The effect of perioperative allergic conjunctivitis on corneal lymphangiogenesis after corneal transplantation.

INTRODUCTION: Perioperative allergic conjunctivitis accelerates the speed of corneal allograft rejection. This study examines the effect of allergic conjunctivitis, with and without dexamethasone treatment, on the early inflammatory response and lymphangiogenesis in the host cornea following corneal transplantation. METHODS: Allogeneic fully MHC-mismatched C57Bl/6 strain donor corneas were transplanted into naive A/J mice and into A/J mice with active allergic conjunctivitis. Further groups of allograft recipients with allergic conjunctivitis were treated post-operatively with twice daily topical dexamethasone 0.1% or phosphate-buffered saline. Mice were killed on days 2 and 6 and corneas were examined by (i) fluorescent immunohistochemistry of frozen sections using anti-CD11b, anti-F4/80 and anti-Gr-1 antibodies, or (ii) whole-mount staining with anti-LYVE-1 antibody. Lymphatic ingrowth and numbers of cells infiltrating the host cornea were compared between groups. RESULTS: There were significantly higher numbers of CD11b(+) cells and LYVE-1(+) vessels in the host cornea at day 2 in allergic compared with naive recipients, but no differences between naive and allergic recipients at day 6. In allergic eyes, dexamethasone treatment significantly inhibited LYVE-1 expression at days 2 and 6, and significantly improved allograft survival in recipients with allergic conjunctivitis if maintained for a week. CONCLUSIONS: The innate immune response to allogeneic corneal tissue is more vigorous in the presence of allergic conjunctivitis than in naive eyes and is associated with accelerated lymphatic ingrowth to host cornea. Topical dexamethasone inhibits lymphatic ingrowth and this may be one mechanism by which topical steroid enhances graft survival.

Increased mast cell numbers in the conjunctiva of glaucoma patients: a possible indicator of preoperative glaucoma surgery inflammation.

PURPOSE: Inflammation is a risk factor for scarring after trabeculectomy surgery. Mast cells are important mediators of inflammation and scarring in allergic eye disease. This exploratory project investigated the presence of mast cells in the conjunctiva of glaucoma patients having trabeculectomy surgery. METHODS: Conjunctival biopsies from glaucoma patients belonging to specific groups: medically treated glaucoma (M, n=6), repeat glaucoma surgery (S, n=8), and uveitic glaucoma (U, n=7). The control group (C, n=8) was retinal detachment patients undergoing repair surgery for the first time. Immunohistochemistry techniques stained for the presence of the intracellular mast cell enzyme tryptase. RESULTS: The median mast cell tryptase-positive counts for all glaucoma groups (M, S, and U) ranged from 0.102-0.113 cells/mm2 compared to 0.064 cells/mm2 for group C. This was statistically significant comparing group S to group C (P=0.0063), but not when comparing groups U or M to group C. The mast cell tryptase-positive counts did not significantly differ among the groups. CONCLUSIONS: Mast cell numbers were significantly increased in glaucoma patients who have previously undergone surgery (group S). Mast cell activity may contribute to the scarring process and the increased risk of excessive conjunctival scarring after trabeculectomy surgery. Further investigation needs to be performed to evaluate this potential role.

Aqueous humor alloreactive cell phenotypes, cytokines and chemokines in corneal allograft rejection.

As biopsies are not taken at the time of human corneal allograft rejection, most information on the early cellular changes in rejection is from animal models. We examined the phenotype of alloreactive cells present in the human anterior chamber during corneal graft rejection by flow cytometry and quantified aqueous humor levels of cytokines and chemokines using cytometric bead array. Aqueous and peripheral blood samples were taken from patients with graft endothelial rejection (n = 11) and from control patients undergoing cataract surgery (n = 8). CD45(+)CD4(+), CD45(+)CD8(+) and CD45(+)CD14(+) cells were found in aqueous during rejection; no CD45(+) cells were seen in control samples. Higher proportions of CD45(+) cells found in aqueous during rejection were CD14(+), denoting monocyte/macrophage lineage, than were CD4(+) or CD8(+). Large elevations were seen in aqueous levels of IL-6, MCP-1 and IP-10 during rejection compared with controls; smaller but still statistically significant increases were seen in MIP-1alpha and eotaxin. The role of CD14(+) cells in allorejection is unclear as is the potential of these chemokines and their receptors as therapeutic targets. Aqueous humor samples offer a unique opportunity to analyze components of the allogeneic response in direct contact with donor tissue but without artifacts inherent in examination of tissue.

