T-cell lineages, repertoire selection and tolerance induction

Alternatives to immunosuppressive drugs in human islet transplantation

Although intensive insulin therapy has resulted in improved metabolic control and decreases in the incidence of complications, the occurrence of severe hypoglycemia remains an issue, as does the continued potential for complications. Islet transplantation, a promising treatment for type I diabetes, has been shown to improve blood sugar levels and decrease or even abrogate the incidence of hypoglycemia. The lack of tissue availability and the toxic effects of immunosuppressants, however, limit the application of islet transplantation as a cure for diabetes. This article discusses possible alternatives to immunosuppressive drugs in human islet transplantations.

T cell receptor specificity is critical for the development of epidermal gammadelta T cells

A particular feature of gammadelta T cell biology is that cells expressing T cell receptor (TCR) using specific Vgamma/Vdelta segments are localized in distinct epithelial sites, e.g., in mouse epidermis nearly all gammadelta T cells express Vgamma3/Vdelta1. These cells, referred to as dendritic epidermal T cells (DETC) originate from fetal Vgamma3+ thymocytes. The role of gammadelta TCR specificity in DETC's migration/localization to the skin has remained controversial. To address this issue we have generated transgenic (Tg) mice expressing a TCR delta chain (Vdelta6.3-Ddelta1-Ddelta2-Jdelta1-Cdelta), which can pair with Vgamma3 in fetal thymocytes but is not normally expressed by DETC. In wild-type (wt) Vdelta6.3Tg mice DETC were present and virtually all of them express Vdelta6.3. However, DETC were absent in TCR-delta(-/-) Vdelta6.3Tg mice, despite the fact that Vdelta6.3Tg gammadelta T cells were present in normal numbers in other lymphoid and nonlymphoid tissues. In wt Vdelta6.3Tg mice, a high proportion of in-frame Vdelta1 transcripts were found in DETC, suggesting that the expression of an endogenous TCR-delta (most probably Vdelta1) was required for the development of Vdelta6.3+ epidermal gammadelta T cells. Collectively our data demonstrate that TCR specificity is essential for the development of gammadelta T cells in the epidermis. Moreover, they show that the TCR-delta locus is not allelically excluded.

Xenotransplantation: application of disease resistance

1. Organ transplantation is now clinically routine for patients with end-stage organ failure. One major limitation in transplantation is chronic rejection involving the loss of the graft despite the use of immunosuppressive agents. Haematopoietic stem cell (HSC) chimerism, achieved through bone marrow transplantation (BMT), induces donor-specific tolerance to transplanted organs and prevents chronic rejection. 2. A second major limitation to organ transplantation is the donor shortage. Xenotransplantation, the transplantation of organs between different species, would have the ability to increase the availability of donor organs. 3. Current immunosuppressive therapies do not prevent the rejection of xenografts. Therefore, the only reliable method for achieving donor-specific tolerance to xenografts may require HSC chimerism. 4. In order to justify the use of BMT to induce transplantation tolerance in patients with non-life-threatening diseases, the morbidity and mortality associated with current conditioning regimens must be addressed. 5. The use of partial conditioning regimens to promote engraftment of xenogeneic HSC and the development of donor-specific tolerance may eventually make xenotransplantation in humans a clinical reality. 6. Additional advantages of xenotransplantation are the ability to genetically engineer the donor xenograft and resistance of some xenografts to infection by human viruses because of the species specificity of most viruses. 7. The clinical application of disease resistance for HIV and hepatitis B virus is the focus of the present review.

Defect in Th1-like cells of nonresponders to hepatitis B vaccine

Peripheral blood lymphocytes from nonresponders to hepatitis B vaccine (HBsAg) failed to undergo a proliferative response to recombinant HBsAg in vitro, whereas cells from responders proliferated vigorously. The lack of proliferative response was not due to defective antigen presentation in that MHC-identical responder and nonresponder antigen presenting cells were equally effective in stimulating responder T cells. Nonresponder T cells did not proliferate in response to antigen-pulsed MHC identical responder antigen presenting cells. The present study demonstrated that: 1) there were no detectable (1 in < 20 x 10(4) HBsAg-precursor T cells in any of the nonresponders, while in responders the frequency of HBsAg-precursor T cells ranged from 1 in 3.2 x 10(3) to 1 in 40 x 10(3); 2) nonresponder cell cultures did not secrete IL-2 in response to HBsAg stimulation; 3) exogenous recombinant IL-2 did not restore the proliferative response of the T cells in HBsAg-pulsed cultures of nonresponders. These results suggest that the cellular basis for the lack of response to HBsAg is a defect in HBsAg-specific Th1-like cells; either there is an absence of the Th1 cells or cells with TCR specificity for HBsAg are present but are unresponsive to the HBsAg peptide-MHC complex (i.e., anergy or tolerance).

Antigen localisation regulates immune responses in a dose- and time-dependent fashion: a geographical view of immune reactivity

This review summarises experimental evidence to illustrate that induction of immune reactivity depends upon antigen reaching and being available in lymphoid organs in a dose- and time-dependent manner. If antigen reaches lymph organs in a localised staggered manner and with a concentration gradient, a response is induced in the draining lymph node. Antigen-presenting cells are of critical importance to transport antigen from the periphery to local organised lymphoid tissue. If antigen is all over the lymphoid system, then it deletes all specific cells in the thymus or induces them within a few days; because of their limited life-span they then die off, leaving the repertoire depleted of this specificity. If antigen does not reach lymphoid organs it is ignored by immune cells. Once a response is induced, activated but not resting T cells will reach antigen outside lymphoid organs, whereas activated B cells differentiate into plasma cells in an inducing environment, mostly in lymphoid tissue including bone marrow, but also in chronic lymphoid-like infiltrations in peripheral organs. In immunopathology (when the infectious agent is known) or in autoimmunity (when the triggering infectious agent is not known or not recognised) lymphoid tissue may become organised close to the antigen (e.g. in organ-specific autoimmune diseases) and may thereby maintain an autoantigen-driven disease-causing immune response for a long time. The notion that native T cells get induced or silenced in the periphery may be questioned because induction can only occur in lymphoid organs providing anatomical structures where critical cell-cell interactions are properly guided and where, therefore, cells are likely to meet sufficiently frequently and in a critical milieu. Since overall immune reactivity critically depends upon the localisation of antigens in a dose- and time-dependent manner, it seems more likely-but this remains to be shown-that activated T cells may get exhausted in non-lymphoid peripheral tissues, whereas they are usually maintained in lymphoid organs. The critical role of antigen in regulating immune responses also has relevance for our understanding of immunological defence against epithelial and mesenchymal tumours, against many infectious diseases and for understanding autoimmunity and immunological memory. Collectively the data indicate that antigen, dependent upon localisation, dose and time, seems to be the simplest regulator of immune responses.

