Herpesvirus DNA is detected by nonhematopoietic cells in the nucleus and by TLR9 in plasmacytoid dendritic cells. However, it is not clear how myeloid cells, which are important for the control of viral infections, sense herpesvirus DNA. Horan et al. (p. 2311) found that the proposed DNA sensor IFN-γ–inducible 16 (IFI16) was necessary for IFN-β production in human macrophages following infection with HSV-1 or CMV. Viral infection caused IFI16 to colocalize with stimulator of IFN genes (STING) and with viral genomic DNA in the cytoplasm. Cytoplasmic recognition of herpesvirus DNA was responsible for IFN-β induction, which was independent of viral nuclear entry. To determine how the herpesvirus DNA might become exposed to the cytosol for detection by IFI16, the authors examined the ubiquitin–proteasome pathway. HSV-1 capsids were found to be ubiquitinated and degraded by the proteasome in the cytosol of macrophages, after which viral DNA was released for recognition by IFI16 or other pattern recognition receptors. The identification of this process by which human myeloid cells recognize and respond to herpesvirus infection enhances our understanding of the innate immune response to DNA viruses and suggests that intracellular viral DNA detection pathways differ based on cell type.

The Vitamin A metabolite all-trans retinoic acid (RA) is important for the maturation and differentiation of a variety of immune cells, but its cell-intrinsic role in the generation of CD8+ effector and memory T cells remains unclear. In this issue, Allie et al. (p. 2178) analyzed CD8+ T cell differentiation following vaccinia virus (VV) infection in mice expressing a dominant negative RA receptor (RARαDN). Relative to wild-type mice, RARαDN mice had a reduced VV-specific CD8+ T cell response that included downregulation of several molecules associated with short-lived effector cell (SLEC) activity but upregulation of TNF-α production. These mice also demonstrated increased differentiation of memory precursor effector cells (MPECs). Experiments using VV-infected mixed bone marrow chimeric mice revealed that the MPEC skewing in RARαDN mice was a CD8+ T cell–intrinsic phenotype, as was the impairment in SLEC activity. Most of these effects were maintained during the contraction phase of the CD8+ T cell response. VV-specific CD8+ T cells deficient in RA signaling also demonstrated a skewing toward the central memory phenotype >60 d postinfection. Taken together, these data reveal the importance of RA signaling for proper differentiation of CD8+ T cells into effector and effector memory populations but caution that Vitamin A supplementation during vaccination may impair optimal central memory responses.

NKT cells and other innate-like T lymphocytes share a phenotype upon maturation that is characterized by memory-like effector responses. Unlike naive conventional T cells, they do not need to differentiate to secrete effector cytokines upon activation, but they do require the expression of promyelocytic zinc finger transcription factor (PLZF). Based on the observations that: 1) signaling lymphocyte activation molecule (SLAM) and Ly108 (SLAM family receptor) were important for the development of iNKT cells and 2) thymic Ly108 was phosphorylated in a contact-dependent manner, Dutta et al. (p. 2121) examined how Ly108 affects PLZF expression. They found that CD4+CD8+ double positive thymocytes that had not yet undergone selection could be costimulated through both the TCR and Ly108 to produce levels of PLZF similar to those found in NKT cells. Stimulation through the TCR alone or the TCR and CD28 were not able to increase PLZF in a comparable manner. Ly108 costimulation increased levels of Egr-2 expression and Egr-2 binding to the promoter of the PLZF gene. Ly108-deficient mice had reduced numbers of CD4+ T cells that expressed PLZF. The authors have therefore identified a pathway by which the SLAM receptor Ly108 enhances PLZF production and plays a role in the development of T cell subsets.

Leishmania causes significant disease and suffering but as of yet there is no vaccine that protects against this intracellular parasite. Instead of choosing to immunize against one target Ag, Dey et al. (p. 2138) chose a different approach, knocking out the Leishmania donovani amastigote-specific protein p27 (Ldp27) and infecting the host with these reduced-virulence parasites as an immunization protocol. Ldp27 encodes a component of the active cytochrome c oxidase complex in the intracellular amastigote form of the parasite. Ldp27−/− parasites were able to establish an infection but only survived 20 wk in host mice. Separate groups of mice were challenged with virulent parasites 12 wk and 20 wk post Ldp27−/− immunization, and protection was seen in both groups. Ldp27−/− immunization activated splenocytes to produce NO and enhanced the leishmaniacidal response. Not only were Th1 CD4+ and CD8+ T cell responses seen in response to immunization, but adoptive transfer of T cells from long-term immunized mice provided protection against virulent challenge in naive mice. Although the Ldp27−/− parasites were derived from L. donovani parasites, immunization could confer protection against L. major and L. braziliensis as well. Thus, the authors have developed a safe immunogen that provides long-term protection against a variety of Leishmania species.

