HSC
HSC MPP IL-7 IL-7
CLP
sinusoid or arteriole ProB IL-7
HSC IL-7 Mesenchymal stem cell
ProB
Figure 1. BM Cells Produce IL-7 Mesenchymal stem/progenitor cells, CAR cells, epithelial cells, all of which are HSC niche-forming cells, produce IL-7. HSCs, MPPs, and CLPs are located in close proximity to IL-7+ BM cells in a CXCL12-CXCR4-dependent manner. ProB cells expand in response to IL-7 produced by various IL-7+ BM cells.
profound reduction in the number of Ly6D+ CLPs and more mature B lineage cells, suggesting that CAR cells indeed produce IL-7 together with CXCL12 and play an important role in B cell development in the BM. Using a lineage tracing approach, it was determined that IL-7+ MSCs can at least give rise to osteoblasts and adipocytes, which express low or no IL-7, indicating that some IL-7+ BM stromal cells are MSCs that have multi-lineage differentiation potential. Ding and Morrison (2013) have proposed that the endosteum may play a role in CLP development or maintenance because approximately 30% of IL-7Ra+ lineage BM cells, including CLPs, are adjacent to the endosteum region. In addition, the deletion of CXCL12 in osteoblasts results in the depletion of CLPs. Gomes et al. (2016) make the following argument: first, more than 1178 Immunity 45, December 20, 2016
80% of Ly6D+ CLPs were positioned > 30 mm away from the endosteum; second, the conditional deletion of IL-7 in osteoblasts rarely affected B lymphopoiesis. Therefore, it is possible that the osteoblasts and other cells in the endosteum attract lymphoid precursors via the CXCL12-CXCR4 axis and indirectly affect lymphopoiesis in some conditions. At minimum, osteoblasts are not a major source of IL-7, which is an indispensable cytokine for B lymphopoiesis. Interestingly, some epithelial cells express IL-7. If IL-7 is deleted from epithelial cells, the numbers of proB and preB cells are significantly reduced, suggesting that IL-7+ epithelial cells physiologically contribute to B lymphopoiesis. Similar to CAR cells, which are components of HSC niches, the majority of IL-7+ BM stromal cells express high levels of CXCL12. Therefore, it appears
as though HSCs, MPPs, and B cell progenitors share the same microenvironment (Figure 1). In fact, HSCs and Flt3+ MPPs are in close proximity to IL-7+ BM stromal cells. In some cases, HSCs and MPPs interact with the same IL-7+ BM stromal cells. In addition, the deletion of CXCL12 in IL-7+ cells results in a significant reduction in the numbers of HSCs, MPPs, CLPs, and B cell progenitors in the BM (Gomes et al., 2016). It is important to note that the decrease in the number of HSCs in the absence of CXCL12 in IL-7+ BM stromal cells is less profound than that in mice without CXCL12 in mesenchymal LEPR+ cells. Therefore, IL-7 LEPR+ mesenchymal cells have a significant role in the formation of HSC niches. Furthermore, factors other than CXCL12, such as SCF, are likely to play a role in the maintenance of HSC numbers in the BM. This notion is supported by the observation that the conditional deletion of SCF from IL-7+ BM cells results in a more significant loss of HSCs than the deletion of CXCL12 in the same IL-7+ cells. When HSCs and MPPs share the same microenvironment, what is the differentiation cue for HSCs? It is possible that the circadian fluctuation of CXCL12 expression influenced by adrenergic signals promotes the differentiation of HSCs (Me´ndez-Ferrer et al., 2008), as HSCs may exit from the HSC niche when CXCL12 expression is low. However, this scenario cannot explain how HSC and MPP quantities are tightly regulated when these two populations share the same niche. It is necessary to determine the different roles between IL-7+ and IL-7 MSCs. Another important unresolved issue is whether any instructive stimulation directs HSCs and/or MPPs toward lymphoid lineage specification or commitment. Gomes et al. (2016) provide evidence that more BM stromal cells produce IL-7 than previously predicted. More importantly, the authors prove that HSCs and MPPs physically share the same niche, suggesting that self-renewal potential of HSCs, as well as their differentiation, are supported by BM cells producing both CXCL12 and IL-7. Technological advances have made it possible to uncover patterns that differ from generally accepted schemes or current models. Further studies will clarify the precise mechanisms by which
Immunity
Previews the homeostasis maintained.
