[Treatment via autotransfusion of UV-irradiated blood in patients with nonspecific lung diseases]

An analysis of 53 patients with unspecific pulmonary diseases revealed changes of the main immunity values that were more pronounced in the older age groups. It was established that autotransfusion of UV-irradiated blood produced a normalizing effect on immunity values. The authors recommend this method in the clinical practice of unspecific pulmonary diseases, in particular, of the older age groups and intolerance of drug therapy.

Production of hapten-specific T cell hybridomas and their use to study the effect of ultraviolet B irradiation on the development of contact hypersensitivity

We produced a series of T cell hybridomas that produce IL-2 when cultured with syngeneic APC coupled to FITC or TNP. These hybridomas are hapten specific and Ia restricted. The hybridomas were used to detect hapten-bearing APC in draining lymph nodes of mice sensitized with trinitrochlorobenzene or FITC in vivo. Hapten-bearing APC capable of stimulating the hybridomas were detectable in draining lymph nodes of hapten-painted mice within 3 h after sensitization. The ability of lymph node APC to stimulate the hybridomas peaked at 24 h and declined by 48 h. The dendritic cell subpopulation was the subpopulation of cells that were found in the regional lymph nodes of hapten-painted animals that were capable of stimulating the hybridomas to produce IL-2. Prior treatment of the skin with low dose UVB irradiation before epicutaneous application of contact sensitizers significantly reduced the capacity of hapten-bearing APC to stimulate the hybridomas. This observation was corroborated by results obtained from flow microfluorometry analysis of lymph node cells from FITC-sensitized mice. Lymph node dendritic cells obtained from FITC-painted mice contain a brightly staining group of cells by flow microfluorometry analysis. Lymph node dendritic cells from FITC-painted, UVB-irradiated mice did not contain this brightly staining population. These results indicate that low dose, local UVB irradiation may affect APC migration and/or function. We believe that these hybridomas will prove to be useful tools in the study of the development and regulation of contact hypersensitivity.

Radiation hormesis. A new concept in radiological science

Low-dose ionizing radiation caused definite stimulation of immune reactions both in humans and mice. The PFC reaction in response to SRBC immunization and the NK activity of the splenocytes were significantly enhanced after low-dose whole body irradiation. Activation of the T lymphocytes, especially the TH, with increased production of IL-2, might be a critical step in the whole process of immunoenhancement. A single dose of 75 mGy X-rays caused significant lowering of hypothalamic M-Enk content as well as serum corticosterone level. The increased serum testosterone level would exert an inhibitory influence on the CRF-ACTH-CS system to keep the blood corticosterone at a lower than normal level which might facilitate the immune reactions in the SRBC-immunized animals. The increased catecholamines in the spleen would probably reinforce this effect resulting in immunoenhancement. Low-dose ionizing radiation caused increased repair of the genetic material at both the molecular and subcellular levels. The UDS of human and murine lymphocytes was augmented by single or continuous low-dose irradiation. The stimulation of DNA polymerase activity might be responsible for such effects. Exposure to very small doses of low LET radiation could induce in different tissues an adaptive response which alleviated chromosome damage caused by subsequent larger dose radiation. Such an adaptive response could be induced both in vivo and in vitro in different animal species. The induced adaptive response faded away after 3 cell cycles could be re-induced by a second exposure to low-dose radiation. The mechanism of the inductive process needs further study.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of interleukin-2, feeder cells, and timing of irradiation on the radiosensitivity of human T lymphocytes assessed by the colony-forming assay

The radiosensitivity of human lymphocytes was investigated by the method of colony formation in the absence of interleukin-2 (IL2) and feeder cells, both of which enhance growth of T-cell colonies. The shape of the survival curve and the radiosensitivity was shown to depend upon the ability of lymphocytes to produce IL2: the survival curve for lymphocytes that were the most competent producers of IL2 is the closest to linearity; the lymphocytes that were poor producers show biphasic survival curves. The radiosensitivity of the lymphocytes from the first group is less than that of the latter, when the comparison is based on the first part of the biphasic survival curve. This is more easily seen when cultures are irradiated 24 h after stimulation by phytohemagglutinin (the time of the peak IL2 production) than when cultures are irradiated 2 h before stimulation. This study demonstrates that growth conditions influence the response of lymphocytes to irradiation and that optimal growth conditions result in a linear survival curve.

