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.     

In vivo administration of major histocompatibility complex class I-specific peptides from influenza virus induces specific cytotoxic T cell hyporesponsiveness

We have been investigating the immunogenicity of two class I major histocompatibility complex-specific peptides with a sequence derived from influenza virus nucleoprotein specific for K^d and one for D^b. Peptide-modified splenocytes are unable to immunize for a primary cytotoxic T (Tc) cell response in vivo, or secondary response in vitro. Peptide-modified stimulator cells can boost virus-primed splenocytes for a strong secondary response in vitro. Animals primed with syngeneic peptide-modified splenocytes upon challenge with virus in vivo do not generate strong secondary Tc cell responses on day 3 after challenge in contrast to virus primed animals. Day 6 responses of virus-challenged, peptide-primed animals are reduced as compared to unprimed mice. This hyporesponsiveness is independent of CD8⁺ T cells in the priming population and can be elicited with tumor cell lines. The data are discussed in the framework of the two-signal model of immune induction.     

Leishmania major infection in mice primes for specific major histocompatibility complex class I-restricted CD8+ cytotoxic T cell responses

This report shows that lymphoid tissues of mice which have resolved a primary infection with Leishmania major contain parasite-specific major histocompatibility complex (MHC) class I-restricted cytolytic CD8⁺ T cell precusors that can be expanded after specific restimulation in vitro with syngeneic antigen-presenting cells pulsed with a cyanogen bromide digest of L. major. In H-2^b mice, two distinct populations of CD8⁺ T cells were identified which both lysed target cells pulsed with L. major-derived peptides but were restricted by a different H-2^b class I gene product. Interestingly, these two populations appear to recognize different parasite-derived peptides. It is noteworthy that one K°-restricted CD8⁺ T cell line was able to specifically lyse syngeneic macrophages infected with viable L. major, indicating that some L. major-derived peptides may reach the MHC class I pathway of presentation from the phagolysosomal compartment where the parasites are confined in infected macrophages. The importance of these parasite-specific MHC class I restricted cytolytic CD8⁺ T cells for the elimination of L. major by the infected host remains to be determined.     

T cell development in a major histocompatibility complex class II-deficient patient

In this report we show that the major histocompatibility complex (MHC) class II-negative thymus of a bare lymphocyte syndrome (BLS) patient contains a reduced CD4⁺ CD8^? T cell population when compared to thymocytes derived from a MHC class II-expressing thymus. Of these CD4⁺ CD8^? BLS thymocytes, approximately only one third co-expressed the CD3 antigen, moreover at a lower expression level when compared to control thymocytes. This suggests a partial maturation of the CD4⁺ CD8^? T cells in the absence of MHC class II expression. Among the BLS thymocytes, CD4⁺ CD8⁺ thymocytes could easily be detected. Noteworthy, the number of CD4^? CD8⁺ thymocytes was significantly increased. CD4⁺ CD8^? T cells could also be found among the BLS peripheral blood mononuclear cells, albeit at reduced numbers. Despite the absence of peripheral MHC class II expression, the majority of these CD4⁺ CD8^? T cells co-expressed the CD45RO marker. In the BLS patient, thymocytes as well as peripheral CD4⁺ CD8^? T cells were not restricted in the use of the available T cell receptor (TcR) V gene family pool. However, the lack of detectable levels of thymic and peripheral MHC class II antigen expression in the BLS patient had altered the CD4^?skewing patterns of TcR V gene families which were present in normal individuals. In conclusion, the lack of MHC class II expression in the BLS patient does not completely inhibit the CD4+ CD8^? T cell development.     

Structural basis for a major histocompatibility complex class Ib-restricted T cell response

In contrast to antigen-specific immunity orchestrated by major histocompatibility complex (MHC) class Ia molecules, the ancestrally related nonclassical MHC class Ib molecules generally mediate innate immune responses. Here we have demonstrated the structural basis by which the MHC class Ib molecule HLA-E mediates an adaptive MHC-restricted cytotoxic T lymphocyte response to human cytomegalovirus. Highly constrained by host genetics, the response showed notable fine specificity for position 8 of the viral peptide, which is the sole discriminator of self versus nonself. Despite the evolutionary divergence of MHC class Ia and class Ib molecules, the structure of the T cell receptor-MHC class Ib complex was very similar to that of conventional T cell receptor-MHC class Ia complexes. These results emphasize the evolutionary 'ambiguity' of HLA-E, which not only interacts with innate immune receptors but also has the functional capacity to mediate virus-specific cytotoxic T lymphocyte responses during adaptive immunity.     

Transection of major histocompatibility complex class I-induced neurites by cytotoxic T lymphocytes.

