6 - Antigens and Antigen Presentation

Peptides presented by the major histocompatibility complex (MHC) are derived from the degradation of cellular proteins. Thus, the repertoire of these peptides (the MHC peptidome) should correlate better with the cellular protein degradation scheme (the degradome) than with the cellular proteome. To test the validity of this statement and to determine whether the majority of MHC peptides are derived from short lived proteins, from defective ribosome products, or from regular long lived cellular proteins we analyzed in parallel the turnover kinetics of both MHC peptides and cellular proteins in the same cancer cells. The analysis was performed by pulse-chase experiments based on stable isotope labeling in tissue culture followed by capillary chromatography and tandem mass spectrometry. Indeed only a limited correlation was observed between the proteome and the MHC peptidome observed in the same cells. Moreover a detailed analysis of the turnover kinetics of the MHC peptides helped to assign their origin to normal, to short lived or long lived proteins, or to the defective ribosome products. Furthermore the analysis of the MHC peptides turnover kinetics helped to direct attention to abnormalities in the degradation schemes of their source proteins. These observations can be extended to search for cancer-related abnormalities in protein degradation, including those that lead to loss of tumor suppressors and cell cycle regulatory proteins.

From the initial observations by Zinkernagel and Doherty over 40 years ago of the need for T cells to recognize "altered self," our understanding of TCR recognition of peptide+ MHC has made significant advances. However, alongside a more detailed understanding of the interaction comes additional questions around precisely why T cells must limit themselves to MHC, when a greater range of ligand binding is demonstrably possible, and mechanistically, how MHC restriction achieves the necessary T cell survival and activation signals. The answer may well lie in the study of noncanonical or poorly signaling TCRs to understand the absolute requirements for effective TCR-pMHC recognition, continued advances in structural biology providing resolution of multimolecular complexes, and cryo-EM providing information on the dynamics of molecular localization and organization before and after TCR ligation of pMHC.

Major Histocompatibility Complex (MHC) molecules play a crucial role in inducing an adaptive immune response. T-cell epitopes require compatible MHC molecules to form MHC-peptide Complexes (pMHC) that activate the T-cell Receptors (TCR) of T-lymphocyte clones. MHCs are polymorphic molecules with wide varieties of gene alleles. There are two classes of MHC molecules, class I and II. Both classes have three classical loci HLA-A, -B, and –C are present in class I and HLA-DP, -DQ, and -DR in class II. To induce a compatible T-lymphocyte clone, the T-cell epitope requires the association of the compatible MHC molecule to form pMHC. Each MHC variant possesses a different groove that is capable of binding a different range of antigenic epitopes. Without the compatible MHC molecule, a T cell clone cannot be activated by a particular viral epitope. With the aim of preventing viral transmission, the efficiency of a viral vaccine is related to the existence of specific MHC alleles in the individual. This article proposes the roles of the MHC molecule to prevent viral infection. In addition, the association of the viral receptor molecule with the viral infection will also be discussed.

The U21 gene product from human herpesvirus 7 binds to and redirects class I major histocompatibility complex (MHC) molecules to a lysosomal compartment. The molecular mechanism by which U21 reroutes class I MHC molecules to lysosomes is not known. Here, we have reconstituted the interaction between purified soluble U21 and class I MHC molecules, suggesting that U21 does not require additional cellular proteins to interact with class I MHC molecules. Our results demonstrate that U21, itself predicted to contain an MHC class I-like protein fold, interacts tightly with class I MHC molecules as a tetramer, in a 4:2 stoichiometry. These observations have helped to elucidate a refined model describing the mechanism by which U21 escorts class I MHC molecules to the lysosomal compartment. In this report, we show that the human herpesvirus 7 (HHV-7) immunoevasin U21, itself a class I MHC-like protein, binds with high affinity to class I MHC molecules as a tetramer and escorts them to lysosomes, where they are degraded. While many class I MHC-like molecules have been described in detail, this unusual viral class I-like protein functions as a tetramer, associating with class I MHC molecules in a 4:2 ratio, illuminating a functional significance of homooligomerization of a class I MHC-like protein.

