Quick recovery in the generation of self-reactive CD4low natural killer (NK) T cells by an alternative intrathymic pathway when restored from acute thymic atrophy

The production of alpha/beta and gamma/delta double negative (DN) T-cells and their role in the maintenance of pregnancy

The ability of the thymus gland to convert bone marrow-derived progenitor cells into single positive (SP) T-cells is well known. In this review we present evidence that the thymus, in addition to producing SP T-cells, also has a pathway for the production of double negative (DN) T-cells. The existence of this pathway was noted during our examination of relevant literature to determine the cause of sex steroid-induced thymocyte loss. In conducting this search our objective was to answer the question of whether thymocyte loss is the end product of a typical interaction between the reproductive and immune systems, or evidence that the two systems are incompatible. We can now report that "thymocyte loss" is a normal process that occurs during the production of DN T-cells. The DN T-cell pathway is unique in that it is mediated by thymic mast cells, and becomes functional following puberty. Sex steroids initiate the development of the pathway by binding to an estrogen receptor alpha located in the outer membrane of the mast cells, causing their activation. This results in their uptake of extracellular calcium, and the production and subsequent release of histamine and serotonin. Lymphatic vessels, located in the subcapsular region of the thymus, respond to the two vasodilators by undergoing a substantial and preferential uptake of gamma/delta and alpha/beta DN T- cells. These T- cells exit the thymus via efferent lymphatic vessels and enter the lymphatic system.The DN pathway is responsible for the production of three subsets of gamma/delta DN T-cells and one subset of alpha/beta DN T-cells. In postpubertal animals approximately 35 % of total thymocytes exit the thymus as DN T-cells, regardless of sex. In pregnant females, their levels undergo a dramatic increase. Gamma/delta DN T-cells produce cytokines that are essential for the maintenance of pregnancy.

Immunologic states of autoimmune diseases

The etiology and immunologic states of autoimmune diseases have mainly been discussed without consideration of extrathymic T cells, which exist in the liver, intestine, and excretion glands. Because extrathymic T cells are autoreactive and are often simultaneously activated along with autoantibody-producing B-1 cells, these extrathymic T cells and B-1 cells should be introduced when considering the immunologic states of autoimmune diseases. The immunologic states of autoimmune diseases resemble those of aging, chronic GVH disease, and malarial infection. Namely, under all these conditions, conventional T and B cells are rather suppressed concomitant with thymic atrophy or involution. In contrast, extrathymic T cells and B-1 cells are inversely activated at this time. These facts suggest that the immunologic states of autoimmune diseases should be reevaluated by introducing the concept of extrathymic T cells and autoantibody-producing B-1 cells, which might be primordial lymphocytes in phylogeny.

Protection against malaria due to innate immunity enhanced by low-protein diet

Mice were fed ad libitum with a normal diet (25% protein) or low-protein diets (0-12.5% protein) for a wk and then infected with a nonlethal or lethal strain of Plasmodium yoelii, that is, blood stage infection. The same diet was continued until recovery. Mice fed with a normal diet showed severe parasitemia during nonlethal infection, but survived the infection. They died within 2 wk in the case of lethal infection. However, all mice fed with low-protein diets survived without apparent parasitemia (there were small peaks of parasitemia) in cases of both nonlethal and lethal strains. These surviving mice were found to have acquired potent innate immunity, showing the expansion of NK1.1 -TCRint cells and the production of autoantibodies during malarial infection. Severe combined immunodeficiency (scid) mice, which lack TCRint cells as well as TCRhigh cells, did not survive after malarial infection of lethal strain of P. yoelii, even when low-protein diets were given. These results suggest that low-protein diets enhanced innate immunity and inversely decreased conventional immunity, and that these immunological deviations rendered mice resistant against malaria. The present outcome also reminds us of our experience in the field study of malaria, in which some inhabitants eventually avoided contracting malaria even after apparent malarial infection.

Extrathymic T cells expand in nude mice following different allogeneic stimuli

We studied extrathymic lymphocyte populations expanded in nude mice after allogeneic stimuli. These were either cells from different tissues or Immunoglobulin (Ig). Although the cells transferred, obtained from Thy-1.1+ donors, were able to induce similar increase in the nude host Thy-1.2+ population, the expanded populations could be qualitatively distinguished from each other by their different expression of mature T cell molecules and by their functional profile. The extrathymic lymphocytes expanded in animals receiving allogeneic fetal thymocytes (FT) were preferentially CD4+ cells and could confer a functional immunocompetent system to the nude host, able to reject allogeneic skin grafts. In contrast, allogeneic adult red blood cells (RBC) led to the expansion of a CD8+ population and to an auto-reactive profile, resulting in the rejection of syngeneic skin grafts by most of the nude hosts. Neither of these profiles was achieved with the other stimuli. These findings support the view that different activation pathways and/or regulatory interactions may lead to the development of distinct extrathymic populations.

