Presentation of soluble antigen to human T cells by products of multiple HLA-linked loci: analysis of antigen presentation by a panel of cloned, autologous, HLA-mutant Epstein-Barr virus-transformed lymphoblastoid cell lines

Epstein-Barr virus-transformed human B lymphoblastoid cell lines (EBV-LCL) can present soluble antigens to antigen-primed T lymphocytes. In this study, we used HLA antigen-loss mutants of an EBV-LCL line (LCL 721) to demonstrate that the presentation of a soluble antigen from Candida albicans (CAN) by EBV-LCL to primed T cells can be restricted by multiple HLA determinants. Haplotype-deletion mutants that contained only the maternal or only the paternal HLA-haplotype were used to demonstrate the preferential role of autologous HLA antigens in presenting soluble antigens to Candida-primed T cells from the donor of LCL-721, and to T cells from her mother and father. Immunoselected mutants of LCL-721 showing a variety of distinct phenotypes that are deficient in HLA-DR, DQ, or DP antigen expression were tested as antigen-presenting cells. The antigen-presenting ability of these class II deficient EBV-LCL variants weakened with progressive loss of class II HLA determinants expressed on the cell surface. Our study, therefore, provides evidence for multiple HLA restriction determinants, including HLA-DR, DQ, and DP. Furthermore, LCL lacking all HLA-DR, DQ, and DP expression because of homozygous deletion of these MHC class II genes still presented CAN and Tetanus toxid (TET), although to a much lesser degree than presented by LCL-721. This suggests that determinants other than DR, DQ, and DP which are expressed on these EBV-LCL may also function as restriction elements for the proliferative T-cell response to soluble antigens.     

Antigen-specific CD4+ T cells recognize epitopes of protective antigen following vaccination with an anthrax vaccine

Detection of antigen-specific CD4+ T cells is facilitated by the use of fluorescently labeled soluble peptide-major histocompatibility complex (MHC) multimers which mirror the antigen specificity of T-cell receptor recognition. We have used soluble peptide-MHC class II tetramers containing peptides from the protective antigen (PA) of Bacillus anthracis to detect circulating T cells in peripheral blood of subjects vaccinated with an anthrax vaccine. PA-specific HLA class II-restricted T lymphocytes were isolated which displayed both TH1- and TH2-like characteristics, indicating heterogeneity of the lymphocyte lineage within the CD4+ response. Presentation of antigen to these T-cell clones by HLA-matched antigen-presenting cells exposed to the intact PA protein confirmed that the identified epitopes are indeed naturally processed by the human immune system. Specific tetramer-derived T-cell profiling may be useful for monitoring helper CD4+ T-cell responses to anthrax vaccination.     

Both HLA-DR restricted and nonrestricted functions of adherent cells are required for induction of T lymphocyte proliferative responses

There appeared to be two distinct functions of adherent cells in human T lymphocyte proliferative responses to purified protein derivative (PPD) The first is antigen-presenting ability which is mediated by antigen-presenting cells (APC) among adherent cells By employing antiserum blocking pretreatment of APC, it was revealed that HLA-DR antigens are involved in this function and that identity or partial identity of HLA-DR antigens between APC and T lymphocytes is required for T lymphocyte antigen recognition The second function is mediated by the soluble factor produced by adherent cells and is HLA-DR nonrestricted

Although it remains nuclear whether APC and adherent cells producing the soluble factor belong to the same cell population, this second function might lead T lymphocytes to proliferate as long as T lymphocytes recognize antigen (PPD) via APC     

