The inhibitory receptor PD-1 is upregulated on activated T cells, and its binding of ligands PD-L1 or PD-L2 limits T cell proliferation and cytokine secretion. Blockade of PD-1 has shown promise in treating many types of viral infections by enhancing T cell-mediated clearance of virus, but it has also been linked to severe immunopathology. To gain insights into the role of PD-1 in the T cell response to Mycobacterium tuberculosis infection, Barber et al. (p. 1598) infected wild-type (WT) and PD-1 knockout (KO) mice via aerosol exposure. Whereas WT mice were able to contain M. tuberculosis infection, PD-1 KO mice succumbed by 35 d postinfection with large necrotic lesions and elevated levels of bacteria in their lungs. PD-1 KO mice were found to have significantly increased M. tuberculosis-specific effector CD4+ T cell numbers compared with WT mice, and depletion of CD4+ T cells rescued PD-1 KO mice from early M. tuberculosis-related mortality. In contrast, the depletion of CD8+ T cells only slightly decreased M. tuberculosis susceptibility. PD-L1 KO mice also exhibited increased susceptibility to M. tuberculosis infection, but to a lesser degree than that observed in PD-1 KO mice, indicating that PD-1–mediated immunopathology was modulated by more than one PD-1 ligand. Collectively, these data demonstrate that PD-1 plays an important role in controlling the detrimental aspects of the CD4+ T cell-mediated response to M. tuberculosis infection.

The cell surface molecule CD22 undergoes endocytosis and transports ligands to endosomes. Because its expression is restricted to B cells, CD22 shows promise as a means to deliver cytotoxic drugs to B cell lymphomas via linkage with glycan ligands or anti-CD22. To better understand the fate of endocytosed CD22 and its therapeutic cargos, O'Reilly et al. (p. 1554) compared the CD22-mediated endocytosis of glycan ligand-based cargos with anti-CD22 Abs. To their surprise, CD22 was not ultimately targeted for degradation following internalization, as previously reported, but was instead recycled back to the cell surface. Similar to previous reports, CD22 ligand-based cargo and Abs bound to CD22 were endocytosed and delivered to endocytic compartments. However, whereas glycan-based ligands accumulated in endosomes, intracellular levels of anti-CD22 Ab did not accumulate beyond 50% of total anti-CD22 Ab levels. Subsequently, it was determined that endocytosed CD22-bound Abs were not degraded in the endosomes, but rather remained bound to CD22 and were returned to the cell surface due to constitutive recycling of CD22. These findings have implications for CD22-mediated B cell therapy, as they indicate that CD22 can act as a recycling receptor.

Maternal-derived grafts invoke a more tolerant alloimmune response than do paternal-derived grafts, which is believed to be linked to fetal/neonatal exposure to noninherited maternal Ags (NIMA) during pregnancy and breast-feeding. To study this phenomenon, Akiyama et al. (p. 1442) compared three groups of anti-Kb TCR transgenic mice, of which only the positive control mice (IMA) expressed a Kb transgene. Importantly, Kb-positive IMA mice and their Kb-deficient experimental littermates (NIMA) were exposed to the Kb Ag during gestation and through breast-feeding, whereas nonexposed (NE) mice from Kb-deficient mothers were not. As expected, all IMA mice and most NIMA mice accepted Kb allogeneic heart transplants indefinitely, whereas NE mice exhibited acute rejection. IMA mice lacked detectable numbers of anti-Kb TCR transgenic T cells owing to self-Ag deletion, but, surprisingly, NIMA Kb-specific T cell numbers were at normal levels compared with those in NE mice. Upon exposure to Kb-expressing cells ex vivo, NIMA Kb-specific splenic T cells produced allograft tolerance-associated cytokines IL-10 and IL-4. In vivo, the selective depletion of T cell populations defined CD8+ T cells as mediators of Kb-induced allograft rejection and CD4+ T cells as mediators of tolerance. Significantly, the adoptive transfer of splenic Foxp3+CD4+CD25high T cells from NIMA mice into Kb -allograft recipient NE mice markedly improved their survival, thereby demonstrating that NIMA-associated transplantation tolerance arises from the induction of regulatory CD4+ T cells. These findings have implications for the design of tolerance protocols in allotransplantation.

Most vaccine strategies aimed at the bacterium Yersinia pestis have focused on eliciting humoral immunity. To interrogate the relevance of promoting a T cell response, Lin et al. (p. 1675) used the B cell-deficient mouse strain μMT to test vaccination with the Y. pestis strain D27 engineered to coexpress E. coli LpxL, which attenuates D27 by promoting an inflammatory innate immune response. Single immunization of μMT mice with live D27-pLpxL provided complete protection against a lethal intranasal challenge with a low dose, but not a high dose, of D27. However, when mice received a booster vaccination 30 d post primary inoculation, a 76% survival rate for high-dose D27 challenge was achieved. Surprisingly, the lung bacterial burden of high-dose–challenged prime/boost mice was not decreased compared with prime-only mice, and tests revealed no statistical difference in bacterial growth or dissemination. However, prime/boost mice had an increased frequency of IL-17–producing pulmonary CD4+ T cells, and IL-17–neutralizing mAb reduced the survival differential of high-dose–challenged prime/boost and prime-only mice, despite an unaltered bacterial burden. These data ascribe a role for IL-17 in cell-mediated defense against pulmonary Y. pestis infection, although further studies are required to define the underlying mechanisms.