Effects of exogenous cytokines on the ethanol-mediated suppression of murine thymocyte proliferation

Although attempts have been made to assess the effect of ethanol on murine thymocyte proliferation, the mechanism which accounts for the immunosuppressive effect of ethanol on the thymocyte proliferation has not been elucidated. Thus, a mouse model was used to determine (1) whether there is a similarity in the effect of ethanol exposure in vitro and in vivo on the proliferative response of thymocytes to phytohemagglutinin (PHA), (2) whether ethanol exposure affects the responsiveness of thymocytes to exogenous interleukin (IL)-1 and IL-2, and (3) whether ethanol affects IL-1 production by peritoneal macrophages. We found that the proliferative response of thymocytes from mice fed on an ethanol-containing diet was significantly inhibited (P < 0.05) compared to that in mice fed on maltose or standard diets. We also observed that low concentrations of ethanol (12.5 mM) appeared to enhance the mitogenic response of thymocytes to PHA, but the response was not significantly greater than that of controls (P > 0.05). Ethanol at higher concentrations (25-100 mM) significantly suppressed the mitogenic response of thymocytes to PHA (P < 0.05) in a dose-dependent manner. Our data also revealed that (1) ethanol did not significantly suppress IL-1 secretion by adherent macrophages stimulated by LPS, and (2) the addition of exogenous IL-1 was insufficient to restore full responsiveness in thymocytes from ethanol-fed mice. Taken together, these results suggest that the suppressive effect of ethanol on thymocyte proliferation is not mediated by insufficient IL-1. Finally, we present novel evidence that addition of exogenous IL-2 completely restores the impaired proliferative response of thymocytes from ethanol-fed mice to control levels. In summary, our results demonstrate that ethanol inhibits thymocyte proliferation in response to PHA, and that the inhibition is not due to insufficient IL-1. We also report that addition of exogenous IL-2 is sufficient to restore full proliferative capacity to thymocytes from ethanol-fed mice.

Spontaneous generation of human CD8+ TCR alpha beta+ cells derived from precursors within the double negative compartment

Flow cytometric analysis demonstrated that fresh human thymocytes contain only a low level of mature CD8+ TCR alpha beta + or CD8+ TCR gamma delta + cells and they consist consist of approximately 70% double positive (DP) and approximately 10% double negative (DN) cells. These unfractionated thymocytes could be selectively expanded in vitro by stimulation with 12-O-tetradecanoylphorbol 13-acetate (TPA) and PHA in the presence of IL-2. The majority of the cells expanded from unfractionated thymocytes expressed CD3, TCR alpha beta and CD8 molecules after long-term culture (18 days). When highly purified DN thymocytes were expanded over a period of 18 days in the presence of DP cells, they also co-expressed CD3, TCR alpha beta and CD8 molecules on their surface. However, when purified DN thymocytes were expanded alone, that is, in the absence of DP cells for 18 days, they expressed CD3-associated TCR gamma delta, but not CD8 or TCR alpha beta. Despite the expression of measurable levels of IL-2 alpha and beta receptors, as well as a significant level of TCR alpha beta, purified DP cells failed to proliferate. These findings provide the first evidence, in humans, that the progression of precursor cells in the DN compartment to a later stage of differentiation can be induced outside the thymus and that DP cells can affect the development of TCR expression in proliferating DN thymocytes.

Altered major histocompatibility complex restriction in the NK1.1+Ly-6Chi autoreactive CD4+ T cell subset from class II-deficient mice

We previously demonstrated selective enrichment of major histocompatibility complex (MHC) class II-specific autoreactive T cells in a subset of mouse CD4+ thymocytes. Here we show that a significant fraction of these autoreactive cells in the normal adult thymus expresses NK1.1 and high levels of Ly-6C and also exhibits flexibility in MHC restriction. In normal mice, this NK1.1+Ly-6Chi subfraction accounts for 10-50% of the CD4+ autoreactive subset and is enriched for MHC class II-restricted autoreactive cells as determined by mixed leukocyte reaction frequency analysis, similar to NK1.1-Ly-6C-CD4+ autoreactive cells. In contrast, in the thymus of class II-deficient littermate mice, NK1.1+Ly-6Chi cells account for most of the mature heat stable antigen (HSA)-CD4+ fraction and exhibit MHC-restricted non-class II autoreactivity. Thus, NK1.1+Ly-6ChiCD4+ T cells show flexibility in MHC class restriction, but their autoreactivity remains MHC dependent.

Repeated treatment with chimeric anti-CD4 antibody in multiple sclerosis

We treated 21 multiple sclerosis patients with two to four doses of cM-T412, a chimeric monoclonal antibody against the CD4 antigen found on helper/inducer T lymphocytes. The mean number (+/- standard error) of circulating CD4 lymphocytes decreased from 888 (+/- 81) cells/mm3 at baseline to 246 (+/- 18) after treatment. At 1 year after the last treatment, the CD4 count had recovered to only 335 (+/- 32). The antibody had no effect on CD8 lymphocytes, B lymphocytes, or other leukocytes. Side effects were minimal. Despite the prolonged depletion of CD4 lymphocytes, no opportunistic infections occurred. Only 1 patient had a possible allergic reaction. Most patients were clinically stable, but a few progressed. We conclude that repeated treatment with cM-T412 is effective in reducing the number of circulating CD4 lymphocytes and has no limiting side effects.

