T Cell Stimulation In Vivo by Lipopolysaccharide (LPS)

Lipopolysaccharide (LPS) from gram-negative bacteria causes polyclonal activation of B cells and stimulation of macrophages and other APC. We show here that, under in vivo conditions, LPS also induces strong stimulation of T cells. As manifested by CD69 upregulation, LPS injection stimulates both CD4 and CD8⁺ T cells, and, at high doses, stimulates naive (CD44^lo) cells as well as memory (CD44^hi) cells. However, in terms of cell division, the response of T cells after LPS injection is limited to the CD44^hi subset of CD8⁺ cells. In contrast with B cells, proliferative responses of CD44^hi CD8⁺ cells require only very low doses of LPS (10 ng). Based on studies with LPS-nonresponder and gene-knockout mice, LPS-induced proliferation of CD44^hi CD8⁺ cells appears to operate via an indirect pathway involving LPS stimulation of APC and release of type I (α, β) interferon (IFN-I). Similar selective stimulation of CD44^hi CD8⁺ cells occurs in viral infections and after injection of IFN-I, implying a common mechanism. Hence, intermittent exposure to pathogens (gram-negative bacteria and viruses) could contribute to the high background proliferation of memory–phenotype CD8⁺ cells found in normal animals.     

Small intestinal CD103+ dendritic cells display unique functional properties that are conserved between mice and humans

A functionally distinct subset of CD103⁺ dendritic cells (DCs) has recently been identified in murine mesenteric lymph nodes (MLN) that induces enhanced FoxP3⁺ T cell differentiation, retinoic acid receptor signaling, and gut-homing receptor (CCR9 and α4β7) expression in responding T cells. We show that this function is specific to small intestinal lamina propria (SI-LP) and MLN CD103⁺ DCs. CD103⁺ SI-LP DCs appeared to derive from circulating DC precursors that continually seed the SI-LP. BrdU pulse-chase experiments suggested that most CD103⁺ DCs do not derive from a CD103⁻ SI-LP DC intermediate. The majority of CD103⁺ MLN DCs appear to represent a tissue-derived migratory population that plays a central role in presenting orally derived soluble antigen to CD8⁺ and CD4⁺ T cells. In contrast, most CD103⁻ MLN DCs appear to derive from blood precursors, and these cells could proliferate within the MLN and present systemic soluble antigen. Critically, CD103⁺ DCs with similar phenotype and functional properties were present in human MLN, and their selective ability to induce CCR9 was maintained by CD103⁺ MLN DCs isolated from SB Crohn's patients. Thus, small intestinal CD103⁺ DCs represent a potential novel target for regulating human intestinal inflammatory responses.     

A Role for Fas in Negative Selection of Thymocytes In Vivo

To seek information on the role of Fas in negative selection, we examined subsets of thymocytes from normal neonatal mice versus Fas-deficient lpr/lpr mice injected with graded doses of antigen. In normal mice, injection of 1–100 μg of staphylococcal enterotoxin B (SEB) induced clonal elimination of SEB-reactive Vβ8⁺ cells at the level of the semi-mature population of HSA^hi CD4⁺ 8⁻ cells found in the thymic medulla; deletion of CD4⁺ 8⁺ cells was minimal. SEB injection also caused marked elimination of Vβ8⁺ HSA^hi CD4⁺ 8⁻ thymocytes in lpr/lpr mice. Paradoxically, however, elimination of these cells in lpr/lpr mice was induced by low-to-moderate doses of SEB (≤1 μg) but not by high doses (100 μg). Similar findings applied when T cell receptor transgenic mice were injected with specific peptide. These findings suggest that clonal elimination of semi-mature medullary T cells is Fas independent at low doses of antigen but Fas dependent at high doses. Previous reports documenting that negative selection is not obviously impaired in lpr/lpr mice could thus reflect that the antigens studied were expressed at only a low level.     

