Failure of T cell receptor V beta negative selection in an athymic environment

Pneumonia, wheezing and asthma were more common in children after thymectomy due to open-heart surgery

AIM

This nationwide study evaluated the clinical impact that an early thymectomy, during congenital heart defect (CHD) surgery, had on the health of children and adolescents.

METHODS

The subjects were patients aged 1-15 years who had undergone CHD surgery at the University Children's Hospital, Helsinki, where all CHD surgery in Finland is carried out, from 2006 to 2018. The parents or the cases and population-based controls, matched for sex, age and hospital district, completed electronic questionnaires. We excluded those with low birth weights or a known immunodeficiency. Adjusted odds ratios (aOR) and 95% confidence intervals (CI) were calculated for prespecified outcomes.

RESULTS

We received responses relating to 260/450 (58%) cases and 1403/4500 (31%) controls and excluded 73 cases with persistent cardiac or respiratory complaints after surgery. The CHD group reported more recurrent hospitalisations due to infections (aOR 6.3, 95% CI 3.0-13) than the controls and more pneumonia episodes (aOR 3.5, 95% CI 2.1-5.6), asthma (aOR 2.5, 95% CI 1.5-4.1) and wheezing (aOR 2.1, 95% CI 1.5-2.9).

CONCLUSION

Hospitalisation due to infections, pneumonia, wheezing and asthma was more common in children after a thymectomy due to open-heart surgery than population-based controls, underlining the importance of immunological follow-ups.

Use of V(D)J recombination excision circles to identify T- and B-cell defects and to monitor the treatment in primary and acquired immunodeficiencies

T-cell receptor excision circles (TRECs) and kappa-deleting recombination excision circles (KRECs) are circular DNA segments generated in T and B cells during their maturation in the thymus and bone marrow. These circularized DNA elements persist in the cells, are unable to replicate, and are diluted as a result of cell division, thus are considered markers of new lymphocyte output. The quantification of TRECs and KRECs, which can be reliably performed using singleplex or duplex real-time quantitative PCR, provides novel information in the management of T- and B-cell immunity-related diseases. In primary immunodeficiencies, when combined with flow cytometric analysis of T- and B-cell subpopulations, the measure of TRECs and KRECs has contributed to an improved characterization of the diseases, to the identification of patients’ subgroups, and to the monitoring of stem cell transplantation and enzyme replacement therapy. For the same diseases, the TREC and KREC assays, introduced in the newborn screening program, allow early disease identification and may lead to discovery of new genetic defects. TREC and KREC levels can also been used as a surrogate marker of lymphocyte output in acquired immunodeficiencies. The low number of TRECs, which has in fact been extensively documented in untreated HIV-infected subjects, has been shown to increase following antiretroviral therapy. Differently, KREC number, which is in the normal range in these patients, has been shown to decrease following long-lasting therapy. Whether changes of KREC levels have relevance in the biology and in the clinical aspects of primary and acquired immunodeficiencies remains to be firmly established.

The kinetics of early T and B cell immune recovery after bone marrow transplantation in RAG-2-deficient SCID patients

The kinetics of T and B cell immune recovery after bone marrow transplantation (BMT) is affected by many pre- and post-transplant factors. Because of the profoundly depleted baseline T and B cell immunity in recombination activating gene 2 (RAG-2)-deficient severe combined immunodeficiency (SCID) patients, some of these factors are eliminated, and the immune recovery after BMT can then be clearly assessed. This process was followed in ten SCID patients in parallel to their associated transplant-related complications. Early peripheral presence of T and B cells was observed in 8 and 4 patients, respectively. The latter correlated with pre-transplant conditioning therapy. Cells from these patients carried mainly signal joint DNA episomes, indicative of newly derived B and T cells. They were present before the normalization of the T cell receptor (TCR) and the B cell receptor (BCR) repertoire. Early presentation of the ordered TCR gene rearrangements after BMT occurred simultaneously, but this pattern was heterogeneous over time, suggesting different and individual thymic recovery processes. Our findings early after transplant could suggest the long-term patients' clinical outcome. Early peripheral presence of newly produced B and T lymphocytes from their production and maturation sites after BMT suggests donor stem cell origin rather than peripheral expansion, and is indicative of successful outcome. Peripheral detection of TCR excision circles and kappa-deleting recombination excision circles in RAG-2-deficient SCID post-BMT are early markers of T and B cell reconstitution, and can be used to monitor outcome and tailor specific therapy for patients undergoing BMT.

Arthritis and pneumonitis produced by the same T cell clones from mice with spontaneous autoimmune arthritis

SKG mice, a newly established model of rheumatoid arthritis (RA), spontaneously develop autoimmune arthritis accompanying extra-articular manifestations, such as interstitial pneumonitis. To examine possible roles of T cells for mediating this systemic autoimmunity, we generated T cell clones from arthritic joints of SKG mice. Two distinct CD8(+) clones were established and both showed in vitro autoreactivity by killing syngeneic synovial cells and a variety of MHC-matched cell lines. Transfer of each clone to histocompatible athymic nude mice elicited joint swelling and histologically evident synovitis accompanying the destruction of adjacent cartilage and bone. Notably, the transfer also produced diffuse severe interstitial pneumonitis. Clone-specific TCR gene messages in the inflamed joints and lungs of the recipients gradually diminished, becoming hardly detectable in 6-11 months; yet, arthritis and pneumonitis continued to progress. Thus, the same CD8(+) T cell clones from arthritic lesions of SKG mice can elicit both synovitis and pneumonitis, which chronically progress and apparently become less T cell dependent in a later phase. The results provide clues to our understanding of how self-reactive T cells cause both articular and extra-articular lesions in RA as a systemic autoimmune disease.

Evidence of the extrathymic development of tyrosinase-related protein-2-recognizing CD8+ T cells with low avidity

The majority of the human tumour-associated antigens characterized to date are derived from non-mutated self-proteins. However, nothing is known about the development of autoreactive and tumour-associated antigen-recognizing T cells. Tyrosinase-related protein (TRP)-2 is a non-mutated melanocyte differentiation antigen and TRP-2-recognizing CD8+ T cells are known to show responses to melanoma both in humans and mice. In addition, TRP-2-reactive T cells with low avidity have been suggested to be readily induced from the spleen cells of naïve mice. On the other hand, recent reports suggest that self antigen-reactive CD8+ T cells can be positively selected in the periphery. In this study, we tested the possibility that TRP-2-reactive CD8+ T cells in naïve mice could develop via the extrathymic pathway. As a consequence, TRP-2-reactive CD8+ T cell precursors in naïve C57BL/6 mice were suggested to express both interleukin-2 (IL-2) receptor beta chain (IL-2Rbeta) and CD44 molecules, in a manner similar to that of extrathymically developed T cells. Furthermore, IL-2Rbeta+ CD44+ CD8+ T cells were detected in the adult thymectomized and bone marrow-reconstituted mice, and functional TRP-2-reactive T cells were generated from their spleen cells. Overall, these results suggest that low avidity CD8+ T cells recognizing TRP-2 can be developed extrathymically.

