Evidence for a regulatory role of the T8 (CD8) antigen in antigen-specific and anti-T3-(CD3)-induced lytic activity of allospecific cytotoxic T lymphocyte clones

Anti-CD8 antibodies can trigger CD8+ T cell effector function in the absence of TCR engagement and improve peptide-MHCI tetramer staining

CD8(+) T cells recognize immunogenic peptides presented at the cell surface bound to MHCI molecules. Ag recognition involves the binding of both TCR and CD8 coreceptor to the same peptide-MHCI (pMHCI) ligand. Specificity is determined by the TCR, whereas CD8 mediates effects on Ag sensitivity. Anti-CD8 Abs have been used extensively to examine the role of CD8 in CD8(+) T cell activation. However, as previous studies have yielded conflicting results, it is unclear from the literature whether anti-CD8 Abs per se are capable of inducing effector function. In this article, we report on the ability of seven monoclonal anti-human CD8 Abs to activate six human CD8(+) T cell clones with a total of five different specificities. Six of seven anti-human CD8 Abs tested did not activate CD8(+) T cells. In contrast, one anti-human CD8 Ab, OKT8, induced effector function in all CD8(+) T cells examined. Moreover, OKT8 was found to enhance TCR/pMHCI on-rates and, as a consequence, could be used to improve pMHCI tetramer staining and the visualization of Ag-specific CD8(+) T cells. The anti-mouse CD8 Abs, CT-CD8a and CT-CD8b, also activated CD8(+) T cells despite opposing effects on pMHCI tetramer staining. The observed heterogeneity in the ability of anti-CD8 Abs to trigger T cell effector function provides an explanation for the apparent incongruity observed in previous studies and should be taken into consideration when interpreting results generated with these reagents. Furthermore, the ability of Ab-mediated CD8 engagement to deliver an activation signal underscores the importance of CD8 in CD8(+) T cell signaling.

CD8 alpha is expressed by human monocytes and enhances Fc gamma R-dependent responses

Background

CD8α enhances the responses of antigen-specific CTL activated through TCR through binding MHC class I, favoring lipid raft partitioning of TCR, and inducing intracellular signaling. CD8α is also found on dendritic cells and rat macrophages, but whether CD8α enhances responses of a partner receptor, like TCR, to activate these cells is not known. TCR and FcR, use analogous or occasionally interchangeable signaling mechanisms suggesting the possibility that CD8α co-activates FcR responses. Interestingly, CD8α+ monocytes are often associated with rat models of disease involving immune-complex deposition and FcR-mediated pathology, such as arthritis, glomerulonephritis, ischaemia, and tumors. While rat macrophages have been shown to express CD8α evidence for CD8α expression by mouse or human monocytes or macrophages was incomplete.

Results

We detected CD8α, but not CD8β on human monocytes and the monocytic cell line THP-1 by flow cytometry. Reactivity of anti-CD8α mAb with monocytes is at least partly independent of FcR as anti-CD8α mAb detect CD8α by western blot and inhibit binding of MHC class I tetramers. CD8α mRNA is also found in monocytes and THP-1 suggesting CD8α is synthesized by monocytes and not acquired from other CD8α+ cell types. Interestingly, CD8α from monocytes and blood T cells presented distinguishable patterns by 2-D electrophoresis. Anti-CD8α mAb alone did not activate monocyte TNF release. In comparison, TNF release by human monocytes stimulated in a FcR-dependent manner with immune-complexes was enhanced by inclusion of anti-CD8α mAb in immune-complexes.

Conclusion

Human monocytes express CD8α. Co-engagement of CD8α and FcR enhances monocyte TNF release, suggesting FcR may be a novel partner receptor for CD8α on innate immune cells.

The cytoplasmic domain of rat NKR-P1 receptor interacts with the N-terminal domain of p56(lck) via cysteine residues

