Mutual antagonism of TGF-beta and Interleukin-2 in cell survival and lineage commitment of induced regulatory T cells.

Transforming growth factor beta (TGF-ß)- and Interleukin-2 (IL-2)-mediated signaling enables the generation and expansion of induced regulatory T (iTreg) cells that carry high hopes for the treatment of chronic inflammatory and autoimmune diseases. Knowledge about factors stabilizing their lineage commitment and lifespan, however, is limited. Here, we investigated the behavior of iTreg cells, derived from apoptosis-defective mouse mutants, during activated cell autonomous cell death, triggered by cytokine-deprivation, or activation-induced cell death (AICD) after restimulation of the T-cell receptor, and compared these responses with those of effector T cells. We observed that iTreg cells were much more sensitive to IL-2-deprivation but poorly susceptible to AICD. In fact, when apoptosis was compromised, T-cell receptor (TCR)-religation resulted in methylation-independent, ERK- and PI3K/mTOR-mediated loss of Foxp3 expression, impaired suppressive capacity and effector cytokine production. Although iTreg cells prevented colitis induction they rapidly lost Foxp3-GFP expression and gained ability to produce effector cytokines thereby imposing Th1 cell fate on resident effector cells. Surprisingly, iTreg cell conversion itself was limited by TGF-ß-mediated Bim/Bcl2L11-dependent apoptosis. Hence, the very same cytokine that drives the generation of iTreg cells can trigger their demise. Our results provide novel insights in iTreg cell biology that will assist optimization of iTreg-based therapy.

CD4(+)CD8(+)TCR(low) thymocytes express low levels of glucocorticoid receptors while being sensitive to glucocorticoid-induced apoptosis.

While signaling by either the TCR or glucocorticoid receptor (GR) can induce apoptosis in thymocytes, recent studies have shown that combining these signals results in survival of CD4(+)CD8(+) thymocytes. Although glucocorticoids (GC) in this way may directly affect T cell selection, no data are available addressing GR expression in thymocyte subsets and in individual cells within subsets. We studied GR expression by combining immunofluorescence cell surface staining for CD4, CD8 and TCR with intracellular staining of GR in four-color cytometry. Significant differences of GR expression were observed in various thymocyte subsets, although a homogeneous distribution of GR expression in individual thymocyte subsets emerged. The highest GR expression was found in CD4(-)CD8(-)TCR(-) thymocytes, and decreased during development via the CD4(-)CD8(+)TCR(-) subpopulation into the CD4(+)CD8(+)TCR(low) subset. Interestingly, the latter population, although expressing less than half the GR density of CD4(-)CD8(-)TCR(-) cells, is the most sensitive subset to GC-induced apoptosis. Up-regulation of TCR expression by the CD4(+)CD8(+)TCR(low) subset to CD4(+)CD8(+)TCR(high) cells was accompanied by a parallel increase in GR expression. The latter finding and the presence of a homogeneous distribution of GR in each thymocyte subset provides an experimental basis for the concept that GR can antagonize TCR-mediated signals at a constant rate relative to TCR expression.

The cell cycle inhibitor p16(INK4A) sensitizes lymphoblastic leukemia cells to apoptosis by physiologic glucocorticoid levels.

The cyclin-dependent kinase inhibitor p16(INK4A) is frequently inactivated in childhood T-cell acute lymphoblastic leukemia. To investigate possible consequences of this genetic alteration for tumor development, we conditionally expressed p16(INK4A) in the T-cell acute lymphoblastic leukemia line CCRF-CEM, which carries a homozygous deletion of this gene. In agreement with its reported function, p16(INK4A) expression was associated with hypophosphorylation of the retinoblastoma protein pRB and stable cell cycle arrest in G(0)/G(1), documenting that the pRB/E2F pathway is functional in these cells. Unexpectedly, p16(INK4A) expression increased the sensitivity threshold for glucocorticoid (GC)-induced apoptosis from therapeutic to physiologic levels. As a possible explanation for this phenomenon, we found that p16(INK4A)-arrested cells had elevated GC receptor expression associated with enhanced GC-mediated transcriptional activity and increased responsiveness of the GC-regulated cyclin D3 gene. These data are supported by our previous findings that GC receptor levels critically influence GC sensitivity and imply that p16(INK4A) inactivation, in addition to allowing unrestricted proliferation, represents a mechanism by which lymphoid tumor cells might escape cell death triggered by endogenous GC.