Quantitation of T-cell neogenesis in vivo after allogeneic bone marrow transplantation in adults

Following myeloablative therapy, it is unknown to what extent age-dependent thymic involution limits the generation of new T cells with a diverse repertoire. Normal T-cell receptor gene rearrangement in T-cell progenitors results in the generation of T-cell receptor rearrangement excision circles (TRECs). In this study, a quantitative assay for TRECs was used to measure T-cell neogenesis in adult patients with leukemia who received myeloablative therapy followed by transplantation of allogeneic hematopoietic stem cells. Although phenotypically mature T cells had recovered by 1 to 2 months after bone marrow transplantation (BMT), TREC levels remained low for 3 months after BMT. T-cell neogenesis became evident by 6 months, and normal levels of adult thymic function were restored at 6 to 12 months after BMT. Subsequent leukemia relapse in some patients was associated with reduced TREC levels, but infusion of mature donor CD4(+) T cells resulted in rapid restoration of thymic function. These studies demonstrate that T-cell neogenesis contributes to immune reconstitution in adult patients and suggest that thymic function can be manipulated in vivo. (Blood. 2001;98:1116-1121)

Reevaluation of the origin of CD44(high) "memory phenotype" CD8 T cells: comparison between memory CD8 T cells and thymus-independent CD8 T cells

CD44(high)CD8 T cells in naive mice, which increase with age, and are often referred to as memory CD8 T cells. However, since thymus-independent CD8 T cells have also been shown to be CD44(high), the origin of the CD44(high)CD8 T cells in naive mice remains unclear. In this study, we compared the characteristics of memory CD8 T cells and thymus-independent CD8 T cells in TCR transgenic mice to clarify the origin of the CD44(high)CD8 T cells in naive normal mice. The memory and thymus-independent CD8 T cells showed differences in surface molecules, spontaneous cell death, cytokine production, and response to IL-2R binding of cytokines. Importantly, the "memory phenotype" CD8 T cells in naive normal mice showed similar characteristics to the thymus-independent CD8 T cells, but differed greatly from "true" memory CD8 T cells in the TCR transgenic mice. Therefore, we conclude that a significant part of the CD44(high) memory phenotype CD8 T cells in naive normal mice represents thymus-independent CD8 T cells, which may participate in age-related changes in immune responses.

Unusual cytotoxic activities of thymus-independent, self-antigen-specific CD8(+) T cells

We compared the cytotoxic activities of thymus-dependent and thymus-independent CD8(+) T cells. Thymus-dependent CD8(+) T cells, which are foreign antigen specific, acquired cytotoxic activity to tumor cells with a basal dose of the antigen peptides and to hybridoma cells expressing anti-TCR mAb only after differentiation into effector cytotoxic T lymphocytes (CTL). In contrast, thymus-independent CD8(+) T cells, which have been shown to be self-antigen specific, never showed cytotoxic activity to the target cells with a basal dose of the self-antigen peptide, while they could lyse hybridoma cells expressing anti-TCR mAb even without prior antigenic stimulation. Furthermore, the ex vivo cytotoxic activity of thymus-independent CD8(+) T cells was also observed against the target cells with high doses of the antigen peptides, which were not lysed by freshly isolated thymus-dependent CD8(+) T cells. Thus it is revealed that thymus-independent, self-antigen-specific CD8(+) T cells already acquire mature CTL functions in situ but have an increased threshold of TCR-mediated signaling for activation. These differences in cytotoxic activities between thymus-dependent and thymus-independent CD8(+) T cells suggest distinct roles of the two subsets of CD8(+) T cells in vivo.

TCR-independent activation of extrathymically developed, self antigen-specific T cells by IL-2/IL-15

Naive intrathymically developed T cells, which express foreign Ag-specific TCR, do not express IL-2R. After antigenic stimulation, they express high affinity IL-2R, which enables IL-2 to be used as an autocrine growth factor. On the contrary, extrathymically developed T cells, which express self Ag-specific TCR but are unresponsive to antigenic stimulation, spontaneously express low affinity IL-2R. In this study, we compared the responses of these two subsets of T cells to IL-2R stimulation and examined the influences of TCR-mediated signaling on the responses. IL-2 or IL-15 augmented the proliferative response of Ag-stimulated, intrathymically developed T cells. On the other hand, extrathymically developed T cells proliferated in response to IL-2 or IL-15, independently of Ag stimulation. Furthermore, both IL-2 and IL-15 induced IFN-gamma production of these T cells, which is strikingly augmented by the presence of IL-12. These results revealed functional differences between intrathymically developed, foreign Ag-specific T cells and extrathymically developed, self Ag-specific T cells. The latter can be activated by some inflammatory cytokines, in an Ag-independent manner, similar to NK cells.

Positive and negative selection of alphabetaTCR+ T cells in thymectomized adult radiation bone marrow chimeras

BACKGROUND

The mature T-cell repertoire is characterized by the negative selection of potentially autoreactive T cells and the positive selection of T cells restricted to antigen-recognition in the context of self-MHC molecules. It is currently believed that the thymus is critical for these selection events. Although alpha(beta)T cell receptor (TCR)+ T cells have been reported in thymectomized recipients, whether this represents clonal expansion of residual T cells or de novo generation of new T cells in the absence of a thymus has not been definitively evaluated.

METHODS

In the current study, development of the T cell repertoire was evaluated in adult radiation bone marrow chimeras prepared after complete surgical thymectomy.

RESULTS

CD4+ and CD8+ T cells were present and exhibited donor-specific TCR-Vbeta expression and self-tolerance, indicative of negative selection. Positive selection was confirmed with the demonstration of host MHC restriction and the presence of donor-derived CD8+ T cells after the transplantation of marrow from Class I deficient donors into normal recipients.

CONCLUSIONS

These data provide evidence, for the first time, that the development of a functional T-cell repertoire can occur in adult recipients without the thymic microenvironment.