Retinal transplantation: progress and problems in clinical application.

There is currently no real treatment for blinding disorders that stem from the degeneration of cells in the retina and affect at least 50 million individuals worldwide. The excitement that accompanied the first studies showing the potential of retinal cell transplantation to alleviate the progress of blindness in such diseases as retinitis pigmentosa and age-related macular degeneration has lost some of its momentum, as attempts to apply research to the clinic have failed so far to provide effective treatments. What these studies have shown, however, is not that the approach is flawed but rather that the steps that need to be taken to achieve a viable, clinical treatment are many. This review summarizes the course of retinal transplant studies and points to obstacles that still need to be overcome to improve graft survival and efficacy and to develop a protocol that is effective in a clinical setting. Emphasis is given particularly to the consequences of introducing transplants to sites that have been considered immunologically privileged and to the role of the major histocompatibility complex classes I and II molecules in graft survival and rejection.

The HMG I proteins: dynamic roles in gene activation, development, and tumorigenesis.

The high mobility group I, Y, and I-C proteins are low-molecular-weight, nonhistone chromosomal proteins that play a general role modulating gene expression during development and the immune response. Consistent with their role in early development, all three proteins are expressed at high levels during embryogenesis, and their expression is markedly diminished in differentiated cells. Exceptions to the general repression of these genes in adult tissues involve (1) A burst of synthesis of the HMG I protein during the immune response (during lymphocyte activation and preceding cytokine/adhesion molecule gene expression), (2) A constitutive expression of the HMG I and Y proteins in photoreceptor cells, and (3) Derepression of HMG I, Y, and often I-C expression in neoplastic cells. Work from several laboratories has now uncovered how these proteins participate in gene activation: (1) By altering the chromatin structure around an inducible gene-and thus influencing accessibility of the locus to regulatory proteins-(2) By facilitating the loading of transcription factors onto the promoters, and (3) By bridging adjacent transcription factors on a promoter via protein/protein interactions. Despite the similar structures and biochemical properties of the three proteins, the work has also provided clues to a division of labor between these proteins. HMG I and Y have demonstrable roles in enhanceosome formation, whereas HMG I-C has a specific role in adipogenesis. C-terminal truncations of HMG I-C and wild-type HMG Y appear to function in a manner analogous to oncogenes, as assessed by cellular transforation assays and transgenic mice. Future work should clearly define the similarities and differences in the biological roles of the three proteins, and should evolve to include attempts at pharmaceutical intervention in disease, based upon structural information concerning HMG I interactions with DNA and with regulatory proteins.

The architectural transcription factor high mobility group I(Y) participates in photoreceptor-specific gene expression.

The nonhistone chromosomal proteins high mobility group I(Y) [HMG I(Y)] have been shown to function as architectural transcription factors facilitating enhanceosome formation on a variety of mammalian promoters. Specifically, they have been shown to act as a "molecular glue" mediating protein-protein and protein-DNA contacts within the enhanceosome complex. HMG I(Y) proteins are expressed at high levels in embryonic and transformed cells and have been implicated in transcriptional regulation in these cells. Terminally differentiated cells, however, have been reported to express only minimal, if any, HMG I(Y). In contrast to these observations, we show here that adult mouse retinal photoreceptors, which are terminally differentiated cells, express high levels of these proteins. Using retinoblastoma cells as an approximate model, we further demonstrate in transiently transfected cells that inhibition of HMG I(Y) expression and mutation of HMG I(Y) binding sites significantly reduce rhodopsin promoter activity. DNase I footprint analysis indicates that HMG I protein interacts with a discrete site within the rhodopsin proximal promoter. This site overlaps with the binding site for Crx, a paired-like homeodomain transcription factor that is essential for photoreceptor functioning and that when mutated causes several forms of human photoreceptor degeneration. Both biochemical and functional experiments demonstrate that HMG I(Y) physically associate with Crx and that their interaction with DNA is required for high-level transcription of the rhodopsin gene. These data provide the first demonstration that HMG I(Y) can be important for gene activation in terminally differentiated cells.

Functional domains of the cone-rod homeobox (CRX) transcription factor.