NKR-P1 is a type II transmembrane protein which acts as an activation receptor on natural killer (NK) cells. The cytoplasmic domains of the CD4, CD8 and 4-1BB receptors contain the sequence Cys-X-Cys-Pro which is directly involved in coupling to another pair of cysteines in the N-terminal domain of the src family tyrosine kinase p56(lck). The cytoplasmic domain of NKR-P1 in rodents also contains the Cys-X-Cys-Pro sequence, but the capacity of the receptor to bind p56(lck) is presently unknown. We tested for direct coupling between these proteins using both protein biochemistry and the yeast two-hybrid technique. Immunoprecipitation studies showed that p56(lck) can be co-immunoprecipitated with NKR-P1 from a rat NK tumor cell line. In addition, the cytoplasmic domain of NKR-P1 interacted with the N-terminal domain of p56(lck) in yeast as assessed by reporter gene activation. Integrity of the cysteine pairs in both proteins was critical in mediating the interaction. The experiments suggest that the association of p56(lck) with NKR-P1 is somewhat weaker than the p56(lck) association with CD8alpha, but of much lower avidity than between CD4 and p56(lck). This could reflect a higher activation threshold for the NKR-P1 and CD8 receptors, which are involved in cytolytic responses, compared to CD4 which is involved in T cell helper function.

Human intestinal epithelial cell-induced CD8+ T cell activation is mediated through CD8 and the activation of CD8-associated p56lck

The activation of CD8+ suppressor T cells by normal intestinal epithelial cells in antigen-specific or allogeneic mixed cell culture systems has significant implications for the regulation of mucosal immune responses. In this study, we found that the capacity of epithelial cells to induce CD8+ suppressor T cell activation appeared to be linked to the binding of CD8 molecules on the T cell surface. This appears to be mediated by a non-class I molecule expressed on the epithelial cell surface, which binds to CD8 and results in the activation of the CD8-associated src-like tyrosine kinase, p56lck. Epithelial cell-stimulated p56lck activation is an early event (in contrast to monocytes) and is essential for T cell activation, since proliferation could be completely abrogated by pretreatment of T cells with genestein or herbamycin, both of which are protein tyrosine kinase inhibitors. Pretreatment of T cells with anti-CD8 or of intestinal epithelial cells with an anti-epithelial cell mAb B9 inhibited p56lck activation and further confirmed that CD8 on the T cell and a CD8 ligand on the epithelial cell were involved in this T cell activation event. The specificity of this reaction was confirmed in experiments in which murine transfectants 3G4 and 3G8, expressing CD4 or CD8, respectively, were used. Coculture of 3G8 with epithelial cells but not with monocytes activated p56lck in this cell line, whereas p56lck was preferentially activated in 3G4 cells when monocytes were used as the stimulator cells. Although stimulation through CD8- and CD8-associated p56lck was important for epithelial cell-induced T cell activation, T cell proliferation could not be induced by cross-linking CD8 alone with monoclonal antibody anti-CD8. These data suggest that a second signal, possibly through the T cell antigen receptor since activation of the T cell receptor-associated kinase fyn was also seen, is required for epithelial cell-driven T cell proliferation.

Generation of a novel CD8+ cytotoxic T lymphocyte that requires soluble factor to lyse autologous antigen-presenting cells

We reported the existence of high and low responders to the streptococcal cell wall antigen (SCW) in the human population. To analyze the mechanism of the low responsiveness to SCW at the cellular level, we established SCW-specific CD4+ T cell lines. During the course of generation of a SCW-specific CD4+ T cell line restricted by HLA-DQ from a low responder, we obtained autoreactive CD8+ cytotoxic T lymphocytes as a cell line (HYCD8). They proliferated in the presence of autologous monocytes and IL-2, without SCW. HYCD8 lysed autologous monocytes and Epstein-Barr virus-transformed B lymphoblastoid cell line (BLCL). This cytotoxic activity was specifically inhibited by an anti-HLA class I framework monoclonal antibody and restricted by HLA-B52 or B54 specificity, as judged by killing activity against panel cells and HLA class I-transfected BLCL. It was unique to HYCD8 that the HLA class I-restricted cytotoxicity was observed only in the presence of soluble factor with low molecular mass (< 10(4) Da) produced mainly by B cells, which could not be replaced by known cytokines and their mixtures. We thus describe novel HLA class I-restricted cytotoxic CD8+ T cells that kill antigen-presenting cells in a soluble factor-dependent manner.

