Major histocompatibility complex (MHC) class I KbDb -/- deficient mice possess functional CD8+ T cells and natural killer cells

Classical major histocompatibility complex class I molecules in motoneurons: new actors at the neuromuscular junction

Major histocompatibility complex (MHC) class I molecules have fundamental functions in the immune system. Recent studies have suggested that these molecules may also have non-immune functions in the nervous system, in particular related to synaptic function and plasticity. Because adult motoneurons express mRNAs for MHC class I molecules, we have examined their subcellular expression pattern in vivo and their role for the synaptic connectivity of these neurons. We observed immunoreactivity for classical MHC class I (Ia) protein in motoneuron somata, but the predominant expression was found in axons and presynaptically at neuromuscular junctions (NMJs). Peripheral nerve lesion induced a strong increase of motoneuron MHC class Ia (H2-K(b)/D(b)) mRNA, indicating a role for MHC class Ia molecules during regeneration. Accordingly, there was an accumulation of MHC class Ia proteins at the cut ends and in growth cones of motor axons after lesion. In K(b-/-)D(b-/-) mice (lacking MHC class Ia molecules), the time course for recovery of grip ability in reinnervated muscles was significantly delayed. Muscles from K(b-/-)D(b-/-) mice displayed an increased density and a disturbed distribution of NMJs and fewer terminal Schwann cells/NMJ compared with wild-type mice. A population of Schwann cells in sciatic nerves expressed the paired Ig receptor B, which binds to MHC class I molecules. These results provide the first evidence that neuronal MHC class Ia molecules are present in motor axons, that they are important for organization of NMJs and motor recovery after nerve lesion, and that their actions may be mediated via Schwann cells.

Self-class I MHC molecules support survival of naive CD8 T cells, but depress their functional sensitivity through regulation of CD8 expression levels

Previous studies have suggested that naive CD8 T cells require self-peptide–major histocompatability complex (MHC) complexes for maintenance. However, interpretation of such studies is complicated because of the involvement of lymphopenic animals, as lymphopenia drastically alters naive T cell homeostasis and function. In this study, we explored naive CD8 T cell survival and function in nonlymphopenic conditions by using bone marrow chimeric donors and hosts in which class I MHC expression is absent or limited to radiosensitive versus radioresistant cells. We found that long-term survival of naive CD8 T cells (but not CD4 T cells) was impaired in the absence of class I MHC. However, distinct from this effect, class I MHC deprivation also enhanced naive CD8 T cell responsiveness to low-affinity (but not high-affinity) peptide–MHC ligands. We found that this improved sensitivity was a consequence of up-regulated CD8 levels, which was mediated through a transcriptional mechanism. Hence, our data suggest that, in a nonlymphopenic setting, self-class I MHC molecules support CD8 T cell survival, but that these interactions also attenuate naive T cell sensitivity by dynamic tuning of CD8 levels.

Preclinical characterization of 1-7F9, a novel human anti-KIR receptor therapeutic antibody that augments natural killer-mediated killing of tumor cells

Inhibitory-cell killer immunoglobulin-like receptors (KIR) negatively regulate natural killer (NK) cell-mediated killing of HLA class I-expressing tumors. Lack of KIR-HLA class I interactions has been associated with potent NK-mediated antitumor efficacy and increased survival in acute myeloid leukemia (AML) patients upon haploidentical stem cell transplantation from KIR-mismatched donors. To exploit this pathway pharmacologically, we generated a fully human monoclonal antibody, 1-7F9, which cross-reacts with KIR2DL1, -2, and -3 receptors, and prevents their inhibitory signaling. The 1-7F9 monoclonal antibody augmented NK cell-mediated lysis of HLA-C-expressing tumor cells, including autologous AML blasts, but did not induce killing of normal peripheral blood mononuclear cells, suggesting a therapeutic window for preferential enhancement of NK-cell cytotoxicity against malignant target cells. Administration of 1-7F9 to KIR2DL3-transgenic mice resulted in dose-dependent rejection of HLA-Cw3-positive target cells. In an immunodeficient mouse model in which inoculation of human NK cells alone was unable to protect against lethal, autologous AML, preadministration of 1-7F9 resulted in long-term survival. These data show that 1-7F9 confers specific, stable blockade of KIR, boosting NK-mediated killing of HLA-matched AML blasts in vitro and in vivo, providing a preclinical basis for initiating phase 1 clinical trials with this candidate therapeutic antibody.

Yersinia pestis endowed with increased cytotoxicity is avirulent in a bubonic plague model and induces rapid protection against pneumonic plague

An important virulence strategy evolved by bacterial pathogens to overcome host defenses is the modulation of host cell death. Previous observations have indicated that Yersinia pestis, the causative agent of plague disease, exhibits restricted capacity to induce cell death in macrophages due to ineffective translocation of the type III secretion effector YopJ, as opposed to the readily translocated YopP, the YopJ homologue of the enteropathogen Yersinia enterocolitica Oratio8. This led us to suggest that reduced cytotoxic potency may allow pathogen propagation within a shielded niche, leading to increased virulence. To test the relationship between cytotoxic potential and virulence, we replaced Y. pestis YopJ with YopP. The YopP-expressing Y. pestis strain exhibited high cytotoxic activity against macrophages in vitro. Following subcutaneous infection, this strain had reduced ability to colonize internal organs, was unable to induce septicemia and exhibited at least a 10(7)-fold reduction in virulence. Yet, upon intravenous or intranasal infection, it was still as virulent as the wild-type strain. The subcutaneous administration of the cytotoxic Y. pestis strain appears to activate a rapid and potent systemic, CTL-independent, immunoprotective response, allowing the organism to overcome simultaneous coinfection with 10,000 LD(50) of virulent Y. pestis. Moreover, three days after subcutaneous administration of this strain, animals were also protected against septicemic or primary pneumonic plague. Our findings indicate that an inverse relationship exists between the cytotoxic potential of Y. pestis and its virulence following subcutaneous infection. This appears to be associated with the ability of the engineered cytotoxic Y. pestis strain to induce very rapid, effective and long-lasting protection against bubonic and pneumonic plague. These observations have novel implications for the development of vaccines/therapies against Y. pestis and shed new light on the virulence strategies of Y. pestis in nature.

H2-K(b) and H2-D(b) regulate cerebellar long-term depression and limit motor learning

There are more than 50 class I MHC (MHCI) molecules in the mouse genome, some of which are now known to be expressed in neurons; however, the role of classical MHCI molecules in synaptic plasticity is unknown. We report that the classical MHCI molecules, H2-K(b) and H2-D(b), are co-expressed by Purkinje cells (PCs). In the cerebellum of mice deficient for both H2-K(b) and H2-D(b) (K(b)D(b-/-)), there is a lower threshold for induction of long-term depression (LTD) at parallel fiber to PC synapses. This change may be a result of additional glutamate release observed at K(b)D(b-/-) CF to PC synapses, which are thought to "train" the cerebellar circuit. A behavioral correlate of cerebellar LTD is motor learning; acquisition and retention of a Rotarod behavioral task is significantly better in K(b)D(b-/-) mice than in WT cohorts. These physiological and behavioral phenotypes in K(b)D(b-/-) mice reveal a surprising role for classical MHCI molecules in synaptic plasticity and motor learning.

