Adaptation of innate lymphoid cells to a micronutrient deficiency promotes type 2 barrier immunity

How the immune system adapts to malnutrition to sustain immunity at barrier surfaces, such as the intestine, remains unclear. Vitamin A deficiency is one of the most common micronutrient deficiencies and is associated with profound defects in adaptive immunity. Here, we found that type 3 innate lymphoid cells (ILC3s) are severely diminished in vitamin A-deficient settings, which results in compromised immunity to acute bacterial infection. However, vitamin A deprivation paradoxically resulted in dramatic expansion of interleukin-13 (IL-13)-producing ILC2s and resistance to nematode infection in mice, which revealed that ILCs are primary sensors of dietary stress. Further, these data indicate that, during malnutrition, a switch to innate type 2 immunity may represent a powerful adaptation of the immune system to promote host survival in the face of ongoing barrier challenges.

Immature thymocytes undergoing receptor rearrangements are resistant to an Atm-dependent death pathway activated in mature T cells by double-stranded DNA breaks

Immature CD4(+)CD8(+) thymocytes rearrange their T cell receptor (TCR)-alpha gene locus to generate clonotypic alpha/beta TCR, after which a few cells expressing selectable TCR are signaled to further differentiate into mature T cells. Because of requirements for self-tolerance, immature CD4(+)CD8(+) thymocytes are programmed to die in the thymus in response to a variety of stimuli that do not induce death of mature T cells. We now demonstrate that, in contrast to all previously described stimuli, immature CD4(+)CD8(+) thymocytes are selectively more resistant than mature T cells to apoptotic death induced by DNA intercalating agents. Importantly, we demonstrate that DNA intercalating agents induce double-stranded DNA breaks in both immature thymocytes and mature T cells, but immature thymocytes tolerate these DNA breaks, whereas mature T cells are signaled to die by an Atm-dependent but p53-independent death mechanism. Thus, our results indicate that absence of an Atm-dependent but p53-independent pathway allows immature thymocytes to survive double-stranded DNA breaks. It is likely that the unique ability of immature thymocytes to survive DNA-damaging intercalating agents reflects their tolerance of double-stranded DNA breaks that occur normally during antigen receptor gene rearrangements.

Programming for cytotoxic effector function occurs concomitantly with CD4 extinction during CD8(+) T cell differentiation in the thymus

CD4(+) T cells are generally specialized to function as helper cells and CD8(+) T cells are generally specialized to function as cytotoxic effector cells. To explain how such concordance is achieved between co-receptor expression and immune function, we considered two possibilities. In one case, immature CD4(+)CD8(+) thymocyte precursors might first down-regulate expression of one co-receptor molecule, with the remaining co-receptor molecule subsequently activating the appropriate helper or cytotoxic functional program. Alternatively, we considered that the same intrathymic signals that selectively extinguished expression of one or the other co-receptor molecule might simultaneously initiate the appropriate helper or cytotoxic functional program. In the present study, we attempted to distinguish between these alternatives by examining thymocyte precursors of CD8(+) T cells for expression of Cathepsin C and Cathepsin W, molecules important for cytotoxic effector function. We report in developing thymocytes that Cathepsin C and Cathepsin W are expressed coordinately with extinction of CD4 co-receptor expression. We conclude that CD4 extinction and initiation of the cytotoxic functional program occurs simultaneously during differentiation of CD8(+) T cells in the thymus.

Coreceptor reversal in the thymus: signaled CD4+8+ thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells

A central paradigm of T cell development is that CD4+8+ (DP) thymocytes differentiate into CD4+ or CD8+ T cells in response to intrathymic signals that extinguish transcription of the inappropriate coreceptor molecule. Contrary to this prevailing paradigm, we now demonstrate that signaled DP thymocytes initially terminate CD8 transcription even when differentiating into CD8+ T cells. Remarkably, thymocytes that have selectively terminated CD8 transcription can be signaled by IL-7 to differentiate into CD8+ T cells by silencing CD4 transcription and reinitiating CD8 transcription, events we refer to as "coreceptor reversal." These observations significantly alter our understanding of CD8+ T cell differentiation and lead to a new perspective ("kinetic signaling") on CD4/CD8 lineage determination in the thymus. These observations also suggest a novel mechanism by which bipotential cells throughout development can determine their appropriate cell fate.

