Selectively expanding subsets of T cells in mice by injection of interleukin-2/antibody complexes: implications for transplantation tolerance

The biological activity of interleukin (IL)-2 and other cytokines in vivo can be augmented by binding to certain anti-cytokine monoclonal antibodies (mAb). Here, we review evidence on how IL-2/anti-IL-2 mAb complexes can be used to cause selective stimulation and expansion of certain T-cell subsets. With some anti-IL-2 mAbs, injection of IL-2/mAb complexes leads to expansion of CD8 T effector cells but not CD4 T regulatory cells (Tregs); these complexes exert less adverse side effects than soluble IL-2 and display powerful antitumor activity. Other IL-2/mAb complexes have minimal effects on CD8 T cells but cause marked expansion of Tregs. Preconditioning mice with these complexes leads to permanent acceptance of MHC-disparate pancreatic islets in the absence of immunosuppression.

T cell memory

T cell memory induced by prior infection or vaccination provides enhanced protection against subsequent microbial infections. The processes involved in generating and maintaining T cell memory are becoming better understood due to recent technological advances in identifying memory T cells and monitoring their behavior and function in vivo. Memory T cells develop in response to a progressive set of cues-starting with signals from antigen-loaded, activated antigen-presenting cells (APCs) and inflammatory mediators induced by the innate immune response, to the poorly defined subsequent signals triggered as the immune response wanes toward homeostasis. The persistence of the resting memory T cells that eventually develop is regulated by cytokines. This chapter discusses recent findings on how memory T cells develop to confer long-term protective immunity.

NKT cells derive from double-positive thymocytes that are positively selected by CD1d

CD1d-reactive NKT cells are a separate T cell sublineage. Instructive models propose that NKT cells branch off the mainstream developmental pathway because of their T cell antigen receptor specificity, whereas stochastic models would propose that they develop from precursor cells committed to this sublineage before variable-gene rearrangement. We show here that immature double-positive (DP) thymocytes form the canonical rearranged Valpha gene of NKT cells at nearly equivalent frequencies in the presence or absence of CD1d expression. After interacting with CD1d in the thymus, these cells give rise to expanded populations of NKT cells-including both CD4+ and double-negative lymphocytes in the thymus and periphery-that express this alpha chain. These results confirm the existence of a DP intermediate for CD1d-reactive NKT cells. They also show that the early developmental stages of these T cells are not governed by a distinct mechanism, which is consistent with the TCR-instructive model of differentiation.

Long-term persistence and reactivation of T cell memory in the lung of mice infected with respiratory syncytial virus

In mice acutely infected with respiratory syncytial virus (RSV), more than 20% of pulmonary CD8(+) T cells, but only 2-3% of CD8(+) T cells in the draining lymph node secreted interferon-gamma in response to a single peptide. Surprisingly, the percentage of virus-specific T cells in the lung remained at these high levels long after the acute infection. Pulmonary memory T cells were further studied in a sensitive adoptive transfer system, which allows visualizing polyclonal CD4(+) and CD8(+) virus-specific memory T cell responses. Fifty days after infection, persisting RSV-specific pulmonary T cells remained CD69(hi) CD62L(lo), but had returned to a resting memory state according to functional criteria. In the absence of neutralizing antibodies reinfection first induced cell division among virus-specific memory T cells 3 days after infection predominantly in the local lymph node. However, divided cells then rapidly accumulated in the lung without significantly increasing in the lymph node. These results suggest rapid export of reactivated cells from the lymph node to the target organ. Thus, although memory T cells can be maintained in the infected organ after a localized virus infection, amplification of a recall response appears to be most effective in organized lymphoid tissue.

IL-7 is critical for homeostatic proliferation and survival of naive T cells

In T cell-deficient conditions, naive T cells undergo spontaneous "homeostatic" proliferation in response to contact with self-MHC/peptide ligands. With the aid of an in vitro system, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naive T cells in vitro. Of these cytokines, only IL-7 was found to be critical; thus, naive T cells underwent homeostatic proliferation in IL-4(-) and IL-15(-) hosts but proliferated minimally in IL-7(-) hosts. In addition to homeostatic proliferation, the prolonged survival of naive T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7(-) hosts. These findings indicate that naive T cells depend on IL-7 for survival and homeostatic proliferation.

Generation and maintenance of memory T cells

Typical immune responses lead to the prominent clonal expansion of antigen-specific T cells followed by their differentiation into effector cells. Most effector cells die at the end of the immune response but some of the responding cells survive and form long-lived memory cells. The factors controlling the formation and survival of memory T cells are discussed. Recent evidence suggests that T memory cells arise from a subset of effector cells. The longevity of T memory cells may require continuous contact with cytokines, notably IL-15 for CD8(+) cells.

Role of self-major histocompatibility complex/peptide ligands in selection and maintenance of a diverse T cell repertoire

Positive selection has long been thought to be a devise for producing a repertoire of T cells that can efficiently recognize foreign peptides in the context of self-major histocompatibility complex (MHC) molecules. However, in the light of recent evidence that long-term survival of mature T cells requires continuous contact with self-MHC molecules, the possibility for an additional role for positive selection has emerged: to generate a repertoire of T cells that can be maintained in the periphery through contact with self-MHC/peptide ligands. In support of this idea, our recent work suggests that positive selection is highly peptide specific and, more important, that mature T cells require extrathymic contact with the same MHC/peptide ligands that initially induced positive selection in the thymus in order for prolonged survival and to undergo homeostatic proliferation in response to T cell deficiency.

Transcription factor PU.1 is necessary for development of thymic and myeloid progenitor-derived dendritic cells

Dendritic cells (DCs) are a heterogeneous population of cells that are specialized for Ag processing and presentation. These cells are believed to derive from both myeloid- and lymphoid-committed precursors. Normal human PBMC-derived, human CD14+ cell (monocyte)-derived, and mouse hematopoietic progenitor-derived DCs were shown to express the hematopoietic cell-restricted, ets family transcription factor PU.1. These populations represent myeloid progenitor-derived DCs. Hematopoietic progenitor cells from PU.1 gene-disrupted (null) mice were unable to generate MHC class IIhigh, CD11c+ myeloid-derived DCs in vitro. Mouse thymic DCs are proposed to be derived from a committed lymphoid progenitor cell that can give rise to T cells as well as DCs. Previously, we showed that CD4 and CD8 T cells developed in PU.1 null mice in a delayed manner and in reduced number. We examined the thymus of 10- to 12-day-old PU.1 null mice and found no evidence of DEC-205+, MIDC-8+ DCs in this tissue. Our findings indicate that PU.1 regulates the development of both thymic and myeloid progenitor-derived populations of DCs, and expand its known role in hematopoietic development.

