A new HLA-linked T cell membrane molecule, related to the beta chain of the clonotypic receptor, is associated with T3

Cell Surface B2m-Free Human Leukocyte Antigen (HLA) Monomers and Dimers: Are They Neo-HLA Class and Proto-HLA?

Cell surface HLA-I molecules (Face-1) consist of a polypeptide heavy chain (HC) with two groove domains (G domain) and one constant domain (C-domain) as well as a light chain, B2-microglobulin (B2m). However, HCs can also independently emerge unfolded on the cell surface without peptides as B2m-free HC monomers (Face-2), B2m-free HC homodimers (Face 3), and B2m-free HC heterodimers (Face-4). The transport of these HLA variants from ER to the cell surface was confirmed by antiviral antibiotics that arrest the release of newly synthesized proteins from the ER. Face-2 occurs at low levels on the normal cell surface of the lung, bronchi, epidermis, esophagus, breast, stomach, ilium, colorectum, gall bladder, urinary bladder, seminal vesicles ovarian epithelia, endometrium, thymus, spleen, and lymphocytes. They are upregulated on immune cells upon activation by proinflammatory cytokines, anti-CD3 antibodies, antibiotics (e.g., ionomycin), phytohemagglutinin, retinoic acid, and phorbol myristate acetate. Their density on the cell surface remains high as long as the cells remain in an activated state. After activation-induced upregulation, the Face-2 molecules undergo homo- and hetero-dimerization (Face-3 and Face-4). Alterations in the redox environment promote dimerization. Heterodimerization can occur among and between the alleles of different haplotypes. The glycosylation of these variants differ from that of Face-1, and they may occur with bound exogenous peptides. Spontaneous arthritis occurs in HLA-B27+ mice lacking B2m (HLA-B27+ B2m-/-) but not in HLA-B27+ B2m+/- mice. The mice with HLA-B27 in Face-2 spontaneous configuration develop symptoms such as changes in nails and joints, hair loss, and swelling in paws, leading to ankyloses. Anti-HC-specific mAbs delay disease development. Some HLA-I polyreactive mAbs (MEM series) used for immunostaining confirm the existence of B2m-free variants in several cancer cells. The upregulation of Face-2 in human cancers occurs concomitantly with the downregulation of intact HLAs (Face-1). The HLA monomeric and dimeric variants interact with inhibitory and activating ligands (e.g., KIR), growth factors, cytokines, and neurotransmitters. Similarities in the amino acid sequences of the HLA-I variants and HLA-II β-chain suggest that Face-2 could be the progenitor of both HLA classes. These findings may support the recognition of these variants as a neo-HLA class and proto-HLA.

Homotypic and heterotypic in cis associations of MHC class I molecules at the cell surface

Through the presentation of peptide antigens to cytotoxic T lymphocytes, major histocompatibility complex (MHC) class I molecules mediate the adaptive immune response against tumors and viruses. Additional non-immunological functions include the heterotypic association of class I molecules with cell surface receptors, regulating their activities by unknown mechanisms. Also, homotypic associations resulting in class I dimers and oligomers - of unknown function - have been related to pathological outcomes. In this review, we provide an overview of the current knowledge about the occurrence, biochemical nature, and dynamics of homotypic and heterotypic associations of class I molecules at the cell surface with special focus on the molecular species that take part in the complexes and on the evidence that supports novel biological roles for class I molecules. We show that both heterotypic and homotypic class I associations reported in the literature describe not one but several kinds of oligomers with distinctive stoichiometry and biochemical properties.

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Major histocompatibility complex class I molecules form homotypic and heterotypic associations at the cell surface.

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Associations show distinctive stoichiometry and biochemical properties.

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Associations might regulate immunological and non-immunological processes.

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Heterotypic association with cell surface receptors might regulate receptor's activity.

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Homotypic associations have been related to pathological outcomes.

Characterization of signaling function and expression of HLA class I molecules in medulloblastoma

Although known for the important function in the immune system, MHC class I molecules are increasingly ascribed an alternative role in modifying signal transduction. In medulloblastoma, HLA class I molecules are associated with poor prognosis, and can induce ERK1/2 activation upon engagement with ligands that bind to incompletely assembled complexes (so called open conformers). We here demonstrate that ERK1/2 activation in medulloblastoma can occur in the absence of endogenously synthesized β2m, formally excluding involvement of closed HLA class conformation. In addition, several experimental observations suggest that heterogeneity of HLA class I expression may be a reflection of the status of original cells before transformation, rather than a consequence of immune-based selection of HLA-loss mutants. These results contribute to our understanding of an immune system-independent role of HLA class I in the pathology of medulloblastoma, and cancer in general.

Open conformers: the hidden face of MHC-I molecules

A pool of MHC-I molecules present at the plasma membrane can dissociate from the peptide and/or the light chain, becoming open MHC-I conformers. Whereas peptide-bound MHC-I molecules have an important role in regulating adaptive and innate immune responses, through trans-interactions with T cell and NK cell receptors, the function of the open MHC-I conformers is less clear but seems to be related to their inherent ability to cis-associate, both with themselves and with other receptors. Here, we review data indicating the open MHC-I conformers as regulators of ligand-receptor interactions and discuss the biological implications for immune and non-immune cells. The likelihood that the MHC-I heavy chains have hidden functions that are determined by the amino acid sequence of the alpha1 and alpha2 domains are discussed.