CD3+CD16+NK1.1+B220+ large granular lymphocytes arise from both alpha-beta TCR+CD4-CD8- and gamma-delta TCR+CD4-CD8- cells

Cultivation of CD4-CD8- double negative (DN) mouse thymocytes and splenocytes with recombinant interleukin 2 (IL2) in the absence of other stimulation results in the generation of DN-CD3/TCR+CD16+NK1.1+B220+ large granular lymphocytes (LGL). Purified DN alpha-beta TCR+ thymocytes and splenocytes are CD16+IL2R alpha-IL2R beta+NK1.1+B220-CD5high. These cells are unique in that they express both CD16 and T cell receptor (TCR) which are usually mutually exclusive. In addition, they express the natural killer (NK) marker, NK1.1. Cultivation of these cells with IL2 for several days results in the generation of DN alpha-beta TCR+CD16+NK1.1+B220+CD5- LGL, suggesting that DN alpha-beta TCR+ cells in thymus and spleen are the precursors of the DN LGL reported previously. DN gamma-delta TCR+CD16-NK1.1-B220-CD5high thymocytes and splenocytes also give rise to DN gamma-delta TCR+CD16+NK1.1+B220+CD5- LGL which, as shown previously with DN alpha-beta TCR+ LGL cells, are cytotoxic against NK-sensitive YAC-1 cells. Cytotoxic activity is also induced through either CD16 or the gamma-delta TCR. DN alpha-beta TCR+ and DN gamma-delta TCR+ LGL cells are thus similar in phenotype to TCR- NK cells. DN alpha-beta TCR+ thymocytes express low levels of the gamma subunit of the high affinity immunoglobulin E receptor (Fc epsilon RI gamma) molecule, an essential component of CD16 expression. Fc epsilon RI gamma expression is greatly enhanced after cultivation with IL2, resulting in a higher surface expression of CD16. In contrast to DN alpha-beta TCR+ thymocytes, DN gamma-delta TCR+ thymocytes do not express detectable CD16 or Fc epsilon RI gamma mRNA but expression of both is induced by cultivation with IL2, leading to the expression of CD16 on the surface. Whereas CD16 molecules on both DN alpha-beta TCR+ and DN gamma-delta TCR+ LGL are associated with only Fc epsilon RI gamma homodimers, the TCR on these cells are associated with an Fc epsilon RI gamma homodimer and/or CD3 zeta-Fc epsilon RI gamma heterodimers. These results demonstrate that the Fc epsilon RI gamma subunit is a component of the TCR in a fraction of T lineage cells.

Effect of dietary vitamin E and selenium deficiency on chicken splenocyte proliferation and cell surface marker expression

Beginning at hatching, chicks were fed a Basal diet, without vitamin E or selenium (Se) or the same diet supplemented with vitamin E (100 IU/kg) and Se (0.2 ppm). The effect of these treatments on the expression of cell surface markers (CT-1a, CD3, CD4, CD8, sIgs, and Ia) defining specific thymocyte and peripheral blood leukocyte (PBL) subpopulations were examined using flow cytometric analyses. In parallel studies the effect of the dietary deficiencies on splenocyte proliferative responses to ConA or PHA stimulation was examined. The mean expression of CD3 and CT-1a per cell was increased while CD8 and CD4 expression was decreased on thymocytes from chicks fed the Basal diet. The proportion of double negative (CD4-, CD8-) thymocytes and single positive CD8+ thymocytes was significantly decreased while single positive CD4+ and double positive (CD4+, CD8+) thymocytes were significantly increased by the dietary vitamin E and Se deficiencies. The dietary deficiencies resulted in a decreased proportion of peripheral T cells and specifically decreased the number of CD4+ PBL. The proliferative response to both ConA and PHA was impaired by the vitamin E and Se dietary deficiencies. The proliferative response could be fully reconstituted but only after vitamin E and Se supplementation for periods longer than 1 week. Plasma SeGSHpx and alpha-tocopherol levels paralleled the mitogen responsiveness observed. These results support the conclusion that vitamin E and Se deficiencies may affect both the maturation of specific lymphocyte subpopulations and the functional and proliferative capabilities of the peripheral lymphocytes.

Early thymic regeneration after irradiation

Whole body irradiation produces profound thymic atrophy. After sublethal irradiation, regeneration begins promptly and the earliest regeneration is from radioresistant intrathymic precursors. The progeny of these precursors expand rapidly and restore thymic cellularity to near normal within 2 weeks. We have used monoclonal antibodies specific for a variety of differentiation markers of the T lineage to analyze the early events in thymic regeneration. A three-color flow microfluorometric analysis revealed that the majority of the cells found early in the regenerative process have the phenotype of mature T cells. These include CD4-/CD8-; CD3hi as well as CD4+/CD8-; CD3hi and Cd4-/CD8+; CD3hi. The proportion of cells with mature phenotypes declines rapidly between day 6 and day 12. Not all of the early appearing cells have mature phenotypes. Among the early cells that do not express CD3 are both CD4 and CD8 single positive cells that express HSA and resemble the intrathymic precursors found in other systems. In these mice CD4 single positive predominate. There are other cells that are HSA positive but express low levels of CD4 and very low levels of Thy-1. These appear to include the earliest members of the T-lineage. In addition to relatively mature conventional T cells and early progenitors, the early developing population includes cells that express markers of the T-cell lineage including the T-cell receptor but do not express Thy-1. These Thy-1 negative T cells comprise a significant number of the earliest cells found after regeneration.

Phenotype analysis of cycling and postcycling thymocytes: evaluation of detection methods for BrdUrd and surface proteins

We present a comparison of two different methods for simultaneous detection of bromodeoxyuridine and cell surface markers. Both methods use enzymatic generation of single-strand DNA with nuclease. The biological system used is the murine thymus, in which in vivo DNA synthetizing cells were labeled by injection of BrdUrd and analyzed at different time points after the nucleoside pulse. The surface proteins detected were CD4 and CD8 differentiation markers and the T-cell receptor. Extraction of DNA-associated proteins with 0.1N HCl and detergent is necessary for the action of EcoR1 and Exonuclease III, but this treatment destroys phycocyanins and induces cell aggregation, as shown using the doublet-discrimination module. For DNAse I action, cells could be treated with paraformaldehyde and a low concentration of Tween 20, and this treatment was adequate for surface staining preservation (even with phycocyanins) and BrdUrd detection. Both methods were adequate for cell cycle studies, but only 7-amino-actinomycin D could be used as total DNA dye after DNAse action, and good results needed long (48-72 h) incubation in the fixative-detergent mixture. The DNAse I method now allows three-color staining (two surface markers and Brd-Urd), analyzed in a one laser-cytometer for the study of the phenotype of cycling cells, and of their progeny, in vivo and in cell cultures. It also allows the quantitative analysis of cell surface receptor densities in conditions similar to fresh cells.

Positive selection of V beta 8+ CD4-8- thymocytes by class I molecules expressed by hematopoietic cells

A small subset of T cells of mature phenotype express the alpha/beta T cell receptor, but not CD4 and CD8 coreceptors (alpha/beta double-negative [DN] cells). The repertoire of V beta usage of alpha/beta DN cells is strongly biased towards V beta 8 expression, suggesting that the formation of the population is subject to selection. We now report that deficiency of class I expression leads to a strongly depressed frequency of V beta 8+ DN cells, but has little effect on V beta 8- DN cells. Studies of hematopoietic chimeras between class I+ and class I- mice demonstrated that expression of class I molecules by hematopoietic cells is necessary and sufficient for selection of most V beta 8 DN cells. The lack of a role for class I expression by thymic epithelial cells suggests that the mechanism of selection of these cells by class I differs significantly from the mechanism of selection of conventional T cells. Models to explain the selection of these cells as well as their possible function in vivo are discussed.