Idiosyncratic alterations of TCR size distributions affecting both CD4 and CD8 T cell subsets in aging mice

We have used a spectratyping method, which displays the size distribution for the complementarity-determining region 3 (CDR3) for T cells utilizing a specific TCR-Vbeta gene, to examine the effects of aging on the TCR repertoire of (BALB/c x C57BL/6)F1 hybrid mice. Although the size distributions from T cells of 8-month-old mice were typically symmetrically shaped around one or two bands of intermediate size, spectratypes from mice 16 or 24 months of age were frequently distorted, with specific size classes either over- or underrepresented compared to normal young controls. Each of 12 mice tested at 16 or 24 months of age had a skewed spectratype for at least one of the 24 Vbeta families examined, and some mice had more than 50% of their spectratypes skewed significantly, as judged by a chi2 test. Comparable age-associated skewing of the T cell repertoire occurred in the CD4 and CD8 subsets, and every mouse over 16 months of age exhibited at least one skewed Vbeta family in both the CD4 and CD8 populations. Although the mice were genetically identical and raised in common facilities, their spectratype patterns were nonetheless idiosyncratic: i.e., the specific set of abnormalities was distinct for each individual old mouse. Whether these distortions of the TCR repertoire in middle-aged and older mice lead to alterations in immune function remains to be determined.

Positive selection of extrathymically developed T cells by self-antigens

Most T cells develop through the thymus, where they undergo positive and negative selection. Some peripheral T cells are known to develop in the absence of thymus, but there is insufficient information about their selection. To analyze the selection of extrathymically developed T cells, we reconstituted thymectomized male or female recipient mice with bone marrow cells of mice transgenic for male H-Y antigen-specific T cell receptor (TCR). It was revealed that the T cells bearing self-antigen-specific TCR were not deleted in thymectomized male recipients. More importantly, the absence of H-Y antigen-specific T cells in thymectomized female recipients suggests positive selection of extrathymically developed T cells by the self-antigen. The extrathymically developed T cells in male mice expressed interleukin (IL)-2 receptor beta chain (IL-2Rbeta) and intermediate levels of CD3 (CD3(int)) but were natural killer cell (NK)1.1(-). They rapidly produced interferon gamma but not IL-4 after TCR cross-linking. Furthermore, a similar pattern of cytokine production was observed in CD3(int)IL-2Rbeta+NK1.1(-) cells in normal mice which have been shown to develop extrathymically. These results suggest that extrathymically developed CD3(int)IL-2Rbeta+NK1. 1(-) cells in normal mice are also positively selected by self-antigens.

Identification of a committed T cell precursor population in adult human peripheral blood

Here, we report data concerning the discovery in adult human peripheral blood of a precursor cell population able to differentiate into CD4+CD3+ alpha beta + mature T cells. These cells, which represent 0.1-0.5% of total peripheral blood mononuclear cells (PBMC), express substantial levels of CD4, but lack CD3 surface expression. At a molecular level, they express the pre-T cell receptor alpha (pT alpha) gene, CD3-gamma, CD-delta and CD-epsilon, and RAG-1 recombination enzyme and have initiated rearrangements in the T cell receptor (TCR)-beta locus (D-J). Moreover, low levels of CD3 epsilon protein, but not of TCR-beta chain, can be detected in their cytoplasm. Our results suggest that CD4+CD3- cells identified in peripheral blood are different from CD3-CD4+CD8- thymocytes and may contain precursors of an extrathymic T cell differentiation pathway.

Discordant xenogeneic neonatal thymic transplantation can induce donor-specific tolerance

The limited supply of human organs for transplantation necessitates the development of methods leading to acceptance of xenografts. To avoid the hazards of the high-dose chronic immunosuppressive pharmacotherapy which would otherwise be required for successful xenografting, it would be desirable to induce permanent tolerance to xenogeneic donors. We have recently demonstrated that xenogeneic donor-specific tolerance can be induced by transplanting fetal pig thymic and hematopoietic tissue into thymectomized, T cell-depleted, and natural killer-cell-depleted mice, or into natural killer cell-depleted nude mice. We have now extended these studies by replacing fetal tissue with neonatal pig thymic and hematopoietic tissue, and by examining the in vivo responses of reconstituted mice to pig skin grafts. Neonatal tissue was studied because it might be more practicable than fetal tissue for the purpose of transplantation to primates. BALB/c nu/nu mice transplanted with neonatal (<24-hr-old) pig thymus and spleen fragments developed circulating mouse CD4+ cells. The pig thymus grafts were necessary for mouse T-cell development, as CD4 recovery did not occur in recipients of neonatal pig splenic tissue alone. The CD4+ cells that developed included Vbeta8.1/2+ T cells in similar proportions as in BALB/c mice, and Vbeta11+ and Vbeta5+ CD4 T cells were deleted almost as completely as in normal BALB/c mice. This deletion was detected among CD4 single-positive graft thymocytes. In 9 of 12 evaluable animals, mixed lymphocyte responses demonstrated tolerance to donor-type pig SLA antigens, with responsiveness to alloantigens and/or third-party pig xenoantigens. Furthermore, grafting of neonatal pig thymus conferred the ability to reject allogeneic mouse skin in 7 of 10 animals. In addition, 7 of 10 animals accepted paternal (donor SLA-matched) skin (median survival time [MST] > 100 days), whereas 4 of 4 animals rejected third-party SLA-mismatched pig skin (MST=40.5 days). We conclude that neonatal pig thymi transplanted to BALB/c nu/nu mice can support the development of mouse CD4+ cells that are functional and specifically tolerant to donor-type pig antigens.

A minimal conditioning approach to achieve stable multilineage mouse plus rat chimerism

Transplantation of untreated rat bone marrow into lethally irradiated (950 cGy) mouse recipients results in durable xenogeneic (rat-->mouse) chimerism and confers donor-specific transplantation tolerance for subsequent xenografts. The purpose of the present study was to characterize the minimal dose of total body irradiation (TBI) which would allow engraftment of rat bone marrow in mouse recipients. We report here that durable and stable lymphohaematopoietic cross-species chimerism can be achieved using a less than totally ablative radiation-based conditioning approach. The percentage of B10 mouse recipients which engrafted with rat bone marrow cells correlated with the dose of TBI. Engraftment of rat bone marrow stem cells occurred in all animals receiving 750 cGy prior to bone marrow transplantation, while no engraftment was detected at doses less than 650 cGy. Although most of the recipients were repopulated with mixed mouse and rat multilineage chimerism, some exhibited a predominance of rat cells. Although mixed xenogeneic rat/mouse chimeras prepared by lethal TBI produced only mouse derived RBC (red blood cells), chimeras prepared by sublethal conditioning produced both rat and mouse RBC. Only animals with detectable chimerism exhibited specific functional transplantation tolerance to donor xenoantigens, as assessed in vitro by mixed lymphocyte reaction assay. This model may offer an in vivo approach to study the role of species-specific growth factors in stem cell biology as well as the mechanisms for the induction of tolerance across species barriers.