The role of H2-O and HLA-DO in major histocompatibility complex class II-restricted antigen processing and presentation

The function of major histocompatibility complex (MHC) class II molecules is to sample exogenous antigens for presentation to CD4+ T helper cells. After synthesis in the endoplasmic reticulum, class II molecules are directed into the endosomal system by association with the invariant chain (Ii), which is sequentially cleaved, generating class II dimers loaded with Ii-derived peptides (CLIP). These class II-peptide complexes are physiological substrates for H2-M/HLA-DM, a resident of the endosomal/lysosomal system which facilitates the removal of CLIP from newly synthesised class II alpha beta dimers. Exchange of CLIP for antigenic class II-binding peptides is also promoted by the action of H2-M/HLA-DM, resulting in stable peptide-class II complexes that are transported to the cell surface for presentation to CD4+ T cells. Recent evidence suggests that this H2-M/HLA-DM-mediated 'peptide editing' is influenced by another MHC class II-encoded molecule, H2-O/HLA-DO. This non-polymorphic alpha beta heterodimer is associated with H2-M/HLA-DM during intracellular transport and within the endosomal system of B cells. H2-O/HLA-DO alters the peptide exchange function of H2-M/HLA-DM in a pH-dependent manner, so that H2-M/HLA-DM activity is limited to more acidic conditions, corresponding to lysosomal compartments. Indeed, H2-O/HLA-DO may serve to limit the presentation of antigens after fluid phase uptake by B cells, while augmenting presentation of antigens internalised via membrane Ig receptors. Such a mechanism may maintain the fidelity of the B-cell-CD4+ T-cell interaction, counteracting self reactivity arising from less stringent lymphocyte activation. Here, data evaluating the role of H2-O/HLA-DO shall be reviewed and its putative function discussed.

The peptide ligands mediating positive selection in the thymus control T cell survival and homeostatic proliferation in the periphery

Positive selection to self-MHC/peptide complexes has long been viewed as a device for skewing the T cell repertoire toward recognition of foreign peptides presented by self-MHC molecules. Here, we provide evidence for an alternative possibility, namely, that the self-peptides controlling positive selection in the thymus serve to maintain the longevity of mature T cells in the periphery. Surprisingly, when total T cell numbers are reduced, these self-ligands become overtly stimulatory and cause naive T cells to proliferate and undergo homeostatic expansion.

Thymic selection by a single MHC/peptide ligand: autoreactive T cells are low-affinity cells

In H2-M- mice, the presence of a single peptide, CLIP, bound to MHC class II molecules generates a diverse repertoire of CD4+ cells. In these mice, typical self-peptides are not bound to class II molecules, with the result that a very high proportion of H2-M- CD4+ cells are responsive to the various peptides displayed on normal MHC-compatible APC. We show here, however, that such "self" reactivity is controlled by low-affinity CD4+ cells. These cells give spectacularly high proliferative responses but are virtually unreactive in certain other assays, e.g., skin graft rejection; responses to MHC alloantigens, by contrast, are intense in all assays. Possible explanations for why thymic selection directed to a single peptide curtails self specificity without affecting alloreactivity are discussed.

A role for Fas in negative selection of thymocytes in vivo

To seek information on the role of Fas in negative selection, we examined subsets of thymocytes from normal neonatal mice versus Fas-deficient lpr/lpr mice injected with graded doses of antigen. In normal mice, injection of 1-100 microg of staphylococcal enterotoxin B (SEB) induced clonal elimination of SEB-reactive Vbeta8+ cells at the level of the semi-mature population of HSAhi CD4+ 8- cells found in the thymic medulla; deletion of CD4+ 8+ cells was minimal. SEB injection also caused marked elimination of Vbeta8+ HSAhi CD4+ 8- thymocytes in lpr/lpr mice. Paradoxically, however, elimination of these cells in lpr/lpr mice was induced by low-to-moderate doses of SEB ( Collapse

Key Words Collapse

MESH Headings

• Amino Acid Sequence
• Animals
• Antibodies, Monoclonal/immunology
• Antibodies, Monoclonal/metabolism
• CD4-Positive T-Lymphocytes/immunology
• CD8-Positive T-Lymphocytes/immunology
• Clone Cells/immunology
• Enterotoxins/immunology
• Flow Cytometry
• Immune Tolerance/immunology
• Mice
• Mice, Inbred Strains
• Mice, Transgenic
• Molecular Sequence Data
• Ovalbumin/immunology
• Peptide Fragments/immunology
• Receptors, Antigen, T-Cell/genetics
• Thymus Gland/physiology
• fas Receptor/physiology

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Grants

• P01 AG001743 NIA NIH HHS
• R37 CA038355 NCI NIH HHS
• CA25803 NCI NIH HHS
• CA38355 NCI NIH HHS
• AI21487 NIAID NIH HHS

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Affiliation(s)

H Kishimoto Department of Immunology, IMM4, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, USA
C D Surh
J Sprent

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Altered antigen presentation in mice lacking H2-O

HLA-DM catalyzes the release of MHC class II-associated invariant chain-derived peptides (CLIP) from class II molecules. Recent evidence has suggested that HLA-DO is a negative regulator of HLA-DM in B cells, but the physiological function of HLA-DO remains unclear. Analysis of antigen presentation by B cells from mice lacking H2-O (the mouse equivalent of HLA-DO), together with biochemical analysis using purified HLA-DO and HLA-DM molecules, suggests that HLA-DO/H2-O influences the peptide loading of class II molecules by limiting the pH range in which HLA-DM is active. This effect may serve to decrease the presentation of antigens internalized by fluid-phase endocytosis, thus concentrating the B cell-mediated antigen presentation to antigens internalized by membrane immunoglobulin.

Thymic selection by a single MHC/peptide ligand produces a semidiverse repertoire of CD4+ T cells

The influence of individual peptides in thymic selection was examined in H2-M- mice, in which positive selection is directed to a single peptide, class II-associated invariant chain peptide (CLIP) bound to H2-A(b). Two sensitive in vivo approaches showed that 70%-80% of CD4+ T cells undergoing positive selection to CLIP+H2-A(b) have self-reactivity to the various peptides expressed on wild-type H2-M+ antigen-presenting cells. When these self-reactive T cells were depleted, the residual CD4+ cells displayed a polyclonal repertoire in terms of alloreactivity, responses to foreign protein antigens, and Vbeta usage. Nevertheless, studies with two T cell receptor transgenic lines suggested that the repertoire of CD4+ cells induced by CLIP was less diverse than the repertoire of CD4+ cells in normal mice. Generation of a fully diverse T cell repertoire thus requires positive selection against multiple peptides.