Distinct sites of production and deposition of the putative cell death marker clusterin in the human thymus

Clusterin is a multifunctional protein endowed with cell-aggregating, complement-inhibitory, and lipid-binding properties. Since several studies have demonstrated highly increased clusterin gene expression in epithelial and nervous tissues regressing as a consequence of tissue involution and apoptotic cell death, clusterin is also considered as a specific marker of dying cells. To determine whether clusterin expression is also upregulated during thymocyte death occurring during the negative selection process we analyzed the cellular distribution of clusterin mRNA and protein by in situ hybridization and immunocytochemistry in the human thymus. We observed that the expression of clusterin mRNA was confined to cells present in the thymic medulla, concentrated mainly around Hassal's bodies. Immunostaining of adjacent sections with antikeratin Ab revealed that cells containing clusterin mRNA were predominantly epithelial. By contrast no clusterin mRNA was found in thymocytes by in situ hybridization and Northern blot analysis of total RNA from purified thymocyte populations. Clusterin protein colocalized with the membrane attack complex of complement and vitronectin in the center of the largest Hassal's bodies, but was not detectable by immunocytochemistry in or at the surface of epithelial cells. Our results demonstrate that clusterin gene expression does not take place in apoptotic thymocytes, and therefore that clusterin synthesis by the dying cell is probably not a prerequisite to its death. However, synthesis of clusterin by medullary epithelial cells may be related to their terminal differentiation, and, furthermore, its presence in Hassal's bodies raises the possibility that the secreted protein is involved in the disposal of cell debris resulting from thymocyte apoptosis.

Apoptosis in the development of the immune system: growth factors, clonal selection and bcl-2

The mammalian immune system is essential for surviving challenge infections with a great range of potential pathogens. The protective effect produced is dependent on many different types of cells which require flexible and independent production and regulation. In particular, many important responses are carried out by lymphocytes, which recognise foreign antigen through exquisitely specific receptors: i.e. surface immunoglobulin (sIg) on B lymphocytes and the T cell receptor (TCR) on T lymphocytes. Each lymphocyte displays receptors with a single specificity, allowing cells with particular specificities to be regulated independently. Since millions of different Igs and TCRs are expressed, the precise selection and regulation of each T and B cell population to produce a useful self-tolerant repertoire is a very complex process. Control of cell populations can, in theory, be exercised at a number of levels, including modulation of active cell death by apoptosis. Recent research has demonstrated that regulation of apoptosis is indeed a crucial element in the control of the immune system in general, and in the development of the TCR and Ig repertoires in particular. The molecular analysis of apoptosis now takes a high priority and the proto-oncogene bcl-2 appears to be responsible for specific suppression of apoptosis in several important situations. It is also clear that malfunctions affecting apoptosis, and in particular bcl-2, can result in significant progression towards malignancy.

Selection of murine T cell receptor alpha beta and gamma delta cells in organ cultures established from 14-day embryos

The expression of minor lymphocyte stimulatory locus (Mls) determinants in combination with murine major histocompatibility complex (MHC) class II molecules, leads to the destruction of lymphocytes bearing specific V region-encoded T cell receptor (TcR) products. A much studied example is the elimination of V beta 6+ cells in IE+/Mls-1a mice, in which deletion can be detected 7-10 days after birth but is not fully operational earlier in embryonic life. Here we investigate this transitional period in development and show that selective deletion of V beta 6 occurs in vitro, approximately 1 week after organ cultures are established from 14 day embryos. These unmanipulated organ cultures receive no additional cell immigrants after day 14, suggesting that the cellular elements mediating negative selection (or their direct precursors), are already resident in the fetal thymus by day 14 of gestation. Hence, the developmental timing of the outset of rigorous negative selection of V beta 6 is not dictated by the postnatal entry of deleting elements into the thymus, but perhaps by the maturation of the pre-existing environment. Using a parallel organ-culture approach we have looked at the development of V delta 4 and V gamma 3, TcR gamma delta+ cells in a variety of mouse strains. These receptors have recently been reported to be subject of MHC and non-MHC linked selection, respectively. We find that after an initial period of expansion, the number of V gamma 3-expressing cells dramatically declines. However, this selective loss of V gamma 3 cells is not contingent on the C57BL/6 mouse strain (in contrast to a previous report). These findings are discussed in the context of current models of ontogeny and repertoire selection.

Selective manipulation of the human T-cell receptor repertoire expressed by thymocytes in organ culture

A recently described organ culture system for human thymocytes is shown to support the generation of a diverse T-cell receptor repertoire in vitro: thymocytes of the alpha beta lineage, including representatives of the V beta families 5.2/5.3, 6.7, and 8, accounted for the majority of T-cell receptor-positive cells throughout a 3-week culture period. Thymocytes bearing gamma delta receptors were also identified, particularly among the CD4 CD8 double-negative subset. The T-cell receptor repertoire expressed in organ culture responded to experimental manipulation with staphylococcal enterotoxins. Staphylococcal enterotoxin D (a powerful activator of human peripheral T cells expressing V beta 5.2/5.3 receptors) caused a marked reduction of V beta 5.2/5.3 expression, as determined with the V beta-specific antibody 42/1C1. Evidence is presented that this loss of V beta 5.2/5.3 expression resulted from the selective deletion of activated thymocytes by apoptosis, in concert with T-cell receptor modulation. These effects of staphylococcal enterotoxin D were specific (since staphylococcal enterotoxin E did not influence V beta 5.2/5.3 expression) and V beta-selective (since expression of V beta 6.7 remained unaffected by staphylococcal enterotoxin D). On the basis of these observations, we suggest that thymic organ culture provides a powerful approach to study the generation of the human T-cell repertoire.