Major histocompatibility complex (MHC) molecules are peptide receptors. Their function is to collect peptides inside the cell and transport them to the cell surface, where the complex of peptide and MHC molecule may be recognized by T lymphocytes (Rammensee et al. 1993). In normal cells MHC-associated peptides are derived from normal, self proteins. During their differentiation, T cells become tolerant — by elimination or inactivation (anergy) — to complexes of self MHC and self peptides (Von Boehmer 1992). Thus when new peptides derived from infectious agents or changed self proteins occur later, these can be recognized by T cells (Zinkernagel and Dohenty 1974). However, overexpressed, aberrantly expressed, and even normal self proteins can induce T cell responses under conditions which revert anergy (Nanda and Sercarz 1995). Since the specific immune response is regulated by T cells, the trimolecular complex of T cell receptor (TCR), MHC molecule, and peptides serves as a control switch for the immune system. Control of tumor growth and tumor dissemination by the immune system, particularly by the cellular immune system, is the basis for development of antitumor vaccines (Foley 1953; Prehn and Main 1957; Klein et al. 1960; Old et al. 1962). The role MHC molecules and their associated peptides play in tumor contol is important for development of targeted antigen-specific vaccines for cancer therapy.KeywordsMajor Histocompatibility ComplexHuman Leukocyte AntigenMajor Histocompatibility Complex ClassHuman Leukocyte Antigen ClassMajor Histocompatibility Complex MoleculeThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Herpesviruses have evolved numerous strategies to evade detection by the immune system. Notably, most of the herpesviruses interfere with viral antigen presentation to cytotoxic T lymphocytes (CTLs) by removing class I major histocompatibility complex (MHC) molecules from the infected cell surface. Clearly, since the herpesviruses have evolved an extensive array of mechanisms to remove class I MHC molecules from the cell surface, this strategy serves them well. However, class I MHC molecules often serve as inhibitory ligands for NK cells, so viral downregulation of all class I MHC molecules should leave the infected cell open to NK cell attack. Some viruses solve this problem by selectively downregulating certain class I MHC products, leaving other class I products at the cell surface to serve as inhibitory NK cell ligands. Here, we show that human herpesvirus 7 (HHV-7) U21 binds to and downregulates all of the human class I MHC gene products, as well as the murine class I molecule H-2K(b). HHV-7-infected cells must therefore possess other means of escaping NK cell detection.

More than a decade has passed since the publication of the first report implicating T cells in the bone loss induced by ovariectomy (ovx).1 Since then there has been extraordinary progress in the understanding of the regulatory network that links the hemopoietic and the mesenchymal compartments of the bone marrow (BM), the interactions between the immune system and bone, and the role of lymphocytes as mediators of the effects of calciotrophic hormones in bone. Collectively this body of knowledge has led to the firm establishment of "osteoimmunology" as a novel discipline and a promising area of investigation. The objective of this perspective is to revisit the exciting hypothesis that T cells play a pivotal role in the mechanism of ovx-induced bone loss. The bone-sparing activity of estrogens is due to a repression of bone remodeling coupled with a balancing effect on bone formation and resorption.2 The dominant acute effect of estrogen is the blockade of new osteoclast formation, cells which arise by cytokine-driven proliferation and differentiation of monocyte precursors that circulate within the hematopoietic cell.3 This process is facilitated by BM stromal cells (SCs), which provide physical support for nascent osteoclasts and produce soluble and membrane-associated factors essential for the proliferation and differentiation of osteoclast precursors. This inhibitory effect on osteoclastogenesis is associated with a repressive effect on osteoblastogenesis.4 Another critical effect of estrogen is that of increasing osteoclast apoptosis5-7 while blocking the apoptosis of osteoblasts and osteocytes.2, 8 It has been proposed that the effects of estrogens on the generation of osteoblasts and the lifespan of osteoblasts and osteoclasts result from extranuclear actions of the endoplasmic reticulum (ER) and activation of cytoplasmic kinases.2, 7 The decline of ovarian function at menopause results in decreased production of estrogen and a parallel increase in pituitary follicle-stimulating hormone (FSH) levels. The combined effects of estrogen deprivation and elevated FSH production cause a marked stimulation of bone resorption9 and a period of rapid bone loss that is central to the pathogenesis of postmenopausal osteoporosis. In mice the acute effects of menopause are modeled by ovx, a procedure that stimulates bone resorption by increasing osteoclast formation3 and lifespan.5-7 This initial phase of bone loss is followed by a slower but more prolonged loss of mainly cortical bone due to incomplete refilling of the resorption cavities due to insufficient osteoblast (OB) activity and lifespan.10 An expansion of the osteoclastic pool is therefore the key mechanism responsible for the bone loss that occurs early after ovx. The net bone loss caused by ovx is limited, in part, by an increase in bone formation resulting from stimulated osteoblastogenesis.11 This compensation is fueled by an expansion of the pool of BM–SCs, increased commitment of such pluripotent precursors toward the osteoblastic lineage,11 and enhanced proliferation of early OB precursors.4 Subsequent escalations in OB apoptosis12, 13 extensions of osteoclast lifespan5, 6 and increased secretion of cytokines that suppress bone formation, such as interleukin 7 (IL-7) and tumor necrosis factor (TNF), are the likely reasons for why bone formation does not increase as much as resorption after ovx. The stimulatory effect of ovx on SCs is equally relevant for osteoclastogenesis as one of the consequences of estrogen deprivation is the formation of osteoblastic cells with an increased osteoclastogenic activity14; ie, the capacity to support osteoclast formation. Evidence now suggests that T cells play a pivotal role in the mechanism of ovx-induced bone loss. Core observations include reports showing that ovx fails to induce trabecular and cortical bone loss in: T-cell–deficient nude mice,1, 15-17, wild-type (WT) mice deleted of T cells by injection of anti–T cell antibodies,18 mice treated with Abatacept19 (an agent that blocks T cell costimulation and induces T cell anergy and apoptosis)20, 21 and mice lacking the T cell costimulatory molecule CD40 ligand (CD40L).18 The fact that nude mice are protected against ovx-induced bone loss has been confirmed independently.22 By contrast, Lee and colleagues23 showed that nude mice are protected against the loss of cortical, but not trabecular bone induced by ovx. In another study, T-cell–deficient and B-cell–deficient mice were found to lose bone after ovx.24 The discrepancy between our reports1, 15-19, 25 and those of others23, 24 is likely explained by differences in the experimental design, the lack of B cells in some models, and compensatory mechanisms such as an increase in natural killer (NK) cells producing the osteoclastogenic factor IL-17.26 Both CD4+ and CD8+ cells have been found to play a role in ovx-induced bone loss. CD4+ cells include the TH1, Th2, and Th17 subsets. Th17 are regarded as the most osteoclastogenic subset of T CD4+ cells because they produce high levels of IL-17, receptor activator of NF-κB ligand (RANKL), and TNF, and low levels of interferon gamma (IFNγ).27, 28 The differentiation of Th17 is inhibited by estrogen via a direct effect mediated by estrogen receptor alpha (ERα).29 The role of Th17 in ovx-induced bone loss remains to be determined because one study reported that IL-17R–null mice are more susceptible to ovx-induced bone loss than controls,30 while another group found these mice to be protected from ovx-induced bone loss.26 More abundant information is available about regulatory T cells (Tregs), a population capable of suppressing the effector function of TH1, Th2, and Th17 T cells. Tregs are defined by the expression of the transcription factor FoxP3. Tregs inhibit monocyte differentiation into osteoclasts in vitro and in vivo, and blunt bone resorption31, 32 through the secretion of IL-4, IL-10, and transforming growth factor beta 1 (TGFβ1).33 Attesting to the relevance of Tregs, studies have shown that estrogen increases the relative number of Tregs.27, 34 Moreover, transgenic mice overexpressing Tregs develop progressive high bone mass due to inhibition of bone resorption, and are protected against ovx-induced bone loss.35 Moreover, adoptive transfer of Tregs into T cell–deficient mice increases bone mass, indicating that Tregs directly affect bone homeostasis without the need to engage other T cell lineages.35 Two mechanisms have been described to explain how T cells contribute to ovx-induced bone loss (Fig. 1). The first involves an increase in T cell activation, leading to enhanced production of TNF by BM T cells. The second is a regulatory crosstalk between T cells and SCs, resulting in enhanced production of osteoclastogenic cytokines by SCs. Schematic representation of the role of T cells in the mechanism by which ovx promotes osteoclastogenesis, osteoblastogenesis, and hemopoiesis. Estrogen deficiency promotes T cell activation by increasing the interaction of antigen (Ag)-loaded MHC molecules with bone marrow macrophages (BMM) and dendritic cells (DC) with the T cell receptor (TCR). The Ags are likely to be non-self peptides derived from the intestinal macrobiota. T cell activation also requires at least two costimulatory signals provided by the binding of BMM and DC-expressed CD40 and CD80 to the T cell surface molecules CD40L and CD28, respectively. A critical upstream event is the increased production of reactive oxygen species (ROS) that activate DCs by increasing their expression of CD80. The expansion of T cells in the BM is partially driven by an ovx-induced increase in the thymic output of naïve T cells. Activated T cells secrete TNF that stimulates osteoclast formation primarily by potentiating the response to RANKL. In addition, T cell–expressed CD40L and DLK1/FA-1 increase the osteoclastogenic activity of SC by blunting their secretion of OPG and augmenting their production of RANKL, M-CSF, and other proinflammatory factors. The survival of naïve T cells and some memory T cells requires the low-affinity engagement of the T cell receptor (TCR) by a diverse repertoire of self-antigens (Ags) bound to major histocompatibility complex (MHC) molecules expressed on Ag-presenting cells.36, 37 By contrast, binding of foreign Ags such as bacterial Ags with MHC molecules is followed by high-affinity interactions with TCRs that drive T cell activation. Both low-affinity interactions with self-Ags and high-affinity foreign Ag/MHC/TCR interactions are referred to as "Ag presentation." Ovx induces T cell expression of activation markers and promotes T cell proliferation, expansion, and acquisition of effector functions. These are all features of T cells exposed to foreign Ags.36 The gastrointestinal tract is colonized for life with 100 trillion indigenous bacteria, creating a diverse ecosystem known as the microbiota, whose contributions to human health are profound.38 The macrobiota is likely to represent the source of foreign Ag that drives the expansion of T cells induced by ovx. Although direct verification of a role for macrobiota in T cell expansion is still lacking, compelling supportive data are available. For example, adoptive transfer of T cells into T cell–deficient mice is followed by rapid engraftment and expansion of donor T cells into