Antigen-Presenting Cells in Human Decidual Tissue

ABSTRACT: The responses of peripheral blood human T lymphocytes supported by decidual antigen-presenting cells (DAPCs) to a variety of immunogenic stimuli were studied and compared to those of T cells supported by peripheral blood antigen-presenting cells (PAPCs). Antigen-presenting cells were isolated from early normal decidual tissue or peripheral blood by elution with ethylenediamine tetraacetic acid of cells that after Ficoll-Paque separation bear receptors for and have bound to fibronectin. DAPCs pulsed with soluble or particulate antigens induced proliferation of T cells with an efficiency equivalent to PAPCs. Decidual tissue APCs also showed the ability to stimulate auto- and alloreactivity. Treatment with anti-human lymphocyte antigen (HLA) class II antibody and ultraviolet radiation resulted in substantial inhibition of the accessory cell function of DAPCs as well as of PAPCs. Bromodeoxyuridine and light treatment of alloreactive T cells generated in vitro was used to demonstrate that DAPCs primed with a synthetic polypeptide antigen (T,G)-A-L can stimulate only HLA class II-compatible T lymphocytes.     

Major histocompatibility complex class II-associated peptides determine the binding of the superantigen toxic shock syndrome toxin-1

Superantigens bind to major histocompatibility complex (MHC) class II proteins and interact with variable parts of the T cell antigen receptor (TCR) β-chain. Cross-linking the TCR with MHC class II molecules on the antigen-presenting cell by the superantigen leads to T cell activation that plays an essential role in pathogenesis. Recent crystallographic data have resolved the structure of the complexes between HLA-DR1 and staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1), respectively. For TSST-1, these studies have revealed possible contact sites between the superantigen and the HLA-DR1-bound peptide. Here, we show that TSST-1 binding is dependent on the MHC-II-associated peptides by employing variants of T2 mutant cells deficient in loading of peptides to MHC class II molecules as superantigen-presenting cells. On HLA-DR3-transfected T2 cells, presentation of TSST-1, but not SEB, was dependent on HLA-DR3-associated peptides. Thus, although these superantigens can be recognized in the context of multiple MHC class II alleles and isotypes, they clearly bind to specific subsets of MHC molecules displaying appropriate peptides.     

A recombinant single-chain human class II MHC molecule (HLA-DR1) as a covalently linked heterotrimer of α chain, β chain,and antigenic peptide,with immunogenicity in vitro and reduced affinity for bacterial superantigens

Major histocompatibility complex (MHC) class II molecules bind to numerous peptides and display these on the cell surface for T cell recognition. In a given immune response, receptors on T cells recognize antigenic peptides that are a minor population of MHC class II-bound peptides. To control which peptides are presented to T cells, it may be desirable to use recombinant MHC molecules with covalently bound antigenic peptides. To study T cell responses to such homogenous peptide-MHC complexes, we engineered an HLA-DR1 cDNA coding for influenza hemagglutinin, influenza matrix, or HIV p24 gag peptides covalently attached via a peptide spacer to the N terminus of the DR1 β chain. Co-transfection with DR α cDNA into mouse L cells resulted in surface expression of HLA-DR1 molecules that reacted with monoclonal antibodies (mAb) specific for correctly folded HLA-DR epitopes. This suggested that the spacer and peptide did not alter expression or folding of the molecule. We then engineered an additional peptide spacer between the C terminus of a truncated β chain (without transmembrane or cytoplasmic domains) and the N terminus of full-length DR α chain. Transfection of this cDNA into mouse L cells resulted in surface expression of the entire covalently linked heterotrimer of peptide, β chain, and α chain with the expected molecular mass of approximately 66 kDa. These single-chain HLA-DR1 molecules reacted with mAb specific for correctly folded HLA-DR epitopes, and identified one mAb with [MHC + peptide] specificity. Affinity-purified soluble secreted single-chain molecules with truncated α chain moved in electrophoresis as compact class II MHC dimers. Cell surface two-chain or single-chain HLA-DR1 molecules with a covalent HA peptide stimulated HLA-DR1-restricted HA-specific T cells. They were immunogenic in vitro for peripheral blood mononuclear cells. The two-chain and single-chain HLA-DR1 molecules with covalent HA peptide had reduced binding for the bacterial superantigens staphylococcal enterotoxin A and B and almost no binding for toxic shock syndrome toxin-1. The unique properties of these engineered HLA-DR1 molecules may facilitate our understanding of the complex nature of antigen recognition and aid in the development of novel vaccines with reduced superantigen binding.     