The paired-like homeodomain transcription factor CRX (cone-rod homeobox) is involved in regulating photoreceptor gene expression and rod outer segment development. Mutations in CRX have been associated with several retinal degenerative diseases. These conditions range from Leber congenital amaurosis (a severe cone and rod degeneration of childhood onset) to adult onset cone-rod dystrophy and retinitis pigmentosa (an adult onset condition that primarily affects rods). The goal of this study is to better understand the molecular basis of CRX function and to provide insight into how mutations in CRX cause such a variety of clinical phenotypes. We performed deletion analysis in conjunction with DNA binding and transient transfection-based transactivation studies to identify the functional domains within CRX. DNA binding requires a complete homeodomain. Furthermore, truncated proteins that did not contain an intact homeodomain failed to demonstrate detectable expression in tissue culture upon transfection. Transactivation analysis indicated that both the OTX tail and the WSP domain are important for controlling positive regulatory activity of CRX. Interestingly, the mapped CRX transactivation domains were also critical when coexpressed with NRL. Specifically, the synergy between CRX and NRL was constant regardless of which CRX variant was used.

CD4+ T cell responses to self- and mutated p53 determinants during tumorigenesis in mice.

We analyzed CD4+ T helper responses to wild-type (wt) and mutated (mut) p53 protein in normal and tumor-bearing mice. In normal mice, we observed that although some self-p53 determinants induced negative selection of p53-reactive CD4+ T cells, other p53 determinants (cryptic) were immunogenic. Next, BALB/c mice were inoculated with J774 syngeneic tumor cell line expressing mut p53. BALB/c tumor-bearing mice mounted potent CD4+ T cell responses to two formerly cryptic peptides on self-p53. This response was characterized by massive production of IL-5, a Th2-type lymphokine. Interestingly, we found that T cell response was induced by different p53 peptides depending upon the stage of cancer. Mut p53 gene was shown to contain a single mutation resulting in the substitution of a tyrosine by a histidine at position 231 of the protein. Two peptides corresponding to wt and mutated sequences of this region were synthesized. Both peptides bound to the MHC class II-presenting molecule (Ed) with similar affinities. However, only mut p53.225-239 induced T cell responses in normal BALB/c mice, a result strongly suggesting that high-affinity wt p53.225-239 autoreactive T cells had been eliminated in these mice. Surprisingly, CD4+ T cell responses to both mut and wt p53.225-239 peptides were recorded in J774 tumor-bearing mice, a phenomenon attributed to the recruitment of low-avidity p53.225-239 self-reactive T cells.

Transgenic mice expressing a truncated form of the high mobility group I-C protein develop adiposity and an abnormally high prevalence of lipomas.

Chromosomal translocations in human lipomas frequently create fusion transcripts encoding high mobility group (HMG) I-C DNA-binding domains and C-terminal sequences from different presumed transcription factors, suggesting a potential role for HMG I-C in the development of lipomas. To evaluate the role of the HMG I-C component, the three DNA-binding domains of HMG I-C have now been expressed in transgenic mice. Despite the ubiquitous expression of the truncated HMG I-C protein, the transgenic mice develop a selective abundance of fat tissue early in life, show marked adipose tissue inflammation, and have an abnormally high incidence of lipomas. These findings demonstrate that the DNA-binding domains of HMG I-C, in the absence of a C-terminal fusion partner, are sufficient to perturb adipogenesis and predispose to lipomas. We provide data supporting the central utility of this animal model as a tool to understand the molecular mechanisms underlying the development of one of the most common kind of human benign tumors.

Identification of a stimulus-dependent DNase I hypersensitive site between the Ialpha and Calpha exons during immunoglobulin heavy chain class switch recombination.

The complete humoral response to foreign antigen depends upon two distinct recombination events within the heavy chain locus of immunoglobulin. The first recombination event takes place in what will become the antigen combining site of the antibody molecule, encoded by V, D and J segments. The second recombination event involves the looping-out of large spans of DNA which separate the various clusters of heavy chain exons which define the different immunoglobulin isotypes, or classes. While a great deal has been learned about the nature of the VDJ recombinase, very little is known about the nature of the class-switch recombinase. Using a cell system where class-switch recombination occurs primarily to the IgA locus, we have looked for stimulus-dependent changes in the chromatin structure of the IgA locus which might result from interactions between components of the recombinase and cis-elements within the region. We present evidence that strongly suggests that the class-switch recombinase interacts between the Ialpha and Calpha exons of IgA, just upstream of the highly reiterated DR1 and DR2 elements. However, although multiple potential SMAD-4 sites are located precisely within the DNase I hypersensitive site and 160 bp upstream of that site, we failed to detect any evidence of DNA/protein interactions near the hypersensitive site. Moreover, recombinant SMAD-3/4 proteins fail to interact with these sites with appreciable affinity in vitro. These data suggest that some other structural alteration at this site (e.g. RNA/DNA hybrid) may mediate the nuclease sensitivity.