Misfolding of major histocompatibility complex class I molecules in activated T cells allows cis-interactions with receptors and signaling molecules and is associated with tyrosine phosphorylation

Knowledge of the origin and biochemical status of beta(2)-microglobulin-free or misfolded major histocompatibility complex (MHC)-I molecules is essential for understanding their pleiotropic properties. Here we show that in normal human T cells, misfolding of MHC-I molecules is turned on upon activation and cell division and is proportional to the level of proliferation. Immunoprecipitation showed that a number of proteins are associated with MHC-I heavy chains at the surface of activated T cells, including the CD8alphabeta receptor and the chaperone tandem calreticulin/ERp57, associations that rely upon the existence of a pool of HC-10-reactive molecules. Biochemical analysis showed that misfolded MHC-I molecules present at the cell surface are fully glycosylated mature molecules. Importantly, misfolded MHC-I molecules are tyrosine phosphorylated and are associated with kinase activity. In vitro kinase assays followed by reprecipitation indicated that tyrosine phosphorylation of the class I heavy chain is probably mediated by a Src tyrosine kinase because Lck was found associated with HC-10 immunocomplexes. Finally, we show that inhibition of tyrosine phosphorylation by using the Src-family tyrosine kinase inhibitor PP2 resulted in enhanced release of MHC-I heavy chains from the cell surface of activated T cells and a slight down-regulation of cell surface W6/32-reactive molecules. This study provides new insights into the biology of MHC-I molecules and suggests that tyrosine phosphorylation may be involved in the regulation of MHC-I misfolding and expression.

Soluble nonclassical HLA generated by the metalloproteinase pathway

Soluble human leukocyte antigens (HLA-A, -B, and -C) proteins can be generated by a membrane-bound metalloproteinase (MPase). The MPase-mediated pathway produces soluble nonconformed HLA proteins susceptible to further degradation, and also HLA proteins with high affinity peptides stable at physiologic temperatures. Accessibility of classical HLA to the MPase cleavage inversely correlates with stability of heavy chain (HC) interactions with beta2-microglobulin (beta(2)m). Whether a MPase is involved in release of soluble nonclassical HLA or CD1 proteins is unknown. We have investigated this question with transfectants expressing full-length HLA proteins. Native surface HLA-E and -G complexes, similar to HLA-A2, were unstable at low pH and dissociated giving rise to beta(2)m-free HC. Furthermore, HLA-E and -G proteins, similar to HLA-A2, were readily released from cell surface into supernatants as soluble 37-kilodalton beta(2)m-free HC. However, the stability of surface CD1d complexes was not affected by pH changes and no soluble CD1d was detected. Because beta(2)m-free CD1d HC were expressed on cells, the lack of cleaved soluble products cannot be explained by high stability of native complexes. Instead, absence of a CD1d-specific MPase in these cells or its impaired interactions with substrate HC may be responsible.

Allosteric crosstalk between peptide-binding, transport, and ATP hydrolysis of the ABC transporter TAP

The transporter associated with antigen processing (TAP) is essential for intracellular transport of protein fragments into the endoplasmic reticulum for loading of major histocompatibility complex (MHC) class I molecules. On the cell surface, these peptide-MHC complexes are monitored by cytotoxic T lymphocytes. To study the ATP hydrolysis of TAP, we developed an enrichment and reconstitution procedure, by which we fully restored TAP function in proteoliposomes. A TAP-specific ATPase activity was identified that could be stimulated by peptides and blocked by the herpes simplex virus protein ICP47. Strikingly, the peptide-binding motif of TAP directly correlates with the stimulation of the ATPase activity, demonstrating that the initial peptide-binding step is responsible for TAP selectivity. ATP hydrolysis follows Michaelis-Menten kinetics with a maximal velocity V(max) of 2 micromol/min per mg TAP, corresponding to a turnover number of approximately 5 ATP per second. This turnover rate is sufficient to account for the role of TAP in peptide loading of MHC molecules and the overall process of antigen presentation. Interestingly, sterically restricted peptides that bind but are not transported by TAP do not stimulate ATPase activity. These results point to coordinated dialogue between the peptide-binding site, the nucleotide-binding domain, and the translocation site via conformational changes within the TAP complex.

Implications of HLA-E allele expression and different HLA-E ligand diversity for the regulation of NK cells

The interaction of HLA-E with CD94/NKG2A is dependant on the binding of HLA class I signal sequence derived peptides to HLA-E. In the caucasoid population two HLA-E alleles are observed at equal frequencies. Here we study the functional differences between the two HLA-E molecules with regard to cell surface expression, peptide binding, and potential to inhibit lytic activity of a CD94/NKG2A(+) NK cell line. In contrast to the HLA-E(R) allele, the HLA-E(G) allele shows considerable cell surface expression even in the absence of endogenous HLA class I signal sequence derived HLA-E ligands. Eighteen HLA-E allele/HLA-E ligand combinations were analyzed. No correlation between cell surface expression of HLA-E and NK cell inhibition was observed. The peptides present in the signal sequences of HLA-B15, -Cw0402, and -Cw7 bound to both HLA-E alleles but did not lead to an inhibition of NK cell lysis. In our experimental system the peptides A2 and G were not effective with regard to NK cell inhibition when bound to the HLA-E(R) allele. These results may be of functional significance particularly in the placenta where the only HLA-E ligands are derived from HLA-G and -C.

