Natural killer (NK) cells recognize the absence of self MHC class I as a way to discriminate normal cells from cells in distress. In humans, this "missing self" recognition is ensured by inhibitory receptors such as KIR, which dampen NK cell activation upon interaction with their MHC class I ligands. We show here that NK cells lacking inhibitory KIR for self MHC class I molecules are present in human peripheral blood. These cells harbor a mature NK cell phenotype but are hyporesponsive to various stimuli, including MHC class I-deficient target cells. This response is in contrast to NK cells that express a single inhibitory KIR specific for self MHC class I, which are functionally competent when exposed to the same stimuli. These results show the involvement of KIR-MHC class I interactions in the calibration of NK cell effector capacities, suggesting its role in the subsequent "missing self" recognition.

Naturally acquired immunity to malaria develops slowly, requiring several years of repeated exposure to be effective. The cellular and molecular factors underlying this observation are only partially understood. Recent studies suggest that chronic Plasmodium falciparum (Pf) exposure may induce functional exhaustion of lymphocytes, potentially impeding optimal control of infection. However, it remains unclear if the “atypical” memory B-cells (MBCs) and “exhausted” CD4 T-cells described in humans exposed to endemic malaria are driven by Pf per se or by other factors commonly associated with malaria, such as co-infections and malnutrition. To address this critical question we took advantage of a ‘natural’ experiment near Kilifi, Kenya, and compared profiles of B and T-cells of children living in a rural community where Pf-transmission is on-going, to the profiles of age-matched children living under similar conditions in a nearby community where Pf transmission ceased 5 years prior to this study. We found that continuous exposure to Pf drives the expansion of “atypical” MBCs. Persistent Pf-exposure was associated with an increased frequency of CD4 T-cells expressing phenotypic markers of exhaustion, both PD-1 alone and PD-1 in combination with LAG-3. This expansion of PD-1 expressing and PD-1/LAG-3 co-expressing CD4 T-cells was largely confined to CD45RA+ CD4 T-cells. The percentage of CD45RA+CD27+ CD4 T-cells co-expressing PD-1 and LAG-3 was inversely correlated with frequencies of activated and classical MBCs. Together, these results suggest that Pf infection per se drives the expansion of atypical MBCs and phenotypically “exhausted” CD4 T-cells, which has been reported in other endemic areas.

IFN-␥ secretion by natural killer (NK) cells is pivotal to

The host response following malaria infection depends on a fine balance between levels of pro-inflammatory and anti-inflammatory mediators resulting in the resolution of the infection or immune-mediated pathology. Whilst other components of the innate immune system contribute to the pro-inflammatory milieu, T cells play a major role. For blood-stage malaria, CD4+ and γδ T cells are major producers of the IFN-γ that controls parasitemia, however, a role for TH17 cells secreting IL-17A and other cytokines, including IL-17F and IL-22 has not yet been investigated in malaria. TH17 cells have been shown to play a role in some protozoan infections, but they also are a source of pro-inflammatory cytokines known to be involved in protection or pathogenicity of infections. In the present study, we have investigated whether IL-17A and IL-22 are induced during a Plasmodium chabaudi infection in mice, and whether these cytokines contribute to either protection or to pathology induced during the infection. Although small numbers of IL-17- and IL-22-producing CD4 T cells are induced in the spleens of infected mice, a more pronounced induction is observed in the liver, where increases in mRNA for IL-17A and, to a lesser extent, IL-22 were observed and CD8+ T cells, rather than CD4 T cells, are a major source of these cytokines in this organ. Although the lack of IL-17 did not affect the outcome of infection or pathology, lack of IL-22 resulted in 50% mortality within 12 days after infection with significantly greater weight loss at the peak of infection and significant increase in alanine transaminase in the plasma in the acute infection. As parasitemias and temperature were similar in IL-22 KO and wild-type control mice, our observations support the idea that IL-22 but not IL-17 provides protection from the potentially lethal effects of liver damage during a primary P. chabaudi infection.

BackgroundEpstein-Barr virus (EBV) is the major environmental factor associated with Hodgkin's lymphoma (HL), a common lymphoma in young adults. Natural killer (NK) cells are key actors of the innate immune response against viruses. The regulation of NK cell function involves activating and inhibitory Killer cell Immunoglobulin-like receptors (KIRs), which are expressed in variable numbers on NK cells. Various viral and virus-related malignant disorders have been associated with the presence/absence of certain KIR genes in case/control studies. We investigated the role of the KIR cluster in HL in a family-based association study.MethodologyWe included 90 families with 90 HL index cases (age 16–35 years) and 255 first-degree relatives (parents and siblings). We developed a procedure for reconstructing full genotypic information (number of gene copies) at each KIR locus from the standard KIR gene content. Out of the 90 collected families, 84 were informative and suitable for further analysis. An association study was then carried out with specific family-based analysis methods on these 84 families.Principal FindingsFive KIR genes in strong linkage disequilibrium were found significantly associated with HL. Refined haplotype analysis showed that the association was supported by a dominant protective effect of KIR3DS1 and/or KIR2DS1, both of which are activating receptors. The odds ratios for developing HL in subjects with at least one copy of KIR3DS1 or KIR2DS1 with respect to subjects with neither of these genes were 0.44[95% confidence interval 0.23–0.85] and 0.42[0.21–0.85], respectively. No significant association was found in a tentative replication case/control study of 68 HL cases (age 18–71 years). In the familial study, the protective effect of KIR3DS1/KIR2DS1 tended to be stronger in HL patients with detectable EBV in blood or tumour cells.ConclusionsThis work defines a template for family-based association studies based on full genotypic information for the KIR cluster, and provides the first evidence that activating KIRs can have a protective role in HL.

