Homeostatic proliferation of a Qa-1b-restricted T cell: a distinction between the ligands required for positive selection and for proliferation in lymphopenic hosts

Ribosylation of the CD8αβ heterodimer permits binding of the nonclassical major histocompatibility molecule, H2-Q10

The CD8αβ heterodimer plays a crucial role in the stabilization between major histocompatibility complex class I molecules (MHC-I) and the T cell receptor (TCR). The interaction between CD8 and MHC-I can be regulated by posttranslational modifications, which are proposed to play an important role in the development of CD8 T cells. One modification that has been proposed to control CD8 coreceptor function is ribosylation. Utilizing NAD⁺, the ecto-enzyme adenosine diphosphate (ADP) ribosyl transferase 2.2 (ART2.2) catalyzes the addition of ADP-ribosyl groups onto arginine residues of CD8α or β chains and alters the interaction between the MHC and TCR complexes. To date, only interactions between modified CD8 and classical MHC-I (MHC-Ia), have been investigated and the interaction with non-classical MHC (MHC-Ib) has not been explored. Here, we show that ADP-ribosylation of CD8 facilitates the binding of the liver-restricted nonclassical MHC, H2-Q10, independent of the associated TCR or presented peptide, and propose that this highly regulated binding imposes an additional inhibitory leash on the activation of CD8-expressing cells in the presence of NAD⁺. These findings highlight additional important roles for nonclassical MHC-I in the regulation of immune responses.

Cutting edge: innate memory CD8+ T cells are distinct from homeostatic expanded CD8+ T cells and rapidly respond to primary antigenic stimuli

Innate memory phenotype (IMP) CD8(+) T cells are nonconventional αβ T cells exhibiting features of innate immune cells and are significantly increased in the absence of ITK. Their developmental path and function are not clear. In this study, we show hematopoietic MHC class I (MHCI)-dependent generation of Ag-specific IMP CD8(+) T cells using bone marrow chimeras. Wild-type bone marrow gives rise to IMP CD8(+) T cells in MHCI(-/-) recipients, resembling those in Itk(-/-) mice, but distinct from those derived via homeostatic proliferation, and independent of recipient thymus. In contrast, MHCI(-/-) bone marrow does not lead to IMP CD8(+) T cells in wild-type recipients. OTI IMP CD8(+) T cells generated via this method exhibited enhanced early response to Ag without prior primary stimulation. Our findings suggest a method to generate Ag-specific "naive" CD8(+) IMP T cells, as well as demonstrate that they are not homeostatic proliferation cells and can respond promptly in an Ag-specific fashion.

Positive selecting cell type determines the phenotype of MHC class Ib-restricted CD8+ T cells

Several studies have demonstrated an apparent link between positive selection on hematopoietic cells (HCs) and an "innate" T-cell phenotype. Whereas conventional CD8(+) T cells are primarily selected on thymic epithelial cells (TECs) and certain innate T cells are exclusively selected on HCs, MHC class Ib-restricted CD8(+) T cells appear to be selected on both TECs and HCs. However, whether TEC- and HC-selected T cells represent distinct lineages or whether the same T-cell precursors have the capacity to be selected on either cell type is unknown. Using an M3-restricted T-cell receptor transgenic mouse model, we demonstrate that not only are MHC class Ib-restricted CD8(+) T cells capable of being selected on either cell type but that selecting cell type directly affects the phenotype of the resulting CD8(+) T cells. M3-restricted CD8(+) T cells selected on HCs acquire a more activated phenotype and possess more potent effector functions than those selected on TECs. Additionally, these two developmental pathways are active in the generation of the natural pool of M3-restricted CD8(+) T cells. Our results suggest that these two distinct populations may allow MHC class Ib-restricted CD8(+) T cells to occupy different immunological niches playing unique roles in immune responses to infection.

