Killer cells reactive to altered-self antigens can also be alloreactive

The Thymus in Chagas Disease: Molecular Interactions Involved in Abnormal T-Cell Migration and Differentiation

Chagas disease, caused by the protozoan parasite T. cruzi, is a prevalent parasitic disease in Latin America. Presently, it is spreading around the world by human migration, thus representing a new global health issue. Chronically infected individuals reveal a dissimilar disease progression: while nearly 60% remain without apparent disease for life, 30% develop life-threatening pathologies, such as chronic chagasic cardiomyopathy (CCC) or megaviscerae. Inflammation driven by parasite persistence seems to be involved in the pathophysiology of the disease. However, there is also evidence of the occurrence of autoimmune events, mainly caused by molecular mimicry and bystander activation. In experimental models of disease, is well-established that T. cruzi infects the thymus and causes locally profound structural and functional alterations. The hallmark is a massive loss of CD4⁺CD8⁺ double positive (DP) thymocytes, mainly triggered by increased levels of glucocorticoids, although other mechanisms seem to act simultaneously. Thymic epithelial cells (TEC) exhibited an increase in extracellular matrix deposition, which are related to thymocyte migratory alterations. Moreover, medullary TEC showed a decreased expression of AIRE and altered expression of microRNAs, which might be linked to a disrupted negative selection of the T-cell repertoire. Also, almost all stages of thymocyte development are altered, including an abnormal output of CD4⁻CD8⁻ double negative (DN) and DP immature and mature cells, many of them carrying prohibited TCR-Vβ segments. Evidence has shown that DN and DP cells with an activated phenotype can be tracked in the blood of humans with chronic Chagas disease and also in the secondary lymphoid organs and heart of infected mice, raising new questions about the relevance of these populations in the pathogenesis of Chagas disease and their possible link with thymic alterations and an immunoendocrine imbalance. Here, we discuss diverse molecular mechanisms underlying thymic abnormalities occurring during T. cruzi infection and their link with CCC, which may contribute to the design of innovative strategies to control Chagas disease pathology.

Both positive and negative effects on immune responses by expression of a second class II MHC molecule

It is perplexing why vertebrates express a limited number of major histocompatibility complex (MHC) molecules when theoretically, having a greater repertoire of MHC molecules would increase the number of epitopes presented, thereby enhancing thymic selection and T cell response to pathogens. It is possible that any positive effects would either be neutralized or outweighed by negative selection restricting the T cell repertoire. We hypothesize that the limit on MHC number is due to negative consequences arising from expressing additional MHC. We compared T cell responses between B6 mice (I-A(+)) and B6.E(+) mice (I-A(+), I-E(+)), the latter expressing a second class II MHC molecule, I-E(b), due to a monomorphic Eα(k) transgene that pairs with the endogenous I-Eβ(b) chain. First, the naive T cell Vβ repertoire was altered in B6.E(+) thymi and spleens, potentially mediating different outcomes in T cell reactivity. Although the B6 and B6.E(+) responses to hen egg-white lysozyme (HEL) protein immunization remained similar, other immune models yielded differences. For viral infection, the quality of the T cell response was subtly altered, with diminished production of certain cytokines by B6.E(+) CD4(+) T cells. In alloreactivity, the B6.E(+) T cell response was significantly dampened. Finally, we observed markedly enhanced susceptibility to experimental autoimmune encephalomyelitis (EAE) in B6.E(+) mice. This correlated with decreased percentages of nTreg cells, supporting the concept of Tregs exhibiting differential susceptibility to negative selection. Altogether, our data suggest that expressing an additional class II MHC can produce diverse effects, with more severe autoimmunity providing a compelling explanation for limiting the expression of MHC molecules.

Alloreactivity: an old puzzle revisited

Alloreactivity, defined as a strong primary T cell response against allelic variants of major histocompatibility complex (MHC) molecules in the species, has been a long-standing puzzle in immunology with some of its details remaining unclear up to now. Here I shall provide a historical overview of how our understanding of alloreactivity has evolved and propose an interpretation that considers alloreactivity to be a mixture of four mechanistically distinct prototypes of T cell response, namely, self-restricted peptide specific, allorestricted peptide specific, alloreactive peptide dependent and alloreactive peptide independent. The relative contribution of each prototype to a given alloresponse is dependent on the extent of disparity (i.e. the number and nature of amino acid substitutions in the docking surface for T cell receptor) between the MHC molecule that the T cell recognizes as self and the stimulating MHC molecule.

Anti H-2Dd alloreactivity mediated by herpes-simplex-virus specific cytotoxic H-2k T lymphocytes is associated with H-2Dk

Herpes-simplex-virus (HSV) specific, H-2k-restricted, immune cytotoxic T lymphocytes also lyse noninfected H-2d target cells. Genetic mapping studies revealed that HSV-specific Dk-restricted CTL cross-react with allogeneic targets expressing Dd alloantigens. Cold target inhibition experiments indicate that only a minority of HSV-specific CTL mediate cross-reactive cytolysis. The data give an example of where the phenomenon of H-2-restricted versus nonrestricted responsiveness is not due to distinct subsets of T cells but solely depends on the antigenic determinants recognized.

The Tritope Model for restrictive recognition of antigen by T-cells II. Implications for ontogeny, evolution and physiology

Based on the Tritope Model of the TCR [Cohn, M., 2005c. The Tritope Model for restrictive recognition of antigen by T-cells. I. What assumptions about structure are needed to explain function? Mol. Immunol. 42, 1419-1443], a set of functional and evolutionary problems surrounding restrictive recognition of antigen are discussed. These include the origin of allele-specific recognition, the selection pressures for polygeneism and polymorphism, the TCR signaling interactions, the centrality of effector T-helper (eTh)-dependence for activation, the role of haplotype exclusion, "nonclassical" MHC-elements, alloreactivity versus xenoreactivity, etc. Further, a set of observations believed to support the Standard Model are reinterpreted.