Regulation of murine lymphokine production in vivo. Ultraviolet radiation exposure depresses IL-2 and enhances IL-4 production by T cells through an IL-1-dependent mechanism

The exposure of experimental animals to the inflammatory effects of ultraviolet radiation (UVR) is known to cause depressions in their ability to initiate and effectuate various types of cellular immune responses. Contact-type and delayed-type hypersensitivity, plus the ability to generate protective forms of anti-viral and anti-tumor immunity, are all affected by the prior exposure of normal animals to the effects of this physical agent. Presently, the cellular and molecular mechanism(s) responsible for mediating the changes in immune function observed in UVR-exposed animals is not fully understood. Herein we report that one reproducible consequence of exposing normal mice to low doses of UVR is a dramatic change in the pattern of lymphokines secreted by their activated T cells. Lymphocytes isolated from UVR-exposed donors produce/secrete greatly reduced levels of the T cell lymphokines IL-2 and IFN-gamma activation in vitro with protein Ag of the polyclonal T cell stimulant anti-CD3. The secretion of IL-4 by these lymphocyte cultures, however, is consistently elevated in comparison to normal controls. Further studies determined that a similar change in lymphokine production was induced when mice were treated with either bacterial LPS or rIL-1 beta, a cytokine known to be elevated in vivo after UVR or LPS exposure. The ability of IL-1 to facilitate a change in the capacity of T lymphocytes to produce/secrete lymphokines after in vitro activation does not appear to represent a direct effect of this cytokine on lymphocyte or accessory cell targets because addition of IL-1 beta to cultures of Ag-primed lymphocytes obtained from normal donors was incapable of altering the pattern of lymphokine production. Collectively, our present results add further support to the hypothesis that UVR-induced elevations in endogenous IL-1 are, in part, responsible for the immunomodulatory effects of UVR. These findings provide compelling evidence that UVR, plus other agents capable of endogenously stimulating the production of IL-1, may function to alter the expression of different effector mechanisms in vivo. This could be facilitated through selective reductions in lymphokines produced by Th-1-type cells (IL-2 and IFN-gamma) and a simultaneous augmentation in a lymphokine produced by Th-2-type cells (IL-4).

Efficient natural defense mechanisms against Listeria monocytogenes in T and B cell-deficient allogeneic bone marrow radiation chimeras. Preactivated macrophages are the main effector cells in an early phase after bone marrow transfer

Radiation chimeras in the early phase after bone marrow transplantation are a good model to study the efficiency of the body's nonspecific defense system represented by macrophages (M phi), polymorphonuclear cells (PMN), and NK cells. These cell types are present in large numbers in spleen and liver at that time, whereas the specific immune system represented by T and B cells is functionally deficient. We previously reported enhanced activities in vitro of M phi (and PMN) from recipient animals in an early phase after allogeneic bone marrow transfer. We here demonstrate that these activities result in enhanced spontaneous resistance against Listeria monocytogenes in vivo: CFU of L. monocytogenes in spleen and liver 48 h after infection were about 1 or 2 to 4 log steps less than in untreated control mice of donor or host haplotype. This enhanced resistance decreased over the 4-mo period after marrow transfer. Preactivated M phi were identified as the most important effector cells. Isolated from spleen and peritoneal cavity, they performed enhanced killing of phagocytosed Listeria. Such preactivated M phi occurred in recipient animals after transfer of allogeneic but not of syngeneic bone marrow. The precise mechanism of M phi activation in the allogeneic radiation chimera in the complete absence of any detectable T cell function is not clear at present. However, these preactivated M phi display an important protective effect against L. monocytogenes: chimeras could eliminate Listeria without acquisition of positive delayed-type sensitivity when infected with 10(3) bacteria. An inoculum of 5 . 10(3) L. monocytogenes resulted either in prolonged survival compared with normal mice of the recipient haplotype or in definitive survival accompanied by a positive delayed-type sensitivity. We concluded that enhanced nonspecific immune functions can in part compensate for the defective specific immune system after bone marrow transfer.

Is the NAD-poly (ADP-ribose) polymerase system the trigger in radiation-induced death of mouse thymocytes?