The development of functionally responsive T cells

The work reviewed in this article separates T cell development into four phases. First is an expansion phase prior to TCR rearrangement, which appears to be correlated with programming of at least some response genes for inducibility. This phase can occur to some extent outside of the thymus. However, the profound T cell deficit of nude mice indicates that the thymus is by far the most potent site for inducing the expansion per se, even if other sites can induce some response acquisition. Second is a controlled phase of TCR gene rearrangement. The details of the regulatory mechanism that selects particular loci for rearrangement are still not known. It seems that the rearrangement of the TCR gamma loci in the gamma delta lineage may not always take place at a developmental stage strictly equivalent to the rearrangement of TCR beta in the alpha beta lineage, and it is not clear just how early the two lineages diverge. In the TCR alpha beta lineage, however, the final gene rearrangement events are accompanied by rapid proliferation and an interruption in cellular response gene inducibility. The loss of conventional responsiveness is probably caused by alterations at the level of signaling, and may be a manifestation of the physiological state that is a precondition for selection. Third is the complex process of selection. Whereas peripheral T cells can undergo forms of positive selection (by antigen-driven clonal expansion) and negative selection (by abortive stimulation leading to anergy or death), neither is exactly the same phenomenon that occurs in the thymic cortex. Negative selection in the cortex appears to be a suicidal inversion of antigen responsiveness: instead of turning on IL-2 expression, the activated cell destroys its own chromatin. The genes that need to be induced for this response are not yet identified, but it is unquestionably a form of activation. It is interesting that in humans and rats, cortical thymocytes undergoing negative selection can still induce IL-2R alpha expression and even be rescued in vitro, if exogenous IL-2 is provided. Perhaps murine thymocytes are denied this form of rescue because they shut off IL-2R beta chain expression at an earlier stage or because they may be uncommonly Bcl-2 deficient (cf. Sentman et al., 1991; Strasser et al., 1991). Even so, medullary thymocytes remain at least partially susceptible to negative selection even as they continue to mature.(ABSTRACT TRUNCATED AT 400 WORDS)

CD8 gamma delta cells: presence in the adult rat thymus and generation in vitro from CD4-/CD8- thymocytes in the presence of interleukin 2

Three to fifteen percent of peripheral T cells in adults express the recently described gamma delta T-cell antigen receptor (TcR) heterodimer. A small subpopulation of gamma delta cells express the CD8 accessory molecule. In this study, we analyzed the potential of highly purified CD4-/CD8-, double negative (DN) rat precursor thymocytes to give rise to gamma delta cells. We observed that in the presence of interleukin 2 (IL-2) and concanavalin A (ConA), both DN and CD8 cells expressing the gamma delta TcR were generated in vitro. We then examined the rat thymus for these cells and confirmed the presence of a previously undescribed CD8 TcR-alpha beta- subset in the rat thymus, expressing high levels of TcR-gamma and delta messages with no detectable TcR-alpha transcripts, similar to the cells generated in vitro in the presence of IL-2 and ConA.

Development and function of T cells in mice rendered interleukin-2 deficient by gene targeting

Interleukin-2 (IL-2) is a lymphocytotropic hormone which is thought to have a key role in the immune response of mammalian cells. It is produced by a subpopulation of activated T-lymphocytes and acts in vitro as the principal auto- and paracrine T-cell growth factor (for reviews see refs 1-3). IL-2 is, however, not the sole T-cell growth factor, nor does it act exclusively on T cells, also promoting growth of NK cells and differentiation of B cells. A role for IL-2 in T-cell development has been postulated but remains controversial. Here we test the requirement for IL-2 in vivo using IL-2-deficient mice generated by targeted recombination. We find that mice homozygous for the IL-2 gene mutation are normal with regard to thymocyte and peripheral T-cell subset composition, but that a dysregulation of the immune system is manifested by reduced polyclonal in vitro T-cell responses and by dramatic changes in the isotype levels of serum immunoglobulins.

Ontogeny of the response of mouse thymocytes to interleukin 1 and interleukin 2

In the present report we demonstrate that the in vitro proliferative response of the newborn thymocytes to interleukin (IL) 1 and IL 2, which is remarkably stronger than the adult thymocyte response, is associated with a considerable increase of CD4-CD8- cells expressing a gamma/delta T cell receptor (TcR). By polymerase chain reaction analysis we show that the V gamma gene segment usage in the adult and newborn responding cells reflects the developmentally regulated expression of the V gamma gene segments, suggesting that the increase in TcR gamma/delta+ cells results from the polyclonal expansion of pre-existing clones. Surprisingly, although the fetal thymocyte populations contain higher numbers of TcR gamma/delta+ cells than the adult and newborn ones, the highest proliferative response to IL 1 and IL 2 is obtained with the newborn thymocytes. Non mutually exclusive hypotheses are discussed to explain these results.

Cell death mechanisms and the immune system

The immune system provides good models for cell death, a phenomenon now recognized to be of fundamental importance in many fields of biology. Cell death is strikingly polymorphic: it can proceed via necrosis (as in complement-mediated cell death) or apoptosis, but the latter displays different patterns (in the receptor-mediated death of some thymocytes, in cell death mediated by TNF alpha or by cytotoxic T cells), perhaps reflecting different pathways of control of a common core mechanism. Even though there are differences in the morphological and metabolic changes associated with the different patterns of apoptosis, some recurrent sequences of events are observed in almost all dying cells. The metabolic state of a cell often seems to play a major role in determining if and how this cell will die in given external circumstances. The nature of molecules causally involved in the dying cell can now be approached in some systems.

In vivo modulation of the distribution of thymocyte subsets: effects of estrogen on the expression of different T cell receptor V beta gene families in CD4-, CD8- thymocytes

Estrogen treatment of mice has been shown to deplete CD4+, CD8+ double-positive (DP) thymocytes and to alter the relative proportion of CD4+ and CD8+ single-positive (SP) thymocytes. In this work, we have studied the effect of the steroid hormone 17 beta-estradiol (E2) on the different subsets of CD4-/CD8- double-negative (DN) thymocytes by analyzing the expression of CD5, CD3-epsilon and of several V beta gene family products of the T cell antigen receptor (TCR). After in vivo administration of E2 a significant decrease in the number and proportion of dull CD5+, CD3-, beta-TCR- DN thymocytes was observed. In contrast E2 treatment significantly increased the proportion of bright CD5+, CD3+, beta-TCR+ DN cells. The E2-induced increase in DN/TCR+ cells was observed for subsets expressing V beta 6, V beta 8, and V beta 11, but not V beta 3 gene products of the TCR. Thus, estrogen administration results in a selective inbalance of the DN thymocyte subsets by depleting an immature, dull CD5+, CD3-, TCR beta- DN subset, while enriching a mature, bright CD5+, CD3+, TCR beta+ DN subset of cells. In addition to TCR beta+ DN thymocytes, an increased proportion of CD4+ and CD8+ SP thymocytes expressing V beta 8, V beta 6, and V beta 11, but not V beta 3, TCR proteins was also observed after E2 administration. An involvement of intrathymic cytokine production in mediating the hormone action is suggested by the ability of estrogen to increase the levels of IL-1 alpha mRNA of intact thymus. Our data suggest that estrogen exerts its effects on a broad range of immature cells, including dull CD5+, CD3-, beta-TCR- DN and DP thymocytes.