An in depth analysis of the concept of "polyspecificity" assumed to characterize TCR/BCR recognition

A workshop group developed the concept of a "polyspecific" TCR/BCR in the framework of today's consensus model. They argue that the individual TCR/BCR combining site is composed of a packet of specificities randomly plucked from the repertoire, hence it is "polyspecific." This essay analyzes the conclusions of the workshop and suggests an alternative. "Polyspecificity" must be dissected into its two component parts, specificity and degeneracy. The TCR and the BCR must be treated differently because the TCR recognizes allele-specifically the MHC-encoded restricting element (R) that serves as the platform presenting peptide (P). Only the anti-P paratope of the TCR behaves analogously to the BCR paratope. The two paratopes are selected to recognize a shape-determinant referred to as an epitope or ligand. The paratope is functionally unispecific in recognition, not polyspecific, with respect to shape; it is degenerate in recognition with respect to chemistry. The recognized shape-determinant can be the product of many chemically different substances, peptide, carbohydrate, lipid, steroid, nucleic acid, etc. Such a degenerate set is functionally treated by the paratope as one shape/epitope/ligand and, in no sense, can a paratope recognizing such a degenerate set be described as "polyspecific." Degeneracy and specificity are concepts that must be distinguished. The two positions are analyzed in this essay, the experiments used to support the view that the paratope of the TCR/BCR is polyspecific, are reinterpreted, and an alternative framework with its accompanying nomenclature, is presented.

TCRβ Chain That Forms Peptide-Independent Alloreactive TCR Transfers Reduced Reactivity with Irrelevant Peptide/MHC Complex

A major feature of the TCR repertoire is strong alloreactivity. Peptides presented by allogeneic MHC are irrelevant for recognition by a subset of alloreactive T cells. To characterize peptide-independent TCRs at the molecular level, we forced the expression of a TCRbeta chain isolated from a peptide-independent alloreactive CD8+ T cell line. The alloreactive TCR repertoire in the transgenic mouse was peptide dependent. However, analysis of essential TCR contacts formed during the recognition of self-MHC-restricted Ag showed that fewer contacts with peptide were established by the transgenic TCRbeta chain, and that this was compensated by additional contacts formed by endogenous TCRalpha chains. Thus, reduced interaction with the peptide appears to be a transferable feature of the peptide-independent TCRbeta chain. In addition, these findings demonstrate that reactivity to peptides is preferred over the reactivity to MHC during the formation of the TCR repertoire.

Early T cell response to allografts occurring prior to alloantigen priming up-regulates innate-mediated inflammation and graft necrosis

The early inflammatory response within organ allografts is initiated by ischemia/reperfusion (I/R) and promotes subsequent alloantigen-primed T cell recruitment into and rejection of the graft. Polymorphonuclear leukocyte (PMN)-mediated tissue damage is a primary component of the early inflammation in allograft rejection. We sought to compare and elucidate the mechanism of early PMN infiltration into cardiac isografts and allografts. Despite identical production of PMN attractant chemokines, PMN infiltration following reperfusion into syngeneic and allogeneic grafts was not equivalent. PMN infiltration into isografts peaked at 9 to 12 hours post-transplant and quickly resolved. In contrast, PMN infiltration into allografts continued to elevated levels, peaking at 24 hours post-reperfusion. This amplified PMN infiltration into allografts did not resolve until 72 hours post-reperfusion and was accompanied by marked parenchymal necrosis. This early innate inflammatory response was regulated by IFN-gamma-producing CD8+ T cells present in the recipient before detectable alloantigen T cell priming. Co-culture with CD62L(low) CD8+ T cells, but not CD62L(high) CD8+ or CD62L(low) CD4+ T cells, harvested from naïve animals induced allogeneic endothelial cells to express IFN-gamma-dependent chemokines. These data demonstrate CD8+ T cell-mediated attack on the vascular endothelium of allografts within hours following organ reperfusion that amplifies innate immune-mediated intra-graft inflammation and necrosis.

In a Transgenic Model of Spontaneous Autoimmune Diabetes, Expression of a Protective Class II MHC Molecule Results in Thymic Deletion of Diabetogenic CD8+ T Cells

H-2(d) mice expressing both the influenza virus hemagglutinin (HA) as a transgene-encoded protein on pancreatic islet beta cells (InsHA), as well as the Clone 4 TCR specific for the dominant H-2K(d)-restricted HA epitope, can be protected from the development of spontaneous autoimmune diabetes by expression of the H-2(b) haplotype. Protection occurs due to the deletion of K(d)HA-specific CD8+ T cells. This was unexpected as neither the presence of the InsHA transgene nor H-2(b), individually, resulted in thymic deletion. Further analyses revealed that thymic deletion required both a hybrid MHC class II molecule, Ebeta(b) Ealpha(d), and the K(d) molecule presenting the HA epitope, which together synergize to effect deletion of CD4+CD8+ thymocytes. This surprising example of protection from autoimmunity that maps to a class II MHC molecule, yet effects an alteration in the CD8+ T cell repertoire, suggests that selective events in the thymus represent the integrated strength of signal delivered to each cell through recognition of a variety of different MHC-peptide ligands.