The Unenlarged Lymph Nodes of HIV-1–infected, Asymptomatic Patients with High CD4 T Cell Counts Are Sites for Virus Replication and CD4 T Cell Proliferation. The Impact of Highly Active Antiretroviral Therapy

The efficacy of triple drug therapy for HIV-1 infection encourages its early use to prevent damage to the immune system. We monitored the effects of such therapy on 12 patients with 14–75-mo histories of minimal disease, i.e., CD4⁺ counts constantly >500/μl and little or no lymph node enlargement. In this way, we could first determine the extent of viral replication and immunoarchitectural changes in unenlarged nodes early in disease, and second follow the response to triple therapy in plasma and lymphoid tissue in tandem. As is known for lymph nodes with more advanced disease, the germinal centers showed productively infected T cells, i.e., CD4⁺CD1a⁻CD68⁻ cells labeling intensely for HIV-1 RNA after in situ hybridization. The unenlarged nodes also showed extensive HIV-1 RNA retention on a well-preserved, follicular dendritic cell (FDC) network, and the follicles were abnormal. There were numerous CD8⁺ cells, many expressing TIA-1 granule antigen. Also, in contrast to normal follicles, CD4⁺ T cell proliferation was active, with marked increases in the number of cycling, Ki-67⁺CD4⁺CD45R0⁺ cells. After 28 d and 3 mo of therapy, productively infected T cells decreased dramatically and often were not apparent. The labeling of the FDC network for viral RNA also decreased, but not for gag protein. We conclude that HIV-1 replicates and accumulates in lymphoid organs before damage of the immune system, that at this stage of disease de novo production of T cells occurs in the lymphoid tissue, and that the infection is sensitive to triple drug therapy in both plasma and lymph nodes.     

Lineage Commitment in the Thymus: Only the Most Differentiated (TCRhibcl-2hi) Subset of CD4+CD8+Thymocytes Has Selectively Terminated CD4 or CD8 Synthesis

Lineage commitment is a developmental process by which individual CD4⁺CD8⁺ (double positive, DP) thymocytes make a decision to differentiate into either CD4⁺ or CD8⁺ T cells. However, the molecular event(s) that defines lineage commitment is controversial. We have previously proposed that lineage commitment in DP thymocytes can be molecularly defined as the selective termination of CD4 or CD8 coreceptor synthesis. The present study supports such a molecular definition by showing that termination of either CD4 or CD8 synthesis is a highly regulated event that is only evident within the most differentiated DP subset (CD5^hiCD69^hiTCR^hibcl-2^hi). In fact, essentially all cells within this DP subset actively synthesize only one coreceptor molecule. In addition, the present results identify three distinct subpopulations of DP thymocytes that define the developmental progression of the lineage commitment process and demonstrate that lineage commitment is coincident with upregulation of TCR and bcl-2. Thus, this study supports a molecular definition of lineage commitment and uniquely identifies TCR^hibcl-2^hi DP thymocytes as cells that are already committed to either the CD4 or CD8 T cell lineage.     

Concurrent Generation of Effector and Central Memory CD8 T Cells during Vaccinia Virus Infection

It is generally thought that during the contraction phase of an acute anti-viral T cell reponse, the effector T cells that escape activation-induced cell death eventually differentiate into central memory T cells over the next several weeks. Here we report that antigen-specific CD8T cells with the phenotype and function of central memory cells develop concomitantly with effector T cells during vaccinia virus (vv) infection. As soon as 5 days after an intraperitoneal infection with vv, we could identify a subset of CD44^hi and CD62L⁺ vv-specific CD8 T cells in the peritoneal exudate lymphocytes. This population constituted approximately 10% of all antigen-specific T cells and like central memory T cells, they also expressed high levels of CCR7 and IL-7R but expressed little granzyme B. Importantly, upon adoptive transfer into naïve congenic hosts, CD62L⁺, but not CD62L⁻ CD8 T cells were able to expand and mediate a rapid recall response to a new vv challenge initiated 6 weeks after transfer, confirming that the CD62L⁺ vv-specific CD8 T cells are bonafide memory cells. Our results are thus consistent with the branched differentiation model, where effector and memory cells develop simultaneously. These results are likely to have implications in the context of vaccine design, particularly those based on vaccinia virus recombinants.     

Antigen-specific peripheral shaping of the natural regulatory T cell population

Although regulatory T (T reg) cells are thought to develop primarily in the thymus, the peripheral events that shape the protective T reg cell population are unclear. We analyzed the peripheral CD4⁺ T cell receptor (TCR) repertoire by cellular phenotype and location in mice with a fixed TCRβ chain. We found that T reg (Foxp3⁺) cells showed a marked skewing of TCR usage by anatomical location in a manner similar to antigen-experienced (CD44^hiFoxp3⁻) but not naive (CD44^loFoxp3⁻) cells, even though CD44^hi and T reg cells used mostly dissimilar TCRs. This was likely unrelated to peripheral conversion, which we estimate generates only a small percentage of peripheral T reg cells in adults. Conversion was readily observed, however, during the immune response induced by Foxp3⁻ cells in lymphopenic hosts. Interestingly, the converted Foxp3⁺ and expanded Foxp3⁻ TCR repertoires were different, suggesting that generation of Foxp3⁺ cells is not an automatic process upon antigen activation of Foxp3⁻ T cells. Retroviral expression of these TCRs in primary monoclonal T cells confirmed that conversion did not require prior cellular conditioning. Thus, these data demonstrate that TCR specificity plays a crucial role in the process of peripheral conversion and in shaping the peripheral T reg cell population to the local antigenic landscape.     

