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Lineage⁻Sca-1⁺c-Kit⁻ (LSK⁻) cells are an atypical lymphoid progenitor population that expands in the spleens of mice infected with Plasmodium yoelii. Although these cells are capable of differentiating into mature B cells, Ab-secreting cells, and germinal center (GC) and memory B cells, the factors that support the differentiation of LSK⁻ cells into B cells postinfection (p.i.) have not been defined. In this issue, Ghosh et al. (p. 1783) showed that at days 7 and 11 p.i. with P. yoelii 17X, LSK⁻ cells were the primary source of splenic IL-17, a cytokine that promotes the formation of tertiary lymphoid tissue by stromal cells. Compared with infected wild-type (WT) controls, infection of mice deficient in IL-17 signaling (Il-17ra^−/− mice) resulted in reductions in splenic B cell populations and parasite-specific IgG in the serum. Adoptive transfer of splenic LSK⁻ cells from naive WT or Il-17ra^−/− mice into either WT or Il-17ra^−/− recipients prior to infection demonstrated that, regardless of donor cell background, recovery of LSK⁻ cells and LSK⁻ derived B cell populations was significantly lower at day 11 p.i. in Il-17ra^−/− versus WT recipients, suggesting that IL-17 derived from LSK⁻ cells acts extrinsically on other cells to promote differentiation of LSK⁻ cells into B cells in response to P. yoelii infection. In vitro and in vivo studies revealed that IL-17RA–expressing podoplanin⁺CD31⁻ stromal cells, which are fibroblast reticular cells (FRCs), responded to IL-17 to promote the development of B cells from LSK⁻ cells, and this effect was mediated by production of CXCL12, a chemokine that plays a critical role in B cell lymphopoiesis. Gene expression assays of FRCs from infected WT and Il-17ra^−/− mice showed that Il-17ra^−/− FRCs are reduced in genes encoding growth factors facilitating lymphopoiesis, such as Cxcl13, Flt3, Il-6, and Baff, and adhesion proteins, such as Vcam-1 and gp38. Taken together, these studies demonstrate that following infection with P. yoelii, LSK⁻ cell derived IL-17 modulates the expression of chemoattractants, growth factors, and adhesion factors by stromal cells, thereby facilitating the differentiation of LSK⁻ cells into B cells. Future work will be needed to determine whether a similar pathway exists in humans.

Septic shock is associated with immune dysfunction of both the innate and adaptive responses. Specifically, studies have demonstrated that T cells become rapidly exhausted after the onset of sepsis, and this is associated with an increased risk of an adverse outcome, although the mechanism underlying this exhaustion remains unknown. As a link between cellular metabolism and T cell effector function has recently been established, Venet et al. (p. 1606) investigated whether alterations in T cell metabolism could play a role in sepsis-induced T cell dysfunction. A metabolomic study of purified lymphocytes demonstrated significant differences between the basal metabolic status of T cells from septic patients and those from healthy controls, such that septic patients had significant decreases in T cell oxidative phosphorylation (OXPHOS) and glycolysis. Furthermore, following stimulation, T cells from septic patients failed to induce aerobic glycolysis, OXPHOS, GLUT1 expression, glucose uptake, and proliferation to levels seen in healthy controls. These deficiencies were attributed to a significant reduction in mTOR expression in T cells of septic patients, whereas HIF-1α expression and Akt phosphorylation remained unchanged when compared with healthy controls, suggesting a key role for mTOR dysfunction in the metabolic alterations seen in T lymphocytes of septic patients. Consistent with previous studies demonstrating that IL-7 plays an important role in T cell function through activation of the PI3K–Akt–mTOR pathway, treatment of T cells from septic patients with recombinant IL-7 rescued mTOR activation, GLUT1 expression, glucose uptake, and proliferation. Confirming a central role for mTOR in this process, addition of rapamycin, a specific mTOR inhibitor, reversed the beneficial effect of IL-7 on T cells from septic patients. Metabolism in T cells from septic individuals thus is shifted toward a catabolic profile, and this shift is closely linked to defects in effector function that are likely dependent on alterations in mTOR expression or activity. Furthermore, this study suggests that targeting metabolism may be a therapeutic avenue to treat sepsis-induced immune alterations.