Structural relatedness of distinct determinants recognized by monoclonal antibody TP25.99 on beta 2-microglobulin-associated and beta 2-microglobulin-free HLA class I heavy chains

The association of HLA class I heavy chains with beta2-microglobulin (beta2m) changes their antigenic profile. As a result, Abs react with either beta2m-free or beta2m-associated HLA class I heavy chains. An exception to this rule is the mAb TP25.99, which reacts with both beta2m-associated and beta2m-free HLA class I heavy chains. The reactivity with beta2m-associated HLA class I heavy chains is mediated by a conformational determinant expressed on all HLA-A, -B, and -C Ags. This determinant has been mapped to amino acid residues 194-198 in the alpha3 domain. The reactivity with beta2m-free HLA class I heavy chains is mediated by a linear determinant expressed on all HLA-B Ags except the HLA-B73 allospecificity and on <50% of HLA-A allospecificities. The latter determinant has been mapped to amino acid residues 239-242, 245, and 246 in the alpha3 domain. The conformational and the linear determinants share several structural features, but have no homology in their amino acid sequence. mAb TP25.99 represents the first example of a mAb recognizing two distinct and spatially distant determinants on a protein. The structural homology of a linear and a conformational determinant on an antigenic entity provides a molecular mechanism for the sharing of specificity by B and TCRs.

Analysis by NMR spectroscopy of the structural homology between the linear and the cyclic peptide recognized by anti-human leukocyte antigen class I monoclonal antibody TP25.99*

The anti-human leukocyte antigen (HLA) class I monoclonal antibody (mAb) TP25.99 has a unique specificity since it recognizes both a conformational and a linear determinant expressed on the beta(2)-mu-associated and beta(2)-mu-free HLA class I heavy chains, respectively. Previously, we reported the identification of a cyclic and a linear peptide that inhibits mAb TP25.99 binding to the beta(2)-mu-associated and beta(2)-mu-free HLA class I heavy chains (S. A. Desai, X. Wang, E. J. Noronha, Q. Zhou, V. Rebmann, H. Grosse-Wilde, F. J. Moy, R. Powers, and S. Ferrone, submitted for publication). The linear X(19) and cyclic LX-8 peptides contain sequence homologous to residues 239-242, 245, and 246 and to residues 194-198, respectively, of HLA class I heavy chain alpha(3) domain. Analysis by two-dimensional transfer nuclear Overhauser effect spectroscopy of the induced solution structures of the linear X(19) and cyclic LX-8 peptides in the presence of mAb TP25.99 showed that the two peptides adopt a similar structural motif despite the lack of sequence homology. The backbone fold is suggestive of a short helical segment followed by a tight turn, reminiscent of the determinant loop region (residues 194-198) on beta(2)-mu-associated HLA class I heavy chains. The structural similarity between the linear X(19) and cyclic LX-8 peptides and the lack of sequence homology suggests that mAb TP25.99 predominantly recognizes a structural motif instead of a consensus sequence.

Soluble CD8 stabilizes the HLA class I molecule by promoting beta2M exchange: analysis in real-time

Human soluble CD8 (sCD8) is secreted by activated CD8+/- cytotoxic T lymphocytes (CTLs). The immunological role of sCD8 is poorly defined, however. We have studied the influence of sCD8 on HLA class I interactions by real-time analysis. Using an optical biosensor we demonstrated that the binding of sCD8 to HLA-A2 promotes exchange of beta2-microglobulin (beta2m) in order to stabilize the complex. Kinetic analysis showed that sCD8 significantly increased the affinity (K(A)) of HLA-A2 for immobilized human beta2m; from 1.14 +/- 0.04 x 10(9) M(-1) in its absence, to 2.18 +/- 0.21 x 10(9) M(-1) following preincubation with sCD8. This suggests that the sCD8:HLA class I complex is unlikely to be degraded at the cell surface. Even in the presence of exogenous peptide (HLA-A2 specific or nonspecific), sCD8 has a stabilizing influence on the HLA class I molecule. These findings point to an immunosuppressive role for sCD8, because the binding of sCD8 to HLA class I would block the binding site for CTL-bound CD8 and, therefore, interfere with T cell activation and proliferation. This may have particular significance in pathological situations where elevated levels of sCD8 are found in extracellular fluids, and sCD8 may provide an alternative approach for immunosuppressive therapy.

The metalloproteinase-mediated pathway is essential for generation of soluble HLA class I proteins by activated cells in vitro: proposed mechanism for soluble HLA release in transplant rejection

We and others have found donor-derived soluble beta2m-associated HLA class I proteins (sHLA/beta2m) in the serum of allograft recipients with acute and chronic rejection. Whether appearance of sHLA/beta2m and upregulated expression of donor cell-bound HLA/beta2m during allograft rejection are related events is unknown. Activation-induced upregulation of in vitro HLA/beta2m expression correlates with the surface expression of another form of HLA class I, namely beta2m-free HLA heavy chains (beta2m-free HC). We have shown that beta2m-free HC, but not beta2m-associated HC, are then cleaved by a specific membrane-bound metalloproteinase and released into supernatants as soluble 36 kDa proteins. We show now that activated peripheral blood lymphocytes produce predominantly the 36 kDa form of sHLA proteins which is present in supernatants as both beta2m-free HC and sHLA/beta2m. Importantly, the metalloprotease inhibitor BB-94 blocked not only the release of soluble beta2m-free HC, but also the appearance of sHLA/beta2m in cell supernatants. Low levels of 36 kDa beta2m-free HC were also present in human plasma of healthy donors. These data suggest an important role for the HLA class I-specific metalloproteinase in vivo in healthy individuals and during allograft rejection in the generation of soluble beta2m-free and beta2m-associated HLA proteins.