Turnover of CD4+ and CD8+ T Lymphocytes in HIV-1 Infection as Measured by Ki-67 Antigen

We investigated CD4⁺ and CD8⁺ T cell turnover in both healthy and HIV-1–infected adults by measuring the nuclear antigen Ki-67 specific for cell proliferation. The mean growth fraction, corresponding to the expression of Ki-67, was 1.1% for CD4⁺ T cells and 1.0% in CD8⁺ T cells in healthy adults, and 6.5 and 4.3% in HIV-1–infected individuals, respectively. Analysis of CD45RA⁺ and CD45RO⁺ T cell subsets revealed a selective expansion of the CD8⁺ CD45RO⁺ subset in HIV-1–positive individuals. On the basis of the growth fraction, we derived the potential doubling time and the daily turnover of CD4⁺ and CD8⁺ T cells. In HIV-1–infected individuals, the mean potential doubling time of T cells was five times shorter than that of healthy adults. The mean daily turnover of CD4⁺ and CD8⁺ T cells in HIV-1–infected individuals was increased 2- and 6-fold, respectively, with more than 40-fold interindividual variation. In patients with <200 CD4⁺ counts, CD4⁺ turnover dropped markedly, whereas CD8⁺ turnover remained elevated. The large variations in CD4⁺ T cell turnover might be relevant to individual differences in disease progression.     

Infected Cell Protein (ICP)47 Enhances Herpes Simplex Virus Neurovirulence by Blocking the CD8+ T Cell Response

The herpes simplex virus (HSV) infected cell protein (ICP)47 blocks CD8⁺ T cell recognition of infected cells by inhibiting the transporter associated with antigen presentation (TAP). In vivo, HSV-1 replicates in two distinct tissues: in epithelial mucosa or epidermis, where the virus enters sensory neurons; and in the peripheral and central nervous system, where acute and subsequently latent infections occur. Here, we show that an HSV-1 ICP47⁻ mutant is less neurovirulent than wild-type HSV-1 in mice, but replicates normally in epithelial tissues. The reduced neurovirulence of the ICP47⁻ mutant was due to a protective CD8⁺ T cell response. When compared with wild-type virus, the ICP47⁻ mutant expressed reduced neurovirulence in immunologically normal mice, and T cell–deficient nude mice after reconstitution with CD8⁺ T cells. However, the ICP47⁻ mutant exhibited normal neurovirulence in mice that were acutely depleted of CD8⁺ T cells, and in nude mice that were not reconstituted, or were reconstituted with CD4⁺ T cells. In contrast, CD8⁺ T cell depletion did not increase the neurovirulence of an unrelated, attenuated HSV-1 glycoprotein (g)E⁻ mutant. ICP47 is the first viral protein shown to influence neurovirulence by inhibiting CD8⁺ T cell protection.     

Human interleukin 17-producing cells originate from a CD161+CD4+ T cell precursor

We demonstrate that CD161 is a highly up-regulated gene in human interleukin (IL) 17 T helper cell (Th17) clones and that all IL-17–producing cells are contained in the CD161⁺ fraction of CD4⁺ T cells present in the circulation or in inflamed tissues, although they are not CD1-restricted natural killer T cells. More importantly, we show that all IL-17–producing cells originate from CD161⁺ naive CD4⁺ T cells of umbilical cord blood, as well as of the postnatal thymus, in response to the combined activity of IL-1β and IL-23. These findings implicate CD161 as a novel surface marker for human Th17 cells and demonstrate the exclusive origin of these cells from a CD161⁺CD4⁺ T cell progenitor.     