of
blood
cells
is
REFERENCES Ding, L., and Morrison, S.J. (2013). Nature 495, 231–235. Gomes, A.C., Hara, T., Lim, V.Y., Herndler-Brandstetter, D., Nevius, E., Sugiyama, T., Tani-ichi, S., Schlenner, S., Richie, E., Rodewald, H.-R., Flavell,
R.A., et al. (2016). Immunity 45, this issue, 1219– 1231. Lai, A.Y., and Kondo, M. (2008). Semin. Immunol. 20, 207–212. Me´ndez-Ferrer, S., Lucas, D., Battista, M., and Frenette, P.S. (2008). Nature 452, 442–447. Morrison, S.J., and Scadden, D.T. (2014). Nature 505, 327–334.
Omatsu, Y., Sugiyama, T., Kohara, H., Kondoh, G., Fujii, N., Kohno, K., and Nagasawa, T. (2010). Immunity 33, 387–399. Tokoyoda, K., Egawa, T., Sugiyama, T., Choi, B.I., and Nagasawa, T. (2004). Immunity 20, 707–718. Zhou, B.O., Yue, R., Murphy, M.M., Peyer, J.G., and Morrison, S.J. (2014). Cell Stem Cell 15, 154–168.
Precision Targeting: Mast Cells Wipe Out Infected Bladder Epithelia Hongnga T. Le1 and Booki Min1,* 1Department of Immunology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.002
Exfoliating infected bladder epithelium is a powerful host defense mechanism that reduces bacterial burden. In this issue of Immunity, Choi et al. (2016) present a function of mast cells that orchestrates the infected epithelial cell exfoliation via cytolytic granules. Uropathogenic E. coli (UPEC) is the causative agent of the majority of urinary tract infections (UTIs), the most common bacterial infection in human (Brumbaugh and Mobley, 2012). Bacteria that enter the periurethral region infect the bladder (termed cystitis), then ascend to the kidney to cause pyelonephritis (Sivick and Mobley, 2010). The infection begins from bacteria attachment to the bladder epithelial cells (BECs). UPEC express a number of adhesion molecules that promote the attachment, colonization, and invasion. FimH is a type 1 pilus that binds to mannosylated uroplakins on the surface of urothelium, which then triggers bacterial internalization (Mulvey et al., 2000). UPEC infection causes massive exfoliation of infected and damaged BECs between 6 and 12 hr of infection, and this shedding process has been proposed as the first-line host defense mechanism that attempts to clear pathogens. FimH-mediated attachment (and/or invasion) seems essential for the induction of exfoliation, as mutants that express type 1 pili lacking the FimH pilus do not cause the exfoliation. Exfoliation is reported to be an apoptosis-like cell death
that involves host DNA fragmentation and caspase activation (Mulvey et al., 1998). Despite the extensive investigation on UPEC-induced pathogenesis and immunity, cellular mechanisms underlying the exfoliation event have remained incomplete. In this issue of Immunity, Choi et al. (2016) sought a host factor(s) mediating BEC exfoliation after experimental UPEC infection and found that mast cells (MCs) played a vital role in exfoliating BECs, thus lowering bacterial burden. As in vitro UPEC infection in a BEC line culture was not sufficient to cause cell death and shedding, they set out searching for host factors. MCs, which normally reside in the underlying lamina propria, are rapidly repositioned directly underneath the infected urothelium prior to the loss of BEC tight junctions and shedding. To validate that MCs were responsible for the BEC exfoliation, they undertook two complementary approaches. First, they examined UPEC-induced BEC exfoliation in MC-deficient KitW-sh/W-sh mice. Unlike WT animals, the BECs of MC-deficient mice remained intact, and the exfoliation was completely restored only after adop-
tive transfer of bone marrow-derived MCs into the KitW-sh/W-sh mice. Second, they utilized Mcpt5-cre-DTR mice, in which MCs are selectively depleted with repeated diphtheria toxin administration. Conditional MC depletion substantially reduced BEC exfoliation after UPEC infection. In support of host defense mechanism of exfoliation, the lack of MC-mediated BEC exfoliation in these animals resulted in higher bacterial burden. What is the signal mobilizing MCs to the site of infection? BECs exposed to UPEC released copious amounts of interleukin1b (IL-1b). IL-1b regulates migration of multiple cell types including neutrophils. The authors hypothesized that IL-1b produced by UPEC-infected BECs recruits MCs. In vitro migration assay showed a potent chemoattractant effect of IL-1b in MCs. In vivo, recombinant IL-1b was directly administered into the bladders of WT or Il1r / naive animals. MCs accumulated in the basement membrane of the bladder of WT but not of Il1r / mice, indicating that IL-1b is involved in MC mobilization. Moreover, Il1r / bone marrow-derived MCs did not migrate