CD8 coreceptor extinction in signaled CD4(+)CD8(+) thymocytes: coordinate roles for both transcriptional and posttranscriptional regulatory mechanisms in developing thymocytes

T-cell development in the thymus is characterized by changing expression patterns of CD4 and CD8 coreceptor molecules and by changes in CD4 and CD8 gene transcription. In response to T-cell receptor (TCR) signals, thymocytes progress through developmental transitions, such as conversion of CD4(+)CD8(+) (double-positive [DP]) thymocytes into intermediate CD4(+)CD8(-) thymocytes, that appear to require more-rapid changes in coreceptor expression than can be accomplished by transcriptional regulation alone. Consequently, we considered the possibility that TCR stimulation of DP thymocytes not only affects coreceptor gene transcription but also affects coreceptor RNA stability. Indeed, we found that TCR signals in DP thymocytes rapidly destabilized preexisting CD4 and CD8 coreceptor RNAs, resulting in their rapid elimination. Destabilization of coreceptor RNA was shown for CD8alpha to be dependent on target sequences in the noncoding region of the RNA. TCR signals also differentially affected coreceptor gene transcription in DP thymocytes, terminating CD8alpha gene transcription but only transiently reducing CD4 gene transcription. Thus, posttranscriptional and transcriptional regulatory mechanisms act coordinately in signaled DP thymocytes to promote the rapid conversion of these cells into intermediate CD4(+)CD8(-) thymocytes. We suggest that destabilization of preexisting coreceptor RNAs is a mechanism by which coreceptor expression in developing thymocytes is rapidly altered at critical points in the differentiation of these cells.

Signals involved in CD4/CD8 lineage commitment: current concepts and potential mechanisms

A competent immune system requires that mature T cells express TCR and CD4/CD8 coreceptors with matching MHC specificities. Such matching of TCR and coreceptor specificity is induced in the thymus at the CD4(+)8(+)stage of development and is referred to as lineage commitment. The process by which immature CD4(+)8(+)thymocytes are signaled to undergo lineage commitment continues to be the subject of intense investigation and discussion. Here, we review the major models by which lineage commitment is thought to occur and discuss the experimental results on which they were based.

Positive selection as a developmental progression initiated by alpha beta TCR signals that fix TCR specificity prior to lineage commitment

During positive selection, immature thymocytes commit to either the CD4+ or CD8+ T cell lineage ("commitment") and convert from short-lived thymocytes into long-lived T cells ("rescue"). By formal precursor-progeny analysis, we now identify what is likely to be the initial positive selection step signaled by alpha beta TCR, which we have termed "induction". During induction, RAG mRNA expression is downregulated, but lineage commitment does not occur. Rather, lineage commitment (which depends upon the MHC class specificity of the alpha beta TCR) only occurs after downregulation of RAG expression and the consequent fixation of alpha beta TCR specificity. We propose that positive selection can be viewed as a sequence of increasingly selective developmental steps (induction-->commitment-->rescue) that are signaled by alpha beta TCR engagements of intrathymic ligands.