Generation of B220low B cells and production of autoantibodies in mice with experimental amyloidosis: association of primordial T cells with this phenomenon

To investigate the immunological state in amyloidosis, mice were twice intraperitoneally injected (2-week interval) with casein emulsified in complete Freund's adjuvant. Two weeks after the treatment, amyloid deposits were detected in the spleen and other organs of these mice. The number of lymphocytes yielded by the liver and spleen increased significantly. The most affected lymphocyte subset was found to be B cells, namely, the total number of B cells increased and unusual B220low B cells were newly generated in the liver and spleen. In other words, not only normal B220high B cells but also unusual B220low B cells were detected in these organs of mice with amyloidosis. In parallel with this phenomenon, autoantibodies against denatured DNA were detected in sera. Since such autoantibodies are known to accompany the functional activation of NKT cells, NKT cell-deficient mice were used for the induction of amyloidosis. Such mice showed less formation of amyloidosis and lower levels of autoantibodies in sera. Athymic nude mice were NKT cell-deficient but NK1.1- TCRint cells were present. These athymic mice showed an intermediate induction of amyloidosis. The cytokine profile seen in mice with amyloidosis was the Th0 type, showing simultaneous production of IL-4 and IFNgamma. These results suggest that the generation of B220low B cells and the production of autoantibodies in aid of primordial T cells may be major immunological mechanisms in amyloidosis mice.

Age-related bias in function of natural killer T cells and granulocytes after stress: reciprocal association of steroid hormones and sympathetic nerves

Stress-associated immune responses were compared between young (8 weeks of age) and old (56 weeks) mice. Since stress suppresses the conventional immune system (i.e. T and B cells) but inversely activates the primordial immune system (i.e. extrathymic T cells, NKT cells, and granulocytes), these parameters were analysed after restraint stress for 24 h. The thymus became atrophic as a function of age, and an age-related increase in the number of lymphocytes was seen in the liver. Although the number of lymphocytes in both the thymus and liver decreased as the result of stress, the magnitude was much more prominent in the thymus. To determine stress-resistant lymphocyte subsets, two-colour immunofluorescence tests were conducted in the liver and spleen. NKT cells were found to be such cells in the liver of young mice. On the other hand, an infiltration of granulocytes due to stress was more prominent in the liver of old mice than in young mice. Liver injury as a result of stress was prominent in young mice. This age-related bias in the function of NKT cells and granulocytes seemed to be associated with a difference in the responses of catecholamines (high in old mice) and corticosterone (high in young mice) after stress. Indeed, an injection of adrenaline mainly induced the infiltration of granulocytes while that of cortisol activated NKT cells. The present results suggest the existence of age-related bias in the function of NKT cells and granulocytes after stress and that such bias might be produced by different responses of sympathetic nerves and steroid hormones between young and old mice.

Syngeneic pregnancy induces overexpression of natural killer T cells in maternal liver

Conditions such as stress, infection, autoimmune disease, etc. elevate the number and function of extrathymic T cells that are generated mainly in the liver. As primitive, self-reactive clones of T cells that coexpress receptors of the natural killer (NK) lineage, they mediate cytotoxicity against altered self, malignant and infected cells and have the unique potential to rapidly secrete large amount of T helper 1 (Th1) or Th2 cytokines. To elucidate whether some of these changes occur even during the syngeneic pregnancy, we made phenotypic and functional characterization of mononuclear lymphatic cells (MNLCs) isolated from the liver and spleen of pregnant C57BL/6 mice, testing their cytotoxicity against syngeneic thymocytes as well as against NK- and lymphokine-activated killer (LAK)-sensitive targets. The data have shown that on the sixteenth day of syngeneic pregnancy TCRint, NK1.1+ and IL-2Rbeta+ cells were accumulated in the liver, while the quantities of CD4+ and CD8+ T cells and total number classical NK (NK1.1+CD3- or IL-2Rbeta+CD3-) cells were increased in the spleen. Pregnancy-activated hepatic and splenic MNLCs were more cytotoxic against syngeneic thymocytes, YAC-1 and P815 targets, suggesting that the maternal liver is a main producer of autoreactive NKT clones, which subsequently augment NK- and LAK cell-mediated cytotoxicity in the liver and spleen.

Role of NK1.1+ and AsGm-1+ cells in oral immunoregulation of experimental colitis

NK1.1 and AsGm-1 expressing cells play a role in immunomodulation. Our purpose was to determine the role of NK1.1+ and AsGm-1+ expressing cells in the inflammatory/tolerance paradigm in experimental colitis. Oral tolerance towards colitis-extracted proteins had previously been shown to alleviate experimental colitis. Colitis was induced in C57/B6 mice by intracolonic instillation of trinitrobenzenesulfonic acid (TNBS). Oral tolerance was induced via five oral doses of proteins extracted from TNBS-colitis colonic wall. Clinical, macroscopic, and microscopic scores were used for colitis assessment. To evaluate the putative role of AsGm-1 in tolerance induction, depletion of AsGm-1 expressing cells was performed. To evaluate the mechanism of tolerance induction, liver-associated NKT lymphocytes were harvested 14 days following tolerance induction, and cultured with concanavalin A (con A) and colitis-extracted proteins. T cell subsets were measured by flow cytometry. Cytokine expression was measured by intracellular staining and enzyme-linked immunosorbent assay (ELISA). Orally tolerized mice exhibited significant alleviation of the clinical, macroscopic, and microscopic parameters of colitis, with increased CD4+ILA+/CD4+IFNgamma+ lymphocyte ratio, increased IL-4, and decreased IFNgamma and IL-12 serum levels. In contrast, orally fed mice that were AsGm-1 depleted showed evidence of severe colitis. These mice exhibited significant decreased CD4 +IL4+/CD4+IFNgamma+ ratios, and an increase in IFNgamma and IL-12, with decreased IL-4 levels. NKT cells harvested from tolerized mice secreted high levels of antiinflammatory cytokines. In contrast, in nontolerized mice, NKT cells mainly secreted proinflammatory cytokines. In a tolerized environment, both NK1.1 and AsGm-1 expressing cells are essential for disease alleviation. In contrast, in a nontolerized environment, AsGm-1 expressing cells support an antiinflammatory immune paradigm, while NKT lymphocytes support a proinflammatory shift.

