Analysis by limiting dilution of interleukin 2-producing T cells in murine ontogeny

In vitro HgCl2 exposure of immune cells at different stages of maturation: effects on phenotype and function

This is the first study to investigate the hypothesis that the immunotoxic effects of inorganic mercury may be modulated by inherent differences in the responsiveness of immune cells related to the age of the donor. We exposed cells from lymph nodes, spleen, and thymus, collected from 7- and 10-day-old CD.1 pups, as well as from adult CD.1 mice, in terms of the effects of mercury in vitro on responses to Con-A stimulation with respect to proliferation, cytokine production, and cell phenotype. The effects of mercury on proliferation were age and organ dependent, while effects on cytokine production were only age dependent. Effects of mercury were observed only on splenocyte T-cell subpopulations and only in cells from 10-day-old pups and from adults. Mercury had no effect on IFN-gamma and IL-4 production by splenocytes from 7-day-old pups, but significantly decreased release of these cytokines by splenocytes from 10-day-old pups and adults. Hg did not affect IL-4 production by lymph node cells or thymocytes. In lymph node cells Hg affected IFN-gamma production only at 7 days. These data indicate that inherent properties of immune cells at different stages of development may influence the response to immunotoxicants.

Partial correction of the TH2/TH1 imbalance in neonatal murine responses to vaccine antigens through selective adjuvant effects

We have recently shown that neonatal responses to a pannel of vaccine antigens and presentation systems differed qualitatively from adult responses by a bias towards a TH2 pattern. Here we report that a selected adjuvant comprising block copolymers in a water-in-oil emulsion can induce balanced TH1/TH2 responses in BALB/c mice primed at 1 week of age with an immunodominant tetanus peptide vaccine. However, using this specific TH1-driving adjuvant only at time of boosting was not sufficient to fully circumvent the persisting influence of TH2-biased neonatal responses. Unexpectedly also, a significant local toxicity was observed in newborn and young mice, whereas only mild reactions occurred in adults. Thus, although the induction of strong TH1 responses in the neonatal period can be achieved using specific adjuvants, through modulation of the immunological environment present at time of priming, whether such immunization strategies would be safe in the neonatal period remains to be demonstrated. These observations should be taken into consideration in the development of novel vaccines that will have to be already effective early in life.

Neonatal and early life immune responses to various forms of vaccine antigens qualitatively differ from adult responses: predominance of a Th2-biased pattern which persists after adult boosting

Induction of neonatal immune responses to vaccine antigens is believed to be of limited efficacy because of immune immaturity and particular susceptibility to tolerogenic signals during this period of life. To characterize particular features of neonatal immune responses to vaccine antigens, we assessed the capacity of BALB/c mice at different stages of immunological maturation to respond to a selection of vaccine antigens and presentation systems. Significant B and T cell responses to vaccine antigens (tetanus and measles virus peptides, tetanus toxoid, live viral attenuated measles virus, canarypox recombinant measles vector or bacillus Calmette-Guérin) were obtained as early as the first week of life. However, these neonatal responses differed qualitatively from adult responses by a decreased IgG2a/IgG1 ratio of vaccine-specific antibodies, the secretion of significantly higher interleukin-5 and lower interferon-gamma levels by vaccine-specific T cells and an impaired induction of cytotoxic T cell precursors. This pattern of biased Th2 versus Th1 responses induced upon early exposure to vaccines was not reversed by decreasing the doses of vaccine antigens. It did not disappear with aging and was still reflected in adult responses to booster immunization with the corresponding antigen. Thus, neonatal immunization can induce significant vaccine specific responses with a predominance of a Th2 pattern which can persist in boosted adult mice.

Seeding of neonatal lymph nodes by T cells and identification of a novel population of CD3−CD4+ cells