Differential expression of CD8 alpha and CD8 beta associated with MHC-restricted and non-MHC-restricted cytolytic effector cells

The differential expression of the alpha and beta chains of the CD8 glycoprotein was examined in three functionally distinct cytolytic effector cell populations: (i) T cells (CD3+ CD56-), (ii) NK cells (CD56+ CD3-), and (iii) non-MHC-restricted T cells (CD56+ CD3+). Twenty-four percent of T cells were CD8+, and they consistently coexpressed both CD8 alpha and CD8 beta. Moreover, CD8+ T cells uniformly expressed high-density CD8 alpha. Forty percent of NK cells were CD8+ but the vast majority (approximately 75%) expressed only CD8 alpha without CD8 beta. In addition, CD8+ NK cells uniformly expressed low-density CD8 alpha. In comparison, 75% of non-MHC-restricted T lymphocytes were CD8+ but they displayed an intermediate phenotype: 60% coexpressed CD8 alpha and CD8 beta while 40% expressed only CD8 alpha. Within this population, CD8 alpha was expressed at high density, similar to that of T cells. Following IL-2 activation, enhancement of non-MHC-restricted cytotoxicity was not associated with any changes in either the quantitative or qualitative pattern of expression of CD8 alpha or CD8 beta by these cells. Addition of either anti-CD8 alpha or anti-CD8 beta mAb did not alter non-MHC-restricted cytotoxicity of either CD56+ CD3- or CD56+ CD3+ effector cells. However, within the CD56+ cell population, non-MHC-restricted cytotoxicity was almost entirely found within the CD8- and CD8 alpha + beta- populations, and both subsets displayed a similar level of killing. In contrast, CD8 alpha+ beta+ cells exhibited very little non-MHC-restricted cytotoxicity. Thus, the coexpression of CD8 alpha and CD8 beta in conjunction with the TCR/CD3 complex appears to characterize MHC restricted cells while the expression of CD8 alpha alone is associated with non-MHC-restricted cytotoxicity. Taken together, these findings suggest that neither CD8 alpha nor CD8 beta is involved in the initial phases of target cell binding or recognition during NK cell-mediated lysis. However, the selective expression of CD8 alpha by a large fraction of non-MHC-restricted effector cells suggests that this antigen may play a different functional role in this unique subset of cytolytic lymphocytes.

A binding site for the T-cell co-receptor CD8 on the alpha 3 domain of HLA-A2

Adhesion measurements between CD8 and 48 point mutants of HLA-A2.1 show that the CD8 alpha-chain binds to the alpha 3 domain of HLA-A2.1. Three clusters of alpha 3 residues contribute to the binding, with an exposed, negatively charged loop (residues 223-229) playing a dominant role. CD8 binding correlates with cytotoxic T-cell recognition and sensitivity to inhibition by anti-CD8 antibodies. Impaired alloreactive T-cell recognition of an HLA-A2.1 mutant with reduced affinity for CD8 is not restored by functional CD8 binding sites on an antigenically irrelevant class I molecule. Therefore, complexes of CD8 and the T-cell receptor bound to the same class I major histocompatibility complex molecule seem to be necessary for 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.

Interplay between the TCR/CD3 complex and CD4 or CD8 in the activation of cytotoxic T lymphocytes

Interactions between CTL and target cells occur in the absence of specific antigen recognition and precede subsequent interaction of the TCR with its specific antigen. This antigen-independent adhesion progresses through two different pathways, one involving the interaction of CD2 with LFA-3 on the target cell, the second the interaction of LFA-1 with ICAM-1. Such antigen-independent adhesions are critical for the activation of T cells via the TCR. Also, CD4 and CD8 can serve as adhesion molecules by binding to monomorphic determinants expressed on class II and class I MHC antigens, respectively, on the target cells, but compared to LFA-1 and CD2 antigens their contribution to conjugate formation is minor. CD4 and CD8 are required for effective T-cell activation in situations where the intrinsic affinity of the TCR or antigen expression is low, suggesting that CD4 and CD8 enhance the avidity of T cells for target cells by binding to class II and class I antigen, respectively. However, CD4 and CD8 are also involved in post-binding events that lead to CTL activation and subsequent lysis of the target cells. On the other hand, blocking of anti-TCR/CD3 mAb-induced CTL reactivity by anti-CD4/CD8 mAbs does not necessarily involve an interference with the binding of CD4 and CD8 to their respective ligands and it has been proposed that the TCR and CD4 or CD8 form functional complexes that are required for optimal T-cell activation. It is still unclear whether blocking by anti-CD4/CD8 mAbs is based on the prevention of complex formation of the TCR with CD4 or CD8, since formation of such complexes has yet to be demonstrated. The alternative hypothesis, that anti-CD4/anti-CD8 mAbs can directly confer negative regulatory signals to the CTL is not supported by our studies with antibody-directed lysis mediated by a CD4+, CD8+ CTL clone. Anti-CD4/CD8 mAbs can also inhibit T-cell cytotoxicity induced by other T-cell surface activation antigens such as CD2 or Tp103. In these situations, the triggering may involve signals transferred via CD3 requiring functional CD3/CD8 or CD3/CD4 complexes. Although most studies investigating the sequence of events leading to T-cell activation are carried out with CTL, preliminary data indicate that the same mechanisms described here for CTL activation are probably also valid for the interactions of T-helper cells with APC or B cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Polymorphism in the alpha 3 domain of HLA-A molecules affects binding to CD8

