The regulation of the immune system

Fundamental questions about the processes of immune recognition are now being asked with the powerful techniques of molecular biology and biochemistry. The extent to which these disciplines have converged with cellular immunology was particularly striking at a recent meeting on the regulation of the immune response.

Mouse endogenous superantigens: Ms and Mls-like determinants encoded by mouse retroviruses

Commonly used inbred mouse strains express different combinations of integrated mouse mammary tumor proviruses (MMTV). This appendix summarizes the proviruses that have been detected. The reported functional properties of those MMTV proviral products which have been identified as superantigens are also summarized, including the ability to elicit primary or secondary T cell responses and to induce Vb-specific clonal deletion during T cell differentiation. In addition, the amino acid sequences of putative ORF gene products of different MMTV are compared.

Cutting edge: telomerase activation in human T lymphocytes does not require increase in telomerase reverse transcriptase (hTERT) protein but is associated with hTERT phosphorylation and nuclear translocation

Capacity for cellular replication is critically important for lymphocyte function and can be regulated by telomerase-dependent maintenance of telomere length. In contrast to most normal human somatic cells that do not express telomerase due to the failure to transcribe telomerase reverse transcriptase (hTERT), lymphocytes express telomerase in a highly regulated fashion yet constitutively transcribe hTERT during development and activation. Here, we report that hTERT protein is present in both thymocytes and blood T cells at equivalent levels despite their substantial differences in telomerase activity, and that induction of telomerase activity in resting CD4(+) T cells is not dependent on net hTERT protein increase. Moreover, hTERT is phosphorylated and translocated from cytoplasm to nucleus during CD4(+) T cell activation. Thus, human T lymphocytes regulate telomerase function through novel events independent of hTERT protein levels, and hTERT phosphorylation and nuclear translocation may play a role in regulation of telomerase function in lymphocytes.

Lineage-specific telomere shortening and unaltered capacity for telomerase expression in human T and B lymphocytes with age

Age effects on telomere length and telomerase expression in peripheral blood lymphocytes were analyzed from 121 normal individuals age newborn to 94 years and revealed several new findings. 1) Telomere shortening was observed in CD4+ and CD8+ T and B cells with age. However, the rate of telomere loss was significantly different in these populations, 35 +/- 8, 26 +/- 7, and 19 +/- 7 bp/year for CD4+ and CD8+ T and B cells, respectively. In addition, CD4+ T cells had the longest average telomeres at all ages, followed by B cells, with CD8+ T cell telomeres the shortest, suggesting that these lymphocyte populations may have different replicative histories in vivo. 2) Telomerase activity in freshly isolated T and B cells was indistinguishably low to undetectable at all ages but was markedly increased after Ag and costimulatory receptors mediated stimulation in vitro. Furthermore, age did not alter the magnitude of telomerase activity induced after stimulation of T or B lymphocytes through Ag and costimulatory receptors or in response to PMA plus ionomycin treatment. 3) The levels of telomerase activity induced by in vitro stimulation varied among individual donors but were highly correlated with the outcome of telomere length change in CD4+ T cells after Ag receptor-mediated activation. Together, these results indicate that rates of age-associated loss of telomere length in vivo in peripheral blood lymphocytes is specific to T and B cell subsets and that age does not significantly alter the capacity for telomerase induction in lymphocytes.

The role of telomerase expression and telomere length maintenance in human and mouse

The molecular regulation of telomere length has been well elucidated by a series of elegant studies over the past decade. More recently, experimental evidence has accrued that addresses the challenging question of if and how telomere length regulation may contribute to normal human aging or to human disease. Recent studies in mice have provided a mammalian precedent indicating that telomerase deficiency can lead to in vivo dysfunction, most probably as a consequence of progressive telomere shortening. In humans, the evidence that telomere shortening might lead to in vivo dysfunction is far less direct, although the recent description of telomerase deficiency and telomere shortening associated with the DKC syndrome is suggestive of such a link. Methodologies exist and continue to be developed that are increasingly capable of manipulating telomerase activity and telomere length in human cells. It remains to be determined whether scientifically rigorous and (equally important) medically ethical approaches will emerge to directly assess the ability of telomere length modulation to correct functional disorders of human cellular function ex vivo or more challenging still, in vivo.

