Tissue-specific and cell surface expression of human major histocompatibility complex class I heavy (HLA-B7) and light (beta 2-microglobulin) chain genes in transgenic mice

Beyond HLA-A*0201: new HLA-transgenic nonobese diabetic mouse models of type 1 diabetes identify the insulin C-peptide as a rich source of CD8+ T cell epitopes

Type 1 diabetes is an autoimmune disease characterized by T cell responses to β cell Ags, including insulin. Investigations employing the NOD mouse model of the disease have revealed an essential role for β cell-specific CD8(+) T cells in the pathogenic process. As CD8(+) T cells specific for β cell Ags are also present in patients, these reactivities have the potential to serve as therapeutic targets or markers for autoimmune activity. NOD mice transgenic for human class I MHC molecules have previously been employed to identify T cell epitopes having important relevance to the human disease. However, most studies have focused exclusively on HLA-A*0201. To broaden the reach of epitope-based monitoring and therapeutic strategies, we have looked beyond this allele and developed NOD mice expressing human β(2)-microglobulin and HLA-A*1101 or HLA-B*0702, which are representative members of the A3 and B7 HLA supertypes, respectively. We have used islet-infiltrating T cells spontaneously arising in these strains to identify β cell peptides recognized in the context of the transgenic HLA molecules. This work has identified the insulin C-peptide as an abundant source of CD8(+) T cell epitopes. Responses to these epitopes should be of considerable utility for immune monitoring, as they cannot reflect an immune reaction to exogenously administered insulin, which lacks the C-peptide. Because the peptides bound by one supertype member were found to bind certain other members also, the epitopes identified in this study have the potential to result in therapeutic and monitoring tools applicable to large numbers of patients and at-risk individuals.

Xenogeneic beta 2-microglobulin substitution affects functional binding of MHC class I molecules by CD8+ T cells

NK cells and CD8+ T cells bind MHC-I molecules using distinct topological interactions. Specifically, murine NK inhibitory receptors bind MHC-I molecules at both the MHC-I H chain regions and beta2-microglobulin (beta2m) while TCR engages MHC-I molecules at a region defined solely by the class I H chain and bound peptide. As such, alterations in beta2m are not predicted to influence functional recognition of MHC-I by TCR. We have tested this hypothesis by assessing the capability of xenogeneic beta2m to modify the interaction between TCR and MHC-I. Using a human beta2m-transgenic C57BL/6 mouse model, we show that human beta2m supports formation and expression of H-2K(b) and peptide:H-2K(b) complexes at levels nearly equivalent to those in wild-type mice. Despite this finding, the frequencies of CD8+ single-positive thymocytes in the thymus and mature CD8+ T cells in the periphery were significantly reduced and the TCR Vbeta repertoire of peripheral CD8+ T cells was skewed in the human beta2m-transgenic mice. Furthermore, the ability of mouse beta2m-restricted CTL to functionally recognize human beta2m+ target cells was diminished compared with their ability to recognize mouse beta2m+ target cells. Finally, we provide evidence that this effect is achieved through subtle conformational changes occurring in the distal, peptide-binding region of the MHC-I molecule. Our results indicate that alterations in beta2m influence the ability of TCR to engage MHC-I during normal T cell physiology.

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.

Streptozotocin-induced diabetic nephropathy in rats: the role of inflammatory cytokines

The role of inflammatory cytokines in the pathogenesis of diabetic nephropathy has been studied in streptozotocin-induced diabetic rats. Rat kidneys were examined by light and electron microscopy and kidney homogenates were also analyzed by Western blot and flow cytometry for the expression of markers of inflammation namely, CD4+ and CD8+ T cells, macrophages, MHC classes I and II, the proinflammatory cytokines tumor necrosis factor-alpha, interferon-gamma and nitric oxide (NO). Light and electron microscope examination revealed infiltration of mononuclear cells throughout the renal parenchyma, with the glomeruli being more severely affected especially at 8 months after disease induction. Western blot and flow cytometric analyses revealed the infiltrating cells to be CD4+ T cells, CD8+ T cells and macrophages. Western blot analyses also revealed increased expression of the proinflammatory and Th1 cytokines tumor necrosis factor-alpha, interferon-gamma as well as nitric oxide. Using flow cytometry, we have shown that the difference in expression of CD4+ T cells in control and diabetic kidneys is more significant at 1 month than at 8 months, while expression of CD8+ T cells is more significant at 8 months. We speculate therefore that diabetic nephropathy is probably initiated and driven by a Th1 process. CD8+ T cells, however, become more significant at later stages of the disease when tissue loss is evident. Since NO induction also occurs only after 8 months, we hypothesize that NO might be significant for the later stages of the disease. Our data implicate inflammation in the pathogenesis of diabetic nephropathy in view of the overexpression of the proinflammatory cytokines TNF-alpha and IFN-gamma and the cells that secrete them in the early and late phases of the disease.