Mature T cells first appear in the thymus of the mouse a few days before birth, about 7-8 days after the thymus was colonized by stem cells. These mature cells are exported to the peripheral lymphoid organs beginning at about the time of birth, but because the number is very small at this stage, little is known about the phenotype or function of these early emigrants. We have examined the cells that accumulate in the peripheral lymph nodes (LN) during the first week of life to understand better the initial seeding of the periphery by T cells. Our studies showed that a high proportion of neonatal LN cells were CD4+, but that the majority of these were CD3- during the first few days of life. The CD3- population did not increase greatly in number after birth and rapidly diminished in proportion as the number of CD3+ cells increased. These CD3-CD4+CD8- cells were found to be Thy-1loCD44+ and to lack surface expression of heat-stable antigen. B220 and Mac-1. They had lymphoid morphology, did not phagocytose latex, and did not exhibit any precursor activity for cells of hemopoietic lineages. Their origin (intra- or extrathymic) as well as their function and physiological role, therefore, remains unknown. CD3+ T cells, both CD4+CD8- and CD4-CD8+, were present in low numbers during the first 1-2 days of life, but at post-natal day 3, a sharp increase in the accumulation of these cells occurred in both LN and spleen. By day 3 the CD4:CD8 ratio in LN was about 2:1, as in the adult. Crude estimates of the rate of export from the thymus from day 3 onwards gave values around 1% of thymocytes per day, i.e. close to our previous estimates for young adult thymus. We found no evidence of particularly high levels of emigration from the thymus during the first week after birth. Both CD4+CD8- and CD4-CD8+ T cell subsets were present in the LN as early as 1 day post-natally with CD4-CD8+ predominating among LN T cells, even though CD3+CD4+CD8- cells predominated over CD3+CD4-CD8+ cells in the thymus. By day 3 the ratio had changed to 2:1 (as in the adult). T cells, therefore, appear to emigrate from the thymus from about the time of birth with a dramatic increase around day 3 after birth.

Peripheral T lymphocytes: expansion potential and homeostatic regulation of pool sizes and CD4/CD8 ratios in vivo

Peripheral T lymphocytes are self-renewable cell populations since, when transferred into syngeneic T cell-deficient athymic mice, they expand in the absence of exogenous antigen stimulation. Quantification of the expansion potential of CD4+ cells by transfer of the same population into successive host mice shows that these cells are able to divide up to 56 times in vivo. Therefore, as a population, CD4+ cells can increase in size 8 x 10(5)-fold, an expansion potential of similar magnitude to that previously reported for colony-forming units. Injection of different numbers of T cells at different CD4/CD8 ratios is followed by T cell accumulation to a similar plateau in recipient nude mice. This indicates that peripheral T lymphocytes are tightly regulated by homeostatic mechanisms that control pool sizes and CD4/CD8 ratios, in a manner independent of the cell input into the peripheral compartment. This kinetic behavior of mature T cells permits the maintenance at the periphery of any T cell specificity previously selected in the thymus. The expansion capacity of peripheral T cells may also allow extensive modulation of peripheral T cell specificities, which would confer a major role to post-thymic selection of mature peripheral T cell repertoires.

Neonatal tolerance induction to Mlsa. I. Tolerance to Mlsa is restricted by shared MHC determinants

In this paper we have examined the influence of MHC (major histocompatibility complex) on neonatal tolerance to Mlsa (minor lymphocyte stimulating). By employing a novel approach we have shown that tolerance to Mlsa is restricted by shared MHC determinants. Thus, neonatal Mlsb mice, injected at birth with spleen cells from Mlsa mice, were tested as adults for Mlsa specific responses by interleukin-2 limiting dilution analysis, a technique which allows us to discriminate between responses to MHC + Mlsa and to MHC alone. Tolerance to Mlsa was in the context of any MHC type examined--donor, host, and third-party MHC products. These results show that tolerance to Mlsa is restricted by shared MHC determinants and extend previous studies indicating that activation of Mlsa responses is similarly restricted.

Unidirectional responses to Mls determinants in vivo. Polyclonal T-cell responses to a single common determinant of Mls in different efficiencies?

Polyclonal anti-Mls responses of peripheral (mature) and thymic (immature) lymphocytes were studied in vivo in terms of local host-versus-graft and graft-versus-host reactions. The level of stimulatory activity differed between the various types of Mls antigens, with Mlsa and Mlsd having the highest levels. Mlsc a lower level, and Mlsb the lowest level. The immunogenicity of Mlsa and Mlsd may not be identical, since Mlsd mice responded to the Mlsa determinant, while Mlsa mice did not react with the Mlsd determinants. This suggests that the anti-Mls response is unidirectional. The fact that Mlsb mice made tolerant at birth to Mlsd (Mlsa) antigens behave like Mlsd (Mlsa) mice when responding to Mls antigens supports this suggestion. Furthermore, thymus cells from the Mlsa-tolerized BALB/c (Mlsb) mice were unresponsive to both Mlsa and Mlsd antigens, while those made tolerant to Mlsd were responsive to Mlsa. These results indicate that the polyclonal response to the strongly immunogenic Mlsa,d antigens is unidirectional, and that the immunogenicity of Mlsa and Mlsd is not identical. Based on the results of tolerance experiments and data on T-cell clones, we suggest that the difference in immunogenicity between Mlsa,b,d antigens is due to different efficiencies of the responding T-cell populations, probably because of a quantitative difference in a single common antigenic determinant expressed in each Mls haplotype, which results in different levels of stimulation of T cells according to the avidity for the determinant.