BackgroundHost innate immunity contributes to malaria clinical outcome by providing protective inflammatory cytokines such as interferon-γ, and by shaping the adaptive immune response. Plasmodium falciparum (Pf) is the etiologic agent of the most severe forms of human malaria. Natural Killer (NK) cells are lymphocytes of the innate immune system that are the first effectors to produce interferon-γ in response to Pf. However, the molecular bases of Pf-NK cell recognition events are unknown. Our study focuses on the role of Pf erythrocyte membrane protein 1 (PfEMP1), a major Pf virulence factor. PfEMP1 is expressed on parasitized-erythrocytes and participates to vascular obstruction through the binding to several host receptors. PfEMP1 is also a pivotal target for host antibody response to Pf infection.Methodology/Principal FindingsUsing genetically-engineered parasite mutant strains, a human genetic deficiency, and blocking antibodies, we identified two receptor-ligand pairs involved in two uncoupled events occurring during the sensing of Pf infection by NK cells. First, PfEMP1 interaction with one of its host receptor, chondroitin sulfate A, mediates the cytoadhesion of Pf-infected erythrocytes to human NK cell lines, but is not required for primary NK cell activation. Second, intercellular adhesion molecule-1 (ICAM-1), another host receptor for PfEMP1, is mandatory for NK cell interferon-γ response. In this case, ICAM-1 acts via its engagement with its host ligand, LFA-1, and not with PfEMP1, consistent with the obligatory cross-talk of NK cells with macrophages for their production of interferon-γ.Conclusion/SignificancePfEMP1-independent but ICAM-1/LFA-1-dependent events occurring during NK cell activation by Pf highlight the fundamental role of cellular cooperation during innate immune response to malaria.

Malaria, caused by the infection with parasites of the germs Plasmodium, is one of the three most important infectious diseases worldwide, along with tuberculosis and infection with human immunodeficiency virus. Natural killer (NK) cells are lymphocytes classically involved in the early defense against viral infections and intracytoplasmic bacterial infections and are also implicated during the course of tumor development and allogeneic transplantation. These cells display important cytotoxic activity and produce high levels of proinflammatory cytokines. In both mouse and human models of malaria, NK cells appear to be a major source of interferon-gamma during the early phase of infection. In humans, indirect signaling through monocytes/macrophages required to optimally stimulate NK cell activity. However, the in vivo functions of NK cells during malaria are still enigmatic, and many issues remain to be dissected, such as the molecular basis of the direct recognition of iRBCs by NK cells.

BackgroundMultiparameter flow cytometry has revealed extensive phenotypic and functional heterogeneity of CD4 T cell responses in mice and humans, emphasizing the importance of assessing multiple aspects of the immune response in correlation with infection or vaccination outcome. The aim of this study was to establish and validate reliable and feasible flow cytometry assays, which will allow us to characterize CD4 T cell population in humans in field studies more fully.Methodology/Principal FindingsWe developed polychromatic flow cytometry antibody panels for immunophenotyping the major CD4 T cell subsets as well as broadly characterizing the functional profiles of the CD4 T cells in peripheral blood. We then validated these assays by conducting a pilot study comparing CD4 T cell responses in distinct populations of healthy adults living in either rural or urban Kenya. This study revealed that the expression profile of CD4 T cell activation and memory markers differed significantly between African and European donors but was similar amongst African individuals from either rural or urban areas. Adults from rural Kenya had, however, higher frequencies and greater polyfunctionality among cytokine producing CD4 T cells compared to both urban populations, particularly for “Th1” type of response. Finally, endemic exposure to malaria in rural Kenya may have influenced the expansion of few discrete CD4 T cell populations with specific functional signatures.Conclusion/SignificanceThese findings suggest that environmentally driven T cell activation does not drive the dysfunction of CD4 T cells but is rather associated with greater magnitude and quality of CD4 T cell response, indicating that the level or type of microbial exposure and antigenic experience may influence and shape the functionality of CD4 T cell compartment. Our data confirm that it is possible and mandatory to assess multiple functional attributes of CD4 T cell response in the context of infection.