Transgenic expression of interleukin 7 restores T cell populations in nude mice

The thymic lesion of the nude mouse causes a profound block in T cell development. The failure of most T cells to mature in nude mice is likely to reflect a requirement for signals elaborated in the normal thymus. Interleukin 7 (IL-7), a lymphokine that is normally expressed in the thymus and has been implicated in T cell maturation, might be central to this process. To test this possibility, we introduced a transgene directing lymphoid expression of IL-7 into nude mice and found that it substantially alleviates the block in T cell maturation caused by the thymic defect. IL-7 transgenic nude mice have increased numbers of peripheral cells expressing the T cell marker Thy-1, the T cell antigen receptor complex, and the co-receptors CD4 and CD8. The IL-7 transgene also restores T cell-specific proliferation and activation responses to the peripheral cells of transgene-rescued nude mice. Such findings point toward a fundamental role for IL-7 in the thymic maturation of T cells.

FK506 inhibits the differentiation of developing thymocytes but not negative selection of T cell receptor V beta 5+ and V beta 11+ T lymphocytes in vivo

To examine the influence of FK506 on lymphocyte development, we employed a syngeneic bone marrow transplantation model using MHC-disparate B10 (H-2b, I-Ab) and B10.BR (H-2k, I-Ak, I-Ek) mice. B10 mice, which do not express class II I-E, do not delete any known T cell receptor (TCR)-V beta, while B10.BR mice (MHC class II I-Ek, I-Ak) delete V beta 5+ and V beta 11+ TCR. Continuous daily treatment of syngeneically reconstituted B10 mice with FK506 delayed the development of thymocytes from the CD4+CD8+ to CD4+CD8- stage, while no effect was observed at the earlier CD4-CD8- to CD4+CD8+ stage. At the same time, there was a significant reduction in TCRhigh thymocytes compared with untreated, syngeneically reconstituted controls. These results suggest that FK506 treatment interfered with thymic positive selection. We also examined whether FK506 treatment would influence negative selection. Levels of expression of V beta 5+ and V beta 11+ T cells in FK506-treated B10.BR-->B10.BR recipients were similar to those observed in unmanipulated, syngeneically reconstituted B10.BR-->B10.BR controls. This was not due to the inhibition of clonal proliferation by FK506, since 35 days after drug withdrawal complete recovery of the peripheral Thy1.2+ population was observed, while the percentages of V beta 5+ and V beta 11+Thy1.2+ T cells were maintained at values similar to controls. Surprisingly, clonal proliferation stimulated by monoclonal antibody against V beta 5 and V beta 11 TCRs was observed in CsA-treated, syngeneically reconstituted B10.BR mice but not in FK506-treated mice, suggesting that CsA may be more likely to induce autoreactivity. Differences in thymic architecture between FK506- and CsA-treated animals further suggested that the drugs may differ in their effects on T cell development in vivo.

Thymus-derived cytokine(s) including interleukin-7 induce increase of T cell receptor alpha/beta+ CD4-CD8- T cells which are extrathymically differentiated in athymic nude mice

Extrathymic T cell differentiation pathways have been reported, although the thymus is the main site of T cell differentiation. The thymus is also known to produce several cytokines that induce proliferation of thymocytes. In the present study, we investigated the influence of thymus-derived cytokines on extrathymic T cell differentiation by intraperitoneal implantation with a diffusion chamber which encloses fetal thymus (we named it fetal thymus-enclosed diffusion chamber, FTEDC) in athymic BALB/c nu/nu mice. Increase in number of T cells bearing T cell receptor (TcR) alpha/beta was detected in lymph nodes and spleens of FTEDC-implanted nude mice 1 week after implantation, whereas no such increase was detected in control nude mice implanted with a diffusion chamber without thymus. The FTEDC-induced increase of T cells was suppressed by intraperitoneal injection of anti-interleukin-7 monoclonal antibody (mAb). The TcR alpha/beta T cells in FTEDC-implanted BALB/c nu/nu mice preferentially expressed V beta 11, although V beta 11-positive T cells are deleted in the thymus of euthymic BALB/c mice by clonal elimination of self-super-antigen Dvb11-specific T cells. TcR alpha/beta T cells in FTEDC-implanted nude mice were of CD4-CD8- phenotype and showed no proliferative response against anti-TcR monoclonal antibody stimulation. These results suggest that the thymus can induce extrathymic T cell differentiation through the influence of thymus-derived cytokine(s) including interleukin-7, and that such extrathymically differentiated T cells have acquired only a little or no ability for proliferation when they recognize antigen by their TcR.

Characterization of the T cells in aged rat bone marrow

In this study, we determined the characteristics of CD3-positive (CD3+) T cells existing in rat bone marrow (BM). In contrast to splenic T cells, BM CD3+ T cells are composed of a higher proportion of CD8+ T cells, and the number of both cell types increased with age. Such CD3+ T cells in aged rats showed a similar usage of TCR V beta as splenic T cells, suggesting that BM CD3+ T cells are thymus-dependent and composed of an ordinary population in view of the expression of the TCR beta-chain. Purified T cells obtained from aged rat BM showed a markedly proliferative response by stimulation with immobilized anti-CD3 mAb, as did splenic T cells. However, the addition of BM non-T cells completely inhibited the response of both BM and splenic T cells in vitro. These results suggest that T cells in rat BM are negatively regulated by BM non-T cells in their response to the TCR-mediated signal not to disrupt the microenvironment of the BM.

Direct evidence for anergy in T lymphocytes tolerized by oral administration of ovalbumin

The present study investigated bystander suppression, specific suppression and anergy as mechanisms for oral tolerance. Oral tolerance was induced in mice by a single gastric intubation of 20 mg ovalbumin (OVA) and was evaluated in vitro by the absence of T lymphocyte proliferative responses to OVA after priming by OVA-complete Freund's adjuvant (CFA). T lymphocyte unresponsiveness was antigen specific, systemic and was not affected by the vehicle used for immunization. T lymphocytes derived from tolerant popliteal lymph nodes (PLN) responded to an acetone precipitate (AP) of mycobacteria present in CFA; this response was not suppressed by co-culture with OVA, thereby arguing against a mechanism of bystander suppression in our system. Responses of PLN T lymphocytes derived from OVA-CFA primed, non-tolerant mice, or those of an OVA-specific T lymphocyte line, were not suppressed by PLN or spleen cells derived from OVA tolerant mice. These results excluded the possibility that oral tolerance was induced and maintained by a mechanism of specific suppression. At the cellular level, we found that OVA-tolerant T lymphocytes did not produce interleukin-2 (IL-2) nor express IL-2 receptor in response to OVA stimulation in vitro; both observations are indicative of a state of anergy. Incubation of OVA-tolerant PLN T lymphocytes together with murine recombinant IL-2 for 5 days, released anergic T lymphocytes and a concomitant OVA-specific proliferative response of CD4+ T cells was detected. Taken together, our experimental system excludes the involvement of bystander or specific suppression in the induction of oral tolerance to OVA, and provides direct evidence to show that oral tolerance results from specific T lymphocyte anergy.

