Recent thymic emigrants (CD4+) continuously migrate through lymphoid organs: within the tissue they alter surface molecule expression

Pharmacological Inhibition of MALT1 Protease Leads to a Progressive IPEX-Like Pathology

Genetic disruption or short-term pharmacological inhibition of MALT1 protease is effective in several preclinical models of autoimmunity and B cell malignancies. Despite these protective effects, the severe reduction in regulatory T cells (Tregs) and the associated IPEX-like pathology occurring upon congenital disruption of the MALT1 protease in mice has raised concerns about the long-term safety of MALT1 inhibition. Here we describe the results of a series of toxicology studies in rat and dog species using MLT-943, a novel potent and selective MALT1 protease inhibitor. While MLT-943 effectively prevented T cell-dependent B cell immune responses and reduced joint inflammation in the collagen-induced arthritis rat pharmacology model, in both preclinical species, pharmacological inhibition of MALT1 was associated with a rapid and dose-dependent reduction in Tregs and resulted in the progressive appearance of immune abnormalities and clinical signs of an IPEX-like pathology. At the 13-week time point, rats displayed severe intestinal inflammation associated with mast cell activation, high serum IgE levels, systemic T cell activation and mononuclear cell infiltration in multiple tissues. Importantly, using thymectomized rats we demonstrated that MALT1 protease inhibition affects peripheral Treg frequency independently of effects on thymic Treg output and development. Our data confirm the therapeutic potential of MALT1 protease inhibitors but highlight the safety risks and challenges to consider before potential application of such inhibitors into the clinic.

Elevated levels of interferon-γ production by memory T cells do not promote transplant tolerance resistance in aged recipients

Immunosenescence predisposes the elderly to infectious and autoimmune diseases and impairs the response to vaccination. We recently demonstrated that ageing also impedes development of transplantation tolerance. Unlike their young counterparts (8-12 weeks of age) aged male recipients (greater than 12 months of age) transplanted with a full MHC-mismatched heart are resistant to tolerance mediated by anti-CD45RB antibody. Surprisingly, either chemical or surgical castration restored tolerance induction to levels observed using young recipients. Based on the strong impact of endocrine modulation on transplant tolerance, we explored the impact of ageing and castration on the immune system. Here we report a significant increase in the percentage of T cells that produce interferon-γ (IFN-γ) in aged male versus young male animals and that the overall increase in IFN-γ production was due to an expansion of IFN-γ-producing memory T cells in aged animals. In contrast to IFN-γ production, we did not observe differences in IL-10 expression in young versus old male mice. We hypothesized that endocrine modulation would diminish the elevated levels of IFN-γ production in aged recipients, however, we observed no significant reduction in the percentage of IFN-γ+ T cells upon castration. Furthermore, we neutralized interferon-γ by antibody and did not observe an effect on graft survival. We conclude that while elevated levels of interferon-γ serves as a marker of tolerance resistance in aged mice, other as yet to be identified factors are responsible for its cause. Defining these factors may be relevant to design of tolerogenic strategies for aged recipients.

Child hospitalization due to severe malaria is associated with the ICAM-1Kilifi allele but not adherence patterns of Plasmodium falciparum infected red blood cells to ICAM-1

This study aimed at determining whether the predisposition of a mutation at position 179 of the ICAM-1 gene to child hospitalization due to malaria was mediated by changes in adherence properties of IRBCs to ICAM-1. ICAM-1 genotypes were determined by nested polymerase chain reaction of isolated DNA from filter blood spots followed by Restriction Fragment Length Polymorphism (RFLP). Plasmodium falciparum adherence assays were done on immobilized purified ICAM-1. Our data indicate that the homozygosity for the ICAM-1(Kilifi) mutation occurs at a frequency of 22.3% in Magugu-Babati, Northern Tanzania. Our results show that there are no differences in IRBC binding profiles across genotypes. We show in this study that homozygosity for the ICAM-1(Kilifi) is associated with child hospitalization (X(2)=14.47, p<0.001). We have further shown that hospitalization was not associated with cytoadherence (X(2)=0.17, p=0.68). We conclude that the ICAM-1(Kilifi) allele occurs at a high frequency in Tanzania and that associations of this allele with higher child hospitalization frequencies is independent of cytoadherence patterns of IRBC isolated from ICAM-1 genotypes, implying that any associations reported to exist between the ICAM-1(Kilifi) mutation and severe malaria are unlikely to be mediated through altered IRBC cytoadherence properties.