Recombinase-activating gene (RAG) 2-mediated V(D)J recombination is not essential for tumorigenesis in Atm-deficient mice

The majority of Atm-deficient mice die of malignant thymic lymphoma by 4-5 mo of age. Cytogenetic abnormalities in these tumors are consistently identified within the Tcr alpha/delta locus, suggesting that tumorigenesis is secondary to aberrant responses to double-stranded DNA breaks that occur during V(D)J recombination. Since V(D)J recombination is a recombinase-activating gene (RAG)-dependent process, we generated Rag2(-/-)Atm(-/-) mice to assess the requirement for RAG-dependent recombination in thymic lymphomagenesis. In contrast to expectation, the data presented here indicate that development of malignant thymic lymphoma in Atm(-/-) mice is not prevented by loss of RAG-2 and thus is not dependent on V(D)J recombination. Malignant thymic lymphomas in Rag2(-/-)Atm(-/-) mice occurred at a lower frequency and with a longer latency as compared with Atm(-/-) mice. Importantly, cytogenetic analysis of these tumors indicated that multiple chromosomal abnormalities occurred in each tumor, but that none of these involved the Tcr alpha/delta locus. Nonmalignant peripheral T cells from TCR-transgenic Rag2(-/-)Atm(-/-) mice also revealed a substantial increase in translocation frequency, suggesting that these translocations are early events in the process of tumorigenesis. These data are consistent with the hypothesis that the major mechanism of tumorigenesis in Atm(-/-) mice is via chromosomal translocations and other abnormalities that are secondary to aberrant responses to double-stranded DNA breaks. Furthermore, these data suggest that V(D)J recombination is a critical, but not essential, event during which Atm-deficient thymocytes are susceptible to developing chromosome aberrations that predispose to malignant transformation.

The role of B7 costimulation in CD4/CD8 T cell homeostasis

The effect of B7-mediated costimulation on T cell homeostasis was examined in studies of B7-1 (CD80) and B7-2 (CD86) transgenic as well as B7-deficient mice. B7 overexpression in transgenic mice resulted in marked polyclonal peripheral T cell hyperplasia accompanied by skewing toward an increased proportion of CD8 single-positive cells and a decreased proportion of CD4 single-positive cells in thymus and more markedly in peripheral T cells. B7-induced T cell expansion was dependent on both CD28 and TCR expression. Transgenic overexpression of B7-1 or B7-2 resulted in down-regulation of cell surface CD28 on thymocytes and peripheral T cells through a mechanism mediated by intercellular interaction. Mice deficient in B7-1 and B7-2 exhibited changes that were the reciprocal of those observed in B7-overexpressing transgenics: a marked increase in the CD4/CD8 ratio in peripheral T cells and an increase in cell surface CD28 in thymus and peripheral T cells. These reciprocal effects of genetically engineered increase or decrease in B7 expression indicate that B7 costimulation plays a physiological role in the regulation of CD4+ and CD8+ T cell homeostasis.

Constitutive and regulated expression of telomerase reverse transcriptase (hTERT) in human lymphocytes

