Citations

AbstractT cells are produced through 2 mechanisms, thymopoiesis and proliferative expansion of postthymic T cells. Thymic output generates diversity of the pool, and proliferation achieves optimal clonal size of each individual T cell. To determine the contribution of these 2 mechanisms to the formation of the initial T-cell repertoire, we examined neonates of 30 to 40 weeks' gestation. Peripheral T cells were in a state of high proliferative turnover. In premature infants, 10% of T cells were dividing; the proliferation rates then declined but were still elevated in mature newborns. Throughout the third trimester, concentrations of T-cell–receptor excision circles (TRECs) were 10 per 100 T cells. Stability of TREC frequencies throughout the period of repertoire generation suggested strict regulation of clonal size to approximately 10 to 20 cells. Neonatal naive CD4+ and CD8+ T cells were explicitly responsive to IL-7; growth-promoting properties of IL-15 were selective for newborn CD8+ T cells. Neonatal T cells expressed telomerase and, in spite of the high turnover, built up a telomeric reserve. Thus, proliferative expansion, facilitated by increased cytokine responsiveness, and thymopoiesis complement each other as mechanisms of T-cell production in neonates. Maintaining optimal clonal size instead of filling the space in a lymphopenic host appears to regulate homeostatic T-cell proliferation during fetal development.

CD28 is the quintessential costimulatory molecule expressed on CD4+ and CD8+ T cells. During chronic infections and the normal aging process, CD28 expression is lost, compromising the functional activity of T cells. CD28 loss is promoted by replicative stress, particularly in the presence of tumor necrosis factor–α, owing to an inoperative CD28 initiator element. It is currently unknown whether CD28 loss is irreversible. The present study examined cytokines for their ability to reinduce CD28 expression. CD4+CD28null T cells constitutively expressed interleukin-12 (IL-12) α and β receptors, which were functional and allowed for the up-regulation of the signal transducer and activator of transcription–4 (STAT-4)–dependent gene CD161. Costimulation of the T-cell and IL-12 receptors induced the transcription of CD28 in approximately 50% of CD4+CD28null T-cell clones and lines. IL-12 by itself did not restore CD28 expression. Up-regulation of CD28 after IL-12 exposure correlated with the reassembly of the CD28–initiator protein complex. The re-expressed CD28 was functional and restored the ability of CD4+CD28null T cells to express CD25 and CD40 ligand. Our data suggest that IL-12 may, in part, functionally rescue senescent CD4+ T cells.

Activation of CD4+ T cells is governed by interplay between stimulatory and inhibitory receptors; predominance of stimulatory signals favors autoimmune reactions. In patients with rheumatoid arthritis, expression of the critical costimulatory molecule, CD28, is frequently lost. Instead, CD4+CD28null T cells express killer immunoglobulin-like receptors (KIRs) with a preferential expression of the stimulatory receptor, CD158j. The frequency of CD4+CD28null T cells in rheumatoid arthritis (RA) correlates with the risk for more severe disease. Moreover, the KIR2DS2 gene, which encodes for CD158j, is a genetic risk factor for rheumatoid vasculitis. CD158j signals through the adaptor molecule, KARAP/DAP12, to positively regulate cytotoxic activity in NK cells. However, the majority of CD4+CD28null T cell clones lacked the expression of KARAP/DAP12. Despite the absence of KARAP/DAP12, CD158j was functional and augmented interferon-γ production after T cell receptor stimulation. Cross-linking of CD158j resulted in selective phosphorylation of c-Jun NH2-terminal protein kinase (JNK) and its upstream kinase, MKK4 that led to the expression of ATF-2 and c-Jun, all in the absence of extracellular signal–regulated kinase (ERK)1/2 phosphorylation. Mutation of the lysine residue within the transmembrane domain of CD158j abolished JNK activation, suggesting that an alternate adaptor molecule was being used. CD4+CD28null T cells expressed DAP10 and inhibition of phosphatidylinositol 3-kinase, which acts downstream of DAP10, inhibited JNK activation; however, no interaction of DAP10 with CD158j could be detected. Our data suggest that CD158j in T cells functions as a costimulatory molecule through the JNK pathway independent of KARAP/DAP12 and DAP10. Costimulation by CD158j may contribute to the autoreactivity of CD4+CD28null T cells in RA.