Blood Lymphocytes, Monocytes and NK Cells Modulate Their Expression of CD44, ICAM‐1, LFA‐1 and MHC Class II After Arrival Into Lymphoid Organs

Adhesion molecules expressed on surface membranes of lymphocytes and other leukocytes enable their entry into the lymphoid and other tissues. However, little is known about molecules that govern the transit of leukocytes through the parenchyma of lymphoid organs proper. We show that in comparison to blood leukocytes, the corresponding cells isolated from lymphoid organs, i.e., lymph nodes and spleen, have a significantly augmented expression of certain surface molecules. The helper and cytotoxic subsets of T cells, as well as B cells, display the increased expression of CD44, ICAM-1 and LFA-1, whereas B cells additionally show the augmented expression of MHC class II. In comparison with blood monocytes, splenic monocytes show the increased expression of ICAM-1 and MHC class II molecules. When compared with blood NK cells, splenic NK cells only show the increased expression of ICAM-1. The molecules, which we show to be up regulated upon the entry of leukocytes into lymphoid organs, could be involved in their retention within the tissue via cell-cell or cell-extracellular matrix interactions and in control of their transit through lymphoid tissues.

Role of ovarian hormones in age-associated thymic involution revisited

A commonly held view that ovarian hormones are causally involved in age-associated thymic involution has been recently challenged. In particular, their relevance in the progression of thymic involution has been disputed. To reassess this issue 10-month-old rats with well advanced thymic involutive changes were ovariectomized (Ovx), and after 1 month thymic cellularity, thymocyte development and levels of recent thymic emigrants (RTEs) were examined in peripheral blood and spleen. In addition, the distribution of major conventional and regulatory T-cell subsets was analyzed in the same peripheral lymphocyte compartments. Ovariectomy increased thymic weight and cellularity above the levels in both 10-month-old and age-matched controls indicating that ovarian hormone ablation not only prevented further progression of thymic involution, but also reversed it. The increased thymic cellularity was accompanied by altered thymocyte differentiation/maturation culminating in increased thymic output of naïve T cells as indicated by elevated levels of both CD4+ and CD8+ RTEs in peripheral blood and spleen. The changes in T-cell development produced: (i) a disproportional increase in cellularity across thymocyte subsets, so that relative proportions of cells at all maturational stages preceding the CD4+CD8+ T cell receptor (TCR)alphabeta(low) stage were reduced; the relative numbers of CD4+CD8+ TCRalphabeta(low) cells entering positive selection and their immediate CD4+CD8+ TCRalphabeta(high) descendents were increased, while those of the most mature CD4+CD8- and CD4-CD8+ TCRalphabeta(high) cells remained unaltered; (ii) enhanced cell proliferation across all thymocyte subsets and (iii) reduced apoptosis of cells within the CD4+CD8+ thymocyte subset. The augmented thymic output of naïve T cells in Ovx rats most likely reflected an early disinhibition of thymocyte development followed by increased positive/reduced negative selection, at least partly, due to raised thymocyte surface Thy-1 expression. The greater number of CD4+CD25+Foxp3+ cells in both thymus and peripheral blood suggested augmented thymic production of these cells. In addition, an increased CD4+/CD8+ cell ratio was found in the spleen of Ovx rats. Thus, ovarian hormone ablation led not only to increased diversity of the T-cell repertoire, but also to a new balance among distinct T-cell subsets in the periphery.