Human telomerase consists of two essential components, telomerase RNA template (hTER) and telomerase reverse transcriptase (hTERT), and functions to synthesize telomere repeats that serve to protect the integrity of chromosomes and to prolong the replicative life span of cells. Telomerase activity is expressed selectively in germ-line and malignant tumor cells but not in most normal human somatic cells. As a notable exception, telomerase is expressed in human lymphocytes during development, differentiation, and activation. Recent studies have suggested that regulation of telomerase is determined by transcription of hTERT but not hTER. The highly regulated expression of telomerase in lymphocytes provides an opportunity to analyze the contribution of transcriptional regulation of hTERT and hTER. We report here an analysis of hTERT expression by Northern and in situ hybridization. It was found that hTERT mRNA is expressed at detectable levels in all subsets of human lymphocytes isolated from thymus, tonsil, and peripheral blood, regardless of the status of telomerase activity. hTERT expression is regulated as a function of lineage development, differentiation, and activation. Strikingly, however, telomerase activity in these cells is not correlated strictly with the levels of hTERT and hTER transcripts. The absence of correlation between telomerase activity and hTERT mRNA could not be attributed to the presence of hTERT splice variants or to detectable inhibitors of telomerase activity. Thus, transcriptional regulation of hTERT is not sufficient to account for telomerase activity in human lymphocytes, indicating a likely role of posttranscriptional factors in the control of enzyme function.

Telomere length regulation in mice is linked to a novel chromosome locus

Little is known about the mechanisms that regulate species-specific telomere length, particularly in mammalian species. The genetic regulation of telomere length was therefore investigated by using two inter-fertile species of mice, which differ in their telomere length. Mus musculus (telomere length >25 kb) and Mus spretus (telomere length 5-15 kb) were used to generate F1 crosses and reciprocal backcrosses, which were then analyzed for regulation of telomere length. This analysis indicated that a dominant and trans-acting mechanism exists capable of extensive elongation of telomeres in somatic cells after fusion of parental germline cells with discrepant telomere lengths. A genome wide screen of interspecific crosses, using M. spretus as the recurrent parent, identified a 5-centimorgan region on distal chromosome 2 that predominantly controls the observed species-specific telomere length regulation. This locus is distinct from candidate genes encoding known telomere-binding proteins or telomerase components. These results demonstrate that an unidentified gene(s) mapped to distal chromosome 2 regulates telomere length in the mouse.

The role of B7-CD28 co-stimulation in tumor rejection

The role of B7 co-stimulatory signaling in in vivo tumor rejection remains incompletely characterized. In particular, the relative competence of B7-1 (CD80) and B7-2 (CD86) to provide effective co-stimulus is not well defined, and the identification of the T cell co-stimulatory receptor that mediates B7 co-stimulation in tumor rejection has not been addressed. These issues were studied by assessing rejection of B7-negative or B7-transfected tumor cells in CD28-expressing or CD28-deficient hosts. B7-negative EL4 tumor cells grew progressively in normal syngeneic C57BUL6 (B6) mice. In contrast EL4 cells transfected with either full length B7-1 or full length B7-2 were rejected, indicating that both B7-1 and B7-2 are competent to mediate rejection of EL4 tumor cells. Expression of truncated B7-1 or B7-2 products, with complete deletion of cytoplasmic domains, was as effective as expression of full length B7-1 or B7-2 in mediating rejection. In contrast to the rejection of B7-transfected EL4 cells observed in CD28-expressing syngeneic hosts, B7-1- and B7-2-positive EL4 cells as well as control EL4 cells grew progressively in CD28-deficient mice, demonstrating the requirement for host expression of CD28 in B7-mediated tumor rejection. These results indicate that interaction of host CD28 with co-stimulatory extracellular B7-1 or B7-2 ligands expressed on tumor cells can play a necessary role in mediating tumor rejection.