Immunity 45, December 20, 2016 ª 2016 Elsevier Inc. 1179
Previews One Niche to Rule Both Maintenance and Loss of Stemness in HSCs Motonari Kondo1,* 1Department of Molecular Immunology, Toho University School of Medicine, Tokyo 143-8540, Japan *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.003
Hemato-lymphopoiesis initiated by hematopoietic stem cells (HSCs) is tightly regulated by factors present in the bone marrow (BM) niche. Using genetically modified mice, Gomes et al. (2016) show that IL-7 is produced by HSC niche-forming cells and find that the same niche that controls HSC self-renewal can also support differentiation. Hematopoietic stem cells (HSCs) continuously give rise to all classes of blood cells, divided into lymphoid and myeloid cell lineages, through multiple lineage restriction steps in the bone marrow (BM). HSCs reside in specialized microenvironments called HSC niches that are composed of BM stromal cells, such as CXCL12-abundant reticular (CAR) cells, mesenchymal stem/stromal cells (MSCs), and endothelial cells (Morrison and Scadden, 2014). It has been suggested that other cells types, such as osteoblasts in the endosteum region of the BM, are also involved in the formation of HSC niches, although this concept is controversial (Morrison and Scadden, 2014). The conditional deletion of key factors necessary for the maintenance of HSCs in the BM, including stem cell factor (SCF) and CXCL12, has uncovered ‘‘true’’ HSC niche cells in the BM (Morrison and Scadden, 2014). Upon the initiation of HSC maturation, HSCs gradually lose their self-renewal potential and become multipotent progenitors (MPPs). The upregulation of Flt3 (or Flk-2) occurs during this maturation process (Lai and Kondo, 2008). However, it is unclear how and where HSCs begin maturation in the BM. If HSCs migrate away from HSC niches, they cannot maintain stemness. Therefore, it is possible that the exit of HSCs from HSC niches is a differentiation cue. Another question is whether a lineage-specific microenvironment exists in the BM, such as a lymphoid niche that can support HSC maturation and differentiation into the lymphoid lineage. In this issue of Immunity, Gomes et al. (2016) sought to determine the location, HSC matura-
tion, and differentiation in the BM. They unexpectedly find that the same HSC niches that control HSC self-renewal also regulate HSC differentiation into MPPs. Interleukin 7 (IL-7) is an indispensable cytokine for the development of T cells (in both humans and mice) and B cells (in mice only). The chemokine CXCL12 has also been suggested to be necessary for the proliferation and maintenance of B cell precursors. However, the precise relationship between IL-7and CXCL12-producing BM stromal cells is unclear. To dissociate the role of CXCL12 in maintenance of HSCs and their maturation, Gomes et al. (2016) conditionally deleted the gene encoding CXCL12 receptors, Cxcr4, using Flk2-Cre mice crossed with floxed Cxcr4 mice. Using BM cells from these Cxcr4 conditional knockout mice and wild-type mice to examine hematopoietic cell development in adult BM microenvironments, the authors developed BM chimeras and found that numbers of MPPs and common lymphoid progenitors (CLPs) were markedly reduced in the absence of CXCR4. In accordance with this observation, the production of mature T, B, and natural killer (NK) cells was also significantly reduced. Cxcr4-deficient MPPs and CLPs were found in the spleen, suggesting that the retention of these cells in the BM is impaired. In addition, both Cxcr4-sufficient and -deficient CLPs could normally give rise to B and NK cells during in vitro culture. Therefore, the authors concluded that the CXCL12-CXCR4 axis is necessary for the maintenance and retention of MPPs and CLPs in the BM.