Rat stem cells developing in irradiated SCID mice fail to become tolerized and cause lethal graft-versus-host disease

Graft-versus-host disease (GVHD) is prominent in irradiated hosts given whole allogeneic bone marrow cells but is generally undetectable when T-depleted stem cells are transferred; under these conditions, the mature T cells arising from the donor stem cells become tolerant to host antigens and fall to cause GVHD. We show here that a radically different situation can occur when hosts are reconstituted with xenogeneic stem cells. When lightly irradiated, adult C.B-17 SCID mice injected with Lewis rat fetal liver (FL) cells show near-total repopulation with rat-derived lymphohemopoietic cells, including T and B cells. However, in marked contrast to chimeras prepared with allogeneic mouse FL cells, rat FL-->SCID chimeras develop severe and often lethal chronic GVHD. In these rat-->mouse chimeras, the rat T cells show limited tolerance to host mouse antigens as determined by various parameters including mixed lymphocyte reaction and cytotoxic T lymphocyte assays in vitro, adoptive transfer of T cells to secondary SCID hosts, and the lack of V beta deletion to endogenous host mtv antigens. GVHD in irradiated rat-->SCID chimeras is most prominent with Lewis FL but also applies to Fisher 344 and Wistar Furth FL cells. The failure of newly formed rat T cells in rat-->SCID chimeras to become fully tolerant to host mouse antigens appears to be due to depletion of host antigen-presenting cells by irradiation. Thus, rat-->SCID chimeras generated by transplanting rat FL cells into unirradiated neonatal SCID mice fail to develop GVHD, and the rat T cells display self-tolerance. As allogeneic H-2-different mouse FL-->irradiated SCID chimeras display strong self-tolerance, presumably through recognition of host antigens on thymic epithelial cells, the implication is that mouse thymic epithelial cells are tolerogenic only for mouse and not for rat immature T cells.

Antigen presentation in retroviral vector-mediated gene transfer in vivo

We have examined mechanisms involved in gene transfer, protein expression, and antigen presentation after direct administration of retroviral vectors using a variety of antigen systems. We have identified transduced infiltrating cells at the injection site, and the majority of the infiltrating cells were of the monocyte/macrophage lineage. We found that the splenic dendritic cell fraction contained proviral DNA, expressed antigenic proteins, and was able to present antigens efficiently to the immune system. Furthermore, the dendritic cell fractions from retroviral vector-immunized mice were able to prime naive T cells in vitro, and adoptive transfer of in vitro-transduced dendritic cell fractions elicited antigen-specific cytotoxic T lymphocytes. These data suggest a role for dendritic cells in induction of immune responses elicited by retroviral vector-mediated gene transfer.

Antigen compartmentation and T helper cell tolerance induction

The process of antigen recognition depends in part on the amount of peptide antigen available and the affinity of the T cell receptor for a particular peptide-major histocompatibility complex (MHC) molecule complex. The availability of self antigen is limited by antigen processing, which is compartmentalized such that peptide antigens presented by MHC class I molecules originate in the cytoplasm, whereas peptide antigens presented by MHC class II molecules are acquired from the endocytic pathway. This segregation of the antigen-processing pathways may limit the diversity of antigens that influence the development and selection of, e.g., CD4-positive, MHC class II-specific T cells. Selection in this case might involve only a subset of self-encoded proteins, specifically those that are plasma membrane bound or secreted. To study these aspects of immune development, we engineered pigeon cytochrome for expression in transgenic mice in two forms: one in which it was expressed as a type II plasma membrane protein, and a second in which it was targeted to the mitochondria after cytoplasmic synthesis. Experiments with these mice clearly show that tolerance is induced in the thymus, irrespective of antigen compartmentation. Using radiation bone marrow chimeras, we further show that cytoplasmic/mitochondrial antigen gains access to the MHC class II pathway by direct presentation. As a result of studying the anatomy of the thymus, we show that the amount of antigen and the affinity of the TCR affect the location and time point of thymocytes under-going apoptosis.

Bone marrow-derived cells fail to induce positive selection in thymus reaggregation cultures

The requirements for inducing positive selection of T cells were examined in thymus reaggregation cultures, a system in which dispersed populations of immature CD4+8+ cells and purified thymic epithelial cells (TEC) are reaggregated in tissue culture. Studies with TEC from mice selectively lacking major histocompatibility complex (MHC) class I (I-II+), class II (I+II-), or both class I and II (I-II-) molecules showed that class II expression was essential for the differentiation of CD4+8+ cells into CD4+8- cells. Unexpectedly, the generation of TCRhi CD4-8+ cells from CD4+8+ cells was apparent with I-II+ TEC but not with I-II- TEC, perhaps reflecting cross-reactive specificity of CD4-8+ cells for class II molecules. Significantly, the failure of I-II- TEC to generate TCRhi CD4+8- or CD4-8+ cells could not be overcome by adding MHC+ bone marrow-derived cells. These findings, together with experiments on purified subsets of TEC, suggest that positive selection in thymus reaggregation cultures is an exclusive property of cortical TEC.

Antigen presentation and T cell development in H2-M-deficient mice

HLA-DM (DM) facilitates peptide loading of major histocompatibility complex class II molecules in human cell lines. Mice lacking functional H2-M, the mouse equivalent of DM, have normal amounts of class II molecules at the cell surface, but most of these are associated with invariant chain-derived CLIP peptides. These mice contain large numbers of CD4+ T cells, which is indicative of positive selection in the thymus. Their CD4+ cells were unresponsive to self H2-M-deficient antigen-presenting cells (APCs) but were hyperreactive to wild-type APCs. H2-M-deficient APCs failed to elicit proliferative responses from wild-type T cells.