Cytotoxic T lymphocytes (CTL) expressing the CD8 glycoprotein recognize peptide antigens presented by class I major histocompatibility complex (MHC) molecules. This correlation and the absence of CD8 polymorphism led to the hypothesis that CD8 binds to a conserved site of class I MHC molecules. Using a cell-cell binding assay we previously demonstrated specific interaction between human class I MHC (HLA-A,B,C) molecules and CD8. Subsequent analysis of the products of 17 HLA-A,B alleles revealed a natural polymorphism for CD8 binding in the human population. Two molecules, HLA-Aw68.1 and HLA-Aw68.2, which do not bind CD8, have a valine residue at position 245 whereas all other HLA-A,B,C molecules have alanine. Site-directed mutagenesis shows that this single substitution in the alpha 3 domain is responsible for the CD8 binding phenotype and also affects recognition by alloreactive and influenza-specific CTL. Our results indicate that CD8 binds to the alpha 3 domain of class I MHC molecules.

Substitution at residue 227 of H-2 class I molecules abrogates recognition by CD8-dependent, but not CD8-independent, cytotoxic T lymphocytes

The CD8 (Lyt 2) molecule is a phenotypic marker for T lymphocytes that recognize and react with major histocompatibility complex (MHC) class I molecules. Antibody blocking experiments and gene transfection studies indicate that CD8 binds to a determinant on MHC class I molecules on the target cells, facilitating interaction between effector T lymphocytes and the target cell. The CD8 molecule may also be involved in transmembrane signalling during T-cell activation. The existence of CD8- cytotoxic T lymphocytes (CTL) and class I-reactive CTL that are not inhibited by antibody to CD8 suggests that at least some CTL do not require the CD8 molecule to interact with and lyse target cells. We have recently demonstrated that cells transfected with an H-2Dd gene that carries a mutation at residue 227 are not killed by primary CTL8. Here we show that although this mutation abrogates recognition by primary CTL, it does not affect recognition by CD8-independent CTL, suggesting that residue 227 of class I molecules might contribute to a determinant that is the ligand of the CD8 molecule.

Regulatory role of the CD8 antigen in both CD3 and CD2 monoclonal antibody-induced nonspecific cytotoxicity of class I- and class II-allospecific cytotoxic T cell clones

We investigated the function of the CD8 moiety in antigen-specific and alternative activation of HLA class I- and HLA class II-allospecific CD8+ cytotoxic T lymphocyte (CTL) clones. Monoclonal antibodies (mAb) directed against the CD8 structure were only found to inhibit antigen-specific cytotoxicity of class I-allospecific CD8+ CTL clones and not of a class II-allospecific CD8+ CTL clone. However, cytotoxicity induced by CD3 mAb (used at suboptimal concentrations) or CD2 mAb in both types of CTL clone was blocked by CD8 mAb. The class II-allospecific CD8+ CTL clone was uniformly more difficult to inhibit than the class I-allospecific CD8+ CTL clones and, moreover, also easier to induce to exert nonspecific cytotoxicity by CD2 mAb and CD3 mAb. The absence of CD8 mAb blocking of antigen-specific cytotoxicity of the class II-specific CD8+ CTL clone is, therefore, assumed to result from too strong a triggering signal to be overcome by the down-regulatory signal of the CD8 antigen. These combined findings suggest a down-regulatory function of CD8 not only in T cell receptor (TcR)/CD3 activation, but also in TcR/CD3-controlled alternative activation routes such as the CD2 activation pathway.