Damage to neurites with transection of axons and spheroid formation is commonly noted in the central nervous system during viral and autoimmune diseases such as multiple sclerosis, but it remains open whether such changes are caused primarily by immune mechanisms or whether they are secondary to inflammation. The present experiments explored whether neurites can be directly attacked by cytotoxic T lymphocytes (CTLs). Cultured murine neurons induced by interferon-gamma and tetrodotoxin to express major histocompatibility complex class I were pulsed with a dominant peptide of the lymphochoriomeningitis virus envelope glycoprotein (GP33) and then confronted with GP33-specific CD8(+) CTLs. Within 3 hours the neurites developed cytoskeleton breaks with adjacent solitary neuritic spheroids, as documented by confocal examination of the cytoskeletal marker beta-tubulin III. At the same time cytoskeleton staining of the neuronal somata showed no damage. The CTLs selectively attacked neurites and induced segmental membrane disruption 5 to 30 minutes after the establishment of peptide-specific CTL-neurite contact, as directly visualized by live confocal imaging. Thus, major histocompatibility complex class I/peptide-restricted CD8(+) T lymphocytes can induce lesions to neurites, which might be responsible for axonal damage during neuroinflammatory diseases.     

Peptide anchor residue glycosylation: effect on class I major histocompatibility complex binding and cytotoxic T lymphocyte recognition

This study extends our previous observation that glycopeptides bind to class I major histocompatibility complex (MHC) molecules and elicit carbohydrate-specific CTL responses. The Sendai virus nucleoprotein wild-type (WT) peptide (FAPGNYPAL) binds H-2D^b using the P5-Asn as an anchor. The peptide K2 carrying a P5 serine substitution did not bind D^b. Surprisingly, glycosylation of the serine (K2-O-GlcNAc) with N-acetylglucosamine (GlcNAc), a novel cytosolic O-linked glycosylation, partially restored peptide binding to D^b. We argue that the N-acetyl group of GlcNAc may fulfil the hydrogen bonding requirements of the D^b pocket which normally accomodates P5-Asn. Glycosylation of the P5-Asn residue itself abrogated binding similar to K2, probably for steric reasons. The peptide K2-O-GlcNAc readily elicited D^b-restricted cytotoxic T lymphocytes (CTL), which did not cross-react with K2 or WT. However, all D^b-restricted CTL raised against K2-O-GlcNAc cross-reacted strongly with another glycopeptide, K3-O-GlcNAc, where the GlcNAc substitution is on a neighboring P4-Ser. Furthermore, D^b-restricted CTL clones raised against K2-O-GlcNAc or K3-O-GlcNAc displayed a striking TCR conservation. Our interpretation is that the carbohydrate of K2-O-GlcNAc not only mediates binding to D^b, but also interacts with the TCR in such a way as to mimic K3-O-GlcNAc. This unusual example of molecular mimicry extends the known effects of peptide glycosylation from what we and others have previously reported: glycosylation may create a T cell neo-epitope, or, conversely, abrogate recognition. Alternatively, glycosylation may block peptide binding to MHC class I and finally, as reported here, restore binding, presumably through direct interaction of the carbohydrate with the MHC molecule.     

T cell recognition of donor major histocompatibility complex class I peptides during allograft rejection.

LEW (RTI1) recipients of DA (RTIav1) skin and kidney allografts were tested for the capacity of their T lymphocytes to proliferate to three 22-24-amino acid peptides from the hypervariable regions of the RTI-Aav1 classical MHC class I molecule. Ten days after rejecting second-set DA kidney allografts, spleen cells (but interestingly not lymph node cells) from LEW recipients showed strong, LEW antigen-presenting cell-dependent, CD4+ T cell proliferation to peptide 1 (from the alpha helical region of the alpha 1 domain). CD8+ T cells showed no response to peptide 1. There was no response by the spleen cells to peptide 2 (from the beta sheet of the alpha 2 domain) or peptide 3 (from the alpha helical region of the alpha 2 domain). Immunization of LEW rats with pure RTI-Aav1 class I H chain in Freund's adjuvant gave responses identical to that seen after grafting, i.e. good CD4+ T cell proliferation to peptide 1, but none to peptides 2 and 3. However, immunization of LEW rats with peptides 1, 2 and 3 in Freund's adjuvant resulted in good CD4+ T cell proliferation responses to each of the peptides. These data demonstrate that indirect allorecognition can be stimulated by allograft rejection, and emphasize that the physiological processing of donor antigens will influence which peptides will be important in indirect allorecognition in transplantation.     