Thymic selection and cell division

Cell division during thymic selection was studied with a system in which purified populations of T cell antigen receptor (TCR)- CD4+8+ (double-positive [DP]) cells and fetal thymic epithelial cells (TEC) were reaggregated in tissue culture. In this system, immature DP cells differentiate into mature single-positive (SP) CD4+8- and CD4-8+ TCRhi cells within 3-4 d, indicative of positive selection. By adding the DNA precursor, bromodeoxyuridine, to the cultures and staining cells for bromodeoxyuridine incorporation, T cell division in reaggregation cultures was found to be high on day 1, low on day 2, and high on days 4-5. Cell separation studies established that cell division on day 1 was restricted to DP blast cells. In the absence of blast cells, small DP cells failed to proliferate and differentiated into SP cells without cell division, thus indicating that proliferation is not an essential component of positive selection. This applied to SP cells generated within the first 2-3 d. Surprisingly, the SP cells generated later in culture showed a high rate of cell division; the proliferating SP cells were TCRhi and included both CD4+8- and CD4-8+ cells. Turnover of TCRhi SP cells was also prominent in the normal neonatal thymus and in TEC reaggregation cultures prepared with adult lymph node T cells. We speculate that division of mature SP cells in the perinatal thymic microenvironment is driven by stimulatory cytokines released from TEC. Such proliferation could be a device to expand the mature T cell repertoire before export to the periphery.

Upregulation of surface markers on dying thymocytes

Using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) to detect cells undergoing early apoptosis, we have defined the surface markers expressed on CD4+CD8+ thymocytes undergoing spontaneous or steroid-induced apoptosis in tissue culture. Some surface markers, e.g., CD4, CD8, and heat stable antigen, are downregulated on apoptotic thymocytes. Surprisingly, however, other markers are upregulated; this applies to T cell receptor beta/CD3, CD69, and CD25 expression. Upregulation of these markers is restricted to a discrete subset of apoptotic cells.

T-cell apoptosis detected in situ during positive and negative selection in the thymus

Because of positive and negative selection to molecules of the major histocompatibility complex (MHC), only a small proportion of the massive numbers of T cells generated in the thymus are selected for export. Immature thymocytes have a rapid turnover, and it has long been assumed that most thymocytes die in situ, presumably from apoptosis. This has yet to be proved, however, and conventional staining techniques have shown only minimal evidence of cell death in the normal thymus. Using a method for detecting cells with DNA strand breaks, we now present direct evidence for apoptosis in the normal thymus. In sections of thymus from adult mice, apoptotic cells are scattered throughout the cortex and are engulfed locally by F4/80+ macrophages. Apoptosis in the thymic cortex is not reduced in MHC-deficient mice, which suggests that T-cell death is primarily a reflection of lack of positive selection rather than negative selection. Direct evidence for apoptosis due to negative selection was obtained by crossing a V beta 5 transgenic line to I-E+ and I-E- mice: I-E+ mice are known to eliminate V beta 5+ T cells in the thymus whereas I-E- mice do not. In marked contrast to I-E- mice, the medulla of I-E+ V beta 5 transgenic mice contains dense aggregates of apoptotic cells; these cells are engulfed by a distinct population of F4/80- MAC-3+ macrophages. Negative selection of V beta 5+ cells is thus restricted to the medulla.

Profound atrophy of the bone marrow reflecting major histocompatibility complex class II-restricted destruction of stem cells by CD4+ cells

The effector functions of CD4+ cells in vivo are presumed to reflect a combination of lymphokine-mediated bystander reactions and direct cytotoxic T lymphocyte activity. To assess the relative importance of these two mechanisms, we studied the effects of transferring small doses of purified unprimed CD4+ cells to lightly irradiated (600 cGy) recipients expressing major histocompatibility complex class II (Ia) differences. Within the first week after transfer, the host marrow was rapidly repopulated with hemopoietic cells. Thereafter, however, the donor CD4+ cells caused massive destruction of hemopoietic cells, both in marrow and spleen. Marrow aplasia did not affect stromal cells and was prevented by coinjecting donor but not host bone marrow. The use of allotypic markers and fluorescence-activated cell sorter analysis indicated that the destructive effects of CD4+ cells were directed selectively to host Ia+ hemopoietic cells, including stem cells; donor hemopoietic cells and Ia- host T cells were spared. No evidence could be found that the ongoing destruction of host cells impaired the capacity of donor stem cells to repopulate marrow, spleen, or thymus. Moreover, CD4+ cells failed to destroy host-type hemopoietic cells from Ia-deficient mice. Tissue destruction by CD4+ cells thus did not seem to reflect a bystander reaction. We conclude that, under defined conditions, CD4+ cells can manifest extremely potent Ia-restricted CTL activity in vivo, probably through recognition of covert Ia expression on stem cells and/or their immediate progeny.

B7 expression on thymic medullary epithelium correlates with epithelium-mediated deletion of V beta 5+ thymocytes

Recent evidence suggests that I-E+ thymic epithelium, especially medullary epithelium, can induce partial deletion of superantigen-reactive T cells expressing TcR V beta 5, V beta 11, and V beta 17. To seek further information on this issue, we constructed bone marrow chimeras in which MHC class II I-E is expressed on thymic epithelial cells at various levels and locations; the chimeras were reconstituted with stem cells from TcR V beta 5 transgenic mice. Intrathymic deletion of V beta 5 T cells was restricted to relatively mature T cells (expressing high TcR levels), and the degree of deletion correlated with the density of I-E expression in the thymic medulla rather than in the thymic cortex; selective I-E expression in medullary epithelium caused prominent deletion. Interestingly, immunostaining of normal and chimeric mice revealed that expression of B7 (the ligand for CD28) is largely restricted to a subset of medullary epithelial cells; these cells are I-E+ and co-express a specific carbohydrate bound by the lectin UEA-1. B7 expression was lower in thymuses of class II-deficient mice (A beta b-/-) and T-cell-deficient mice (SCID), suggesting that B7 expression is up-regulated during CD4+ thymocyte selection. In support of this idea, B7 expression in the thymus was restored to a normal level in bone marrow reconstituted SCID mice. Because B7 expression correlates with a costimulatory signal for T cells, selective expression of B7 and related antigens on I-E+ medullary epithelium may explain why these cells play a more prominent role in V beta deletion than cortical epithelium.