Agents that induce DNA strand breaks evoke a drop in the NAD content in mouse thymocytes. A decrease in the endogenous NAD content that occurs immediately after gamma-irradiation of thymocytes is entirely attributed to the activation of poly(ADP-ribosylation). The addition of 5 mM benzamide before irradiation prevents the postirradiation drop of the NAD level but has no effect on chromatin degradation and cell death. In contrast to liver nuclei, pre-incubation of mouse thymic nuclei with NAD had no effect on the subsequent chromatin endonucleolysis by Ca2+/Mg2+-dependent endonuclease. It is suggested that the NAD-poly(ADP-ribose) polymerase system is probably not the trigger in the radiation-induced programmed death of mouse thymocytes, but may merely be indicative of the radiation response of these cells.

On the role of Ca, Mg-dependent nuclease in the postirradiation degradation of chromatin in lymphoid tissues

The characteristics of the postirradiation degradation of chromatin in thymocytes in vivo were compared with the features of chromatin fragmentation in isolated thymocyte nuclei in vitro by endogenous chromatin-bound nucleases. Nuclease which degrades chromatin produces in vivo fragments of nucleosomal size; the double-strand breaks appear as the result of the accumulation of single-strand breaks with 3'-OH ends; the nuclease is inhibited by Zn2+ and DTNB and its activity is depressed by cycloheximide pretreatment. In experiments on in vitro degradation of chromatin in isolated thymocyte nuclei similar properties were observed for the Ca, Mg-dependent, but not for acid nuclease. The results bring further evidence of the involvement of an enzyme of the Ca, Mg-dependent nuclease-type in chromatin degradation in irradiated thymocytes.

[Immune status indices of medical radiologists]

The number of T- and B-lymphocytes of the peripheral blood was studied in 59 medical radiologists. A decrease in the number of T-lymphocytes was noted, its degree being associated with an individual annual dose of irradiation, the period of contact with radiation and age. The number of B-lymphocytes was changed to a lesser degree. It is assumed that a previously reported decrease in the body resistance to environmental hazards depends, to a certain extent, on immunodeficiency detected in this work as well as on the deficiency of metabolism of C and B1 vitamins reported earlier.

Cell proliferation and thymocyte subset reconstitution in sublethally irradiated mice: compared kinetics of endogenous and intrathymically transferred progenitors

After sublethal (6 Gy) whole-body irradiation, the C57BL/Ba (Thy-1.1) murine thymus regenerated in two waves, on days 3-10 and 25-32, separated by a severe relapse. The second phase of depletion-reconstitution reproduced the first one, in a less synchronous manner. The depletion affected all cell subsets, but CD4+ CD8- cells decreased later than immature cells. Cell proliferation, measured by BrdUrd incorporation, started on day 3 after irradiation and concerned CD4- CD8-, CD4- CD8+, and CD4+ CD8+ cells, sequentially. CD4+ CD8- cells never represented a significant percentage of cycling cells. When irradiation was immediately followed by an intrathymic injection of 10(5) C57BL/Ka (Thy-1.2) bone marrow cells, the relapse in thymus reconstitution was no longer observed. Detected with anti-Thy-1.2 antibodies, donor cells started cycling on day 14 and showed only one wave of proliferation. In these chimeras, recipient thymocytes behave exactly like thymocytes of solely irradiated mice. Intrathymically transferred CD4- CD8- thymocytes (10(5] showed the same proliferation kinetics as endogenous cells, with a peak in number on day 10 but completely disappeared from the thymus on days 14-21. These data reflect maturational differences between intrathymic and bone marrow precursor cells and suggest different radiosensitivities not linked to proliferative status. The resting state of the thymus immigrants was shown by the absence of Thy-1 acquisition by bone marrow cells continuously labeled for 10 days with BrdUrd in vivo before intrathymic transfer. When such labeled bone marrow cells were injected in the thymus, only the minor BrdUrd- subset gave rise to Thy-1+ cells.

Chromosome aberrations in peripheral lymphocytes and radiation dose to active bone marrow in patients treated for cancer of the cervix