Trypanosoma cruzi infection does not impair major histocompatibility complex class I presentation of antigen to cytotoxic T lymphocytes

Trypanosoma cruzi (T. cruzi), the etiological agent of Chagas' disease, lives free within the cytoplasm of infected host cells. This intracellular niche suggests that parasite antigens may be processed and presented on major histocompatibility complex (MHC) class I molecules for recognition by CD8⁺ T cells. However, the parasite persists indefinitely in the mammalian host, indicating its success at evading immune clearance. It has been shown that T. cruzi interferes with processing and presentation of antigenic peptides in the MHC class II pathway. This investigation sought to determine whether interference in MHC class I processing and presentation occurs with T. cruzi infection. Surface expression of MHC class I molecules was found to be unaffected or up-regulated by T. cruzi infection in vitro. A model system employing a β-galactosidase (β-gal)-specific murine cytotoxic T lymphocyte (CTL) line (0805B) showed: (i) in vitro infection of mouse peritoneal macrophages or J774 cells with T. cruzi did not inhibit MHC class I presentation of exogenous peptide (a nine-amino acid epitope of β-gal) to the CTL line, (ii) in vitro infection of a β-gal-expressing 3T3 cell line (LZEJ) with T. cruzi did not inhibit MHC class I presentation of the endogenous protein to the CTL line and (iii) mouse renal adenocarcinoma cells infected with T. cruzi and subsequently infected with adenovirus expressing β-gal were able to present antigen to the β-gal-specific CTL line. These findings indicate that the failure of the immune response to clear T. cruzi does not result from global interference by the parasite with MHC class I processing and presentation. Parasites engineered to express β-gal were unable to sensitize infected antigen-presenting cells in vitro to lysis by the CTL 0805B line. This was probably due to the intracellular localization of the β-gal within the parasite and its inaccessibility to the host cell cytoplasm.     

Analysis of immunological tolerance to major histocompatibility complex antigens. I. High frequencies of tolerogen-specific cytotoxic T lymphocyte precursors in mice neonatally tolerized to class I major histocompatibility complex antigens

Injection of (CBA X A)F1 cells into neonatal CBA mice rendered them tolerant to skin grafts of (CBA X A)F1 origin. Limiting dilution analysis revealed a very low frequency of tolerogen-inducible cytotoxic T lymphocyte precursors (CTL-P) in spleens of tolerant mice. Two in vitro procedures allowed, however, the induction of tolerogen-specific CTL-P of high frequencies in tolerant mice: (a) the "by-pass" activation of spleen cells from tolerant mice by concanavalin A under short-term bulk culture conditions followed by culture of limiting numbers of activated responder cells, and (b) absorption of spleen cells from tolerant mice on monolayers of tolerogen-activated T cells from normal syngeneic mice. Furthermore, spleen cells from tolerant mice, recently challenged with a tolerogen-bearing skin graft, specifically suppressed the activation of tolerogen-reactive splenic CTL-P from normal CBA mice under limiting dilution conditions. These data confirm the presence of tolerogen-specific CTL-P of high frequency in tolerant mice and suggest their functional inactivation through a suppressive mechanism.     

Induction of cytotoxic T lymphocytes specific to the molecule of the class I major histocompatibility complex for subcutaneous immunization in pads

T lymphocytes from immune lymph nodes, specific to the molecule of the class I major histocompatibility complex, were found to contain cytotoxic T lymphocyte precursors which mature to become effector cytotoxic T lymphocytes only in the presence of helper cells and L3T4⁺, but not Lyt2⁺ T helpers. The findings indicate that a subcutaneous injection of alloantigen of the class I major histocompatibility complex for immunization in the pads leads to the creaction of the type of microenvironment of the lymph nodes which prevents activation of Lyt2⁺ T helpers or leads to the activation of their functionally negligible part. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 119, N ^o 2, pp. 190–193, February, 1995 Presented by N. N. Trapeznikov, Member of the Russian Academy of Medical Sciences     

Identification of tumor antigen-specific cytotoxic T lymphocytes cross-recognizing allogeneic major histocompatibility class I molecules

Adoptive immunotherapy of cancer utilizes tumor antigen-specific cytotoxic T lymphocytes (CTL) as mediators of a targeted anti-tumor effect. In this study, we show that such CTL can be able to cross-recognize allogeneic major histocompatibility complex (MHC) molecules in a phenomenon of molecular mimicry. A self histo-leukocyte antigen (HLA) A*0201-restricted CTL specific for peptide MT27-35 from the human differentiation antigen Melan-A/MART-1 was shown to cross-recognize allogeneic A*0220 molecules which differ from syngeneic A*0201 for a single amino acid substitution at position 66 of the antigen-binding groove. A*0220 molecules were recognized on a variety of human cells of different histological origin but not on COS-7 cells. A second self-A*0201-restricted CTL, specific for peptide D10/6-271 encoded by the tumor-specific DAM-gene family, was shown to cross-recognize allogeneic B*3701 molecules which differ from syngeneic A*0201 by 32 amino acids in the peptide antigen-binding cleft. B*3701 molecules were recognized on a variety of cell types including COS-7 cells. These data raise new safety issues for clinical trials of cancer immunotherapy using adoptive transfer of in vitro generated, allogeneic CTL with specific anti-tumor activity.     