the host. This process is driven by foreign Ags.36 Attesting to a role of the macrobiota, the expansion of transferred T cells into T cell–deficient mice is greatly reduced in host mice raised in a germ-free environment.36 Moreover, mice maintained in germ-free conditions display increased bone mass due to the lack of immune cell activation.39 T cells are key inducers of bone-wasting because ovx increases T cell TNF production to a level sufficient to augment RANKL-induced osteoclastogenesis.1 This effect is due to an increased number of TNF-producing T cells15 and enhanced production of TNF per cell.18, 40 Ovx also increases the population of premature senescent CD4 + CD28–T cells,40 a lineage that produces high levels of TNF. The presence of increased levels of T cell–produced TNF in the BM of ovx animals is well documented.15, 19, 40, 41 Studies have also shown that menopause increases T cell activation and T cell production of TNF and RANKL in humans.42, 43 The role of TNF in ovx-induced bone loss has been demonstrated in multiple models. For example, ovx fails to induce bone loss in TNF-null mice and in animals lacking the p55 TNF receptor.15 Likewise, transgenic mice insensitive to TNF due to the overexpression of a soluble TNF receptor,44 and mice treated with the TNF inhibitor TNF binding protein45 are protected from ovx-induced bone loss. The specific relevance of T cell TNF production in vivo was demonstrated by the finding that although reconstitution of nude recipient mice with T cells from WT mice restores the capacity of ovx to induce bone loss, reconstitution with T cells from TNF-deficient mice does not.15 The mechanism by which estrogen deficiency expands the pool of TNF-producing T cells is summarized in Figure 1 and involves reactivation of thymic function and induction of T cell activation in the BM. T cell activation is driven by enhanced Ag presentation by macrophages and dendritic cells (DCs).46, 47 The thymus undergoes progressive structural and functional decline with age, coinciding with increased circulating sex-steroid levels at puberty.48 By middle age most parenchymal tissue is replaced by fat, and in both mice and humans fewer T cells are produced and exported to secondary lymphoid organs. However, the thymus continues to generate new T cells even into old age.49, 50 In fact, active lymphocytic thymic tissue has been documented in adults up to 107 years of age.51 Under severe T cell depletion secondary to human immunodeficiency virus (HIV) infection, chemotherapy, or bone marrow transplant, an increase in thymic output (known as thymic rebound) becomes critical for long-term restoration of T cell homeostasis. For example, middle-aged women treated with autologous bone marrow transplants develop thymic hypertrophy and a resurgence of thymic T cell output, which contributes to the restoration of a wide T cell repertoire,52 although the intensity of thymic rebound declines with age. The mechanism driving thymic rebound is not completely understood, but one factor involved is IL-7.53 Both androgens and estrogen suppress thymic function.54, 55 Accordingly, castration reverses thymic atrophy and increases export of recent thymic emigrants to the periphery,56 whereas sex steroids inhibit thymic regeneration by promoting thymocyte apoptosis and arresting thymocyte/prelymphocyte differentiation.57 Restoration of thymic function after castration occurs in young58 as well as in very old rodents.59, 60 In accordance with the notion that estrogen deficiency induces a rebound in thymic function, ovx increases the thymic export of naïve T cells.61 Indeed, stimulated thymic T cell output accounts for ∼50% of the increase in the number of T cells in the periphery. Moreover, thymectomy decreases the bone loss induced by ovx by ∼50%, thus demonstrating that the thymus plays a previously unrecognized role in the pathogenesis of ovx-induced bone loss in mice.61 The remaining bone loss is a consequence of the peripheral expansion of naïve and memory T cells. This finding suggests the tantalizing hypothesis that estrogen deficiency–induced thymic rebound may be responsible for the exaggerated bone loss in young women undergoing surgical menopause62 or for the rapid bone loss characteristic of women in their first 5 to 7 years after natural menopause.63 Indeed, an age-related decrease in estrogen deficiency–induced thymic rebound could mitigate the stimulatory effects of sex steroid deprivation and explain why the rate of bone loss in postmenopausal women diminishes as aging progresses.63 The most upstream effects of ovx in the BM are to stimulate the production of reactive oxygen species (ROS) and to impair the generation of antioxidants.10, 19, 64, 65 In response to ovx, ROS are produced by most BM cells, including T cells.41 ROS play an important role in postmenopausal bone loss by generating a more oxidized bone microenvironment.66, 67 Multiple enzymatic pathways regulate the intracellular redox state through modulation of ROS levels.68 Ovx blunts the BM levels of glutathione (GSH), a critical ROS scavenger, and reduces expression of APE1/Ref-1 and Prx-1 proteins, which collectively limit the production of intracellular ROS.69 ROS have important direct effects on osteoblasts and osteoclasts; these effects have been addressed elsewhere.7, 70, 71 However, additional pivotal effects of ROS include expanding the pool of mature DCs that express the costimulatory molecule CD80, and increasing DC-mediated Ag presentation.19 Antioxidants potently inhibit DC differentiation and their ability to activate T cells,72, 73 in part by suppressing expression of MHC class II and costimulatory molecules in response to Ag.74 N-acetyl-cysteine (NAC), which acts as an intracellular scavenger of ROS by restoring intracellular concentrations of GSM, can block DC maturation75 and DC-mediated T cell activation.76 In vivo support for a role of ROS is provided by experiments demonstrating that administration of antioxidants prevents ovx-induced bone loss,19, 