Interestingly, Gomes et al. (2016) demonstrated that Ly6D+ CLPs are located in the proximity of IL-7+ BM stromal cells in a CXCL12-CXCR4dependent manner. In the presence of the CXCR4 antagonist AMD3100, this proximity of Ly6D+ CLPs to IL-7+ BM stromal cells disappears. In addition, the phosphorylation of a signal molecule, STAT5, in Ly6D+ CLPs is dependent on the proximity of Ly6D+ CLPs to IL-7+ BM stromal cells. Gomes et al. (2016) more directly clarified this using Il7-GFP-Cxcl12-DsRed double-reporter mice, in which approximately 60% of CXCL12+ BM stromal cells express IL-7. These results seem to contradict the results obtained by Tokoyoda et al. (2004), who showed that IL-7 and CXCL12 are produced by distinct BM stromal cells. Cells that produce high levels of CXCL12 were defined as CAR cells, which are components of HSC niches (Omatsu et al., 2010). The difference in these studies may be attributed to differences in the experimental approach taken by the two research teams. Tokoyoda et al. (2004) detected IL-7 using anti-IL-7 antibodies by immunofluorescence microscopy, whereas Gomes et al. (2016) used Il7-ECFP or GFP reporter mice and visualized GFP and/or CFP with specific antibodies or by flow cytometry, which could enhance the signal. Therefore, Gomes et al. (2016) might be able to detect lower levels of IL-7 than the levels that can be detected using the methods of Tokoyoda et al (2004). More importantly, the conditional deletion of IL-7 in CAR cells, which share functional similarities with MSCs that express leptin receptor (LEPR) (Zhou et al., 2014), results in a
Immunity 45, December 20, 2016 ª 2016 Elsevier Inc. 1177
Immunity
Previews
IL-7 ProB IL-7
CAR cell HSC
HSC MPP IL-7 IL-7
CLP
sinusoid or arteriole ProB IL-7
HSC IL-7 Mesenchymal stem cell
ProB
Figure 1. BM Cells Produce IL-7 Mesenchymal stem/progenitor cells, CAR cells, epithelial cells, all of which are HSC niche-forming cells, produce IL-7. HSCs, MPPs, and CLPs are located in close proximity to IL-7+ BM cells in a CXCL12-CXCR4-dependent manner. ProB cells expand in response to IL-7 produced by various IL-7+ BM cells.