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.

Prothymocyte seeding in the thymus

A major gap in our understanding of T lymphocyte development is the process of stem cell differentiation into T lymphocyte precursors. An important question is whether bone marrow-derived stem cells become committed to T lymphoid lineage within the bone marrow, or whether this occurs once cells have entered the microenvironment of the thymus. Attempts to identify a haemapoietic precursor of thymocytes in mice, a "prothymocyte", have involved cell transfer experiments involving isolated and selected populations of bone marrow stem cells, as well as transformed or continuous cell lines representing early stage in mouse T cell development. Current information on the properties of stem cells which can seed the thymus is reviewed in this paper, and the possibility that progenitor T cells may be identified by their expression of receptor(s) which localise them into the thymus is considered.

T-cell development in the fetus and the invariant series hypothesis

Comparison of the timing of appearance of certain T-cell markers in the intrathymic development of T cells during gestation reveals a common sequence of expression in several species. Here Richard Aspinall and colleagues put forward a hypothesis concerning this 'invariant series' of markers that shares the same timing of expression across species barriers. It is proposed that T-cell markers that are members of the invariant series are very important in deciding the fate of a developing thymocyte.

Characterization of an acute T lymphoblastic leukemia expressing the gamma/delta T-cell receptor

Leukemic cells of a 20 year old patient, suffering from acute lymphoblastic leukemia, were characterized by surface marker and functional analysis. A significant cell population within this type of leukemia expresses concomitantly the CD4 and CD8 antigen on the same cell and might represent a new differentiation stage of T-cells with the gamma/delta receptor. The leukemic cells show a distinct pattern of growth response to mitogens and lymphokines, which might correlate to their differentiation stage. Moreover, a "natural killer"-like activity can be induced in these cells by IL-2.

Expression of T cell receptor genes in the thymus: localization of transcripts in situ and comparison of mature and immature subsets

We have analyzed the expression of T cell receptor (TcR) genes in the thymus using in situ RNA hybridizations with probes to the constant regions of the TcR alpha, beta, gamma and delta chains. Localization of transcripts revealed low TcR alpha mRNA levels in the thymus cortex and very low levels in the subcapsular region. In contrast, TcR beta message was very abundant in the cortex. TcR gamma or delta mRNA+ thymocytes showed a scattered, predominantly cortical localization. In contrast to gamma, TcR delta transcripts were abundant in the subcapsular region. Control experiments with sorted TcR alpha/beta or gamma/delta cells revealed a detection efficiency of 75%-85% for the respective TcR mRNA and data on TcR gene expression in mature, CD3+ thymocytes were consistent with previous reports. The analysis of immature, CD3- thymocyte subsets, however, revealed a virtual absence of TcR alpha transcripts and an unexpectedly high proportion of cells (14%-29%) expressing the gene for the TcR delta chain. The data are discussed in view of current models of lineage relationships in the thymus.

Changes in the relative abundance of type I and type II lck mRNA transcripts suggest differential promoter usage during T-cell development

The lck gene, which encodes the lymphoid cell-specific tyrosine protein kinase p56lck, is expressed from two widely separated promoters. The proximal promoter gives rise to a type I lck transcript, and the distal promoter gives rise to a type II transcript. We found that the ratio of the two transcripts changed during T-cell maturation. Type I lck mRNA was twofold more abundant than the type II transcript in early fetal thymocytes. In the adult, the type I and type II lck mRNAs were present in approximately equal amounts in immature thymocytes expressing the heat-stable antigen. In contrast, there was five- to ninefold more type II lck than type I lck mRNA in more mature thymocytes that did not express the heat-stable antigen and in splenic T cells. This change in relative transcript abundance probably reflects activation of the distal promoter and inactivation of the proximal promoter during T-cell maturation in the thymus. It is possible that the two promoters are regulated by different trans-acting factors whose expression is regulated during T-cell maturation.

Changes in the relative abundance of type I and type II lck mRNA transcripts suggest differential promoter usage during T-cell development

The lck gene, which encodes the lymphoid cell-specific tyrosine protein kinase p56lck, is expressed from two widely separated promoters. The proximal promoter gives rise to a type I lck transcript, and the distal promoter gives rise to a type II transcript. We found that the ratio of the two transcripts changed during T-cell maturation. Type I lck mRNA was twofold more abundant than the type II transcript in early fetal thymocytes. In the adult, the type I and type II lck mRNAs were present in approximately equal amounts in immature thymocytes expressing the heat-stable antigen. In contrast, there was five- to ninefold more type II lck than type I lck mRNA in more mature thymocytes that did not express the heat-stable antigen and in splenic T cells. This change in relative transcript abundance probably reflects activation of the distal promoter and inactivation of the proximal promoter during T-cell maturation in the thymus. It is possible that the two promoters are regulated by different trans-acting factors whose expression is regulated during T-cell maturation.

Study of the thymocyte cell cycle by bivariate analysis of incorporated bromodeoxyuridine and DNA content

The in vivo cell cycle of normal murine thymocytes was studied by bivariate analysis of bromodeoxyuridine and total DNA content in the 24 h following a single injection of the thymidine analogue. Bromodeoxyuridine incorporation was strictly limited to cells in S phase and 98% of S phase cells were labeled, demonstrating high efficiency and specificity. Cell-cycle parameters were determined by measuring the DNA content of bromodeoxyuridine-labeled cells, related to their distribution in the different phases. The changes of this distribution as a function of time reflected the progression of the cells along the cell cycle. The duration of total cycle, S phase, and both G2/M and G1 was 10 h, 6.5 h and 1.5 h, respectively. All thymocytes labeled in S phase entered G2/M, divided and returned to the G0/G1. Seventy percent of them remained in the resting state, and the other 30% re-entered a second S phase. Cell-cycle parameters of isolated CD4-CD8- cells were also determined. No evidence of cell loss during S or G2/M phase was found, suggesting that intrathymic cell death is not directly linked to the proliferative phases of differentiation.