64, 70 while depletion of glutathione by buthionine sulfoximine (BSO), which inhibits glutathione synthesis, enhances bone loss.64 Bone loss caused by BSO has significant similarities to bone loss induced by estrogen deficiency, as both processes are TNF-dependent.77 A second, direct upstream effect of estrogen deficiency is to blunt BM levels of TGFβ,78 a powerful repressor of T cell activation. TGFβ acts as an immunosuppressant by inhibiting T cell activation and T cell production of inflammatory cytokines, including IFNγ. Demonstrating the relevance of the repressive effects of TGFβ on T cell function, mice with T cell–specific blockade of TGFβ signaling were found to be completely resistant to the bone-sparing effects of estrogen.16 Gain of function experiments confirmed that elevation of the systemic levels of TGFβ prevents ovx-induced bone loss and bone turnover.16 The key downstream mechanism by which ovx increases Ag presentation by macrophages is a stimulatory effect on the expression of the gene encoding Class II Transactivator (CIITA). The product of CIITA is a non-DNA binding factor induced by IFNγ that functions as a transcriptional coactivator at the MHC II promoter.79 Increased CIITA expression in macrophages results from ovx-mediated increases in IFNγ production by CD4+ T cells and the responsiveness of CIITA to IFNγ.46 This cytokine was initially described as an anti-osteoclastogenic cytokine because it is a potent inhibitor of osteoclastogenesis in vitro.80 The notion that IFNγ is an inhibitor of bone resorption was reinforced by the finding that silencing of IFNγR−/− signaling leads to a more rapid onset of collagen-induced arthritis and bone resorption81 as compared to WT controls, and by the report that IFNγ decreases serum calcium and osteoclastic bone resorption in nude mice.82, 83 Several mechanisms have been proposed to explain the anti-osteoclastogenic activity of IFNγ, including inhibition of RANKL signaling through the degradation of TNF receptor–associated factor 6 (TRAF6),80 stimulation of apoptosis mediated by Fas/FasL signals,84 and inhibition of RANK and c-Fms gene expression.85 However, the finding that IFNγ is an effective treatment for osteopetrosis both in humans86 and rodents87 demonstrates that the net effect of IFNγ in vivo is to stimulate osteoclastic bone resorption. In keeping with a net pro-resorptive effect of IFNγ in vivo are reports demonstrating that IFNγ−/− and IFNγR−/− mice are protected against ovx-induced bone loss.17, 46 Mice lacking IFNγ production are also protected against infection-induced alveolar bone loss,88 whereas in erosive tuberculoid leprosy and psoriatic arthritis IFNγ production correlates positively with tissue destruction.89 In addition, randomized controlled trials have shown that IFNγ does not prevent bone loss in patients with rheumatoid arthritis (RA),90 nor the bone-wasting effect of cyclosporine A.91 Finally, disruption of IFNγ signaling in vivo results in a strong and sustained inhibition of markers of osteoclastic activity.92 These opposing in vitro and in vivo effects of IFNγ are explained by the fact that IFNγ influences osteoclast formation via both direct and indirect effects.17 IFNγ directly blocks osteoclast formation through targeting of maturing osteoclast.93 However, IFNγ is also a potent inducer of antigen presentation and thus of T cell activation. Therefore, when IFNγ levels are increased in vivo, activated T cells secrete pro-osteoclastogenic factors and this activity offsets the anti-osteoclastogenic effects of IFNγ.17 It should also be mentioned that it is now recognized that IFNγ affects bone turnover by promoting the commitment of BM–SCs into the osteoblastic lineage and their differentiation into mature osteoblasts.94 Accordingly, treatment with IFNγ reverses ovx-induced bone loss by promoting bone formation.92 Another mechanism by which estrogen regulates T cell TNF production is by repressing the production of IL-7, a potent lymphopoietic cytokine and an inducer of bone destruction in vivo.95 Attesting to the relevance of this factor, IL-7 levels are significantly elevated following ovx,61, 96 and in vivo IL-7 blockade is effective in preventing ovx-induced bone destruction.96 The elevated BM levels of IL-7 contribute to the expansion of the T cell population in peripheral lymphoid organs through several mechanisms. First, IL-7 directly stimulates T cell proliferation.61 Second, IL-7 increases antigen presentation by upregulating the production of IFNγ. Third, IL-7 and TGFβ regulate The in TGFβ signaling characteristic of estrogen deficiency may to stimulate IL-7 thus driving the of osteoclastogenic cytokine production and bone In estrogen deficiency, IL-7 bone loss by suppressing bone formation and thus bone formation from resorption. A that the crosstalk between T cells and SCs is relevant for ovx-induced bone loss was provided by the finding that activated T cells induce SC apoptosis via the ligand a which blunts the compensatory increase in bone formation that bone loss in ovx More abundant information is available about the T crosstalk driven by the also known as is a key surface ligand expressed on T CD40L to and several CD40 is expressed on cells, hemopoietic and cells of the osteoblastic CD40L has been to because T cells, through the production of the anti-osteoclastogenic factor by B mice a reduced bone due to exaggerated bone bone has also been found in by a in which CD40L production is due to a in the CD40L However, mice lacking T cell–expressed CD40L are protected against hormone bone the that CD40L may in while promoting bone resorption conditions of bone Studies with mice and with WT mice treated with the CD40L have that silencing of CD40L completely prevents ovx-induced bone A mechanism was shown to be First, silencing of CD40L blocks the activation of T cells and the resulting production of TNF. Second, CD40L is for ovx to increase the proliferation and the differentiation of SCs and their capacity to support osteoclast formation