Renal transplant patients show variations in their self-reactive repertoires: a serial study

We addressed the question of whether allo-transplantation (Tx) induces breakdown of tolerance to self-antigens or alteration of the autoreactive T cell repertoire in humans. The serial variation of T cell autoreactivity was studied in the peripheral blood of 12 renal transplant patients, by autologous limiting dilution assay and autologous mixed lymphocyte reaction. Ten of 12 patients presented a positive response in autologous peripheral blood mononuclear cells in the post-Tx period, in contrast to four of 12 patients before Tx (P = 0.038). Multi-hit kinetics was found in 57% of the assays analyzed, indicating frequent regulatory control of the autologous response. Quantitative analysis performed in eight patients showed an increase in precursor frequency at >1 year post-Tx in five patients. These data indicate that autoreactivity increases or develops following Tx, in humans. Post-Tx events such as alloreactivity, infections or immunosuppression could interfere with the balance of autoreactive and regulatory cells, leading to changes in the T cell repertoires to self-antigens and eventually breakdown of self-tolerance. Further investigation is needed to elucidate whether post-Tx autoreactivity contributes to rejection, plays a regulatory role over alloreactivity or both, at separate times.

On the Lifespan of Virgin T Lymphocytes

To study the lifespan of virgin T lymphocytes, we removed the thymus from adult female mice and then, at various times afterward, tested their ability to mount an immune response to a newly encountered Ag, the male Ag H-Y. We found that unprimed thymectomized mice were able to generate a primary response to H-Y for some time after thymectomy but lost this ability at ∼6 mo. In contrast, mice that were primed to H-Y just after thymectomy continued to display immunological memory to H-Y for >1 year. These experiments show that primary immune responses disappear in the absence of a thymus.

The MHC reactivity of the T cell repertoire prior to positive and negative selection

T cell antigen receptors (TCRs) on mature T cells react with peptide antigens presented by self-MHC proteins and also frequently cross-react with foreign MHC proteins. The fundamental question whether MHC reactivity is inherent in the germline TCR sequences or is imposed by thymic selection was addressed here by inducing nonselective maturation of immature thymocytes in the absence of MHC molecules. MHC reactivity in the preselection repertoire is very high, but no higher than in the normal repertoire. Cross-reactivity of clones with multiple MHC molecules occurred to a similar extent in the preselection and MHC-selected repertoires. The results establish the MHC reactivity of the germline TCR repertoire, indicate the minimum fraction of immature thymocytes that must undergo negative selection, and suggest that some TCR-MHC contacts may be conserved.

Flexibility in T-cell receptor ligand repertoires depends on MHC and T-cell receptor clonotype

T-cell receptors (TCR) recognize peptides complexed to self-major histocompatibility complex (MHC) molecules. Recognition of peptide/MHC ligands by the TCR is highly peptide specific. However, certain TCRs can also recognize sequence-related and -unrelated ('mimicry') epitopes presented by homologous MHC molecules. Using two human, human leucocyte antigen-DR1 (HLA-DR1)-restricted T-cell clones specific for HA p307-319, we identified several diverse combinations of peptide-MHC complexes that are functionally equivalent in their ability to trigger T-cell stimulation. These findings demonstrate that a single TCR can productively interact with different peptides complexed to self- as well as non-self-MHC molecules. This extended reactivity is human leucocyte antigen (HLA) allele and TCR clonotype dependent, as the peptide repertoire recognized depends on the presenting HLA-DR molecule and varies among different TCRs that both recognize the HA p307-319/DR1 complex. Importantly, certain peptide analogues can completely change the HLA-restriction pattern of the TCR: T-cell recognition of the wild-type peptide that was absent in the context of a non-self HLA-DR molecule, was restored by complementing substitutions in altered peptide ligands, that could not be presented by the original restriction element. This mechanism may play an important role in allorecognition.

Molecular basis for the recognition of two structurally different major histocompatibility complex/peptide complexes by a single T-cell receptor

2C is a typical alloreactive cytotoxic T lymphocyte clone that recognizes two different ligands. These ligands are adducts of the allo-major histocompatibility complex (MHC) molecule H-2Ld and an endogenous octapeptide, and of the self-MHC molecule H-2Kb and another peptide. MHC-binding and T-cell assays with synthetic peptides in combination with molecular modeling studies were employed to analyze the structural basis for this crossreactivity. The molecular surfaces of the two complexes differ greatly in densities and distributions of positive and negative charges. However, modifications of the peptides that increase similarity decrease the capacities of the resulting MHC peptide complexes to induce T-cell responses. Moreover, the roles of the peptides in ligand recognition are different for self- and allo-MHC-restricted T-cell responses. The self-MHC-restricted T-cell responses were finely tuned to recognition of the peptide. The allo-MHC-restricted responses, on the other hand, largely ignore modifications of the peptide. The results strongly suggest that adaptation of the T-cell receptor to the different ligand structures, rather than molecular mimicry by the ligands, is the basis for the crossreactivity of 2C. This conclusion has important implications for T-cell immunology and for the understanding of immunological disorders.

Viral peptide specific cytotoxic T lymphocytes are of high avidity to host-MHC but only low avidity to donor-MHC after allogeneic bone marrow transplantation

In the thymus maturing T lymphocytes are positively selected for efficient interaction with self-MHC molecules. Consequently, mature peripheral T cells recognize foreign (microbial) antigens in association with self-MHC molecules (known as MHC restricted recognition). In experimental bone marrow transplantation (BMT) lymphohaemopoietic stem cells from an MHC disparate donor transfused to an irradiated host give rise to mature T lymphocytes with host-MHC restriction specificity. While experiments with T cell receptor transgenic mice have largely confirmed this concept, many studies using genetically unmanipulated animals analysing polyclonal T cell repertoires have also shown donor-MHC restricted T cell activities after allogeneic BMT. To analyse this discrepancy we generated 18 virus specific cytotoxic T cell (CTL) clones, 16 from F1 into parent and two from fully allogeneic bone marrow chimeras, and analysed the MHC restriction specificity in proliferation and cytotoxicity assays. The cytotoxicity of all the clones was primarily host-MHC restricted. However, the CTL clones proliferated to viral antigen presented by both donor- or host-MHC. Our model allowed CTL cloning by cross-specific stimulation with antigen plus either donor-MHC or else host-MHC. Interestingly, even the 14 CTL clones which had been raised with donor-MHC systematically killed host-MHC but not donor-MHC expressing cells. Thus, after BMT, CTLs may proliferate crossreactively to donor-MHC but cytolysis is predominantly directed to host-MHC expressing cells. Since lytic CTL activity probably reflects high avidity CTL interaction necessary for viral clearance in vivo, the data suggest that the donor-MHC restricted CTL activity may not be protective and that virus may escape CTL surveillance in donor cells.