An international study of cervical cancer patients reported a doubling of the risk for leukemia following radiotherapy. To evaluate the extent of residual chromosome damage in circulating T-cell lymphocytes in this population, approximately 200 metaphases were examined from each of 96 irradiated and 26 nonirradiated cervical cancer patients treated more than 17 years ago (average 23 years). Radiation dose averaged over the total red bone marrow was estimated to be 8.1 Gy. The type and frequency of stable and unstable chromosome aberrations were quantified in 24,117 metaphases. Unstable aberrations did not differ significantly between irradiated and nonirradiated patients (P greater than 0.5). Stable aberrations (i.e., translocations, inversions, or chromosomes with deleted segments), however, were significantly higher among irradiated (2.8 per 100 cells) compared to nonirradiated (0.7 per 100 cells) women (P less than 10(4). The frequency of these stable aberrations was found to increase significantly with increasing dose to the bone marrow. These data indicate that a direct relationship between radiation dose and extent of damage to somatic cells persists in populations and can be detected many years after partial-body radiation exposure. The stable aberration rate in irradiated cervical cancer patients was 50 to 75% lower than those observed 25 years or more after radiation exposure in atomic bomb survivors and in ankylosing spondylitis patients treated with radiotherapy. The average marrow dose was only 1 Gy in the examined atomic bomb survivors and 3.5 Gy in the ankylosing spondylitis patients. It appears, then, that a very high dose delivered to the pelvic cavity in fractionated doses resulted in far fewer persistent stable aberrations than lower doses delivered either in acute whole-body exposure or in fractionated doses to the spinal column and sacroiliac joints. The higher radiation dose and the concentration of that dose in a smaller area of the body appear to be responsible for the lower rate of persistent aberrations observed in cervical cancer patients.

Late tissue-specific toxicity of total body irradiation and busulfan in a murine bone marrow transplant model

Total body irradiation (TBI) and busulfan were compared for late effects in a murine model of bone marrow transplantation (BMT). Male C57BL/6 mice were given fractionated TBI or busulfan given in 4 equal daily doses followed by infusion of 10(7) syngeneic bone marrow cells. Total doses of 16.4 Gy TBI and 3.4 mg busulfan were chosen for their equivalence in inducing near complete engraftment of allogeneic marrow from donor mice of the LP strain. The two treatment groups had a late wave of mortality starting at about 80 weeks after transplantation. Specific tissue damage was manifested in bone marrow stem cells, splenic T-cell precursors, hair greying and cataract formation for both TBI and busulfan but to varying degrees. Severe nephrotoxicity and anemia were observed only after TBI. Although both busulfan and TBI kill early marrow stem cells and are effective preparative agents in bone marrow transplantation, their effects on other stem cell and organ systems are not similar. In addition, many of the injuries seen are late to occur. The delayed expression of injury deserves careful long-term evaluation of BMT recipients before the therapeutic potential of effective preparative regimens can be fully appreciated.

6-Thioguanine resistant T-lymphocyte determination as a possible indicator of human ionizing radiation exposure

We used the autoradiographic assay to assess human in vivo somatic cell gene mutation at the hypoxanthine guanine phosphoribosyl transferase (hgprt) locus in T-lymphocytes. Cells able to incorporate tritiated thymidine in vitro in 6-thioguanine containing short-term cultures were enumerated in order to determine 6-thioguanine resistant (TGr) variant frequencies in cryopreserved lymphocytes from control individuals and 3 persons suspected of 60Co exposure from an accident in Cd. Juárez, México. The data indicate that the lymphocyte TGr variant frequency assay may be potentially usefull for human population monitoring following accidental exposures to ionizing radiation.

Rapid recovery of Langerhans cell alloreactivity, without induction of autoreactivity, after in vivo ultraviolet A, but not ultraviolet B exposure of human skin

For therapeutic medical, cosmetic, and recreational reasons, humans expose themselves to increasing amounts of UVA. However, little is known of the photobiologic events associated with cutaneous carcinogenesis and photoaging that occur as a result of UVA exposure. UVB exposure of human skin abrogates the function of epidermal CD1+DR+ Langerhans cells and induces the appearance of CD1-DR+ non-Langerhans cell APC. This non-Langerhans cell APC population activates autoreactive immunoregulatory T cells that lead to suppressor-effector T cell function. In this report we show that, similarly to UBV, UVA exposure abrogates the function of CD1+DR+ Langerhans cells. However, in contrast to UVB, there is rapid recovery of Langerhans cell antigen-presenting cell activity and that CD1-DR+ non-Langerhans cell APC failed to appear to a significant degree. In keeping with the lack of CD1-DR+ epidermal cells, UVA exposed epidermal cells harvested 3 days after exposure functioned similarly to normal epidermis in that they activated alloreactive T cells but not autoreactive T cells in the absence of added Ag. This was in contrast to UVB irradiated epidermal cells that potently activate autoreactive T cells and contain CD1-DR+ cells. Thus, although both UVA and UVB initially depletes and inactivates CD1+DR+ Langerhans cells, the subsequent APC function of epidermal cells exposed to UVA differ profoundly from that of cells exposed to UVB. UVA radiation is less carcinogenic than UVB; differences in host responses to UV tumors may be linked to the rapid recovery of Langerhans cell function and the lack of induction of CD1-DR+ non-Langerhans cell APC after UVA exposure.

