Inhibition of an allospecific T cell hybridoma by soluble class I proteins and peptides: estimation of the affinity of a T cell receptor for MHC

To investigate the molecular basis of the interaction between the T cell receptor and the MHC class I antigen in an allogeneic response, a soluble counterpart of the murine class I molecule, H-2Kb, was genetically engineered. Cells secreting this soluble molecule, H-2Kb/Q10b, inhibited stimulation of an H-2Kb-reactive T cell hybridoma by cells transfected with H-2Kbm10, a weak stimulus, but not by H-2Kb- or H-2Kbm6-transfected cells. Soluble purified H-2Kb/Q10b protein also blocked T cell stimulation. In addition, a peptide from the wild-type H-2Kb molecule spanning the region of the bm10 mutation specifically inhibited activation of the T cell hybridoma by H-2Kbm10 cells, thus suggesting that amino acid residues 163-174 of H-2Kb define a region important for T cell receptor binding. An estimate for the Kd of the T cell receptor for soluble H-2Kb/Q10b was 10(-7) M, while the Kd for soluble peptide 163-174 was 10(-4) M.     

Beta-chain broadens range of CD8 recognition for MHC class I molecule.

It is known that the alpha-chain of CD8 binds to a negatively charged loop composed of residues 223 to 229 on MHC class I Ag and that binding of CD8 alpha enhances Ag recognition of T cells. We have recently shown that the mouse CD8 alpha homodimer does not bind to either the HLA class I alpha 3 domain or a mutant of H-2Kb Ag containing a substitution of glutamine for methionine at residue 224, which brings this residue toward the human consensus. Here we report a complementary study of the CD8 beta-chain. The functional role of the CD8 beta-chain was analyzed by using four T cell hybridoma lines expressing mouse CD8 alpha and transfected with the mouse CD8 beta gene. As compared with the lines expressing only CD8 alpha, allorecognition of the chimeric H-2Kb Ag that contains the HLA class I alpha 3 domain was enhanced in lines expressing both CD8 alpha and -beta. This enhancement was blocked by either anti-CD8 mAb or anti-HLA class I alpha 3 domain mAb. In addition, we show that CD8 alpha beta binds the H-2Kb mutant Ag at residue 224. These results suggest that the beta-chain allows the CD8 alpha beta heterodimer to recognize the chimeric H-2Kb Ag. A model for the role of the beta-chain is presented.     

Class II MHC molecules and the HIV gp 120 envelope protein interact with functionally distinct regions of the CD4 molecule.

A murine T cell hybridoma with a receptor specific for the class I molecule H-2 Dd was transfected with an expressible cDNA for human CD4. Expression of the human class II MHC molecule HLA-DP on Dd-positive murine fibroblasts resulted in a greatly enhanced response of the CD4-positive T cell hybridoma, measured either by lymphokine production or by rosette formation. Inhibition of these functional assays with anti-CD4 monoclonal antibodies implicated the two amino-terminal domains of CD4 in an interaction with the HLA-DP molecule. This interaction was blocked by incubation with recombinant gp120 envelope protein of HIV. In contrast, recombinant soluble CD4 did not inhibit and was able to prevent the inhibition by gp120. Anti-CD4 antibody blocking experiments clearly indicated that distinct regions of CD4 interact respectively with gp120 and with class II MHC molecules.     

Blockade of transgenic gamma delta T cell development in beta 2-microglobulin deficient mice.

The gamma delta T cell receptor (TCR) of the hybridoma KN6 recognizes the self molecule encoded by a class I gene which maps within the TL region of the major histocompatibility complex (MHC) of H-2b mice. Mice transgenic (Tg) for this TCR were crossed with mice genetically deficient in beta 2-microglobulin (beta 2m). No mature Tg gamma delta T cells were detected in the thymus or the spleen of the beta 2m- gamma delta Tg mice. We conclude that interaction between the Tg gamma delta TCR and a beta 2m-associated molecule (probably an MHC class I molecule) is required for the generation of mature Tg gamma delta T cells.     