T cell receptor contact to restricting MHC molecules is a prerequisite for peripheral interclonal T cell competition

T cell survival and homeostatic proliferation in the periphery requires T cell receptor (TCR) binding to restricting major histocompatability complex (MHC)-encoded molecules, as well as the availability of certain lymphokines. However, the exact mechanisms by which these signals interrelate and contribute to homeostasis are not understood. By performing T cell transfers into TCR transgenic hosts we detected a hierarchical order of homeostatic proliferation for T cells differing in MHC restriction, such that OT1 cells (K(b) restricted) proliferated in P14 (D(b)-restricted TCR) recipients, but not vice versa. Using K(b) mutant mice, we demonstrated that proliferation of OT1 cells in P14 recipients, as well as the ability of host OT1 cells to hinder the proliferation of donor P14 cells, were dependent on OT1-TCR binding to K(b) molecules. However, interclonal T cell competition was not mediated simply by competition for physical access to the MHC-bearing cell. This was shown in parabiotic pairs of OT1 and K(b) mutant mice in which P14 cells failed to proliferate, even though the OT1 cells could not interact with half of the APCs in the system. Thus, we conclude that the interaction between the TCR and restricting MHC molecule influences the ability to compete for trophic resources not bound to the stimulating APC. This mechanism allows a local competitiveness that extends beyond a T cell's specificity.

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.

Use of the CD107 mobilization assay reveals that cytotoxic T lymphocytes with novel MHC-Ib restriction are activated during Listeria monocytogenes infection

Detection of cytotoxic activity by pathogen-specific T cells of unknown antigenic specificity is difficult due to the limitations of using infected cells, instead of peptide-pulsed cells, as targets. We report here that the recently described CD107 mobilization assay readily allowed for the ex vivo detection of cytotoxic T lymphocytes (CTL) with a novel MHC-Ib restriction that specifically recognized Listeria monocytogenes-infected macrophages. The CD107 mobilization assay is likely to be a useful tool for detection of CD8(+) T cells that recognize a wide variety of intracellular pathogens.

Critical role of Qa1b in the protection of mature dendritic cells from NK cell-mediated killing

Molecular interactions in natural killer (NK) cell-mediated killing of dendritic cells (DC) have under recent years come under scrutiny. Upon stimulation with IFN-gamma or lipopolysaccharide, DC become relatively resistant to NK cell-mediated lysis. In the present study, we investigated the role of Qa1(b) on DC and its receptor NKG2A on NK cells in the protection of mature DC from NK cells. We demonstrate that while both NKG2A+ and NKG2A- NK cells can efficiently lyse unstimulated DC, NKG2A+ NK cells but not NKG2A- NK cells are largely impaired in their ability to lyse mature DC. Similarly, mature DC from mice expressing H-2D(b), whose leader peptide sequence binds and stabilizes Qa1(b), were resistant to NK cell-mediated killing, suggesting that stable Qa1(b) expression contributes to the protection of mature DC. This finding was further validated by the demonstration that addition of the Qdm leader peptide could protect TAP1-/- DC from NK cell-mediated lysis both in vitro and in vivo. The present data suggest that stable expression of Qa1 on the surface of mature DC contributes to the protection of DC from NK cell-mediated lysis.

Xenogeneic β2-Microglobulin Substitution Alters NK Cell Function

Recently, it has been shown that human beta(2)-microglobulin (h-beta(2)m) blocks the association between the NK cell inhibitory receptor Ly49C and H-2K(b). Given this finding, we therefore sought to assess the immunobiology of NK cells derived from C57BL/6 (H-2(b)) mice expressing exclusively h-beta(2)m. Initial analysis revealed that the Ly49C expression profile of NK cells from h-beta(2)m(+) mice was modified, despite the fact that H-2K(b) expression was normal in these mice. Moreover, the NK cells were not anergic in that IL-2 treatment of h-beta(2)m(+) NK cells in vitro enabled efficient lysis of prototypic tumor cell lines as well as of syngeneic h-beta(2)m(+) lymphoblasts. This loss of self-tolerance appeared to correlate with the activation status of h-beta(2)m(+) NK cells because quiescent h-beta(2)m(+) transplant recipients maintained h-beta(2)m(+) grafts but polyinosine:polycytidylic acid-treated recipients acutely rejected h-beta(2)m(+) grafts. NK cell reactivity toward h-beta(2)m(+) targets was attributed to defective Ly49C interactions with h-beta(2)m:H-2K(b) molecules. With regard to NK cell regulatory mechanisms, we observed that h-beta(2)m:H-2K(b) complexes in the cis-configuration were inefficient at regulating Ly49C and, furthermore, that receptor-mediated uptake of h-beta(2)m:H-2K(b) by Ly49C was impaired compared with uptake of mouse beta(2)m:H-2K(b). Thus, we conclude that transgenic expression of h-beta(2)m alters self-MHC class I in such a way that it modulates the NK cell phenotype and interferes with regulatory mechanisms, which in turn causes in vitro-expanded and polyinosine:polycytidylic acid-activated NK cells to be partially self-reactive similar to what is seen with NK cells derived from MHC class I-deficient mice.

Signalling through TEC kinases regulates conventional versus innate CD8(+) T-cell development

Recent data from three laboratories have identified the TEC kinases, ITK and RLK, as crucial regulators of CD8(+) T-cell development into the conventional lymphocyte lineage. In the absence of ITK and RLK, CD4(+)CD8(+) thymocytes upregulate the T-box transcription factor eomesodermin, and develop into mature CD8(+) T cells that resemble memory cells, exhibit immediate effector cytokine production and depend on IL-15. Furthermore, the selection of these non-conventional 'innate' T cells results from interactions with haematopoietic cells in the thymus. These findings lead to the hypothesis that altered TCR signalling, together with distinct co-stimulatory signals, is the basis for the development of non-conventional T-cell lineages.

Natural Regulation of Immunity to Minor Histocompatibility Antigens

MHC-matched hemopoietic stem cell transplantation is commonly used for the treatment of some forms of leukemia. Conditioning regimens before transplant act to reduce the burden of leukemic cells and the graft-vs-leukemia (GvL) effect can eliminate residual disease. The GvL effect results largely from the recognition of minor histocompatibility Ags by donor T cells on recipient tissues. These Ags are generally widely expressed and also provoke graft-vs-host (GvH) disease. Manipulation of immunity to promote GvL while curtailing GvH would greatly improve clinical outcome. To develop strategies that may achieve this, the parameters which control immunity to minor histocompatibility Ags need to be defined. In this study, we have analyzed responses to the mouse HY minor histocompatibility Ag using hemopoietic cell and skin grafts as surrogate GvL and GvH targets, respectively. We show that natural regulation of CD8 T cell responses to HY operates at multiple levels. First, CD4 T cell help is required for primary CD8 responses directed at hemopoietic cells. However, although CD4 T cells of H2(k) mouse strains recognize HY, they provide ineffective help associated with a proportion of recipients developing tolerance. This was further investigated using TCR-transgenic mice which revealed H2(k)-restricted HY-specific CD4 T cells are highly susceptible to regulation by CD25(+) regulatory T cells which expand in tolerant recipients. A second level of regulation, operating in the context of skin grafts, involves direct inhibition of CD8 T cell responses by CD94/NKG2 engagement of the nonclassical MHC class I molecule Qa1.