Murine neonatal recent thymic emigrants are phenotypically and functionally distinct from adult recent thymic emigrants

In contrast to adults, the murine neonatal CD4+ compartment contains a high frequency of recent thymic emigrants (RTEs). However, the functional capabilities of these cells in neonates are relatively unknown. Moreover, it has not been determined whether RTEs from neonates and adults are comparable. Here we have directly compared neonatal and adult CD4+ RTEs for the first time, using a transgenic mouse strain that allows for the identification and purification of RTEs. Our data demonstrate that RTEs from murine neonates and adults are phenotypically and functionally distinct. In particular, although the magnitude of RTEs cytokine responses from both age groups is dependent on the conditions of activation, neonatal RTEs always exhibited higher levels of effector Th1/Th2 cytokine production than adult RTEs. In addition, neonatal, but not adult, RTEs showed early proliferation in response to stimulation with interleukin-7 alone. This was associated with faster kinetics of interleukin-7Ralpha down-regulation and higher levels of pSTAT5 in neonatal RTEs. These quantitative and qualitative differences in the neonatal and adult RTEs populations may at least partially explain the diverse responses that are elicited in vivo in neonates in response to different conditions of antigen exposure.

CD26/dipeptidyl peptidase 4-deficiency alters thymic emigration patterns and leukcocyte subsets in F344-rats age-dependently

As CD26 (dipeptidyl peptidase 4/DPP4) rapidly truncates incretins N-terminally, including glucagon-like peptide-1, DPP4-inhibitors have been developed for treatment of diabetes type 2. To some extent this is surprising, as CD26/DPP4 is also deeply involved in immune regulation. Long-term pharmacological studies are hampered by off-target inhibition of DPP4-homologues. Therefore, we studied the effects of genetic CD26/DPP4-deficiency by investigating blood, spleen and thymus leucocyte subpopulations of wild-type and CD26-deficient F344-rats at different ages. In young animals at 1 and 3 months of age, there were no differences in leucocyte subsets, while in older animals the T cell composition was changed significantly. From the age of 6 months onwards, reduced numbers of recent thymic emigrants and memory T cells, and consequently an increased amount of naive T cells were observed in CD26-deficient rats. In addition, the architecture of the thymus was altered, as observed by a reduced density of lymphocytes in the medulla. Furthermore, the number of proliferating cells in the thymus was decreased in CD26-deficient rats at a higher age. Moreover, CD26-deficiency resulted in markedly reduced numbers of B cells in later life. Additionally, an age- but not CD26-dependent increase of regulatory T cells and a decrease of natural killer cell numbers were detected in the blood and spleen. Our findings indicate an important role of CD26 in maintaining lymphocyte composition, memory T cell generation and thymic emigration patterns during immunosenescence, with possible implications for using DPP4-inhibitors.

The role of sex steroids and gonadectomy in the control of thymic involution

A major underlying cause for aging of the immune system is the structural and functional atrophy of the thymus, and associated decline in T cell genesis. This loss of naïve T cells reduces adaptive immunity to new stimuli and precipitates a peripheral bias to memory cells against prior antigens. Whilst multiple mechanisms may contribute to this process, the temporal alliance of thymic decline with puberty has implicated a causative role for sex steroids. Accordingly ablation of sex steroids induces profound thymic rejuvenation. Although the thymus retains some, albeit highly limited, function in healthy adults, this is insufficient for resurrecting the T cell pool following cytoablative treatments such as chemo- and radiation-therapy and AIDS. Increased risk of opportunistic infections and cancer relapse or appearance, are a direct consequence. Temporary sex steroid ablation may thus provide a clinically effective means to regenerate the thymus and immune system in immunodeficiency states.

Environmental and intrinsic factors lead to antigen unresponsiveness in CD4(+) recent thymic emigrants from aged mice

Naive CD4 cells from aged mice respond inefficiently to Ag, but the factors that underlie the age-associated defects remain unclear. We have used two approaches to isolate recent thymic emigrants (RTE) in young and aged mice and have compared their capacity to respond to antigenic stimulation ex vivo. An in situ intrathymic CFSE injection labeled developing thymocytes and allowed the identification of RTE in secondary lymphoid tissues. Analysis of CFSE-labeled RTE and control unlabeled naive CD4 cells indicated that cells from aged mice were defective in their ability to increase intracellular Ca(2+) concentration following TCR cross-linking. Aged naive and RTE CD4 also secreted less IL-2 and proliferated less than that of comparable young CD4 populations. Defects in effector generation in aged RTE were overcome by the addition of IL-2 to cultures. RTE from both polyclonal and TCR transgenic mice were compromised, indicating that defects were independent of TCR specificity. In the second model, the cotransfer of congenic marker-labeled young and aged BM cells into young and aged syngeneic hosts revealed that hyporesponsiveness in aged RTE was caused by a combination of defects intrinsic to CD4 progenitors and defects induced by the aged environment. Depletion of peripheral CD4 cells in aged mice led to production of new RTE that were not defective. The results of this study suggest that defects induced by environmental and lineage intrinsic factors act together to reduce responses to Ag in aged naive CD4 cells and that these defects can be overcome in aged CD4 cells produced during recovery from lymphopenia.