B7 expression on thymic medullary epithelium correlates with epithelium-mediated deletion of V beta 5+ thymocytes

Recent evidence suggests that I-E+ thymic epithelium, especially medullary epithelium, can induce partial deletion of superantigen-reactive T cells expressing TcR V beta 5, V beta 11, and V beta 17. To seek further information on this issue, we constructed bone marrow chimeras in which MHC class II I-E is expressed on thymic epithelial cells at various levels and locations; the chimeras were reconstituted with stem cells from TcR V beta 5 transgenic mice. Intrathymic deletion of V beta 5 T cells was restricted to relatively mature T cells (expressing high TcR levels), and the degree of deletion correlated with the density of I-E expression in the thymic medulla rather than in the thymic cortex; selective I-E expression in medullary epithelium caused prominent deletion. Interestingly, immunostaining of normal and chimeric mice revealed that expression of B7 (the ligand for CD28) is largely restricted to a subset of medullary epithelial cells; these cells are I-E+ and co-express a specific carbohydrate bound by the lectin UEA-1. B7 expression was lower in thymuses of class II-deficient mice (A beta b-/-) and T-cell-deficient mice (SCID), suggesting that B7 expression is up-regulated during CD4+ thymocyte selection. In support of this idea, B7 expression in the thymus was restored to a normal level in bone marrow reconstituted SCID mice. Because B7 expression correlates with a costimulatory signal for T cells, selective expression of B7 and related antigens on I-E+ medullary epithelium may explain why these cells play a more prominent role in V beta deletion than cortical epithelium.

Rat T cell responses to superantigens. II. Allelic differences in V beta 8.2 and V beta 8.5 beta chains determine responsiveness to staphylococcal enterotoxin B and mouse mammary tumor virus-encoded products

The previous paper in this series demonstrates that rat T cells developing de novo in the presence of mouse mammary tumor virus (Mtv) antigens in rat-->severe combined immunodeficiency (SCID) mouse xenochimeras display a distinct pattern of V beta-restricted deletion; this deletion pattern is remarkably similar to that occurring during thymic development of mouse T cells in Mtv+ strains. In addition, T cells developing in the absence of Mtv antigens in these rat-->mouse xenochimeras are tolerant of host antigens, but show strong primary proliferative responses in cultures stimulated with Mtv-7+ (Mlsa) mouse cells; like the mouse, these rat T cell responses are dominated by V beta 6 and V beta 8 T cells. Here, we continue analysis of rat T cell responses to superantigens; we show that T cells from Lewis and Fischer 344 rats expressing V beta 8.2 display an important all-or-nothing difference in their responses to Mtv-7 superantigens. This all-or-none strain difference in the response to Mtv-7 applies also to the response by V beta 8.2 and V beta 8.5 T cells to the soluble superantigen staphylococcal enterotoxin B. Because these two rat strains express different alleles of these two V beta 8 family members, this finding identifies additional, hitherto unreported residues of the T cell receptor beta chain important in T cell responses to superantigens.

Rat T cell response to superantigens. I. V beta-restricted clonal deletion of rat T cells differentiating in rat-->mouse chimeras

T cells of mice display V beta-specific reactivity for a spectrum of mouse mammary tumor virus (Mtv) antigens; confrontation with these antigens during ontogeny causes substantial "holes" in the T cell repertoire. Since endogenous Mtv antigens are rare in other species, the question arises whether V beta-specific recognition of Mtv antigens is unique to mice. To examine this question, rat T cells were allowed to differentiate from stem cells in severe combined immunodeficiency (SCID) mice. These rat-->mouse xenochimeras were prepared under a variety of conditions. The results show that rat T cells are strongly reactive to mouse Mtv antigens, both in terms of tolerogenicity and immunogenicity. In fact, the V beta specificity of rat and mouse T cells for Mtv antigens is almost indistinguishable.

Molecular mimicry in primary biliary cirrhosis. Evidence for biliary epithelial expression of a molecule cross-reactive with pyruvate dehydrogenase complex-E2

Sera from patients with primary biliary cirrhosis (PBC) react with enzymes of the 2-oxo dehydrogenase pathways, particularly PDC-E2. These enzymes are present in all nucleated cells, yet autoimmune damage is confined to biliary epithelial cells. Using a panel of eight mouse monoclonal antibodies and a human combinatorial antibody specific for PDC-E2, we examined by indirect immunofluorescence and confocal microscopy sections of liver from patients with PBC, progressive sclerosing cholangitis, and hepatocarcinoma. The monoclonal antibodies gave typical mitochondrial immunofluorescence on biliary epithelium and on hepatocytes from patients with either PBC, progressive sclerosing cholangitis, or hepatocarcinoma. However, one of eight mouse monoclonal antibodies (C355.1) and the human combinatorial antibody reacted with great intensity and specificity with the luminal region of biliary epithelial cells from patients with PBC. Simultaneous examination of these sections with an antiisotype reagent for human IgA revealed high IgA staining in the luminal region of biliary epithelial cells in patients with PBC. IgG and IgA antibodies to PDC-E2 were detected in the bile of patients with PBC but not normal controls. We believe that this data may be interpreted as indicating that a molecule cross-reactive with PDC-E2 is expressed at high levels in the luminal region of biliary epithelial cells in PBC.

Intrathymic and extrathymic tolerance in bone marrow chimeras

Parent-->F1 bone marrow (BM) chimeras provide a useful model for studying self tolerance induction. When prepared with supralethal irradiation (1300 cGy) and conditioned with anti-T cell antibodies, parent-->F1 BM chimeras are devoid of host BM-derived cells; host H-2 expression is apparent in both the intrathymic and extrathymic environments but is limited to non BM-derived cells. When parent-->F1 chimeras are injected with T cells from normal parental strain mice, the expression of host H-2 antigens on nonprofessional APC might be expected to induce tolerance through induction of clonal anergy. In practice, this does not occur. Instead, a small proportion of the injected T cells is induced to proliferate and differentiate into effector cells. Tolerance is not seen. Similarly, tolerance is not apparent when thymectomized parent-->F1 chimeras are given parental strain thymus grafts. These findings suggest that the expression of host H-2 antigens in the post-thymic environment of chimeras is not intrinsically tolerogenic for mature T cells or recent thymic emigrants. Interestingly, post-thymic tolerance does occur when parental strain T cells differentiate in the endogenous thymus of chimeras. Thus, when mature CD8+ cells are prepared from thymus vs lymph nodes (LN) of parent-->F1 chimeras, tolerance to host class I antigens is more marked in LN than thymus; this applies to cytotoxic T lymphocyte (CTL) precursors, generated by limiting dilution analysis. It would appear therefore that many of the host-reactive CTL precursors generated in the thymus of chimeras undergo tolerance induction (deletion or irreversible inactivation) in the post-thymic environment. We suggest that such tolerance is a reflection of a covert form of tolerance induced in the thymus: intrathymic contact with host antigens on thymic epithelial cells (TEC) in chimeras does not delete typical CTL precursors, but these cells are rendered "semi-tolerant". When cultured in vitro in the presence of lymphokines, the cells are able to recover and differentiate into CTL. In vivo, however, the cells recognize antigen in the periphery in the relative absence of lymphokines and the cells die. Although host class I expression on TEC in chimeras deletes only a small proportion of CTL precursors, contact with TEC induces strong tolerance of CD8+ cells in terms of helper-independent proliferative responses in vitro and induction of lethal graft-versus-host disease in vivo. We postulate that these latter responses are controlled by high-affinity T cells, whereas typical CTL generated in LDA are predominantly low-affinity cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Exclusion of circulating T cells from the thymus does not apply in the neonatal period