Differentiation of thymocytes from CD3-CD4-CD8- through CD3-CD4-CD8+ into more mature stages induced by a thymic stromal cell clone

We have investigated the capacity of our established thymic stromal cell clone (MRL104.8a) or its derived factor(s) to induce the differentiation of immature thymocytes. Culture of purified adult murine double-negative (CD4-CD8-, indicated here as CD4-8-) thymocytes on the MRL104.8a thymic stromal cell monolayer for 1 day resulted in the induction of an appreciable percentage of CD4-8+ thymocytes. A bone marrow-derived stromal cell monolayer or a L929 fibroblast monolayer failed to generate CD4-8+ cells. This differentiation could also be induced by a semipurified sample of the MRL104.8a culture supernatant, which contained a thymic stroma-derived T-cell growth factor capable of contributing to the growth of double-negative immature thymocytes. CD4-8+ thymocytes generated 1 day after coculture with the MRL104.8a cells or the sample containing thymic stroma-derived T-cell growth factor were found to be CD3- and J11d+, excluding the possibility of expansion of mature (CD3+4-8+) thymocytes present in the thymus. More importantly, when the culture period was extended to 2 or 3 days, an appreciable number of CD4+8+ and single-positive (CD4+) cells were generated on the MRL104.8a monolayer. Thus, these results provide the direct demonstration that CD3-4-8- immature thymocytes are promoted to differentiate through a rapidly cycling intermediate (CD3-4-8+) into double- and single-positive cells by a specialized thymic stromal component.

Murine spontaneous T-cell leukemia constitutively expressing IL-2 receptor--a model for human T-cell malignancies expressing IL-2 receptor

We describe a new, spontaneously occurring BALB/c-derived murine T-cell leukemia. The leukemic cells, designated LB, grow rapidly and progressively in the syngeneic host with no signs of effective immunological resistance. LB cells expressed the Thy-1+, Lyt-2+, L3T4-, CD3- class-I+, CD25+ (IL-2 receptor, IL-2R), class-II-, gp70- phenotype. As LB cells express IL-2, as indicated by staining with 2 distinct anti-CD25 IL-2R monoclonal antibodies (MAbs), the therapeutic efficacy of IL-2-diphtheria toxin-related protein was tested on this leukemic model. IL-2-diphtheria toxin, but not diphtheria toxin, efficiently inhibited the proliferation of LB cells. The proliferation of a murine myeloma cell line, which does not express IL-2R, was not inhibited by IL-2-diphtheria toxin. The possible implantation of this animal model in fundamental and practical studies is discussed.

Tolerance-related V beta clonal deletions in normal CD4-8-, TCR-alpha/beta + and abnormal lpr and gld cell populations

We have analyzed tolerance-related clonal deletion of Mls-and I-E-reactive thymocytes at the RNA level using a multi-V beta probe RNAse protection assay, and used this phenomenon to identify the maturation stage of the abnormally expanded CD4-8-, TCR-alpha/beta + subset in lpr and gld homozygous mice, and of the phenotypically similar minor thymocyte subset found in normal mice. Essentially complete V beta clonal deletions were detected in lpr and gld cells of all appropriate background strains. Substantial, but not complete, V beta clonal deletions were also detected in the CD4-8- TCR-alpha/beta + subset of normal mice. Since expression of CD4/CD8 is required for V beta clonal deletions to occur, we conclude that lpr and gld cells, and at least a portion of CD4-8- TCR-alpha/beta + thymocytes in normal mice, are derived by secondary loss of CD4/CD8 accessory molecules from more mature CD4+8+ precursors. One possible interpretation of these findings is that such CD4/CD8 loss may affect a class of self-reactive thymocytes that have escaped direct clonal deletion. Exportation and expansion of such cells in the periphery may be an important contributory factor in the induction of systemic autoimmunity.

T cell receptor expression on immature thymocytes with in vivo and in vitro precursor potential

Immature CD8-CD4- double-negative (DN) thymocytes differentiate intrathymically into CD8+CD4- and CD8-CD4+ thymocytes and migrate to the periphery. This differentiation proceeds through several intermediate phenotypic changes in the expression of CD8 and CD4. We have recently established the existence of a CD8loCD4lo cell population in murine thymus that can repopulate the irradiated thymus in vivo and differentiate rapidly in vitro to CD8+CD4+ double-positive (DP) cells. The CD8loCD4lo cells score as DN upon direct cytofluorometric analysis, yet are distinct from true DN cells by various criteria. Experimental evidence strongly suggests that they are descendants of true DN in the maturation pathway. In the experiments presented here, we further characterize this CD8loCD4lo thymocyte population. Northern blot and RNA protection analysis reveal that these cells transcribe full length mRNA for the T cell receptor (TcR)alpha chain, unlike the less mature interleukin 2 receptor-positive DN thymocytes. Surface expression of the TcR-associated CD3 molecule occurs on approximately 15% of these cells at low levels characteristic of immature cells. In the course of in vitro differentiation a vast majority (approximately 80%) of these cells convert to CD8+CD4+ and significant numbers of the brightly staining DP convertants (11%-34% on day 1 and 48%-68% on day 2) express immature levels of CD3. Our results indicate that CD8lo, CD4lo cells might be the first thymic subset to rearrange TcR alpha chain genes and express TcR alpha/beta heterodimer on the surface at levels characteristic of immature cells. Furthermore, the surface expression of TcR persists on the in vitro progeny of these thymocytes.

Positive selection of the T cell repertoire: where and when does it occur?

The T cell repertoire is shaped by both positive and negative influences. T lymphocytes that express the V beta 6 variable region are positively selected in the thymus by cells expressing major histocompatibility complex (MHC) class II E molecules. To identify these cells, we have quantitated V beta 6+ T lymphocytes in a set of transgenic mice showing variant patterns of E expression in the thymus. We demonstrate that class II molecules must be expressed on epithelial cells of the cortex for positive selection to occur. Using a direct assay of unmanipulated thymocytes, we show that positive selection is manifest only as a rather late event in thymocyte differentiation, after the maturation of cortical double-positives into single-positives.

Analysis of T-cell subpopulations in human IL-2R alpha transgenic mice: expansion of Thy1.2- thymocytes and depletion of double-positive T-cell precursors

Transgenic mice carrying the human IL-2R alpha/p55 gene under the control of the SV40 enhancer/promoter were used to study the relevance of IL-2R in T-cell development. Serological analysis of the mouse lines obtained indicated transient, regulated expression of the human p55 gene, mainly confined to the early thymus, but which was also detected in lower amounts in the spleen. These mice showed degenerated thymuses, with an increased number of Thy1.2- double-negative precursor cells; they also had specific depletion of double-positive thymocytes. Transgene expression led to an increased number of intermediate-affinity IL-2 receptors (possibly ascribed to deregulated expression of IL-2R beta/p75) in transgenic thymocytes older than 12 weeks. These results suggest the occurrence of strong linkage between the IL-2/IL-2R system elements and thymic differentiation/maturation; they lend support to the idea of functionality of IL-2R expressed transiently in early stages of T-cell development.