profound reduction in the number of Ly6D+ CLPs and more mature B lineage cells, suggesting that CAR cells indeed produce IL-7 together with CXCL12 and play an important role in B cell development in the BM. Using a lineage tracing approach, it was determined that IL-7+ MSCs can at least give rise to osteoblasts and adipocytes, which express low or no IL-7, indicating that some IL-7+ BM stromal cells are MSCs that have multi-lineage differentiation potential. Ding and Morrison (2013) have proposed that the endosteum may play a role in CLP development or maintenance because approximately 30% of IL-7Ra+ lineage BM cells, including CLPs, are adjacent to the endosteum region. In addition, the deletion of CXCL12 in osteoblasts results in the depletion of CLPs. Gomes et al. (2016) make the following argument: first, more than 1178 Immunity 45, December 20, 2016
80% of Ly6D+ CLPs were positioned > 30 mm away from the endosteum; second, the conditional deletion of IL-7 in osteoblasts rarely affected B lymphopoiesis. Therefore, it is possible that the osteoblasts and other cells in the endosteum attract lymphoid precursors via the CXCL12-CXCR4 axis and indirectly affect lymphopoiesis in some conditions. At minimum, osteoblasts are not a major source of IL-7, which is an indispensable cytokine for B lymphopoiesis. Interestingly, some epithelial cells express IL-7. If IL-7 is deleted from epithelial cells, the numbers of proB and preB cells are significantly reduced, suggesting that IL-7+ epithelial cells physiologically contribute to B lymphopoiesis. Similar to CAR cells, which are components of HSC niches, the majority of IL-7+ BM stromal cells express high levels of CXCL12. Therefore, it appears
as though HSCs, MPPs, and B cell progenitors share the same microenvironment (Figure 1). In fact, HSCs and Flt3+ MPPs are in close proximity to IL-7+ BM stromal cells. In some cases, HSCs and MPPs interact with the same IL-7+ BM stromal cells. In addition, the deletion of CXCL12 in IL-7+ cells results in a significant reduction in the numbers of HSCs, MPPs, CLPs, and B cell progenitors in the BM (Gomes et al., 2016). It is important to note that the decrease in the number of HSCs in the absence of CXCL12 in IL-7+ BM stromal cells is less profound than that in mice without CXCL12 in mesenchymal LEPR+ cells. Therefore, IL-7 LEPR+ mesenchymal cells have a significant role in the formation of HSC niches. Furthermore, factors other than CXCL12, such as SCF, are likely to play a role in the maintenance of HSC numbers in the BM. This notion is supported by the observation that the conditional deletion of SCF from IL-7+ BM cells results in a more significant loss of HSCs than the deletion of CXCL12 in the same IL-7+ cells. When HSCs and MPPs share the same microenvironment, what is the differentiation cue for HSCs? It is possible that the circadian fluctuation of CXCL12 expression influenced by adrenergic signals promotes the differentiation of HSCs (Me´ndez-Ferrer et al., 2008), as HSCs may exit from the HSC niche when CXCL12 expression is low. However, this scenario cannot explain how HSC and MPP quantities are tightly regulated when these two populations share the same niche. It is necessary to determine the different roles between IL-7+ and IL-7 MSCs. Another important unresolved issue is whether any instructive stimulation directs HSCs and/or MPPs toward lymphoid lineage specification or commitment. Gomes et al. (2016) provide evidence that more BM stromal cells produce IL-7 than previously predicted. More importantly, the authors prove that HSCs and MPPs physically share the same niche, suggesting that self-renewal potential of HSCs, as well as their differentiation, are supported by BM cells producing both CXCL12 and IL-7. Technological advances have made it possible to uncover patterns that differ from generally accepted schemes or current models. Further studies will clarify the precise mechanisms by which
Immunity
Previews the homeostasis maintained.
of
blood
cells
is
REFERENCES Ding, L., and Morrison, S.J. (2013). Nature 495, 231–235. Gomes, A.C., Hara, T., Lim, V.Y., Herndler-Brandstetter, D., Nevius, E., Sugiyama, T., Tani-ichi, S., Schlenner, S., Richie, E., Rodewald, H.-R., Flavell,
R.A., et al. (2016). Immunity 45, this issue, 1219– 1231. Lai, A.Y., and Kondo, M. (2008). Semin. Immunol. 20, 207–212. Me´ndez-Ferrer, S., Lucas, D., Battista, M., and Frenette, P.S. (2008). Nature 452, 442–447. Morrison, S.J., and Scadden, D.T. (2014). Nature 505, 327–334.