Productive infection of neonatal CD8+ T lymphocytes by HIV-1

CD8+ T lymphocytes confer significant but ultimately insufficient protection against HIV infection. Here we report that activated neonatal CD8+ T cells can be productively infected in vitro by macrophage-tropic (M-tropic) HIV-1 isolates, which are responsible for disease transmission, whereas they are resistant to T cell-tropic (T-tropic) HIV strains. Physiological activation of CD8-alpha/beta+ CD4- T cell receptor-alpha/beta+ neonatal T cells, including activation by allogeneic dendritic cells, induces the accumulation of CD4 messenger RNA and the expression of CD4 Ag on the cell surface. The large majority of anti-CD3/B7.1-activated cord blood CD8+ T cells coexpress CD4, the primary HIV receptor, as well as CCR5 and CXCR4, the coreceptors used by M- and T-tropic HIV-1 strains, respectively, to enter target cells. These findings are relevant to the rapid progression of neonatal HIV infection. Infection of primary HIV-specific CD8+ T cells may compromise their survival and thus significantly contribute to the failure of the immune system to control the infection. Furthermore, these results indicate a previously unsuspected level of plasticity in the neonatal immune system in the regulation of CD4 expression by costimulation.

Tales of tails: regulation of telomere length and telomerase activity during lymphocyte development, differentiation, activation, and aging

Telomerase activity and the regulation of telomere length are factors which have been implicated in the control of cellular replication. These variables have been examined during human lymphocyte development, differentiation, activation, and aging. It was found that telomere length of peripheral blood CD4+ T cells decreases with age as well as with differentiation from naive to memory cells in vivo, and decreases with cell division in vitro. These results provide evidence that telomere length correlates with lymphocyte replicative history and residual replicative potential. In contrast, telomere length appears to increase during tonsil B-cell differentiation and germinal center (GC) formation in vivo. It was also found that telomerase activity is highly regulated during T-cell development and B-cell differentiation in vivo, with high levels of telomerase activity expressed in thymocytes and GC B cells, and low levels of telomerase activity in resting mature peripheral blood lymphocytes. Finally, resting lymphocytes retain the ability to upregulate telomerase activity upon activation, and this capacity does not appear to decline with age. Although the precise role of telomerase in lymphocyte function remains to be elucidated, telomerase may contribute to protection from telomere shortening in T and B lymphocytes, and may thus play a critical role in lymphocyte development, differentiation and activation. The future study of telomerase and its regulation of telomere length may enhance our understanding of how the replicative lifespan is regulated in lymphocytes.

Telomere lengthening and telomerase activation during human B cell differentiation

The function of the immune system is highly dependent on cellular differentiation and clonal expansion of antigen-specific lymphocytes. However, little is known about mechanisms that may have evolved to protect replicative potential in actively dividing lymphocytes during immune differentiation and response. Here we report an analysis of telomere length and telomerase expression, factors implicated in the regulation of cellular replicative lifespan, in human B cell subsets. In contrast to previous observations, in which telomere shortening and concomitant loss of replicative potential occur in the process of somatic cell differentiation and cell division, it was found that germinal center (GC) B cells, a compartment characterized by extensive clonal expansion and selection, had significantly longer telomeric restriction fragments than those of precursor naive B cells. Furthermore, it was found that telomerase, a telomere-synthesizing enzyme, is expressed at high levels in GC B cells (at least 128-fold higher than those of naive and memory B cells), correlating with the long telomeres in this subset of B cells. Finally, both naive and memory B cells were capable of up-regulating telomerase activity in vitro in response to activation signals through the B cell antigen receptor in the presence of CD40 engagement and/or interleukin 4. These observations suggest that a novel process of telomere lengthening, possibly mediated by telomerase, functions in actively dividing GC B lymphocytes and may play a critical role in humoral immune response by maintaining the replicative potential of GC and descendant memory B cells.

What does cell death have to do with aging?