Peptide-induced nonresponsiveness of HLA-DP restricted human T cells reactive with Dermatophagoides spp. (house dust mite).

The activation of CD4+ T lymphocytes, which play a central role in allergic inflammation, depends on the recognition of allergen-derived peptides in association with major histocompatibility complex class II gene products. In this report we demonstrate, at a clonal level, that a component of the T-cell repertoire reactive with Dermatophagoides spp. (house dust mite) in atopic individuals, is restricted by HLA-DP class II molecules. This supports the recent results emerging from genetic epidemiologic studies that indicate positive associations between the HLA-DP phenotype and immune responsiveness to a variety of common allergens. Our findings also reveal that the T cells restricted by HLA-DP recognize a species-specific epitope located in the group I allergen of Dermatophagoides pteronyssinus (residues 101-119). Furthermore, we report that the pretreatment of the T cells restricted by HLA-DP with the Der p I peptide renders them nonresponsive to an immunogenic challenge with house dust mite allergen, and the loss of antigen-dependent proliferation is associated with downregulation of membrane expression of the T-cell antigen receptor. The ability to functionally inactivate T cells restricted by HLA-DP, as well as those that recognize allergen in association with HLA-DR class II molecules, suggests that desensitization with allergen-derived peptides may have therapeutic potential in the management of allergic diseases irrespective of their HLA class II association.     

Cross-linking HLA-DR molecules on Th1 cells induces anergy in association with increased level of cyclin-dependent kinase inhibitor p27(Kip1)

HLA class II molecules play pivotal roles in antigen presentation to CD4+ T cells. We investigated signaling via HLA-DR molecules expressed on CD4+ T cells. When HLA-DR or CD3 molecules on cloned CD4+ T cells were cross-linked by solid-phase mAbs, T cells proliferated, and this resulted in anergy. Whereas cross-linking of HLA-DR and CD3 resulted in secretion of the same levels of IFN-gamma and IL-8, secretion of IL-10 induced by cross-linking of HLA-DR was less than that induced by cross-linking of CD3 on CD4+ T cells. Interestingly, expression of p27(Kip1) but not p21(Cip1) increased after stimulation by either anti-HLA-DR or anti-CD3 mAb. This was indeed the case, when T cells were rendered anergic using a soluble form of antigenic peptide. In contrast, T cells stimulated by peptide-pulsed PBMC expressed little p27(Kip1). We propose that signaling via HLA-DR molecules on CD4+ T cells at least in part contributes to the induction of T cell anergy, through the upregulated expression of the p27(Kip1). The implication of our finding is that HLA-DR molecules play a role in human T cell anergy induced by a soluble form of antigenic peptide.     

Human Th1 cell lines recognize the Mycobacterium tuberculosis ESAT-6 antigen and its peptides in association with frequently expressed HLA class II molecules

We have used a synthetic-peptide approach to map epitope regions of the Mycobacterium tuberculosis ESAT-6 antigen recognized by human T cells in relation to major histocompatibility complex (MHC) restriction. ESAT-6-specific CD4+ T-cell lines were established by stimulating peripheral blood mononuclear cells from 25 HLA-DR-typed tuberculosis patients with complete antigen in vitro. The established T-cell lines were then screened for proliferation and interferon-gamma (IFN-gamma) secretion in response to eight overlapping 20-mer peptides covering the ESAT-6 sequence. The response of the T-cell lines to ESAT-6 and peptides from a human leucocyte antigen (HLA)-heterogeneous group of donors suggested the presence of multiple epitopes and promiscuous recognition of the antigen. Analysis of antigen and peptide recognition in the presence of anti-HLA class I and class II antibodies suggested that the T-cell lines recognized ESAT-6 in association with HLA-DR and -DQ molecules. Furthermore, testing of selected T-cell lines with ESAT-6 and the peptides in the presence of autologous and allogeneic HLA-DR- and -DQ-typed antigen-presenting cells identified HLA-DR2, -DR52 and -DQ2 amongst the HLA molecules involved in the presentation of ESAT-6 and its peptides to human Th1 cells. In addition, the T-cell lines were cytotoxic for monocytes and macrophages pulsed with ESAT-6 and peptides. In conclusion, the recognition of ESAT-6 by IFN-gamma-secreting and cytotoxic CD4+ T cells in association with frequently expressed HLA class II molecules supports the application of this antigen to either specific diagnosis or subunit vaccine design.     