The organogenesis of the secondary lymphoid tissues during embryogenesis is dependent upon hematopoietic lymphoid tissue inducer (LTi) cells. The differentiation of multipotent hematopoietic progenitor cells into LTis is incompletely understood, but it is known that the transcription factors Id2 and Rorγt are required. In this issue, Tachibana et al. (p 1450) show that the Runx1/Cbfβ transcription factor complex plays an important role in early LTi development. Mice deficient in either the P1-Runx1 isoform or the Cbfβ2 variant protein exhibited impaired development of Peyer’s patches and lymph nodes. The formation of the anlagen, or initial cluster of cells, involves a fetal liver-derived lineage marker-negative α4β7integrin+ IL-7Rα+ population. In mice deficient for Rorγt, P1-Runx1, or Cbfβ2, this population was significantly reduced compared with that in wild-type mice. Further analyses revealed that this population was divisible into early IL-7Rαmid and more mature IL-7Rαhigh populations. Expression of the P1-Runx1/Cbfβ2 complex was crucial for the development of both populations, whereas Rorγt/Id2 expression was required only for the development of the IL-7Rαhigh population. These data reveal that the P1-Runx1/Cbfβ2 complex plays an important role during both LTi lineage induction and the subsequent initiation of lymphoid tissue organogenesis.

IL-17+RORγt+TCRαβ+ cells, or Th17 cells, are most abundantly localized within the intestinal epithelium. Previous studies indicate that Th17 development requires symbiotic microbiota, but the nature of this relationship is unclear. To test if cognate microbial TCR Ags are required, Lochner et al. (p. 1531) generated mice that expressed enhanced GFP under the control of RORγt+ and a single TCR transgene specific for an MHC class II-restricted male self-Ag. In the intestinal milieu of female (hence Ag-deficient) mice, the number of Th17 cells was normal compared with control mice. However, dissection of the RORγt+ T cell population revealed a lack of regulatory Foxp3+RORγt+ T cells. When the influence of the microbiota was removed by raising the mice as germ-free, both IL-17+RORγt+ and Foxp3+RORγt+ T cell populations were markedly decreased compared with control mice. Inoculation with a single species of intestinal commensal bacteria raised IL-17+RORγt+ T cell numbers, but it took a more complex microbial flora to induce Foxp3+RORγt+ T cell numbers similar to control mice. This work confirms that Th17 development does not require the influence of the intestinal microbiota, yet also shows that the lack of specific microbial Ags negatively affects the generation of Foxp3+ RORγt+ T cell populations, which could lead to an enhanced inflammatory intestinal milieu.

Dendritic cells (DCs) have the capacity to promote both immunogenic and tolerogenic T cell responses. In this issue, two separate groups sought to better define the functional roles of various DC subsets. In the first manuscript, Atibalentja et al. (p. 1421) investigated the extent to which thymic DC subsets capture and present blood-derived MHC class II Ags to thymocytes and the functional outcome. When mice were i.v. injected with peptides derived from hen egg-white lysozyme (HEL), all thymic DC subsets, which included intrathymically-derived CD8α+SirpαCD11chi conventional DC (cDC) and bone marrow (BM)-derived plasmacytoid DC (pDC) and CD8αSirpα+ cDC, captured HEL. However, the cDC populations exhibited a similar efficiency of HEL presentation to T cells that was superior to that of both pDC and immature DC. cDC presentation of HEL Ag induced the deletion of HEL-specific CD4+CD8+ double positive thymocytes and the differentiation of HEL-specific CD4+ single positive T cells into regulatory T cells (Tregs). Although blood DCs could ferry HEL into the thymus, HEL-induced negative selection and Treg induction were not compromised by the depletion of blood APCs, indicating that thymic cDCs primarily mediated blood-derived Ag responses in the thymus. These data reveal that these intrathymic cDC subsets are functionally redundant as the primary APCs of blood-derived MHC class II Ags in the thymus.

In the second article, Björck et al. (p. 1477) investigated whether pDC-induced tolerogenic and inflammatory immune responses were attributable to a single pDC subset or to distinct pDC subsets. A survey of cell surface markers expressed by BM-derived pDCs revealed that the presence or absence of expression of the tetra-membrane spanning protein CD9 defined two phenotypically, functionally, and morphologically distinct CD11c+/B220+ pDC subsets. Both CD9+ and CD9 cells expressed the pDC-specific transcription factor E2-2, as well as IFN regulatory factor 7. However, CpG stimulation revealed that, although CD9+ pDC expressed large amounts of IFN-α, CD9 pDCs produced comparatively very little. Correspondingly, Ag-pulsed CD9+ pDCs induced significant T cell proliferation and promoted CD8+ T cell activation and CTL formation, whereas Ag-pulsed CD9 pDCs induced little T cell proliferation and promoted the generation of Tregs. Electron microscopy showed that CD9+ pDCs contained abundant rough endoplasmic reticulum, whereas CD9 pDCs contained relatively little. Finally, CD9+ pDCs were found to represent recently BM-derived pDCs that matured into CD9 pDCs in peripheral lymphoid tissues. These findings show that pDC-mediated inflammatory and tolerogenic responses are induced by distinct subsets that represent different maturation stages of BM-derived pDCs.

Summaries written by Meredith G. Safford, Ph.D.