Linrong Lu

Inhibition of follicular T-helper cells by CD8+ regulatory T cells is essential for self tolerance

The ability to produce vigorous immune responses that spare self tissues and organs depends on the elimination of autoreactive T and B cells. However, purging of immature and mature self-reactive T and B cells is incomplete and may also require the involvement of cells programmed to suppress immune responses. Regulatory T cells (Treg) belonging to the CD4+ T-cell subset may have a role in preventing untoward inflammatory responses, but T-cell subsets programmed to inhibit the development of autoantibody formation and systemic-lupus-erythematosus-like disease have not yet been defined. Here we delineate a CD8+ regulatory T-cell lineage that is essential for the maintenance of self tolerance and prevention of murine autoimmune disease. Genetic disruption of the inhibitory interaction between these CD8+ T cells and their target Qa-1+ follicular T-helper cells results in the development of a lethal systemic-lupus-erythematosus-like autoimmune disease. These findings define a sublineage of CD8 T cells programmed to suppress rather than activate immunity that represents an essential regulatory element of the immune response and a guarantor of self tolerance.

Qa-1 restriction of CD8 + suppressor T cells

There is increasing evidence that the immune response can be inhibited by several T cell subsets, including NK T cells, CD25+CD4+ T cells, and a subpopulation of CD8+ T cells. Animal model studies of multiple sclerosis have suggested an important role for suppressor CD8+ T cells in protection against disease recurrence and exacerbation. The molecular lynchpin of CD8+ suppressive activity is the murine MHC molecule Qa-1, termed HLA-E in humans. Here we summarize findings from work on Qa-1 that have begun to delineate suppressor CD8+ T cells and their mechanisms of action in the context of self tolerance and autoimmune disease.

Ras-related protein Rab10 facilitates TLR4 signaling by promoting replenishment of TLR4 onto the plasma membrane

The Toll-like receptor (TLR)4 receptor complex, TLR4/MD-2, plays an important role in the inflammatory response against lipopolysaccharide, a ubiquitous membrane component in Gram-negative bacteria. Ligand recognition by TLR4 initiates multiple intracellular signaling pathways, leading to production of proinflammatory mediators and type I IFN. Ligand interaction also leads to internalization of the surface receptor complex into lysosomes, leading to the degradation of TLR4 and the termination of LPS response. However, surface level of TLR4 receptor complex is maintained via continuous replenishment of TLR4 from intracellular compartments like Golgi and endosomes. Here we show that continuous replenishment of TLR4 from Golgi to plasma membrane is regulated by the small GTPase Rab10, which is essential for optimal macrophage activation following LPS stimulation. Expression of Rab10 is inducible by LPS. Blockade of Rab10 function leads to decreased membrane TLR4 expression and diminished production of inflammatory cytokines and interferons upon LPS stimulation. These findings suggest that Rab10 expression provides a mechanism to refine TLR4 signaling by regulating the trafficking rate of TLR4 onto the plasma membrane. In addition, we show that altered Rab10 expression in macrophages influences disease severity in an in vivo model of LPS-induced acute lung injury, suggesting Rab10 as a possible therapeutic target for human acute respiratory distress syndrome (ARDS).

Generation and Regulation of CD8+ Regulatory T Cells

Research into the suppressive activity of CD4+FoxP3+ T regulatory cells (Treg) has defined a sublineage of CD4+ cells that contribute to self-tolerance and resistance to autoimmune disease. Much less attention has been given to the potential contribution of regulatory sublineages of CD8+ cells. Analysis of a small fraction of CD8+ cells that target autoreactive CD4+ cells through recognition of the MHC class Ib molecule Qa-1 in mouse and HLA-E in human has revitalized interest in CD8+ Treg. Here we summarize recent progress and future directions of research into the role of this CD8+ sublineage in resistance to autoimmune disease.