Spontaneous Autoimmune Diabetes in Monoclonal T Cell Nonobese Diabetic Mice

It has been established that insulin-dependent diabetes mellitus (IDDM) in nonobese diabetic (NOD) mice results from a CD4⁺ and CD8⁺ T cell–dependent autoimmune process directed against the pancreatic beta cells. The precise roles that beta cell–reactive CD8⁺ and CD4⁺ T cells play in the disease process, however, remain ill defined. Here we have investigated whether naive beta cell–specific CD8⁺ and CD4⁺ T cells can spontaneously accumulate in pancreatic islets, differentiate into effector cells, and destroy beta cells in the absence of other T cell specificities. This was done by introducing K^d– or I-A^g7–restricted beta cell–specific T cell receptor (TCR) transgenes that are highly diabetogenic in NOD mice (8.3- and 4.1-TCR, respectively), into recombination-activating gene (RAG)-2–deficient NOD mice, which cannot rearrange endogenous TCR genes and thus bear monoclonal TCR repertoires. We show that while RAG-2^−/− 4.1-NOD mice, which only bear beta cell–specific CD4⁺ T cells, develop diabetes as early and as frequently as RAG-2⁺ 4.1-NOD mice, RAG-2^−/− 8.3-NOD mice, which only bear beta cell–specific CD8⁺ T cells, develop diabetes less frequently and significantly later than RAG-2⁺ 8.3-NOD mice. The monoclonal CD8⁺ T cells of RAG-2^−/− 8.3-NOD mice mature properly, proliferate vigorously in response to antigenic stimulation in vitro, and can differentiate into beta cell–cytotoxic T cells in vivo, but do not efficiently accumulate in islets in the absence of a CD4⁺ T cell–derived signal, which can be provided by splenic CD4⁺ T cells from nontransgenic NOD mice. These results demonstrate that naive beta cell– specific CD8⁺ and CD4⁺ T cells can trigger diabetes in the absence of other T or B cell specificities, but suggest that efficient recruitment of naive diabetogenic beta cell–reactive CD8⁺ T cells to islets requires the assistance of beta cell–reactive CD4⁺ T cells.     

4-1BB Costimulatory Signals Preferentially Induce CD8+ T Cell Proliferation and Lead to the Amplification In Vivo of Cytotoxic T Cell Responses

The 4-1BB receptor is an inducible type I membrane protein and member of the tumor necrosis factor receptor (TNFR) superfamily that is rapidly expressed on the surface of CD4⁺ and CD8⁺ T cells after antigen- or mitogen-induced activation. Cross-linking of 4-1BB and the T cell receptor (TCR) on activated T cells has been shown to deliver a costimulatory signal to T cells. Here, we expand upon previously published studies by demonstrating that CD8⁺ T cells when compared with CD4⁺ T cells are preferentially responsive to both early activation events and proliferative signals provided via the TCR and 4-1BB. In comparison, CD28-mediated costimulatory signals appear to function in a reciprocal manner to those induced through 4-1BB costimulation. In vivo examination of the effects of anti-4-1BB monoclonal antibodies (mAbs) on antigen-induced T cell activation have shown that the administration of epitope-specific anti-4-1BB mAbs amplified the generation of H-2^d–specific cytotoxic T cells in a murine model of acute graft versus host disease (GVHD) and enhanced the rapidity of cardiac allograft or skin transplant rejection in mice. Cytokine analysis of in vitro activated CD4⁺ and CD8⁺ Tcells revealed that anti-4-1BB costimulation markedly enhanced interferon-γ production by CD8⁺ T cells and that anti-4-1BB mediated proliferation of CD8⁺ T cells appears to be IL-2 independent. The results of these studies suggest that regulatory signals delivered by the 4-1BB receptor play an important role in the regulation of cytotoxic T cells in cellular immune responses to antigen.     

Disturbed CD4+ T Cell Homeostasis and In Vitro HIV-1 Susceptibility in Transgenic Mice Expressing T Cell Line–tropic HIV-1 Receptors

T cell line–tropic (T-tropic) HIV type 1 strains enter cells by interacting with the cell-surface molecules CD4 and CXCR4. We have generated transgenic mice predominantly expressing human CD4 and CXCR4 on their CD4-positive T lymphocytes (CD4⁺ T cells). Their primary thymocytes are susceptible to T-tropic but not to macrophage-tropic HIV-1 infection in vitro, albeit with a viral antigen production less efficient than human peripheral blood mononuclear cells. Interestingly, even without HIV infection, transgenic mice display a CD4⁺ T cell depletion profile of peripheral blood reminiscent of that seen in AIDS patients. We demonstrate that CD4⁺ T cell trafficking in transgenic mice is biased toward bone marrow essentially due to CXCR4 overexpression, resulting in the severe loss of CD4⁺ T cells from circulating blood. Our data suggest that CXCR4 plays an important role in lymphocyte trafficking through tissues, especially between peripheral blood and bone marrow, participating in the regulation of lymphocyte homeostasis in these compartments. Based on these findings, we propose a hypothetical model in which the dual function of CXCR4 in HIV-1 infection and in lymphocyte trafficking may cooperatively induce progressive HIV-1 infection and CD4⁺ T cell decline in patients.     