Proteolysis and class I major histocompatibility complex antigen presentation

Summary: The class I major histocompatibility complex (MHC class 1) presents 8–10 residue peptides to cytotoxic T lymphocytes. Most of these antigenic peptides are generated during protein degradation in the cytoplasm and are then transported into the endoplasmic reticulum by the transporter associated with antigen processing (TAP), Several lines of evidence have indicated that the proteasome is the major proteolytic activity responsible for generation of antigenic peptides—probably most conclusive has been the finding that specific inhibitors of die proteasome block antigen presentation. However, other proteases (e.g. the signal peptidase) may also generate some epitopes, particularly those on certain MHC class I alleles. The proteasome is responsible for generating the precise C termini of many presented peptides, and appears to be the only activity in ceils that can make this cleavage. In contrast, aminopeptidases in the cytoplasm and endoplasmic reticulum can trim the N terminus of extended peptides to their proper size. Interestingly, the cellular content of proteases involved in the production and destruction of antigenic peptides is modified by inter-feron-γ (IFN-γ) treatment of cells, IFN-γ indicates the expression of three new proteasome β submits that are preferentially incorporated into new proteasomes and alter their pattern of peptidase activities. These changes are likely to enhance the yield of peptides with C termini appropriate for MHC binding and have been shown to enhance the presentation of at least some antigens. IFN-γ also upregulates leucine aminopeptidase, which should promote the removal of N-terminal flanking residues of antigenic peptides. Also, this cytokine downregulates the expression of a metallo-proteinase, thimet oligopeptidase that actively destroys many antigenic peptides. Thus, IFN-γ appears to increase the supply of peptides by stimulating their generation and decreasing their destruction. The specificity and content of these various proteases should determine the amount of peptides available for antigen presentation. Also, the efficiency with which a peptide is presented is determined by the protein's half life (e.g. its ubiquitination rate) and the sequences flanking antigenic peptides, which influence the rates of proteolytic cleavage and destruction.     

Post-proteasomal antigen processing for major histocompatibility complex class I presentation

Peptides presented by major histocompatibility complex class I molecules are derived mainly from cytosolic oligopeptides generated by proteasomes during the degradation of intracellular proteins. Proteasomal cleavages generate the final C terminus of these epitopes. Although proteasomes may produce mature epitopes that are eight to ten residues in length, they more often generate N-extended precursors that are too long to bind to major histocompatibility complex class I molecules. Such precursors are trimmed in the cytosol or in the endoplasmic reticulum by aminopeptidases that generate the N terminus of the presented epitope. Peptidases can also destroy epitopes by trimming peptides to below the size needed for presentation. In the cytosol, endopeptidases, especially thimet oligopeptidase, and aminopeptidases degrade many proteasomal products, thereby limiting the supply of many antigenic peptides. Thus, the extent of antigen presentation depends on the balance between several proteolytic processes that may generate or destroy epitopes.     

The alpha 3 domain of major histocompatibility complex class I molecules plays a critical role in cytotoxic T lymphocyte stimulation

Allogeneic major histocompatibility complex class I molecules induce strong cytotoxic T lymphocyte (CTL) responses whereas xenogeneic molecules do not. We have tested a series of mouse/human hybrid molecules for their ability to stimulate mouse CTL. The molecules with murine alpha 3 domains consistently stimulated stronger CTL responses than those with human alpha 3 domains, independent of the species origin of the N-terminal alpha 1 or alpha 2 domains. We have found that the ability of class I molecules to induce strong cytotoxic responses correlates positively with their ability to stimulate expansion of the CD8+CD4-T cell subset. The results indicate that mouse T cells can recognize class I molecules with human alpha 1 and/or alpha 2 domains, but for efficient stimulation of these T cells it is important that the immunizing molecule contains a murine alpha 3 domain. We suggest that T cell priming requires an efficient interaction of CD8 with the class I alpha 3 domain, and this shows some species restriction.     