Influence of xenogeneic beta2-microglobulin on functional recognition of H-2Kb by the NK cell inhibitory receptor Ly49C

NK cells maintain self-tolerance through expression of inhibitory receptors that bind MHC class I (MHC-I) molecules. MHC-I can exist on the cell surface in several different forms, including "peptide-receptive" or PR-MHC-I that can bind exogenous peptide. PR-MHC-I molecules are short lived and, for H-2K(b), comprise approximately 10% of total MHC-I. In the present study, we confirm that signaling through the mouse NK inhibitory receptor Ly49C requires the presence of PR-K(b) and that this signaling is prevented when PR-K(b) is ablated by pulsing with a peptide that can bind to it with high affinity. Although crystallographic data indicate that Ly49C can engage H-2K(b) loaded with high-affinity peptide, our data suggest that this interaction does not generate an inhibitory signal. We also show that no signaling occurs when the PR-K(b) complex has mouse beta(2)-microglobulin (beta(2)m) replaced with human beta(2)m, although replacement with bovine beta(2)m has no effect. Furthermore, we show that beta(2)m exchange occurs preferentially in the PR-K(b) component of total H-2K(b). These conclusions were reached in studies modulating the sensitivity to lysis of both NK-resistant syngeneic lymphoblasts and NK-sensitive RMA-S tumor cells. We also show, using an in vivo model of lymphocyte recirculation, that engrafted lymphocytes are unable to survive NK attack when otherwise syngeneic lymphocytes express human beta(2)m. These findings suggest a qualitative extension of the "missing self" hypothesis to include NK inhibitory receptors that are restricted to the recognition of unstable forms of MHC-I, thus enabling NK cells to respond more quickly to events that decrease MHC-I synthesis.

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.

Expression of nonclassical MHC class Ib genes: comparison of regulatory elements

Peptide binding proteins of the major histocompatibility complex consist of the "classical" class Ia and "nonclassical" class Ib genes. The gene organization and structure/function relationship of the various exons comprising class I proteins are very similar among the class Ia and class Ib genes. Although the tissue-specific patterns of expression of these two gene families are overlapping, many class Ib genes are distinguished by relative low abundance and/or limited tissue distribution. Further, many of the class Ib genes serve specialized roles in immune responses. Given that the coding sequences of the class Ia and class Ib genes are highly homologous we sought to examine the promoter regions of the various class Ib genes by comparison to the well characterized promoter elements regulating expression of the class Ia genes. This analysis revealed a surprising complexity of promoter structures among all class I genes and few instances of conservation of class Ia promoter regulatory elements among the class Ib genes.

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.

CD8+ T cells are necessary for recognition of allelic, but not locus-mismatched or xeno-, HLA class I transplantation antigens

Although HLA transgenic mice (HLA TgM) could provide a powerful approach to investigate human MHC-specific T cell responsiveness, the extent to which these molecules are recognized by the mouse immune system remains unclear. We established TgM expressing HLA class I alleles A2, B7, or B27 in their fully native form (HLAnat) or as hybrid molecules (HLAhyb) of the HLA alpha1/alpha2 domains linked to the H-2Kb alpha3, transmembrane, and cytoplasmic domains (i.e., to maintain possible species-specific interactions). Comparison of each as xeno- (i.e., by non-TgM) vs allo- (i.e., by TgM carrying an alternate HLA allele) transplantation Ags revealed the following: 1) Although HLAhyb molecules induced stronger xeno-CD8+ T cell responses in vitro, additional effector mechanisms must be active in vivo because HLAnat skin grafts were rejected faster by non-TgM; 2) gene knockout recipients showed that xenorejection of HLAnat and, unexpectedly, HLAhyb grafts doesn't depend on CD8+ or CD4+ T cells or B cells; 3) each HLAhyb strain developed tolerance to "self" but rejected allele- (-B27 vs -B7) and locus- (-B vs -A) mismatched grafts, the former requiring CD8+ T cells, the latter by CD8+ T cell-independent mechanisms. The finding that recognition of xeno-HLAhyb does not require CD8+ T cells while recognition of the identical molecule in a strictly allo context does, demonstrates an alpha1/alpha2 domain-dependent difference in effector mechanism(s). Furthermore, the CD8+ T cell-independence of locus-mismatched rejection suggests the degree of similarity between self and non-self alpha1/alpha2 determines the effector mechanism(s) activated. The HLA Tg model provides a unique approach to characterize these mechanisms and develop tolerance protocols in the context of human transplantation Ags.