CD45RC Isoforms Define Two Types of CD4 Memory T Cells, One of which Depends on Persisting Antigen

The cellular basis of immunological memory remains a controversial area with respect to the identity of memory T cells and the role of persisting antigen. CD4 T cells are phenotypically divided by the expression of high and low molecular weight isoforms of CD45, surface markers that are frequently used to identify “naive” (CD45R^high) and “memory” (CD45R^low) subsets. The latter subset responds rapidly in antigen recall assays but paradoxically has a short life span, a property that is difficult to reconcile with long-term memory. The present study examines these issues using a DTH (delayed-type hypersensitivity) model in which contact sensitivity to dinitrochlorobenzene (DNCB) was transferred to athymic nude rats by recirculating CD4 T cell subsets defined in the rat by the anti-CD45RC mAb OX22. As expected, CD45RC⁺ (but not RC⁻) CD4 T cells from normal unprimed rats transferred a DNCB-specific DTH response, whereas, 4 d after sensitization the CD45RC⁻ (memory) subset alone contained the DNCB reactivity. However, when donor cells were collected from thymectomized rats sensitized two mo earlier, DNCB-specific responses were transferred by both CD45RC⁻ and RC⁺ subsets suggesting that many of the latter had developed from cells with a memory phenotype. This was confirmed when CD45RC⁻ CD4 T cells from 4-d primed rats were parked in intermediate nude recipients and recovered 2 mo later. DNCB-specific activity was now found wholly within the CD45RC⁺ “revertant” subset; the CD45RC⁻ CD4 T cell population was devoid of activity. Importantly, we found that the total switch-back from CD45RC⁻ to RC⁺ could be prevented, apparently by persisting antigen. The results indicate that there are two functionally distinct categories of memory T cells: one, a short-lived CD45R^low type which orchestrates the rapid kinetics, the other, a longer-lived CD45R^high revertant which ensures that immunological memory endures.     

Impact of the TCR Signal on Regulatory T Cell Homeostasis, Function, and Trafficking

Signaling through the T cell antigen receptor (TCR) is important for the homeostasis of naïve and memory CD4⁺ T cells. The significance of TCR signaling in regulatory T (Treg) cells has not been systematically addressed. Using an Ox40-cre allele that is prominently expressed in Treg cells, and a conditional null allele of the gene encoding p56^Lck, we have examined the importance of TCR signaling in Treg cells. Inactivation of p56^Lck resulted in abnormal Treg homeostasis characterized by impaired turnover, preferential redistribution to the lymph nodes, loss of suppressive function, and striking changes in gene expression. Abnormal Treg cell homeostasis and function did not reflect the involvement of p56^Lck in CD4 function because these effects were not observed when CD4 expression was inactivated by Ox40-cre.The results make clear multiple aspects of Treg cell homeostasis and phenotype that are dependent on a sustained capacity to signal through the TCR.     

Processing of Tumor Antigen Differentially Impacts the Development of Helper and Effector CD4+ T-cell Responses

CD4⁺ T cells contribute to the antitumor T-cell response as both effectors that promote tumor rejection and helpers that facilitate the activation of other antitumor effector cells, such as CD8⁺ T cells. Maximal engagement of both effector and helper CD4⁺ T-cell responses is a desirable attribute of cancer vaccines. We have employed the B16F10 murine melanoma model and a series of recombinant adenovirus (Ad) vaccines expressing mutant forms of the tumor antigen, dopachrome tautomerase, to investigate the relationship between antigen processing and the antitumor CD4⁺ T-cell response. Our results have revealed an unexpected dichotomy in the generation of helper and effector CD4⁺ T-cell responses where CD4⁺ T effector responses are dependent upon protein processing and trafficking, whereas CD4⁺ T helper responses are not. The results have important implications for strategies aimed at augmenting antigen immunogenicity by altering intracellular processing and localization.     