New Approaches to Specific Immunomodulation in Transplantation

T cells can recognize foreign MHC antigens by two distinct routes, either directly as intact molecules, or indirectly as processed peptides. Recent evidence strongly suggests that the indirect pathway of allorecognition plays a key role in initiating and sustaining graft rejection. Theoretically, all mismatched HLA alloantigens could generate immunogenic peptides which may be recognized in the context of any of the two self HLA-DR molecules. However, indirect recognition appears to be limited to a single peptide determinant of an allogeneic HLA-DR molecule and restricted by one self HLA-DR molecule. Furthermore, T cells involved in the self-restricted allopeptide recognition express a limited array of T cell receptor variable genes. These findings suggest that selective immune interventions, such as peptide blockade of the self HLA-DR molecule involved in the presentation of the dominant allopeptide, induction of high-zone tolerance or TCR antagonism, may be devised to prevent graft rejection.     

Mechanisms by which HLA-class II molecules protect human B lymphoid tumour cells against NK- and LAK-mediated cytolysis.

We have previously shown that mutant B lymphoblastoid cell lines, totally deficient in expression of human leucocyte antigen (HLA)-class II molecules, but with normal HLA-class I expression, develop enhanced susceptibility to natural killer (NK) and lymphokine-activated killer (LAK) cell lysis. The current investigations were aimed at examining the role of HLA-DR and native peptides occupying the antigen-presenting grooves of HLA-class II molecules in protecting mutants of the same B-lymphoid lines against LAK-mediated lysis. No augmentation in LAK lysis was observed despite using two mutant B-cell lines (9.22.3 and 3.1.0) that lacked HLA-DR. Both these lines expressed HLA-DP and HLA-DQ. However, when using other B-cell lines with point mutations in certain regions of the HLA-DR alpha-chain (78, 80 and 96) significantly increased their susceptibility to LAK lysis despite normal expression of HLA-DR and the other class I and II molecules. Of particular interest was the finding that absence of native peptides in antigen-presenting grooves of all the HLA-class II molecules did not render the mutant B cell (9.5.3) susceptible to LAK lysis. These observations support the concept that there are different NK or LAK clones. Certain LAK clones recognize ''self'' major histocompatibility complex (MHC) antigens (but not the native peptides in their antigen-presenting grooves). Presence of ''self'' MHC antigens inhibits such clones. Conversely, other NK or LAK clones recognize ''non-self'' in the context of MHC antigens. Hence, point mutations at certain specific sites on the MHC molecules or foreign peptides in the antigen-presenting grooves enhances the susceptibility of these cells to LAK clones recognizing ''non-self''.     