Selective gene transfer to T lymphocytes using coreceptor-specific [MLV(HIV)] pseudotype vectors in a transgenic mouse model

The coreceptor usage of HIV-1 envelope proteins (Env) is mainly dependent on a defined variable region within the V3-loop of Env. Thus, retroviral vectors derived from murine leukemia virus (MLV), which have been pseudotyped with HIV-1 envelope proteins holding different V3-loops, enable selective gene delivery into either CXCR4 or CCR5 positive cultured cells. Here, we tested the distribution of CD4/CCR5-tropic [MLV(HIV)]-pseudotype vectors in transgenic mice expressing CD4 and either CXCR4 or CCR5 of human origin. The specificity of gene transfer was analyzed by ex vivo transduction of spleen cells as well as after i.v. or i.p. injection of transgenic mice. Expression of the transferred marker gene EGFP and vector sequences could be detected exclusively in lymphocytes expressing (hu)CD4 and (hu)CCR5, whereas MLV vectors pseudotyped with the VSV-G envelope glycoprotein mediated gene transfer in mice of all genotypes investigated. These data demonstrated that cell-specific gene delivery via [MLV(HIV)]-pseudotyped vectors, as previously shown for cultured cells, is also achievable in vivo.

T-cell transcriptome analysis points up a thymic disorder in idiopathic nephrotic syndrome

BACKGROUND

Idiopathic nephrotic syndrome is a proteinuric disease secondary to the release of a nonidentified circulating glomerular permeability factor by T cells. Because specificities of T-cell activation in idiopathic nephrotic syndrome remain unknown, we evaluated transcriptional activation of T cells in nephrotic patients during proteinuria.

METHODS

Transcriptomes of CD2+ cells were analyzed by serial analysis of gene expression (SAGE) in a nephrotic child during proteinuria relapse and after remission, away from any immunosuppressive treatment. Expression of specific transcripts overexpressed during proteinuria relapse was compared by reverse transcription-polymerase chain reaction (RT-PCR) in CD2+ cells from 11 nephrotic patients during relapse and remission and 11 non nephrotic patients during infection and after recovery.

RESULTS

Differential analysis of CD2+ cell transcriptome identified >200 mRNA tags overexpressed during proteinuria relapse, including many T-cell markers. RT-PCR analysis of expression of specific transcripts indicated that (1) under remission conditions, nephrotic children displayed induction of four transcripts, including IKBKB, and repression of NFKBIA as compared to non nephrotic children after recovery, and (2) proteinuria relapse was associated with induction of L-selectin and T-lymphocyte maturation-associated protein, two markers of T-cell differentiation and recent emigrant/naive T cells.

CONCLUSION

Results indicate that circulating T cells from relapsing nephrotic patients include a significant population of low-mature cells while those from nephrotic patients in remission are characterized by constitutive activation of nuclear factor-kappaB (NF-kappaB), altogether suggesting a thymic dysregulation of apoptosis in nephrotic patients.

Multiparameter evaluation of human thymic function: interpretations and caveats

After the provision of highly active antiretroviral therapy (HAART), the level of circulating CD4+ T cells increases in many adults infected with the human immunodeficiency virus, type 1 (HIV). To study factors involved in immune reconstitution, we have measured thymic abundance by CT scans, circulating naive-phenotype CD4+ T cells by flow cytometry, and T cell receptor (TCR) rearrangement excision circles (TRECs) by quantitative PCR in 40 virologically suppressed, HIV-infected adults and 33 age-matched, HIV-uninfected controls. In HIV-uninfected subjects, naive T cell numbers, thymic abundance, and the frequency of circulating naive CD4+ T cells bearing TRECs decreased with age, as expected. When corrected for this relationship with age, naive T cell numbers correlated significantly with naive T cell TREC frequencies. Virologically suppressed HIV-infected subjects had higher TREC frequencies, and subjects over the age of 39 were more likely to have abundant thymus compared to age-matched, HIV-uninfected adults. Nevertheless, all HIV-infected subjects had reduced absolute numbers of naive T cells, irrespective of thymic size, age, or TREC frequencies. These data illustrate the complex relationship between these measures of thymic size and function and underscore the need to develop more definitive measures of thymic function in the future.