Calreticulin associates with non-HLA-A,-B class I proteins in the human choriocarcinoma cell lines JEG-3 and BeWo

Human placental trophoblast expresses as unusual repertoire of major histocompatibility complex (MHC) class I products that appears to reflect the unique role of this epithelium in mediating feto-maternal relations during pregnancy. Trophoblast is devoid of human leucocyte antigen (HLA)-A,-B antigens but can express one or more non-HLA-A,-B class I proteins. The human choriocarcinoma cell lines JEG-3, BeWo and JAR are widely used as models to study trophoblast. During attempts to isolate non-HLA-A,-B class I from JEG-3 and BeWo by immunoaffinity chromatography using a monoclonal antibody to beta 2-microglobulin we observed a 55,000 MW protein co-purifying with class I. N-terminal amino acid sequencing and immunoblotting using a specific antiserum identified this product as calreticulin, a molecule recently shown to be involved in the assembly of classical class I in human B-lymphoblastoid cells. In our hands JEG-3 and BeWo were found to express 45,000 MW non-HLA-A,-B class I proteins while the 40,000 MW HLA-G product was identified only in JEG-3. Our data suggest that calreticulin associates with non-HLA-A,-B class I heterodimers and with free 45,000 MW non-HLA-A,-B class I H chains in JEG-3. JAR was found to be devoid of detectable class I H chains but contained beta 2-microglobulin and calreticulin. However, calreticulin-beta 2-microglobulin complexes were not detected in JAR. Calreticulin and class I were apparently co-localized within the endoplasmic reticulum of JEG-3 cells whereas only class I was expressed at the cell surface. These studies demonstrate that calreticulin is associated with non-HLA-A,-B class I products in human choriocarcinoma cells.

Production of soluble HLA-class-I molecules by IFN-gamma-induced colon-adenocarcinoma cells

High levels of soluble HLA-class-I molecules (sHLA) were found to be produced in a time- and dose-dependent manner by colon-adenocarcinoma Colo205 cells in response to IFN-gamma stimulation. Among other cytokines tested, only IL-6, TNF, IFN-alpha and IFN-beta showed weak inducibility. IFN-gamma-induced production of sHLA was synergistically enhanced by IL-1 alpha, IL-6 or TNF, and combined treatment with TNF and IL-6 exhibited an additive to synergistic induction. Expression of sHLA is unlikely to result from IFN-gamma-induced enhancement of overall HLA-class-I expression, as at low concentrations IFN-alpha, IFN-beta and IFN-gamma stimulated increased expression of cell membrane HLA-class-I molecules in Colo205 cells with almost equal efficiency, whereas only IFN-gamma induced high level production of sHLA. Immunoblotting with a monoclonal antibody recognizing beta 2-microglobulin-free HLA-class-I heavy chain revealed 3 major forms of sHLA heavy chain, i.e., 45/43-, 37- and 33-kDa molecules, in the culture supernatants of IFN-gamma-induced Colo205 cells. The 45/43-kDa proteins can be partitioned into Triton X-114, representing intact HLA-class-I heavy chains shed from the cell membrane. The hydrophilic 37- and 33-kDa heavy chain, which remained almost exclusively in the aqueous phase after extraction with Triton X-114, could well be due to alternative RNA splicing, with deletion of exon 5 encoding the hydrophobic transmembrane region of membrane-anchored HLA-class-I heavy chain.

Species-specific differences in chaperone interaction of human and mouse major histocompatibility complex class I molecules

Previous studies have shown that immature mouse class I molecules transiently associate with a resident endoplasmic reticulum protein of 88 kD that has been proposed to act as a chaperone for class I assembly. Subsequently, this protein was demonstrated to be identical to calnexin and to associate with immature forms of the T cell receptor complex, immunoglobulin, and human class I HLA heavy chains. In this paper we define further the interaction of human class I HLA heavy chains with chaperone proteins and find key differences with the complexes observed in the mouse system. First, calnexin and immunoglobulin binding protein (BiP) both associate with immature HLA class I heavy chains. The two chaperones are not found within the same molecular complex, suggesting that calnexin and BiP do not interact simultaneously with the same HLA class I heavy chain. Second, only free HLA class I heavy chains, and not beta 2-microglobulin (beta 2m)-associated heavy chains are found associated with the chaperones. Indeed, addition of free beta 2m in vitro induces dissociation of chaperone-class I HLA heavy chain complexes. The kinetics for dissociation of the class I HLA heavy chain-chaperone complexes and for formation of the class I HLA heavy chain-beta 2m complex display a reciprocity that suggests the interactions with chaperone and beta 2m are mutually exclusive. Mouse class I heavy chains expressed in human cells exhibit the mouse pattern of interaction with human chaperones and human beta 2m and not the human pattern, showing the difference in behavior is purely a function of the class I heavy chain sequence.

CD8 and beta 2-microglobulin-free MHC class I molecules in T cell immunoregulation

Intracellular assembly of MHC class I heavy chains with beta 2-microglobulin occurs prior to the expression of the antigen-presenting complex on the cell surface. The association of beta 2-microglobulin with newly synthesized class I heavy chains is thought to be a strict prerequisite for their transport to the cell surface. However, MHC class I molecules not associated with beta 2-microglobulin (beta 2-microglobulin-free class I heavy chains) have been detected on the surface of activated lymphoid cells. These molecules have different conformations. Therefore, their interactions with other membrane proteins and biological functions may be different from those assigned to beta 2-microglobulin-associated MHC class I molecules. The two forms of MHC class I molecules on the surface of activated cells can self-associate and also form complexes with distinct proteins. Upon interaction with the appropriate ligands these molecular complexes transduce signals regulating cell activation. The ligand for beta 2-microglobulin-free class I heavy chains appears to be soluble CD8. A model is presented describing a novel mechanism of immunoregulation mediated by both soluble and membrane-bound forms of CD8 and beta 2-microglobulin-free class I heavy chains.