Background — CD4 T lymphocytes accumulate in unstable plaque. The direct and indirect involvement of these T cells in tissue injury and plaque instability is not understood. Methods and Results — Gene profiling identified perforin, CD161, and members of the killer-cell immunoglobulin-like receptors as being differentially expressed in CD4 + CD28 null T cells, a T-cell subset that preferentially infiltrates unstable plaque. Frequencies of CD161 + and perforin-expressing CD4 T cells in peripheral blood were significantly increased in patients with unstable angina (UA). CD161 appeared on CD4 + CD28 null T cells after stimulation, suggesting spontaneous activation of circulating CD4 T cells in UA. Perforin-expressing CD4 + T-cell clones from patients with UA exhibited cytotoxic activity against human umbilical vein endothelial cells (HUVECs) in redirected cytotoxicity assays after T-cell receptor triggering and also after stimulation of major histocompatibility complex class I–recognizing killer-cell immunoglobulin-like receptors. HUVEC cytolysis was dependent on granule exocytosis, as demonstrated by the paralyzing effect of pretreating CD4 + CD28 null T cells with strontium. Incubation of HUVECs with C-reactive protein (CRP) increased HUVEC lysis in a dose-dependent fashion. Conclusions — In patients with UA, CD4 T cells undergo a change in functional profile and acquire cytotoxic capability. Cytotoxic CD4 T cells effectively kill endothelial cells; CRP sensitizes endothelial cells to the cytotoxic process. We propose that T-cell–mediated endothelial cell injury is a novel pathway of tissue damage that contributes to plaque destabilization. The sensitizing effect of CRP suggests synergy between dysregulated T-cell function and acute phase proteins in acute coronary syndromes.

ABSTRACTElderly individuals are at high risk for morbidity and mortality when infected with influenza virus. Vaccinations with inactivated virus are less effective in the elderly due to the declining competency of the aging immune system. We have explored whether immunological parameters predict poor anti-influenza virus vaccine responses and can be used as biological markers of immunosenescence. One hundred fifty-three residents of community-based retirement facilities aged 65 to 98 years received a trivalent influenza vaccine. Vaccine-induced antibody responses were determined by comparing hemagglutination inhibition titers before and 28 days after immunization. The composition of the T-cell compartment was analyzed by flow cytometry and the sizes of three T-cell subsets, CD4+CD45RO+cells, CD4+CD28nullcells, and CD8+CD28nullcells, were determined. Only 17% of the vaccine recipients were able to generate an increase in titers of antibody to all three vaccine components, and 46% of the immunized individuals failed to respond to any of the three hemagglutinins. The likelihood of successful vaccination declined with age and was independently correlated with the expansion of a particular T-cell subset, CD8+CD28nullT cells. The sizes of the CD4+CD45RO+memory T-cell and CD4+CD28nullT-cell subsets had no effect on the ability to mount anti-influenza virus antibody responses. Frequencies of CD8+CD28nullT cells are useful biological markers of compromised immunocompetence, identifying individuals at risk for insufficient antibody responses.

Rheumatoid arthritis (RA) is a heterogeneous syndrome of which a subset of patients develops vascular inflammation. The genetic determinants that confer risk for rheumatoid vasculitis are not known, but patients with vascular complications are known to have an expansion of CD4+CD28null T cells, a cell population potentially involved in endothelial damage. CD4+CD28null T cell clones isolated from RA patients with vasculitis were found to express killer cell immunoglobulin–like receptors (KIRs) with the stimulatory KIR2DS2 often present in the absence of opposing inhibitory receptors with related specificities. To test the hypothesis that the KIR2DS2 gene is involved in the development of vasculitis, association studies were performed. The KIR2DS2 gene was significantly enriched among patients with rheumatoid vasculitis compared with normal individuals (odds ratio 5.56, P = 0.001) and patients with RA but no vasculitis (odds ratio 7.96, P = 0.001). Also, the distribution of human histocompatibility leukocyte antigen (HLA)-C, the putative ligand for KIRs, was significantly different in patients with rheumatoid vasculitis in comparison with the control populations. These data suggest that HLA class I–recognizing receptors and HLA class I genes are genetic risk determinants that modulate the pattern of RA expression. Specifically, KIR2DS2 in conjunction with the appropriate HLA-C ligand may have a role in vascular damage by regulating CD4+CD28null T cells.