‘Self-Protection’ of Individual CD4+ T Cells against R5 HIV-1 Infection by the Synthesis of Anti-Viral CCR5 Ligands

It is well established that paracrine secretion of anti-viral CCR5 ligands by CD8⁺ and CD4⁺ T cells can block the infection of activated CD4⁺ T cells by R5 and dual-tropic isolates of HIV-1. By contrast, because CD4⁺ T cells can be infected by HIV-1 and at least some subsets secrete anti-viral CCR5 ligands, it is possible that these ligands protect against HIV-1 via autocrine as well as paracrine pathways. Here we use a model primary CD4⁺ T cell response in vitro to show that individual CD4⁺ T cells that secrete anti-viral CCR5 ligands are ‘self-protected’ against infection with R5 but not X4 strains of HIV-1. This protection is selective for CD4⁺ T cells that secrete anti-viral CCR5 ligands in that activated CD4⁺ T cells in the same cultures remain infectable with R5 HIV-1. These data are most consistent with an autocrine pathway of protection in this system and indicate a previously unappreciated selective pressure on the emergence of viral variants and CD4⁺ T cell phenotypes during HIV-1 infection.     

T cell activation enhancement by endogenous pMHC acts for both weak and strong agonists but varies with differentiation state

T cells are extremely sensitive in their ability to find minute amounts of antigenic peptide in the midst of many endogenous peptides presented on an antigen-presenting cell. The role of endogenous peptides in the recognition of foreign peptide and hence in T cell activation has remained controversial for CD8⁺ T cell activation. We showed previously that in a CD8⁺ T cell hybridoma, nonstimulatory endogenous peptides enhance T cell sensitivity to antigen by increasing the coreceptor function of CD8. However, others were not able to detect such enhancement in naive and activated CD8⁺ T cells. Here, we show that endogenous peptides substantially enhance the ability of T cells to detect antigen, an effect measurable by up-regulation of activation or maturation markers and by increased effector function. This enhancement is most pronounced in thymocytes, moderate in naive T cells, and mild in effector T cells. The importance of endogenous peptides is inversely proportional to the agonist activity of the stimulatory peptide presented. Unlike for CD4⁺ T cells, the T cell receptor of CD8⁺ T cells does not distinguish between endogenous peptides for their ability to enhance antigen recognition.     

Distinct Effects of IL-18 on the Engraftment and Function of Human Effector CD8+ T Cells and Regulatory T Cells

IL-18 has pleotropic effects on the activation of T cells during antigen presentation. We investigated the effects of human IL-18 on the engraftment and function of human T cell subsets in xenograft mouse models. IL-18 enhanced the engraftment of human CD8⁺ effector T cells and promoted the development of xenogeneic graft versus host disease (GVHD). In marked contrast, IL-18 had reciprocal effects on the engraftment of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (Tregs) in the xenografted mice. Adoptive transfer experiments indicated that IL-18 prevented the suppressive effects of Tregs on the development of xenogeneic GVHD. The IL-18 results were robust as they were observed in two different mouse strains. In addition, the effects of IL-18 were systemic as IL-18 promoted engraftment and persistence of human effector T cells and decreased Tregs in peripheral blood, peritoneal cavity, spleen and liver. In vitro experiments indicated that the expression of the IL-18Rα was induced on both CD4 and CD8 effector T cells and Tregs, and that the duration of expression was less sustained on Tregs. These preclinical data suggest that human IL-18 may have use as an adjuvant for immune reconstitution after cytotoxic therapies, and to augment adoptive immunotherapy, donor leukocyte infusions, and vaccine strategies.     

Identification of a Committed T Cell Precursor Population in Adult Human Peripheral Blood

Here, we report data concerning the discovery in adult human peripheral blood of a precursor cell population able to differentiate into CD4⁺CD3⁺αβ⁺ mature T cells. These cells, which represent 0.1–0.5% of total peripheral blood mononuclear cells (PBMC), express substantial levels of CD4, but lack CD3 surface expression. At a molecular level, they express the pre-T cell receptor α (pTα) gene, CD3-γ, CD-δ and CD-ε, and RAG-1 recombination enzyme and have initiated rearrangements in the T cell receptor (TCR)-β locus (D–J). Moreover, low levels of CD3ε protein, but not of TCR-β chain, can be detected in their cytoplasm. Our results suggest that CD4⁺CD3⁻ cells identified in peripheral blood are different from CD3⁻CD4⁺CD8⁻ thymocytes and may contain precursors of an extrathymic T cell differentiation pathway.