Glucocorticoid production in the chicken bursa and thymus.

Glucocorticoid (GC) hormones play an important role in thymic T cell selection and in the development of autoimmune diseases. Previous studies have shown that the mammalian thymus itself is able to produce GC. In order to assess the importance of these findings in terms of the evolutionary development of the immune system, we investigated the functional presence of steroidogenic enzymes in primary lymphoid organs of chickens, which represent one of the best studied non-mammalian species. To this end, we attempted to demonstrate enzyme activities of the whole set of steroidogenic enzymes for the synthesis of GC in the bursa of Fabricius and the thymus. We isolated steroidogenic organelles from primary lymphoid tissues, incubated these with radioactive (precursor) steroids in vitro and visualized the resulting products by thin-layer chromatography. Our results show that the chicken bursa as well as the chicken thymus possesses all enzymes and cofactors required for GC production. The observation of GC production in an organ responsible for B cell selection and maturation is a further step in uncovering the yet ill-defined mechanism of B cell selection. These results provide the biochemical basis for the in situ hormonal effects, and underline the general importance of GC hormones on T and B lymphocyte development and selection.

The cell cycle inhibitor p16(INK4A) sensitizes lymphoblastic leukemia cells to apoptosis by physiologic glucocorticoid levels.

The cyclin-dependent kinase inhibitor p16(INK4A) is frequently inactivated in childhood T-cell acute lymphoblastic leukemia. To investigate possible consequences of this genetic alteration for tumor development, we conditionally expressed p16(INK4A) in the T-cell acute lymphoblastic leukemia line CCRF-CEM, which carries a homozygous deletion of this gene. In agreement with its reported function, p16(INK4A) expression was associated with hypophosphorylation of the retinoblastoma protein pRB and stable cell cycle arrest in G(0)/G(1), documenting that the pRB/E2F pathway is functional in these cells. Unexpectedly, p16(INK4A) expression increased the sensitivity threshold for glucocorticoid (GC)-induced apoptosis from therapeutic to physiologic levels. As a possible explanation for this phenomenon, we found that p16(INK4A)-arrested cells had elevated GC receptor expression associated with enhanced GC-mediated transcriptional activity and increased responsiveness of the GC-regulated cyclin D3 gene. These data are supported by our previous findings that GC receptor levels critically influence GC sensitivity and imply that p16(INK4A) inactivation, in addition to allowing unrestricted proliferation, represents a mechanism by which lymphoid tumor cells might escape cell death triggered by endogenous GC.

Bidirectional effects of corticosterone on splenic T-cell activation: critical role of cell density and culture time.

Glucocorticoids inhibit stimulus-induced T-cell proliferation, an early and essential parameter of cellular immunity. It was recently found however that physiological concentrations of glucocorticoids can also accelerate, not only inhibit, rat T-cell mitogenesis. We investigated mechanism(s) underlying mitogenic actions of glucocorticoids on anti-T-cell receptor (TCR)- and concanavalin A (Con A)-induced T-cell proliferation. Surprisingly, the ability of the glucocorticoid corticosterone (CORT) to either enhance or inhibit T-cell proliferation was found to depend primarily on the cell density and the timing of the cultures. At cell densities up to 1 x 10(5) cells/well (i.e. 'low' density), CORT inhibited T-cell proliferation irrespective of the culture time. In contrast, at cell densities of 2 x 10(5) cells/well and higher ('high' density), CORT potently stimulated T-cell mitogenesis during the first 2-3 culture days, but subsequently inhibited the proliferative response after 5-7 days. The glucocorticoid receptor antagonist RU486 completely abolished the effects of CORT. However, production of the main T cell growth factor interleukin (IL)-2 was inhibited by CORT at both 'low' and 'high' cell densities. In addition, irrespective of cell density, T-cell mitogenesis under either control conditions or in presence of CORT was completely blocked by an anti-IL-2-receptor-alpha-chain (IL-2Ralpha) antibody, indicating that T-cell proliferation was dependent on the IL-2 pathway. Immunofluorescence staining of IL-2Ralpha on CD4+ cells after 2-3 days in culture was increased by CORT, but only on cells cultured at 'high' density. Thus, glucocorticoids increase T-cell responsiveness to IL-2 under conditions of 'high' cell density only. We conclude that glucocorticoids may contribute to a more efficient early stage of cellular immune responses under conditions of intimate cell-to-cell contact (i.e. 'high' cell density), a situation likely to be present in vivo, for instance in lymph nodes. Thus, these findings are relevant to our understanding of the glucocorticoid control of immune function.