Interleukin-4 mediates CD8 induction on human CD4+ T-cell clones

CD4 and CD8 antigens are simultaneously expressed on most of the cortical thymocytes, that weakly express the T-cell antigen receptor(TCR)/CD3 complex. Mature peripheral T cells, however, strongly express the TCR complex and are positive for either CD4 or CD8. Nevertheless, a small percentage of peripheral CD3+ T cells express CD4 and CD8 simultaneously. These mature, double positive cells could be intermediates between CD4+CD8+ thymocytes and mature, single positive T cells, or they may originate from single positive T cells that acquire either CD4 or CD8. Here we report that activation and culturing of cloned CD4+ T cells in interleukin-4 (IL-4), results in the acquisition of CD8 due to its de novo synthesis. The IL-4-induced co-expression of CD8 on CD4+ T cells is reversible, in that CD8 disappeared from double positive T-cell clones isolated in IL-4, when they were cultured in IL-2. CD8 induced by IL-4 can be functional as a monoclonal antibody to CD8 inhibited anti-CD3-mediated cytotoxicity by a double positive T-cell clone.

Functional consequences of anti-sense RNA-mediated inhibition of CD8 surface expression in a human T cell clone

An experimental approach for defining the function of CD8 has been developed by linking anti-sense RNA mutagenesis and T cell cloning technologies. We have transfected an anti-sense CD8 episomal expression vector into a CD8+ nontransformed human T cell clone that is specific for the human class I alloantigen HLA-B35. Expression of CD8 on this T cell clone, JH.ARL.1, was selectively and efficiently inhibited. Stimulation of this CD8- variant with specific alloantigen resulted in a marked loss of a number of functional responses, including cytotoxicity, proliferation, IL-2 secretion, and IL-2-R expression. However, these same functional responses could be elicited with stimuli that do not require antigen recognition to activate the T cell (anti-CD3 mAbs, PHA). The results of our study support the hypothesis that CD8 is required for recognition of class I MHC alloantigens that results in activation of T cell functional responses.

Urushiol (poison ivy)-triggered suppressor T cell clone generated from peripheral blood

Allergic contact dermatitis to Toxicodendron radicans (poison ivy) is mediated by the hapten urushiol. An urushiol-specific, interleukin 2 (IL-2)-dependent T cell clone (RLB9-7) was generated from the peripheral blood of a patient with a history of allergic contact dermatitis to T. radicans. This clone proliferated specifically to both leaf extract and pure urushiol. Although the clone had the phenotype CD3+CD4+CD8+, proliferation to antigen was blocked by anti-CD8 and anti-HLA-A, B, C, but not by anti-CD4, suggesting that CD4 was not functionally associated with the T cell receptor. Furthermore, studies with antigen-presenting cells from MHC-typed donors indicated that the clone was MHC class 1 restricted. RLB9-7 was WT31 positive, indicating it bears the alpha beta T cell receptor. The clone lacked significant natural killer cell activity and produced only low levels of IL-2 or gamma-interferon upon antigen stimulation. Addition of RLB9-7 to autologous peripheral blood mononuclear cells in the presence of urushiol inhibited the pokeweed mitogen-driven IgG synthesis. This suppression was resistant to irradiation (2,000 rad) and was not seen when RLB9-7 was added to allogeneic cells, even in the presence of irradiated autologous antigen-presenting cells, suggesting that suppression was MHC restricted and not mediated by nonspecific soluble factors. However, RLB9-7 cells in the presence of urushiol inhibited the synthesis of tetanus toxoid-specific IgG by autologous lymphocytes, indicating that the suppression, although triggered specifically by urushiol, was nonspecific.

The Lyt-2 molecule recognizes residues in the class I alpha 3 domain in allogeneic cytotoxic T cell responses