through enhanced production of factor and RANKL, and secretion of a critical additional mechanism by which T cells bone homeostasis in ovx mice is through a crosstalk between T cells and SCs that results in enhanced osteoclastogenesis to a enhanced In ovx increases the number of activated T cells that the expression of and RANKL by SCs, and ovx the SC production of The net result is a significant increase in the rate of The interaction of CD40L with CD40 on SCs in the of estrogen deficiency to the effects of costimulation on B cell OPG the of osteoclast formation in of bone loss. An additional mechanism of T crosstalk the novel of bone mass antigen has been for a known as This factor can be to generate a soluble known as In conditions DLK1/FA-1 are produced by SCs, B cells, and T cells. These factors stimulate osteoclastogenesis and block by the production of TNF, IL-7, and other inflammatory cytokines by SCs. Ovx increases the production of DLK1/FA-1 by activated CD4+ and CD8+ T cells, resulting in a stimulation of the production of osteoclastogenic cytokines by SCs. Attesting to the relevance of mice are significantly protected against ovx-induced bone The hypothesis that the immune system is pivotal for the bone loss about by menopause remains to be demonstrated in it remains to the of why the immune system in and is involved in the mechanism of ovx-induced bone loss. may by postmenopausal bone loss as an of an event critical for the need to stimulate bone resorption in the This process is essential to the increased for calcium about by The for this event is the acute in estrogen levels A second to the period is in associated with is by proliferation of the and formation of These are by and is now that regulates and to by the production of RANKL and a system that is for bone homeostasis is also a key of The in levels in the period results in reduced tissue levels of RANK and RANKL, with the effect of the for The increase in the and TNF levels in the BM induced by estrogen levels the of calcium from the to the and then to the a state of to the is induced by an increase in the number of A to the of is the loss of such immune and the restoration of a immune This is through a decrease in the resulting from the acute in steroid levels. However, a between immune to the and bone is provided by the that OPG is expressed by human it is to that of ovarian function induces bone loss through an immune response because natural has these key within the immune system to (Fig. Schematic representation of in the period and of the involved cells and The that estrogen and are followed by a stimulation of bone resorption, to and of immune to the Activated T cells drive bone resorption by TNF. A decrease in the number of Tregs induces the of the immune The loss of OPG may contribute to the and the increased bone resorption. A decrease in levels of RANKL and RANK induce the of the for with increased of calcium from the the in the contribute to an event critical for The data the hypothesis that of ovarian function induces bone loss through an immune response because natural has key within the immune system to The data support the hypothesis that the bone loss induced by estrogen deficiency is due to a complex of hormones and cytokines that to the process of bone Ovx T cell TNF production by increasing T cell activity in the and the peripheral lymphoid organs. T cell precursors the BM and to the T cell and expansion in the of from the these new T cells to peripheral lymphoid including the BM Ovx induces T cell activation in the BM in part by directly promoting antigen and in part via stimulation of IL-7 and IFNγ production and of TGFβ The net result of these actions is an increase in the number of TNF-producing T cells. The elevated levels of TNF increase RANKL-induced osteoclast formation. Estrogen deficiency also T cell activation and osteoclastogenesis by leading to an in The combined effect of IFNγ and ROS enhances Ag T cell activation. T cells stimulate RANKL and production by SCs, through CD40L and The that has of

Woodchuck hepatitis virus (WHV), similar to human hepatitis B virus, causes acute liver inflammation that can progress to chronic hepatitis and hepatocellular carcinoma. WHV also invades cells of the host lymphatic system, where it persists for life. We report here that acute and chronic hepadnavirus hepatitis is characterized by a profound difference in the expression of class I major histocompatibility complex (MHC) molecules on the surface of infected hepatocytes and, notably, lymphoid cells. While acute WHV infection is accompanied by the enhanced hepatocyte surface presentation of class I MHC antigen and upregulated transcription of the relevant hepatic genes, inhibition of class I antigen display on liver cells is a uniform hallmark of chronic WHV infection. This inhibition in chronic hepatitis occurs despite augmented (as in acute infection) expression of hepatic genes for class I MHC heavy chain, beta(2)-microglobulin, and transporters associated with antigen processing (TAP1 and TAP2). Further, the class I antigen inhibition is not related to the histological severity of hepatocellular injury, the extent of lymphocytic infiltrations, the level of intrahepatic gamma interferon induction, or the hepatic WHV load. Importantly, the antigen expression is also inhibited on organ lymphoid cells of chronically infected hosts. The results obtained in this study demonstrate that the defective presentation of class I MHC molecules on cells supporting persistent WHV replication is due to viral posttranscriptional interference. This event may diminish the susceptibility of infected hepatocytes to virus-specific T-cell-mediated elimination, hinder virus clearance, and deregulate the class I MHC-dependent functions of the host immune system. This multifarious effect could be critical for perpetuation of liver damage and evasion of the antiviral immunological surveillance in chronic infection and therefore could be supportive of hepadnavirus persistence.