Early postirradiation chromatin degradation in thymocytes

The decrease in the average DNA size in thymocytes starts soon after in vivo irradiation and at approximately 45 min reaches a plateau, thereafter showing only minor changes up to 3 h. This fall in extent of chromatin cleavage coincides with the accumulation of 1.0-1.5 kb DNA fragments. Double-strand breaks generated by endonucleases are not randomly distributed along DNA but clustered in such a way that they give rise to fragments of 1-5 nucleosomes in size. Cycloheximide treatment partially inhibits nuclease activity in nuclear extracts isolated from thymus of irradiated mice. This suggest that DNA fragmentation is an early event in programmed death of thymocytes mediated by irradiation. The data indicate that it requires protein synthesis and that it precedes release of polydeoxyribonucleotides.

[Comparative experimental study of various methods of photomodification++ of the blood]

An experimental analysis of various methods of photomodification of blood has shown that their use in medical practice has principle possibility to obtain effects of various directions at the expense of variation of blood irradiation regimens. UV radiation brings about processes of lipid peroxidation in plasma membrane of lymphocytes, their intensity depends on the presence of oxygen in the surroundings.

The expression and sequences of T cell antigen receptor beta-chain genes in the thymus at an early stage after sublethal irradiation

The sequential appearance of the thymocyte subpopulations and TCR gene messages occurred in the thymus of AKR mice (H-2k, Mlsa) from 7 to 14 days after sublethal irradiation. The thymocytes on day 7 after irradiation were composed of a large number of CD4+CD8+ blast-like cells and a relatively high proportion of CD4-CD8- cells (15 to 25%) but few CD3highCD4+CD8-/CD4-CD8+ cells. Approximately 22% of the CD4-CD8- cells were CD3high and -27% of the CD3highCD4-CD8- cells (-6% of whole CD4-CD8- cells) were F23.1+. The thymocytes on day 7 expressed a large amount of gamma- and delta-chain gene transcripts but reduced levels of alpha- and beta-chain gene transcripts. The V gene repertoire of 18 functional beta-chain cDNA derived from the thymocytes on day 7 was compared with those of 20 functional beta-chain cDNA derived from the thymocytes on day 14 which were composed of a large number of CD3lowCD4+CD8+ small-sized cells and a small number of CD3highCD4+CD8- cells. It is noteworthy that the distribution of V beta genes expressed in the thymocytes on day 7 was much the same as that in the thymocytes on day 14 but significantly different from that in normal BALB/c thymocytes as previously described. Interestingly, neither V beta 8.1 nor V beta 6 genes, which are important for recognition of the product of the Mlsa locus, was detected in these two cDNA libraries. These results suggest that clonal selection of TCR V beta repertoire, irrespective of positive or negative selection, appears to occur at the early stage of T cell differentiation, i.e., on the blast-like CD4+CD8+ thymocytes.

Thymus transplantation and disease prevention in the diabetes-prone Bio-Breeding rat

Bio-Breeding rat T lymphocytes proliferate poorly in response to alloantigen. Transplantation of Bio-Breeding rats with fetal thymus tissue from diabetes resistant rats leads to an improvement in the T cell proliferative response, but only if the thymus contains bone marrow-derived, radiation-resistant thymic antigen presenting cells of the diabetes-resistant phenotype. The current study provides evidence that thymus transplantation leading to the restoration of Bio-Breeding T cell proliferative function can also significantly reduce the incidence of insulitis and prevent the development of diabetes. It appears that a defect in the bone marrow-derived thymic APC population contributes to an abnormal maturation of Bio-Breeding T lymphocytes which in turn predisposes animals to insulitis and diabetic disease.