Expression of the Fc receptor in the mammary gland during lactation in the marsupial Trichosurus vulpecula (brushtail possum)

One of several functions described for the Fc receptor is regulation of IgG isotype transport into milk. The first marsupial homologues of the Fc receptor heavy and light chains, FcRn and beta-2 microglobulin, from the brushtail possum have been cloned and characterised. The level of FcRn mRNA in the possum mammary gland was highest at the start of lactation, and decreased slowly thereafter. Expression of FcRn mRNA did not increase during the switch phase when the concentration of IgG in milk is highest. In contrast, the level of beta-2 microglobulin mRNA in the mammary gland increased during the switch phase when milk IgG concentration also increases. This correlation between beta-2 microglobulin mRNA expression in the mammary gland with the time of active IgG-transfer into milk was also observed in the bovine and murine mammary gland. This suggests that expression of the Fc receptor in the mammary gland is controlled by the expression of beta-2 microglobulin and that its expression is upregulated during the period of highest IgG-transfer into milk.

Xenogeneic and allogeneic anti-MHC immune responses induced by plasmid DNA immunization

Major histocompatibility complex (MHC) proteins are known to be incorporated into the human immunodeficiency virus (HIV-1) envelope as the virion buds from the host cell surface. Studies using simian immunodeficiency virus (SIV) infection of macaques have demonstrated that immunization with uninfected human cells or purified HLA proteins can provide protection from challenge with live SIV when it is grown in human cells expressing the same MHC alleles. Thus the induction of anti-MHC immune responses represents an important option to consider with respect to vaccine design for SIV and HIV. Here we examine plasmid DNA immunization strategies as an alternative to cellular or protein immunogens for the induction of xenogeneic and allogeneic immune responses in C57BL/6 mice and in an HLA transgenic mouse model system, respectively. We compared the immunogenicity of HLA-A2- and HLA-B27-expressing splenocytes with the corresponding plasmid DNA immunogens. Results from the transgenic mouse experiments indicate that plasmid DNA immunization with both class I and class II MHC-encoding vectors can elicit antibody responses recognizing conformationally intact MHC molecules. Our data also show that immunization with class I MHC-encoding DNA immunogens can elicit cytotoxic T-lymphocyte responses, demonstrating the potential to mobilize both antibody and cell-mediated anti-MHC immune responses in the context of this approach to HIV-1 vaccine design.

The use of human leucocyte antigen class I transgenic mice to investigate human immune function

HLA class I transgenic mice are a powerful research tool which have been used as models for human immune responses. This review describes the generation of the different HLA class I transgenic mice, the techniques used to improve expression of the transgene and use of the transgene product in immune responses. It also illustrates how HLA class I transgenic mice have provided insights into the nature of the allogeneic and xenogeneic response, the generation of CTL responses, the development of autoimmune diseases, and their use for the generation of anti-HLA class I antibodies. Despite these advances, the use of available HLA class I transgenic mice as models for human disease and immune responses has been limited. The development of new transgenic strains incorporating multiple human transgenes may allow the potential of HLA class I transgenic mice to be realised.

Distinct Stage-Specific cis-Active Transcriptional Mechanisms Control Expression of T Cell Coreceptor CD8α at Double- and Single-Positive Stages of Thymic Development

Developing thymocytes that give rise to CD8+ (cytotoxic) and CD4+ (helper) αβ-TCR T lymphocytes go through progressive stages of expression of coreceptors CD8 and CD4 from being negative for both (the double-negative stage), to coexpressing both (the double-positive (DP) stage), to a mutually exclusive sublineage-specific expression of one or the other (the single-positive (SP) stage). To delineate the mechanisms underlying regulation of CD8 during these developmental transitions, we have examined expression of a series of mouse CD8α gene constructs in developing T cells of conventional and CD8α “knock-out” transgenic mice. Our results indicate that cis-active transcriptional control sequences essential for stage- and sublineage-specific expression lie within a 5′ 40-kb segment of the CD8 locus, ∼12 kb upstream of the CD8α gene. Studies to characterize and sublocalize these cis sequences showed that a 17-kb 5′ subfragment is able to direct expression of the CD8α gene up to the CD3intermediate DP stage but not in more mature DP or SP cells. These results indicate that stage-specific expression of CD8α in developing T cells is mediated by the differential activity of multiple functionally distinct cis-active transcriptional control mechanisms. It will be important to determine the relationship of “switching” between these cis mechanisms and selection.

NK Cells from Human MHC Class I (HLA-B) Transgenic Mice Do Not Mediate Hybrid Resistance Killing Against Parental Nontransgenic cells

We have investigated the capacity of human MHC class I HLA-B gene products, HLA-B27, -B7 (fully human), and -B7Kb (human-mouse hybrid consisting of the α1 and α2 domains of HLA-B7, and the α3 and cytoplasmic domains of mouse H-2Kb), expressed on mouse NK cells during ontogeny to influence NK recognition of otherwise syngeneic mouse target cells. Despite a high level of surface expression of the transgene (comparable to that of endogeneous H-2DbKb molecules), the direct killing of YAC-1 targets, and the killing of P815 targets in a redirected lysis assay, the NK effectors of these transgenic mice could not mediate hybrid resistance-like killing of nontransgenic C57BL/6 target cells either in vitro or in vivo. Splenocytes from B6-B27 mice could be used to generate CTL lines against a B27-binding peptide, implying that T cells restricted by HLA-B27 developed during ontogeny. NK cells from B6-B27 could lyse B6-B27 Con A lymphoblasts pulsed with Db-binding peptide but not B27-binding peptides. Taken together, our results show that these human HLA-B transgene products cannot function as class I MHC “self” elements for mouse NK cells, even when present throughout ontogeny.