The assembly of inflammatory lesions in rheumatoid arthritis is highly regulated and typically leads to the formation of lymphoid follicles with germinal center (GC) reactions. We used microdissection of such extranodal follicles to analyze the colonizing T cells. Although the repertoire of follicular T cells was diverse, a subset of T cell receptor (TCR) sequences was detected in multiple independent follicles and not in interfollicular zones, suggesting recognition of a common antigen. Unexpectedly, the majority of shared TCR sequences were from CD8 T cells that were highly enriched in the synovium and present in low numbers in the periphery. To examine their role in extranodal GC reactions, CD8 T cells were depleted in human synovium-SCID mouse chimeras. Depletion of synovial CD8 T cells caused disintegration of the GC-containing follicles. In the absence of CD8 T cells, follicular dendritic cells disappeared, production of lymphotoxin-α1β2 markedly decreased, and immunoglobulin (Ig) secretion ceased. Immunohistochemical studies demonstrated that these CD8 T cells accumulated at the edge of the mantle zone. Besides their unique localization, they were characterized by the production of interferon (IFN)-γ, lack of the pore-forming enzyme perforin, and expression of CD40 ligand. Perifollicular IFN-γ+ CD8 T cells were rare in secondary lymphoid tissues but accounted for the majority of IFN-γ+ cells in synovial infiltrates. We propose that CD8+ T cells regulate the structural integrity and functional activity of GCs in ectopic lymphoid follicles.

The immune system is equipped with an extremely large spectrum of structurally diverse receptors to recognize all potential antigens. This fundamental principle of receptor diversity is no longer upheld in patients with rheumatoid arthritis (RA), who have a marked contraction of the T cell receptor repertoire. In this study, the ability of RA patients to produce T cells and to maintain T cell homeostasis was examined. CD4 T cells containing T cell receptor rearrangement excision circles (TREC) were substantially reduced in RA patients; TREC levels in young adult patients matched those of controls 20 years older. Increased self-replication of T cells in RA was indicated by age-inappropriate erosion of telomeres in circulating T cells with almost complete attrition of telomeric reserves in patients 20–30 yr of age. The degree of telomere loss was not related to disease duration or the use of disease-modifying medication and was most pronounced in CD4 + CD45RO null (naive) T cells. The loss of TREC-positive T cells could be a consequence of a primary defect in peripheral T cell homeostasis. Alternatively, RA patients may have impaired thymic function with the increased turnover of peripheral T cells being a secondary compensatory event.

Aberrations in the T cell repertoire with the emergence of oligoclonal populations have been described in patients with rheumatoid arthritis (RA). However, the extent of the repertoire perturbations as well as the underlying mechanisms are not known. We now have examined the diversity of the peripheral CD4 T cell repertoire by determining the frequencies of arbitrarily selected T cell receptor (TCR) β-chain sequences. Healthy individuals displayed a highly diverse repertoire, with a median frequency of individual TCR β-chain sequences of 1 in 2.4 × 10 7 CD4 T cells. In RA patients, the median TCR β-chain frequency was increased 10-fold, indicating marked contraction of the repertoire ( P < 0.001). The loss in TCR diversity was not limited to CD4 memory T cells but also involved the compartment of naive T cells, suggesting that it reflected an abnormality in T cell repertoire formation and not a consequence of antigen recognition in the synovium. Also, control patients with chronic inflammatory disease such as hepatitis C expressed a diverse repertoire indistinguishable from that of normals. Telomere length studies indicated an increased replicative history of peripheral CD4 T cells in RA patients, suggesting an enhanced turnover within the CD4 compartment. Compared with age-matched controls, terminal restriction fragment sizes were 1.7 kilobases shorter ( P < 0.001). These data demonstrate an altered CD4 T cell homeostasis in RA that may contribute to the autoimmune response as well as to the immunodeficiency in these patients.