Functional and physical association of a cell surface phospholipid and interleukin-2 receptor p55(alpha) subunits

A phosphatidylcholine-like phospholipid expressed in the outer leaflet of the cell membrane shortly after mitogenic activation of T-cells is described, based on the binding of monoclonal antibody 90. 60.3. Expression of the 90.60.3 phospholipid antigen in T-cells is activation-dependent. Once expressed, the 90.60.3 phospholipid is in direct physical association with the interleukin-2 (IL-2) binding domain of IL-2 receptor alpha subunits, but does not affect IL-2 binding. The association is specific, because the 90.60.3 phospholipid is not found in association with other domains of IL-2 receptor alpha subunits, or near IL-2 receptor beta or gamma subunits. Culturing cytokine-dependent cell lines in the presence of monoclonal antibody 90.60.3 potentiates IL-2-dependent cell survival and proliferation in a dose-dependent manner. In contrast, IL-4-dependent responses are not potentiated. Taken together, the data suggest that specific plasma membrane phospholipids expressed in the outer leaflet after T-cell activation associate with the IL-2 binding domain of IL-2 receptor alpha subunits (and perhaps other cytokine receptors), and may play a role in regulating receptor mobility or signal transduction.

Unique molecular surface features of in vivo tolerized T cells

Differential expression of surface markers can frequently be used to distinguish functional subsets of T cells, yet a surface phenotype unique to T cells induced into an anergic state has not been described. Here, we report that CD4 T cells rendered anergic in vivo by superantigen can be identified by loss of the 6C10 T cell marker. Inoculation of Vbeta8.1 T cell antigen receptor (TCR) transgenic mice with a Vbeta8.1-reactive minor lymphocyte-stimulating superantigen (Mls-1(a)) induces tolerance to Mls-1(a) by clonal anergy. CD4 lymph node T cells from Mls-1(a) inoculated transgenic mice enriched for the 6C10(-) phenotype neither proliferate nor produce interleukin-2 upon TCR engagement, whereas 6C10(+) CD4 T cells retain responsiveness. Analysis of T cell memory markers demonstrate that 6C10(-) T cells remain 3G11(hi) but express heterogeneous levels of CD45RB, CD62L, CD44, and the CD69 early activation marker, suggesting that T cells at various degrees of activation can be functionally anergic. These studies demonstrate that anergic T cells can be purified based on 6C10 expression permitting examination of issues concerning biochemical and biological features specific to T cell anergy.

OlP-1, a novel protein that distinguishes early oligodendrocyte precursors

Oligodendrocyte development may be divided into three distinct stages: I) commitment of neuroectoderm cells to the oligodendrocyte lineage, II) migration of precursors into the surrounding parenchyma concomitant with increased proliferation, and III) cessation of migration and proliferation and initiation of myelination. Stage II of development has remained enigmatic because of the paucity of known molecules that distinguish these immature migratory cells. We describe a novel surface protein, termed OlP-1, which is restricted in expression to this developmental stage in the mouse. Cytofluorographic comparisons with known developmental markers showed OlP-1 to be expressed primarily by stage II precursors in vitro. Histologic analyses supported this conclusion by showing co-localization of OlP-1 with stage II molecules in vivo. Two conclusions were drawn from these results. First, OlP-1 was a novel protein expressed by murine oligodendrocyte precursors at a point in development that suggested a role in migration or proliferation. Second, dispersal of OlP-1-positive cells throughout the developing brain did not correlate with the location of myelination which, observed days later, progressed in a caudal to rostral manner. These data supported the concept that the final steps of maturation and myelin gene expression may be dependent upon extrinsic factors located predominantly within white matter tracts.

Incomplete endoplasmic reticulum (ER) retention in immature thymocytes as revealed by surface expression of "ER-resident" molecular chaperones

The folding and assembly of nascent proteins in the endoplasmic reticulum (ER) is assisted by molecular chaperones that are themselves retained within the ER. We now report that a number of different ER proteins, including molecular chaperones, are selectively expressed on the surface of immature thymocytes, but their surface expression is extinguished upon further differentiation. Escape from the ER is only possible for newly synthesized ER proteins before they become permanently retained. Thus, the cellular process of ER retention is incomplete in immature thymocytes and provides an explanation for surface expression of partial receptor complexes that transduce differentiative signals during thymic development.