IL-35 is an IL-12 family cytokine that is expressed by regulatory T cells and has immunosuppressive activities. Based on data suggesting that IL-35 can also be expressed in tumors, Wang et al. (p. 2415) hypothesized that this cytokine could promote tumor progression. IL-35 production was first confirmed in human cancer tissues, and its function was then assessed in mice. Murine plasmacytoma or melanoma cells expressing IL-35, when injected into either wild-type or Rag-deficient mice, grew into tumors much faster than did tumor cells lacking IL-35, and tumor growth enhancement was inhibited by a neutralizing anti–IL-35 mAb. In Rag-deficient mice, IL-35–producing tumors demonstrated increased angiogenesis and accumulation of myeloid-derived suppressor cells that were largely responsible for the enhanced growth relative to tumors lacking IL-35. In wild-type mice, tumor-derived IL-35 reduced numbers of intratumoral CD8+ T cells but did not directly impair their proliferation or effector activity. Interestingly, tumor cells expressing IL-35 were more resistant to attack by tumor-specific CD8+ T cells than were tumor cells lacking IL-35. IL-35 produced in the tumor microenvironment may therefore enhance tumor growth and immune evasion through multiple mechanisms and may be a useful target for cancer immunotherapy.

Dendritic cells (DCs) and monocytes/macrophages can be stimulated by ssRNA from HIV-1; however, the involvement of these mononuclear phagocytes in HIV pathogenesis is debated. In an attempt to resolve this controversy, Wonderlich et al. (p. 2188) analyzed the dynamics and functions of rhesus macaque blood and lymph node (LN) mononuclear phagocytes following acute infection with SIVmac251. After analyzing the SIV-induced mobilization and recruitment patterns of plasmacytoid DCs (pDCs), monocytes, and macrophages in the blood and LNs, the authors addressed how SIV infection affected their function. Uridine-rich regions that could act as TLR7/8 ligands were identified in the env coding region of SIVmac251 ssRNA. In all mononuclear phagocytes assayed in uninfected macaques, the uridine-rich sequences stimulated TNF-α production, additionally inducing IFN-α in pDCs and macrophages. SIVmac251 infection impaired responsiveness to these ligands by pDCs and monocytes/macrophages in both blood and LNs. In contrast, myeloid DCs (mDCs) became hyperresponsive to TLR7/8 ligands following infection. Both hyporesponsiveness and hyperresponsiveness of mononuclear phagocytes was reversed by antiretroviral therapy. Interestingly, macaques with increased mDC responsiveness to the SIV-encoded TLR ligands were more likely to control SIV infection long term. This study demonstrates that SIV encodes TLR ligands that elicit different responses from different subtypes of mononuclear phagocytes, including beneficial anti-SIV activity in mDCs.

Successful tumor immunotherapy requires attacking malignant cells and overcoming the suppressive microenvironment that intratumoral regulatory cells create. Conventional αβ regulatory T cells (Tregs) have been extensively studied for their role in negative regulation within the tumor microenvironment. Following observations that γδ Tregs can also play a suppressive role in disease, including cancer, Ye et al. (p. 2403) examined the specific effects of γδ Tregs from breast tumors on innate and adaptive immunity. These intratumoral γδ Tregs induced immunosenesecence in DCs and T cells, altering their phenotypic profiles and functions. This immunosenescence had an amplification effect, as cells induced by γδ Tregs became suppressive themselves. Disruption of TLR8 signaling in γδ Tregs was capable of blocking the suppressive and amplification effects of γδ Tregs both in vitro and in vivo. The authors have identified a novel pathway of intratumoral immunosuppression mediated through TLR8 and γδ Tregs, giving further insight into the tumor microenvironment and how best to aid immunotherapy.

Hematopoietic humanized mice (hu-mice) can be used to model the human immune system; however, these mice have a block in the maturation of human B cells. A similar impairment in B cell reconstitution occurs in patients receiving transplants of cord blood stem cells. To better understand the process of immune reconstitution in hu-mice and transplant recipients, Lang et al. (p. 2090) systematically characterized the kinetics of human T and B cell development in BALB/c-Rag2nullIl2rγnull hu-mice. In these mice, B cell reconstitution consistently preceded T cell reconstitution. Although the majority of splenic and peripheral blood B cells in the hu-mice were immature, a significant fraction of mature B cells could be found in the lymph nodes (LNs) and, as the mice aged, in the spleen. The presence of splenic mature B cells correlated with increasing frequencies of LN and splenic T cells, particularly those with activated and memory phenotypes. Mature human B cells were also linked to improved IgM and IgG responses in response to challenge with both T cell–dependent and –independent Ags. Transfer of exogenous human T cells into hu-mice or depletion of developing human T cells in these mice revealed that T cells were required for human B cell maturation. These data provide a framework for the understanding of human B cell development and B cell reconstitution following stem cell transplantation, and emphasize the importance of T cells in this process.