Activation of human T4 cells by cross-linking class I MHC molecules

These studies examined whether cross-linking class I MHC molecules results in functional or biochemical responses in human T4 cells. The initial studies demonstrated that cross-linking class I MHC molecules either by culturing highly purified T4 cells with immobilized mAb to class I MHC Ag or reacting the T4 cells with mAb to class I MHC Ag and then cross-linking the mAb with goat antimouse Ig (GaMIg) enhanced T4 cell proliferation induced by an immobilized mAb to CD3, OKT3. More-over, immobilized but not soluble mAb to class I MHC Ag enhanced T4 cell proliferation induced by the combination of two mAb to CD2, OKT11, and D66.2. Finally, T4 cells reacted with mAb to CD3 and class I MHC Ag proliferated in the presence of IL-2 when cross-linked with GaMIg more vigorously than T4 cells reacted with either mAb alone. Cross-linking class I MHC molecules was also found to stimulate T4 cells directly. T4 cells reacted with mAb to class I MHC Ag or beta 2 microglobulin and cross-linked with GaMIg proliferated vigorously in the presence of IL-2 or PMA. In addition, it was demonstrated that cross-linking class I MHC molecules by culturing T4 cells with immobilized mAb to class I MHC Ag induced T4 cell proliferation in the presence of IL-2. T4 cell proliferation in the presence of IL-2 and PMA could also be induced by reacting the cells with specific mAb to polymorphic determinants on class I MHC molecules and cross-linking with GaMIg. Cross-linking mAb to CD4 or CD11a did not have a similar functional effect on T4 cells. Finally it was demonstrated that adding GaMIg to T4 cells reacted with mAb to class I MHC Ag but not CD11a resulted in an increase in intracellular calcium concentration. The data demonstrate that cross-linking class I MHC molecules results in the generation of at least one activation signal, a rise in intracellular calcium concentration, and, thereby, stimulates human T4 cells.     

Efficiency of antigen presentation after antigen targeting to surface IgD, IgM, MHC, Fc gamma RII, and B220 molecules on murine splenic B cells

Bispecific heteroconjugate antibodies can bind soluble protein Ag to APC and thereby enhance Ag presentation. We used such antibodies to bind hen egg lysozyme (HEL) to various structures on the surface of normal splenic B cells to determine which structures would provide the best targets for enhanced presentation. We found that HEL was presented efficiently to hybridoma T cells if bound to sIgD, sIgM, or class I or II MHC molecules, but not at all if bound to Fc gamma RII, or B220 molecules on B cells. The efficiency of presentation of HEL was measured as a function of the amount of 125I-HEL bound per cell. HEL was presented with 5 to 10 times greater efficiency when bound to sIg, than when bound to MHC molecules. When compared on the basis of the amount of HEL bound, sIgD and sIgM functioned equally as target structures, as did class I and class II MHC molecules. Large amounts of HEL bound to B220, but no presentation resulted, indicating that focusing HEL to the APC surface was not sufficient for presentation to occur. HEL was internalized rapidly and in large amounts when bound to sIgD or sIgM, but slowly and in small amounts, when bound to class I or class II MHC molecules. Thus, a rapid rate of internalization may in part explain the high efficiency of Ag presentation after binding to sIg. However, the small amount of HEL internalized via MHC molecules was utilized efficiently for presentation. These results indicate that sIgM and sIgD serve equally on normal B cells to focus and internalize Ag and enhance Ag presentation, but that class I or class II MHC molecules can also be used to internalize Ag and enhance Ag presentation, perhaps by a separate intracellular processing pathway.     

H-2Kb antigen expression has no effect on natural killer susceptibility and tumorigenicity of a murine hepatoma