Herpes simplex virus infection of human fibroblasts and keratinocytes inhibits recognition by cloned CD8+ cytotoxic T lymphocytes

CD8+ cytotoxic T lymphocytes (CTL) clones with specificity for herpes simplex virus (HSV) were derived from two donors with genital HSV-2 infection. These CTL clones specifically lysed HSV-infected autologous B lymphoblastoid cells, but not HSV-infected fibroblasts. Exogenous peptide loading sensitized both cell types to lysis by an HSV-specific CTL clone of known specificity. HSV infection rendered fibroblasts refractory to peptide sensitization. HSV infection also rendered fibroblasts and keratinocytes insensitive to lysis by allospecific CD8+ CTL clones. Lysis of B lymphoblastoid cells in this system was only slightly reduced by HSV infection. Reduction of fibroblast allospecific lysis was dose and time dependent and was blocked by acyclovir, indicating the involvement of a late HSV gene product. HSV caused a reduction of fibroblast cell surface HLA class I antigen, at least in part due to reduction of synthesis of heavy chain-beta 2 microglobulin heterodimers. These results suggest that HSV-induced blockade of antigen presentation by cutaneous cells to CD8+ CTL may be a mechanism by which HSV limits or evades the immune response of the host.

Impaired intracellular transport and cell surface expression of nonpolymorphic HLA-E: evidence for inefficient peptide binding

The assembly of the classical, polymorphic major histocompatibility complex class I molecules in the endoplasmic reticulum requires the presence of peptide ligands and beta 2-microglobulin (beta 2m). Formation of this trimolecular complex is a prerequisite for efficient transport to the cell surface, where presented peptides are scanned by T lymphocytes. The function of the other class I molecules is in dispute. The human, nonclassical class I gene, HLA-E, was found to be ubiquitously transcribed, whereas cell surface expression was difficult to detect upon transfection. Pulse chase experiments revealed that the HLA-E heavy chain in transfectants, obtained with the murine myeloma cell line P3X63-Ag8.653 (X63), displays a significant reduction in oligosaccharide maturation and intracellular transport compared with HLA-B27 in corresponding transfectants. The accordingly low HLA-E cell surface expression could be significantly enhanced by either reducing the culture temperature or by supplementing the medium with human beta 2m, suggesting inefficient binding of endogenous peptides to HLA-E. To analyze whether HLA-E binds peptides and to identify the corresponding ligands, fractions of acid-extracted material from HLA-E/X63 transfectants were separated by reverse phase HPLC and were tested for their ability to enhance HLA-E cell surface expression. Two fractions specifically increased the HLA class I expression on the HLA-E transfectant clone.

The origin and fate of beta 2m-free MHC class I molecules induced on activated T cells

We report here that the expression of major histocompatibility complex (MHC) class I heavy chains not associated with beta 2-microglobulin is induced on resting human T cells by a variety of stimuli. These beta 2m-free class I heavy chains are not transported as such from the endoplasmic reticulum but originate from surface beta 2m-associated MHC class I molecules. beta 2m-free class I heavy chains are spontaneously released from the surface of activated cells. Cross-linking of beta 2m-free class I heavy chains with specific monoclonal antibodies results in the rapid down-regulation and internalization of these molecules. In contrast, beta 2m-associated MHC class I molecules display a different pattern of modulation. Previously, we reported that beta 2m-free class I heavy chains interact with CD8 molecules expressed on the same activated T cells. We propose that interactions between these molecules are involved in a mechanism regulating the function of activated T cells.

T-cell activation. IV. Evidence for a functional linkage between MHC class I, interleukin-2 receptor, and interleukin-4 receptor molecules

The aim of the present work was to study regulatory interactions between MHC class I molecules and the interleukin (IL)-2, IL-3, and IL-4 receptors and functional interactions between the receptors for IL-2 and IL-4. Our major observations were: (1) quiescent splenic T cells exposed to specific anti-MHC class I antibodies become responsive to IL-2 and IL-4 stimulation; (2) T-cell clones (CTLL-2 and HT-1) grown at high cell density or low IL-2 concentrations become refractory to IL-2 and IL-4 stimulation. After exposure to anti-class I antibodies the refractory cells recover responsiveness to lymphokine-induced proliferation; (3) IL-2 receptor expression is non-inducible in class I-negative T-lymphoma cells, but is inducible following class I gene transfection of the cells; (4) exposure of T-cells and clones to IL-2 receptor antibody increases the responsiveness to IL-4 stimulation; (5) IL-2 and IL-4 act synergistically at low and substimulatory lymphokine levels; and (6) IL-3 responsiveness of hemopoietic cells is not influenced by exposure to anti-MHC class I antibody. It is concluded that class I molecules are of importance for the functional expression of the receptors for IL-2 and IL-4 and that these receptors are functionally interrelated.