Group 3 innate lymphoid cells (ILC3s) are well known to play a role in maintaining the intestinal epithelial barrier during inflammation, primarily through their production of IL-22. The role played by this cytokine during homeostasis in the healthy gut is less fully elucidated and, in this issue, Savage et al. (p. 1912) generated an IL-22 reporter mouse to assess the expression of IL-22 during development and under basal conditions. In the intestine of healthy 6-wk-old mice, IL-22 expression was detected in CD4⁺ αβ T cells, γδ T cells, and ILCs residing in small intestinal lymphoid tissues (SILT), but the highest levels of IL-22 expression were seen in lymphoid tissue inducer (LTi) ILC3s. IL-22 expression was also present at low levels in the SILT of prenatal mice but dramatically increased in LTi cells to levels similar to those of adult animals following weaning (postnatal day 21). The increased expression of IL-22 following weaning was dependent on microbial flora, as treatment of mice with broad-spectrum antibiotics from the fetal stage through postweaning reduced the number of LTi cells expressing high levels of IL-22 in the SILT. Consistent with this, MyD88 deficiency in either CD11c⁺ or Villin⁺ cells reduced IL-22–expressing LTi cells, indicating that in the healthy intestine, LTi cells resident in the SILT were dependent on MyD88 signaling for their activation. Furthermore, IL-22–expressing LTi cells in healthy SILT were found to colocalize with a rare, activated subset of macrophages that constitutively expressed IL-12/23 p40, a cytokine that can induce IL-22 production from LTis, and that produced more IL-23 in response to TLR stimulation. This population also dramatically increased in number in the SILT following weaning Finally, treatment of neonatal mice with IL-23 or CpG induced IL-22 expression in nearly all LTi cells. In summary, these data identify a previously unrecognized homeostatic relationship between IL-22–expressing LTi cells and macrophages in the SILT and highlight the importance of the intestinal flora for maintaining steady-state IL-22 expression.

The zinc finger transcription factor Thpok is necessary for thymic development and peripheral differentiation of CD4⁺ effector T cells, but its potential involvement in the activity of regulatory T cells (Tregs) has remained unclear. By disrupting the genes encoding both Thpok and the related transcription factor LRF in mice, Carpenter et al. (p. 1716) identified an overlapping requirement for these two transcription factors in Treg differentiation and function. Disruption of Thpok in double-positive thymocytes reduced generation of thymic Treg precursors and resultant Treg differentiation, and this effect was amplified by the additional disruption of Lrf. In male mice with Treg-specific deletion of both Thpok and Lrf (double knockout [dKO] mice), but of neither alone, a lethal inflammatory syndrome developed that was similar to that observed in Foxp3-deficient Scurfy mice, indicating an important but redundant role of these transcription factors in maintaining immune homeostasis. As in conventional CD4⁺ T cells, Thpok and LRF appeared to be important for maintaining the CD4⁺CD8⁻ phenotype of Tregs; however, unlike in conventional T cells, Thpok- and LRF-mediated effects on Treg activity did not involve inhibiting Runx3 and were independent of their effects on CD4 expression. Thpok was found to stabilize Foxp3 expression in Tregs, and Thpok and LRF together promoted Treg survival and supported Foxp3-induced expression of CD25 and Nrp1. A comparison of gene expression profiles in dKO Tregs versus controls revealed reduced expression of genes associated with IL-2 signaling and effector differentiation in dKO Tregs. These data suggest that in addition to their known importance in the differentiation of conventional CD4⁺ T cells, Thpok and LRF are also necessary, via distinct mechanisms, for the differentiation and maintenance of CD4⁺ Tregs.

Although Langerhans cells (LCs) in the epidermis are known to act as APCs, the presence of multiple dendritic cell (DC) populations in the skin has complicated the analysis of these cells’ functions. In this issue, Strandt et al. (p. 1626) generated transgenic mice in which Ag presentation was inducible and limited to LCs to assess LC function in vivo. In these mice, tamoxifen (TAM) treatment induced transgenic expression of Ags, including nonsecreted GFPOVA, in LCs but no other DC subsets. LCs presenting GFPOVA in vivo could stimulate endogenous OVA-specific CD8⁺ T cells under steady-state conditions, inducing these CTLs to proliferate, kill target cells, and secrete IFN-γ. However, these mice were resistant to cutaneous challenge with OVA during the contraction phase of the CTL response, suggesting the induction of tolerance. This tolerance was confirmed to be Ag specific and was dependent on regulatory T cells (T_regs), as T_reg depletion reversed tolerance, allowing expansion of IFN-γ–secreting, cytotoxic CD8⁺ T cells in OVA-challenged mice. To investigate whether the activation state of LCs affected the nature of the CTL response, LCs were stimulated in vivo with anti-CD40 and polyinosinic-polycytidylic acid concurrently with TAM treatment, which induced presentation of GFPOVA, and then challenged with cutaneous OVA as before. OVA-specific CD8⁺ T cells expanded by these activated LCs did not exhibit tolerance but were instead strongly activated by cutaneous OVA challenge, suggesting that a memory response had been induced. Supporting this idea, CD62L and IL-7R/CD127, which are expressed on central memory T cells, were shown to be upregulated on CTLs primed by activated LCs but not on CTLs primed by steady-state LCs. This in vivo system allows isolated analysis of LC Ag presentation and identifies conditions under which these cells can either tolerize CTLs or promote from them a robust recall response.