Thymic and peripheral apoptosis of antigen-specific T cells might cooperate in establishing self tolerance

Aside from CD4+CD8+ double-positive (DP) thymocytes, the subpopulations of T lineage cells affected by negative selection are unknown. To address whether this process occurs in more mature cell types, we have compared the responses of purified single-positive (SP) murine thymocytes and peripheral T cells to the superantigen staphylococcal enterotoxin B (SEB) utilizing as antigen-presenting cells (APC) a fibroblast cell line expressing transfected I-Ek class II molecules. Whereas approximately 70% of SEB-reactive SP thymocytes, either CD4+ or CD8+, undergo programmed cell death (apoptosis) and, therefore, negative selection, CD4+ and CD8+ antigen-specific peripheral T cells are predominantly activated and proliferate to APC+SEB. Thus, mature thymocytes and peripheral T cells, with identical patterns and levels of expression of CD4, CD8 and T cell receptor (TCR), are programmed to elicit different responses following TCR stimulation. Unexpectedly, however activation of peripheral T cells was preceded by deletion of a large fraction of V beta 8+ T lymphocytes (SEB specific). This surprising phenomenon was also observed in in vivo studies: in fact, administration of SEB to adult mice resulted in depletion of the majority of antigen-specific T cells from the peripheral lymphoid tissues analyzed (lymph nodes and spleen). This depletion is the consequence of deletion as indicated by program cell death of V beta 8+ T cells and is followed by proliferation of the remaining SEB-reactive T cells. Clonal elimination of peripheral T cells may represent a mechanism by which tolerance to self antigens never expressed in and/or exported to the thymus is achieved.

The optimal number of major histocompatibility complex molecules in an individual

A straightforward argument is presented to calculate the number of different major histocompatibility complex (MHC) molecules in an individual that maximizes the probability of mounting immune responses against a large number of foreign peptides. It is assumed that increasing the number of MHC molecules per individual, n, has three different effects: (i) it increases the number of foreign peptides that can be presented; (ii) it increases the number of different T-cell receptors (TCRs) positively selected in the thymus; but (iii) it reduces the number of TCRs by negative selection. The mathematical analysis shows that n = 1/f maximizes the number of different TCRs that pass through positive and negative selection and that n = 2/f maximizes the probability to mount immune responses against a large fraction of foreign peptides. Here f is the fraction of TCRs deleted by one MHC molecule. Both results depend on approximations that are discussed in the paper. The model presented has implications for our understanding of the evolutionary forces acting on the MHC.

HLA-DR peptide-induced alloreactive T cell lines reveal an HLA-DR sequence that can be both "dominant" and "cryptic": evidence for allele-specific processing

Previously, we reported on a T cell line, ThoU6, which we obtained through stimulation of DPw3+ cells with a synthetic "DR3 peptide" with a sequence identical to the third hypervariable region of the DRB1*0301 chain. This T cell line recognizes both the synthetic peptide presented by DPw3 as well as DR3+ DPw3+ stimulator cells. This implies that the synthetic DR3 peptide has a natural counterpart in DR3-positive cells. Here we describe the recognition pattern of another T cell line that was sensitized with the same synthetic DR3 peptide. This T cell line, BieU6, shows both HLA-DRw13/Dw18 (self)-restricted recognition of the synthetic DR3 peptide and allorecognition towards DR13/Dw19, a molecule which is highly homologous to Dw18, in the absence of synthetic peptide. These results suggest that the epitope formed by the Dw18 molecule plus the synthetic DR3 peptide and recognized by T cell line BieU6 mimics the Dw19 molecule. The potential role for a Dw19-specific peptide is discussed. The inability of T cell line BieU6 to recognize Dw18+ DR3+ cells indicates that, in this case, the synthetic DR3 peptide is "cryptic", i.e. does not have a natural counterpart that is effectively presented to T cells. Mapping of the shortest peptides recognized by T cell lines ThoU6 and BieU6 indicate that these sequences are fully overlapping. We, therefore, suggest that the antigen-presenting molecules, HLA-DPw3 and HLA-Dw18, differ in their accessibility for self peptides derived from the third hypervariable region of DR molecules. These observations may be explained by allele-specific processing.

Identification of an autologous insulin B chain peptide as a target antigen for H-2Kb-restricted cytotoxic T lymphocytes

We have examined the CD8+ peripheral T cell repertoire of C57BL/6 (H- 2b) mice for cytotoxic T lymphocyte (CTL) reactivities to insulin, using in vitro immunization with a chymotryptic digest of reduced bovine insulin. The results presented in this study demonstrate that potentially autoreactive H-2Kb-restricted cytotoxic T cells specific for an autologous insulin B chain peptide are present in the preimmune splenic T cell repertoire. The immunogenic peptide comprises residues 7- 15 from the insulin B chain and has features in common with naturally processed Kb-restricted peptides identified by others. The minimal peptide sequence recognized by these cytotoxic T cells is 10-15, which is highly conserved in mammalian species and constitutes a self-peptide in mice. The presence of class I major histocompatibility complex- restricted CTLs with potentially autoreactive specificities in preimmune animals raises the possibility of a role for such cells in autoimmune disease states. Possible mechanisms for the in vivo expansion of insulin peptide-specific CTLs are discussed.