Analyzing the migration of labeled T cells in vivo: an essential approach with challenging features

T cells are involved in the pathogenesis of many diseases. To exert a pathological effect, T cells enter the tissues. We show that the determination of their entry site requires isolation of the respective T cell population, injection into genetically un-manipulated animals, and identification of the cells in vivo at various time points after injection. We indicate variables influencing in vivo migration experiments artificially, and outline how resulting problems can be either avoided or taken into account. Reviewing experiments performed according to the outlined criteria reveals two types of migration patterns for T cell subsets in vivo: 1). Naïve and memory T cells enter lymphoid and non-lymphoid organs in comparable numbers, but selectively accumulate in lymphoid tissues over time, 2). Effector T cells, too, enter lymphoid and non-lymphoid organs in comparable numbers. However, most of them die within 24 hours. Depending on the presence of cytokines, chemokines and extracellular matrix compounds they are able to survive, thereby preferentially accumulating in their target tissues. This information might help to understand the role of migration in the pathogenesis of T cell mediated diseases.

Lymphocyte function-associated antigen-1 and intercellular adhesion molecule-1 expression on B-cell subsets and the effects of splenectomy-experimental studies

There is an abundance of data dealing with recirculation of T cells in the rats, but relatively little is known about the traffic of B cells. The adhesion molecules expressed on the surface membrane are of great significance for recirculation of lymphocytes. However, very little is known about the expression of various adhesion molecules on B-cell subsets. Here we show that in normal rats various adhesion molecules are differentially expressed on B-cell subsets and that the level of their expression changes after the entry of B lymphocytes from the blood into the lymphoid tissues. In splenectomized rats, the surface expression of LFA-1 and ICAM-1 is selectively reduced on B-cell subsets in blood and lymph node, which is accompanied by a selective increase in the number of all B-cell subsets in the blood. The decreased surface expression of adhesion molecules results in faster migration of B lymphocytes through lymph nodes with subsequent accumulation of these cells in the blood.

Dynamics of T cells and TCR excision circles differ after treatment of acute and chronic HIV infection

We quantified T cell proliferation and thymic function in primary HIV infection (PHI; n = 19) and chronic HIV infection (CHI; n = 14) by measuring Ki67 staining and TCR excision circle (TREC) number. After antiretroviral therapy of PHI there is a profound decrease in the number and percentage of Ki67(+) T cells (<6% Ki67(+)) with no significant increase in TREC per million cells and a transient increase in TREC per milliliter. In contrast, after antiretroviral therapy of CHI there is a reduction in the percentage but little change in the total number of Ki67(+)CD4(+) T cells associated with increases in both TREC per million cells and TREC per milliliter. Using a mathematical model that accounts for proliferation, death, and redistribution of T cells, we find that redistribution is consistent with the TREC changes observed during treatment of PHI and that an increase in thymic output is needed to explain the increase in TREC during treatment of CHI. Consideration of TREC per milliliter shows that changes in proliferation alone cannot explain the changes in TREC. In addition, although increased proliferation of memory cells in HIV infection has been established, we find no difference in TREC per million CD45RA(-) "memory" T cells between healthy and infected individuals (p = 0.154 for CD4(+); p = 0.383 for CD8(+)). Finally, although the number of TREC per million cells is always much lower in memory T cells than in naive T cells, in the setting of HIV infection, given that memory cells make up a larger proportion of total T cells, we find that 50% of TREC per milliliter in CD4(+) T cells is harbored in the CD45RA(-) "memory" subset of our infected subjects.