Involvement of major histocompatibility complex class I antigens in T cell activation

During the last few years ample evidence has been collected indicating a regulatory role for major histocompatibility complex class I antigens (Ag) in T cell activation. However, due to differential effects (stimulatory and inhibitory) of anti-class I antibodies (Ab) observed under different conditions, no coherent scheme of the mechanism of action of these Ag has emerged. Here, we present evidence that the mode of action of anti-class I Ab depends upon the presence or absence of monocytes/macrophages (M phi) in the culture. The Ab inhibit Ag presentation by binding to M phi. Coating of tetanus toxin -pulsed M phi with anti-class I Ab is sufficient to suppress T cell activation. On the contrary, these Ab enhance lectin- as well as phorbol ester-induced T cell activation in the absence of M phi. Cross-linking of class I Ag on T cell surface mobilizes cytoplasmic Ca2+, and also enhances the CD3-induced Ca2+ flux inside the cells indicating a functional relationship between CD3 and class I Ag. Though surface modulation and immunoprecipitation experiments do not indicate any physical association between these two types of molecules on the T cell surface, capping studies show that cross-linking of class I Ag induces an association of these Ag with CD3. Binding of anti-CD3 Ab enhances the strength of association between CD3 and class I Ag, and the former co-caps completely with the latter. Based on these observations we propose that during antigen presentation M phi (or Ag-presenting cells) and T cells, besides interacting via peptide--class II Ag/CD3--T cell receptor complex formation, also interact through class I Ag. The latter interaction may stabilize the contact formation between T cells and Ag-presenting cell and support T cell activation.

Activation of human CD8-positive T cells via the CD8/HLA class I complex

Cross-linking of CD8 and HLA class I molecules with appropriate monoclonal antibodies (mAb) and goat anti-mouse Ig (GaMIg) antibody resulted in a marked proliferation of resting human CD8 cells in the presence of interleukin-2 (IL-2). These cells also expressed IL-2 receptor (IL-2R), transferrin receptor, HLA-DR and -DQ antigens. Activation of the cross-linked CD8 cells is apparently independent of accessory monocytes. Various anti-CD8 and anti-HLA class I mAb recognizing nonpolymorphic antigenic determinants were examined for the efficacy of activating CD8 cells. Among mAb specific for HLA class I molecules, PA2.6, MB40.5, BB7.7, A1.4, and W6/32 mAb markedly stimulated the proliferation of cross-linked CD8 cells, whereas BBM.1, Q1/28, and HC10 mAb were found inactive. Footprinting analysis of HLA class I molecules suggested that the activity of these anti-HLA class I mAb appeared to be related to the corresponding peptides they protect from enzymatic digestion. In contrast to the anti-HLA class I mAb, all anti-CD8 mAb examined (C8, OKT8A, and anti-Leu-2a) induced the proliferation of CD8-HLA class I cross-linked cells with similar efficacy. These results suggest that physical interaction between CD8 and at least one specific region of HLA class I molecules can trigger the activation of resting human CD8 cells.

T cell activation. II. Activation of human T lymphoma cells by cross-linking of their MHC class I antigens

The present work demonstrates that antibody-induced cross-linking of MHC class I antigens on Jurkat T lymphoma cells leads to a rise in intracellular calcium (Cai2+) and, in the presence of phorbol ester (PMA), to IL-2 production and IL-2 receptor expression. The rise in Cai2+ exhibited a profile very different from that obtained after anti-CD3 antibody-induced activation suggesting that activation signals are transduced differently after binding of anti-CD3 antibody and class I cross-linking, respectively. However, when Cai2+ was examined in individual Jurkat cells by means of a digital image processing system no differences were observed after cross-linking with anti-CD3 and anti-MHC class I antibodies, respectively. Two CD3-negative mutant lymphoma lines were nearly totally refractory to class I cross-linking. Taken together our results may indicate the existence of a functional linkage between the T cell receptor complex and MHC class I molecules.

Lack of a role of monocytes in the inhibition by monoclonal antibodies to monomorphic and polymorphic determinants of HLA class I antigens of PHA-P-induced peripheral blood mononuclear cell proliferation

This study aimed at characterizing the mechanism(s) underlying the regulatory role of distinct determinants of HLA Class I antigens in PHA-P-induced T cell proliferation and the involvement of monocytes in this phenomenon. The anti-HLA-A2,A28 monoclonal antibodies (MoAb) CR11-351, the MoAb Q6/64 to a determinant restricted to the gene products of the I antigens HLA-B locus, and the MoAb CR10-215 and W6/32 to distinct monomorphic determinants of HLA Class I antigens were found to inhibit PHA-P-induced peripheral blood mononuclear cell (PBMC) proliferation in a dose-dependent fashion. The inhibition is specific and reflects neither inhibition of PHA-P binding to cells nor a toxic effect of the anti-HLA Class I MoAb. The latter differed in the concentration required to induce inhibition, in the influence of the concentration of PHA-P used as mitogen, in the differential effect on the donors used as a source of PBMC, and/or in the requirement of the Fc portion to induce inhibition. At variance with the information in the literature, the inhibitory effect of anti-HLA Class I MoAb on PHA-P-induced PBMC proliferation neither reflected their interaction with accessory cells nor was mediated by suppressor factors released by monocytes stimulated with PHA-P in the presence of anti-HLA Class I MoAb. Therefore, the regulatory role of HLA Class I antigens in T cell proliferation is not likely to be mediated by monocytes and/or factors released from them, but may reflect an involvement of these molecules in T cell activation pathways.