Recent reports suggested a correlation between decreased expression of tumor cell MHC class I Ag and increased susceptibility to NK cells. These studies led to the hypothesis that tumor cells displaying reduced levels of MHC class I Ag have reduced tumorigenicity in vivo because they are eliminated from the host by endogenous NK cells. The present studies use the murine hepatoma BW7756 and a spontaneous H-2Kb loss variant, Hepa-1, to test this hypothesis. The parental BW7756 tumor is highly malignant in syngeneic C57L/J hosts while Hepa-1 cells do not give rise to tumors, suggesting that the loss of H-2Kb Ag expression correlates with decreased tumorigenicity and NK susceptibility. Hepa-1 cells were therefore transfected with an H-2Kb gene to generate H-2Kb Ag expressing clones. The resulting clones were tested for tumorigenicity. Syngeneic or NK-deficient C57BL/6-beige/beige mice challenged with Hepa-1 or the H-2Kb transfectants rejected the cells, suggesting that reexpression of H-2Kb Ag does not restore tumorigenicity and that NK cells are not involved in Hepa-1 rejection. In vitro H-2Kb Ag-negative and -positive Hepa-1 cells are equally susceptible to tilorone-boosted NK cells, indicating that MHC class I Ag expression also does not affect in vitro NK susceptibility. Tumor challenged athymic nude and sublethally irradiated syngeneic mice develop tumors demonstrating that T cells are probably responsible for rejection of the Hepa-1 tumor, and that H-2Kb Ag expression has no effect on rejection. Inasmuch as the expression of H-2Kb Ag on Hepa-1 cells does not effect tumorigenicity or in vitro NK susceptibility, the previously reported association between reduced MHC class I Ag levels and increased NK susceptibility is not universally applicable.     

H-2 antigen expression and sensitivity of BL6 melanoma cells to natural killer cell cytotoxicity

The sensitivity of H-2b-high and H-2b-low variants of BL6 melanoma to the cytotoxic action of NK and lymphokine-activated killer cells was investigated. BL6 mouse melanoma cells lack detectable H-2Kb and had low levels of expression of H-2Db Ag. The BL6T2 variant cells, obtained after treatment of BL6 cells with mutagen N-methyl-N-nitro-N'-nitro-soguanidine, had relatively high levels of expression of class I H-2b Ag. Poly(I:C)-stimulated spleen cells of nude mice were highly cytotoxic for BL6T2, whereas H-2b-low BL6 cells were less sensitive to NK activity in an 18-h 51Cr-release assay. Similar results were obtained after 4-h incubation of radio-labeled tumor cells with IL-2-activated effector cells. In contrast, both lines were equally sensitive to lysis by purified granules derived from rat large granular lymphocytes (LGL) or by macrophages. By using various clones selected from BL6 or BL6T2 cells, it was found that BL6 or BL6T2 clones with low H-2b Ag expression were less sensitive to lysis by NK cells than H-2b-high clones. After IFN treatment of either BL6 or BL6T2, the target cells became more resistant to lysis by either NK cells or by purified LGL granules. IFN-treated BL6 cells had substantially increased expression of H-2b Ag and in this respect became similar to untreated BL6T2. However, IFN-treated BL6 cells were more resistant than BL6T2 cells to lysis by NK cells and LGL granules, suggesting that augmentation of H-2b Ag expression and NK resistance could be two independent IFN-induced effects. With a cold target inhibition assay, it was found that BL6T2 or its H-2 positive clones were highly competitive and inhibited the cytotoxic activity of NK and lymphokine-activated killer cells against radiolabeled YAC-1 and BL6T2, whereas BL6 cells or H-2-negative clones of BL6T2 and BL6 lines showed poor competitive ability. Thus, our data indicate that the NK resistance of H-2-low BL6 cells may be due to a paucity of NK recognizable determinants. N-Methyl-N-nitro-N'-nitroguanidine treatment of BL6 melanoma cells was associated with an increase in class I H-2b Ag expression and NK sensitivity, suggesting the involvement of class I MHC Ag in the sensitivity of tumor cells to NK cell-mediated cytotoxicity.     

Distinct roles for L3T4 in T cell activation

The mutually exclusive expression of L3T4 and Lyt-2 on murine T cells and the correlation of their expression to the major histocompatibility complex (MHC) restriction of the T cell antigen receptor (Ti) have led to the hypothesis that these surface molecules are related to recognition of class II and class I MHC antigens, respectively. It has been suggested that these T cell surface molecules interact with nonpolymorphic determinants on MHC antigens. We have studied the role of L3T4 in activation of an H-2Dd-specific T cell hybridoma. This novel hybridoma allowed the separate evaluation of the specificities of Ti and L3T4 and the examination of their roles in T cell activation. Antibody-blocking experiments have demonstrated that L3T4 was involved in triggering this T cell hybridoma only if the antigen-bearing cell expressed Ia. The apparent requirement for an L3T4-Ia interaction reflected the amount of available H-2Dd antigen. It appears that the L3T4-Ia interaction influences T cell activation during suboptimal antigenic stimulation. We have begun to examine the role of L3T4 in lectin and anti-Ti monoclonal antibody stimulation of the same T cell hybridoma. These experiments have suggested a distinct role for L3T4 in the absence of Ia, as a mediator of a negative signal for activation.     