Immune-privileged embryonic Swiss mouse STO and STO cell-derived progenitor cells: major histocompatibility complex and cell differentiation antigen expression patterns resemble those of human embryonic stem cell lines

Embryonic mouse STO (S, SIM; T, 6-thioguanine resistant; O, ouabain resistant) and 3(8)21-enhanced green fluorescent protein (EGFP) cell lines exhibit long-term survival and hepatic progenitor cell behaviour after xenogeneic engraftment in non-immunosuppressed inbred rats, and were previously designated major histocompatibility complex (MHC) class I- and class II-negative lines. To determine the molecular basis for undetectable MHC determinants, the expression and haplotype of H-2K, H-2D, H-2L and I-A proteins were reassessed by reverse transcriptase-polymerase chain reaction (RT-PCR), cDNA sequencing, RNA hybridization, immunoblotting, quantitative RT-PCR (QPCR), immunocytochemistry and flow cytometry. To detect cell differentiation (CD) surface antigens characteristic of stem cells, apoptotic regulation or adaptive immunity that might facilitate progenitor cell status or immune privilege, flow cytometry was also used to screen untreated and cytokine [interferon (IFN)-gamma]-treated cultures. Despite prior PCR genotyping analyses suggestive of H-2q haplotypes in STO, 3(8)21-EGFP and parental 3(8)21 cells, all three lines expressed H-2K cDNA sequences identical to those of d-haplotype BALB/c mice, as well as constitutive and cytokine-inducible H-2K(d) determinants. In contrast, apart from H-2L(d[LOW]) display in 3(8)21 cells, H-2Dd, H-2Ld and I-Ad determinants were undetectable. All three lines expressed constitutive and cytokine-inducible CD34; however, except for inducible CD117([LOW]) expression in 3(8)21 cells, no expression of CD45, CD117, CD62L, CD80, CD86, CD90.1 or CD95L/CD178 was observed. Constitutive and cytokine-inducible CD95([LOW]) expression was detected in STO and 3(8)21 cells, but not in 3(8)21-EGFP cells. MHC (class I(+[LOW])/class II-) and CD (CD34+/CD80-/CD86-/CD95L-) expression patterns in STO and STO cell-derived progenitor cells resemble patterns reported for human embryonic stem cell lines. Whether these patterns reflect associations with mechanisms that are regulatory of immune privilege or functional tissue-specific plasticity is unknown.

Effective control of chronic gamma-herpesvirus infection by unconventional MHC Class Ia-independent CD8 T cells

Control of virus infection is mediated in part by major histocompatibility complex (MHC) Class Ia presentation of viral peptides to conventional CD8 T cells. Although important, the absolute requirement for MHC Class Ia–dependent CD8 T cells for control of chronic virus infection has not been formally demonstrated. We show here that mice lacking MHC Class Ia molecules (K^b−/−xD^b−/− mice) effectively control chronic γ-herpesvirus 68 (γHV68) infection via a robust expansion of β₂-microglobulin (β₂-m)-dependent, but CD1d-independent, unconventional CD8 T cells. These unconventional CD8 T cells expressed: (1) CD8αβ and CD3, (2) cell surface molecules associated with conventional effector/memory CD8 T cells, (3) TCRαβ with a significant Vβ4, Vβ3, and Vβ10 bias, and (4) the key effector cytokine interferon-γ (IFNγ). Unconventional CD8 T cells utilized a diverse TCR repertoire, and CDR3 analysis suggests that some of that repertoire may be utilized even in the presence of conventional CD8 T cells. This is the first demonstration to our knowledge that β₂-m–dependent, but Class Ia–independent, unconventional CD8 T cells can efficiently control chronic virus infection, implicating a role for β₂-n–dependent non-classical MHC molecules in control of chronic viral infection. We speculate that similar unconventional CD8 T cells may be able to control of other chronic viral infections, especially when viruses evade immunity by inhibiting generation of Class Ia–restricted T cells.

In this paper the authors identify a β2-microglobulin–dependent but major histocompatibility complex (MHC) Class Ia– and CD1-independent class of CD8 T cells that effectively control chronic γ-herpesvirus infection in mice. The important point that should be of general interest to the readers of PLoS Pathogens is that an effective CD8 T cell response develops during chronic infection of mice lacking MHC Class Ia molecules. Enormous efforts have gone into characterizing the role of conventional CD8 T cells that recognize viral peptides together with MHC Class Ia molecules during chronic viral infection, and many vaccine approaches focus solely on this response. This paper shows that additional types of CD8 T cells can operate during chronic infection, and that indeed, conventional MHC Class Ia–restricted T cells may be dispensable for control of chronic herpesvirus infection. The authors believe this is a fundamentally important point because it raises the question of whether unconventional CD8 T cells are important for control of other chronic viral infections such as infection with HIV, Hepatitis C virus, Hepatitis B virus, or human herpesviruses.

Highly conserved pattern of recognition of influenza A wild-type and variant CD8+ CTL epitopes in HLA-A2+ humans and transgenic HLA-A2+/H2 class I-deficient mice

As an in vivo model for studying human MHC (HLA) class I-restricted CTL responses to viral infection, we established a series of HLA Tg mice expressing HLA-A2, -B7 or -B27 human/mouse hybrid genes on a background deficient for H2 class I (Tg HLA(hyb)/H2 class I DKO). To determine whether CTL recognition of influenza A (flu) infection in Tg HLA-A2(hyb)/H2 DKO mice is similar to HLA-A2+ humans, we compared the HLA-A2-restricted Tg mouse and human CD8+ T-cell responses to an immunodominant flu epitope (wild-type [WT] M1 58-66), as well as a variant of this peptide (var. M1 58-66). Similar to HLA-A2+ humans, our results show WT M1 58-66 is likely the dominant CTL epitope recognized in infected Tg HLA-A2(hyb)/H2 DKO mice. Var. M1 58-66 was also recognized by WT peptide-reactive T cells from both HLA-A2+ humans and Tg mice, although slightly less efficiently than WT M1 58-66 in both cases. Reduced variant recognition was shown to be associated with reduced peptide/A2 binding, as well as a more limited repertoire of utilized TCR Vbeta chains. The similar pattern of recognition and cross reaction observed here for the WT and variant M1 58-66 epitopes with HLA-A2 by human and Tg HLA mouse CTLs indicates that A2-dependent events of Ag processing, presentation and recognition are well-conserved between species. These findings demonstrate that this Tg HLA-A2/H2 DKO model will aid identification and development of epitopes as vaccines for numerous viral and tumor antigens for the HLA-A2 supertype.

Restricted MHC-peptide repertoire predisposes to autoimmunity

MHC molecules associated with autoimmunity possess known structural features that limit the repertoire of peptides that they can present. Such limitation gives a selective advantage to TCRs that rely on interaction with the MHC itself, rather than with the peptide residues. At the same time, negative selection is impaired because of the lack of negatively selecting peptide ligands. The combination of these factors may predispose to autoimmunity. We found that mice with an MHC class II–peptide repertoire reduced to a single complex demonstrated various autoimmune reactions. Transgenic mice bearing a TCR (MM14.4) cloned from such a mouse developed severe autoimmune dermatitis. Although MM14.4 originated from a CD4⁺ T cell, dermatitis was mediated by CD8⁺ T cells. It was established that MM14.4⁺ is a highly promiscuous TCR with dual MHC class I/MHC class II restriction. Furthermore, mice with a limited MHC–peptide repertoire selected elevated numbers of TCRs with dual MHC class I/MHC class II restriction, a likely source of autoreactivity. Our findings may help to explain the link between MHC class I responses that are involved in major autoimmune diseases and the well-established genetic linkage of these diseases with MHC class II.