A novel CD3-J11d+ subset of CD4+CD8- cells repopulating thymus in radiation bone marrow chimeras

Sequential appearance of T cell subpopulations occurs in the thymus of irradiated AKR (H-2k, Thy-1.1) mice at an early stage after transplantation with bone marrow cells of C3H/HeN (H-2k, Thy-1.2) mice. The donor-derived thymocytes were first detected on day 8 after bone marrow reconstitution. Although most of the thymocytes were CD4-CD8- cells, an appreciable level of CD4+CD8- cells was detected in the thymus at this stage. The early appearing CD4+CD8- cells were a novel subset of thymocytes that were J11d+CD3-. From day 10 to day 21 the proportion of CD4+CD8-CD3-J11d+ cells decreased while the proportion of CD4+CD8+ cells and CD4+CD8-CD3+J11d- cells increased. The CD4+CD8-CD3- cells seem to diversify to form CD4+CD8+ thymocytes after short-term culture in vitro. These results suggested the existence of a differential pathway from CD4-CD8- cells to CD4+CD8+ cells via CD4+CD8- cells in thymus.

Characterization of the subset of immature thymocytes which can undergo rapid in vitro differentiation

Recently, we reported that thymocytes expressing the CD8 molecule on their surface can give rise to CD4+CD8+ double-positive and CD4+ single-positive progeny following intrathymic transfer into an irradiated host mouse. Thymcoytes expressing a high density of CD8, referred to as CD8hi, and those expressing a low density of the molecule, CD8lo, were both able to differentiate in vivo. In this study we examined the ability of these CD8+ thymocytes populations and of CD4-CD8- double-negative thymocytes to change their phenotype during brief in vitro culture. CD8+ thymocytes were prepared by anti-CD4 plus complement lysis followed by positive selection of the survivors on anti-CD8-coated plates. After 16 h of culture, greater than 60% of CD8+ thymocytes became double-positive. Both CD8hi and CD8lo cells were able to show this in vitro change: about 30% of the former and about 80% of the latter became double-positive. In contrast to this, double-negative thymocytes which had been depleted of cells expressing low densities of CD8 did not show such a phenotypic conversion in vitro. Further panning experiments suggested that all of the CD8+ thymocytes actually express a low surface density of the CD4 molecule which is undetectable in our cytofluorometric assays.

A murine early thymocyte developmental sequence is marked by transient expression of the interleukin 2 receptor

Precursors of all T-lineage cells are found in the population of thymocytes that lacks the CD4 and CD8 surface markers. These "double-negative" thymocytes are heterogeneous and can be divided into discrete subpopulations based on their expression of other surface markers. We have determined the relative maturity of these subpopulations based on the extent of rearrangement and expression of their T-cell receptor genes, their cell cycle status, and their thymus reconstitution capacity. Within the subpopulation of double negatives expressing high levels of the heat-stable antigen, the additional markers phagocytic glycoprotein 1 (Pgp-1) and interleukin 2 receptor (IL-2R) can be used to define the sequence IL-2R- Pgp-1+----IL-2R+ Pgp-1-----IL-2R- Pgp-1-, which occurs before the expression of CD4 and CD8. Transient expression of the IL-2R marks an important developmental point in the sequence just prior to a burst of cell proliferation and a loss of thymus reconstitution ability. The earliest cells in this sequence are already partially rearranged for genes in the C beta 1 region. IL-2R expression marks a second wave of T-cell antigen receptor of beta-chain gene rearrangement and the initiation of T-cell antigen receptor alpha- and beta-chain gene expression.

Functional responsiveness in vitro and in vivo of alpha/beta T cell receptors expressed by the B2A2 (J11d)- subset of CD4-8- thymocytes

B2A2-CD4-8- cells represent a rare subpopulation of thymocytes normally comprising 0.5% of the total adult thymus. These cells are known to express CD3-associated T cell receptor (TcR) alpha/beta molecules. In the present study we have examined the functional capacity of alpha/beta heterodimers on B2A2-CD4-8- cells. In the presence of monoclonal antibody (mAb) specific for either murine CD3 or TcR expressing the V beta 8-encoded beta chain (F23.1), B2A2-CD4-8- cells proliferated. Such proliferation was blocked by mAb to interleukin 2 receptor (IL 2R), suggesting an autocrine mechanism involving IL 2 production and subsequent utilization. IL 2 and also IL 3 production by mAb-stimulated B2A2-CD4-8- cells was directly confirmed. Furthermore, a panel of B2A2-CD4-8- clones were derived to assess the role of the TcR in cytolysis. Many clones were isolated which killed Fc receptor-bearing P815 target cells only in the presence of F23.1 mAb. Finally, in vivo treatment of neonatal mice with F23.1 mAb resulted in a marked reduction of V beta 8+ B2A2-CD4-8- thymocytes. Collectively, these results indicate that the TcR alpha/beta complex on CD4-CD8-B2A2- cells is fully capable of transducing signals that lead to proliferation, lymphokine production and cytolysis in vitro, as well as to disappearance of this subset from the thymus in vivo.

Intrathymic deletion of self-reactive cells prevented by neonatal anti-CD4 antibody treatment

T-cell differentiation in the thymus involves the coordinate expression of genes encoding the alpha and beta chains of the major histocompatibility complex-restricted heterodimeric antigen receptor (TCR) complex, as well as other functionally important molecules such as CD4 and CD8. The repertoire of TCR expressed by T cells is generally thought to be influenced by positive and/or negative selection events occurring when TCRs on developing T cells interact with self-antigens and major histocompatibility complex components. Using a model system in which specific antigen-reactive cells can be monitored by virtue of their preferential expression of certain TCR beta-chain variable (V beta) domains, it has been shown that self-reactive T cells are clonally deleted during development. We report here that clonal deletion of V+ beta 6 cells in Mlsa mice can be prevented by in vivo neonatal administration of monoclonal antibodies directed against CD4. Furthermore, as anti-CD4 monoclonal antibody treatment resulted in the reappearance of V+ beta 6 cells in the mature CD8+ T-cell subset, it is likely that clonal deletion acts on the CD4+CD8+ thymocyte subset and that this subset is an intermediate stage in the differentiation pathway of both CD4+ and CD8+ T-cell lineages.