A receptor that subserves reovirus binding can inhibit lymphocyte proliferation triggered by mitogenic signals

A novel surface receptor complex involved in inhibition of T-cell proliferation is described. Biochemical isolation revealed two non-covalently associated proteins of about M(r) 65,000 (p65) and 95,000 (p95). These polypeptides may be related. The p65 form is expressed after cellular activation and replication and is recognized by monoclonal antibody (mAb) 87.92.6 or reovirus hemagglutinin as unnatural ligands. The p95 species is associated with tyrosine kinase enzymatic activity. Receptor ligation results in rapid alteration of the phosphotyrosine content of cellular substrates, and this activity correlates with antiproliferative effects. The inhibition of proliferation is a time-dependent reversible arrest at the G1-S phase of the cell cycle. Activation through the T-cell receptor, protein kinase C, or addition of cytokines does not reverse the antiproliferative effect. This receptor complex may define novel features of T-cell proliferation.

Genetically engineered grafts to study xenoimmunity: a role for indirect antigen presentation in the destruction of major histocompatibility complex antigen deficient xenografts

BACKGROUND

The genetic engineering of xenogeneic donor species for transplantation may provide a means of attenuating the potent immune response elicited by tissues from foreign species. Because of their well-established role in allograft rejection, a logical target for genetic manipulation is the genes encoded by the major histocompatibility complex (MHC). In the current study we examined whether skin, heart, or pancreatic islet xenografts harvested from lines of transgenic mice rendered deficient in MHC antigen expression by gene disruption would exhibit a survival benefit when transplanted to xenogeneic rat recipients. In addition, we characterized the in vitro response of rat T cells to normal and MHC-deficient mouse cells.

METHODS

Skin, heart, and pancreatic islet grafts were harvested from control C57Bl/6 and each of three lines of mice deficient in MHC antigen expression. MHC-deficient lines included (1) mice selectively lacking MHC class I antigens (CID), produced by disruption of the beta-2 microglobulin gene; (2) mice lacking MHC class II expression (CIID), produced by targeting the I-A beta-chain gene; and (3) mice devoid of both class I and class II molecules (CI,IID).

RESULTS

In contrast to the prolonged survival that has been observed for certain allografts deficient in MHC antigen expression, we did not detect significant extension of survival in the case of xenografts. Using in vitro assays of T-cell function, we demonstrated that rats that rejected grafts lacking MHC expression evidenced sensitization of T cells specific for graft antigens presented by rat antigen-presenting cells.

CONCLUSIONS

The strategies of gene targeting of donor species to produce less immunogenic xenografts may be hampered by the presence of a strong response through the indirect pathway of immunity. Immune intervention directed at the indirect antigen presentation pathway may be of benefit in xenotransplantation.

Delayed allograft rejection by T cell receptor V beta 8.1 transgenic mice peripherally tolerized to Mls-1

One commonly studied model system for peripheral tolerance is the antigen-specific unresponsiveness of T cells from mice previously inoculated with superantigens such as Mls-1a. In this study, we used a TcR V beta 8.1 transgenic mouse model to investigate whether mice peripherally tolerized to Mls-1a exhibit delayed skin allograft rejection. We report dramatic prolongation of skin allograft survival in V beta 8.1 transgenic but not in non-transgenic mice tolerized to Mls-1a. Peripherally induced unresponsiveness to Mls-1a can, therefore, be considered true tolerance.

Induction of interleukin 12 responsiveness is impaired in anergic T lymphocytes

The cytokine, interleukin 12 (IL-12), stimulates both natural killer cells and T cells to proliferate and to secrete interferon gamma (IFN-gamma). The T cell proliferative response to IL-12 must be induced and is evident after T cell receptor-mediated stimulation. As reported here, tolerant CD4+ T cells and clones, that are anergic for IL-2 production, are also anergic for induction of the proliferative response to IL-12. Murine T helper 1 clones tolerized in vitro, as well as anergic CD4+ T cells isolated from mice tolerized to the Mls-1a antigen (Ag) in vivo, demonstrated defective induction of proliferation to IL-12 upon restimulation with Ag. IL-12-enhanced production of IFN-gamma was observed in both control and anergic cells after Ag/antigen-presenting cell (APC) activation, although total IFN-gamma secretion by anergic cells was less than that produced by control cells, even in the presence of IL-12. These data indicate that T cell clonal anergy results in profound inhibition of proliferative responses, since the autocrine growth factor, IL-2, is not produced, and the APC-derived cytokine, IL-12, is not an effective stimulus for anergic T cell proliferation.