Mouse endothelial cells cross-present lymphocyte-derived antigen on class I MHC via a TAP1- and proteasome-dependent pathway

In vivo studies suggest that vascular endothelial cells (ECs) can acquire and cross-present exogenous Ag on MHC-I but the cellular mechanisms underlying this observation remain unknown. We tested whether primary female mouse aortic ECs could cross-present exogenous male Ag to the T cell hybridoma, MHH, specific for HYUty plus D(b). MHC-I-deficient male spleen cells provided a source of male Ag that could not directly stimulate the MHH cells. Addition of male but not female MHC-I-deficient spleen cells to wild-type syngeneic female EC induced MHH stimulation, demonstrating EC cross-presentation. Lactacystin treatment of the donor male MHC-I-deficient spleen cells, to inhibit proteasome function, markedly enhanced EC cross-presentation showing that the process is most efficient for intact proteins rather than degraded peptide fragments. Additional experiments revealed that this EC Ag-processing pathway is both proteasome and TAP1 dependent. These studies demonstrate that cultured murine aortic ECs can process and present MHC-I-restricted Ag derived from a separate, live cell, and they offer insight into the molecular requirements involved in this EC Ag presentation process. Through this pathway, ECs expressing cross-presented peptides can participate in the effector phase of T cell-mediated inflammatory responses such as autoimmunity, anti-tumor immunity, and transplant rejection.

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.

Natural killer cell education in mice with single or multiple major histocompatibility complex class I molecules

The ability of murine NK cells to reject cells lacking self MHC class I expression results from an in vivo education process. To study the impact of individual MHC class I alleles on this process, we generated mice expressing single MHC class I alleles (K^b, D^b, D^d, or L^d) or combinations of two or more alleles. All single MHC class I mice rejected MHC class I–deficient cells in an NK cell–dependent way. Expression of K^b or D^d conveyed strong rejection of MHC class I–deficient cells, whereas the expression of D^b or L^d resulted in weaker responses. The educating impact of weak ligands (D^b and L^d) was further attenuated by the introduction of additional MHC class I alleles, whereas strong ligands (K^b and D^d) maintained their educating impact under such conditions. An analysis of activating and inhibitory receptors in single MHC class I mice suggested that the educating impact of a given MHC class I molecule was controlled both by the number of NK cells affected and by the strength of each MHC class I–Ly49 receptor interaction, indicating that NK cell education may be regulated by a combination of qualitative and quantitative events.

Homing phenotypes of tumor-specific CD8 T cells are predetermined at the tumor site by crosspresenting APCs

Expression of tissue-specific homing molecules directs antigen-experienced T cells to particular peripheral tissues. In studies using soluble antigens that focused on skin and gut, antigen-presenting cells (APCs) within regional lymphoid tissues were proposed to be responsible for imprinting homing phenotypes. Whether this occurs in other sites and after physiologic antigen processing and presentation is unknown. We define in vivo imprinting of distinct homing phenotypes on monospecific T cells responding to antigens expressed by tumors in intracerebral, subcutaneous, and intraperitoneal sites with efficient brain-tropism of CD8 T cells crossprimed in the cervical lymph nodes (LNs). Multiple imprinting programs could occur simultaneously in the same LN when tumors were present in more than one site. Thus, the identity of the LN is not paramount in determining the homing phenotype; this critical functional parameter is dictated upstream at the site of antigen capture by crosspresenting APCs.

Strong memory CD8+ T cell responses against immunodominant and three new subdominant HLA-B27-restricted influenza A CTL epitopes following secondary infection of HLA-B27 transgenic mice

We previously showed that the known HLA-B27-restricted influenza A epitope identified from human studies, NP.383-391, was recognized by CTLs following influenza A infection of transgenic (Tg) HLA-B27/H2 class I-deficient (H2 DKO) mice. Here, we examined the kinetics of the primary NP.383-391-specific response in Tg HLA-B27/H2 DKO mice at the site of respiratory infection, along with the profile of additional influenza A epitopes recognized. While the temporal kinetics of the Tg HLA-B27/NP.383-391-specific CD8+ T cell response paralleled the H2-D(b)/NP.366-374-specific response of non-Tg H2b mice, the magnitude was less. Using epitope prediction programs, we identified three novel B27-restricted influenza A epitopes, PB2.702-710, PB1.571-579, and PB2.368-376, recognized during both the primary and secondary response to infection. Although the secondary NP.383-391-specific response was dominant, PB1.571-579 and PB2.368-376 stimulated stronger proliferative expansion in memory T cells. Our results indicate a broader B27/influenza A CTL repertoire than previously known. Together with results for other HLA class I alleles, this information will become important in improving vaccine strategies for influenza A and other human pathogens.

Direct effects of T-bet and MHC class I expression, but not STAT1, on peripheral NK cell maturation

The homeostatic maturation of NK cells is severely impaired in mice lacking the transcription factor T-bet, and the expression of the NK cell maturation marker killer cell lectin-like receptor G1 (KLRG1) has been shown to be dependent on MHC class I molecules. Interferon (IFN)-gamma signaling via the signal transducer and activator of transcription (STAT)1 is vital for T-bet and MHC class I induction. Here we investigated the relationship between STAT1, T-bet, and MHC class I molecules with regard to the phenotypic maturation of peripheral NK cells. We demonstrate that, to varying degrees, the maturation status of peripheral NK cells is impaired in naive mice with deficiencies in STAT1, T-bet, or MHC class I molecules. We find that in naive animals, the expression of wild-type levels of MHC class I molecules in trans is sufficient to restore the maturation profiles of STAT1(-/-) NK cells in vivo. In contrast, expression of T-bet is required in cis for normal NK cell maturation to occur. Additionally, we demonstrate that the activation-induced maturation of NK cells during the course of murine cytomegalovirus (MCMV) infection does not require expression of MHC class I molecules or STAT1 but is severely delayed in the absence of T-bet.

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

Naive T cells proliferate in response to self MHC molecules after transfer into lymphopenic hosts, a process that has been termed homeostatic proliferation (HP). Previous studies have demonstrated that HP is driven by low level signaling induced by interactions with the same MHC molecules responsible for positive selection in the thymus. Little is known about the homeostatic regulation of T cells specific for class Ib molecules, including Qa-1 and H2-M3, though it has been suggested that their capacity to undergo homeostatic expansion may be inherently limited. In this study, we demonstrate that naive 6C5 TCR transgenic T cells with specificity for Qa-1(b) have a capacity similar to conventional T cells to undergo HP after transfer into sublethally irradiated mice. Proliferation was largely dependent on the expression of beta(2)-microglobulin, and experiments with congenic recipients expressing Qa-1(a) instead of Qa-1(b) demonstrated that HP is specifically driven by Qa-1(b) and not through cross-recognition of classical class I molecules. Thus, the same MHC molecule that mediates positive selection of 6C5 T cells is also required for HP. Homeostatic expansion, like positive selection, occurs in the absence of a Qa-1 determinant modifier, the dominant self-peptide bound to Qa-1 molecules. However, experiments with TAP(-/-) recipients demonstrate a clear distinction between the ligand requirements for thymic selection and HP. Positive selection of 6C5 T cells is dependent on TAP function, thus selection is presumably mediated by TAP-dependent peptides. By contrast, HP occurs in TAP(-/-) recipients, providing an example where the ligand requirements for HP are less stringent than for thymic selection.

Infection-induced expansion of a MHC Class Ib-dependent intestinal intraepithelial gammadelta T cell subset

Salmonella species invade the host via the intestinal epithelium. Hence, intestinal intraepithelial lymphocytes (iIELs) are potentially the first element of the immune system to encounter Salmonella during infection. In this study, we demonstrate, in a mouse model, the expansion of a CD8alphabeta(+)CD94(-)TCRgammadelta(+) T cell subset within the iIEL population in response to oral infection with virulent or avirulent Salmonella. This population can be detected 3 days following infection, represents up to 15% of the TCRgammadelta(+) iIELs, and is dependent on the MHC class Ib molecule T23 (Qa-1). Qa-1 is expressed by intestinal epithelial cells and thus accessible for iIEL recognition. Such cells may play a role in the early immune response to Salmonella.