A single T cell receptor bound to major histocompatibility complex class I and class II glycoproteins reveals switchable TCR conformers

Major histocompatibility complex class I (MHCI) and MHCII proteins differ in structure and sequence. To understand how T?cell receptors (TCRs) can use the same set of variable regions to bind both proteins, we have presented a comparison of a single TCR bound to both MHCI and MHCII ligands. The TCR adopts similar orientations on both ligands with TCR amino acids thought to be evolutionarily conserved for MHC interaction occupying similar positions on the MHCI and MHCII helices. However, the TCR antigen-binding loops use different conformations when interacting with each ligand. Most importantly, we observed alternate TCR core conformations. When bound to MHCI, but not MHCII, Vα disengages from the Jα β strand, switching Vα's position relative to Vβ. In several other structures, either Vα or Vβ undergoes this same modification. Thus, both TCR V-domains can switch among alternate conformations, perhaps extending their ability to react with different MHC-peptide ligands.     

T cell receptor specificity for major histocompatibility complex proteins

The ligands for alpha beta T cell receptors (alphabetaTCRs) are usually major histocompatibility complex (MHC) proteins bound to peptides. Although there is evidence that T cell receptor variable regions have been selected evolutionarily to bind MHC, the rules governing this interaction have not previously been apparent. However, recent solved structures of T cell receptors with related variable regions bound to MHC plus peptides suggest that some amino acids in variable region CDR1 and CDR2s almost always react in a consistent way with MHC. These amino acids may therefore have been selected evolutionarily to predispose T cell receptors toward recognition of MHC ligands.     

Structural requirements for the peptide-induced conformational change of free major histocompatibility complex class I heavy chains.

In an attempt to define the structural features of peptides which are important for inducing the folding of free class I heavy chains in the absence of beta 2-microglobulin, and to determine whether they are the same as those required to form stable major histocompatibility complex (MHC): peptide adducts, we have used a panel of peptides related to the Db-binding nonamer ASNENMDAM (influenza nucleoprotein residues 366-374) with altered primary structures, and a number of other peptides which have the Db-binding "motif". In this way, we have shown that in addition to the "anchor" residues which define this motif, the alpha amino and carboxyl groups at the N and C termini also play a major role in both inducing the conformational change in free heavy chain (HC) and formation of a stable Db:peptide complex. We also show that the importance of the key residues is affected by the primary sequence "context" in which they appear. In addition, we have extended our original finding that naturally processed epitopes induce a conformational change in free HC to the H2Kb HC, and show that the effect does not require the presence of the class I alpha 3 domain.     

Differential half-life of major histocompatibility complex encoded class I molecules in T and B lymphoblasts

Major histocompatibility complex encoded class I molecules have been reported to be internalized in T lymphocytes but not in B lymphocytes. In order to better understand the physiology of these molecules, we investigated their kinetics of disappearance and the fate of antibody bound to them in T and B lymphoblasts. Metabolically labelled H-2K molecules were immunoprecipitated from the surface and from inside the cells after different periods of chase and analyzed by gel electrophoresis. In T lymphoblasts, there was a rapid disappearance of both surface and intracellular H-2K molecules with a half-life of about 5 hr. After a 20 hr chase, only lower mol. wt products were immunoprecipitated. In contrast, in B lymphoblasts, H-2K molecules were more stable with a half-life of greater than 20 hr. Bound anti-H-2K antibodies were degraded in T but not in B lymphoblasts. These results suggest that class I molecules and antibodies bound to them do not recycle back to the cell surface but are degraded after internalization in T lymphoblasts, whereas these molecules are less degraded in B lymphoblasts.     

Natural killer cell receptors for major histocompatibility complex class I and related molecules in cytomegalovirus infection

Downmodulation of major histocompatibility complex (MHC) class I molecules by cytomegalovirus (CMV) impairs the engagement of specific leucocyte-inhibitory receptors, rendering infected cells vulnerable to natural killer (NK) cells. Members of the murine Ly49 and human KIR families, CD85j (ILT2 or leucocyte Ig-like receptor-1), as well as the CD94/NKG2A-inhibitory killer lectin-like receptor (KLR) fulfil this surveillance role. On the other hand, NK-activating receptors specific to ligands expressed on virus-infected cells may overcome the control by inhibitory receptors. In this regard, NKG2D and Ly49H lectin-like molecules trigger NK-cell functions recognizing, respectively class I-related stress-inducible molecules and the m157 murine CMV glycoprotein. Among a variety of immune evasion strategies, CMV promotes the synthesis of class I surrogates and selectively preserves the expression of some class I molecules in infected cells; moreover, CMV interferes with the expression of ligands for NKG2D. We herein review these aspects of the host-pathogen interaction, discussing a number of open issues.