Altered circadian rhythms of the stress hormone and melatonin response in lupus-prone MRL/MP-fas(Ipr) mice.

The immune system interacts with the hypothalamo-pituitary-adrenal axis via so-called glucocorticoid increasing factors, which are produced by the immune system during immune reactions, causing an elevation of systemic glucocorticoid levels that contribute to preservation of the immune reactions specificities. Previous results from our laboratory had already shown an altered immuno-neuroendocrine dialogue via the hypothalamo-pituitary-adrenal axis in autoimmune disease-prone chicken and mouse strains. In the present study, we further investigated the altered glucocorticoid response via the hypothalamo-pituitary-adrenal axis in murine lupus. We established the circadian rhythms of corticosterone, dehydroepiandrosterone-sulfate, adrenocorticotropic hormone and melatonin, as well as the time response curves after injection of interleukin-1 of the first three parameters in normal SWISS and lupus-prone MRL/MP-fas(Ipr) mice. The results show that lupus-prone MRL/ MP-fas(Ipr) mice do not react appropriately to changes of the light/dark cycle, circadian melatonin rhythms seem to uncouple from the light/dark cycle, and plasma corticosterone levels are elevated during the resting phase. Diurnal changes of dehydroepiandrosterone-sulfate and adrenocorticotropic hormone were normal compared to healthy controls. These data indicate that MRL/ MP-fas(Ipr) mice not only show an altered glucocorticoid response mediated via the hypothalamo pituitary adrenal axis to IL-1, but are also affected by disturbances of corticosterone and melatonin circadian rhythms. Our findings may have implications for intrathymic T cell development and the emergence of autoimmune disease.

Glucocorticoids regulate TCR-induced elevation of CD4: functional implications.

CD4 serves as a coreceptor during Ag recognition by the TCR. This interaction results in a marked increase in the sensitivity of a T cell to Ag presented by MHC class II molecules. Here we report that activation of T cells either by plate-bound mAb (anti-TCR, anti-CD3) or soluble activators (staphylococcal enterotoxin A, Con A) is associated with an (up to 3-fold) increase in CD4 cell surface expression on CD25+ cells, which was maximal after 72-96 h. Incubation with the glucocorticoid hormone corticosterone (CORT) shifted the enhancement of CD4 expression to a point about 24 h earlier than that observed in control cultures. In parallel, the proliferative response of these CORT-treated cells was profoundly enhanced. An involvement of increased CD4 expression in this enhanced proliferative response was evidenced by the observation that T cell proliferation in CORT-treated cultures was much less sensitive to inhibition by an inhibitory, nondepleting anti-CD4 mAb than that in control cultures. TCR down-regulation was, however, not affected by CORT. Thus, based on this study and previous reports we propose that both TCR-mediated signals and glucocorticoids are important physiological regulators of CD4 expression. In addition, these findings may be of significance for the sensitivity of CD4+ cells to HIV infection upon T cell activation, as the efficacy of primary patient HIV entry depends on the level of surface CD4.

Glucocorticoid production in the murine thymus.

Glucocorticoid hormones are known to act as important modulatory factors in the development of autoimmune diseases, and to play an important role in thymic T-cell selection. There seems to be a finely balanced equilibrium between the apoptosis-inducing effects of glucocorticoid and T cell receptor ligand binding. Here we are investigating whether glucocorticoid-induced T cell apoptosis is mainly dependent on circulating glucocorticoid levels or if the thymus itself is able to produce glucocorticoids. To this end, we attempted to demonstrate enzyme activities of the whole set of steroidogenic enzymes for the synthesis of glucocorticoids in murine thymic tissue. We isolated steroidogenic organelles from thymic tissue, incubated these with radioactive (precursor) steroids in vitro, and visualized the resulting products by thin-layer chromatography. Our results show that the thymus possesses all enzymes and cofactors required for glucocorticoid production. However, an intact thymic architecture is necessary for glucocorticoid production, since 11beta-hydroxylase was not detected in irradiated thymi or in a thymic epithelial cell line. The results of these experiments show that the whole glucocorticoid metabolism takes place within the thymus. This finding provides the biochemical basis for the in situ effects of glucocorticoid hormones on thymocyte development and selection.