Finally! The Brambell receptor (FcRB). Mediator of transmission of immunity and protection from catabolism for IgG

F. W. Rogers Brambell was the father of the field of transmission of immunity, which he entered 50 years before the present era. As part of his quantitative and temporal studies on transmission, he defined the first Fc receptor system for IgG, and furthermore recognized the link between transmission of passive immunity from mother to young and protection from catabolism for IgG. This article provides a historical overview of the efforts of Professor Brambell and summarizes the subsequent elaboration of the details of the physiology and molecular biology of this remarkable receptor system.

The protection receptor for IgG catabolism is the beta2-microglobulin-containing neonatal intestinal transport receptor

More than 30 years ago, Brambell published the hypothesis bearing his name [Brambell, F. W. R., Hemmings, W. A. & Morris, 1. C. (1964) Nature (London) 203, 1352-1355] that remains as the cornerstone for thinking on IgG catabolism. To explain the long survival of IgG relative to other plasma proteins and its pattern of increased fractional catabolism with high concentrations of IgG, Brambell postulated specific IgG "protection receptors" (FcRp) that would bind IgG in pinocytic vacuoles and redirect its transport to the circulation; when the FcRp was saturated, the excess unbound IgG then would pass to unrestricted lysosomal catabolism. Brambell subsequently postulated the neonatal gut transport receptor (FcRn) and showed its similar saturable character. FcRn was recently cloned but FcRp has not been identified. Using a genetic knockout that disrupts the FcRn and intestinal IgG transport, we show that this lesion also disrupts the IgG protection receptor, supporting the identity of these two receptors. IgG catabolism was 10-fold faster and IgG levels were correspondingly lower in mutant than in wild-type mice, whereas IgA was the same between groups, demonstrating the specific effects on the IgG system. Disruption of the FcRp in the mutant mice was also shown to abrogate the classical pattern of decreased IgG survival with higher IgC concentration. Finally, studies in normal mice with monomeric antigen-antibody complexes showed differential catabolism in which antigen dissociates in the endosome and passes to the lysosome, whereas the associated antibody is returned to circulation; in mutant mice, differential catabolism was lost and the whole complex cleared at the same accelerated rate as albumin, showing the central role of the FcRp to the differential catabolism mechanism. Thus, the same receptor protein that mediates the function of the FcRn transiently in the neonate is shown to have its functionally dominant expression as the FcRp throughout life, resolving a longstanding mystery of the identity of the receptor for the protection of IgG. This result also identifies an important new member of the class of recycling surface receptors and enables the design of protein adaptations to exploit this mechanism to improve survivals of other therapeutic proteins in vivo.

Regulation of meiotic chromatin loop size by chromosomal position

At meiotic prophase, chromatin loops around a proteinaceous core, with the sizes of these loops varying between species. Comparison of the morphology of sequence-related inserts at different sites in transgenic mice demonstrates that loop size also varies with chromosomal geography. Similarly, chromatin loop lengths differ dramatically for interstitially and terminally located hamster telomeric sequences. Sequences, telomeric or otherwise, located at chromosome termini, closely associate with the meiotic proteinaceous core, forming shorter loops than identical interstitial sequences. Thus, we present evidence that different chromatin packaging mechanisms exist for interstitial versus terminal chromosomal regions, which act separately from those operating at the level of the DNA sequence. Chromosomal position plays the dominant role in chromatin packaging.

Human CD8 transgene regulation of HLA recognition by murine T cells

A series of human CD8 transgenic (hCD8 Tg) mice with differential expression in the thymus and periphery were produced to investigate CD8 coreceptor regulation of repertoire selection and T cell responses. Expression of hCD8 markedly enhanced responses to both HLA class I molecules and hybrid A2/Kb molecules providing functional evidence for a second interaction site, outside of the alpha 3 domain, which is essential for optimal coreceptor function. Peripheral T cell expression of hCD8 was sufficient to augment responsiveness to HLA class I, as hCD8 Tg mice which lacked thymic expression responded as well as mice expressing hCD8 in the thymus and periphery. Both murine CD8+ and CD4+ T cells expressing hCD8 transgenes exhibited markedly enhanced responses to foreign HLA class I, revealing the ability of T cell receptor repertoires selected on either murine class I or class II to recognize human class I major histocompatibility complex (MHC). In contrast to recognition of foreign class I, thymic expression of hCD8 transgenes was absolutely required to enhance recognition of antigenic peptide restricted by self-HLA class I. Thus, our studies revealed disparate requirements for CD8 coreceptor expression in the thymus for selection of a T cell repertoire responsive to foreign MHC and to antigenic peptides bound to self-MHC, providing a novel demonstration of positive selection that is dependent on human CD8.