When Lockshin and Zakeri discussed the relevance of apoptosis to aging 7 years ago, the common view was that apoptosis would have primarily a negative impact on aging by destroying essential and often irreplaceable cells. That view has now changed to one that acknowledges that there are two general ways in which apoptosis can play a role in aging: (1) elimination of damaged and presumably dysfunctional cells (e.g., fibroblasts, hepatocytes), which can then be replaced by cell proliferation, thereby maintaining homeostasis, and (2) elimination of essential post-mitotic cells (e.g., neurons, cardiac myocytes), which cannot be replaced, thereby leading to pathology. Evidence exists in two systems (fibroblasts and thymocytes/lymphocytes) that there are age-related decreases in the potential for apoptosis, although the molecular bases for the decreases in these two systems appear to differ. Upon becoming senescent, fibroblasts lose the ability to down-regulate expression of the bcl-2 gene in response to an apoptotic signal; thus, apoptosis is blocked even though an initiating signal has been received. In contrast, thymocytes/lymphocytes lack the ability to initiate the signal because of down-regulation of the cell surface receptor Fas. There is limited information available for other tissue types, and nothing is known about why and how age-related changes occur. An interesting observation is that the frequency of up-regulation of the bcl-2 gene as a result of chromosome translocation in otherwise normal B cells increases with age; the functional consequences of this phenomenon during aging are not known. The role of apoptosis in regulating cell number is also a promising area of research. The studies on liver damage and neoplastic lesions suggest an extremely important role for apoptosis in controlling cancer. This may be particularly important in the prostate where hypertrophy and/or cancer are a virtual certainty with ever-increasing age. It is not known whether the ability to undergo apoptosis declines in the prostate with increasing age, but it appears possible that it may, thus explaining the loss of control over cell number in this tissue. A particularly important area of research is whether apoptosis plays a role in the changing balance between bone formation and resorption observed during osteoporosis. Monica Driscoll has already pointed out that, "regulation and execution of cell death is an absolutely critical process that interfaces with nearly every aspect of life. Future investigation of the links of cell death to cellular aging and the aging of organisms should be an exciting enterprise." The results currently available do suggest that apoptosis is a process that may be important in aging, at least in some tissues, and the mechanism of its regulation, in particular, needs to be understood. Several tumor suppressor gene and oncogene products are involved in signal transduction associated with apoptosis, but it remains to be shown which of these, if any, are actually involved in "on-off" switches for apoptosis. Where great progress has been made is in understanding the events occurring after binding of either Fas ligand or tumor necrosis factor to their respective receptors. However, one area about which little is known is the identity of the signals that initiate this process in response to intracellular damage. Through continuing research on cell death mechanisms, funded by the NIA, we hope to provide answers to such fundamental questions as: 1. Are there age-related changes in apoptosis, and what role, if any, do these have in the aging process? 2. If age-related changes in apoptosis do occur, what molecular mechanisms are altered to produce these changes? 3. Can approaches be developed to improve the detection and elimination of damaged cells in vivo in tissues where cell replacement is possible? 4. Can death of damaged cells be attenuated or delayed in nonrenewable tissues, and, if so, is it advantageous to the org

Telomere length, telomerase activity, and replicative potential in HIV infection: analysis of CD4+ and CD8+ T cells from HIV-discordant monozygotic twins

To address the possible role of replicative senescence in human immunodeficiency virus (HIV) infection, telomere length, telomerase activity, and in vitro replicative capacity were assessed in peripheral blood T cells from HIV+ and HIV- donors. Genetic and age-specific effects on these parameters were controlled by studying HIV-discordant pairs of monozygotic twins. Telomere terminal restriction fragment (TRF) lengths from CD4+ T cells of HIV+ donors were significantly greater than those from HIV- twins. In contrast, telomere lengths in CD8+ T cells from HIV+ donors were shorter than in HIV- donors. The in vitro replicative capacity of CD4+ cells from HIV+ donors was equivalent to that of HIV- donors in response to stimulation through T cell receptor CD3 and CD28. Little or no telomerase activity was detected in freshly isolated CD4+ or CD8+ lymphocytes from HIV+ or HIV- donors, but was induced by in vitro stimulation of both HIV+ and HIV- donor cells. These results suggest that HIV infection is associated with alterations in the population dynamics of both CD4+ and CD8+ T cells, but fail to provide evidence for clonal exhaustion or replicative senescence as a mechanism underlying the decline in CD4+ T cells of HIV-infected donors.