The cellular lesion responsible for exaggerated IgE synthesis accompanying allergic breakthrough

Appropriate levels of IgE are maintained by a cellular and molecular network composed of (1) a suppressive, Ly-1+, CD4+ T cell-dependent arm that is activated by inappropriate high levels of IgE and (2) an enhancing, CD8+ T cell-dependent arm that controls this suppression in a feedback regulatory manner. Ly-1+ T cells also function to counterbalance (inhibit) the activity of these latter CD8+ T cells. It has been previously shown that Ly-1+ T cells can reverse low-dose irradiation-induced enhancement of IgE antibody responses (i.e., allergic breakthrough). We have analyzed lymphocytes isolated from mice subjected to low-dose irradiation to determine which component of this network is defective in such animals. Stimulation of normal lymphocytes with IgE in vitro resulted in the release of lymphokines that suppress IgE antibody responses. In contrast, similar stimulation of lymphocytes from irradiated mice did not elicit secretion of such suppressive lymphokines, unless the cells were depleted of CD8+ T cells or reconstituted with normal Ly-1+ T cells. Because Ly-1+ T cells of irradiated mice could not reconstitute the response, we conclude that this functional subset of CD4+ T cells, which normally controls CD8+ T cell activity in this network, is defective in animals that exhibit irradiation-induced allergic breakthrough.

Immunity to coccidiosis: T-cell control of infection with Eimeria vermiformis in mice does not require co-operation with inflammatory cells

The necessity for co-operation between lymphocytes and myeloid-derived inflammatory cells in the mediation of anti-coccidial immunity was investigated using mice infected with Eimeria vermiformis. Reciprocal exchange of immune lymphocytes between H-2 compatible strains of contrasting susceptibility to infection (resistant BALB/B and susceptible C57BL/10) resulted in successful transfer of immunity in both homologous and heterologous exchanges. Recipients of immune cells, whatever their original response phenotype, expressed a high degree of immunity to infection, indicating that the differential susceptibility of the strains is a property of their lymphoid cells and is not attributable to their capacity to mount inflammatory responses. This conclusion was confirmed by the successful adoptive transfer of immunity into NIH mice previously exposed to 600 rad X-irradiation; at this level of irradiation inflammatory responsiveness is severely depressed. Additional confirmation that strain-response phenotype is lymphocyte dependent and that immune lymphocytes can mediate their effects against E. vermiformis without the intervention of inflammatory cells was obtained from studies on the mucosal mast cell response to infection. No correlation existed between the development of intestinal mastocytosis, an index of T-cell-mediated inflammatory responsiveness, and the expression of resistance to E. vermiformis in BALB/c (resistant), C57BL/10 (susceptible) and NIH (susceptible) mice.

A role for Lyt-2+ T cells in resistance to cutaneous leishmaniasis in immunized mice

The role of Lyt-2+ T cells in immunologic resistance to cutaneous leishmaniasis was analyzed by comparing infection patterns in resistant C57BL/6 mice and susceptible BALB/c mice induced to heal their infections after sub-lethal irradiation or i.v. immunization, with similar mice treated in vivo with anti-Lyt-2 antibodies. Administration of anti-Lyt-2 mAb resulted in a dramatic reduction in the number of lymphoid cells expressing the Lyt-2+ phenotype. Such treatment led to enhanced disease in both resistant C57BL/6 and irradiated BALB/c mice, as assessed by lesion size, but did not affect the capacity of these mice to ultimately resolve their infections. In contrast, anti-Lyt-2 treatment totally blocked the induction of resistance in i.v. immunized mice. These results suggest, that Lyt-2+ T cells may play a role in immunity to a Leishmania major infection and that their relative importance to resistance may depend on how resistance is induced.

Does T-cell tolerance require a dedicated antigen-presenting cell?

Almost 30 years ago Burnet proposed that the immune system maintained self-tolerance by deleting autoreactive lymphocytes. Recently it has become clear that for T cells this step occurs in the thymus, where developing T cells first express their antigen-specific receptors. Here a T-cell which encounters its antigen disappears--if it is not dead, it at least stops expressing its receptors. In the periphery by contrast, encounter with antigen leads to activation and proliferation of the responding T-cell. There are two possible explanations for this difference. Either the antigen-presenting cells in the thymus are different from those in the periphery and instead of producing positive signals they directly or indirectly kill the thymocytes; or the T cells themselves are different, and like immature B cells, may die after encounter with antigen. We tested the first possibility and found that dendritic cells from spleen, which are the most potent activators of mature T cells, are also the most potent inactivators of young developing T cells. Thus it is not the antigen-presenting cell which determines whether a T-cell responds or dies, but the T-cell itself or its thymic environment.