A regulatory element in the beta 2-microglobulin promoter identified by in vivo footprinting

Expression of the beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I genes is coordinately regulated. By ligation-mediated polymerase chain reaction, we have analyzed in vivo factor binding to the promoter region of the murine beta 2-m gene. In adult spleen, in which beta 2-m is expressed, strong protection was found in three elements. Two of these elements, the beta 2-m NF-kappa B binding site and the interferon consensus sequence, are homologous to the regulatory elements of the MHC class I genes and were also found to be protected in spleen. A third protected element, PAM, identified in this work, is unique to the beta 2-m gene. None of the elements showed protection in brain tissue, in which neither the beta 2-m nor the MHC class I gene is expressed. In vivo footprinting was also performed with F9 embryonal carcinoma cells, in which expression of the beta 2-m and MHC class I genes is induced at a low level only upon stimulation with retinoic acid (RA). No in vivo protection was detected before and after RA treatment of F9 cells, indicating that RA induction of beta 2-m (and MHC class I) expression occurs without detectable in vivo factor occupancy, whereas EL4 T lymphocytes expressing beta 2-m at a high level exhibited strong protection similar to that in spleen. Despite the lack of in vivo occupancy, the nuclear factors specific for each of the three elements were present in brain tissue and F9 cells as well as in spleen tissue and EL4 cells. We show that PAM, an element identified by its in vivo protection, binds nuclear factors ranging from 40 to 50 kDa in size and is capable of enhancing transcription of a reporter in F9 and other cells. Taken together, these results indicate that in vivo factor occupancy for the beta 2-m and MHC class I promoters is coordinated and occurs through a mechanism other than mere expression of relevant factors.

A regulatory element in the beta 2-microglobulin promoter identified by in vivo footprinting

Expression of the beta 2-microglobulin (beta 2-m) and major histocompatibility complex (MHC) class I genes is coordinately regulated. By ligation-mediated polymerase chain reaction, we have analyzed in vivo factor binding to the promoter region of the murine beta 2-m gene. In adult spleen, in which beta 2-m is expressed, strong protection was found in three elements. Two of these elements, the beta 2-m NF-kappa B binding site and the interferon consensus sequence, are homologous to the regulatory elements of the MHC class I genes and were also found to be protected in spleen. A third protected element, PAM, identified in this work, is unique to the beta 2-m gene. None of the elements showed protection in brain tissue, in which neither the beta 2-m nor the MHC class I gene is expressed. In vivo footprinting was also performed with F9 embryonal carcinoma cells, in which expression of the beta 2-m and MHC class I genes is induced at a low level only upon stimulation with retinoic acid (RA). No in vivo protection was detected before and after RA treatment of F9 cells, indicating that RA induction of beta 2-m (and MHC class I) expression occurs without detectable in vivo factor occupancy, whereas EL4 T lymphocytes expressing beta 2-m at a high level exhibited strong protection similar to that in spleen. Despite the lack of in vivo occupancy, the nuclear factors specific for each of the three elements were present in brain tissue and F9 cells as well as in spleen tissue and EL4 cells. We show that PAM, an element identified by its in vivo protection, binds nuclear factors ranging from 40 to 50 kDa in size and is capable of enhancing transcription of a reporter in F9 and other cells. Taken together, these results indicate that in vivo factor occupancy for the beta 2-m and MHC class I promoters is coordinated and occurs through a mechanism other than mere expression of relevant factors.

Far upstream regions of class II MHC Ea are necessary for position-independent, copy-dependent expression of Ea transgene

The chromatin upstream of the class II MHC Ea gene contains specific, DNase I hypersensitive (DH) sites (groups I-V), overlapping and extending the promoter proximal and distal control regions. To determine whether the Ea DH groups I-V define a functionally important chromatin domain or locus control region (LCR), we have used wild type Ead gene constructs to generate transgenic mouse lines from strains that do not express an endogenous Ea gene product. Constructs contained either DH groups I-V 'Longs' or DH groups I-II 'Shorts', of the hypersensitive sites defined within 20 kb 5' of Ea. We show that position-independent, copy number-dependent expression of the Ead gene occurs only with the Long construct (8/8 transgenic mouse lines, over a range of copy numbers, 1-30 copies); in contrast, the Short constructs are subject to position-dependent effects. This suggests that the region delineated by Ea DH groups I-II is necessary but not sufficient as an LCR, which requires the presence of the upstream regions containing DH III-V for complete position-independent, copy number-dependent expression. These results introduce an immunologically-important, putative LCR which can be used to target genes to cells of the B cell lineage, as well as to other class II MHC expressing cells, and highlight the importance of chromatin structure analysis as a means to locate DNA regions of regulatory interest which are dispersed over a large distance.