A novel downstream positive regulatory element mediating transcription of the human high mobility group (HMG) I-C gene.

The high mobility group (HMG) I proteins are small, non-histone chromosomal proteins that promote gene activation during development and within rapidly dividing cells. They do so by facilitating enhanceosome formation on inducible genes, via both protein/DNA and protein/protein interactions. The HMG I-C gene is tightly regulated, normally being expressed exclusively during embryonic development. However, HMG I-C expression is also observed frequently in a number of tumor types, and this expression has been shown to contribute to the malignant transformation process. With the aim of dissecting pathways that lead to aberrant expression of HMG I-C in tumor cells, we have analyzed HMG I-C gene regulation in the human hepatoma cell line PLC/PRF/5. One of the two HMG I-C transcripts detected in this cell line originates from a novel downstream initiation site at nucleotide -161 relative to the first methionine. Transcription from the downstream initiation site is mediated by a PRE located between nt -222 and -217. We show here that the Sp1 and Sp3 transcription factors interact with the PRE and transactivate the HMG I-C promoter in a cooperative fashion. This study provides the first characterization of this downstream HMG I-C promoter.

High mobility group I/Y protein functions as a specific cofactor for Oct-2A: mapping of interaction domains.

The octamer motif (ATTTGCAT) present in several eukaryotic promoters and enhancers is now known to influence the transcription of several genes by interacting with members of a broad family of homeodomain proteins. The promoter of the human class II MHC gene HLA-DRA contains a conserved octamer element that can bind (among other proteins) the transcription factor Oct-2A and the high mobility group proteins (HMG) I/Y. We have previously determined that HMG I(Y) and Oct-2A cooperatively activate HLA-DRA gene expression, most likely due to the ability of HMG I(Y) to selectively recruit Oct-2A to the octamer motif. In this report, we present results of our investigations of the mechanisms of cooperative transactivation of HLA-DRA transcription by Oct-2A and HMG I(Y). We show that both the amino- and the carboxy-terminal domains of Oct-2A are required for HLA-DRA transactivation. Experiments using domain-swap chimeras of the Oct-1 and Oct-2A polypeptides indicate that cooperative activation of the DRA gene by HMG I(Y) and Oct-2A requires the carboxy-terminal domain (CTD) of Oct-2A. However, HMG I(Y) physically interacts with the conserved POU domains of both Oct-1 and Oct-2A. We therefore postulate that the nature of the CTD attached to the POU homeodomain influences the outcome of interaction with HMG I(Y). These studies support the view that HMG I(Y) is an important cofactor for HLA-DRA gene activation by Oct-2A and provide insights into its mechanism of action.

HLA-DRA promoter polymorphism and diversity generation within the immune system.

Multiple major histocompatibility complex (MHC) alleles exist at most class I and II loci. Polymorphism of MHC polypeptides may reflect either different levels of selective pressure operating on each molecule or different mutation rates at different loci. To gain further insight into this issue, we sequenced the non-coding promoter region of the HLA-DRA gene from several Epstein-Barr virus-transformed B cell lines and compared the extent of polymorphism found in this region with the known polymorphism of the HLA-DQB promoter. Our results indicate that the HLA-DRA promoter displays a low level of polymorphism while the promoter of HLA-DQB exhibits a nucleotide substitution rate fivefold greater than that of DRA. Moreover, through phylogenetic analysis, the HLA-DRA promoter was found to have diverged much less than the associated alleles of HLA-DRB1 and -DQA1. Taken together, these results suggest that the HLA-DRA promoter is highly conserved and may be under a stronger functional constraint than the promoter regions of other MHC class II genes.

Polymorphic nucleotides within the human IL-4 promoter that mediate overexpression of the gene.