Compartmentalization of the peripheral immune system

The periphery of the immune system--as opposed to the central lymphoid organs--contains inhomogeneously distributed B and T cells whose phenotype, repertoire, developmental origin, and function are highly divergent. Nonconventional lymphocytes bearing a phenotype that is rare in the blood, spleen, or lymph nodes of undiseased individuals are encountered at high frequency in different localizations, e.g., alpha/beta TCR+CD4-CD8- cells in the bone marrow and gut epithelium, particular invariant gamma/delta TCR+CD4-CD8 alpha+CD8 beta- and gamma/delta TCR+CD4-CD8 alpha-CD8 beta- T cells in various epithelia, or CD5+ B cells in the peritoneum. The antigen receptor repertoire is different in each localization. Thus, different gamma/delta TCR gene products dominant in each site, and the proportion of cells expressing transgenic and endogenous alpha/beta TCR and immunoglobulin gene products follows a gradient, with a maximum of endogenous gene expression in the peritoneum, intermediate values in other peripheral lymphoid organs (spleen, lymph nodes), and minimum values in thymus and bone marrow. Forbidden T cells that bear self-superantigen-reactive V beta gene products are physiologically detected among alpha/beta TCR+CD4-CD8- lymphocytes of the bone marrow, as well as in the gut. Violating previous ideas on self-tolerance preservation, self-peptide-specific gamma/delta T cells are present among intestinal intraepithelial lymphocytes, and CD5+ B cells produce low-affinity crossreactive autoantibodies in a physiological fashion. It appears that, in contrast to the bulk of T and B lymphocytes, certain gamma/delta and alpha/beta T cells found in the periphery, as well as most CD5+ B cells, do not depend on the thymus or bone marrow for their development, respectively, but arise from different, nonconventional lineages. In addition to divergent lineages that are targeted to different organs guided by a spatiotemporal sequence of tissue-specific homing receptors, local induction or selection processes may be important in the diversification of peripheral lymphocyte compartments. Selection may be exerted by local antigens, antigen-presenting cells whose function varies in each anatomical localization, cytokines, and cell-matrix interactions, thus leading to the expansion and maintenance of some clones, whereas others are diluted out or deleted. The spatial compartmentalization of lymphocytes in different microenvironments has major functional consequences and leads to a partial fragmentation of immunoregulatory circuits at the local level. Lymphocytes residing in certain antigen-exposed compartments are likely to combat tissue-specific pathogens or self-proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo administration of interleukin-2 turns on anergic self-reactive T cells and leads to autoimmune disease

One major mechanism of self tolerance involves the deletion of T cell clones in the thymus. In athymic mice, tolerance to self antigens must be generated extrathymically. T cells with self-reactive receptors undergo either peripheral clonal deletion or become unresponsive (i.e. anergic). The unresponsive state of human and mouse T cell clones in vitro can be reversed by the addition of exogenous interleukin (IL)-2, thus transforming anergic T cells to an activated state. Here it is shown that the in vivo delivery of IL-2 to athymic BALB/c nu/nu mice abrogates the anergic state of self-reactive V beta 3+ and V beta 11+ T cells [which are normally deleted in the minor lymphocyte stimulatory (Mls)-1b-, I-E(+)-expressing euthymic counterparts]. Thus, V beta 3+ and V beta 11+ T cells from IL-2-treated nude mice proliferate in response to T cell receptor cross-linking and acquire effector functions as measured by their ability to deliver aid to B cells upon specific stimulation. This activation correlates with the development of autoimmune manifestations (DNA autoantibodies, rheumatoid factors, erythroleukopenia and minimal change nephritis) in these IL-2-treated mice.

Infection breaks T-cell tolerance

Clonal deletion or clonal anergy establish tolerance in T cells that bear potentially autoreactive antigen receptors. Here we report that concomitant infection with the nematode Nippostrongylus brasiliensis breaks an established T-cell tolerance induced by injection of mice with Staphylococcus enterotoxin B (SEB). CD4+ T cells from SEB-tolerant mice did not produce either interleukin-2 or interleukin-4 when challenged in vitro with SEB. N. brasiliensis infection of SEB-primed animals resulted in a normal expansion of SEB-tolerant CD4+V beta 8+ T cells in vivo as well as an equivalent increase of SEB-reactive, interleukin-4-producing CD4+V beta 8+ T cells both in SEB-tolerant and in normal animals. Thus, infection with N. brasiliensis circumvented the tolerance established with SEB. Activation of anergic, potentially autoreactive CD4+ T cells by infectious agents seems to be a major pathway for the initiation of autoimmune diseases. Our results suggest that infectious agents may break tolerance in potentially autoreactive CD4+ T cells by activation of alternative reaction pathways.

A fail-safe mechanism for maintaining self-tolerance

Using cytotoxic T lymphocyte (CTL) responses to the class I histocompatibility antigen Qa1 and to the minor histocompatibility antigen H-Y, we show that the immune system maintains a peripheral screening process that is able to tolerize a wide variety of potentially autoimmune CTL. The critical factor is the presence or absence of specific T helper cells. If T help is available, CTL precursors that recognize antigen are activated. In the absence of help, they are tolerized. Thus, T helper cells are guardians of peripheral tolerance in CTL.

Bone marrow cells of athymic nude mice express functional T cell receptor alpha chain transcripts rearranged to V delta 2, 3, 4, 5, 6 genes

From bone marrow cells (BMC) of athymic nude mice, T cell receptor (TcR) alpha chain transcripts were selectively amplified by polymerase chain reaction (PCR) using V delta 2-, V delta 3-, V delta 4-, V delta 5-, V delta 6- and C alpha-specific primers. Amplified DNA fragments were cloned, and 32 randomly selected clones from 5 PCR were sequenced. Twenty-three distinct rearrangement events were detected, of which 87% (20/23) were in-frame. All five tested V delta genes (V delta 2, 3, 4, 5, 6) rearranged in-frame to J alpha-C alpha. N-region diversity in V delta-J alpha junctions present in most clones was limited to two to five nucleotides. P-nucleotide additions in this region were also detected. The V delta 5 gene located 3' of C delta in reversed transcriptional orientation was rearranged to J alpha by inversion. The J alpha usage pattern of the sequenced clones was strongly biased towards rearrangement of the most 5' genes (located nearest to C delta) of the J alpha cluster: the most 5' J alpha (J alpha TA1) was used by 30% of all clones, and 78% of all J alpha rearranged to V delta were located in the 5' 12 kb of the 60-kb J alpha cluster. As distinct V delta/C delta and V alpha/C alpha TcR usage patterns are prevalent in peripheral T cell populations, our data suggest that these TcR usage patterns results from repertoire selections operating in alpha beta and gamma delta T cell lineages, but not from preferential V delta-C delta and V alpha-C alpha rearrangement patterns.