Occupancy of upstream regulatory sites in vivo coincides with major histocompatibility complex class I gene expression in mouse tissues

The major histocompatibility complex (MHC) class I HLA-B7 transgene carrying a 660-bp upstream sequence is expressed in the mouse with tissue specificity that parallels that of the expression of endogenous mouse MHC class I (H-2) genes. We have performed in vivo genomic footprinting for the HLA-B7 transgene and the endogenous H-2Kb gene. We show that the upstream region of both the transgene and the endogenous gene was extensively occupied in spleen tissue, where these genes are expressed at high levels. In contrast, no occupancy was detected in brain tissue, where expression of these genes is virtually absent. Sites exhibiting in vivo protection correspond to cis elements previously shown to bind to nuclear factors in vitro, including the constitutive enhancer region I and the interferon response element. The strongest tissue-specific protection was detected at site alpha, located downstream from the interferon response element. Site alpha bound a constitutively expressed nuclear factor(s) in vitro that exhibited an overlapping specificity which may involve a nuclear hormone receptor, RXR, and an AP-1-related factor. Site alpha was functional in vivo, as it enhanced MHC class I transcription in lymphocytes. These results show that the tissue-specific occupancy of the MHC class I regulatory sequences in vivo correlates with their expression and suggest that in vivo occupancy is controlled by a mechanism other than the mere presence of factors capable of binding to these sites. Our results suggest that a sequence present in the 660-bp upstream region in a human leukocyte antigen gene directs tissue-specific occupancy of MHC class I genes in vivo, independently of their position and copy number, illustrating a potential advantage of using a transgene for delimitation of the sequence requirement for in vivo occupancy.

Occupancy of upstream regulatory sites in vivo coincides with major histocompatibility complex class I gene expression in mouse tissues

The major histocompatibility complex (MHC) class I HLA-B7 transgene carrying a 660-bp upstream sequence is expressed in the mouse with tissue specificity that parallels that of the expression of endogenous mouse MHC class I (H-2) genes. We have performed in vivo genomic footprinting for the HLA-B7 transgene and the endogenous H-2Kb gene. We show that the upstream region of both the transgene and the endogenous gene was extensively occupied in spleen tissue, where these genes are expressed at high levels. In contrast, no occupancy was detected in brain tissue, where expression of these genes is virtually absent. Sites exhibiting in vivo protection correspond to cis elements previously shown to bind to nuclear factors in vitro, including the constitutive enhancer region I and the interferon response element. The strongest tissue-specific protection was detected at site alpha, located downstream from the interferon response element. Site alpha bound a constitutively expressed nuclear factor(s) in vitro that exhibited an overlapping specificity which may involve a nuclear hormone receptor, RXR, and an AP-1-related factor. Site alpha was functional in vivo, as it enhanced MHC class I transcription in lymphocytes. These results show that the tissue-specific occupancy of the MHC class I regulatory sequences in vivo correlates with their expression and suggest that in vivo occupancy is controlled by a mechanism other than the mere presence of factors capable of binding to these sites. Our results suggest that a sequence present in the 660-bp upstream region in a human leukocyte antigen gene directs tissue-specific occupancy of MHC class I genes in vivo, independently of their position and copy number, illustrating a potential advantage of using a transgene for delimitation of the sequence requirement for in vivo occupancy.

Tissue-specific expression of the HLA-DRA gene in transgenic mice

Transgenic mice were produced containing a 33 kilobase (kb) DNA fragment encompassing the five exons and all the known regulatory regions of the class II HLA-DRA gene. The transgene displayed regulated expression [constitutive and interferon-gamma (IFN)-gamma induced] of the human products in most mouse tissues. The tissue distribution of the DRA transgene products more closely resembled that of their mouse homologues, the endogenous H-2 Ea products, than the wider distribution of DRA products in humans. This was evident in several tissues (endothelia of small vessels, especially those of glomerular capillaries, Kupffer cells, and epithelial cells lining the gastrointestinal tract), known to differentially express class II molecules in the two species. Thus, the wider human specific pattern of expression requires an exact cis/trans complementation which is incompletely reconstituted in transgenic mice, suggesting that human-specific cis-acting elements may have arisen during evolution to direct the expression of class II genes to those anatomical regions which usually lack them in the mouse. The only example of aberrant expression of the DRA gene in the present series of transgenic mice was in the dendritic and/or epithelial cells of the thymic cortex, which displayed greatly reduced DR alpha levels in spite of a normal expression of the endogenous E alpha molecules.