Tumorigenicity and H-2 expression of papillomavirus-transformed mouse cell lines

Tumorigenicity in immunocompetent syngeneic mice and H-2 class I antigen expression of BPV1-transformed mouse cell lines had no correlation. H-2 expression was examined using monoclonal anti-(H-2Kb) and anti-(H-2Db) antibodies in immunofluorescence staining for flow cytometry analysis and by determining the sensitivity of the cells to cytolysis by allostimulated spleen cells. Nontumorigenic cell lines were as resistant as tumorigenic cell lines to natural killer activity. The results indicate that in our model defence by natural killer cells is not a decisive factor. The results also show that instead of or in addition to H-2 class I antigens other factors (e.g. the presence or absence of virus-specific antigens) are important in determining the tumorigenicity of BPV1-transformed cell lines.

Ability of pure resting CD8+ human T cells to respond to alloantigen

The ability of highly purified resting human CD8 cells to respond to alloantigens in vitro was examined. Necessary conditions for induction of interleukin 2 receptors (IL-2R), IL-2 production, proliferative responses, and various effector functions were determined. Allogeneic non-T cells induced IL-2R expression in a high proportion of resting CD4 and CD8 cells, but only CD4 cells produced detectable amounts of IL-2. CD8 cells also became IL-2 responsive upon stimulation with purified resting allogeneic CD4 or CD8 cells, indicating that HLA class I+, II- cells alone may initiate activation of resting CD8 cells. The activated CD8 cells needed the presence of simultaneously activated CD4 cells or exogenous IL-2 to be able to synthesize DNA. Effector functions like cytotoxicity, mixed lymphocyte culture (MLC) suppression, or gamma interferon (IFN-gamma) production were also only detectable when the CD8 cells were activated in the presence of IL-2.

Proliferative response of peripheral blood mononuclear cells to autologous and allogeneic muscle in patients with polymyositis/dermatomyositis

We examined the proliferative responses of peripheral blood mononuclear cells (PBMC) to autologous and homologous muscle homogenates in 21 patients with early, active, untreated polymyositis/dermatomyositis (PM/DM), 8 patients with chronic PM/DM, 10 patients with myopathies other than PM/DM, 7 patients with connective tissue diseases without myositis, and 12 healthy individuals. PBMC from patients with PM/DM and from control subjects were incubated with various dilutions of autologous and homologous muscle homogenates. PBMC from patients with active PM/DM underwent significant proliferation on exposure to both the autologous muscle and the homologous muscle homogenates. In contrast, PBMC from patients with chronic PM/DM, other myopathies, connective tissue diseases without myositis, and from healthy individuals did not respond to either autologous or homologous muscle. Our findings demonstrate that the PBMC of patients with PM/DM are sensitized to muscle.

Recognition of xeno-(HLA, SLA) major histocompatibility complex antigens by mouse cytotoxic T cells is not H-2 restricted: a study with transgenic mice

Cytotoxic T lymphocytes (CTLs) recognize antigens in the context of major histocompatibility complex (MHC) class I gene products. The T-cell receptor (TCR) that mediates this MHC-restricted antigen recognition recognizes short peptide fragments rather than the intact antigen. Presentation of peptides to the TCR may thus be a major function of the MHC. An intriguing question emerging from this model is whether peptide presentation also applies to foreign MHC antigens and which of the available MHC molecules can present preferentially the peptides of the foreign MHC molecule. Allo- and xenoreactive CTLs might either recognize native MHC class I molecules or peptides presented by self MHC or by the foreign class I MHC itself. The finding that synthetic peptides corresponding to MHC class I regions are recognized by allo- and xenoreactive CTLs suggests that recognition of foreign MHC by CTLs might involve degraded fragments presented by syngeneic class I molecules. We used MHC transgenic mice as a tool to study these questions. The CTL responses against human (HLA) antigen B27 were analyzed by using HLA-B27 transgenic mice with various H-2 haplotypes. We report here that mouse xeno-MHC-specific (anti-B27) CTLs are perfectly able to kill human and mouse cells expressing the appropriate xenoantigen and that in primary and secondary responses to xeno-MHC, the mouse T-cell repertoire does not use self-H-2 as a restriction element. Absence of H-2 restriction was confirmed by the lack (less than 1/10(6] of H-2-restricted HLA-specific CTL precursors. Therefore, H-2-restricted recognition of xeno-MHC antigens cannot be generalized as part of a classical MHC class I-specific response. These results indicate that xenoreactive CTLs usually recognize intact MHC molecules or MHC peptides preferentially presented by their native MHC molecule. We suggest the latter possibility.

Positive selection of antigen-specific T cells in thymus by restricting MHC molecules

Thymus-derived lymphocytes (T cells) recognize antigen in the context of class I or class II molecules encoded by the major histocompatibility complex (MHC) by virtue of the heterodimeric alpha beta T-cell receptor (TCR). CD4 and CD8 molecules expressed on the surface of T cells bind to nonpolymorphic portions of class II and class I MHC molecules and assist the TCR in binding and possibly in signalling. The analysis of T-cell development in TCR transgenic mice has shown that the CD4/CD8 phenotype of T cells is determined by the interaction of the alpha beta TCR expressed on immature CD4+8+ thymocytes with polymorphic domains of thymic MHC molecules in the absence of nominal antigen. Here we provide direct evidence that positive selection of antigen-specific, class I MHC-restricted CD4-8+ T cells in the thymus requires the specific interaction of the alpha beta TCR with the restricting class I MHC molecule.