The required interaction between monocytes and peripheral blood T lymphocytes (T-PBL) upon activation via CD2 or CD3. Role of HLA class I molecules from accessory cells and the differential response of T-PBL subsets

Previously, we have shown that paraformaldehyde-fixed monocytes are able to fully complement, in terms of [3H]dThd incorporation, a primary stimulus delivered to purified T cells by monoclonal antibodies (mAb) reacting with CD3 or CD2 molecules. Here, we show that depending on the stimulus used (CD3 mAb or different pairs of CD2 mAb) HLA class I molecules from monocytes are directly involved in complementary signals provided to T cells. This was evidenced by the following observations: (a) mAb reacting with the heavy or light chain of class I molecules, or their Fab fragments, completely blocked proliferation of peripheral blood lymphocytes (PBL) activated by CD3 mAb; (b) mAb against the heavy chain of HLA class I but not against beta 2-microglobulin partially blocked (approximately equal to 50%) PBL activation by the CD2 "GT2 + T111" mAb pair but did not block activation by CD2 "D66 + T111" mAb; (c) this pattern of inhibition was observed when anti-class I mAb were used in the soluble phase or when they were bound to monocytes subsequently fixed with paraformaldehyde and cultivated with purified autologous T cells; (d) fixed monocytes are able to restore interleukin (IL) 2 receptor expression on purified T cells stimulated by CD3 mAb or CD2 "GT2 + T111", contrary to anti-HLA class I mAb-pretreated monocytes. The inhibitory effects of anti-HLA class I mAb bound to monocytes were not found to be reversed by recombinant IL2 or recombinant IL1. We assume that HLA class I would be involved in two or more signals delivered to T cells by monocytes, the requirement in those signals depending on the initial stimulus applied to T cells.(ABSTRACT TRUNCATED AT 250 WORDS)

HLA class I molecules are associated with CD1a heavy chains on normal human thymus cells

The molecules encoded by the major histocompatibility complex play a pivotal role in regulatory interactions between cells of the immune system, which can result in the activation and function of T cells. The function of the CD1 molecules, which are homologous to the major histocompatibility complex-encoded molecules but are encoded on human chromosome 1, is not known. HLA class I molecules and CD1a heavy chains share the ability to associate with several different cell-surface molecules. We show here, by several technical approaches, that HLA class I molecules are associated with CD1a heavy chains on the surface of normal thymus cells. The functional significance of this association during T-cell differentiation is discussed.

Physical association between the CD8 and HLA class I molecules on the surface of activated human T lymphocytes

Immune recognition by cytotoxic effector T cells requires participation of the CD8 and major histocompatibility complex class I antigens. We found that the CD8 molecule is noncovalently associated with the HLA class I heavy chain on the surface of human T cells activated by Con A. Accordingly, anti-CD8 monoclonal antibodies precipitated a heterodimer containing polypeptides of 32 and 43 kDa from the lysates of activated T cells. The 43-kDa chain of this heterodimer can be adsorbed from cell lysates with anti-HLA-A, -B, and -C antibodies. Endoglycosidase F treatment and chymotryptic peptide mapping identified a structural similarity between this 43-kDa molecule and the HLA class I heavy chain precipitated by the anti-HLA-A, -B, and -C antibody W6/32. Analysis of anti-CD8 precipitates under nonreducing and reducing conditions indicated a lack of interchain disulfide bonding between the CD8 and HLA heavy chain molecules. The CD8-HLA heavy chain complex was also detected in mixed lymphocyte cultures and a cloned cytotoxic T-lymphocyte line but not in purified natural killer cells. The present study indicates that CD8 is complexed with HLA heavy chain on the same cells, and the complex may have functional relevance in the T-cell recognition process.

Intermolecular complexes between three human CD1 molecules on normal thymus cells

The first cluster of differentiation (CD1) defines at least three distinct human thymic cell-surface differentiation antigens-CD1a, CD1b, and CD1c. We looked for structural homology of the three CD1 heavy chains at their peptide level by two-dimensional peptide maps. We show here that the CD1a Mr 49,000 heavy chain and the CD1b Mr 45,000 heavy chain appear to be more homologous to each other than to the CD1c Mr 43,000 heavy chain and that only one tyrosil peptide is common to the three heavy chains. Study of the CD1 heavy chains from several individuals reveals a very limited polymorphism of these molecules. We also demonstrate here that CD1a or CD1a-like molecules and other CD1 molecules can form intermolecular complexes on the surface of normal thymus cells. Molecules that are structurally very similar to CD1a molecules are associated noncovalently either with CD1c molecules or with CD1b molecules, and only CD1a molecules can associate covalently with CD8 molecules. In contrast, we could not find these intermolecular complexes on the surface of leukemic T-cell lines in culture.

An in vivo functional immune system lacking polyclonal T-cell surface expression of the CD3/Ti(WT31) complex

A polyclonal T-cell receptor complex (TCR) expression defect (as detected with monoclonal antibody WT31) has been found in two children belonging to an otherwise healthy Spanish family. One of the sibs (V, who had been vaccinated with attenuated poliomyelitis virus) showed clinical signs of immunodeficiency with an autoimmune syndrome, but the other (older) sib (D, vaccinated with attenuated rubella, measles, mumps, and poliomyelitis viruses) has been symptomless throughout life. In contrast to both sibs' normal expression of other peripheral leucocyte markers, as measured by flow cytometry (including CD1, CD2, CD4, CD8, and CD16), only about 6% of CD2+ polyclonal T cells expressed surface antigen-specific T-cell receptor (Ti/WT31), and only about 23% weakly expressed surface CD3 determinants. On the remaining CD2+ T cells in each sib the expression of Ti and CD3 was undetectable; the defect in CD3 expression is very likely secondary to the defect in Ti expression. Natural killer (NK) activity was not increased in any of the sibs, ruling out a high content of NK cells among their CD2+ lymphocytes. Functional data indicate that CD3-mediated T-cell activation with anti-CD3 monoclonals and Ti-mediated responses to allogeneic and tetanus toxoid antigens were severely depressed, whereas activation via CD2 was normal in the T lymphocytes of both sibs. Genes encoding for Ti alpha, beta, and gamma chains did not show major alterations by southern blot analysis, and polyclonal beta chain genes rearrangements were detected in both children's T-cell blasts. Family clustering suggests a genetic pathogenesis, but linkage to HLA or other blood group markers has not been found. Sib V had a concomitant autoimmune disease and died after a severe autoimmune haemolytic anaemia, indicating a relationship between the TCR and generation of autoimmune clones. However, the resistance of both individuals to infection and to vaccination with attenuated viruses, and the fact that sib D has been symptomless to date questions the relative importance of the TCR in the immune response against infection, and suggests that alternative T-cell activation pathways and non-specific defence mechanisms (external surfaces--bound and/or cellular) may suffice under certain circumstances.