Unique sensitivity to alpha-galactosylceramide of NKT cells in the uterus

It was previously reported that NKT cells, which are mainly present in the liver of mice, are also present in the uterus and that these uterine NKT cells are associated with abortion under overactivated conditions. In this study, we further examined their phenotypic and functional properties. In parallel with the progression of pregnancy, the number of uterine lymphocytes increased. This increase accompanied an increase in the number of TCRalphabeta(+) T cells and NKT cells in the uterus, although the number of NKT cells decreased at late pregnancy. Approximately one-third of the TCRalphabeta(+) cells were NKT cells at the early pregnant stage, while this ratio decreased toward late pregnancy. These uterine NKT cells were found to respond to alpha-galactosylceramide (alpha-GalCer) differently in comparison with liver NKT cells. In contrast to the apoptotic response of liver NKT cells on day 1 after alpha-GalCer injection, uterine NKT cells expanded prominently without such apoptosis. The majority of liver NKT cells were CD4(+). In contrast, almost all of the uterine NKT cells were double negative CD4(-)8(-). However, similar to liver NKT cells, uterine NKT cells used an invariant chain of Valpha14Jalpha281 gene for TCRalpha. The resistance against apoptosis might be due to the high expression of bcl-2 on uterine NKT cells after alpha-GalCer activation. Other evidence was that macrophages which existed in the pregnant uterus carried an activation marker, CD69, and could potentially produce many cytokines by their activation. The present results suggest that uterine NKT cells and surrounding macrophages are quite different in function from those in the liver.

Abundance of unconventional CD8(+) natural killer T cells in the large intestine

Natural killer T (NKT) cells are mainly present in the liver and thymus, and the majority of these T cells express either a CD4(+) or a double-negative (DN) CD4(-)8(-) phenotype. In the present study, we examined whether such NKT cells were present in the intestine. NKT cells were rare in all sites of the small intestine, including an intraepithelial site. However, a considerable number of NKT cells were found at an intraepithelial site in the large intestine. This result was confirmed by both immunofluorescence and immunohistochemistry. In contrast to conventional NKT cells, NKT cells in the large intestine were CD8(+) or DN CD4(-)8(-). In the case of conventional NKT cells, their existence is known to depend on non-classical MHC class I-like antigens (i. e. CD1d) but not on classical MHC class I antigens. However, the NKT cells in the large intestine were independent of the presence of both CD1d and classical MHC class I antigens. These results were obtained using knockout mice lacking the corresponding genes and molecules. NKT cells in the large intestine were mainly alpha betaTCR(+) (> 75 %) but did not use an invariant chain of Valpha14Jalpha281, which is preferentially used by conventional NKT cells. These NKT cells did not bias the TCR-Vbeta usage toward Vbeta8. These findings suggest that the large intestine is a site in which unconventional NKT cells carrying the CD8(+) phenotype (or DN CD4(-)8(-)) are abundant and that these cells are independent of MHC and MHC-like antigens.

Extrathymic pathways of T-cell differentiation and immunomodulation

It is well established that the thymus is an essential organ for the support of T-cell differentiation. However, some T cells, termed extrathymic T cells, have been found to differentiate without such support by the thymus. The major sites of these T cells are the intestine and liver. Subsequent studies have revealed that extrathymic T cells are also present in the uterus and exocrine glands (e.g., the salivary gland). Depending on the sites, extrathymic T cells have some distinct properties as well as some common properties. For example, all extrathymic T cells have a TCR-CD3 complex similar to thymus-derived T cells. Extrathymic T cells comprise both alpha beta T cells and gamma delta T cells. Although extrathymic T cells are very few in number at any extrathymic sites in youth, they increase in number as a function of age. This phenomenon seems to occur in parallel with thymic involution. Even in youth, extrathymic T cells are activated in number and function by stress, in autoimmune diseases, and during pregnancy. Acute thymic atrophy always accompanies this activation. Therefore, reciprocal regulation between extrathymic T cells and thymus-derived T cells might be present. We hypothesize that extrathymic T cells are intimately associated with innate immunity and that the mechanisms underlying autoimmune diseases and intracellular infection (e.g., malaria) cannot be properly understood without introducing the concept of extrathymic T cells.