Predominant variable region gene usage by gamma/delta T cell receptor-bearing cells in the adult thymus

Previous studies have indicated that the diversity of gamma genes expressed by gamma/delta-bearing murine T cells is limited, but comparable information concerning the expressed diversity of delta genes is lacking. In this study, we have investigated the rearrangement and expression of delta and gamma genes in T cell hybridomas that express gamma/delta T cell receptors. Three productive delta chain cDNA clones were isolated (delta 7.3, delta 7.1, and delta 2.3) that encode new variable region sequences. Two of the delta cDNAs differ significantly from those observed in the V alpha repertoire. In addition, one cDNA expressed a new J delta region (J delta 2), which was localized between J delta 1 and C delta genes. Using these and other delta gene probes and gamma gene probes, we found that five independent hybridomas expressed four different V delta s and three different V gamma s. However, analysis of an enriched population of gamma/delta-expressing cells from the adult thymus suggests that only a few V delta genes and one V gamma gene are used by the majority of the cells. These results suggest that important components of receptor chain that contribute to specificity (i.e., the germline V gene sequences) are relatively nondiverse in the thymic gamma/delta population.

Alloreactive immune responses of transgenic mice expressing a foreign transplantation antigen in a soluble form

Transfection of cells with the H-2Kk gene lacking the transmembrane and cytoplasmic segments resulted in secretion of the H-2Kk protein, as determined by immunoprecipitation with monoclonal anti-H-2Kk antibodies. Transgenic (H-2b X H-2d)F1 mice were established carrying integrated copies of the modified H-2Kk gene. Expression of the soluble H-2Kk antigen in the transgenic mice was demonstrated in cell supernatants of biosynthetically labeled splenic and thymic Con A blasts as well as bone marrow-derived macrophages. Soluble H-2Kk molecules were also present in the sera of the transgenic animals. No cell-surface expression of the H-2Kk antigen could be observed. In spite of the presence of the soluble H-2Kk molecules in the transgenic mice, the animals were able to generate H-2Kk-specific cytolytic T cells as well as antibody responses when stimulated with cell-surface-bound H-2Kk antigens. These responses were indistinguishable from those of the nontransgenic littermates. Possible explanations for the observed lack of tolerance are discussed.

The foreign antigen binding site and T cell recognition regions of class I histocompatibility antigens

Most of the polymorphic amino acids of the class I histocompatibility antigen, HLA-A2, are clustered on top of the molecule in a large groove identified as the recognition site for processed foreign antigens. Many residues critical for T-cell recognition of HLA are located in this site, in positions allowing them to serve as ligands to processed antigens. These findings have implications for how the products of the major histocompatibility complex (MHC) recognize foreign antigens.

Recognition requirements for the activation, differentiation and function of T-helper cells specific for class I MHC alloantigens

The present review has focused on the specificity of the T-helper cell populations initiating MHC class I alloreactions. In contrast to conventional immune responses against soluble antigens, responses against membrane-bound class I alloantigens are initiated by two distinct antigen-specific T-helper cell populations that can be distinguished by their Lyt phenotype, MHC restriction specificity, antigen specificity, and requirement for thymically determined self-recognition. Alloresponses were shown to be a composite consisting of two distinct components: one mediated by L3T4+ Th cells and very similar to conventional self + X responses; and one mediated by Lyt2+ Th cells and unique to alloresponses against MHC class I antigens. As would befit an unusual Th cell population, the recognition/response spectrum of Lyt2+ Th cells was highly unusual and was found to be the basis for much of the uniqueness we attribute to immune alloreactions, including rapid rejection of tissue allografts in vivo.

Role of MHC class-I antigens and the CD3 complex in the lysis of autologous human tumours by T-cell clones

Peripheral blood lymphocytes (PBL) of 4 patients with malignant effusions were stimulated for 6 days with purified autologous tumour cells, before isolation of the lymphoblasts and cloning by limiting dilution in interleukin-2 (IL-2). Forty-five clones were analyzed for cytotoxicity (CTX) against autologous, allogeneic tumour and erythromyeloid K562 cells of known status with respect to expression of major histocompatibility complex (MHC) antigens, estimated by reaction with the W6/32 (anti HLA, -A, -B, -C monomorphic) and TDR 31.1 (anti HLA-DR) monoclonal antibodies (MAb). All 45 clones were CD3+. Twenty-five (56%) of them were cytotoxic for at least one target; 24 were autoreactive (restricted in 7); 17 were alloreactive; 16 were K562 reactive. Under comparable conditions autoreactivity was partially blocked by W6/32 in 12/20 effector:target combinations; alloreactivity in 8/13 and K562 reactivity in 0/14. Modulation of effector cell surface CD3 antigens by OKT3 monitored by flow cytometry reduced autoreactivity in 9/14 combinations, alloreactivity in 2/6 and K562 reactivity in 0/4. W6/32 blocking and T3 modulation of cytotoxicity were almost invariably concordant against the same target. The data suggest that, to accomplish lysis of autologous and allogeneic tumour targets, certain clones require MHC recognition and a functional CD3 complex, while for others with similar target cell repertoires, there is no such requirement. It is possible that T-cell clones responding to a tumour-associated antigen (TAA) in the context of self MHC antigens can also respond to an allogeneic class-I product in the absence of TAA, and/or that aberrant class-I antigen expression on autologous tumours accounts for the alloreactivity.

Mammalian T-lymphocyte antigen receptor genes: genetic and nongenetic potential to generate variability

T lymphocytes of higher vertebrates are able to specifically recognize a seemingly unlimited number of foreign antigens via their receptors, the T cell antigen receptors (TCRs). T lymphocytes mature by passing through the thymus and acquire antigen specificity by expressing the TCR molecules on their cell surface. Genetic and somatic diversification mechanisms give rise to the enormous degree of TCR variability observed in mature T cells: germline and combinatorial diversity as well as junctional and the so-called N-region diversity. In contrast to the situation in immunoglobulin genes somatic hypermutation does not seem to play a significant role in TCR diversification. It is argued here that the enzyme terminal nucleotidyl-transferase is potentially a major factor in generating the immense diversity. We propose furthermore that this enzyme ensures the flexibility of T cell responses to novel antigens by random insertion of so-called N-region nucleotides. Apart from the physiological functions of TCR genes any involvement in the etiology of T cell neoplasia remains to be proven.