CD8 T cells can reject major histocompatibility complex class I-deficient skin allografts

Following transplantation, recipient T cells can recognize and respond to donor antigens expressed directly on donor cells, and can respond to donor-derived peptides that have been processed and presented in the context of recipient MHC through the indirect pathway. Indirectly primed CD4(+) T cells have been well studied in transplantation, but little information is available regarding whether indirectly primed CD8(+) T cells participate in rejection. To address this, we placed MHC class I-deficient D(b)K(b) knockout skin grafts onto allogeneic H-2 (k) SCID recipients followed by adoptive transfer of purified H-2 (k) CD8(+) T cells. The MHC class I-deficient grafts were rejected and only CD8(+) T cells were detectable in the recipient lymphoid organs and in the skin grafts. Immunohistochemical analysis showed that CD8(+) T cells were found in close proximity to vascular endothelial cells and to recipient infiltrating macrophages, suggesting specific interactions. The data demonstrate that cross-primed polyclonal CD8(+) T cells can function as active participants in the effector phase of rejection. The findings confirm and extend previous studies using a monoclonal TCR transgenic T cell and shed light on mechanisms of acute and chronic graft injury that are potentially relevant to human transplant recipients.

Low Number of H-2Dd-Negative Haematopoietic Cells in Mixed Bone Marrow Chimeras Convey In Vivo Tolerance to H-2Dd-Negative Cells But Fail to Prevent Resistance to H-2Dd-Negative Leukaemia

Natural killer (NK) cells kill cells lacking self major histocompatibility complex (MHC) class I. This missing self reactivity is beneficial in haploidentical bone marrow transplantations to cure leukaemia, in which donor-derived NK cells reject MHC disparate leukaemia cells and prevent relapse. To understand the role of NK cells in transplantation, we have studied NK cell tolerance in mice receiving mixed bone marrow transplants with limiting number of the MHC disparate component. Using an MHC class I (Dd) transgenic mouse model, we generated bone marrow chimeras carrying mixtures of Dd-positive and -negative cells. NK reactivity against Dd-negative cells (missing self) was assayed by outgrowth of lymphoma cells, stability of the chimerism in vivo and killing of Concanavalin A blasts in vitro. Up to 20% Dd-negative haematopoietic cells reduced, but did not abrogate, rejection of Dd-negative tumours and killing of Dd-negative T-cell blasts. In contrast, the ratios between Dd-positive and -negative cells were stable in vivo, suggesting tolerance to normal cells. Our data suggest that NK cell tolerance to normal cells and tumours in mixed MHC environments is differentially regulated, tolerance to normal cells being more easily induced. These results are important in relation to the role of NK cells in antileukaemic reactions after bone marrow transplantation.

Thymic Selection and Peripheral Activation of CD8 T Cells by the Same Class I MHC/Peptide Complex

Thymic selection is controlled by the interaction between TCR and MHC/peptide. Strength and quality of the signal determine whether thymocytes are selected or deleted. The factors that contribute to this signal remain poorly defined. Here we show that fetal thymic organ cultures (FTOCs) derived from OT-I transgenic mice (the OT-I TCR is restricted by K(b)-SIINFEKL) on a K(b)D(b-/-) background support positive selection, but only when provided with soluble H-2K(b)-SIINFEKL complexes. Selection of CD8 T cells is independent of the valency of the ligand or its capability to coengage CD8 molecules. Both CD8alphaalpha and CD8alphabeta T cells are selected by H-2K(b)-SIINFEKL, but only CD8alphabeta cells are capable of releasing IFN-gamma in response to the same ligand. The alpha(4)beta(7) integrin is up-regulated on postselection thymocytes from FTOCs. After adoptive transfer, FTOC-derived OT-I CD8 T cells divide in response to the agonist peptide SIINFEKL. These results establish that CD8 T cells responsive to their nominal peptide-Ag can be generated in FTOC supplemented with soluble MHC class I molecules equipped with the same peptide.

MHC-dependent and -independent modulation of endogenous Ly49 receptors on NK1.1+ T lymphocytes directed by T-cell receptor type

Natural killer (NK) T lymphocytes are thought to act as regulatory cells directing early events during immune responses. Murine NKT cells express inhibitory receptors of the Ly49 family. These receptors have a well-established and crucial role in modulating NK cell activities, but their physiological role in regulating NKT cells is not well understood, nor is the influence of major histocompatibility (MHC) ligands on endogenous Ly49 expression. We have further investigated how the expression of inhibitory NK receptors is regulated on NKT cells, and demonstrate a non-random expression of ligated Ly49 molecules on CD1d-restricted NKT cells. The nature of the T-cell receptor on the NKT cell crucially determines the profile of expressed Ly49 isoforms. Further, we show that MHC class I ligands efficiently modulate the expression levels of the inhibitory receptors, and the frequencies of cells positive for the Ly49 members. In addition, we find a several-fold increase in Ly49C/I-expressing NKT cells in adult thymus, apparently independent of MHC class I molecules. Abundant expression of Ly49 receptors on NKT cells, and the striking differences found in Ly49 isoform patterns on NKT-cell subsets differing in T-cell receptor expression, suggest that the pattern of Ly49 expression is tuned to fit the T-cell receptor and to emphasize further a role for these receptors in NKT immunity.

Elimination in vivo of developing T cells by natural killer cells

Natural killer cells gauge the absence of self class I MHC on susceptible target cells by means of inhibitory receptors such as members of the Ly49 family. To initiate killing by natural killer cells, a lack of inhibitory signals must be accompanied by the presence of activating ligands on the target cell. Although natural killer cell–mediated rejection of class I MHC–deficient bone marrow (BM) grafts is a matter of record, little is known about the targeting in vivo of specific cellular subsets by natural killer cells. We show here that development of class I MHC–negative thymocytes is delayed as a result of natural killer cell toxicity after grafting of a class I MHC–positive host with class I MHC–negative BM. Double positive thymocytes that persist in the presence of natural killer cells display an unusual T cell receptor–deficient phenotype, yet nevertheless give rise to single positive thymocytes and yield mature class I MHC–deficient lymphocytes that accumulate in the class I MHC–positive host. The resulting class I MHC–deficient CD8 T cells are functional and upon activation remain susceptible to natural killer cell toxicity in vivo. Reconstitution of class I MHC–deficient BM precursors with H2-K^b by retroviral transduction fully restores normal thymic development.

Class Ia MHC-deficient BALB/c mice generate CD8+ T cell-mediated protective immunity against Listeria monocytogenes infection

CD8(+) T cells are required for protective immunity against intracellular pathogens such as Listeria monocytogenes. In this study, we used class Ia MHC-deficient mice, which have a severe reduction in circulating CD8(+) T cells, to determine the protective capacity of class Ib MHC-restricted T cells during L. monocytogenes infection. The K(b-/-)D(b-/-) mutation was backcrossed onto a C.B10 (BALB/c congenic at H-2 locus with C57BL/10) background, because BALB/c mice are more susceptible to Listeria infection than other commonly studied mouse strains such as C57BL/6. C.B10 K(b-/-)D(b-/-) mice immunized with a sublethal dose of L. monocytogenes were fully protected against a subsequent lethal infection. Adoptive transfer of Listeria-immune splenocyte subsets into naive K(b-/-)D(b-/-) mice indicated that CD8(+) T cells were the major component of this protective immune response. A CD8(+) T cell line isolated from the spleen of a Listeria-infected class Ia MHC-deficient mouse was shown to specifically recognize Listeria-infected cells in vitro, as determined by IFN-gamma secretion and cytotoxicity assays. Adoptive transfer of this T cell line alone resulted in significant protection against L. monocytogenes challenge. These results suggest that even a limited number of class Ib MHC-restricted T cells are sufficient to generate the rapid recall response required for protection against secondary infection with L. monocytogenes.