Limiting dilution analysis of the response of murine L3T4+ spleen cells to alloantigen

Cell sorter-purified small splenic L3T4+ cells from B6 mice were clonally expanded under limiting dilution (LD) conditions by coculture for 4-6 days with irradiated allogeneic stimulator cells in culture medium supplemented with various growth factor preparations. Proliferating L3T4+ cell clones were detected by [3H]thymidine uptake; interleukin 2 (IL-2) production of restimulated L3T4+ cell clones was measured in a sensitive colorimetric assay. IL-3 (but not IL-1 or IL-4) supported clonal expansion in vitro of many L3T4+ cell clones produced IL-2. The data were consistent with the hypothesis that only a single titrated precursor cell was limiting in the system. In the response to class II (bm12) H-2 alloantigen, 1 in 40-200 L3T4+ cells was induced to clonal growth; in the response to class I (bm1) H-2 alloantigen, a tenfold lower frequency (1 in 600-800) of inducible L3T4+ B6 cells was measured. A fraction of the generated L3T4+ cell clones showed IL-2-independent growth: anti-IL-2 receptor monoclonal antibodies (MoAb) (7D4 and PC61.5) blocked the proliferation of about 80% of the IL-2-producing L3T4+ cell clones, while about 20% of these clones seemed resistant to inhibition of proliferation by these MoAb. We have thus defined an LD system with high cloning efficiency for L3T4+ cells that does not depend on exogenous IL-2 supplements.

Limiting dilution analysis of interleukin-2-producing mature T cells. Interleukin 2 secretion is an exclusive property of L3T4+ lymphocytes

Evaluation of lymphokine production by individual activated T cells is necessary to characterize their growth requirements. We have studied interleukin 2 (IL-2) secretion by mature T lymphocytes using a high resolution assay system with the following characteristics: (a) a threshold of IL-2 detection 25 times lower than classic IL-2 titration; (b) the ability to discriminate between IL-2 and IL-4 activities; (c) absence of 'in situ' IL-2 absorption; (d) IL-2 production revealed at the single cell level. By this method an average of 10% of spleen cells, and 75% of L3T4+ cells were detected as producers in a concanavalin A (Con A)-dependent T cell activation system. Our results also suggest the complete restriction of IL-2 secretion to cells with this phenotype. Therefore, factors other than IL-2 must play a major role in Lyt 2+ mitogen-driven, helper-independent T cell proliferation.

Activation of abundant precursors associated with the response of Mlsa H-2k spleens to Mlsc antigen

We have determined the frequency of splenic interleukin 2-producing T-cell precursors that respond to H-2-identical, Mls-disparate stimulating cells. Gene products of the Mlsa and Mlsd alleles are highly stimulatory in primary mixed lymphocyte reactions and activate substantial numbers of T-cell precursors in normal spleens. Although the product of the Mlsc allele has been described as weakly stimulatory in primary mixed lymphocyte reactions, we detected abundant T-cell precursors responsive to Mlsc under our limiting dilution conditions; H-2k Mlsa T-cell precursors responding to Mlsc were as frequent (2.605 per 1000) as H-2k Mlsb or Mlsc T-cells responsive to Mlsd (1.975 per 1000). In contrast, using the frequency of self-responding cells as a threshold, we were unable to detect precursors for Mlsa, Mlsb, Mlsc, or Mlsd in H-2b mice, for Mlsb in H-2k mice, or for Mlsc in H-2k Mlsb or Mlsd mice. Thus, we conclude that in select strain combinations and under our experimental conditions Mlsc stimulators can activate T-cell precursors that are as abundant as precursors reactive to strong stimulators Mlsa and Mlsd.

