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Two of the three main categories of innate lymphoid cells (ILCs) in humans, group 1 and 3 cells (ILC1 and ILC3), have been observed to express natural cytotoxicity receptors, but it is not known if these receptors are expressed on ILC2. NKp30 is one such receptor expressed mainly by human NK cells as three isoforms, of which two are stimulatory and one, NKp30c, is considered regulatory. Salimi et al. (p. 45) have now found that NKp30 is expressed on human ILC2 and have analyzed its function on these cells. Large fractions of both cultured ILC2 lines and ex vivo human ILC2 expressed NKp30, particularly the NKp30c isoform. Crosslinking NKp30 on ILC2 with one of its ligands, B7-H6, or incubation of ILC2 with B7-H6–expressing tumor cells stimulated production of type 2 cytokines including IL-13, IL-5, and GM-CSF. Type 2 cytokine production could be inhibited by blocking NKp30 binding using Galectin-3 or Abs specific for NKp30 or B7-H6. Stimulation of ILC2 via NKp30 crosslinking involved the canonical NF-κB signaling pathway and resulted in increased expression of genes encoding type 2 cytokines. To address the physiological relevance of the interaction of NKp30 expressed on ILC2 with B7-H6, lesional skin biopsies from atopic dermatitis patients were compared with healthy control skin. Relative to healthy skin, atopic dermatitis lesions had higher expression of B7-H6 with a broader distribution among the skin layers. Incubation of a keratinocyte cell line with cytokines including IL-13 and IFN-γ led to upregulation of B7-H6 and resulted in dose-dependent stimulation of IL-13 production from cocultured ILC2. These data present a mechanism for the stimulation of ILC2-mediated type 2 immune responses and suggest that NKp30 could serve as a therapeutic target for diseases such as atopic dermatitis.

γδ T cells bearing certain Vγ-chains home to different tissues and serve specialized functions; for example, Vγ4⁺ T cells produce IL-17 and some Vγ1⁺ T cells produce IL-4. Genetic knockouts of one subset have also been shown to affect the phenotype of other γδ T cell subsets as well as homeostatic serum Ig levels, suggesting that γδ T cells influence other immune cells. In this issue, Huang et al. (p. 217) examined the effect that deficiencies in different γδ T cell subsets had on B cell development and homeostasis. Developing B cells in the bone marrow appeared largely unaltered between wild-type, B6.TCR-δ^−/−, B6.TCR-Vγ1^−/−, and B6.TCR-Vγ4^−/−/6^−/− mice, but mature B cell populations were affected. Specifically, B6.TCR-Vγ4^−/−/6^−/− mice had increased IgG1-producing plasma cells and decreased numbers of transitional, follicular, and CD5⁺ B1 B cells and almost no marginal zone B cells. In contrast, B6.TCR-δ^−/− mice had relatively normal proportions of B cell populations. Genetic ablation of IL-4 reversed the B cell defects observed in B6.TCR-Vγ4^−/−/6^−/− mice. Adoptive transfer of B6.TCR-Vγ4^−/−/6^−/− γδ T cells, which are mostly Vγ1⁺ T cells, reduced marginal zone B cells in B6.TCR-δ^−/− recipients, an effect that was abrogated if the transferred cells were also IL-4^−/−. In vivo CD45 labeling suggested that splenic γδ T cells were relatively exposed to the circulation, potentially residing in the marginal zone. Splenic γδ T–B cell conjugates showed an enrichment of marginal zone B cells and higher CD40L and ICOS expression on γδ T cells, again suggesting that these cells may directly interact. Taken together, these results suggest that γδ T cell subsets directly influence the differentiation of B cells, possibly through cytokine secretion or cognate interaction.