H-2-specific antibodies induced by injection of syngeneic leukemia cells

AKR/J mice immunized with several syngeneic leukemia cells contained antibodies in their sera which reacted with certain AKR leukemia cell lines, depending on their H-2 expression, and precipitated H-2K antigens from lysates of leukemia cells. Precipitation of H-2K was not due to virus-specific antibodies: it could not be blocked by prior absorption with H-2-negative leukemias, but was blocked by certain allogeneic lymphocytes. Tumor-specific H-2K antibodies did not react with H-2K from normal AKR lymphocytes either on the cell surface or after detergent solubilization; however, they did react with H-2K from mitogen-activated AKR and BALB.K lymphoblasts. Since both these latter cells were also lysed by AKR-Gross/MuLV-specific and H-2Kk-restricted cytotoxic T lymphocytes, we consider the possibility that antibodies detecting conformational alterations induced in H-2Kk molecules by viral association may be present in syngeneic AKR antileukemia sera.

Reactive T cells in the immune repertoire: self-restricted and allo-restricted helper T-cell clones to Epstein-Barr virus

Helper T-cell clones were generated by stimulation with autologous or allogeneic lymphoblastoid B cells (B-LCL) transformed by the Epstein-Barr virus (EBV). Some of these T-cell clones were allo-reactive and others were specific to EBV-transformed B-LCL. Helper T-cell clones specific to EBV-transformed B-LCL were restricted either by class-I or by class-II HLA molecules of self. T-cell clones restricted by class-I HLA molecules were stained by OKT3 and OKT8 monoclonal antibodies (MAbs), whereas class-II-restricted clones stained with OKT3 and OKT4. Not all helper T-cell clones specific to EBV-transformed B-LCL were restricted to self: one clone restricted by allo-HLA antigen was established. This finding suggests that in humans, as in mice, some T cells in the T-cell repertoire can be allo-restricted. This allo restriction may represent cross-reactivity of T cells, whereby "self + X" equals "allo + Y." Activation of these cross-reacting T cells restricted by allogeneic HLA molecules during infectious mononucleosis will give a T-cell response which may appear unrestricted by self HLA molecules. This mechanism helps to explain, at least in part, the HLA unrestricted cytotoxicity to B-LCL observed in infectious mononucleosis.

Cross-reactive recognition by antigen-specific, major histocompatibility complex-restricted T cells of a mitogen derived from Mycoplasma arthritidis is clonally expressed and I-E restricted

In order to determine whether or not the major histocompatibility complex (MHC)-encoded restriction element used by a T cell in the recognition of its primary antigen affected its ability to be cross-reactively stimulated by MAS (a soluble product of Mycoplasma arthritidis), a panel of cloned, soluble antigen-specific I-A- and I-E-restricted T cells were tested for their ability to cross-reactively recognize and respond to MAS. Initial studies indicated that all of the cloned T cells tested were capable of responding to MAS in the presence of genetically E alpha E beta-expressing (I-E+), but not E alpha E beta-non-expressing (I-E-) accessory cells (AC). However, subsequent studies demonstrated that the ability of most of these T cell clones to mount proliferative responses to MAS in the presence of I-E+ AC was dependent upon the presence of Lyt-1+2- T cells in the irradiated spleen cells which were used as AC sources. When T cell-depleted, I-E+ populations of spleen cells or an I-E+ antigen-presenting line (WEHI-5) were used as AC sources, only 6 of the 34 clones tested were found to be directly responsive to MAS. Subsequent to stimulation by MAS plus I-E product, these MAS-reactive T cell clones were capable of "recruiting" bystander T cells to proliferate. Finally, the ability of a given T cell clone to respond to MAS plus I-E product did not appear to be influenced by the restriction element used by that clone in its response to other antigens since both I-A-restricted and I-E-restricted T cell clones were responsive to MAS plus I-E in equivalent proportions. Thus, the data presented indicated that I-E-restricted T cell reactivity to MAS is a clonally expressed property of T cells that is independent of their conventional antigen specificities and MHC restriction patterns.

Correlations between T-cell specificity and the structure of the antigen receptor

The derived amino-acid sequences of the heterodimeric antigen receptors expressed by a series of murine T-cell clones are presented. A comparison of the receptor sequences indicates that several mechanisms for generating receptor diversity can influence T-cell specificity, including junctional diversity, combinatorial joining, and combinatorial chain associations.

Trans-stimulation of T cells: characterization of targets and involvement in loss of alloreactivity

The rapid loss of alloreactivity within populations of antigen-primed, in vitro propagated T cells cannot be explained by the appearance of suppressor cells nor by the dilution effect of the proliferative antigen-specific T cells alone. The involvement of trans-stimulation in loss of alloreactivity, i.e. the recruitment of non-antigen-specific T cells into proliferation in an antigen-dependent and specific fashion, was assessed. Susceptibility to trans-stimulation was found to correlate directly with state of activation at the outset of assay. Large T cells (low buoyant density) but not small T cells (high buoyant density) are susceptible to trans-stimulation. Moreover, in vitro pre-activation of small T cells by mitogen confers susceptibility to trans-stimulation. Analysis of the alloreactivity in Percoll fractions of antigen-primed lymph node T cells revealed activity in both large- and small-T-cell fractions with some enrichment in the latter. The small T cells, refractive to trans-stimulation, are diluted out of the population within the early weeks of antigen-mediated in vitro propagation, accounting for a rapid loss of considerable alloreactivity. The loss of all detectable alloreactivity within antigen-selected populations suggests that the state of activation conferring sensitivity to trans-stimulation must be maintained, and that neither the antigen nor the culture conditions employed met this requirement.