Gld and lpr mice: single gene mutant models for failed self tolerance

Mice homozygous for the gld or lpr mutations develop autoimmunity, and a lymphoproliferative disorder involving accumulation of huge numbers of unusual CD4-CD8-TCR alpha beta lo T cells. Here we review our past work with gld mice, and attempt to explain lymphoproliferation in terms of current models of T cell maturation and self-tolerance induction. The availability of molecular probes to the gene products of lpr and gld should shortly lead to a better understanding of the acquisition of self tolerance during T cell maturation and of autoimmunity.

Reduced CD3-mediated protein tyrosine phosphorylation in anergic CD4+ and CD8+ T cells

Mice inoculated i.v. with superantigens exhibit long lived Ag-specific T cell tolerance. An in vitro model for this phenomenon is the ensuing unresponsiveness of Th1 T cell clones activated via the TCR/CD3 complex in the absence of co-stimulation. We have previously demonstrated alterations in TCR-mediated early protein tyrosine phosphorylation events in Th1 clones anergic for IL-2 production. In this study, we demonstrate unresponsiveness in CD4+ and CD8+ T cells from V beta 8.1 transgenic mice inoculated i.v. with the superantigen Mls-1a. The unresponsiveness of both CD4+ and CD8+ T cells involves defective IL-2 production upon restimulation, with CD4+ T cells exhibiting an additional defect in IL-2 utilization. The transgenic model allowed study of T cell signaling in a relatively homogeneous population of unresponsive cells without elaborate purification of Ag-reactive populations. Both CD4+ and CD8+ T cells exhibit altered tyrosine phosphorylation of two protein substrates upon CD3-mediated restimulation. The substrates involved, p38 and p75, are of identical size to substrates similarly affected in anergic Th1 clones. Altered tyrosine phosphorylation is therefore closely associated with defective IL-2 production in these three anergic T cell types, and may play a role in the maintenance of anergy.

Reduced CD3-mediated protein tyrosine phosphorylation in anergic CD4+ and CD8+ T cells

Mice inoculated i.v. with superantigens exhibit long lived Ag-specific T cell tolerance. An in vitro model for this phenomenon is the ensuing unresponsiveness of Th1 T cell clones activated via the TCR/CD3 complex in the absence of co-stimulation. We have previously demonstrated alterations in TCR-mediated early protein tyrosine phosphorylation events in Th1 clones anergic for IL-2 production. In this study, we demonstrate unresponsiveness in CD4+ and CD8+ T cells from V beta 8.1 transgenic mice inoculated i.v. with the superantigen Mls-1a. The unresponsiveness of both CD4+ and CD8+ T cells involves defective IL-2 production upon restimulation, with CD4+ T cells exhibiting an additional defect in IL-2 utilization. The transgenic model allowed study of T cell signaling in a relatively homogeneous population of unresponsive cells without elaborate purification of Ag-reactive populations. Both CD4+ and CD8+ T cells exhibit altered tyrosine phosphorylation of two protein substrates upon CD3-mediated restimulation. The substrates involved, p38 and p75, are of identical size to substrates similarly affected in anergic Th1 clones. Altered tyrosine phosphorylation is therefore closely associated with defective IL-2 production in these three anergic T cell types, and may play a role in the maintenance of anergy.