Extrathymic positive selection of αβ T-cell precursors in nude mice

T lymphocytes expressing alpha beta T-cell receptors with sufficient affinity to major histocompatibility complex (MHC) molecules expressed on thymus epithelial cells are positively selected and mature to functional T cells. But several studies have demonstrated that athymic nude mice grafted with MHC-incompatible thymuses developed T cells specific for nude host rather than thymic MHC. We examined this paradox by analysing the specificity of T lymphocytes derived from nude mice. We report here that nude T lymphocyte precursors transferred to allogeneic SCID (severe combined immunodeficiency) mice with a functioning thymus (but lacking T or B cells) generated host MHC-restricted effector T cells but also contained T cells restricted to donor MHC. If nude T cells were depleted from nude lymphohaemopoietic donor cells before or after transfer, only host MHC-specific T cells matured. The results may explain the unusual MHC specificities of nude T lymphocytes described in earlier studies and demonstrate two separate differentiation steps: in nude mice, T cells may be positively selected for self-MHC restriction specificity extrathymically; then a functional thymus is required for efficient T cell maturation.

Cross-species transplantation tolerance: rat bone marrow-derived cells can contribute to the ligand for negative selection of mouse T cell receptor V beta in chimeras tolerant to xenogeneic antigens (mouse + rat----mouse)

Mixed xenogeneic bone marrow reconstitution (mouse + rat----mouse) results in stable mixed lymphopoietic chimerism (1-48% rat), long-term survival, and the induction of stable functional donor-specific transplantation tolerance to xenoantigens in vivo. To examine the role of negative selection of potentially xenoreactive T lymphocytes during tolerance induction across a species barrier, mixed xenogeneic chimeras (mouse + rat----mouse) were prepared and analyzed using a mixture of mouse and rat bone marrow cells for relative T cell receptor (TCR)-V beta expression on mouse T cells. In mixed xenogeneic chimeras (B10 mouse + rat----B10 mouse), T cell maturation proceeded normally in the presence of rat bone marrow-derived elements, and functional donor-specific tolerance to rat xenoantigens was present when assessed by mixed lymphocyte reactivity in vitro. V beta 5, which is expressed at high (undeleted) levels in normal B10 mice, was consistently deleted in B10 recipients of Wistar Furth (WF), but not F344 rat bone marrow, whereas the coadministration of either F344 rat or WF rat bone marrow with B10 mouse bone marrow cells resulted in a significant decrease in expression of TCR-V beta 11. Taken together, these data demonstrate for the first time that rat bone marrow-derived cells can contribute in a strain-specific manner to the ligand for negative selection of specific mouse TCR-V beta during tolerance induction across a species barrier.

Rearrangement and junctional-site sequence analyses of T-cell receptor gamma genes in intestinal intraepithelial lymphocytes from murine athymic chimeras

The molecular organization of rearranged T-cell receptor (TCR) gamma genes intraepithelial lymphocytes (IEL) was studied in athymic radiation chimeras and was compared with the organization of gamma gene rearrangements in IEL from thymus-bearing animals by polymerase chain reaction and by sequence analyses of DNA spanning the junction of the variable (V) and joining (J) genes. In both thymus-bearing mice and athymic chimeras, IEL V-J gamma-gene rearrangements occurred for V gamma 1.2, V gamma 2, and V gamma 5 but not for V gamma 3 or V gamma 4. Sequence analyses of cloned V-J polymerase chain reaction-amplified products indicated that in both thymus-bearing mice and athymic chimeras, rearrangement of V gamma 1.2 and V gamma 5 resulted in in-frame as well as out-of-frame genes, whereas nearly all V gamma 2 rearrangements were out of frame from either type of animal. V-segment nucleotide removal occurred in most V gamma 1.2, V gamma 2, and V gamma 5 rearrangements; J-segment nucleotide removal was common in V gamma 1.2 but not in V gamma 2 or V gamma 5 rearrangements. N-segment nucleotide insertions were present in V gamma 1.2, V gamma 2, and V gamma 5 IEL rearrangements in both thymus-bearing mice and athymic chimeras, resulting in a predominant in-frame sequence for V gamma 5 and a predominant out-of-frame sequence for V gamma 2 genes. These findings demonstrate that (i) TCR gamma-gene rearrangement occurs extrathymically in IEL, (ii) rearrangements of TCR gamma genes involve the same V gene regardless of thymus influence; and (iii) the thymus does not determine the degree to which functional or nonfunctional rearrangements occur in IEL.

Rearrangement and junctional-site sequence analyses of T-cell receptor gamma genes in intestinal intraepithelial lymphocytes from murine athymic chimeras

The molecular organization of rearranged T-cell receptor (TCR) gamma genes intraepithelial lymphocytes (IEL) was studied in athymic radiation chimeras and was compared with the organization of gamma gene rearrangements in IEL from thymus-bearing animals by polymerase chain reaction and by sequence analyses of DNA spanning the junction of the variable (V) and joining (J) genes. In both thymus-bearing mice and athymic chimeras, IEL V-J gamma-gene rearrangements occurred for V gamma 1.2, V gamma 2, and V gamma 5 but not for V gamma 3 or V gamma 4. Sequence analyses of cloned V-J polymerase chain reaction-amplified products indicated that in both thymus-bearing mice and athymic chimeras, rearrangement of V gamma 1.2 and V gamma 5 resulted in in-frame as well as out-of-frame genes, whereas nearly all V gamma 2 rearrangements were out of frame from either type of animal. V-segment nucleotide removal occurred in most V gamma 1.2, V gamma 2, and V gamma 5 rearrangements; J-segment nucleotide removal was common in V gamma 1.2 but not in V gamma 2 or V gamma 5 rearrangements. N-segment nucleotide insertions were present in V gamma 1.2, V gamma 2, and V gamma 5 IEL rearrangements in both thymus-bearing mice and athymic chimeras, resulting in a predominant in-frame sequence for V gamma 5 and a predominant out-of-frame sequence for V gamma 2 genes. These findings demonstrate that (i) TCR gamma-gene rearrangement occurs extrathymically in IEL, (ii) rearrangements of TCR gamma genes involve the same V gene regardless of thymus influence; and (iii) the thymus does not determine the degree to which functional or nonfunctional rearrangements occur in IEL.