Atopy, which predisposes individuals to develop asthma, severe systemic anaphylaxis, and atopic dermatitis, is usually associated with dramatically elevated total serum IgE levels and is thought to be controlled by a major susceptibility gene and multiple minor susceptibility genes. A recent sib-pair analysis revealed a tight linkage between markers on 5q31.1 and a major susceptibility gene controlling total serum IgE levels. Due to its location within this cluster and its biologic role in Ig class switching and Th2 cell differentiation, the IL-4 gene has emerged as one major candidate for the atopy gene. In one model, polymorphisms within IL-4 regulatory elements might result in overexpression of the gene, amplifying Th2 cell differentiation and class switching to IgE. In support of this model, we report that the human IL-4 promoter exists in multiple allelic forms that exhibit distinct transcriptional activities in IL-4-positive T cells. A particular allele has an unusually high transcriptional activity. A nucleotide substitution within a recently described OAP40 element located just upstream of an NF-AT site (P sequence) appears to be largely responsible for the increased promotor strength of this particular allelic form of the IL-4 promoter. In EMSAs, this substitution results in a markedly enhanced affinity for sequence-specific complexes exhibiting an AP-1 specificity. The identification of allelic nucleotides, which results in overexpression of the IL-4 gene, provides specific targets for a comprehensive screening of atopic and nonatopic individuals and may provide a clue for genetic predisposition for atopy.

Inhibition of NF-AT-dependent transcription by NF-kappa B: implications for differential gene expression in T helper cell subsets.

Activation of individual CD4+ T cells results in differential lymphokine expression: interleukin 2 (IL-2) is preferentially produced by T helper type 1 (TH1) cells, which are involved in cell-mediated immune responses, whereas IL-4 is synthesized by TH2 cells, which are essential for humoral immunity. The Ca(2+)-dependent factor NF-ATp plays a key role in the inducible transcription of both these lymphokine genes. However, while IL2 expression requires the contribution of Ca(2+)- and protein kinase C-dependent signals, we report that activation of human IL4 transcription through the Ca(2+)-dependent pathway is diminished by protein kinase C stimulation in Jurkat T cells. This phenomenon is due to mutually exclusive binding of NF-ATp and NF-kappa B to the P sequence, an element located 69 bp upstream of the IL4 transcription initiation site. Human IL4 promoter-mediated transcription is downregulated in Jurkat cells stimulated with the NF-kappa B-activating cytokine tumor necrosis factor alpha and suppressed in RelA-overexpressing cells. In contrast, protein kinase C stimulation or RelA overexpression does not affect the activity of a human IL4 promoter containing a mouse P sequence, which is a higher-affinity site for NF-ATp and a lower-affinity site for RelA. Thus, competition between two general transcriptional activators, RelA and NF-ATp, mediates the inhibitory effect of protein kinase C stimulation on IL4 expression and may contribute to differential gene expression in TH cells.

Expression of the chemokine RANTES by a human bronchial epithelial cell line. Modulation by cytokines and glucocorticoids.

The chemokine RANTES is a potent chemoattractant for eosinophils, lymphocytes, and monocytes, and has been detected recently in the epithelium of human airways mucosa. We have studied, therefore, the expression of RANTES mRNA and protein in the human bronchial epithelial cell line BEAS-2B. Using Northern blot analysis, RANTES mRNA was not detectable in unstimulated BEAS-2B cells. Incubation of cells with TNF-alpha or IFN-gamma induced the expression of RANTES mRNA and protein within 16 h. The combination of TNF-alpha and IFN-gamma displayed a marked synergism in inducing RANTES expression. Pretreatment of cells with the glucocorticoid budesonide (10(-10)-10(-7) M) for 24 h inhibited expression of RANTES mRNA and protein stimulated by either TNF-alpha or TNF-alpha plus IFN-gamma in a concentration- and time-dependent manner. Nonglucocorticoid steroids did not inhibit RANTES mRNA expression. Production of RANTES by epithelium could contribute to the mechanism of selective cellular recruitment occurring in the airways during inflammation, thus playing a relevant role in the pathogenesis of diseases such as asthma, rhinitis, and polyposis. The down-regulation of RANTES production by glucocorticoids in epithelial cells may contribute to the efficacy of these compounds in reducing cellular infiltration and, ultimately, to their anti-inflammatory properties.

Mapping of the interaction site of the defective transcription factor in the class II major histocompatibility complex mutant cell line clone-13 to the divergent X2-box.