Induction of cytokine receptors by glucocorticoids: functional and pathological significance.

Current concepts on the role of glucocorticoid hormones in the regulation of inflammatory and immune responses depict this role as being inhibitory. Over the past decade, however, a large variety of studies have shown that glucocorticoids also exert stimulatory effects on immune function, suggesting that the present concept of the role of glucocorticoids in the immune system in not sufficient and needs to be extended. Here, Jan Wiegers and Hans Reul ask how these apparently paradoxical effects fit together and what their functional and pathological significance might be.

Altered neuroimmunoendocrine communication during a condition of chronically increased brain corticotropin-releasing hormone drive.

Presently, it is clear that the brain, immune system, and endocrine system build a complex network of interactions at various levels. Inflammation, which may be regarded as a stressful challenge, initiates apart from immunological, autonomic, and neuroendocrine responses also profound behavioral (e.g., immobility, social disinterest) changes. Key mediators herein are corticotropin-releasing hormone (CRH) and cytokines, such as interleukin-1 beta (IL-1 beta). Currently, the behavioral changes, collectively termed sickness behavior, are thought to be adaptive responses to support the body's efforts to fight the infection. Using in vivo microdialysis and biotelemetry in freely moving animals, we have studied the monoaminergic circuits in the brain implicated in the regulation of physiological and behavioral responses to a peripheral inflammatory challenge (see also chapter of Linthorst and Reul in this volume). To expand our insight into the relationship between hypersecretion of CRH and physiological and behavioral abnormalities associated with stress-related disorders, a series of experiments was conducted with long-term centrally CRH-infused rats. These rats showed reduced body weight gain, decreased food intake, elevated plasma ACTH and corticosterone levels, thymus involution and immunosuppression, but, paradoxically, enhanced IL-1 beta mRNA expression in spleen macrophages. After a peripheral endotoxic challenge on the seventh day of treatment, the CRH-infused rats produced aberrant (i.e., blunted and/or delayed) HPA axis, fever, behavioral, and hippocampal serotonergic responses. However, endotoxin-induced plasma IL-1 and IL-6 bioactivities were significantly enhanced in these animals. The data show that chronically elevated central CRH levels as occurring during chronic stress result in defective central nervous system and immune system responses to an acute (inflammatory) challenge. These observations provide evidence that chronic CRH hypersecretion is an important factor in the etiology of stress-related disorders.

Glucocorticoids accelerate anti-T cell receptor-induced T cell growth.

To study steroid regulation of cell-mediated immunity, we used anti-TCR-stimulated rat splenic lymphocyte mitogenesis as our experimental paradigm. Surprisingly, we found that the principal glucocorticoid of the rat, corticosterone (CORT), potently enhanced anti-TCR-induced lymphocyte proliferation after 2 to 3 days in culture, followed by inhibited cell growth after 5 to 7 days. Thus, glucocorticoids appeared to accelerate anti-TCR-induced lymphocyte mitogenesis. This effect occurred at physiologic concentrations (50-1000 nM), which are known to be released in vivo after an immune challenge. Kinetic experiments showed that CORT had to be present within 60 min after the initiation of TCR activation to produce maximal enhancing effects; a delay of 2 h or more left CORT ineffective. The lymphocytes incubated with CORT may have an increased sensitivity to IL-2 because 1) CORT suppressed IL-2 production throughout the culture period, and 2) an anti-IL-2R mAb completely blocked both control and CORT-treated anti-TCR-induced lymphocyte proliferation. Although the IL-2R alpha- and beta-chain mRNA concentrations were not altered in CORT-treated splenocyte cultures, we observed by FACS analysis an increased expression of the IL-2R alpha-chain on CORT-treated TCR alpha beta + and CD4+ T cells after 48 to 72 h of culture, suggesting an increased sensitivity of these T cells to IL-2 during the phase of enhanced proliferation. These results demonstrate a clear distinction between the enhancing effects of glucocorticoids on anti-TCR-induced lymphocyte proliferation and their well known inhibitory actions. Thus, the present study expands the regulatory role of glucocorticoids in cellular immunity, adding a novel effective stimulatory component to their inhibitory properties.

