Giuliana Magri

B cell–helper neutrophils stimulate the diversification and production of immunoglobulin in the marginal zone of the spleen

Neutrophils utilize immunoglobulins (Igs) to clear antigen, but their role in Ig production is unknown. Here we identified neutrophils around the marginal zone (MZ) of the spleen, a B cell area specialized in T-independent Ig responses to circulating antigen. Neutrophils colonized peri-MZ areas after post-natal mucosal colonization by microbes and enhanced their B-helper function upon receiving reprogramming signals from splenic sinusoidal endothelial cells, including interleukin 10 (IL-10). Splenic neutrophils induced Ig class switching, somatic hypermutation and antibody production by activating MZ B cells through a mechanism involving the cytokines BAFF, APRIL and IL-21. Neutropenic patients had fewer and hypomutated MZ B cells and less preimmune Igs to T-independent antigens, which indicates that neutrophils generate an innate layer of antimicrobial Ig defense by interacting with MZ B cells.Neutrophils use immunoglobulins to clear antigen, but their role in immunoglobulin production is unknown. Here we identified neutrophils around the marginal zone (MZ) of the spleen, a B cell area specialized in T cell–independent immunoglobulin responses to circulating antigen. Neutrophils colonized peri-MZ areas after postnatal mucosal colonization by microbes and enhanced their B cell–helper function after receiving reprogramming signals, including interleukin 10 (IL-10), from splenic sinusoidal endothelial cells. Splenic neutrophils induced immunoglobulin class switching, somatic hypermutation and antibody production by activating MZ B cells through a mechanism that involved the cytokines BAFF, APRIL and IL-21. Neutropenic patients had fewer and hypomutated MZ B cells and a lower abundance of preimmune immunoglobulins to T cell–independent antigens, which indicates that neutrophils generate an innate layer of antimicrobial immunoglobulin defense by interacting with MZ B cells.

Innate lymphoid cells integrate stromal and immunological signals to enhance antibody production by splenic marginal zone B cells

Innate lymphoid cells (ILCs) regulate stromal cells, epithelial cells and cells of the immune system, but their effect on B cells remains unclear. Here we identified RORγt+ ILCs near the marginal zone (MZ), a splenic compartment that contains innate-like B cells highly responsive to circulating T cell–independent (TI) antigens. Splenic ILCs established bidirectional crosstalk with MAdCAM-1+ marginal reticular cells by providing tumor-necrosis factor (TNF) and lymphotoxin, and they stimulated MZ B cells via B cell–activation factor (BAFF), the ligand of the costimulatory receptor CD40 (CD40L) and the Notch ligand Delta-like 1 (DLL1). Splenic ILCs further helped MZ B cells and their plasma-cell progeny by coopting neutrophils through release of the cytokine GM-CSF. Consequently, depletion of ILCs impaired both pre- and post-immune TI antibody responses. Thus, ILCs integrate stromal and myeloid signals to orchestrate innate-like antibody production at the interface between the immune system and circulatory system.

NKp46 and DNAM-1 NK-cell receptors drive the response to human cytomegalovirus-infected myeloid dendritic cells overcoming viral immune evasion strategies

Information on natural killer (NK)-cell receptor-ligand interactions involved in the response to human cytomegalovirus (HCMV) is limited and essentially based on the study of infected fibroblasts. Experimental conditions were set up to characterize the NK response to HCMV-infected myeloid dendritic cells (DCs). Monocyte-derived DCs (moDCs) infected by the TB40/E HCMV strain down-regulated the expression of human leukocyte antigen class I molecules and specifically activated autologous NK-cell populations. NKG2D ligands appeared virtually undetectable in infected moDCs, reflecting the efficiency of immune evasion mechanisms, and explained the lack of antagonistic effects of NKG2D-specific monoclonal antibody. By contrast, DNAM-1 and DNAM-1 ligands (DNAM-1L)-specific monoclonal antibodies inhibited the NK response at 48 hours after infection, although the impact of HCMV-dependent down-regulation of DNAM-1L in infected moDCs was perceived at later stages. moDCs constitutively expressed ligands for NKp46 and NKp30 natural cytotoxicity receptors, which were partially reduced on HCMV infection; yet, only NKp46 appeared involved in the NK response. In contrast to previous reports in fibroblasts, human leukocyte antigen-E expression was not preserved in HCMV-infected moDCs, which triggered CD94/NKG2A(+) NK-cell activation. The results provide an insight on key receptor-ligand interactions involved in the NK-cell response against HCMV-infected moDCs, stressing the importance of the dynamics of viral immune evasion mechanisms.

Intestinal IgA production and its role in host‐microbe interaction

Complex and diverse communities of bacteria establish mutualistic and symbiotic relationships with the gut after birth. The intestinal immune system responds to bacterial colonization by acquiring a state of hypo‐responsiveness against commensals and active readiness against pathogens. The resulting homeostatic balance involves a continuous dialog between the microbiota and lymphocytes with the intermediation of epithelial and dendritic cells. This dialog causes massive production of immunoglobulin A (IgA), a non‐inflammatory antibody specialized in mucosal protection. Here, we discuss recent advances on the regulation of intestinal IgA responses and their role in host‐microbe interaction.