Inhibition of abnormal T cell development and autoimmunity in gld mice by transgenic T cell receptor beta chain

Mice homozygous for the gld (generalized lymphoproliferative disease) mutation developed systemic autoimmune disease and severe lymphadenopathy due to an age-related accumulation in the peripheral lymphoid organs of polyclonal T cells bearing a unique phenotype (CD4-CD8-TCR alpha beta+B220+). These T cells overexpress T cell receptor (TcR) alpha beta chain RNA, proto-oncogenes c-myb and fyn, and proliferate poorly in response to TcR-mediated stimulation. The origin of these T cells is poorly understood. To study the influence of a functionally rearranged TcR beta chain on the T cell developmental abnormality of the gld mutation and autoimmunity, we have backcrossed TcR V beta 8.1-transgenic mice to C3H-gld/gld to homozygosity (transgenic gld mice). In transgenic gld mice, lymphadenopathy was markedly inhibited and the accumulation of CD4-CD8- T cells did not occur, although the remaining T cells overexpressed c-myb and proliferated poorly in response to TcR occupancy. These features indicate that the pattern of proto-oncogene expression and abnormal function persist in phenotypically normal T cells in transgenic gld mice, and that these characteristics can be dissociated from the accumulation of CD4-CD8- T cells. The hypergammaglobulinemia and anti-double-stranded DNA (anti-dsDNA) antibody production was partially improved in transgenic gld mice, supporting the critical role of T cells in abnormal B cell activation described in autoimmunity-prone mice. To investigate further the mechanisms underlying the inhibition of CD4-CD8- T cell accumulation in transgenic gld mice, the fetal ontogeny of T cells in transgenic mice was compared with that of non-transgenic mice. In transgenic thymus, development of TcR alpha beta+ cells was accelerated as detected by earlier expression of CD4, CD8 and TcR in fetal thymus. In contrast, the number of TcR gamma delta+ cells was reduced. We suggest that altered T cell development in transgenic mice directly or indirectly inhibits the accumulation of abnormal T cells in gld mice.

Mechanisms of autoimmunity in the context of T-cell tolerance: insights from natural and transgenic animal model systems

There are a number of mechanisms which cooperate to produce and maintain T-cell tolerance. First, and perhaps most important, is the clonal deletion in the thymus of T cells with high affinity for self antigens. However, to ensure that a wide repertoire of T cells is available in the periphery to combat foreign antigens, the threshold of clonal deletion may be set low enough so that T cells whose TCR's have sub-threshold affinity for self antigens mature and migrate to the periphery. T cells which recognize self antigen-derived peptides not expressed or presented in the thymus will also fail to be deleted. For those self-reactive T cells which are not deleted in the thymus, other mechanisms may produce tolerance, including an undefined alteration of signalling pathways which produces clonal anergy, and lowering the avidity of the TCR for its ligand by downregulating coreceptor and accessory molecules. Active suppression of T-cell responses in another well-described phenomenon whose mechanism is undefined. From our observations with the model systems discussed here, we have observed three distinct mechanisms by which T-cell tolerance can be circumvented, allowing autoimmune phenomena to occur. These mechanisms may have relevance for different types of autoimmune diseases seen in humans. In gld mice, the autoimmune disease seems to be related to a global defect in T-cell differentiation and function, which allows for the expansion of autoimmune B cells. While we showed that clonal deletion of V beta-bearing T cells is appropriate in certain cases, aberrant lymphokine secretion by the abnormal T cells or disruption of immune system regulation are most probably responsible for allowing autoantibody production. While human lupus erythematosis shares much of the pathology of lpr and gld mice, there is no expansion of T cells with a similar phenotype in human lupus. There are environmental factors which must play a role in the development of human lupus, since the incidence of the disease does not follow an absolute genetic pattern. The escape from clonal deletion and subsequent reactivation of autoimmune T cells which we observed in V beta 8.1 TCR-transgenic mice can be a model for human autoimmune diseases such as multiple sclerosis and type I diabetes, in which T cells are directed against a specific autoantigen. According to this model, susceptibility loci for autoimmune disease such as the MHC would function by producing different repertoires of T cells which in some cases could gain autoreactivity following activation.(ABSTRACT TRUNCATED AT 400 WORDS)