Memory phenotype of CD8+ T cells in MHC class Ia-deficient mice

B6.K(b-)D(b-) mice are devoid of class Ia but express normal levels of class Ib molecules. They have low levels of CD8 T cells in both the thymus as well as peripheral T cell compartments. Although the percentage of splenic CD8 alpha alpha T cells is increased in these animals, approximately 90% of CD8 T cells are CD8 alpha beta. In contrast to B6 animals, most of the CD8 T cells from these mice have a memory phenotype (CD44(high)CD122(high) CD62L(low)) including both CD8 alpha beta and CD8 alpha alpha subsets. In the thymus of B6.K(b-)D(b-) animals, there is a decrease in the percentage of SP CD8 T cells, although most are CD44(low), similar to that seen in B6 mice. The spleens from day 1-old B6 and B6.K(b-)D(b-) mice have a relatively high proportion of CD44(high)CD62L(low) CD8 T cells. However, by day 28 most CD8 T cells in B6 mice have a naive phenotype while in B6.K(b-)D(b-) mice the memory phenotype remains. Unlike CD44(high) cells that are found in B6 animals, most CD44(high) cells from B6.K(b-)D(b-) mice do not secrete IFN-gamma rapidly upon activation. The paucity of CD8 T cells in B6.K(b-)D(b-) mice might be due in part to their inability to undergo homeostatic expansion. Consistent with this, we found that CD8 T cells from these animals expand poorly in X-irradiated syngeneic hosts compared with B6 CD8 T cells that respond to class Ia Ags. We examined homeostatic expansion of B6 CD8 T cells in single as well as double class Ia knockout mice and were able to estimate the fraction of cells reactive against class Ia vs class Ib molecules.

A peptide from heat shock protein 60 is the dominant peptide bound to Qa-1 in the absence of the MHC class Ia leader sequence peptide Qdm

The MHC class Ib molecule Qa-1 binds specifically and predominantly to a single 9-aa peptide (AMAPRTLLL) derived from the leader sequence of many MHC class Ia proteins. This peptide is referred to as Qdm. In this study, we report the isolation and sequencing of a heat shock protein 60-derived peptide (GMKFDRGYI) from Qa-1. This peptide is the dominant peptide bound to Qa-1 in the absence of Qdm. A Qa-1-restricted CTL clone recognizes this heat shock protein 60 peptide, further verifying that it binds to Qa-1 and a peptide from the homologous Salmonella typhimurium protein GroEL (GMQFDRGYL). These observations have implications for how Qa-1 can influence NK cell and T cell effector function via the TCR and CD94/NKG2 family members, and how this effect can change under conditions that cause the peptides bound to Qa-1 to change.

Human MHC class I transgenic mice deficient for H2 class I expression facilitate identification and characterization of new HLA class I-restricted viral T cell epitopes

Although mice transgenic (Tg) for human MHC (HLA) class I alleles could provide an important model for characterizing HLA-restricted viral and tumor Ag CTL epitopes, the extent to which Tg mouse T cells become HLA restricted in the presence of endogenous H2 class I and recognize the same peptides as in HLA allele-matched humans is not clear. We previously described Tg mice carrying the HLA-B27, HLA-B7, or HLA-A2 alleles expressed as fully native (HLA(nat)) (with human beta(2)-microglobulin) and as hybrid human/mouse (HLA(hyb)) molecules on the H2(b) background. To eliminate the influence of H2(b) class I, each HLA Tg strain was bred with a H2-K(b)/H2-D(b)-double knockout (DKO) strain to generate mice in which the only classical class I expression was the human molecule. Expression of each HLA(hyb) molecule and HLA-B27(nat)/human beta(2)-microglobulin led to peripheral CD8(+) T cell levels comparable with that for mice expressing a single H2-K(b) or H2-D(b) gene. Influenza A infection of Tg HLA-B27(hyb)/DKO generated a strong CD8(+) T cell response directed at the same peptide (flu nucleoprotein NP383-391) recognized by CTLs from flu-infected B27(+) humans. As HLA-B7/flu epitopes were not known from human studies, we used flu-infected Tg HLA-B7(hyb)/DKO mice to examine the CTL response to candidate peptides identified based on the B7 binding motif. We have identified flu NP418-426 as a major HLA-B7-restricted flu CTL epitope. In summary, the HLA class I Tg/H2-K/H2-D DKO mouse model described in this study provides a sensitive and specific approach for identifying and characterizing HLA-restricted CTL epitopes for a variety of human disease-associated Ags.

Stem cell medicine encounters the immune system

Recent progress in deriving human embryonic stem (hES) cells and defining their capacity to differentiate has inspired hope that they could become a source of replacement cells for damaged or diseased tissues. We review the immunological barriers to transplanting hES cells and consider several potential solutions, including stem-cell banking, modification of the immunogenicity of donor cells and induction of tolerance to the graft. We evaluate the probable efficacy of these approaches with a view to facilitating the use of hES cells in clinical practice.

Class I negative CD8 T cells reveal the confounding role of peptide-transfer onto CD8 T cells stimulated with soluble H2-Kb molecules

Crosslinking of the T cell receptor has been proposed to be a prerequisite for T cell activation. Although the evidence supports this notion for CD4 T cells, the situation for CD8 T cells is less clear. Soluble class I monomers have been used to determine activation requirements in vitro with contradictory results. The possibility of transfer of peptide from soluble class I molecules onto class I molecules present on the surface of CD8 T cells, with ensuing presentation to other CD8 T cells, has been widely ignored. We show that monomeric and tetrameric class I molecules as well as free peptide can stimulate naive CD8 T cells in vitro. We generate and characterize CD8 T cells that express the OT-I T cell receptor (for K(b)/SIINFEKL) yet lack K(b) and D(b) molecules, and show that their activation requirements differ from their class I positive counterparts when stimulated with soluble K(b) molecules. By eliminating the confounding effect of peptide transfer, we unmask the true activation requirements for naive CD8 T cells and show that multivalent engagement of T cell receptors, as well as costimulation, is required for optimal stimulation.

Positive selection of MHC class Ib-restricted CD8(+) T cells on hematopoietic cells

Unlike conventional CD8(+) T cells, major histocompatibility complex (MHC) class Ib-restricted CD8(+) T cells show an activated phenotype in uninfected mice and respond rapidly to foreign invaders. The underlying factors that contribute to these differences are not well understood. We show here that the activated phenotype of MHC class Ib-restricted CD8(+) T cells was partially acquired as a result of interactions in the thymus and reflected an increased capacity to be selected via interactions with MHC molecules on hematopoietic cells. Using bone marrow-chimeric mice, we have shown that MHC class Ib-restricted, but not MHC class Ia-restricted, CD8(+) T cells specific for Listeria monocytogenes were efficiently selected when MHC class I was expressed only on hematopoietic cells. Thus, the distinct functional properties of MHC class Ib-restricted versus MHC class Ia-restricted CD8(+) T cells may result, at least in part, from the different ways in which they are positively selected in the thymus.

