The retroelement Lx9 puts a brake on the immune response to virus infection.

The notion that mobile units of nucleic acid known as transposable elements can operate as genomic controlling elements was put forward over six decades ago1,2. However, it was not until the advancement of genomic sequencing technologies that the abundance and repertoire of transposable elements were revealed, and they are now known to constitute up to two-thirds of mammalian genomes3,4. The presence of DNA regulatory regions including promoters, enhancers and transcription-factor-binding sites within transposable elements5-8 has led to the hypothesis that transposable elements have been co-opted to regulate mammalian gene expression and cell phenotype8-14. Mammalian transposable elements include recent acquisitions and ancient transposable elements that have been maintained in the genome over evolutionary time. The presence of ancient conserved transposable elements correlates positively with the likelihood of a regulatory function, but functional validation remains an essential step to identify transposable element insertions that have a positive effect on fitness. Here we show that CRISPR-Cas9-mediated deletion of a transposable element-namely the LINE-1 retrotransposon Lx9c11-in mice results in an exaggerated and lethal immune response to virus infection. Lx9c11 is critical for the neogenesis of a non-coding RNA (Lx9c11-RegoS) that regulates genes of the Schlafen family, reduces the hyperinflammatory phenotype and rescues lethality in virus-infected Lx9c11-/- mice. These findings provide evidence that a transposable element can control the immune system to favour host survival during virus infection.

Denisovan, modern human and mouse TNFAIP3 alleles tune A20 phosphorylation and immunity.

Resisting and tolerating microbes are alternative strategies to survive infection, but little is known about the evolutionary mechanisms controlling this balance. Here genomic analyses of anatomically modern humans, extinct Denisovan hominins and mice revealed a TNFAIP3 allelic series with alterations in the encoded immune response inhibitor A20. Each TNFAIP3 allele encoded substitutions at non-catalytic residues of the ubiquitin protease OTU domain that diminished IκB kinase-dependent phosphorylation and activation of A20. Two TNFAIP3 alleles encoding A20 proteins with partial phosphorylation deficits seemed to be beneficial by increasing immunity without causing spontaneous inflammatory disease: A20 T108A;I207L, originating in Denisovans and introgressed in modern humans throughout Oceania, and A20 I325N, from an N-ethyl-N-nitrosourea (ENU)-mutagenized mouse strain. By contrast, a rare human TNFAIP3 allele encoding an A20 protein with 95% loss of phosphorylation, C243Y, caused spontaneous inflammatory disease in humans and mice. Analysis of the partial-phosphorylation A20 I325N allele in mice revealed diminished tolerance of bacterial lipopolysaccharide and poxvirus inoculation as tradeoffs for enhanced immunity.

Cytosolic Recognition of RNA Drives the Immune Response to Heterologous Erythrocytes.

The archetypal T cell-dependent antigen is sheep red blood cells (SRBCs), which have defined much of what we know about humoral immunity. Early studies using solubilized or sonicated SRBCs argued that the intact structure of SRBCs was important for optimal antibody responses. However, the reason for the requirement of intact SRBCs for the response to polyvalent protein antigen remained unknown. Here, we report that the immune response to SRBCs is driven by cytosolic recognition of SRBC RNA through the RIG-I-like receptor (RLR)-mitochondrial anti-viral signaling adaptor (MAVS) pathway. Following the uptake of SRBCs by antigen-presenting cells, the MAVS signaling complex governs the differentiation of both T follicular cells and antibody-producing B cells. Importantly, the involvement of the RLR-MAVS pathway precedes that of endosomal Toll-like receptor pathways, yet both are required for optimal effect.

Cytokine Expression by T Follicular Helper Cells.

T follicular helper (Tfh) cells are a specialized subset of CD4+ T cells located within temporary structures known as germinal centers (GC) formed within B cell follicles of secondary lymphoid organs. In the GC, Tfh cells facilitate the production of high-affinity antibodies through secretion of effector cytokines, such as IL-21 and IL-4, and through cell-to-cell interactions. The flow cytometric-based method described here allows the detection of intracellular cytokines within the Tfh population of secondary lymphoid organs (e.g., spleen, lymph nodes, and lymphoid nodules such as Peyer's patches), enabling the study of Tfh responses to different stimuli in the context of immunity and autoimmunity.

Potent antitumour activity of interleukin-2-Fc fusion proteins requires Fc-mediated depletion of regulatory T-cells.

Interleukin-2 (IL-2) is an established therapeutic agent used for cancer immunotherapy. Since treatment efficacy is mediated by CD8+ and NK cell activity at the tumour site, considerable efforts have focused on generating variants that expand these subsets systemically, as exemplified by IL-2/antibody complexes and 'superkines'. Here we describe a novel determinant of antitumour activity using fusion proteins consisting of IL-2 and the antibody fragment crystallizable (Fc) region. Generation of long-lived IL-2-Fc variants in which CD25 binding is abolished through mutation effectively prevents unwanted activation of CD25+ regulatory T-cells (Tregs) and results in strong expansion of CD25- cytotoxic subsets. Surprisingly, however, such variants are less effective than wild-type IL-2-Fc in mediating tumour rejection. Instead, we report that efficacy is crucially dependent on depletion of Tregs through Fc-mediated immune effector functions. Our results underpin an unexpected mechanism of action and provide important guidance for the development of next generation IL-2 therapeutics.

IL-21 and IL-4 Collaborate To Shape T-Dependent Antibody Responses.

The selection of affinity-matured Ab-producing B cells is supported by interactions with T follicular helper (Tfh) cells. In addition to cell surface-expressed molecules, cytokines produced by Tfh cells, such as IL-21 and IL-4, provide B cell helper signals. In this study, we analyze how the fitness of Th cells can influence Ab responses. To do this, we used a model in which IL-21R-sufficient (wild-type [WT]) and -deficient (Il21r(-/-)) Ag-specific Tfh cells were used to help immunodeficient Il21r(-/-) B cells following T-dependent immunization. Il21r(-/-) B cells that had received help from WT Tfh cells, but not from Il21r(-/-) Tfh cells, generated affinity-matured Ab upon recall immunization. This effect was dependent on IL-4 produced in the primary response and associated with an increased fraction of memory B cells. Il21r(-/-) Tfh cells were distinguished from WT Tfh cells by a decreased frequency, reduced conjugate formation with B cells, increased expression of programmed cell death 1, and reduced production of IL-4. IL-21 also influenced responsiveness to IL-4 because expression of both membrane IL-4R and the IL-4-neutralizing soluble (s)IL-4R were reduced in Il21r(-/-) mice. Furthermore, the concentration of sIL-4R was found to correlate inversely with the amount of IgE in sera, such that the highest IgE levels were observed in Il21r(-/-) mice with the least sIL-4R. Taken together, these findings underscore the important collaboration between IL-4 and IL-21 in shaping T-dependent Ab responses.