Immunological discrimination between self and non-self by precursor depletion and memory accumulation

We study processes by which T-lymphocytes "learn" to discriminate "self" from "non-self". We show that intrinsic features of the T cell activation and proliferation process are sufficient to tolerize (self) reactive T-lymphocyte clones. Self vs non-self discrimination therefore develops without any down-regulatory (e.g. suppressive) interactions. T-lymphocyte clones will expand by proliferation only if the IL2 concentration is high enough to induce a proliferation rate larger than the rate of cell decay. This concentration is the proliferation threshold. Because effector T cells are short-lived the proliferation threshold must be quite high. Such high numbers of cells producing IL2 are achieved only when sufficient (memory) precursors are activated. Self and non-self antigens differ with respect the number of (memory) precursor cells they accumulate, as a result of two processes, i.e. precursor depletion and memory accumulation, and can thus be discriminated. Precursor depletion: the dynamics of long-lived precursors can cause tolerization. In neonatal circumstances precursor influx is still low, newborn cells reacting with self antigens are immediately activated, generating (few), i.e. fewer than the proliferation threshold, effectors that decay rapidly. Thus total lymphocyte numbers remain low, yielding self tolerance. Conversely, large doses of similar antigens introduced in mature systems push "their" lymphocyte clone over the proliferation threshold because a large (accumulated) precursor population is rapidly activated. Small doses are however low zone tolerized. Memory accumulation: peripheral T-lymphocyte populations in fact consist of a mixture of virgin precursors and memory cells. If the formation process of (long-lived) memory cells is taken into account and virgin precursors are made short-lived, the proliferation threshold again accounts for self non-self discrimination. Memory cells accumulate when antigenic restimulation is low; it is low when the antigen concentration and/or the antigen affinity is low. Therefore self antigens, which are present in relatively high concentrations, fail to accumulate high affinity memory cells, and are hence tolerated. Memory cells crossreacting to self antigens with low affinity, however accumulate neonatally, pushing those clones over the proliferation threshold whenever "their" high affinity antigen enters the immune system. Thus the model generates differences in the antigenicity (i.e. memory precursor frequency) of self and non-self.(ABSTRACT TRUNCATED AT 400 WORDS)

Neonatal tolerance induction in the thymus to MHC-class II-associated antigens. II. Significance of MHC antigens in anti-Mls tolerance

Specificity of anti-Mlsa tolerance induced in BALB/c (H-2d, Mlsb) neonates was investigated by a popliteal lymph node (PLN)-swelling assay for the local graft-versus-host (GVH) reaction by injecting tolerant thymus cells into the footpads of several types of F1 hybrid mice. When thymus cells were obtained from 1-week-old normal BALB/c, they evoked enlargement of PLNs of (BALB/c X DBA/2)F1 (H-2d, Mlsb/a) [CDF1] recipients and of other hybrid recipients, heterozygous in Mlsa,c,d alleles, irrespective of the major histocompatibility complex (MHC) haplotypes. The same thymus cells did not cause the response in MHC-heterozygous F1 hybrids when the hybrids were homozygous in Mlsb, identical with BALB/c mice. Therefore, the PLN response to Mls antigens, known to be closely associated with MHC-class II antigens, was not directed to the class II antigens themselves. This enabled us to examine the effects of MHC on tolerance induction to the Mls antigens. When BALB/c neonates were injected with CDF1 bone marrow cells, complete tolerance to Mlsa-H-2d antigens of CDF1 cells was induced in the thymus, while responsiveness to Mlsa antigens in the context of H-2k and H-2b antigens, was not affected. This indicates MHC-restriction of neonatal tolerance to Mls antigens. Furthermore, when Mls and H-2-heterozygous (BALB/c X AKR)F1 (H-2d/k, Mlsb/a) bone marrow cells served as the tolerogen, thymus cells of BALB/c neonates were also tolerized to Mlsa-H-2k antigens as well as to Mlsa-H-2d antigens, which suggests the involvement of MHC, probably class II antigens of tolerance-inducing cells.

Different characteristics of allogeneic and trinitrophenyl-modified H-2-restricted cell-mediated lympholysis: analysis of differences in interleukin-2 dependency using an ontogenical approach

Ontogenical development of in vitro allogeneic cell-mediated lympholysis (CML) and in vitro trinitrophenyl (TNP)-modified CML was studied mainly in correlation with helper cell activity, using C3H/HeN and C57BL/6 mice. Allogeneic and TNP-modified CML were not detected in the spleen of these mice at 1 week after birth. Allogeneic CML was detectable, in parallel with increases in age. This activity in 5-week-old mice was much the same as in the 8-week-old mice but the TNP-modified CML did not appear until 8 weeks after birth. Exogenous interleukin-2 (IL-2) induced sufficient activity of TNP-modified CML, even in spleen cells from 1-week-old mice, while the same treatment had a weak but significant effect on the induction of allogeneic CML in these same cells. Experiments on the mixed lymphocyte reaction (MLR) in the presence of exogenous IL-2 showed that lymphocyte proliferation in response to TNP-modified cells was higher than that in response to allogeneic cells. These results suggest different dependencies on IL-2 between allogeneic and TNP-modified killer precursor cells. Endogenous IL-2 production and proliferative response in MLR showed that helper cells contributing to the TNP-modified CML matured later, compared to allogeneic CML. Different sensitivities to IL-2 in two types of CML, in addition to different ontogenical developments, suggest that cytotoxic T lymphocytes to allogeneic cells differ quantitatively and qualitatively from TNP-modified H-2-restricted killer cells.