Identification of β-lactoglobulin-derived peptides and class II HLA molecules recognized by T cells from patients with milk allergy

BACKGROUND: Cow's milk allergy impairs the health and development of many infants since it deprives them of adequate nutrition. Cow's milk fractions contain many allergens, and beta-lactoglobulin (BLG) is one of the major allergens. OBJECTIVE: The purpose of this study was to determine T cell epitopes, antigen-presenting molecules and cytokine production by T cells in relation to BLG. The results can provide new therapeutic possibilities of using analogue peptides of BLG for infants with cow's milk allergy. METHODS: Using a mixture of a panel of overlapping synthetic peptides that cover the entire BLG molecule, we established polyclonal BLG-specific short-term T cell lines and clones from peripheral blood mononuclear cells of four patients with allergy to cow's milk carrying most of the common human leucocyte antigen (HLA) haplotypes seen in the Japanese population. We then identified the T cell epitopes and antigen-presenting molecules, and measured the production of cytokines interleukin (IL)-4, IL-5 and interferon-gamma in the culture supernatants. RESULTS: The T cell lines established from the four patients responded to seven different peptides. Three of the peptides stimulated the T cells of two donors, regardless of the HLA types. The patterns of inhibition of the proliferative responses of the cell lines by anti-HLA class II antibodies were heterogeneous; three were mainly inhibited by anti-HLA-DR mAbs, and the other was inhibited by anti-HLA-DQ mAbs. High levels of IL-5 were produced by these T cell lines. CONCLUSIONS: Patients' T cells recognized BLG in association with a variety of HLA-DR or -DQ as antigen-presenting molecules. Although some peptides did have a more potent T cell stimulatory activity than others, the T cell receptor ligands formed with the BLG molecule are heterogeneous. Peptides for the desensitization of T cells of the patients with cow's milk allergy need to be designed keeping in mind the different requirements in different ethnic groups.     

Intracellular transport and peptide binding properties of HLA class II glycoproteins

Protein antigens internalized by an antigen presenting cell are degraded into peptides, a subset of which binds to the class II glycoproteins encoded by the major histocompatibility complex to form epitopes recognized by specific T cells. Current evidence suggests that the immunogenic peptides are generated in an endosomal, acidic compartment containing internalized antigen, proteinases, and exocytic class II molecules. These exocytic class II glycoproteins are associated during transport from the endoplasmic reticulum to the endosomal compartment with an additional glycoprotein, the invariant chain. Proteolytic degradation of the invariant chain in the endosomal compartment dissociates it from the class II glycoproteins, which only then acquire the capacity to bind peptides. After peptide binding occurs, the class II-peptide complexes are transported to the antigen-presenting cell surface for recognition by T cells.     

The correlation between ovomucoid-derived peptides, human leucocyte antigen class II molecules and T cell receptor-complementarity determining region 3 compositions in patients with egg-white allergy

BACKGROUND: Food allergies are more prevalent in children, due to the immature gastrointestinal epithelial membrane barrier allowing more proteins through the barrier and into circulation. Ovomucoid (OM) is one of the major allergens that is found in egg white. OBJECTIVE: The aim of this study was to determine T cell epitopes, antigen-presenting human leucocyte antigen (HLA) class II molecules of the T cell lines (TCLs) and T cell clones (TCCs), and complementarity determining region (CDR) 3 loops of the T cell receptor (TCR) alpha and beta chains of the TCCs specific to OM. METHODS: We established TCLs and TCCs specific to OM from peripheral blood mononuclear cells (PBMCs) of four atopic patients with egg-white allergy using a mixture of a panel of overlapping synthetic peptides corresponding to the amino acid sequence of the entire OM. We identified the T cell epitopes by antigen-induced proliferative responses, antigen-presenting molecules using allogeneic PBMCs and CDR3 loops of the TCR alpha and beta chains by cloning and sequence analysis. RESULTS: The TCLs and TCCs responded to seven different peptides, and their antigen-presenting molecules were different from each other. Sequence analysis of the TCR alpha and beta gene usage of the TCCs showed marked heterogeneity, and the usage of the CDR3 loop of the TCCs involved heterogenous amino acid residues. Interestingly, TCCs 'IH3.3' and 'YT6.1' recognized the same OM peptides, and had the same TCR Vbeta-Jbeta gene usage. Considering that peptide motifs bind to HLA class II molecules, the electrically charged residue (positive or negative) on the CDR3alpha and the CDR3beta loops of TCR of TCC may form ionic bonds with a charged residue on the HLA class II molecules-peptide complex. CONCLUSIONS: TCCs that have the same TCR gene usage were established from patients who had shown similar hypersensitivity-type, indicating that antigen recognition by a specific TCR is closely associated with the characteristics of each patient's symptoms.     