Spatial distribution, kinetics, signaling and cytokine production during homeostasis driven proliferation of CD4+ T cells

During recovery from lymphopenia, the naïve T-cells undergo homeostasis driven proliferation (HDP) and acquire a memory phenotype. The HDP of T-cells requires signals derived from T-cell-receptor, p56lck kinase, IL-7R and IL-15R. However, the role of other signaling molecules during HDP of CD4+ T-cells remains speculative. The differentiation of naïve T-cells into Th1/Th2/Th17 or Treg populations during HDP is not well understood. Present report describes the spatial and signaling characteristics of HDP of CD4+ T-cells and their cytokine profiles. The HDP of CD4+ T-cells was found to occur only in specific areas (T-cell zones) of secondary lymphoid organs of lymphopenic mice. The inhibitors of MEK and PKC and their combination with inhibitors of PI3kinase and mTOR suppressed mitogen induced T-cell proliferation without affecting their HDP. The CD4+ T-cells taken from reconstituted lymphopenic mice showed activation of proteins involved in NF-kappaB pathway, significantly higher production of pro-inflammatory cytokine IL-6, and lower production of IL-4 as compared to T-cells from normal mice. Plumbagin, a known NF-kappaB blocker inhibited survival as well as HDP of CD4+ T-cells and IL-6 production in activated T-cells. Our results demonstrate the essential role of NF-kappaB during HDP of T-cells.

Polyclonal MHC Ib-restricted CD8+ T cells undergo homeostatic expansion in the absence of conventional MHC-restricted T cells

Naive T cells have the capacity to expand in a lymphopenic environment in a process called homeostatic expansion, where they gain a memory-like phenotype. Homeostatic expansion is dependent on competition for a number of factors, including growth factors and interactions with their selecting self-MHC molecules. In contrast to conventional T cells, it is unclear whether class Ib-restricted CD8+ T cells have a capacity to undergo homeostatic expansion. In this study, we demonstrate that polyclonal MHC Ib-restricted CD8+ T cells can undergo homeostatic expansion and that their peripheral expansion is suppressed by conventional MHC-restricted T cells. The acute depletion of CD4+ T cells in MHC class Ia-deficient Kb-/-Db-/- mice led to the substantial expansion of class Ib-restricted CD8+ T cells. Adoptive transfer of class Ib-restricted CD8+ T cells to congenic lymphopenic recipients revealed their ability to undergo homeostatic expansion in a MHC Ib-dependent manner. To further study the homeostatic expansion of MHC Ib-restricted T cells in the absence of all conventional MHC-restricted T cells, we generated mice that express only MHC Ib molecules by crossing H-2Kb-/-Db-/- with CIITA-/- mice. CD8+ T cells in these mice exhibit all of the hallmarks of naive T cells actively undergoing homeostatic expansion with constitutive memory-like surface and functional phenotype. These findings provide direct evidence that MHC Ib-restricted CD8+ T cells have the capacity to undergo homeostatic expansion. Their peripheral expansion is suppressed under normal conditions by a numerical excess of conventional MHC class Ia- and class II-restricted T cells.

Integration of B cells and CD8+ T in the protective regulation of systemic epithelial inflammation

Mechanisms that control abnormal CD4(+) T cell-mediated tissue damage are a significant factor in averting and resolving chronic inflammatory epithelial diseases. B cells can promote such immunoregulation, and this is thought to involve interaction with MHC II- or CD1-restricted regulatory T cells. The purpose of this study is to genetically define the interacting cells targeted by protective B cells, and to elucidate their regulatory mechanisms in CD4(+) T cell inflammation. Transfer of G alpha i2-/- CD3(+) T cells into lymphopenic mice causes a dose-dependent multi-organ inflammatory disease including the skin, intestine, and lungs. Disease activity is associated with elevated levels of serum TNF-alpha and IFN-gamma, and an activated IL-17 producing CD4(+) T cell population. Mesenteric node B cells from wild type mice suppress disease activity, serum cytokine expression, and levels of CD4(+) T cells producing TNF-alpha IFN-gamma, and IL-17. The protective function of B cells requires genetic sufficiency of IL-10, MHC I and TAP1. Regulatory B cells induce the expansion and activation of CD8(+) T cells, which is correlated with disease protection. These results demonstrate that CD8(+) T cells can ameliorate lymphopenic systemic inflammatory disease, through peptide/MHC I-dependent B cell interaction.