An antigen will be able to stimulate an immune response if the antigen has been presented and recognized by T-cells. Major Histocompatibility Complex (MHC) molecules are molecules that will recognize and present the peptides of the antigen. In humans, two classes of MHC are known to carry out this role, namely Class-I MHC and Class-II MHC, while Class-III MHC has a function that is not directly related to peptide binding. The genes that code for MHC molecules are so polymorphic that each individual is certain to have a different allele (heterozygous). In Class-I MHC, only the α-chain is polymorphic while in Class-II MHC both α and β-chains are polymorphic. MHC polymorphism will affect (1) the ability to create immune responses including antibody production, (2) resistance or susceptibility to infectious diseases, and (3) resistance or susceptibility to autoimmune and allergic diseases. That is why MHC is so polymorphic. The immune system has to fight many different pathogens. By having many different MHC molecules, one can present widely different antigens and can spur an effective immune response.

The T-cell receptor is a cell surface heterodimer consisting of an alpha and a beta chain that binds foreign antigen in the context of a cell surface molecule encoded by the major histocompatibility complex (MHC), thus restricting the T-cell response to the surface of antigen presenting cells. The variable (V) domain of the receptor binds antigen and MHC molecules and is composed of distinct regions encoded by separate gene elements--variable (V alpha and V beta), diversity (D beta) and joining (J alpha and J beta)--rearranged and joined during T-cell differentiation to generate contiguous V alpha and V beta genes. T-helper cells, which facilitate T and B cell responses, bind antigen in the context of a class II MHC molecule. The helper T-cell response to cytochrome c in mice is a well-defined model for studying the T-cell response to restricted antigen and MHC determinants. Only mice expressing certain class II molecules can respond to this antigen (Ek alpha Ek beta, Ek alpha Eb beta, Ev alpha Ev beta and Ek alpha Es beta). Most T cells appear to recognize the C-terminal peptide of cytochrome c (residues 81-104 in pigeon cytochrome c). We have raised helper T cells to pigeon cytochrome c or its C-terminal peptide analogues in four different MHC congenic strains of mice encoding each of the four responding class II molecules. We have isolated and sequenced seven V alpha genes and six V beta genes and analysed seven additional helper T cells by Northern blot to compare the structure of the V alpha and V beta gene segments with their antigen and MHC specificities. We have added five examples taken from the literature. These data show that a single V alpha gene segment is responsible for a large part of the response of mice to cytochrome c but there is no simple correlation of MHC restriction with gene segment use.

Molecular mimicry by major histocompatibility complex molecules and peptides accounts for some alloresponses.

Human cytomegalovirus (HCMV) unique short region 3 (US3) protein, a type I membrane protein, prevents maturation of class I major histocompatibility complex (MHC) molecules by retaining them in the endoplasmic reticulum (ER) and thus helps inhibit antigen presentation to cytotoxic T cells. US3 molecules bind to class I MHC molecules in a transient fashion but retain them very efficiently in the ER nonetheless. The US3 luminal domain is responsible for ER retention of US3 itself, while both the US3 luminal and transmembrane domains are necessary for retaining class I MHC in the ER. We have expressed the luminal domain of US3 molecule in Escherichia coli and analyzed its secondary structure by using nuclear magnetic resonance. We then predicted the US3 tertiary structure by modeling it based on the US2 structure. Unlike the luminal domain of US2, the US3 luminal domain does not obviously interact with class I MHC molecules. The luminal domain of US3 dynamically oligomerizes in vitro and full-length US3 molecules associate with each other in vivo. We present a model depicting how dynamic oligomerization of US3 may enhance its ability to retain class I molecules within the ER.