Endocytosis and dissociation of class I MHC molecules labeled with fluorescent beta-2 microglobulin

Membrane class I MHC molecules of Con-A activated and lymphoma murine cells have been labeled by exchange of the cell's beta 2m with soluble fl-beta 2m. It has previously been shown that this method of labeling is specific and does not affect the biologic properties of class I MHC Ag. With this labeling it has been possible to demonstrate the constitutive endocytosis of class I MHC by fluorescence microscopy and by measuring the resistance to quenching by crystal violet of the internalized fl-beta 2m molecules. We could also follow the kinetics of beta 2m dissociation from the class I molecules at different pH. At pH 5.5, that is the average pH of endosomes, there is considerable dissociation within 15 to 20 min, that is the average recycling half time of class I MHC containing endosomes in activated T cells. Inasmuch as the process is reversible it is likely that, in the recycling endosomes of T cells, class I MHC molecules undergo conformational changes with beta 2m going off and on and with consequent changes of the peptide binding site. This process might be involved in Ag presentation, but, because it is apparently limited to T cells, it would play a role in the presentation of the cell's own TCR in idiotypic interactions between T cells.     

Some cloned murine CD4+ T cells recognize H-2Ld class I MHC determinants directly. Other cloned CD4+ T cells recognize H-2Ld class I MHC determinants in the context of class II MHC molecules.

Murine T lymphocytes recognize nominal Ag presented by class I or class II MHC molecules. Most CD8+ T cells recognize Ag presented in the context of class I molecules, whereas most CD4+ cells recognize Ag associated with class II molecules. However, it has been shown that a proportion of T cells recognizing class I alloantigens express CD4 surface molecules. Furthermore, CD4+ T cells are sufficient for the rejection of H-2Kbm10 and H-2Kbm11 class I disparate skin grafts. It has been suggested that the CD4 component of an anti-class I response can be ascribed to T cells recognizing class I determinants in the context of class II MHC products. To examine the specificity and effector functions of class I-specific HTL, CD4+ T cells were stimulated with APC that differed from them at a class I locus. Specifically, a MLC was prepared involving an allogeneic difference only at the Ld region. CD4+ clones were derived by limiting dilution of bulk MLC cells. Two clones have been studied in detail. The CD4+ clone 46.2 produced IL-2, IL-3, and IFN-gamma when stimulated with anti-CD3 mAb, whereas the CD4+ clone 93.1 secreted IL-4 in addition to IL-2, IL-3, and IFN-gamma. Cloned 46.2 cells recognized H-2Ld directly, whereas recognition of Ld by 93.1 apparently was restricted by class II MHC molecules. Furthermore, cytolysis by both clones 46.2 and 93.1 was inhibited by the anti-CD4 mAb GK1.5. These results demonstrate that CD4+ T cells can respond to a class I difference and that a proportion of CD4+ T cells can recognize class I MHC determinants directly as well as in the context of class II MHC molecules.     