Interferon-gamma and donor MHC class I control alternative macrophage activation and activin expression in rejecting kidney allografts: a shift in the Th1-Th2 paradigm

Organ allografts deficient in interferon-gamma (Ifng) or major histocompatibility complex (MHC) class I products develop accelerated necrosis when rejection develops, depending on perforin and granzymes. Thus Ifng-induced donor class I products deliver inhibitory signals to host inflammatory cells. We used microarrays to investigate whether Ifng-induced donor class I products also control inflammation patterns in mouse kidney allografts. Compared to wild-type (WT) allografts, many transcripts were increased in both Ifng-deficient allografts (Ifng-suppressed transcripts [GSTs]) and class I-deficient allografts (class I-suppressed transcripts [CISTs]), with 73% overlap between GSTs and CISTs. Some GSTs and CISTs reflected increased necrosis, including known injury-induced transcripts. However, many GSTs and CISTs were independent of perforin, granzymes and necrosis, and were associated with alternative macrophage activation (AMA) (e.g. arginase I [Arg1], macrophage elastase [Mmp12] and macrophage mannose receptor 1 [Mrc1]). AMA transcripts were induced despite absence of host interleukin (IL)4 and IL13 receptors. The AMA inducer may be activins, whose genes (inhibin A [InhbA] and inhibin B [InhbB]) were increased in all allografts with AMA. We conclude that in allograft rejection, Ifng acts via donor Ifng receptors (Ifngr) to induce donor class Ia and Ib products, which engage host inflammatory cells to limit perforin-granzyme-mediated damage and prevent AMA associated with inhibition of activin expression. Thus, Ifng may control T helper type 2 (Th2) cell inflammation by induction of class I products.

Regulation of immunity by a novel population of Qa-1-restricted CD8alphaalpha+TCRalphabeta+ T cells

Regulatory mechanisms involving CD8+ T cells (CD8 regulatory T cells (Tregs)) are important in the maintenance of immune homeostasis. However, the inability to generate functional CD8 Treg clones with defined Ag specificity has precluded a direct demonstration of CD8 Treg-mediated regulation. In the present study, we describe the isolation of functional lines and clones representing a novel population of TCRalphabeta+ Tregs that control activated Vbeta8.2+ CD4 T cells mediating experimental autoimmune encephalomyelitis. They express exclusively the CD8alphaalpha homodimer and recognize a peptide from a conserved region of the TCR Vbeta8.2 chain in the context of the Qa-1a (CD8alphaalpha Tregs). They secrete type 1 cytokines but not IL-2. CD8alphaalpha Tregs kill activated Vbeta8.2+ but not Vbeta8.2- or naive T cells. The CD8alphaalpha Tregs prevent autoimmunity upon adoptive transfer or following in vivo activation. These findings reveal an important negative feedback regulatory mechanism targeting activated T cells and have implications in the development of therapeutic strategies for autoimmune diseases and transplantation.

An MHC Class Ib-Restricted TCR That Cross-Reacts with an MHC Class Ia Molecule

TCR transgenic 6C5 T cells recognize an insulin B chain epitope presented by the nonclassical class I MHC molecule, Qa-1(b). Positive selection of these T cells was shown previously to require Qa-1(b). Despite dedicated specificity for Qa-1(b), evidence presented in the current study indicates that 6C5 T cells can cross-recognize a classical class I molecule. Clonal deletion was observed unexpectedly in 6C5.H-2(bxq) mice, which do not express I-E MHC class II molecules and thus should not be subject to superantigen-mediated negative selection. 6C5 T cells were observed to respond in vivo and in vitro to spleen cells from allogeneic H-2(q) mice, and specificity was mapped to D(q). Evidence was obtained for direct recognition of D(q), rather than indirect presentation of a D(q)-derived peptide presented by Qa-1(b). Polyclonal CD8(+) T cells from class Ia-deficient K(b)D(b-/-) mice reacted in vitro to allogeneic spleen cells with an apparent frequency comparable to conventional class Ia-restricted T cells. Our results provide a clear example of a Qa-1-specific TCR that can cross-react with a class Ia molecule and evidence supporting the idea that this may be a common property of T cells selected by class Ib molecules.

MHC class Ib molecules bridge innate and acquired immunity

Our understanding of the classical MHC class I molecules (MHC class Ia molecules) has long focused on their extreme polymorphism. These molecules present peptides to T cells and are central to discrimination between self and non-self. By contrast, the functions of the non-polymorphic MHC class I molecules (MHC class Ib molecules) have been elusive, but emerging evidence reveals that, in addition to antigen presentation, MHC class Ib molecules are involved in immunoregulation. As we discuss here, the subset of MHC class Ib molecules that presents peptides to T cells bridges innate and acquired immunity, and this provides insights into the origins of acquired immunity.