The involvement of the different domains of the MHC class I molecule in CTL recognition was investigated. mAbs specific for the alpha 1/alpha 2 domains of H-2Ld interfered with both the primary and secondary generation and effector function of in vitro Ld-specific CTL. mAbs specific for the alpha 3 domain of H-2Ld interfered with the generation and function of primary in vitro Ld-specific CTL; however, there was no effect on the in vitro generation of secondary CTL and only partial inhibition of their function. In vivo treatment with graft-specific antibodies to both the alpha 3 domain and the alpha 1/alpha 2 domains together resulted in a dramatic enhancement of Ld- or Dd-disparate skin grafts, whereas the individual mAbs showed minimal effects. This suggested that the class I alpha 3 domain is recognized by alloreactive CTL. Several approaches were undertaken to examine whether recognition of the alpha 3 domain determinants is mediated by the Lyt-2 molecule. When mAbs specific for the alpha 3 domain of either H-2Ld or H-2Dd were used in vivo and in vitro, the resulting CTL population was not inhibited by antibody to the alpha 3 domain and was only partially inhibited by antibody to Lyt-2. We therefore observed a correlation between the effects of antibody to the class I alpha 3 domain of the target molecule and antibody to the Lyt-2 molecule on the CTL. To further test the relationship between CTL recognition of the alpha 3 domain and the involvement of Lyt-2, we used a cell expressing a mutation in the alpha 3 domain of the Dd molecule. The mutation resulted in a single amino acid substitution of glu to lys at residue 227 of the alpha 3 domain. Consistent with an earlier report, cells expressing the mutant Dd lys molecule were not lysed by CTL from a primary stimulation against the wild-type Dd glu molecule. However, this same cell line was killed by the Lyt-2-independent secondary Dd-specific CTL generated in the presence of antibody to the alpha 3 domain in vivo and in vitro. Furthermore, cells expressing the mutant Dd lys molecule failed to stimulate a primary response. In conclusion, several independent lines of evidence indicate that residues in the alpha 3 domain of the class I molecule are involved in recognition by the Lyt-2 molecule, and that Lyt-2-mediated recognition can be specifically blocked using mAb to determinants in the alpha 3 domain.

Regulatory properties of LFA-1 alpha and beta chains in human T-lymphocyte activation

Lymphocyte function-associated antigen-1 (LFA-1) is a heterodimer composed of an alpha and beta chain that is expressed on the surface of most leukocytes and is an essential molecule for adhesion reactions between cells participating in the immune response. A putative ligand for LFA-1 is the intercellular adhesion molecule ICAM-1 (refs 3-5). Leukocyte adhesion abnormality is found in patients with LFA-1 deficiency. It is not clear whether binding of ligand to the LFA-1 molecule merely spatially orientates cells towards each other or can also induce signals that regulate cell activation and differentiation. We have recently developed a T-cell proliferation assay which uses immobilized anti-CD3 monoclonal antibodies as stimulant and is independent of LFA-1-mediated cellular adhesion. As there is no interference by anti-LFA-1 monoclonal antibodies with the adhesion-dependent activation steps, this T-cell activation system allows us to investigate whether transmembrane signals are induced by binding of ligand to LFA-1 on T cells. Our data indicate that binding of ligand to LFA-1 results in the transduction of regulatory signal across the plasma membrane, rather like other molecules (CD2, CD4, CD8) (refs 8-11) with signal-modifying properties involved in the adhesion of T cells to target/stimulator cells. Indeed, adhesion molecules might generally be important in signal transduction, even in cells not belonging to the immune system.

Recognition of influenza virus-infected B-cell lines by human influenza virus-specific CTL

The cytotoxic activity on influenza virus-infected Epstein-Barr virus (EBV)-transformed B-lymphoblastoid cell lines (LCL-Flu) and influenza virus-infected phytohemagglutinin lymphoblasts (PHA-Flu) was compared with the use of influenza-A virus-specific cytotoxic T lymphocytes (CTL), generated in short-term bulk cultures. Cold-target inhibition experiments showed that the lysis of PHA-Flu was completely blocked by both cold LCL-Flu and cold PHA-Flu whereas the lysis of LCL-Flu was completely inhibited by cold LCL-Flu, but only partially by cold PHA-Flu, indicating that structures can be recognized on LCL-Flu which are absent from PHA-Flu. Monoclonal antibody (McAb) directed against a monomorphic determinant of major histocompatibility complex (MHC) class I molecules inhibited the lysis of PHA-Flu more strongly than the lysis of LCL-Flu. Since LCL have a high expression of MHC class II molecules compared to PHA lymphoblasts, we examined whether class II-restricted CTL activity was responsible for the (anti)class I McAb-resistant lysis of LCL-Flu. Neither anti-CD4 McAb nor anti-class II McAb inhibited the lysis of LCL-Flu which argues against a contribution of MHC class II-restricted CTL. Depletion of CD16+ cells, containing the majority of the nonspecific cytotoxic cells, did not affect the lysis of LCL-Flu, indicating that the remaining lysis on LCL-Flu was also not due to a nonspecific component. We suggest that cell-type-dependent variations exist in the nature of the immunogenic determinants to which CTL respond.