Positive selection of a Qa-1-restricted T cell receptor with specificity for insulin

The phenotype and development of T cells from transgenic mice expressing a T cell receptor with specificity for insulin presented by the MHC class Ib molecule Qa-1(b) was investigated. Peripheral T cells from the transgenic mice express CD8 and, after activation, kill Qa-1(b)-positive lymphoid target cells in the presence of soluble insulin. Thymic selection requires expression of Qa-1(b) but not the dominant Qa-1-associated peptide, Qdm. In contrast to conventional T cells, selection is at least as efficient when the selecting ligand is expressed only on hematopoietic lineage cells as compared to expression on epithelial cells in the thymus. Our findings suggest that there is a dedicated population of Qa-1-restricted T cells that are selected by interaction with Qa-1 and that the cellular requirements for selection may differ from conventional T cells.

The nonclassical major histocompatibility complex molecule Qa-2 protects tumor cells from NK cell- and lymphokine-activated killer cell-mediated cytolysis

The cytotoxic activity of NK cells is regulated by class I MHC proteins. Although much has been learned about NK recognition of class I autologous targets, the mechanisms of NK self-tolerance are poorly understood. To examine the role of a nonpolymorphic, ubiquitously expressed class Ib Ag, Q9, we expressed it on class I-deficient and NK-sensitive B78H1 melanoma. Presence of this Qa-2 family member on tumor cells partially protected targets from lysis by bulk lymphokine-activated killer (LAK) cells. H-2K(b)-expressing B78H1 targets also reduced LAK cell activity, while H-2D(b) offered no protection. Importantly, blocking with F(ab')(2) specific for Q9 or removal of this GPI-attached molecule by phospholipase C cleavage restored killing to the level of vector-transfected cells. Experiments with LAK cells derived from H2(b) SCID and B6 mice established that NK1.1(+)TCR(-) NK and NK1.1(+)TCR(+) LAK cells were the prevalent cytolytic populations inhibitable by Q9. Treatment of mice with poly(I:C) also resulted in generation of Q9-regulated splenic cytotoxicity. LAK cells from different mouse strains responded to Q9, suggesting that the protective effect of this molecule is not detectably influenced by Ly49 polymorphisms or the presence/absence of Q9 in NK-harboring hosts. We propose that Q9 expressed on melanoma cells serves as a ligand for yet unidentified NK inhibitory receptor(s) expressed on NK1.1(+) NK/T cells.

Dendritic cell maturation overrules H-2D-mediated natural killer T (NKT) cell inhibition: critical role for B7 in CD1d-dependent NKT cell interferon gamma production

Given the broad expression of H-2 class Ib molecules on hematopoietic cells, antigen presentation pathways among CD1d expressing cells might tightly regulate CD1d-restricted natural killer T (NKT) cells. Bone marrow-derived dendritic cells (BM-DCs) and not adherent splenocytes become capable of triggering NK1.1(+)/T cell receptor (TCR)(int) hepatic NKT cell activation when (a) immature BM-DCs lack H-2D(b)-/- molecules or (b) BM-DCs undergo a stress signal of activation. In such conditions, BM-DCs promote T helper type 1 predominant CD1d-restricted NKT cell stimulation. H-2 class Ia-mediated inhibition involves more the direct H-2D(b) presentation than the indirect Qa-1(b) pathway. Such inhibition can be overruled by B7/CD28 interactions and marginally by CD40/CD40L or interleukin 12. These data point to a unique regulatory role of DCs in NKT cell innate immune responses and suggest that H-2 class Ia and Ib pathways differentially control NKT cell recognition of DC antigens.

Murine female reproductive tract intraepithelial lymphocytes display selection characteristics distinct from both peripheral and other mucosal T cells

Despite immense effort, the development of vaccines effective at mucosal sites has proceeded at a faltering pace. Efforts concentrating on humoral immunity but neglecting cellular immunity may be misdirected by ignoring many viral mucosal pathogens. Improved understanding of the development and maintenance of lymphocytes populating the reproductive tract (rtIELs) may inform advances in vaccination strategies for sexually transmitted diseases. Recent studies highlight tissue-specific differences in the development of mucosal immunity and suggest that the local milieu may play a role in selection, maintenance and function of resident lymphocytes. Here, we describe MHC class I and thymus dependence of subpopulations of rtIELs. TCRalphabeta+ CD8alphabeta+ T cells in the periphery, intestine, and female reproductive tract are all developmentally dependent on classical class I MHC and the thymus. TCRalphabeta+ CD8alphaalpha+ are absent from the periphery and the rtIELs, but are present and classical MHC class I-independent, in the intestine. In contrast to intestinal TCRgammadelta+ cells, TCRgammadelta+ rtIELs are CD8 negative and thymus dependent. In contrast to peripheral TCRgammadelta+ cells, murine TCRgammadelta+ rtIELs express not a diverse array of Vdelta genes, but rather, a canonical Vdelta1. In summary, lymphocytes isolated from the murine female reproductive tract have characteristics distinct from both peripheral T cells and those found at other mucosal sites. Therefore, for the purpose of vaccination strategies, the female reproductive tract should be regarded neither as peripheral nor mucosal, but rather as a tissue with distinctive immunological characteristics.

Functional roles of TAP and tapasin in the assembly of M3-N-formylated peptide complexes

H2-M3 is a MHC class Ib molecule with a high propensity to bind N-formylated peptides. Due to the paucity of endogenous Ag, the majority of M3 is retained in the endoplasmic reticulum (ER). Upon addition of exogenous N-formylated peptides, M3 trafficks rapidly to the cell surface. To understand the mechanism underlying Ag presentation by M3, we examined the role of molecular chaperones in M3 assembly, particularly TAP and tapasin. M3-specific CTLs fail to recognize cells isolated from both TAP-deficient (TAP(o)) and tapasin-deficient mice, suggesting that TAP and tapasin are required for M3-restricted Ag presentation. Impaired M3 expression in TAP(o) mice is due to instability of the intracellular pool of M3. Addition of N-formylated peptides to TAP(o) cells stabilizes M3 in the ER and partially restores surface expression. Surprisingly, significant amounts of M3 are retained in the ER in tapasin-deficient mice, even in the presence of N-formylated peptides. Our results define the role of TAP and tapasin in the assembly of M3-peptide complexes. TAP is essential for stabilization of M3 in the ER, whereas tapasin is critical for loading of N-formylated peptides onto the intracellular pool of M3. However, neither TAP nor tapasin is required for ER retention of empty M3.

Induction of M3-restricted cytotoxic T lymphocyte responses by N-formylated peptides derived from Mycobacterium tuberculosis

Major histocompatibility complex (MHC) class I–restricted CD8⁺ T cells play a critical role in the protective immunity against Mycobacterium tuberculosis (Mtb). However, only a few Mtb peptides recognized by MHC class Ia–restricted CD8⁺ T cells have been identified. Information on epitopes recognized by class Ib–restricted T cells is even more limited. M3 is an MHC class Ib molecule that preferentially presents N-formylated peptides to CD8⁺ T cells. Because bacteria initiate protein synthesis with N-formyl methionine, the unique binding specificity of M3 makes it especially suitable for presenting these particular bacterial epitopes. We have scanned the full sequence of the Mtb genome for NH₂-terminal peptides that share features with other M3-binding peptides. Synthetic peptides corresponding to these sequences were tested for their ability to bind to M3 in an immunofluorescence-based peptide-binding assay. Four of the N-formylated Mtb peptides were able to elicit cytotoxic T lymphocytes (CTLs) from mice immunized with peptide-coated splenocytes. The Mtb peptide–specific, M3-restricted CTLs lysed the Mtb-infected macrophages effectively, suggesting that these N-formylated Mtb peptides are presented as the naturally processed epitopes by Mtb-infected cells. Furthermore, T cells from Mtb-infected lungs, spleen, and lymph nodes responded to N-formylated Mtb peptides in an M3-restricted manner. Taken together, our data suggest that M3-restricted T cells may participate in the immune response to Mtb.