Long-term intracerebroventricular corticotropin-releasing hormone administration induces distinct changes in rat splenocyte activation and cytokine expression.

The effects of long-term corticotropin-releasing hormone (CRH) infusion in the lateral ventricle of the rat on hypothalamic-pituitary-adrenocortical (HPA) axis parameters and on the immune system function were studied. Compared with infusion of vehicle, the CRH treatment produced a sustained overactivity of the HPA axis, as evidenced by elevated plasma ACTH and corticosterone levels, increased anterior pituitary POMC messenger RNA (mRNA) expression, and adrenal enlargement. Long-term CRH treatment also inhibited body weight gain and reduced thymus and spleen weight. In the CRH-treated animals, both Concanavalin A (Con A)-induced T lymphocyte proliferation and lipopolysaccharide (LPS)-induced B lymphocyte mitogenesis was largely suppressed. Surprisingly, interleukin-2 (IL-2) levels were higher in supernatants of splenocyte cultures from CRH-treated rats than in those of control animals. However, IL-2 receptor alpha chain (IL-2R alpha) mRNA expression after Con A stimulation was highly suppressed in the CRH-treated animals. In addition, Northern blot analysis of RNA from splenocytes isolated from spleens of CRH-treated rats revealed a marked expression of IL-1 beta mRNA, in contrast to the barely detectable levels of this cytokine in control animals. Moreover, incubation of total splenocytes and spleen macrophages with LPS resulted in an enhanced induction of IL-1 beta mRNA in cells of CRH-treated rats compared with that of control animals. When adrenalectomized rats were treated with CRH or vehicle, the effects of the CRH treatment on T and B cell proliferation, IL-2 production, and IL-1 beta mRNA expression were abolished. Thus, a continuously increased HPA axis drive results in disparate changes in immune system function. Whether the observed changes in cytokine expression should be regarded as physiologically adaptive adjustments in support of immune function or as potentially pathological anomalies remains to be elucidated.

Enhancement of rat splenic lymphocyte mitogenesis after short term preexposure to corticosteroids in vitro.

Numerous previous studies investigating the effects of corticosteroids on immune proliferation reported almost unanimously inhibitory effects. However, in many of these studies, high concentrations of corticosteroids (micromolar range) and long incubation times (days) were used. We have investigated whether corticosteroid hormones at low physiological (nanomolar) vs. high (micromolar) concentrations have distinct effects on immune cells after short term (minutes) as opposed to long term preexposure (hours). When rat splenocytes were preincubated with high concentrations (0.1-1 microM) of corticosterone (CORT) or with the specific glucocorticoid agonist RU 28362 (0.1 microM) for 1-6 h, washed, and then exposed to the T-cell mitogen Concanavalin-A, a time- and dose-dependent inhibition of lymphocyte mitogenesis over the next 3 days of culture was found. Preincubation with the glucocorticoid antagonist RU 486 completely blocked this inhibitory effect of CORT and RU 28362. Preexposure of splenocytes to CORT for 10-60 min did not alter mitogenesis. No differences were observed between intact and adrenalectomized (ADX) rats. However, if splenocytes from ADX rats were used in the presence of the glucocorticoid antagonist RU 486, the proliferative response over a 3-day period was stimulated by short term preexposure (10-30 min) to low concentrations (3-30 nM) of CORT. Under these incubation conditions, the mineralocorticoid aldosterone, in the presence of RU 486; also produced a stimulatory effect on ADX splenic lymphocyte mitogenesis, whereas RU 28362 was not effective. Corticosteroid receptor binding studies revealed the presence of mineralocorticoid (MR) as well as glucocorticoid (GR) receptors (45 and 600 fmol/mg protein, respectively) in the spleen. In conclusion, low physiological concentrations of CORT and aldosterone have novel stimulatory properties on mitogenesis of splenic lymphocytes from ADX rats. These effects become evident in the presence of GR antagonist and persist after short corticosteroid preexposure times. In contrast, after prolonged preincubation with high concentrations of corticosteroids, the well known immunosuppressive action is observed. We postulate that distinct MR- and GR-mediated effects may underlie these differential steroid actions on splenic lymphocyte proliferation.

Prenatal immune challenge alters the hypothalamic-pituitary-adrenocortical axis in adult rats.