Coreceptor function of CD4 in response to MHC class I molecule

Specificity of T cell receptor (TCR) and its interaction with coreceptor molecules play decisive role in successful passing of T lymphocytes via check-points during their development and finally determine the efficiency of adaptive immunity. Genes encoding alpha- and beta-chains of TCR hybridoma 1D1 have been cloned. The hybridoma 1D1 was established by the fusion of BWZ.36CD8alpha cell line with CD8+ memory cells specific to MHC class I H-2Kb molecule. Exploiting retroviral transduction of thymoma 4G4 cells with TCR genes and coreceptors CD4 and CD8, variants of this cell line expressing on the surface CD3/TCR complex and coreceptors, separately or simultaneously have been obtained. The main function of CD4 is stabilization of interaction between TCR and MHC class II molecule. Nevertheless, we have found that CD4 could successfully participate in the activation of transfectants via TCR specific to MHC class I molecule H-2Kb. Moreover, coreceptor CD4 dominates CDS, because the response of transfectants CD4+CD8+ is blocked by antibodies to CD4 and MHC Class II Ab molecule but not to coreceptor CD8. The response of CD4+ cells was not due to cross-reaction between TCR 1D1 with MHC class II molecules, because transfectants do not respond to splenocytes of H-2b knockouted mice with impaired assembly of TCR/beta2-microglobulin/peptide complexes resulting in their absence on the cell surphace. The effect of domination was not due to sequestration of kinase p56lck, because truncated CD4 with the loss of binding motif for p56lck remained functional in 4G4 cells. Results obtained can explain the number of features of intrathymic selection and represent experimental basis for development of new methods of cancer gene therapy.     

Binding of antigen-specific, H-2-restricted T cell hybridomas to antigen-pulsed adherent cell monolayers

Two methods were investigated for studying the binding of radiolabeled hybridoma T cells to antigen (Ag) and H-2 products for which they bore receptors. In both cases hybridoma T cells were labeled with tritiated thymidine. In one method labeled cells were added to adherent splenic cells prepulsed with antigen, and the mixture was incubated overnight at 37 degrees C before nonadherent cells were gently washed away. The percent of adherent hybridoma T cells was then estimated by harvesting the adherent monolayers and measuring tritium counts bound. In a second method radiolabeled hybridoma T cells were added to adherent antigen-pulsed B cell lymphomas or hybridomas for between 15 min and 1 hr at 37 degrees C before removal of nonadherent cells and harvesting of the adherent monolayers. In both cases binding was both antigen- and I-region specific. In the second case binding was also rapid; significant binding could be measured after 15 min incubation. These techniques were used to study subclones of one of our T cell hybridomas that were thought by a functional assay (interleukin 2 release) to have lost receptors for Ag/H-2. It was found that subclones of the hybridoma that no longer secreted interleukin 2 in response to Ag/H-2, even though they continued to secrete interleukin 2 in response to concanavalin A, also no longer bound specifically to Ag-pulsed monolayers of the appropriate H-2 type. This confirmed the idea that these subclones had lost the ability to synthesize receptors for Ag/H-2. It is hoped that assays of this type will be useful in the future for the study of Ag/H-2 receptors on T cells.     

Endocytosis of the class I major histocompatibility antigen via a phorbol myristate acetate-inducible pathway is a cell-specific phenomenon and requires the cytoplasmic domain

Class I major histocompatibility (MHC) antigens are expressed by virtually all mammalian cells, yet their levels of expression and behavior on the cell surface vary in a cell-specific fashion. A panel of lymphoid (both B and T) and nonlymphoid cell lines was used to study the kinetics of internalization of the H-2Ld class I MHC in different cell types. These studies revealed that endocytosis of H-2Ld occurs by both constitutive and PMA-regulated pathways in lymphoid cells, but only by a PMA-refractory pathway in the nonlymphoid cells tested. Transfectant derivatives of the T lymphoma, EL4, which express wild-type or mutant H-2Ld class I MHC antigens, were used to investigate the requirement for the cytoplasmic domain of the class I MHC antigen for its endocytosis in T lymphocytes. These studies showed that modification or deletion of the cytoplasmic domain of H-2Ld abrogates endocytosis via a PMA-regulated pathway. The role of cytoplasmic domain phosphorylation in PMA-inducible endocytosis was examined. The wild-type H-2Ld antigen is phosphorylated in all cell types examined, and this phosphorylation is up-regulated by PMA treatment. In contrast, cytoplasmic domain mutants of H-2Ld fail to be phosphorylated in vivo, in the presence or absence of PMA. The universality of PMA-inducible hyperphosphorylation of the class I MHC antigen among diverse cell types leads us to conclude that phosphorylation of the cytoplasmic domain, while perhaps necessary, is not sufficient for triggering endocytosis via a PMA-inducible pathway. Furthermore, the results with the cytoplasmic domain mutants of H-2Ld suggest that a structural conformation of the class I MHC cytoplasmic domain is required for endocytosis via this route.     