We have previously described a mutant B lymphoblastoid cell line, Clone-13, that expresses HLA-DQ in the absence of HLA-DR and -DP. Several criteria indicated that the defect in this cell line influences the activity of an isotype-specific transcription factor. Indeed, transient transfection of HLA-DRA and DQB reporter constructs indicated that the affected factor operates via cis-elements located between -141 base pairs and the transcription initiation site. A series of hybrid DRA/DQB reporter constructs was generated to further map the relevant cis-elements in this system. Insertion of oligonucleotides spanning the DQB X-box (but not the DQB-W region or the DQB Y-box) upstream of -141 in a DRA reporter plasmid rescued expression to nearly wild-type levels. Substitution promoters were then generated where the entire X-box, or only the X1- or X2-boxes of HLA-DRA were replaced with the analogous regions of HLA-DQB. The DQB X2-box was able to restore expression to the silent DRA reporter construct. Moreover, replacement of the DQB X2-box with the DRA X2-box markedly diminished the activity of the DQB promoter in the mutant cell. None of the hybrid reporter constructs were defective when transfected into the wild-type, HLA-DR/-DQ positive parental cell line, Jijoye. These studies suggest that the divergent X2-box of the class II major histocompatibility complex promoters plays an important role in influencing differential expression of the human class II isotypes.

A novel cysteine-rich sequence-specific DNA-binding protein interacts with the conserved X-box motif of the human major histocompatibility complex class II genes via a repeated Cys-His domain and functions as a transcriptional repressor.

The class II major histocompatibility complex (MHC) molecules function in the presentation of processed peptides to helper T cells. As most mammalian cells can endocytose and process foreign antigen, the critical determinant of an antigen-presenting cell is its ability to express class II MHC molecules. Expression of these molecules is usually restricted to cells of the immune system and dysregulated expression is hypothesized to contribute to the pathogenesis of a severe combined immunodeficiency syndrome and certain autoimmune diseases. Human complementary DNA clones encoding a newly identified, cysteine-rich transcription factor, NF-X1, which binds to the conserved X-box motif of class II MHC genes, were obtained, and the primary amino acid sequence deduced. The major open reading frame encodes a polypeptide of 1,104 amino acids with a symmetrical organization. A central cysteine-rich portion encodes the DNA-binding domain, and is subdivided into seven repeated motifs. This motif is similar to but distinct from the LIM domain and the RING finger family, and is reminiscent of known metal-binding regions. The unique arrangement of cysteines indicates that the consensus sequence CX3CXL-XCGX1-5HXCX3CHXGXC represents a novel cysteine-rich motif. Two lines of evidence indicate that the polypeptide encodes a potent and biologically relevant repressor of HLA-DRA transcription: (a) overexpression of NF-X1 from a retroviral construct strongly decreases transcription from the HLA-DRA promoter; and (b) the NF-X1 transcript is markedly induced late after induction with interferon gamma (IFN-gamma), coinciding with postinduction attenuation of HLA-DRA transcription. The NF-X1 protein may therefore play an important role in regulating the duration of an inflammatory response by limiting the period in which class II MHC molecules are induced by IFN-gamma.

Human X-box-binding protein 1 is required for the transcription of a subset of human class II major histocompatibility genes and forms a heterodimer with c-fos.

A complementary DNA encoding a member of the leucine-zipper class of proteins (human X-box-binding protein, hXBP-1) that binds to the 3' end of the conserved X box (X2) of the HLA-DRA major histocompatibility complex gene was recently described. Further gel-retardation analysis has demonstrated that hXBP-1 also binds to HLA-DPB X2 but not to other X2 sequences. Transient transfection of a mammalian expression vector with the hXBP-1 cDNA inserted in the antisense orientation represses the surface expression of HLA-DR and HLA-DP in Raji cells. Cotransfection of the antisense hXBP-1 vector with a HLA-DRA/chloramphenicol acetyltransferase (but not a HLA-DQB/chloramphenicol acetyltransferase) reporter plasmid decreases chloramphenicol acetyltransferase activity in Raji cells and in gamma-interferon-treated HeLa cells relative to cells cotransfected with a control antisense vector. Moreover, hXBP-1 is shown to form a stable heterodimer with the product of the c-fos protooncogene. These data suggest that the hXBP-1 c-fos heterodimer is critical for the transcription of a subset of the human class II major histocompatibility complex genes and that the regulatory mechanisms for the different class II genes are distinct.