Positive selection of an MHC class-I restricted TCR in the absence of classical MHC class I molecules

The H-2Ld alloreactive 2C T cell receptor (TCR) is commonly considered as being positively selected on the H-2Kb molecule. Surprisingly, 2C TCR+ CD8+ single-positive T cells emerge in massive numbers in fetal thymic organ culture originating from 2C transgenic, H-2KbD(b-/-) (2C+KbD(b-/-)) but not in fetal thymic organ culture from beta2-microglobulin(-/-) 2C transgenic animals. Mature CD8+ T cells are observed in newborn but not in adult 2C+KbD(b-/-) mice. These CD8+ T cells express the alpha4beta7 integrin, which allows them to populate the intestine, a pattern of migration visualized by intrathymic injection of FITC and subsequent accrual of FITC-labeled lymphocytes in the gut. We conclude that the 2C TCR is reactive not only with H-2Ld and H-2Kb, but also with nonclassical MHC class I products to enable positive selection of 2C+ T cells in the fetal and newborn thymus and to support their maintenance in the intestine.

MHC class Ia-restricted T cells partially account for beta2-microglobulin-dependent resistance to Mycobacterium tuberculosis

Recent studies have highlighted the heterogeneous nature of the CD8(+) T cell response during human Mycobacterium tuberculosis infection; MHC class Ia, MHC class Ib and CD1 have all been identified as significant restriction elements. Here we have attempted to define the role of MHC class Ia in resistance to M. tuberculosis infection in mice. The course of M. tuberculosis infection in mice deficient in a single MHC class Ia molecule, either H2-K(b) or H2-D(b), was essentially identical to that observed in wild-type mice. In contrast, mice fully deficient in MHC class Ia molecules (H2-K(b) / H2-D(b) double knockout mice) were substantially more susceptible to M. tuberculosis infection. However, the double knockout mice were not as susceptible as beta 2-microglobulin-deficient mice, which have a broader phenotypic deficit. Thus, antigen presentation via MHC class Ia is an important component in resistance to M. tuberculosis, but its absence only partially accounts for the increased susceptibility of beta 2-microglobulin-deficient mice.

Qa-2-dependent selection of CD8alpha/alpha T cell receptor alpha/beta(+) cells in murine intestinal intraepithelial lymphocytes

Murine intestinal intraepithelial lymphocytes (iIELs) are made up of a heterogeneous mix of T cells with unique phenotypes. Whereas CD8(+) T cells in peripheral lymphoid organs use CD8alpha/beta and are selected on MHC class Ia molecules, a majority of iIELs use CD8alpha/alpha. Here, we report that the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is independent of classical MHC class I molecules K(b) and D(b), as illustrated by their presence in K(b)/D(b) double-knockout mice and in mice lacking a nonclassical MHC class I molecule, CD1d. Most strikingly, their presence is decreased by approximately 70% in mice lacking transporter associated with antigen processing (TAP). The TAP-dependent nonclassical MHC class I molecule Qa-2 is strongly implicated in the presence of these cells, as inferred from the low numbers of CD8alpha/alpha TCR-alpha/beta T cells in mice deficient in Qa-2 genes. Second, a Qa-2-transgenic mouse made in a Qa-2(-) strain showed an increase in the numbers of CD8alpha/alpha cells among its iIELs. Thus, the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is mainly dependent on the nonclassical MHC class I molecule Qa-2.

Analysis of Qa-1(b) peptide binding specificity and the capacity of CD94/NKG2A to discriminate between Qa-1-peptide complexes

The major histocompatibility complex class Ib protein, Qa-1(b), serves as a ligand for murine CD94/NKG2A natural killer (NK) cell inhibitory receptors. The Qa-1(b) peptide-binding site is predominantly occupied by a single nonameric peptide, Qa-1 determinant modifier (Qdm), derived from the leader sequence of H-2D and L molecules. Five anchor residues were identified in this study by measuring the peptide-binding affinities of substituted Qdm peptides in experiments with purified recombinant Qa-1(b). A candidate peptide-binding motif was determined by sequence analysis of peptides eluted from Qa-1 that had been folded in the presence of random peptide libraries or pools of Qdm derivatives randomized at specific anchor positions. The results indicate that Qa-1(b) can bind a diverse repertoire of peptides but that Qdm has an optimal primary structure for binding Qa-1(b). Flow cytometry experiments with Qa-1(b) tetramers and NK target cell lysis assays demonstrated that CD94/NKG2A discriminates between Qa-1(b) complexes containing peptides with substitutions at nonanchor positions P4, P5, or P8. Our findings suggest that it may be difficult for viruses to generate decoy peptides that mimic Qdm and raise the possibility that competitive replacement of Qdm with other peptides may provide a novel mechanism for activation of NK cells.

Persistence of memory CD8 T cells in MHC class I-deficient mice

An understanding of how T cell memory is maintained is crucial for the rational design of vaccines. Memory T cells were shown to persist indefinitely in major histocompatibility complex (MHC) class I-deficient mice and retained the ability to make rapid cytokine responses upon reencounter with antigen. In addition, memory CD8 T cells, unlike naïve cells, divided without MHC-T cell receptor interactions. This "homeostatic" proliferation is likely to be important in maintaining memory T cell numbers in the periphery. Thus, after naïve CD8 T cells differentiate into memory cells, they evolve an MHC class I-independent "life-style" and do not require further stimulation with specific or cross-reactive antigen for their maintenance.

MHC class Ia molecules alone control NK-mediated bone marrow graft rejection

Mice with functionally deleted genes encoding MHC class I heavy (H-2K(b), H-2D(b)) and light (beta2-microglobulin) chains were used in bone marrow cell transfer experiments to study the role of class Ia and Ib molecules in NK cell function. Absence of H-2K(b) and absence of H-2D(b) on bone marrow cells resulted in complete and in almost complete NK-mediated rejection, respectively. Absence of either H-2 class Ib (at least when expressed in H-2 class Ia-deficient mice) or cell surface class Ia free heavy chains did not result in bone marrow rejection. Thus, in C57BL/6 adult mice, the inactivation of NK cells required for bone marrow cell engraftment relies entirely upon-H-2 class Ia molecules. These results imply the existence of an inhibitory receptor which recognizes either directly or indirectly H-2D(b) molecules and further suggest that in C57BL/6 mice the NK cells which do not express a H-2K(b) specific inhibitory receptor necessarily express an H-2D(b)-specific one.

Cutting Edge: TCRαβ+ CD8αα+ T Cells Are Found in Intestinal Intraepithelial Lymphocytes of Mice That Lack Classical MHC Class I Molecules

TCRαβ+ intestinal intraepithelial lymphocytes (IEL) can express either the typical CD8αβ heterodimer or an unusual CD8αα homodimer. Both types of CD8+ IEL require class I molecules for their differentiation, since they are absent in β2m−/− mice. To gain insight into the role of class I molecules in forming TCRαβ+ CD8+ IEL populations, we have analyzed the IEL in mice deficient for either TAP, β2m, CD1, or K and D. We find that K−/−D−/− mice have TCRαβ+ CD8αα+ IEL, although they are deficient for TCRαβ+ CD8αβ+ cells. This indicates that at least some TCRαβ+ CD8αα+ IEL require only nonclassical class I molecules for their development. Surprisingly, the TCRαβ+ CD8αα+ IEL are significantly increased in K−/−D−/− mice, suggesting a complex interaction between CD8+ IEL and class I molecules that might include direct or indirect negative regulation by K and D, as well as positive effects mediated by nonclassical class I molecules.