Analysis of the cellular mechanism underlying inhibition of EAE after treatment with anti-NKG2A F(ab′)2

Autoimmune encephalomyelitis may be ameliorated experimentally by enhancing NK cell-mediated elimination of activated autoreactive T cells through a mutation that interrupts the interaction between Qa-1b and CD94/NKG2A. Here we evaluate the ability of an anti-NKG2A F(ab′)2 Ab to enhance elimination of autoreactive T cells and reduce experimental autoimmune encephalomyelitis (EAE). Anti-NKG2A F(ab′)2 treatment diminishes progression of both myelin oligodendrocyte glycoprotein (MOG)-induced EAE in intact C57BL/6 mice and after adoptive transfer of disease-causing T cells. Analyses of the underlying mechanism revealed that administration of anti-NKG2A F(ab′)2 Ab reduces CD4+ T recall responses to MOG and skews the proportion of IL-17- and IFNγ-producing CD4+ T cells toward the protective IL-4- and IL-10-secreting CD4+ T cell subpopulations. CD94/NKG2A-dependent inhibition of inflammatory damage to spinal cord is associated with decreased infiltration of T cells and reduced microglia activation in the central nervous system. Because anti-NKG2A F(ab′)2 treatment had no detectable effect on the numbers or activity of T and B lymphocytes and NK cells in peripheral lymphoid tissues, this anti-NKG2A-based approach may represent a safe and effective therapy for this CNS disorder.

Tespa1 is involved in late thymocyte development through the regulation of TCR-mediated signaling

Signaling via the T cell antigen receptor (TCR) during the CD4+CD8+ double-positive developmental stage determines thymocyte selection and lineage commitment. Here we describe a previously uncharacterized T cell–expressed protein, Tespa1, with critical functions during the positive selection of thymocytes. Tespa1−/− mice had fewer mature thymic CD4+ and CD8+ T cells, which reflected impaired thymocyte development. Tespa1 associated with the TCR signaling components PLC-γ1 and Grb2, and Tespa1 deficiency resulted in attenuated TCR signaling, as reflected by defective activation of the Erk–AP-1 and Ca2+-NFAT pathways. Our findings demonstrate that Tespa1 is a component of the TCR signalosome and is essential for T cell selection and maturation through the regulation of TCR signaling during T cell development.

IL4I1 Is a Novel Regulator of M2 Macrophage Polarization That Can Inhibit T Cell Activation via L-Tryptophan and Arginine Depletion and IL-10 Production.

Interleukin 4-induced gene-1 (IL4I1) was initially described as an early IL-4-inducible gene in B cells. IL4I1 protein can inhibit T cell proliferation by releasing its enzymatic catabolite, H2O2, and this effect is associated with transient down-regulation of T cell CD3 receptor-zeta (TCRζ) expression. Herein, we show that IL4I1 contributes to the regulation of macrophage programming. We found that expression of IL4I1 increased during bone marrow-derived macrophage (BMDM) differentiation, expression of IL4I1 is much higher in primary macrophages than monocytes, and IL4I1 expression in BMDMs could be induced by Th1 and Th2 cytokines in two different patterns. Gene expression analysis revealed that overexpression of IL4I1 drove the expression of M2 markers (Fizz1, Arg1, YM-1, MR) and inhibited the expression of M1-associated cytokines. Conversely, knockdown of IL4I1 by siRNA resulted in opposite effects, and also attenuated STAT-3 and STAT-6 phosphorylation. Furthermore, IL4I1 produced by macrophages catalyzed L-tryptophan degradation, while levo-1-methyl-tryptophan (L-1-MT), but not dextro-1-methyl-tryptophan, partially rescued IL4I1-dependent inhibition of T cell activation. Other inhibitors, such as diphenylene iodonium (DPI), an anti-IL-10Rα blocking antibody, and a nitric oxide synthase inhibitor, NG-monomethyl-L-arginine, also had this effect. Overall, our findings indicate that IL4I1 promotes an enhanced M2 functional phenotype, which is most likely associated with the phosphorylation of STAT-6 and STAT-3. Moreover, DPI, L-1-MT, NG-monomethyl-L-arginine, and anti-IL-10Rα blocking antibody were all found to be effective IL4I1 inhibitors in vitro.