Engineered promiscuous T helper peptides for the induction of immune responses

Following recognition of antigens by T helper (Th) lymphocytes, T cell help is elicited to induce humoral and cellular immune responses. These antigens are presented as short peptides, T helper peptides (THP), bound to MHC class II molecules. Since both endogenous THP (from antigens of interest) or exogenous THP (not encompassed by the sequence of the antigen of interest) are able to elicit T cell help, we decided to engineer promiscuous exogenous THP capable of binding to several HLA-DR molecules, in order to cover an important proportion of the human population. Some of these exogenous THP were able to bind to all seven HLA-DR molecules tested and were immunogenic in vivo in HLA-DR4 transgenic mice. Among them, peptides p37, p62 and p45 elicited Th1 cytokine profiles in vivo, providing help for the induction of potent CTL responses. Finally, in vitro stimulation assays carried out using human cells, showed that these peptides could induce T cell responses using cells obtained from individuals with a broad spectrum of HLA-DR molecules. Thus, engineered exogenous THP may be a valuable tool for the induction of immune responses in a large proportion of human population.     

Allo-cross-reactivity of a human neuraminidase-specific T cell clone dependent on presentation of an endogenous B cell-specific antigen.

T cells specific for foreign antigen recognize a complex of peptides and self-major histocompatibility complex (MHC) molecules and can also cross-react with allo-MHC molecules. It remains controversial, however, what alloreactive T cells exactly recognize. It has been proposed that alloreactive T cells recognize endogenous peptides presented by allo-MHC molecules. To test this hypothesis, we examined an influenza virus-specific T cell clone (6H5), specific for neuraminidase N2 and restricted by HLA-DR1. In the absence of influenza virus, this clone cross-reacted with HLA-DR1Dw1+ but not with HLA-DR1Dw20+ Epstein-Barr virus-transformed lymphoblastoid cells (B-LCL). Cold target inhibition experiments and the rearrangement pattern of the T cell receptor beta chain indicated that 6H5 was a monoclonal T cell population most likely using the same T cell receptor for both responses. To determine whether determinants other than HLA-DR1Dw1+ B-LCL or activated B cells, but, surprisingly, not to other cell types expressed HLA-DR1Dw1, including monocytes and transfected L cells. These experiments further support the concept that recognition of allogeneic MHC (in this case HLA-DR1Dw1) may result from a cross-reactivity of T cells specific for a complex of foreign antigen and self-MHC (neuraminidase N2 and HLA-DR1Dw20). Furthermore, allorecognition of T cell clone 6H5 appears to depend upon the recognition of a complex of allogeneic MHC and a cell-type specific endogenous peptide presented by activated B cells.     

Definition of the alpha 2 region of HLA-DR molecules involved in CD4 binding.

HLA class II molecules present antigenic peptides to the T cell receptor of CD4+ T lymphocytes and interact with CD4 during the antigen recognition process. A major CD4 binding site encompassing amino acids (aa) 134-148 in the beta 2 domain of HLA-DR has been previously identified and residues located within the alpha 2 subunit of murine MHC class II I-Ad molecules have been shown to contribute to CD4-class II interaction. To characterize the alpha 2 region of HLA-DR molecules involved in the binding of CD4, we have synthesized overlapping linear and cyclic peptides derived from a region encompassing aa 121-143. We demonstrate that two linear peptides (aa 124-138 and 130-143) and a cyclic one (aa 121-138) specifically bind to CD4-sepharose affinity columns. Although cyclic analogues exhibit more ordered populations as detected by circular dichroism measurements, cyclization did not improve the activity of some peptides. Peptide sequence positioning in HLA-DR1 dimer model indicates that alpha 2 residues 124 to 136 form a solvent-exposed loop which faces the beta 2 loop delimited by residues 134-148. These data suggest that one CD4 molecule contacts both alpha 2 and beta 2 loops of the HLA-DR homodimer.     