Reevaluation of T cell receptor excision circles as a measure of human recent thymic emigrants

The human thymus exports newly generated T cells to the periphery. As no markers have been identified for these recent thymic emigrants (RTE), it is presently impossible to measure human thymic output. T cell receptor excision circles (TREC) have been recently used to assess thymic output during both health and disease. Using a mathematical model, we quantify age-dependent changes both in the number of RTE generated per day and in TREC concentration during an 80-year lifespan. Through analyses, we demonstrate that RTE and peripheral T cell division have the same potential to affect TREC concentration at any age in healthy people. T cell death also influences TREC concentration, but to a lesser extent. During aging, our results indicate that thymic involution primarily induces an age-dependent decline in TREC concentrations within both CD4(+) and CD8(+) T cell populations. We further apply this model for studying TREC concentration during HIV-1 infection. Our analyses reveal that a decrease in thymic output is the major contributor to the decline in TREC concentration within CD4(+) T cells, whereas both increased peripheral T cell division and decreased thymic output induce the decline in TREC concentration within CD8(+) T cells. Therefore, we suggest that T cell turnover should be examined together with TREC concentration as a measure of RTE. If peripheral T cell division remains relatively unchanged, then TREC concentration indeed reflects thymic output.

Splenectomy of rats selectively reduces lymphocyte function-associated antigen 1 and intercellular adhesion molecule 1 expression on B-cell subsets in blood and lymph nodes

Splenectomy increases the number of B cells in the blood of humans and animals. It is unknown whether this is due to changes in migration, proliferation, or both. The numbers of naïve (IgD(+)IgM(+)), memory (IgD(-)IgM(high)), newly formed (IgM(high)CD90(high)), early recirculating follicular (IgM(low)CD90(high)), recirculating follicular (IgM(low)CD90(-)), and marginal zone (IgM(high)CD90(-)) phenotype B cells were determined in control and splenectomized rats by flow cytometry. All subsets increased significantly in the blood after splenectomy. Because surface molecules are involved in the regulation of migration and proliferation, their expression (lymphocyte function-associated antigen 1 [LFA-1], intercellular adhesion molecule 1 (ICAM-1), L-selectin, alpha4-integrins, CD44, major histocompatability complex class II, interleukin 2 receptor-alpha chain) was determined on B- and T-cell subsets of both groups. B cells, but not T cells, showed a significantly reduced LFA-1 and ICAM-1 expression in blood and lymph nodes, whereas the expression of the other surface molecules analyzed remained unchanged. The down-regulation of these molecules did not influence the adherence of B cells to high endothelial venules in vitro. In vivo, however, ICAM-1(low)-expressing B cells migrated significantly faster through lymph nodes (ICAM-1(low) 41 +/- 5 hours versus ICAM-1(high) 58 +/- 3 hours), whereas proliferation of B cells in bone marrow, lymph node, and blood remained unchanged. Thus, the presence of one organ is necessary for appropriate expression of LFA-1 and ICAM-1 on B cells in other, distant organs. The more rapid transit of ICAM-1(low) B cells through lymph nodes may be responsible for the increased B-cell number in the blood after splenectomy.

Age-related changes in CCR9+ circulating lymphocytes: are CCR9+ naive T cells recent thymic emigrants?

The chemokine receptor CCR9 is reported to be predominantly expressed by thymocytes as well as by circulating gut-homing and resident T cells in the small intestinal mucosa. Its ligand thymus-expressed chemokine (TECK) is produced by thymic and small intestinal epithelium. Here we report that the proportion of circulating CCR9+ naive T cells (mostly CD4+) declines with age, from approximately 15% of all T cells at birth to around 1% in adults. The proportion of CCR9+ T cells lacking the classical gut-homing receptor alpha4beta7, was much higher in children than in adults. Therefore, circulating CD3+CCR9+CD45RA+ cells have most likely left the thymus quite recently. This notion was supported by the small number of CCR9+ naive T cells which was present shortly after thymectomy. Establishing a phenotypic marker for recent thymic emigrants might provide a powerful tool in the clinical assessment and follow-up after cancer chemotherapy, hematopoietic stem cell transplantation, and during antiretroviral treatment of human immunodeficiency virus (HIV)-infected patients.