We investigated whether non-abortive maternal infections would compromise fetal brain development and alter hypothalamic-pituitary-adrenocortical (HPA) axis functioning when adult. To study putative teratogenic effects of a T cell-mediated immune response versus an endotoxic challenge, 10-d-pregnant rats received a single intraperitoneal injection of 5 x 10(8) human red blood cells (HRBC) or gram-negative bacterial endotoxin (Escherichia coli LPS: 30 micrograms/kg). The adult male progeny (3 mo old) of both experimental groups showed increased basal plasma corticosterone levels. In addition, after novelty stress the HRBC group, but not the LPS group, showed increased ACTH and corticosterone levels. Both groups showed substantial decreases in mineralocorticoid (MR) and glucocorticoid receptor (GR) levels in the hippocampus, a limbic brain structure critical for HPA axis regulation, whereas GR concentrations in the hypothalamus were unchanged and in anterior pituitary were slightly increased. HRBC and LPS indeed stimulated the maternal immune system as revealed by specific anti-HRBC antibody production and enhanced IL-1 beta mRNA expression in splenocytes, respectively. This study demonstrates that a T cell-mediated immune response as well as an endotoxic challenge during pregnancy can induce anomalies in HPA axis function in adulthood. Clinically, it may be postulated that disturbed fetal brain development due to prenatal immune challenge increases the vulnerability to develop mental illness involving inadequate responses to stress.

Differential effects of corticosteroids on rat peripheral blood T-lymphocyte mitogenesis in vivo and in vitro.

The effects of corticosteroids were studied on the concanavalin A (Con A)-induced mitogenesis of peripheral blood T-lymphocytes obtained from intact and adrenalectomized (ADX) Wistar rats. One week of adrenalectomy reduced the proliferative response of T-cells by 65% compared with sham-operated controls. Substitution of ADX rats with subcutaneously implanted 12.5-mg corticosterone (Cort) pellets, which resulted in low circulating Cort levels (17 +/- 3 nM), restored the reduced proliferative capacity to that of sham-ADX animals. In contrast, T-lymphocyte proliferation was nearly absent in ADX rats substituted with high circulating Cort levels (173 +/- 15 nM; 100-mg Cort pellet). In vitro, Cort suppressed the mitogenic response of T-lymphocytes from ADX and sham-ADX animals. The glucocorticoid antagonist RU-486 (500 nM) completely blocked this suppressive effect. However, a 10 times lower concentration of RU-486 reversed the effects of a low (10 nM) Cort concentration from suppression to stimulation. It is concluded that high Cort concentrations in vivo and in vitro suppressed T-lymphocyte mitogenesis but that low concentrations in vivo were stimulatory, whereas this stimulation in vitro occurred only in the presence of antiglucocorticoids. These opposing effects of Cort emphasize a bimodal regulatory role of this hormone in immune regulation that may be mediated by different corticosteroid receptors.

Two specific T cell factors that initiate immune responses in murine allograft systems. A comparison of biologic functions.

It has been suggested that Ag-specific T cell factors play a role in the early phase of cellular immune responses. Two of these factors are studied in this paper. The first factor is specific macrophage arming factor (SMAF), that binds to (arms) macrophages and renders them specifically cytotoxic against tumor cells. The second factor is involved in the induction of an early (2 h) mast cell-dependent hypersensitivity reaction, that precedes the delayed-type hypersensitivity response (mast cell arming T cell factor; MTCF). In this study we compare both factors in an allogeneic murine tumor system (C57BL (H-2b) mice sensitized against SL2 (H-2d) lymphoma cells), both factors were: 1) dependent on T lymphocytes for their production, 2) detectable in serum 2 to 3 days after immunization, and 3) MHC (H-2)-Ag specific. Immunochemical studies showed that both factors have a molecular mass between 45 and 90 kDa and bind to the mAb 14-30 (directed against specific T cell factors), but not to anti-kappa/lambda L chain antibodies. Furthermore, it was shown that SMAF produced in vitro could induce a mast cell-dependent early 2-h hypersensitivity reaction against SL2 tumor cells, and resembled in this way MTCF. We concluded that the biologic activities and immunochemical characteristics of SMAF and MTCF are similar. Both factors are produced during the early stages of the immune response and seem to play a role in the initiation of the cell-mediated immune response.