Characterization of B cells in human thymus

The surface characteristics of B cells present in the human thymus were investigated. Cytofluorometrical and immunohistological studies, using anti-human IgM or anti-B cell monoclonal antibodies (mAbs; anti-Leu 12Ab, anti-Leu 16Ab, or L26), revealed that a small number of B cells are present in the human thymus. The thymic B cells were detected only in a low-density cell population, whereas in a high-density cell fraction, only T cells were found. In 15 cases, all of which the thymi were histologically normal, the percentages of B cells in the low-density fraction were 0.28% to 50% (6.8% in average), and Leu 1+ (CD5+) B cells in the low-density fraction were 0.1% to 26% (3.5% in average); approximately 50% of the thymic B cells were Leu 1+ B cells. These results indicate that B cells, especially Leu 1 (CD5)+ B cells, are also present in the human thymus, as suggested from our previous reports on mice.

The contribution of non-MHC genes to susceptibility to autoimmune diseases

Genetic studies of experimental models of autoimmune diseases, including systemic lupus-like syndromes and organ-specific autoimmunity, provide major information on genetic control of autoimmune diseases. In addition to genes known to be linked to the major histocompatibility complex (MHC), these studies point to multiple genes located outside the MHC that influence the onset and the progression of autoimmune diseases. Identification of these genes and of their interrelationships is now a major task that will be facilitated by recent progress in molecular biology and gene mapping. Among candidate genes, antigen-receptor genes (i.e., immunoglobulin- and T-cell receptor genes) most likely contribute an important part of the autoimmune susceptibility in several of these animal models. Available linkage data suggest a similar involvement of these antigen-receptor genes in several human autoimmune diseases. In addition to a better understanding of pathogenic mechanisms associated with autoimmunity, the knowledge of these disease-predisposing genes is expected to permit a better classification of often complex syndromes as well as the design of new treatments.

The appearance of T cells bearing self-reactive T cell receptor in the livers of mice injected with bacteria

We demonstrated in the present study that with bacterial stimulation, an increased number of alpha/beta T cells proliferated in the liver of mice and that even T cells bearing self-reactive T cell receptor (TCR) (or forbidden T cell clones), as estimated by anti-V beta monoclonal antibodies in conjunction with immunofluorescence tests, appeared in the liver and, to some extent, in the periphery. The majority (greater than 80%) of forbidden clones induced had double-negative CD4-8-phenotype. In a syngeneic mixed lymphocyte reaction, these T cells appear to be self-reactive. Such forbidden clones and normal T cells in the liver showed a two-peak pattern of TCR expression, which consisted of alpha/beta TCR dull and bright positive cells, as seen in the thymus. A systematic analysis of TCR staining patterns in the various organs was then carried out. T cells from not only the thymus but also the liver had the two-peak pattern of alpha/beta TCR, whereas all of the other peripheral lymphoid organs had a single-peak pattern of TCR. However, T cells in the liver were not comprised of double-positive CD4+8+ cells, which predominantly reside in the thymus. The present results therefore suggest that T cell proliferation in the liver might reflect a major extrathymic pathway for T cell differentiation and that this hepatic pathway has the ability to produce T cells bearing self-reactive TCR under bacterial stimulation, probably due to the lack of a double-positive stage for negative selection.

Fatal lymphoreticular disease in the scurfy (sf) mouse requires T cells that mature in a sf thymic environment: potential model for thymic education

Characteristic lesions in mice hemi- or homozygous for the X-linked mutation scurfy (sf) include lymphohistiocytic proliferation in the skin and lymphoid organs, Coombs' test-positive anemia, hypergammaglobulinemia, and death by 24 days of age. The role of the thymus in the development of fatal lymphoreticular disease in the scurfy mouse was investigated. Neonatal thymectomy doubles the life span of scurfy mice, moderates the histologic lesions, and prevents anemia, despite the continued presence of high levels of serum IgG. Animals bred to be nude and scurfy (nu/nu; sf/Y) are viable, fertile, and free of scurfy lesions. Bone marrow from scurfy mice can reconstitute lethally irradiated, H-2-compatible animals but does not transmit scurfy disease. We conclude, from these data, that scurfy lesions are mediated by T lymphocytes that mature in an abnormal (sf) thymic environment.

T cell receptor expression by T cells that mature extrathymically in nude mice

The expression of TCR by T cells that mature extrathymically in nude mice was determined by staining Ig- cells from B10 nude mice that were 5 months of age or older with mAbs specific for CD3, alpha/beta or gamma/delta TCR. Although the majority of Ig- cells in older nude mice express TCR, the distribution of alpha/beta and gamma/delta TCR in relation to CD4 and CD8 expression is markedly different compared to T cells from euthymic mice. Approximately half of the CD3+ T cells found in the spleen and lymph nodes of nude mice express gamma/delta TCR that is equally distributed between CD4-8- double-negative and CD8+ single-positive T cells. These data provide the first quantitative measure of the expression of TCR by T cells that mature in the absence of a thymus and suggest that the extrathymic environment, although not efficient, is permissive for the maturation of T cells that express alpha/beta and gamma/delta TCR.

Interleukin 2 abrogates the nonresponsive state of T cells expressing a forbidden T cell receptor repertoire and induces autoimmune disease in neonatally thymectomized mice

Under physiological conditions, the vast majority of T cells differentiate in the thymus, an organ that provides an optimal microenvironment for T cell maturation and shapes the T cell repertoire via positive and negative selection processes. In the present report, we demonstrate that neonatal thymectomy of CBA/H mice results in a diminution of T cells in peripheral lymphoid organs (spleen, lymph nodes), but is followed by a marked transient (12 wk) increase in Thy-1+ CD3+ cells in the peritoneal cavity. These cells exhibit predominantly a double-negative (CD4-CD8-) phenotype among which products of the T cell receptor (TCR) V beta 11 gene family (i.e., an I-E-reactive TCR normally deleted in I-E-bearing CBA/H mice) are selectively overexpressed. This observation suggests that, under athymic conditions, T cell differentiation and/or accumulation may occur in the peritoneal cavity. Intraperitoneal inoculation of an interleukin 2 (IL-2) vaccinia virus construct that releases high titers of human IL-2 in vivo induces conversion of these double-negative T cells to either CD4+ CD8- or CD4- CD8+ single positives, and allows in vitro stimulation of TCR V beta 11-bearing cells with a clonotypic anti-V beta antibody. Since IL-2 induces autoimmune manifestations (DNA autoantibodies, rheumatoid factors, and interstitial nephritis) in thymectomized CBA/H mice, but not in sham-treated littermates, this lymphokine is likely to enhance the autoaggressive function of T cells that bear forbidden, potentially autoreactive TCR gene products and that are normally deleted in the thymus.

