Systems Scrutiny of iNKT Subsets
Invariant NK T (iNKT) cells can be divided into three effector subsets: NKT1, NKT2, and NKT17, which are presumed to be analogous to the Th1, Th2, and Th17 subsets of helper T cells, respectively. To gain a more complete picture of the genetic profiles of these subsets, Lee et al. (p. 1460) performed RNAseq analysis on iNKT cells from BALB/c Tbet/IL-4 reporter mice sorted into NK T progenitor (NKTp), NKT1, NKT2, and NKT17 subsets. These four subsets were confirmed to have distinct transcriptional profiles, and signature genes specifically expressed in each subset that encoded cytokines, receptors, and/or transcription factors were identified. For example, NKTp cells upregulated Myc-associated genes, which were proposed to regulate these cells’ proliferation. Using multiple bioinformatic tools, the data obtained from the RNAseq analyses were compared with published datasets to identify similarities between the iNKT subsets and other hematopoietic cells. Although NKT cells are thought to be similar to NK cells, only NKT1 cells had a transcriptional program like that of NK cells, and NKT1 signature genes were also found to be enriched in OT-I CD8+ T cells and Th1 cells. Innate lymphoid cells (ILCs) and γδ T cells can be divided into effector subsets parallel to those of iNKT cells, and NKT1, NKT2, and NKT17 cells shared signature genes with the IFN-γ–, IL-4–, and IL-17–producing subsets of these cells, respectively. Interestingly, although NKT1 cells shared signature genes with Th1 cells, NKT2 and NKT17 cells shared very few similarities with Th2 and Th17 cells. Thus, the four subsets of iNKT cells are transcriptionally unique and have profiles that are, somewhat unexpectedly, more similar to those of γδ T cells and ILCs than conventional CD4+ T cells.
ZBTB32 Manages Memory
Memory B cells rapidly expand and differentiate into Ab-producing plasma cells upon re-encountering their cognate Ag. In this issue, Jash et al. (p. 1159) identified a role for the transcription factor ZBTB32, which is known to be upregulated in activated B cells, in keeping these secondary responses under control. High levels of ZBTB32 expression were observed in mouse and human memory B cells but not in naive B cells or polyclonal plasma cells. Although primary Ab responses were not affected by ZBTB32 deficiency, Zbtb32−/− mice demonstrated significantly increased secondary Ab responses to both haptens and protein Ags, relative to Zbtb32+/+ mice. Compared with controls, secondary plasma cells in Zbtb32−/− mice differentiated more rapidly in the spleen, produced higher titers of Abs, and were maintained for a longer duration in the bone marrow in a cell-intrinsic manner. Microarray analysis of resting memory B cells did not detect dramatic differences in gene expression between the cells of Zbtb32−/− and Zbtb32+/+ mice; however, greater differences were observed once these cells were activated or differentiated into secondary plasma cells. The Zbtb32−/− plasma cells expressed increased levels of genes associated with proliferation and survival, despite the fact that ZBTB32 expression was turned off in the secondary plasma cells of Zbtb32-sufficient mice. Further analysis determined that Zbtb32 deficiency prolonged Ab responses by augmenting survival, but not proliferation, of these cells. These data identify a transcriptional mechanism by which B cell recall responses may be kept under control in vivo.
NK and T Cell Memory Maintenance with IL-15
Common γ-chain (γc) cytokines, such as IL-15, IL-2, and IL-7, tightly regulate lymphocyte function and homeostasis. Due to the considerable overlap in their biological activity, the non-redundant roles these cytokines may play in vivo are not fully understood. To shed some light on the specific functions of IL-15 in vivo, DeGottardi et al. (p. 1183) investigated CD4+, CD8+, and NK cell populations in rhesus macaques (RM) following inhibition of IL-15 activity using a “rhesusized” anti-IL-15 mAb. Within the first two weeks of a 6-wk treatment, a near complete loss of circulating and tissue NK cells and CD4+ and CD8+ effector memory T cells (TEM) was observed, while circulating and tissue naive and central memory T cell populations remained unaffected. Whereas depletion of NK cells persisted throughout treatment with the anti-IL-15 mAb, the circulating, but not tissue, CD4 and CD8 TEM compartments were restored by a rapid proliferation of TEM. These data suggest that NK and TEM homeostasis are highly dependent on IL-15 but also indicate that a proportion of TEM can persist in the absence of IL-15 signaling, suggesting that there may be other regulatory molecules involved in modulating TEM populations. Additionally, although there was not a notable increase in plasma IL-7 in mAb-treated RM, TEM from these animals demonstrated increased responsiveness to ex vivo and in vivo stimulation with IL-7, suggesting a role for IL-7 in supporting TEM homeostasis in the absence of IL-15 signaling. Taken together, these data suggest a significant role for IL-15 in the development and survival of NK cells in RM and suggest that although IL-15 is important for maintaining peripheral TEM populations, there is complex interplay between IL-15 and other γc cytokines, such as IL-7, in maintaining peripheral TEM homeostasis. As IL-15 overexpression is hypothesized to drive activation and proliferation of self-reactive T cells in a number of autoimmune disorders, this study provides a useful nonhuman primate model for exploring the impact of the therapeutic application of IL-15 blockade.