The exact role of major histocompatibility complex (MHC) molecules in the peripheral survival of naive T cells is controversial, as some studies have suggested that they are critically required whereas others have suggested that they are not. Here we controlled for some of the features that differed among the earlier studies, and analyzed both the survival and expansion of naive CD4+ T cells transferred into MHC syngeneic, allogeneic, or MHC negative environments. We found that naive T cells transferred into MHC negative or allogeneic environments often fail to survive because of rejection and/or competition by natural killer (NK) cells, rather than failure to recognize a particular MHC allele. In the absence of NK cells, naive CD4+ T cells survived equally well regardless of the MHC type of the host. There was, however, an MHC requirement for extensive space-induced “homeostatic” expansion. Although the first few divisions occurred in the absence of MHC molecules, the cells did not continue to divide or transit to a CD44hi phenotype. Surprisingly, this MHC requirement could be satisfied by alleles other than the restricting haplotype. Therefore, space-induced expansion and survival are two different phenomena displaying different MHC requirements. Memory CD4+ T cells, whose survival and expansion showed no requirements for MHC molecules at all, dampened the space-induced expansion of naive cells, showing that the two populations are not independent in their requirements for peripheral niches.

Genes of the major histocompatibility complex (MHC) offer several assets that make them unique candidates for studies of adaptation in natural populations (Potts & Wakeland, 1990; Hedrick, 1994). The primary role of the MHC is to recognize foreign proteins, present them to specialist immune cells and initiate an immune response (Klein & Figueroa, 1986). The MHC gene family includes highly polymorphic genes encoding a set of transmembrane glycoproteins that are critical to the generation of immune responses (Kennedy et al., 2002). In general, foreign proteins enter cells either by infection or by phagocytosis into antigenpresenting cells such as macrophages. These foreign proteins are broken down into small peptides and loaded onto specific MHC molecules. The MHC molecule comprises an immunoglobulin stalk, which anchors the molecule to the cell surface, and a basket receptor called antigen-recognizing sites (ARS) located in peptide binding region (PBR). A subset of these protein/MHC complexes are then transported to the cell surface and presented for interrogation by the circulating T-cell population. A complex cascade of immune responses is triggered when the T cell binds to the presented peptide. Two major groups of MHC genes can be distinguished. MHC class I genes play an essential role in the immune defense against intracellular pathogens by binding peptides mainly derived from viral proteins and cancer-infected cells. They are expressed on the surface of all nucleated somatic cells. In contrast, MHC class II genes are predominantly involved in monitoring the extracellular environment by presenting peptides mainly derived from parasites (e.g. bacteria, nematodes, cestodes) to the T-cells. They are primarily expressed on antigen-presenting cells of the immune system, such as B cells and macrophages. Although ARS do show a degree of specificity, a single MHC molecule can bind multiple peptides that have common amino acids at particular anchor positions (Altuvia & Margalit, 2004). Genes within the MHC involved in antigen presentation constitute the most polymorphic loci known in vertebrates (Hedrick, 1994). The polymorphism of the MHC-molecules is associated with the diversity of the T-lymphocyte receptors that in turn determine the disease and parasite resistance of an organism and thus may affect the long-term survival rate of populations (Hedrick et al., 1999; Paterson et al., 1998). The ARS show high levels of polymorphism not only in the number of alleles but also in the sequence variation among alleles (Hughes & Yeager, 1998). The general view is that balancing selection is the determinant role in shaping patterns of nucleotide diversity in MHC genes (Bernatchez & Landry, 2003; Hughes & Nei, 1989).

Acute allograft rejection is mediated by host CD8+ cytotoxic T lymphocytes (CTL) targeting graft class I major histocompatibility complex (MHC) molecules. In experimental rodent models, rejection requires differentiation of naive CD8+ T cells into alloreactive CTL within secondary lymphoid organs, whereas in humans, CTL may alternatively develop within the graft from circulating CD8+ effector memory T cells (TEM) that recognize class I MHC molecules on graft endothelial cells (EC). This latter pathway is poorly understood. Here, we show that host CD4+ TEM, activated by EC class II MHC molecules, provide critical help for this process. First, blocking HLA-DR on EC lining human artery grafts in immunodeficient mice reduces CD8+ CTL development within and acute rejection of the artery by adoptively transferred allogeneic human lymphocytes. Second, siRNA knockdown or CRISPR/Cas9 ablation of class II MHC molecules on EC prevents CD4+ TEM from helping CD8+ TEM to develop into CTL in vitro. Finally, implanted synthetic microvessels, formed from CRISPR/Cas9-modified EC lacking class II MHC molecules, are significantly protected from CD8+ T cell–mediated destruction in vivo. We conclude that human CD8+ TEM–mediated rejection targeting graft EC class I MHC molecules requires help from CD4+ TEM cells activated by recognition of class II MHC molecules.