A potential role for microbial superantigens in the pathogenesis of systemic autoimmune disease

We have attempted herein to demonstrate how microbial superantigens could promote an abnormal form of "cognate" T helper-B cell interaction, analogous to that which may occur during GVH disease, leading to B cell activation and systemic autoimmunity. In vitro studies performed at our laboratory and others have demonstrated that resting human B cells bind microbial superantigens and present them to superantigen-reactive autologous T helper cells, resulting in T cell activation and polyclonal IgM and IgG production by the superantigen-bearing B cells. In vitro studies of microbial superantigen-mediated murine T helper-B cell interactions demonstrate preferential help for B cells that have encountered specific antigen. Both in humans and in mice, the cellular interactions involved and the B cell responses induced are highly analogous to those mediated by allospecific T helper-B cell interaction. Finally, the results of studies carried out on T cell-deficient (nude) mice suggest that microbial superantigens may trigger similar T helper cell-dependent polyclonal IgM and IgG responses in vivo. These mice will be studied over time and tested for the development of autoantibodies characteristic of SLE and of autoimmune organ system damage, the occurrence of which are predicted by our model.

Requirement of dendritic cells and B cells in the clonal deletion of Mls-reactive T cells in the thymus

The present study was performed to identify cells responsible for the elimination of T cells reactive with minor lymphocyte-stimulating (Mls) antigens during T cell development. Experiments were carried out in a fetal thymus organ culture (FTOC) system. To examine the tolerance-inducing activity, various populations of cells from adult CBA/J (Mls-1a) mice were injected into deoxyguanosine (dGuo)-treated FTOC of C3H/He (Mls-1b) mice with a microinjector, and 2 d later, the thymus lobes were injected with fetal thymus cells from C3H/He mice as T cell precursors. After 14 d of cultivation, cells were harvested and assayed for the expression of the T cell receptor V beta 6 element. The absence or marked reduction of T cells expressing V beta 6 at high levels (V beta 6high) was regarded as indicating the deletion of Mls-1a-reactive T cells. T cell-depleted populations of thymic as well as splenic cells from CBA/J mice were able to induce clonal deletion. Further characterization of the effector cells was carried out by fractionating the spleen cells before injecting them into dGuo-FTOC. None of the dish-adherent population, dish-nonadherent population, or purified B cells alone were able to induce clonal deletion, whereas the addition of purified B cells to adherent cells restored tolerance inducibility. It was further shown that a combination of CBA/J B cells and C3H/He dendritic cells was effective in eliminating Mls-reactive clones. These results indicate that for the deletion of clones reactive with Mls antigens during T cell development in the thymus, both DC and B cells are required.

Self-reactive T cells are activated by the 65-kDa mycobacterial heat-shock protein in neonatally thymectomized mice

To elucidate the mechanism of autoimmune disease in neonatally thymectomized (NTX) mice, we have investigated the responsiveness of the self-reactive T cells which have not undergone clonal deletion in such animals. Consistent with a recent report (Yuuki et al., Eur. J. Immunol. 1990. 20: 1475), T cells bearing V beta 11-gene products capable of recognizing I-E-encoded molecules were readily detected in the mature T cell pool of NTX BALB/c (I-Ed, Mls-2a) mice. The V beta 11-bearing T cells in NTX mice expressed interleukin 2 receptors and responded normally to signals delivered through the T cell receptor. Notably, these T cells in NTX mice proliferated significantly after culture with the 65-kDa mycobacterial heat-shock protein, whose amino acid sequence is highly homologous to that in eukaryotes. These results suggest that self-reactive T cells in NTX mice may be activated by heat-shock proteins derived from various pathogens and/or stressed autologous cells, resulting in the development of autoimmune diseases in such animals.

Distinct mechanisms of neonatal tolerance induced by dendritic cells and thymic B cells

To assess the role of different types of antigen-presenting cells (APC) in the induction of tolerance, we isolated B cells, macrophages, and dendritic cells from thymus and spleen, and injected these into neonatal BALB/c mice across an Mls-1 antigenic barrier. One week after injection of APC from Mls-1-incompatible mice or from control syngeneic mice, we measured the number of thymic, Mls-1a-reactive, V beta 6+ T cells and the capacity of thymocytes to induce a graft-vs.-host (GVH) reaction in popliteal lymph nodes of Mls-1a mice. Injection of thymic but not spleen B cells deleted thymic, Mls-1a-reactive V beta 6+ T cells and induced tolerance in the GVH assay. The thymic B cells were primarily of the CD5+ type, and fluorescence-activated cell sorter-purified CD5+ thymic B cells were active. Injection of dendritic cells from spleen or thymus also induced tolerance, but the V beta 6 cells were anergized rather than deleted. Macrophages from thymus did not induce tolerance. Dendritic cells and thymic B cells were also effective in inducing tolerance even when injected into Mls-, major histocompatibility complex-incompatible, I-E- mice, but only thymic B cells depleted V beta 6-expressing T cells. Therefore, different types of bone marrow-derived APC have different capacities for inducing tolerance, and the active cell types (dendritic cells and CD5+ thymic B cells) can act by distinct mechanisms.

Role of MHC and antigens in T-cell development

The role of self-peptides in influencing the development of the T-cell repertoire has been the focus of recent studies. The findings suggest that the recognition of self-peptides bound to MHC proteins in the thymus is part of the thymic self-recognition process that results in selective maturation, or positive selection of T cells.

Towards an integrated view of thymopoiesis

One prediction from the complex series of steps in intrathymic T-cell differentiation is that to regulate it the stroma controlling the process must be equally complex: the attraction of precursors, commitment to the T-cell lineage, induction of T-cell receptor (TCR) gene rearrangement, accessory molecule expression, repertoire expansion, major histocompatibility complex (MHC) molecule-based selection (positive and negative), acquisition of functional maturity and migratory capacity must all be controlled. In this review, Richard Boyd and Patrice Hugo combine knowledge of T-cell differentiation with thymic stromal cell heterogeneity to offer an integrated view of thymopoiesis within the thymic microenvironment.

Programmed cell death and extrathymic reduction of Vbeta8+ CD4+ T cells in mice tolerant to Staphylococcus aureus enterotoxin B

Clonal deletion and functional inactivation of self-reactive cells have been invoked as mechanisms underlying intrathymic development of T-cell tolerance. The relative importance of these mechanisms in the development of tolerance of more mature, peripheral T cells either to self or to exogenous antigens is unclear, although recent data relate the development of T-cell tolerance in the periphery to clonal anergy. We have now investigated the induction of extrathymic tolerance using BALB/c mice that were made tolerant to Staphylococcus aureus enterotoxin B, a superantigen which specifically interacts in such mice with T cells bearing V beta 8 antigen receptors. Both euthymic and athymic mice made tolerant to S. aureus enterotoxin B had a markedly reduced number of V beta 8.1,2+ CD4+ peripheral T cells. This reduction was accompanied by genomic DNA fragmentation that is associated with cell death. These results indicate that a deletional mechanism can contribute to the induction of T-cell tolerance in peripheral lymphoid cells.