Soluble HLA-DR molecules in patients with HLA class II versus class I associated disorders

HLA genes have been identified as key genetic factors contributing to many chronic diseases characterized by autoimmune features. The role of HLA encoded molecules in the pathogenesis of these diseases is unresolved. We have now analysed soluble HLA-DR molecules circulating in the serum of patients with different autoimmune diseases and have defined parameters controlling serum levels. Patients with HLA-DR associated diseases were characterized by elevated serum concentrations of HLA-DR molecules and were clearly distinct from patients with HLA-B27 associated disorders. We did not find evidence for a correlation between disease activity, laboratory abnormalities and elevated serum concentrations of soluble HLA-DR molecules. Studies in normal individuals indicated that soluble HLA-DR molecules are at least partially regulated by the HLA haplotype. Highest serum concentrations were found in individuals carrying the HLA-DR3 or HLA-DR4 haplotype raising the possibility that the phenomenon of HLA-disease association reflects differences in the genetic control of soluble HLA-DR molecules. Interferon-gamma treatment caused an increase in serum concentrations of soluble HLA-DR molecules, whereas a decrease of circulating HLA-DR molecules was associated with an immunosuppressive with cyclosporine A. These data suggest that the patient's immunoresponsiveness represents a second important mechanism controlling circulating HLA-DR molecules.     

HLA-peptide complex and immunity-related diseases]

The class II HLA molecule (HLA-DR, -DQ or -DP) is expressed on various antigen presenting cells (APC). Proteolytic fragments of peptides processed by APC that match the physiochemical character of the peptide-binding grooves formed by alpha and beta chains of class II HLA molecules are expressed on the surface of APC and are recognized by T cells. Many class II-binding peptide motifs reported to date follow the 9-mer peptide pattern AxxBxCDxE, with A, B, C, D, and E residues functioning to bind to HLA, thereby designated as "anchors". On the contrary, the x residues are recognized by T cells through the T cell receptor (TCR). Analog peptides with single residue substitutions at x residues changed signals in T cells, leading to quantitative and qualitative (anergy/survival, etc.) changes in human T cell clonal responses. Certain analogs changed signals in monocytes, leading to the upregulation of IL-12. Furthermore, cross-linking of class II HLA molecules on monocytes using mAbs leads to upregulation of monokines, in which HLA-DR, -DQ, and -DP play differential roles. Thus, HLA molecules when recognized by TCR, not only present peptide antigens to T cells but also transmit signals to APC, where the polymorphism and heterogeneity of HLA molecules may play important roles.     

Xenogeneic recognition of soluble and cell surface HLA class II antigens by proliferative murine T cells

In order to characterize the murine anti-human xenogeneic mixed lymphocyte reactions (MLR), we studied T cell proliferative responses against various human lymphoid cells by immunization of mice either with cellular or purified HLA-DR antigens. Data presented here indicated that small amounts of soluble HLA-DR antigen were able to prime mice, and that the xenogeneic MLR depends on the expression of HLA class II antigens on the stimulating cells. Experiments using a mutant cell line clearly showed that HLA-DP molecules were also sufficient in eliciting a primary or a secondary xenogeneic MLR while no secondary proliferative response was obtained with cells expressing only HLA class I molecules. Using a large panel of human cells with various haplotypes, our results also showed that (a) nonpolymorphic determinants of HLA class II antigens trigger dominantly the murine T cells and (b) the xenogeneic response required I-E and L3T4 accessory molecules and was not inhibited with anti I-A and monomorphic anti-HLA class II antigen monoclonal antibodies. Altogether these results suggest that HLA class II antigens act as nominal antigens in triggering a murine anti-human proliferative response.