T Cell Costimulation through CD28 Depends on Induction of the Bcl-xγ Isoform: Analysis of Bcl-xγ–deficient Mice

The molecular basis of CD28-dependent costimulation of T cells is poorly understood. Bcl-xγ is a member of the Bcl-x family whose expression is restricted to activated T cells and requires CD28-dependent ligation for full expression. We report that Bcl-xγ–deficient (Bcl-xγ−/−) T cells display defective proliferative and cytokine responses to CD28-dependent costimulatory signals, impaired memory responses to proteolipid protein peptide (PLP), and do not develop PLP-induced experimental autoimmune encephalomyelitis (EAE). In contrast, enforced expression of Bcl-xγ largely replaces the requirement for B7-dependent ligation of CD28. These findings identify the Bcl-xγ cytosolic protein as an essential downstream link in the CD28-dependent signaling pathway that underlies T cell costimulation.

Glatiramer acetate ameliorates inflammatory bowel disease in mice through the induction of Qa-1-restricted CD8⁺ regulatory cells.

Inflammatory bowel diseases (IBDs) are complex multifactorial immunological disorders characterized by dysregulated immune reactivity in the intestine. Here, we investigated the contribution of Qa‐1‐restricted CD8+ Treg cells in regulating experimental IBD in mice. We found that CD8+ T cells induced by T‐cell vaccination ameliorated the pathological manifestations of dextran sulfate sodium induced IBD when adoptively transferred into IBD mice. In addition, CD8+ cell suppressive activity was induced by vaccination with glatiramer acetate (GA), an FDA‐approved drug for multiple sclerosis (MS). We next showed that GA‐induced CD8+ Treg cells worked in a Qa‐1‐dependent manner and their suppressive activity depends on perforin‐mediated cytotoxicity. Finally, we confirmed the role of CD4+ T cells in dextran sulfate sodium induced colitis progression, and clarified that GA‐induced CD8+ T cells exerted their therapeutic effects on colitis by targeting pathogenic CD4+ T cells. Our results reveal a new regulatory role of Qa‐1‐restricted CD8+ Treg cells in IBD and suggest their induction by GA vaccination as a potential therapeutic approach to IBD.

A Novel Size-Based Sorting Mechanism of Pinocytic Luminal Cargoes in Microglia

Microglia are the resident immune cells in the CNS and play diverse roles in the maintenance of CNS homeostasis. Recent studies have shown that microglia continually survey the CNS microenvironment and scavenge cell debris and aberrant proteins by phagocytosis and pinocytosis, and that reactive microglia are capable to present antigens to T cells and initiate immune responses. However, how microglia process the endocytosed contents and evoke an immune response remain unclear. Here we report that a size-dependent selective transport of small soluble contents from the pinosomal lumen into lysosomes is critical for the antigen processing in microglia. Using fluorescent probes and water-soluble magnetic nanobeads of defined sizes, we showed in cultured rodent microglia, and in a cell-free reconstructed system that pinocytosed proteins become degraded immediately following pinocytosis and the resulting peptides are selectively delivered to major histocompatibility complex class II (MHC-II) containing lysosomes, whereas undegraded proteins are retained in the pinosomal lumen. This early size-based sorting of pinosomal contents relied on the formation of transient tunnel between pinosomes and lysosomes in a Rab7- and dynamin II-dependent manner, which allowed the small contents to pass through but restricted large ones. Inhibition of the size-based sorting markedly reduced proliferation and cytokine release of cocultured CD4+ T cells, indicating that the size-based sorting is required for efficient antigen presentation by microglial cells. Together, these findings reveal a novel early sorting mechanism for pinosomal luminal contents in microglial cells, which may explain how microglia efficiently process protein antigens and evoke an immune response.