Evidence that major histocompatibility complex restriction of foreign transplantation antigens occurs when tolerance is induced in neonatal mice and rats

Studies on the survival of skin-specific antigen (Skn)-incompatible skin grafts in mice rendered tolerant at birth with major histocompatibility complex (MHC)-incompatible lymph node and spleen or bone marrow cells, as well as studies concerned with the survival of third-party skin grafts in rats rendered tolerant at birth with MHC-incompatible bone marrow cells, indicate that MHC restriction of foreign transplantation antigens occurs when tolerance is induced. Thus, evidence is presented that animals rendered tolerant with MHC-incompatible bone marrow cells depleted of mature T lymphocytes will accept any graft that is homozygous for the bone marrow donor's foreign MHC. Evidence has also been obtained that continuous exposure to foreign transplantation antigens in association with an MHC different from that of the graft may induce unresponsiveness to the same antigens in association with the MHC of the graft.

Fine specificity analysis of lactate dehydrogenase B-specific proliferating T cell clones: implications for the mechanism of alloreactivity

T cell clones of C57BL/6 origin which recognize porcine lactate dehydrogenase B (LDH-BP) together with the Ab molecule were characterized in terms of fine specificity for both LDH-B and self-major histocompatibility complex determinants. Using antigen-presenting cells from the Ab-mutant strain B6.C-H-2bm12 (bm12), three clonotypes could be distinguished: the first responds to LDH-BP + bm12, the second fails to respond and the third is alloreactive to bm12. The last clone exhibits additional alloreactivities to A molecules expressed in strains of H-2 haplotypes f, r, s, u, w6, w7, w16, w17 and w23. All three clonotypes give identical response patterns to a panel of 17 different dehydrogenase enzymes, and react to the same tryptic peptide of LDH-BP. Thus, these clones appear to recognize the same LDH-B epitope together with at least 3 different determinants of the Ab molecule. The data suggest that alloreactivity is more closely related to T cell specificity for self-major histocompatibility complex than to specificity for foreign antigen.

Involvement of I-J epitopes in the self- and allo-recognition sites of T cells: blocking of syngeneic and allogeneic mixed lymphocyte reaction-responder cells by monoclonal anti-I-J antibodies

Monoclonal anti-I-Jk antibodies (mAbs) were found to inhibit syngeneic and allogeneic mixed lymphocyte reactions by blocking the responder T cells but not the stimulator cells. Only the responses of H-2k and H-2a strains were inhibited. Three different anti-I-Jk mAbs (1G8, 4B11, and KN34) showed different inhibitory patterns in allogeneic mixed lymphocyte reactions of individual H-2k strains, depending on the H-2 and immunoglobulin heavy chain variable region (IgVH) genes possessed by the stimulator strains. The results indicated that I-J epitopes are involved in the self- and allo-recognition sites of T cells, which are clonally distributed and used to recognize Ia plus IgVH-linked products.

Spinal fluid lymphocytes responsive to autologous and allogeneic cells in multiple sclerosis and control individuals

Spinal fluid lymphocytes from multiple sclerosis (MS) patients and controls were stimulated with either autologous non-T cells or with allogeneic non-T cells followed by stimulation with autologous non-T lymphocytes. Cells responding to these stimuli were cloned and their proliferative responses to autologous and allogeneic MS and normal non-T cells were measured. Large numbers of clones with specific patterns of reaction to both autologous and allogeneic cells were obtained from lymphocytes in MS cerebrospinal fluid (CSF), but only occasionally from cells in control CSF. Patterns of responses among clones from a particular CSF were similar and often identical, which suggested that cells in MS CSF were relatively restricted in their specificities. Surface antigen phenotyping of the clones showed them to be predominantly OKT4+, with 13% OKT8+ and 11% OKT4+8+. Peripheral T cells that were stimulated and cultured in parallel with CSF cells were different in that they usually did not give rise to as many clones nor were their patterns of response similar. Many CSF clones were heteroclitic, that is they responded to particular allogeneic cells but not autologous cells. Lymphocytes in MS CSF thus appear to represent a selected population of cells with a high frequency of responsiveness to autologous and allogeneic antigens. Such responses may be evidence for immune regulation within the central nervous system or could represent responses to altered-self antigens.

Induction of "allogeneic effect"-like reaction by syngeneic TNP-modified lymphoid cells

TNP-substituted SRBC-immune spleen cells, when injected into cyclophosphamide-treated recipients, are recognized by T lymphocytes and produce, in the presence of specific antigen (SRBC), significantly more PFC than nonsubstituted cells. Labeling of immune B cells is more important in producing the augmented responses than is the labeling of immune T cells. TNP determinant has to be bound directly to the transferred immune cells to produce enhanced antibody responses, as when recipients were injected with non-substituted immune cells and TNP-substituted non-immune cells simultaneously, no increase in PFC number was noted (lack of a bystander effect). When recipients were rendered tolerant to TNP, two separate effects were observed, dependent on the mode of inducing unresponsiveness. In mice which were treated with TNP over an extended period of time, lack of recognition of TNP was demonstrated, such that TNP-substituted cells failed, when transferred, to produce an augmented response. When a short-term tolerogenic regime was used, the adoptively transferred TNP-labeled cells gave a very poor response (greater than 95% inhibition) due to in vivo suppression and/or killing. These results, together with the lack of influence of tolerance induced to unrelated hapten (DNP or OX), confirm the antigen specificity of the phenomenon. The reaction observed by us shows a striking resemblance with, but not identical to, the "allogeneic effect" produced by MHC encoded alloantigens. Our results extend the list of analogous immune reactions induced by MHC encoded alloantigens and TNP-derivatized self.