Interactions of small B lymphocytes with unprimed noncytolytic T cells: dissociation between "presentation" and growth induction

The accessory cell requirements in lectin-dependent triggering and growth of unprimed Lyt-2-T lymphocytes were analyzed by quantitatively comparing the ability of small B cells and peritoneal macrophages to either induce reactivity to growth factors or support growth. Lightly or nonirradiated small B cells were 15 to 30-fold less efficient as compared to T cell-depleted peritoneal cell populations, in the support of the lectin-stimulated Lyt-2-T cell proliferation. In contrast, lightly irradiated small B lymphocytes were quantitatively as efficient as macrophages in mediating lectin-driven Lyt-2-T cell proliferation, if relevant supernatants were added into culture. Finally, supernatants derived from cultures where T-small B cell ratios were optimal for growth of responder Lyt-2-lymphocytes were two orders of magnitude less efficient than conditioned medium obtained from cultures containing optimal T-macrophage ratios, in their ability to support growth of activated T cells. We conclude from these experiments that: in contrast to cytolytic T cell precursors, lectin-dependent induction of unprimed Lyt-2- T lymphocytes requires accessory cells; small B cells and macrophages are equally competent in this respect; and growth support by small B cell populations is due to contamination by macrophages which are the only cell type performing this function. We therefore interpret reports on Lyt-2- T cell proliferation upon stimulation with high numbers of small B cells as a two-step process: "presentation" and induction of T cells which is essentially B cell dependent, and factor production ensured by contaminating macrophages.

Postnatal development of functional T cell subsets in the mouse: a frequency analysis of mitogen reactive precursors of proliferating, of cytotoxic and of IL 2 producing T cells

In order to study the postnatal development of functional T cell subsets in the mouse, a mitogen-driven limiting dilution culture system was used for a precursor frequency analysis of proliferating, of cytolytic and of IL 2-producing T cells, respectively, present in spleen and thymus of mice from neonatal to adult age. In adult mice, the majority (up to 100%) of splenic T cells was capable to respond to Concanavalin A. In contrast, an up to tenfold lower frequency of mitogen-reactive precursors was found within positively selected Thy-1+ spleen cells of neonatal mice. Within this fraction of Con A reactive neonatal T cells, there was an apparent imbalance in the CTLp/PTLp ratio within the first to second week after birth. Accordingly, significant numbers of immunocompetent precursors of HTLp were detected in the spleen shortly after birth, while the vast majority of CTLp developed later on. This differential development of CTLp and PTLp was not seen with thymocytes of the same mice, where from the age of two to three days on up to the adult age the frequency of both CTLp and PTLp remained largely unchanged. Analysis of the Lyt-antigen expression, in addition, revealed phenotypical differences between neonatal and adult Thy-1+ spleen cells, such that Lyt-2 antigen density but not the proportion of Lyt-2 positive T cells, was considerably lower in newborn mice. An age related increase in both Lyt-2 antigen density and CTLp frequency was parallelled by the rapid increase in the total number of splenic T cells during the second and third postnatal week, reaching 60-70% of adult values. At this time, a normalisation of the CTLp/PTLp ratio at approximately 0.4 had occurred.

Acquired immunological tolerance of foreign cells is impaired by recombinant interleukin 2 or vitamin A acetate

The susceptibility of newborn mice to the inception of tolerance after exposure to antigen is associated with their deficiency in the production of endogenous interleukin 2 (IL-2). As further evidence of the complicity of IL-2 in the inception and maintenance of tolerance, it is shown here that a solid and long-lasting state of tolerance induced by the intravenous injection into newborn CBA mice of lymphoid cells from (CBA X C57BL/10ScSn)F1 hybrids can be brought to an end by the administration of exogenous IL-2 or by supplementing an otherwise normal diet with vitamin A acetate, the effect of which is to increase the proportion of the moiety of the T-cell population that produces IL-2. These results indicate that certain nonspecific stimuli can influence whether immunological tolerance is maintained.