Although T cells arise in the thymus, migration of mature postthymic T cells back to the thymus is very limited in adult mice and is restricted to activated cells. In neonates, by contrast, we present evidence that circulating CD4+ and CD8+ T cells with a naive/resting phenotype readily enter the thymus after intravenous injection and remain there for prolonged periods. The migration of resting T cells to the neonatal thymus is largely limited to an unusual subset of cells which lacks expression of the lymph node homing receptor, leukocyte-endothelial cell adhesion molecule 1 (LECAM-1) (MEL-14). Migration of mature T cells to the thymus in neonates may be important for self-tolerance induction.

An unusual class II molecule

H-2O is a recently described non-polymorphic mouse MHC class II molecule. Here, Lars Karlsson and colleagues describe the discovery of the H-2Ob and H-2Oa loci and the pattern of expression of H-2O, and speculate on the possible function of this unusual molecule.

Stimulation of mature unprimed CD8+ T cells by semiprofessional antigen-presenting cells in vivo

To test whether unprimed CD8+ cells can recognize class I alloantigens presented selectively on non-bone marrow (BM)-derived cells, unprimed parental strain CD8+ cells were transferred to long-term parent-->F1 BM chimeras prepared with supralethal irradiation. Host class I expression in the chimeras was undetectable on BM-derived cells and, in spleen, was limited to low-level staining of vascular endothelium and moderate staining of follicular dendritic cells (a population of nonhemopoietic cells in germinal centers). Despite this restricted expression of antigen, acute blood-to-lymph recirculation of parental strain T cells through the chimeras led to selective trapping of 95% of CD8+ cells reactive to normal F1 spleen antigen presenting cells (APC) in vitro. Subsequently, a small proportion of the trapped cells entered cell division and gave rise to effector cells expressing strong host-specific CTL activity. The activation of host-specific CD8+ cells was also prominent in double-irradiated chimeras, and cell separation studies showed that the effector cells were generated from resting precursor cells rather than from memory-phenotype cells. It is suggested that the non-BM-derived cells in the chimeras acted as semiprofessional APC. These cells were nonimmunogenic for most host-reactive CD8+ cells but were capable of stimulating a small subset of high-affinity T cells. The possible relevance of the data to the prolonged immunogenicity of vascularized allografts in humans is discussed.

Two subsets of epithelial cells in the thymic medulla

Information was sought on the features of epithelial cells in the murine thymic medulla. The expression of major histocompatibility complex (MHC) molecules on medullary epithelium was defined by light microscopy with the aid of bone marrow chimeras and MHC-transgenic mice. A proportion of medullary epithelial cells was found to show conspicuously high expression of conventional MHC (H-2) class I (K, D, L) and class II (I-A, I-E) molecules. These cells express a high density of the Y-Ae epitope, a complex of an E alpha peptide and I-Ab molecules found on typical bone marrow-derived cells. MHC+ medullary epithelial cells show limited expression of I-O molecules, a class of atypical nonpolymorphic MHC-encoded class II molecules present on B cells. Other medullary epithelial cells express a high density of I-O molecules but show little or no expression of typical MHC class I or II molecules. MHC and I-O expression thus appear to subdivide medullary epithelial cells into two phenotypically distinct subsets. This applies in adults. In the embryonic thymus most medullary epithelial cells express both types of molecules.

Growth of epithelial cells in the thymic medulla is under the control of mature T cells

Epithelial cells in the thymic medulla are conspicuous in normal adult mice, but sparse in the early fetal thymus and the thymus of adult T cell-deficient SCID mice. To examine whether growth of medullary epithelial cells (MEC) depends upon local contact with mature T cells, we used the finding that the SCID thymus is unusually permeable to mature T cells entering from the bloodstream. When SCID mice received multiple injections of mature lymph node T cells from birth, the thymus accumulated large numbers of mature TCR+ T cells of resting phenotype, but contained virtually no immature (CD4+8+) cells. The injected T cells localized in the medullary region of the thymus and led to marked regeneration of MEC. These and other data suggest that the growth of MEC is under the control of mature T cells. Placing MEC under T cell control might be a device for regulating the size and integrity of the medulla, especially during the phase of rapid thymic growth. Maintaining the cellular components of the medulla in proper balance could be critical for ensuring efficient self tolerance induction.

Thymus-grafted SCID mice show transient thymopoiesis and limited depletion of V beta 11+ T cells

To seek direct evidence for the notion that stem cells in the thymus need to be constantly replenished from the bone marrow (BM), fetal (day 15) thymuses from normal BALB/c mice were grafted into T and B cell-deficient C.B-17 SCID mice (both H-2d, I-E+). The thymus grafts in these mice showed normal thymopoiesis for the first 3 wk postgrafting but then developed sudden atrophy with near complete loss of CD4+8+ cells by 4-5 wk. Such atrophy was not seen when the thymus-grafted mice were cotransplanted with normal BM cells. The lymph nodes of SCID mice receiving thymus grafts alone contained mature T cells but virtually no B cells. This lack of B cells was associated with aberrant I-E-restricted V beta deletion: the depletion of V beta 3+ and V beta 5+ T cells was near complete, whereas V beta 11+ cells showed only marginal depletion.

Long-term xenogeneic chimeras. Full differentiation of rat T and B cells in SCID mice

To test whether T and B cell differentiation can proceed across species barriers, rat fetal liver (FL) cells were used to reconstitute SCID mice. Provided that the hosts were conditioned with light irradiation, i.v. injection of FL cells caused near-complete repopulation with rat-derived lymphohematopoietic cells, including myeloid and erythroid cells, Ia+ cells of the macrophage/dendritic cell lineages, and mature T and B cells. In keeping with the known hypersensitivity of SCID cells to irradiation, host hematopoietic cells in the chimeras were almost undetectable, even with hosts exposed to as low as 250 rad. In the case of T cells, the distribution of immature and mature cells in the thymus of rat FL----SCID chimeras closely resembled the normal rat thymus in terms of architecture and expression of CD4, CD8, and alpha beta-TCR molecules. Thymopoiesis was followed by the appearance of large numbers of typical rat CD4+ and CD8+ cells in spleen and lymph nodes. These organs also contained substantial numbers of rat B (mu+) cells. The data thus indicate that the xenogeneic environment of SCID mice is fully capable of sustained de novo differentiation of rat T and B cells.