Role in T-cell activation for HLA class I molecules from accessory cells: further distinction between activation signals delivered to T cells via CD2 and CD3 molecules

The immunological function of major histocompatibility complex molecules, including HLA class I molecules, is to present antigens and/or their processed peptides to various lymphocyte subpopulations. Thus, they play a pivotal role in regulatory interactions between cells of the immune system, which can result in the activation and function of T cells. We looked for a role of major histocompatibility complex molecules during T-cell activation induced by monoclonal antibody (mAb) or combinations of mAb recognizing the two well-characterized T-cell surface molecules CD3 and CD2. To activate T-cell peripheral blood lymphocytes, we used a CD3 mAb or two different pairs of CD2 mAb, CD2 "GT2 + T11(1)" and CD2 "D66 + T11(1)," which, as we have previously shown, deliver different signals of activation to T cells. Anti-HLA class I mAb blocked the activation induced by CD3 mAb or by CD2 GT2 + T11(1), but it did not block activation induced by CD2 D66 + T11(1). We observed this pattern of inhibition according to the stimulus used to activate T cells both when the anti-HLA class I mAbs were added to cultures of whole peripheral blood mononuclear cells and when they were fixed to monocytes only. In the latter case, purified monocytes were first incubated with the anti-HLA mAb (whether whole immunoglobulin or Fab fragment) and then fixed with paraformaldehyde before culture with autologous purified T cells. Anti-HLA class I fixed on monocytes prevented both interleukin 2 (IL-2) receptor expression and IL-2 synthesis on T cells. The inhibitory effects of anti-class I mAb bound to monocytes were not reversed by adding large amounts of recombinant IL-2 or recombinant IL-1, a finding consistent with the observations that accessory cells surface components can fully complement the signals directly delivered to T cells by CD2 or CD3 mAb. We conclude that HLA class I from accessory cells plays an important role in the early phase of T-cell activation when direct contacts between accessory cells and T cells are required.

Structure, expression, and molecular mapping of a divergent member of the class I HLA gene family

A class I gene distinct from HLA-A, -B, or -C was identified in a cosmid clone and transfected into mouse L cells. The gene, placed adjacent to the polyoma enhancer, produced a full-length class I mRNA and high levels of a 43-kDa protein in the cytoplasm. The surface expression of the gene product required its association with human beta 2-microglobulin. The protein was recognized by a xenoantiserum raised against a mixture of human B- and T-cell lines. The product was also serologically reactive with the HLA framework monoclonal antibodies. The complete nucleotide sequence of the gene was determined and a specific oligonucleotide probe was synthesized. This probe was used to identify a full-length mRNA transcript in a B-lymphoblastoid cell line (JY). The gene was mapped within a 190-kilobase Not I restriction fragment located in the telomeric portion of the human major histocompatibility complex. Distinct features of the gene include the structure of the promoter, the position of the translation initiation site, a frameshift mutation at the carboxyl terminus, the insertion of an Alu repeat element in the eighth exon, divergence in the derived amino acid sequence, and the lack of expression of the gene in some cells.

Two forms of the T-cell receptor gamma protein found on peripheral blood cytotoxic T lymphocytes

The T-cell receptor (TCR) gamma polypeptide is expressed associated with CD3 (T3) on the surface of normal human peripheral blood lymphocytes. These cells function as non-MHC-restricted cytotoxic T lymphocytes (CTL)and thus may play an important role in host immune defence. The TCR gamma polypeptide occurs as a dimer in at least two molecular forms based on the absence or presence of disulphide linkage. These forms use TCR gamma polypeptides with strikingly different peptide backbone sizes.

Characterization of T cell receptor gamma chain expression in a subset of murine thymocytes

While much information exists about the structure and function of the clonally distributed T cell receptor (TCR) alpha beta heterodimer, little is known about the gamma protein, the product of a third rearranging TCR gene. An antiserum to a carboxyl-terminal peptide common to several of the murine gamma chain constant regions and a monoclonal antibody to the murine T3 complex were used to identify products of this TCR gene family in a subpopulation of Lyt2-, L3T4- thymocytes. This subpopulation does not express TCR alpha or full-length TCR beta messenger RNA. The gamma chain is a 35-kilodalton (kD) protein that is disulfide-bonded to a 45-kD partner and is associated with the T3 complex. Analysis of the glycosylation pattern of this thymic gamma chain revealed that the major variable region gamma (V gamma) gene transcribed in activated peripheral T cells is absent from this subpopulation. The cells that bear this second T cell receptor may therefore represent a distinct lineage differentiating within the thymus.