Weak base amines can inhibit class I MHC-restricted antigen presentation

This report describes the effects of NH4Cl, CH3NH2, and chloroquine on class I and II MHC-restricted Ag presentation. OVA-specific T-T hybridomas were used to detect processed OVA in association with class I, H-2Kb, and class II, I-Ad/b, molecules on a B lymphoblastoid APC. OVA, internalized by APC under hypertonic conditions, was presented in association with class I and II MHC molecules. Treating the APC with NH4Cl or CH3NH2 inhibited class I- and II-restricted Ag presentation. In contrast, chloroquine markedly inhibited class II, but not class I-restricted Ag presentation. Controls indicated that drug-treated APC were fully competent to interact with T cells and present processing-independent antigenic peptides in association with both class I and II MHC molecules. NH4Cl and CH3NH2 did not inhibit the uptake of radiolabeled Ag by the APC. After the proteolytic removal of H-2Kb from the surface of APC, NH4Cl and CH3NH2-treated and control APC regenerated identical amounts of surface H-2Kb and this regeneration required de novo protein synthesis. These latter results indicate that NH4Cl and CH3NH2 can inhibit Ag presentation without affecting the synthesis, transport, or surface expression of H-2Kb. Also, NH4Cl did not affect the transport of H-2Db to the surface of mutant RMA-S cells that were cultured with exogenous peptides. Taken together these results strongly suggest that NH4Cl and CH3NH2 but not chloroquine can inhibit a critical and early intracellular step in class I-restricted Ag presentation while simultaneously inhibiting class II-restricted Ag presentation.     

An IL 2-secreting T cell hybridoma that responds to a self class I histocompatibility antigen in the H-2D region

A self-reactive T cell hybridoma that secretes IL-2 in response to H-2d haplotype cells resulted from a fusion of BALB/cBy lymph node cells with the AKR thymoma BW5147. The lymph node cells used had been enriched for cells reactive to (TG)-A--L, but neither this antigen nor fetal calf serum were required for stimulation of the hybridoma designated 3DT52.5. The gene product responsible for stimulation mapped to the H-2D region. Allogeneic cells of the b, f, k, q, and s haplotypes failed to stimulate. Not all H-2d haplotype cells were effective stimulators of 3DT52.5. Peritoneal cells and splenic B cells were much more stimulatory than splenic T cells. Most tumor cell lines of H-2d derivation and of B cell or macrophage/monocyte lineage were stimulatory, whereas H-2d T cell lines were not. The capacity to stimulate 3DT52.5 did not correlate with the ability to stimulate I region-restricted hybridomas, or with the ability to be induced to stimulate such hybridomas. Stimulatory cell lines did not apparently produce a soluble factor required for stimulation, and negative cell lines were not inhibitory. The monoclonal antibody 27-11-13, which reacts with H-2D of the b, d, and q haplotypes, inhibited stimulation of 3DT52.5 but did not inhibit stimulation of the sibling hybridoma 3DT18.11, which responds to (TG)-A--L plus I-Ad. Conversely, the monoclonal anti-I-Ad antibody MK-D6 inhibited stimulation of 3DT18.11 but not 3DT52.5. Although it is clear that 3DT52.5 recognizes a class I antigen coded for in the H-2D region, the precise molecular nature of the antigen is unknown. The structure of the antigen receptor on this hybridoma may prove to be of interest when it can be compared with receptors found on T cell hybridomas restricted by class II histocompatibility antigens.     

T cell recognition of nonpolymorphic determinants on H-2 class I molecules

Recognition of polymorphic determinants on class I or class II MHC Ag is required for T lymphocyte responses. Using cell-size artificial membranes (pseudocytes) bearing H-2 class I Ag it is demonstrated that T cells can, in addition, recognize nonpolymorphic determinants on class I proteins. Pseudocytes bearing class I alloantigen stimulate in vitro generation of secondary allogeneic CTL responses. At a suboptimal alloantigen surface density, incorporation of class I molecules identical to those of the responder cells (self-H-2) or from third-party cells resulted in dramatically enhanced responses, whereas incorporation of class II proteins had no effect. The receptor that mediates recognition of conserved class I determinants has not been identified, but results of antibody blocking studies are consistent with the Lyt-2/3 complex of CTL having this role. Thus, class I proteins on Ag-bearing cells can have two distinct roles in T cell activation, one involving recognition of polymorphic determinants by the Ag-specific receptor and the other involving recognition of conserved determinants.     

