The interaction of noncytotoxic decidual natural killer cells (dNK) and extravillous trophoblasts (EVT) at the maternal–fetal interface was studied. Confocal microscopy revealed that many dNK interact with a single large EVT. Filamentous projections from EVT enriched in HLA-G were shown to contact dNK, and may represent the initial stage of synapse formation. As isolated, 2.5% of dNK contained surface HLA-G. However, surface HLA-G–negative dNK contained internalized HLA-G. Activation of dNK resulted in the disappearance of internalized HLA-G in parallel with restoration of cytotoxicity. Surface HLA-G was reacquired by incubation with EVT. This HLA-G cycle of trogocytosis, endocytosis, degradation, and finally reacquisition provides a transient and localized acquisition of new functional properties by dNK upon interaction with EVT. Interruption of the cycle by activation of dNK by cytokines and/or viral products serves to ensure the NK control of virus infection at the interface, and is illustrated here by the response of dNK to human cytomegalo virus (HCMV)-infected decidual stromal cells. Thus, the HLA-G cycle in dNK can provide both for NK tolerance and antiviral immunity.

Imaging immune surveillance by natural killer (NK) cells has revealed that integration of activating and inhibitory signals determines whether or not NK cells stop to kill the target cell or retain a migratory configuration.

Rapid cell-mediated immune responses, characterized by production of proinflammatory cytokines, such as IFN-γ, can inhibit intraerythrocytic replication of malaria parasites and thereby prevent onset of clinical malaria. In this study, we have characterized the kinetics and cellular sources of the very early IFN-γ response to Plasmodium falciparum-infected RBCs among human PBMCs. We find that NK cells dominate the early (12–18 h) IFN-γ response, that NK cells and T cells contribute equally to the response at 24 h, and that T cells increasingly dominate the response from 48 h onward. We also find that although γδ T cells can produce IFN-γ in response to P. falciparum-infected RBCs, they are greatly outnumbered by αβ T cells, and thus, the majority of the IFN-γ+ T cells are αβ T cells and not γδ T cells; γδ T cells are, however, an important source of TNF. We have previously shown that NK cell responses to P. falciparum-infected RBCs require cytokine and contact-dependent signals from myeloid accessory cells. In this study, we demonstrate that NK cell IFN-γ responses to P. falciparum-infected RBCs are also crucially dependent on IL-2 secreted by CD4+ T cells in an MHC class II-dependent manner, indicating that the innate response to infection actually relies upon complex interactions between NK cells, T cells, and accessory cells. We conclude that activation of NK cells may be a critical function of IL-2–secreting CD4+ T cells and that standard protocols for evaluation of Ag-specific immune responses need to be adapted to include assessment of NK cell activation as well as T cell-derived IL-2.

Subsets of NK cells can have distinct functions. Here, we report that >25% of human peripheral blood NK cells express HLA-DR after culture with IL-2. This can be driven by an expansion of a small subset of NK cells expressing HLA-DR, in contrast to previous assumptions that HLA-DR is upregulated on previously negative cells. HLA-DR-expressing NK cells showed enhanced degranulation to susceptible target cells and expressed chemokine receptor CXCR3, which facilitated their enrichment following exposure to CXCL11/I-TAC. Suggesting HLA-DR-expressing NK cells have an important role in an immune response, stimulation of PBMCs with Mycobacterium bovis BCG (BCG) triggered expansion of this subset. Importantly, the magnitude of an individual's NK cell IFN-γ response triggered by BCG was associated with the initial frequency of HLA-DR-expressing NK cells in PBMCs. More directly indicating the importance of HLA-DR-expressing NK cells, enriching the frequency of this subset in PBMCs substantially augmented the IFN-γ response to BCG. Thus, HLA-DR expression marks a distinct subset of NK cells, present at low frequency in circulating blood but readily expanded by IL-2, that can play an important role during immune responses to BCG.

The activating receptor NKG2D recognizes proteins that are not normally expressed at the surface of most cells but are expressed during a cellular “stress” response (e.g., upon induction of the DNA damage pathway). This establishes recognition of “induced self” as an important strategy for surveillance of infections or tumor transformation. However, NKG2D ligands can also be induced on human macrophages by TLR stimulation, which has been far less studied. In this paper, we clarify that LPS, which ligates TLR-4, preferentially upregulated MICA and not MICB; CL097, which ligates TLR-7/8, upregulated both MICA and MICB; and polyinosinic-polycytidylic acid, which ligates TLR-3, upregulated neither. To probe how LPS stimulation triggers MICA expression, we determined that the stability of MICA mRNA was much longer than that of MICB mRNA, but neither was changed by LPS stimulation. This finding suggests that increased levels of MICA mRNA following LPS stimulation resulted from increased transcription. However, it was not sufficient for surface protein expression, which was controlled posttranscriptionally via a separate pathway involving the ataxia telangiectasia mutated/ataxia telangiectasia and Rad3 related kinases. Moreover, LPS stimulation decreased expression of microRNAs (miRNA)—miR-17-5, miR-20a, and miR-93—which target MICA, implicating a novel role for miRNAs in NKG2D ligand expression. Thus, TLR stimulation allows expression of NKG2D ligands through multiple pathways, including downmodulation of specific miRNAs.

Patients with Chronic Sarcoidosis have Reduced CD27 + IgM + IgD + Unswitched Memory B cells and an Expanded Population of Terminal Effector CD8 + CD27 -CD28 -T cells. J Clin Cell Immunol 3:132.