Regulation of telomerase RNA template expression in human T lymphocyte development and activation

Telomeres are unique DNA-protein complexes at the terminals of chromosomes that appear to play a critical role in protecting chromosomal integrity and in maintaining cellular replicative potential. Telomerase is a ribonuclear protein that is capable of elongating telomeres by the addition of telomeric hexanucleotide repeats and therefore contributing to the capacity for cell replication. Telomerase activity is expressed in human germline cells and malignant cells, and it has recently been demonstrated that telomerase activity is highly regulated in normal lymphocytes at specific stages of development and activation. However, these studies have not elucidated whether telomerase activity is regulated at the level of specific gene expression or whether the regulation of telomerase RNA template (hTR) and/or protein components contributes to the regulation of telomerase activity in normal somatic cells. To characterize at a molecular level the regulation of telomerase expression in human T lymphocytes, we analyzed the expression of hTR during lineage development and after in vitro activation. It was found that hTR is expressed in subsets of thymocytes with strong telomerase activity at levels that are consistently higher (1.5 times; p < 0.01) than those found in peripheral blood resting T cells. In addition, hTR is up-regulated two- to fivefold in peripheral blood naive and memory CD4+ T cells after in vitro activation with anti-CD3 plus anti-CD28. These results establish that hTR expression is regulated in normal human T cells during lineage development and after activation, and indicate that regulation of hTR expression may contribute to the regulation of telomerase activity in normal lymphoid cells.

Regulation of telomerase RNA template expression in human T lymphocyte development and activation

Telomeres are unique DNA-protein complexes at the terminals of chromosomes that appear to play a critical role in protecting chromosomal integrity and in maintaining cellular replicative potential. Telomerase is a ribonuclear protein that is capable of elongating telomeres by the addition of telomeric hexanucleotide repeats and therefore contributing to the capacity for cell replication. Telomerase activity is expressed in human germline cells and malignant cells, and it has recently been demonstrated that telomerase activity is highly regulated in normal lymphocytes at specific stages of development and activation. However, these studies have not elucidated whether telomerase activity is regulated at the level of specific gene expression or whether the regulation of telomerase RNA template (hTR) and/or protein components contributes to the regulation of telomerase activity in normal somatic cells. To characterize at a molecular level the regulation of telomerase expression in human T lymphocytes, we analyzed the expression of hTR during lineage development and after in vitro activation. It was found that hTR is expressed in subsets of thymocytes with strong telomerase activity at levels that are consistently higher (1.5 times; p < 0.01) than those found in peripheral blood resting T cells. In addition, hTR is up-regulated two- to fivefold in peripheral blood naive and memory CD4+ T cells after in vitro activation with anti-CD3 plus anti-CD28. These results establish that hTR expression is regulated in normal human T cells during lineage development and after activation, and indicate that regulation of hTR expression may contribute to the regulation of telomerase activity in normal lymphoid cells.

Tissue-specific and growth-regulated expression of CD44 variable exon determinants in the mouse

CD44 is a polymorphic transmembrane glycoprotein widely expressed in lymphocytes and epithelial cells. CD44 polymorphism reflects both posttranslational modification and alternative splicing of up to 10 variably expressed exons in the membrane-proximal CD44 extracellular domain. An analysis of CD44 variable exon-containing isoforms in the mouse was carried out by generating a panel of monoclonal antibodies against variable region determinants of CD44. Immunohistochemical analysis demonstrated selective patterns of expression of CD44 variable exon determinants in normal tissues, and flow cytometric analysis identified expression of CD44 variable exon-dependent determinants in epithelial and lymphoid cell lines. Regulation of alternative splicing was studied by characterization of cell surface expression of CD44 variable exon determinants on HC11 mammary epithelial cells, and it was demonstrated that variably spliced isoforms are selectively regulated as a function of growth phase in vitro. These results demonstrate the tissue-specific and growth-regulated expression of the CD44 isoform at the level of cell surface protein products and identify isoform-specific determinants that can be targeted in analysis of isoform-specific function.