Clonal heterogeneity of HLA-B27 cellular allorecognition. Delineation of immunodominant sites

The fine specificity of nine cytolytic T lymphocyte (CTL) clones obtained after stimulation of HLA-B27- responder lymphocytes with B27.1+ lymphoblastoid cell lines has been analyzed. These clones defined three different reaction patterns when tested against a panel of target cells including those expressing all known HLA-B27 subtypes: (a) specific recognition of HLA-B27.1, B27.2 and B27d, (b) selective reactivity with B27.1, B27d and HLA-B40 and (c) selective recognition of B27.1, B27.2, B27d, B27f and B40. Representative clones within each group were analyzed in detail. Differences in lytic ability of the various susceptible targets within each group were established by cold target inhibition analyses and by blocking experiments with anti-CD3 and anti-CD8 monoclonal antibodies. When correlated with the known structure of the HLA-B27 subtypes, these results demonstrate the critical relevance of amino acid changes within residues 77-81 and at position 152 in modulating allospecific CTL recognition of HLA-B27.1 and suggest that these residues could be involved in the structure of immunodominant regions of this antigen. The observed cross-reactions with HLA-B40, differing from B27.1 in 16 amino acid residues, suggest that the simultaneous occurrence of multiple amino acid changes could have mutually compensatory effects, so that a cross-reactive epitope might result from various combinations of polymorphic residues.

Reconstitution of MHC class I specificity by transfer of the T cell receptor and Lyt-2 genes

The T cell receptor alpha and beta chain genes donated by an H-2 class I-specific, CD8-dependent cytotoxic T cell clone were transferred, alone or in combination with the Lyt-2 gene, into a class II-restricted, CD4+ T cell hybridoma. Two important points emerged. First, the alpha and beta T cell receptor genes endowed the recipient with the H-2 class I specificity of the donor only if the same cell had also been transfected with the Lyt-2 gene. Second, the functional Lyt-2 molecule was expressed on the transfected cells in the absence of the Lyt-3 polypeptide. These results demonstrate that, besides the T cell receptor, the Lyt-2 polypeptide is the only subset-specific molecule required to retarget a class II-reactive, CD4+ T cell line toward H-2 class I molecules.

Effective activation of resting mouse T lymphocytes by cross-linking submitogenic concentrations of the T cell antigen receptor with either Lyt-2 or L3T4

We studied the activation of small resting mouse T lymphocytes by antibodies to the T cell antigen receptor in combination with antibodies to other T cell surface antigens. Solid-phase but not soluble antibodies KJ16-133 and F23.1, both directed to beta chains of the V beta 8 family, activate T cells to proliferate in the presence of growth factors, in a dose-dependent fashion. Antibodies to Lyt-2 and to L3T4 had no activating effect at any concentration. However, submitogenic concentrations of KJ16-133 and of F23.1 synergized with a wide range of concentrations of anti-Lyt-2 and anti-L3T4 to cause T cell proliferation similar or greater in magnitude to that caused by high concentrations of anti-T cell receptor antibody. Synergistic activation was also observed with antibodies to Lyt-1, LFA-1 and H-2 class I antigens but to a significantly lower degree. This was particularly clear in limiting dilution experiments in which the corrected frequencies of T cells proliferating in response to low amounts of anti-T cell receptor antibody together with anti-Lyt-2 were 1/4 to 1/7 for BALB/c T cells. The frequencies of BALB/c T cells responding to high concentrations of anti-T cell receptor antibody alone were between 1/14 and 1/126 and still lower frequencies of T cells proliferated in synergistic responses with anti-LFA-1 or anti-Lyt-1. Synergistic activation leads to the induction of functional cytotoxic cells. We interpret these data as suggestive that cross-linking of the T cell antigen receptor with either Lyt-2 (CD8) or L3T4 (CD4) represents an optimal activating signal for resting T cells. We think that, in physiological T cell activation, cross-linking of the T cell receptor to CD8 or CD4 is induced by their simultaneous binding to major histocompatibility complex (MHC) class I (for CD8) or MHC class II (for CD4) molecules on stimulator cells. We consider the possibility that similar cross-linking requirements may also exist during T cell repertoire selection in ontogeny, thus accounting for the strict coexpression of MHC class I and class II-restricted T cell receptors with CD8 and CD4 molecules, respectively.