Cognate Ag Directs Destructive T Cells
Rejection of transplanted pancreatic islets by CD8+ effector T cells remains a major obstacle to the clinical application of these grafts to reverse diabetes mellitus. Although recent studies have demonstrated that migration of effector T cells to whole organ grafts is primarily driven by cognate Ag rather than chemokines, it is unclear whether the same mechanisms are relevant to transplantation of islets, which are revascularized following the transplant and contain neo-vessels that are likely to be of both donor and recipient origin. In this issue, Zhang et al. (p. 1471) assess the mechanisms underlying Ag-driven CD8+ T cell migration to revascularized pancreatic islet grafts using a mouse model in which OVA-expressing islet cells were transplanted into non-OVA–expressing diabetic recipients. Transfer of OVA-specific effector CD8+ T cells into graft recipients in which donors and recipients either did or did not express the restricting MHC class I molecule revealed that presentation of donor-derived Ag/MHCI complexes was required for driving effector cell migration to islet grafts. Effector cells in which Gαi-coupled cytokine signaling was inhibited migrated to islet grafts as effectively as controls, indicating that Gαi-coupled chemokine signaling was not required for effector T cell migration. Given that inside-out activation of integrin is essential for cell migration, the authors investigated SKAP1, a T cell adapter protein that plays a key role in TCR-triggered integrin activation, as a potential player in Ag-driven T cell migration and graft rejection. Islet cell transplantation into SKAP1-deficient mice revealed that SKAP1 did not play an essential role in Ag-dependent T cell migration, as allograft rejection occurred at the same rate in wild-type and SKAP1-deficient mice. In addition, prolonged graft survival was observed in SKAP1-deficient mice treated with an immunosuppressive regimen, compared with SKAP1-sufficient hosts, suggesting a role for host SKAP1 in allograft rejection via a mechanism yet to be determined. Taken together, these results highlight novel TCR signaling pathways that may play a significant role in T cell-mediated graft rejection and represent novel targets to prolong transplant survival.
Receptors for pIg in the Chicken
The polymeric IgR (pIgR) is a transmembrane protein that ferries pIg across epithelial barriers into the mucosa, after which its ectodomain, or secretory component (SC), remains associated with the pIg and mediates functions that include protecting Abs from degradation and interacting with bacterial factors. The pIgR has been conserved through evolution, but the number of Ig-like domains present in the SC varies from two in bony fish to five in mammals. To better understand the evolution of this receptor and its role in mucosal immunity, Stadtmueller et al. (p. 1408) sought to characterize the structure and dynamics of avian pIgR, as well as the mechanism by which it may bind ligands. The authors purified the chicken pIgR ectodomain, or Gallus gallus SC (ggSC), and used double electron-electron resonance spectroscopy (DEER) to evaluate its structure. Unlike human SC (hSC), which adopts a closed structure in which domains D1 and D5 maintain contact in the absence of ligand, unliganded ggSC, which has 4 Ig-like domains, appeared to adopt several defined conformations that alternated between open and closed states. The ggSC lacks domain D2 of mammalian SC, and generation of an hSC variant lacking D2 revealed that this domain was responsible for keeping hSC in the closed conformation. Although surface plasmon resonance evaluation of ggSC and hSC binding to human polymeric IgA and IgM showed similarities in binding mechanisms, it also identified distinctions that appeared to be unrelated to the presence or absence of D2. This study reveals distinct conformations adopted by the SC portion of pIgR in different species, implying the results of different selective pressures in the evolution of mucosal immunity.