Identification of cis sequences controlling efficient position-independent tissue-specific expression of human major histocompatibility complex class I genes in transgenic mice

We previously reported that genomic major histocompatibility complex class I human leukocyte antigen (HLA)-B7 gene constructs with as little as 0.66 kb of 5'- and 2.0 kb of 3'-flanking DNA were expressed efficiently and appropriately in transgenic mice. To identify and characterize the relevant cis-acting regulatory elements in more detail, we have generated and analyzed a series of transgenic mice carrying native HLA-B7 genes with further 5' truncations or intronic deletions and hybrid constructs linking the 5'-flanking region of B7 to a reporter gene. We were unable to detect a specific requirement for sequence information within introns 2 to 7 for either appropriate constitutive or inducible class I expression in adult animals. The results revealed the presence of cis-acting regulatory sequences between -0.075 kb and -0.66 kb involved in driving efficient copy number-dependent constitutive and gamma interferon-enhanced tissue-specific expression. The region from -0.11 to -0.66 kb is also sufficient to prevent integration site-specific "position effects," because in its absence HLA-B7 expression is frequently detected at significant levels at inappropriate sites. Conserved sequence elements homologous to the H-2 class I regulatory element, or enhancer A, and the interferon response sequence are located between about -151 and -228 bp of the B7 gene. Our results also indicate the existence of sequences downstream of -0.11 kb which can influence the pattern of tissue-specific expression of the HLA-B7 gene and the ability of this gene to respond to gamma interferon.

Identification of cis sequences controlling efficient position-independent tissue-specific expression of human major histocompatibility complex class I genes in transgenic mice

We previously reported that genomic major histocompatibility complex class I human leukocyte antigen (HLA)-B7 gene constructs with as little as 0.66 kb of 5'- and 2.0 kb of 3'-flanking DNA were expressed efficiently and appropriately in transgenic mice. To identify and characterize the relevant cis-acting regulatory elements in more detail, we have generated and analyzed a series of transgenic mice carrying native HLA-B7 genes with further 5' truncations or intronic deletions and hybrid constructs linking the 5'-flanking region of B7 to a reporter gene. We were unable to detect a specific requirement for sequence information within introns 2 to 7 for either appropriate constitutive or inducible class I expression in adult animals. The results revealed the presence of cis-acting regulatory sequences between -0.075 kb and -0.66 kb involved in driving efficient copy number-dependent constitutive and gamma interferon-enhanced tissue-specific expression. The region from -0.11 to -0.66 kb is also sufficient to prevent integration site-specific "position effects," because in its absence HLA-B7 expression is frequently detected at significant levels at inappropriate sites. Conserved sequence elements homologous to the H-2 class I regulatory element, or enhancer A, and the interferon response sequence are located between about -151 and -228 bp of the B7 gene. Our results also indicate the existence of sequences downstream of -0.11 kb which can influence the pattern of tissue-specific expression of the HLA-B7 gene and the ability of this gene to respond to gamma interferon.

Expression of HLA-B27 in transgenic mice is dependent on the mouse H-2D genes

HLA-B27 transgenic mice in the context of various H-2 haplotypes were produced. A high expression of the HLA-B27 antigen was observed in mice homozygous for H-2b, H-2f, H-2s, H-2p, H-2r, and H-2k haplotypes. Mice of the H-2v haplotype expressed HLA-B27 at an intermediate level. Expression of HLA-B27 was minimal in mice of the H-2q and H-2d haplotypes. This was observed both on the B10 background and in DBA/2 or BALB/c mice. Only minimal expression of HLA-B27 could be detected in B10.PL (KuDd) or B10.RKDB (KkSkDdLb) mice, indicating that the low level of HLA-B27 expression maps to the H-2D gene or a very closely linked gene. Integration and transcription of the HLA-B27 gene does not appear to be different between high-expressing haplotypes and low-expressing haplotypes as determined by Southern and Northern blot analysis. However, expression of HLA-B27 on the cell surface correlated with the amount of HLA-B27 and beta 2M that could be immunoprecipitated with an anti-B27 antibody. Therefore, the association of the B27 heavy chain with endogenous beta 2M and subsequent expression on the cell surface are disrupted in mice with some class I H-2D genes. Possible mechanisms that might contribute to this defect in assembly, transport, and expression of class I molecules are discussed.

Immune deficiency due to high copy numbers of an Ak beta transgene

Because allelic polymorphism of the major histocompatibility complex class II antigens affects the immune response at several levels, we wished to characterize the contribution of a particular alpha or beta chain in vivo using transgenic mice. We have established and characterized 12 lines of H-2s/s mice carrying from 1 to 65 copies of an Ak beta transgene. The transgene was coexpressed with the endogenous allele in a tissue-specific manner, and Ak beta mRNA expression correlated well with transgene copy number. High copy number (extreme overexpression) of the transgene was associated with a variety of defects, including a significant reduction in Ia cell-surface expression, a severe decrease in B-cell number, abnormal extramedullary granulopoiesis, and an increased susceptibility to infection. In this paper we describe in detail the phenotype associated with high copy numbers of the Ak beta transgene. The defects we have observed may be relevant to similar phenomena seen in other transgenic mice. In addition, these mice have fortuitously provided a system in which to assess the effect of various levels of class II cell-surface expression in the thymus on selection of the T-cell repertoire.