Omatsu, Y., Sugiyama, T., Kohara, H., Kondoh, G., Fujii, N., Kohno, K., and Nagasawa, T. (2010). Immunity 33, 387–399. Tokoyoda, K., Egawa, T., Sugiyama, T., Choi, B.I., and Nagasawa, T. (2004). Immunity 20, 707–718. Zhou, B.O., Yue, R., Murphy, M.M., Peyer, J.G., and Morrison, S.J. (2014). Cell Stem Cell 15, 154–168.
Precision Targeting: Mast Cells Wipe Out Infected Bladder Epithelia Hongnga T. Le1 and Booki Min1,* 1Department of Immunology, Lerner Research Institute, Cleveland Clinic Foundation, Cleveland, OH 44195, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2016.12.002
Exfoliating infected bladder epithelium is a powerful host defense mechanism that reduces bacterial burden. In this issue of Immunity, Choi et al. (2016) present a function of mast cells that orchestrates the infected epithelial cell exfoliation via cytolytic granules. Uropathogenic E. coli (UPEC) is the causative agent of the majority of urinary tract infections (UTIs), the most common bacterial infection in human (Brumbaugh and Mobley, 2012). Bacteria that enter the periurethral region infect the bladder (termed cystitis), then ascend to the kidney to cause pyelonephritis (Sivick and Mobley, 2010). The infection begins from bacteria attachment to the bladder epithelial cells (BECs). UPEC express a number of adhesion molecules that promote the attachment, colonization, and invasion. FimH is a type 1 pilus that binds to mannosylated uroplakins on the surface of urothelium, which then triggers bacterial internalization (Mulvey et al., 2000). UPEC infection causes massive exfoliation of infected and damaged BECs between 6 and 12 hr of infection, and this shedding process has been proposed as the first-line host defense mechanism that attempts to clear pathogens. FimH-mediated attachment (and/or invasion) seems essential for the induction of exfoliation, as mutants that express type 1 pili lacking the FimH pilus do not cause the exfoliation. Exfoliation is reported to be an apoptosis-like cell death
that involves host DNA fragmentation and caspase activation (Mulvey et al., 1998). Despite the extensive investigation on UPEC-induced pathogenesis and immunity, cellular mechanisms underlying the exfoliation event have remained incomplete. In this issue of Immunity, Choi et al. (2016) sought a host factor(s) mediating BEC exfoliation after experimental UPEC infection and found that mast cells (MCs) played a vital role in exfoliating BECs, thus lowering bacterial burden. As in vitro UPEC infection in a BEC line culture was not sufficient to cause cell death and shedding, they set out searching for host factors. MCs, which normally reside in the underlying lamina propria, are rapidly repositioned directly underneath the infected urothelium prior to the loss of BEC tight junctions and shedding. To validate that MCs were responsible for the BEC exfoliation, they undertook two complementary approaches. First, they examined UPEC-induced BEC exfoliation in MC-deficient KitW-sh/W-sh mice. Unlike WT animals, the BECs of MC-deficient mice remained intact, and the exfoliation was completely restored only after adop-
tive transfer of bone marrow-derived MCs into the KitW-sh/W-sh mice. Second, they utilized Mcpt5-cre-DTR mice, in which MCs are selectively depleted with repeated diphtheria toxin administration. Conditional MC depletion substantially reduced BEC exfoliation after UPEC infection. In support of host defense mechanism of exfoliation, the lack of MC-mediated BEC exfoliation in these animals resulted in higher bacterial burden. What is the signal mobilizing MCs to the site of infection? BECs exposed to UPEC released copious amounts of interleukin1b (IL-1b). IL-1b regulates migration of multiple cell types including neutrophils. The authors hypothesized that IL-1b produced by UPEC-infected BECs recruits MCs. In vitro migration assay showed a potent chemoattractant effect of IL-1b in MCs. In vivo, recombinant IL-1b was directly administered into the bladders of WT or Il1r / naive animals. MCs accumulated in the basement membrane of the bladder of WT but not of Il1r / mice, indicating that IL-1b is involved in MC mobilization. Moreover, Il1r / bone marrow-derived MCs did not migrate
Immunity 45, December 20, 2016 ª 2016 Elsevier Inc. 1179