Natural killer cell-mediated response to human cytomegalovirus-infected macrophages is modulated by their functional polarization

MΦ comprise a heterogeneous population of cells, which contribute to host defense and maintenance of immune homeostasis. MΦ may be infected by human cytomegalovirus (HCMV), which has evolved different strategies to subvert the immune response. In the present study, we comparatively analyzed the natural killer (NK) cell response against HCMV (TB40E)‐infected proinflammatory (M1) and antinflammatory (M2) MΦ, derived from autologous monocytes, cultured in the presence of GM‐CSF and M‐CSF, respectively. M1 MΦ were more resistant to infection and secreted IL‐6, TNF‐α, IFN‐α, and IL‐12; by contrast, in HCMV‐infected M2 MΦ, proinflammatory cytokines, IL‐10, and IFN‐α production were limited and IL‐12 was undetectable. NK cell degranulation was triggered by interaction with HCMV‐infected M1 and M2 MΦ at 48 h postinfection. The response was partially inhibited by specific anti‐NKp46, anti‐DNAM‐1, and anti‐2B4 mAb, thus supporting a dominant role of these activating receptors. By contrast, only HCMV‐infected M1 MΦ efficiently promoted NK cell‐mediated IFN‐γ secretion, an effect partially related to IL‐12 production. These observations reveal differences in the NK cell response triggered by distinct, HCMV‐infected, monocyte‐derived cell types, which may be relevant in the immunopathology of this viral infection.

Inhibition of NKG2D expression in NK cells by cytokines secreted in response to human cytomegalovirus infection

The NKG2D receptor activates natural killer (NK) cell cytotoxicity and cytokine production on recognition of self-molecules induced by cellular stress under different conditions such as viral infections. The importance of NKG2D in the immune response to human cytomegalovirus (HCMV) is supported by the identification of several viral molecules that prevent the expression of NKG2D ligands by infected cells. In this study we report that, paradoxically, a significant, selective, and transient reduction of NKG2D expression on NK cells is detected during HCMV infection of peripheral blood mononuclear cells if needed. Antagonizing type I interferon (IFN), interleukin-12 (IL-12), and IFNgamma prevented HCMV-induced down-regulation of surface NKG2D. Moreover, treatment of purified NK cells with recombinant IFNbeta1 and IL-12 mimicked the effect, supporting a direct role of these cytokines in regulating NKG2D surface expression in NK cells. The loss of NKG2D expression selectively impaired NK-cell cytotoxicity against cells expressing NKG2D ligands but preserved the response triggered through other activating receptors. These results support that down-regulation of NKG2D expression on NK cells by cytokines with a key role in antiviral immune response may constitute a physiologic mechanism to control NK-cell reactivity against normal cells expressing NKG2D ligands in the context of inflammatory responses to viral infections.

IL-12-Dependent Inducible Expression of the CD94/NKG2A Inhibitory Receptor Regulates CD94/NKG2C+ NK Cell Function

The inhibitory CD94/NKG2A and activating CD94/NKG2C killer lectin-like receptors specific for HLA-E have been reported to be selectively expressed by discrete NK and T cell subsets. In the present study, minor proportions of NK and T cells coexpressing both CD94/NKG2A and CD94/NKG2C were found in fresh peripheral blood from adult blood donors. Moreover, CD94/NKG2A surface expression was transiently detected upon in vitro stimulation of CD94/NKG2C+ NK cells in the presence of irradiated allogeneic PBMC or rIL-12. A similar effect was observed upon coculture of NKG2C+ NK clones with human CMV-infected autologous dendritic cell cultures, and it was prevented by an anti-IL-12 mAb. NKG2A inhibited the cytolytic activity of NKG2C+ NK clones upon engagement either by a specific mAb or upon interaction with a transfectant of the HLA class I-deficient 721.221 cell line expressing HLA-E. These data indicate that beyond its constitutive expression by an NK cell subset, NKG2A may be also transiently displayed by CD94/NKG2C+ NK cells under the influence of IL-12, providing a potential negative regulatory feedback mechanism.

The B cell helper side of neutrophils

Neutrophils use opsonizing antibodies to enhance the clearance of intruding microbes. Recent studies indicate that splenic neutrophils also induce antibody production by providing helper signals to B cells lodged in the MZ of the spleen. Here, we discuss the B cell helper function of neutrophils in the context of growing evidence indicating that neutrophils function as sophisticated regulators of innate and adaptive immune responses.

Brief Report: Late-Onset Cryopyrin-Associated Periodic Syndrome Due to Myeloid-Restricted Somatic NLRP3 Mosaicism.

Gain‐of‐function NLRP3 mutations cause cryopyrin‐associated periodic syndrome (CAPS), with gene mosaicism playing a relevant role in the pathogenesis. This study was undertaken to characterize the genetic cause underlying late‐onset but otherwise typical CAPS.

Late onset cryopyrin‐associated periodic syndrome due to myeloid‐restricted somatic NLRP3 mosaicism

Gain‐of‐function NLRP3 mutations cause cryopyrin‐associated periodic syndrome (CAPS), with gene mosaicism playing a relevant role in the pathogenesis. This study was undertaken to characterize the genetic cause underlying late‐onset but otherwise typical CAPS.