Tumor necrosis factor and immune interferon synergistically increase transcription of HLA class I heavy- and light-chain genes in vascular endothelium

Tumor necrosis factor and immune interferon synergistically increase cell-surface expression of class I major histocompatibility complex molecules in cultured human endothelial cells. We report that tumor necrosis factor and interferon gamma each independently increase mRNA levels and together cause a greater-than-additive (i.e., synergistic) increase in steady-state mRNA levels and transcriptional rates of the class I heavy- and light-chain genes. HLA heavy-chain mRNA is equally stable in cytokine-treated and -untreated endothelial cells. Interferon gamma does not increase tumor necrosis factor receptor number or affinity on human endothelial cells. We conclude that the synergistic increase in class I major histocompatibility complex cell-surface expression results principally from the synergistic increase in transcriptional rates. We propose that this increase is caused by the cooperative binding of independently activated transcription factors to the promoter/enhancer sequences of class I genes.

Self-tolerance to HLA focuses the response of immunized HLA-transgenic mice on production of antibody to precise polymorphic HLA alloantigens

HLA class I-transgenic mice express HLA antigen on their tissues and establish self-tolerance to the expressed monomorphic and polymorphic determinants. When challenged with skin grafts and lymphoid cells of a second HLA-transgenic mouse expressing a different HLA molecule, a specific immune response is elicited that is focused on the determinants specified by the allelic HLA differences between donor and recipient transgenic mice. In the studies described, this has led to the production of a number of monoclonal antibodies with specificity for the HLA-Cw3 and HLA-B7 crossreactive groups of class I antigens. These results indicate that immunization of appropriate transgenic strains of mice with murine cells expressing a different HLA allelic transgene should permit the generation of monoclonal antibodies of narrow specificity against virtually any polymorphic epitope on HLA antigens.

Expression and function of HLA-A2.1 in transgenic mice

We have derived a number of transgenic mouse lines which express the human major histocompatibility complex class I gene HLA-A2.1. Two lines carry the complete human HLA-A2.1, the others bear a recombinant gene in which the HLA-A2.1 coding regions are fused to the H-2Kb promoter. Analysis of transgenic spleen cells by immunofluorescence demonstrates that these mouse cells express HLA-A2.1 on their surface in association with mouse beta 2-microglobulin (beta 2m), confirming that HLA-A2 does not require human beta 2m to be expressed at the cell surface. The cells contain more HLA mRNA than endogenous H-2 class I mRNA. There is also a large pool of non-beta 2m-associated HLA heavy chain inside the cell. In contrast the amount of HLA:beta 2m complex is low. Thus, in transgenic mice HLA-A2 seems to compete poorly with H-2 heavy chains for mouse beta 2m. The HLA-A2.1 transgenic mice do not produce influenza-virus-specific cytotoxic T cells (CTL) restricted to the HLA transgene, at least in sufficient numbers to be measured in a direct bulk CTL assay. The dominance of H-2-restricted clones may be the result of quantitative rather than qualitative factors. However, HLA-A2.1 transgenic spleen cells are effective in stimulating an allogeneic CTL response in normal mice. This response is not H-2 restricted. Cold target inhibition studies show that there are at least two populations of CTL, one of which is specific for HLA-A2.1 on mouse cells. This result suggests that at least some allo-CTL are directed against major histocompatibility complex plus "self-peptide".

Multiple enhancer-like sequences in the HLA-B7 gene

Enhancer-like sequences have previously been identified in the promoter region of the mouse major histocompatibility complex (MHC) class I genes. We have screened for such sequences in and around a human MHC class I gene, HLA-B7. Various restriction fragments of the B7 gene were assayed for their ability to enhance transcription of a bacterial chloramphenicol acetyltransferase gene from a simian virus 40 promoter in transiently transfected mouse LTA cells. Our results demonstrate that enhancer activity is located in introns 3 and 5 as well as 5' to the transcription initiation site. RNase protection experiments corroborate the results. Preliminary experiments indicate that B7 enhancers are active in various cell types. The role of these enhancers in B7 gene expression is not known at present. We speculate that the position of the enhancer elements may be related to the occurrence of Hpa II tiny fragment islands.

HLA expression and function in single and double HLA-B27-transgenic mice

The expression and function of HLA antigens in mice single transgenic for HLA-B27.2 (sTGM-B27.2) or double transgenic (dTGM) for HLA-B27.2 and human beta 2-microglobulin (h beta 2m) were compared. B27.2 could be well detected on the cell membrane of lymphocytes of sTGM. However, the expression in sTGM was much lower than in dTGM mice. Nevertheless, also in sTGM mice, the B27-transgene product possessed all functional properties of a class I HLA molecule. This was shown by the recognition and induction of antibodies and cytotoxic T cells, by the induction of "allo"-immunity, including skin graft rejection, and by the ability to present viral antigens. In dTGM, the expression of B27 on peripheral blood lymphocytes, spleen and lymphnode cells was comparable to H-2. However, on thymocytes, a relatively lower expression of HLA than H-2 was observed. This low expression of B27 on thymocytes is in concert with the observation that B27 is expressed only in the medulla of the thymus and not detectable in the cortex.