Long-term xenogeneic chimeras. Full differentiation of rat T and B cells in SCID mice

To test whether T and B cell differentiation can proceed across species barriers, rat fetal liver (FL) cells were used to reconstitute SCID mice. Provided that the hosts were conditioned with light irradiation, i.v. injection of FL cells caused near-complete repopulation with rat-derived lymphohematopoietic cells, including myeloid and erythroid cells, Ia+ cells of the macrophage/dendritic cell lineages, and mature T and B cells. In keeping with the known hypersensitivity of SCID cells to irradiation, host hematopoietic cells in the chimeras were almost undetectable, even with hosts exposed to as low as 250 rad. In the case of T cells, the distribution of immature and mature cells in the thymus of rat FL----SCID chimeras closely resembled the normal rat thymus in terms of architecture and expression of CD4, CD8, and alpha beta-TCR molecules. Thymopoiesis was followed by the appearance of large numbers of typical rat CD4+ and CD8+ cells in spleen and lymph nodes. These organs also contained substantial numbers of rat B (mu+) cells. The data thus indicate that the xenogeneic environment of SCID mice is fully capable of sustained de novo differentiation of rat T and B cells.

Mature murine B and T cells transferred to SCID mice can survive indefinitely and many maintain a virgin phenotype

To seek information on the potential lifespan of normal B and T lymphocytes, lymph node (LN) cells from unprimed mice were transferred to H-2-identical severe combined immunodeficiency (SCID) hosts. At a population level, the donor B and T cells survived for at least 10 mo post-transfer with no reduction in their numbers. In terms of antibody production, LN-injected SCID mice remained responsive to several different antigens and contained unprimed precursors of memory cells for greater than or equal to 6 mo post-transfer. Most of the B and T cells recovered from LN-injected SCID mice expressed the typical virgin phenotype of mature lymphocytes from young mice. These findings suggest that many of the transferred lymphocytes might have remained in interphase as virgin cells from the time of injection. This did not apply to all of the transferred cells, however, because 20-40% of CD4+ cells from long-term SCID hosts displayed a memory phenotype, 7% incorporated 2-bromodeoxyuridine over 5 d, and total numbers of B and T cells increased gradually (twofold) over a 10-mo period. Collectively, the data favor the view that the pool of mature B and T cells in adult mice is largely self sufficient: some of the cells proliferate, presumably in response to environmental antigens, but many mature cells can remain quiescent for prolonged periods. Input of new cells from the primary lymphoid organs continues, but at a much reduced rate relative to young life.

T cell contact with Ia antigens on nonhemopoietic cells in vivo can lead to immunity rather than tolerance

Long-term H-2-heterozygous a----(a x b)F1 bone marrow (BM) chimeras prepared with supralethal irradiation (1,300 rad) are devoid of Ia+ host BM-derived antigen-presenting cells (APC), but show quite strong host Ia expression in germinal centers, probably on follicular dendritic cells (a class of nonhemopoietic stromal cells). To examine whether Ia expression on these non-BM-derived cells is capable of inducing post-thymic tolerance of T cells, thymectomized irradiated (a x b)F1 mice were reconstituted with parent alpha stem cells and then, 6 mo later, given parent alpha thymus grafts. As measured by primary mixed lymphocyte reactions and V beta expression, the CD4+ cells differentiating in the thymus-grafted mice showed no detectable tolerance to the H-2 (Ia) antigens of the host. To examine whether the thymus-grafted mice contained immunologically significant quantities of host Ia antigens, long-term alpha----(alpha x b)F1 chimeras were injected with normal strain alpha CD4+ cells; the donor cells were recovered from thoracic duct lymph of the chimeras and tested for host reactivity in vitro. The results showed that Ia expression in the chimeras was sufficient to cause selective trapping of a substantial proportion of host-Ia-reactive CD4+ cells soon after transfer and, at later stages, to induce strong priming. Tolerance was not seen. The data place constraints on the view that T cell recognition of antigen expressed on cells other than typical BM-derived APC leads to tolerance induction.

A novel class II MHC molecule with unusual tissue distribution

The repertoire of mature class II-restricted T cells is generated through a complex process of selection whereby early T cells confront class II molecules in the thymus, especially on epithelial cells. Expression of class II molecules on such cells is prominent both in the cortex and in the medulla. We have identified a novel class II molecule, H-20, which is expressed only in epithelial cells of the thymic medulla and in B cells. The unusual tissue distribution and the nonpolymorphic nature of H-20 suggest that its function is different from that of classical class II molecules.

Reentry of T cells to the adult thymus is restricted to activated T cells

To seek information on the capacity of mature T cells to migrate to the thymus, mice were injected with Thy-1-marked populations enriched for resting T cells or T blast cells; localization of the donor cells in the host thymus was assessed by staining cryostat sections of thymus and by FACS analysis of cell suspensions. With injection of purified resting T cells, thymic homing was extremely limited, even with injection of large doses of cells. By contrast, in vivo generated T blast cells migrated to the thymus in substantial numbers. Thymic homing by T blasts was greater than 50-fold more efficient than with resting T cells. Blast cells localized largely in the medulla and remained in the thymus for at least 1 mo post-transfer. Interestingly, localization of T blasts in the thymus was 10-fold higher in irradiated hosts than normal hosts. Thymic homing was especially prominent in mice injected with T blasts incubated in vitro with the DNA precursor, 125I-5-iodo-2'deoxyuridine (125IDUR); with transfer of 125IDUR-labeled blasts to irradiated hosts, up to 5% of the injected counts localized in the host thymus. These data suggest that thymic homing by T blasts might be largely restricted to cells in S phase. The physiological significance of blast cell entry to the thymus is unclear. The possibility that these cells participate in intrathymic tolerance induction is discussed.