Cell surface expression and alloantigenic function of a human class I MHC heavy chain gene (HLA-B7) in transgenic mice

We have introduced the gene encoding the heavy chain of the human MHC class I Ag HLA-B7 into transgenic mice. The gene was shown to be expressed at both the RNA and protein level. Cell surface HLA-B7 was detected on whole spleen cells by immunoprecipitation and on purified T cells by flow cytometry (FACS). Normal mice immunized with H-2-syngeneic B7-transgenic spleen cells generated CTL capable of killing transgenic cells and B7-expressing human JY cells. Anti-HLA mAb blocked the killing of JY cells. These results indicate that the human class I Ag HLA-B7 can be expressed at the surface of transgenic spleen cells in the absence of human beta 2-microglobulin, and that a significant fraction exists in a form recognizable by nontransgenic CTL as a major histocompatibility Ag unrestricted by H-2.     

Deletion of antigen-specific immature thymocytes by dendritic cells requires LFA-1/ICAM interactions.

An in vitro assay was used for assessing the participation of various cell surface molecules and the efficacy of various cell types in the deletion of Ag-specific immature thymocytes. Thymocytes from mice expressing a transgenic TCR specific for the male Ag presented by the H-2Db class I MHC molecule were used as a target for deletion. In H-2d transgenic mice, cells bearing the transgenic TCR are not subjected to thymic selection as a consequence of the absence of the restricting H-2Db molecule but, nevertheless, express this TCR on the vast majority of immature CD4+8+ thymocytes. In this report we show that CD4+8+ thymocytes from H-2d TCR-transgenic mice are preferentially killed upon in vitro culture with male APC; DC were particularly effective in mediating in vitro deletion when compared with either B cells or T cells. Deletion of CD4+8+ thymocytes by DC was H-2b restricted and could be inhibited by mAb to either LFA-1 alpha or CD8. Partial inhibition was observed with mAb to ICAM-1, whereas mAb to CD4 and LFA-1 beta were without effect. These results are the first direct evidence of LFA-1 involvement in negative selection and provide further direct support for the participation of CD8/class I MHC interactions in this process. Like the requirements for deletion, activation of mature male-specific CD4-8+ T cells from female H-2b TCR-transgenic mice was also largely dependent on Ag presentation by DC and required both LFA-1/ICAM and CD8/class I MHC interactions; these results support the view that activation and deletion may represent maturation stage-dependent consequences of T cells encountering the same APC. Finally, our results also support the hypothesis that negative selection (deletion) does not require previous positive selection because deletion was observed under conditions where positive selection had not occurred.     

Mapping the binding site on CD8 beta for MHC class I reveals mutants with enhanced binding

In an effective immune response, CD8+ T cell recognition of virally derived Ag, bound to MHC class I, results in killing of infected cells. The CD8alphabeta heterodimer acts as a coreceptor with the TCR, to enhance sensitivity of the T cells to peptide/MHC class I, and is two orders of magnitude more efficient as a coreceptor than the CD8alphaalpha. To understand the important interaction between CD8alphabeta and MHC class I, we created a panel of CD8beta mutants and identified mutations in the CDR1, CDR2, and CDR3 loops that decreased binding to MHC class I tetramers as well as mutations that enhanced binding. We tested the coreceptor function of a subset of reducing and enhancing mutants using a T cell hybridoma and found similar reducing and enhancing effects. CD8beta-enhancing mutants could be useful for immunotherapy by transduction into T cells to enhance T cell responses against weak Ags such as those expressed by tumors. We also addressed the question of the orientation of CD8alphabeta with MHC class I using CD8alpha mutants expressed as a heterodimer with wild-type CD8alpha or CD8beta. The partial rescuing of binding with wild-type CD8beta compared with wild-type CD8alpha is consistent with models in which either the topology of CD8alphaalpha and CD8alphabeta binding to MHC class I is different or CD8alphabeta is capable of binding in both the T cell membrane proximal and distal positions.