Limited options of suitable model systems have made it difficult to assess the ability of adjuvants to enhance human CD8⁺ T cell responses. In this issue, Lissina et al. (p. 256) developed an in vitro approach that allowed qualitative and quantitative comparison of adjuvant effects on Ag-specific CD8⁺ T cell priming. In this model, human dendritic cells (DCs) were mobilized within a culture of unfractionated PBMCs from HLA-A2⁺ individuals, and these cultures were then stimulated with a Melan-A/MART-1 epitope (ELA) that was recognized by a high frequency of naive CD8⁺ T cells. The authors used this system to analyze the ability of the ssRNA40 ligand for TLR8 (TLR8L) to influence CD8⁺ T cell activity, as the nonfunctional nature of mouse TLR8 has prevented study of this ligand in murine systems. PBMC cultures were stimulated with GM-CSF/IL-4 or FLT3L to mobilize DCs and were then exposed to inflammatory cytokines, LPS, or TLR8L in combination with the ELA epitope. ELA-specific CD8⁺ T cells primed in cultures stimulated with FLT3L and TLR8L had significantly greater cytotoxic potential, polyfunctional cytokine responses, Ag sensitivity, and T-bet expression than cells primed under the other conditions. Blockade of IL-12 at the beginning of the culture period reduced T-bet expression and, consequently, the expression of cytotoxic mediators and the polyfunctionality of these cells, indicating that IL-12 was important for TLR8L-stimulated CD8⁺ T cell activity. Thus, TLR8L may prove to be a useful adjuvant for immunotherapy applications, and this culture system can be used to characterize the effectiveness of other adjuvants in stimulating human CD8⁺ T cell responses.

Neutrophils and other cells produce the antimicrobial protein calprotectin (CP), a heterodimer of S100A8 and S100A9 that combats pathogens through mechanisms that include sequestration of Mn and Zn. Clark et al. (p. 336) investigated how CP affects different stages of Aspergillus fumigatus in corneal infection. CP-deficient S100A9^−/− neutrophils were able to phagocytose and kill fungal conidia (spores) similarly to wild-type (WT) neutrophils in vitro but were unable to stunt hyphal growth like WT neutrophils. Corneal fungal burden was higher in S100A9^−/− than WT mice, although neutrophil numbers were equivalent. Administration of recombinant CP to corneas of S100A9^−/− mice reduced hyphal mass. Further experiments were carried out to determine if the protective effect of CP was directly related to limitation of Zn and Mn. In vitro incubation of CP with A. fumigatus led to increased branching and stunted hyphae, similar to that observed following incubation with WT neutrophils. Addition of ZnSO₄ and MnSO₄ rescued hyphal growth and a CP mutant unable to bind Zn and Mn was incapable of anti-fungal activity. A. fumigatus expression of the transcription factor ZafA enhances Zn uptake, and ΔzafA A. fumigatus was more susceptible than the WT fungus to killing by neutrophils or recombinant CP. Infection of WT mouse corneas with ΔzafA A. fumigatus led to lower fungal burden, even though neutrophil recruitment was comparable to infection with WT A. fumigatus. These results demonstrate how Zn and Mn chelation by CP can combat corneal A. fumigatus infection.

CD4⁺ T cell–dependent immunity is impaired in Mycobacterium tuberculosis infection, which may contribute to the lack of bacterial clearance, as compared with the less virulent pathogen M. bovis bacillus Calmette–Guérin (BCG), which can be eliminated by CD4⁺ T cells. Grace and Ernst (p. 357) hypothesized that M. tuberculosis restricts CD4⁺ T cell activation via interference with Ag presentation and addressed this hypothesis by analyzing responses of transgenic CD4⁺ T cells specific for a peptide of the immunodominant Ag85B that is present in both BCG and M. tuberculosis. Relative to infection with M. tuberculosis, infection with BCG resulted in earlier proliferation of CD4⁺ T cells in the lung-draining mediastinal lymph node (MDLN) and earlier arrival of primed T cells in the lung. Priming of these CD4⁺ T cells required the presence of fewer BCG than M. tuberculosis bacteria in the MDLN. When dendritic cells (DCs) were infected with equivalent numbers of BCG or M. tuberculosis, those infected with BCG more efficiently activated the transgenic T cells following transfer into host mice. In vitro experiments revealed that Th1 Ag85B-specific cells consistently secreted more IFN-γ in response to BCG-infected DCs or macrophages than to M. tuberculosis-infected cells. The reduced ability of M. tuberculosis- versus BCG-infected DCs to activate CD4⁺ T cells did not result from differences in expression of MHC II or costimulatory molecules but was instead traced to presentation of lower levels of bacterial Ag. This impaired Ag presentation may influence the persistence of M. tuberculosis infection and thus has implications for the development of novel vaccine strategies to combat this pervasive pathogen.