Evidence for the targeting by 2-oxo-dehydrogenase enzymes in the T cell response of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic autoimmune liver disease that includes the presence of lymphoid infiltrates in portal tracts, high titer autoantibodies against pyruvate dehydrogenase-E2 (PDH-E2) and branched chain ketoacid dehydrogenase-E2 (BCKD-E2), and biliary tract destruction. The mechanism by which the autoimmune response is induced, the specificity of damage to the biliary epithelium, and the role of T cells in PBC are still unknown. To address these issues, we have taken advantage of a mouse mAb, coined C355.1, and studied its reactivity against a panel of liver tissue from normal subjects as well as a panel of liver specimens from patients with PBC, progressive sclerosing cholangitis, and chronic active hepatitis (CAH). C355.1, much like human autoantibodies to PDH-E2, reacts exclusively by immunoblotting with PDH-E2, binds to the inner lipoyl domain of the protein, and inhibits PDH-E2 activity in vitro. In addition, we have also attempted to develop cloned T cell lines that react with PDH-E2 and/or BCKD-E2 using liver biopsies from patients with PBC, compared with CAH. Although monoclonal C355.1 produced typical mitochondrial fluorescence on sections of normal liver, pancreas, lung, heart, thyroid, and kidney, it produced a distinct and intense reactivity when used to stain the bile ducts of patients with PBC. Nine of 13 PBC liver biopsies studied herein contained bile ducts on light microscopy, all of which reacted intensely at a 1:100 culture supernatant dilution of monoclonal C355.1. In contrast, although bile ducts of liver specimens from normals, CAH, and progressive sclerosing cholangitis also reacted with C355.1, such reactivity was exclusively mitochondrial and readily detectable only at a dilution of 1:2. More importantly, we generated CD4+, CD8-, alpha beta TCR+ cloned T cell lines from patients with PBC, but not from CAH, that produced IL-2 specifically in response to PDH-E2 or BCKD-E2.

Evidence for the targeting by 2-oxo-dehydrogenase enzymes in the T cell response of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a chronic autoimmune liver disease that includes the presence of lymphoid infiltrates in portal tracts, high titer autoantibodies against pyruvate dehydrogenase-E2 (PDH-E2) and branched chain ketoacid dehydrogenase-E2 (BCKD-E2), and biliary tract destruction. The mechanism by which the autoimmune response is induced, the specificity of damage to the biliary epithelium, and the role of T cells in PBC are still unknown. To address these issues, we have taken advantage of a mouse mAb, coined C355.1, and studied its reactivity against a panel of liver tissue from normal subjects as well as a panel of liver specimens from patients with PBC, progressive sclerosing cholangitis, and chronic active hepatitis (CAH). C355.1, much like human autoantibodies to PDH-E2, reacts exclusively by immunoblotting with PDH-E2, binds to the inner lipoyl domain of the protein, and inhibits PDH-E2 activity in vitro. In addition, we have also attempted to develop cloned T cell lines that react with PDH-E2 and/or BCKD-E2 using liver biopsies from patients with PBC, compared with CAH. Although monoclonal C355.1 produced typical mitochondrial fluorescence on sections of normal liver, pancreas, lung, heart, thyroid, and kidney, it produced a distinct and intense reactivity when used to stain the bile ducts of patients with PBC. Nine of 13 PBC liver biopsies studied herein contained bile ducts on light microscopy, all of which reacted intensely at a 1:100 culture supernatant dilution of monoclonal C355.1. In contrast, although bile ducts of liver specimens from normals, CAH, and progressive sclerosing cholangitis also reacted with C355.1, such reactivity was exclusively mitochondrial and readily detectable only at a dilution of 1:2. More importantly, we generated CD4+, CD8-, alpha beta TCR+ cloned T cell lines from patients with PBC, but not from CAH, that produced IL-2 specifically in response to PDH-E2 or BCKD-E2.

Structural requirement for autoreactivity on human pyruvate dehydrogenase-E2, the major autoantigen of primary biliary cirrhosis. Implication for a conformational autoepitope

The E2 component (acetyltransferase) of the pyruvate dehydrogenase (PDH) complex is the major mitochondrial autoantigen recognized by autoantibodies in patients with primary biliary cirrhosis (PBC). Previous work, using only a partial length rat liver cDNA clone of PDH-E2, demonstrated that the immunodominant epitope was localized to the lipoic acid binding site. Human PDH-E2, in contrast to rat PDH-E2, has two lipoic acid binding sites. By using a full length human cDNA for PDH-E2, and by preparation of multiple overlapping recombinant fragments, we have determined that three autoreactive determinants are present on human PDH-E2: two cross-reactive lipoyl domains, and an area surrounding the E1/E3 binding region. The dominant epitope was localized to the inner lipoyl domain whereas the outer lipoyl domain only showed a weak cross-reactivity, and only 1/26 PBC sera reacted weakly to the E1/E3 binding region area. By probing recombinant fusion proteins expressed from small restriction fragments of the inner lipoyl domain, we have found that a minimum of 75 amino acids (residues 146-221) were required for detectable autoantibody binding, and that 93 amino acids (residues 128-221) were necessary for characteristically strong antimitochondrial autoantibody recognition. Such a requirement for a large region suggests the possibility that a conformational autoepitope may be recognized. In addition, we have found that absorption of PBC sera with the purified mammalian PDH complex does not remove reactivity against Escherichia coli Ag. The possible implications for such results are discussed.

Structural requirement for autoreactivity on human pyruvate dehydrogenase-E2, the major autoantigen of primary biliary cirrhosis. Implication for a conformational autoepitope

The E2 component (acetyltransferase) of the pyruvate dehydrogenase (PDH) complex is the major mitochondrial autoantigen recognized by autoantibodies in patients with primary biliary cirrhosis (PBC). Previous work, using only a partial length rat liver cDNA clone of PDH-E2, demonstrated that the immunodominant epitope was localized to the lipoic acid binding site. Human PDH-E2, in contrast to rat PDH-E2, has two lipoic acid binding sites. By using a full length human cDNA for PDH-E2, and by preparation of multiple overlapping recombinant fragments, we have determined that three autoreactive determinants are present on human PDH-E2: two cross-reactive lipoyl domains, and an area surrounding the E1/E3 binding region. The dominant epitope was localized to the inner lipoyl domain whereas the outer lipoyl domain only showed a weak cross-reactivity, and only 1/26 PBC sera reacted weakly to the E1/E3 binding region area. By probing recombinant fusion proteins expressed from small restriction fragments of the inner lipoyl domain, we have found that a minimum of 75 amino acids (residues 146-221) were required for detectable autoantibody binding, and that 93 amino acids (residues 128-221) were necessary for characteristically strong antimitochondrial autoantibody recognition. Such a requirement for a large region suggests the possibility that a conformational autoepitope may be recognized. In addition, we have found that absorption of PBC sera with the purified mammalian PDH complex does not remove reactivity against Escherichia coli Ag. The possible implications for such results are discussed.