The regulation and function of the CD4 coreceptor during T lymphocyte development

Comparison of peptide-major histocompatibility complex tetramers and dextramers for the identification of antigen-specific T cells

Fluorochrome-conjugated peptide-major histocompatibility complex (pMHC) multimers are widely used for flow cytometric visualization of antigen-specific T cells. The most common multimers, streptavidin-biotin-based 'tetramers', can be manufactured readily in the laboratory. Unfortunately, there are large differences between the threshold of T cell receptor (TCR) affinity required to capture pMHC tetramers from solution and that which is required for T cell activation. This disparity means that tetramers sometimes fail to stain antigen-specific T cells within a sample, an issue that is particularly problematic when staining tumour-specific, autoimmune or MHC class II-restricted T cells, which often display TCRs of low affinity for pMHC. Here, we compared optimized staining with tetramers and dextramers (dextran-based multimers), with the latter carrying greater numbers of both pMHC and fluorochrome per molecule. Most notably, we find that: (i) dextramers stain more brightly than tetramers; (ii) dextramers outperform tetramers when TCR-pMHC affinity is low; (iii) dextramers outperform tetramers with pMHC class II reagents where there is an absence of co-receptor stabilization; and (iv) dextramer sensitivity is enhanced further by specific protein kinase inhibition. Dextramers are compatible with current state-of-the-art flow cytometry platforms and will probably find particular utility in the fields of autoimmunity and cancer immunology.

The role of CD4 in regulating homeostasis of T helper cells

Intrathymic T cell selection and peripheral activation of mature T cells are crucial for self-recognition and the general immune response to viral, bacterial, and tumor antigens. The T cell coreceptors, CD4 and CD8, contribute to the regulation of these processes. The importance of interactions between CD4 and molecules encoded by the class II major histocompatibility complex (MHC) for thymic T cell selection has been clearly established, however, the role of CD4-MHC class II interactions in T helper (TH) cell differentiation, in the maintenance of homeostasis in the peripheral immune system, and in the generation of memory TH cells is largely unclear. Here, we present evidence for a role of CD4 in controlling homeostasis in the peripheral immune system. We also demonstrate the importance of CD4-MHC class II interactions in inducing these previously not recognized functions of CD4.

Inhibition of contact sensitivity in human CD4+ transgenic mice by human CD4-specific monoclonal antibodies: CD4+ T-cell depletion is not required

Clenoliximab and keliximab are monkey/human chimeric monoclonal antibodies (mAbs) of the immunoglobulin G4 (IgG4) and IgG1 isotypes, respectively, that recognize the same epitope on human CD4. The two mAbs possess identical idiotypes and exhibit equal affinities for CD4. Upon administration of these mAbs to mice that express a human CD4 transgene, but not mouse CD4 (HuCD4/Tg mice), clenoliximab and keliximab exhibited similar kinetics of binding to CD4, and induced the same degree of CD4 modulation from the cell surface, although only keliximab mediated CD4+ T-cell depletion. Epicutaneous sensitization and challenge of HuCD4/Tg mice with the hapten oxazolone resulted in a contact sensitivity response characterized by tissue swelling, and the presence of interferon-gamma (IFN-gamma) and interleukin-4 (IL-4) in the local tissue. Administration of a single 2-mg dose of either clenoliximab or keliximab to HuCD4/Tg mice prior to sensitization significantly reduced post-challenge tissue swelling, and levels of IFN-gamma and IL-4, indicating that CD4+ T-cell depletion is not required for anti-CD4 mAb-mediated inhibition of contact sensitivity. Administration of either mAb prior to challenge failed to inhibit the contact sensitivity response, indicating differential sensitivity of the afferent and efferent phases of the response to inhibition by CD4-specific mAbs. Collectively, these data indicate that CD4 functions as a positive regulatory molecule in the contact sensitivity response.

Negative Regulation of CD4 Lineage Development and Responses by CD5

CD5 deficiency results in a hyper-responsive phenotype to Ag receptor stimulation. Here we show that the development and responses of CD4 lineage T cells are regulated by the function of CD5. Thymocytes expressing the I-Ad-restricted DO11.10 TCR undergo abnormal selection without CD5. In H-2d mice, the absence of CD5 causes deletion of double-positive thymocytes, but allows for efficient selection of cells expressing high levels of the DO11.10 clonotype. By contrast, there is enhanced negative selection against the DO11.10 clonotype in the presence of I-Ab. T cell hybridomas and DO11.10 T cells are more responsive to TCR stimulation in the absence of CD5. Such hypersensitivity can be eliminated by expression of wild-type CD5, but not by a form of CD5 that lacks the cytoplasmic tail. Finally, CD5 deficiency partially suppresses the block of CD4 lineage development in CD4-deficient mice. Taken together, the data support a general role for CD5 as a negative regulator of Ag receptor signaling in the development and immune responses of CD4 lineage T cells.

CD4-Mediated Signals Induce T Cell Dysfunction In Vivo

Triggering of CD4 coreceptors on both human and murine T cells can suppress TCR/CD3-induced secretion of IL-2. We show here that pretreatment of murine CD4+ T cells with the CD4-specific mAb YTS177 inhibits the CD3-mediated activation of the IL-2 promoter factors NF-AT and AP-1. Ligation of CD4 molecules on T cells leads to a transient stimulation of extracellular signal-regulated kinase (Erk) 2, but not c-Jun N-terminal kinase (JNK) activity. Pretreatment with anti-CD4 mAb impaired anti-CD3-induced Erk2 activation. Costimulation with anti-CD28 overcame the inhibitory effect of anti-CD4 Abs, by induction of JNK activation. The in vivo relevance of these studies was demonstrated by the observation that CD4+ T cells from BALB/c mice injected with nondepleting anti-CD4 mAb were inhibited in their ability to respond to OVA Ag-induced proliferation and IL-2 secretion. Interestingly, in vivo stimulation with anti-CD28 mAb restored IL-2 secretion. Furthermore, animals pretreated with anti-CD4 elicited enhanced IL-4 secretion induced by OVA and CD28. These observations suggest that CD4-specific Abs can inhibit T cell activation by interfering with signal 1 transduced through the TCR, but potentiate those delivered through the costimulatory molecule CD28. These studies have relevance to understanding the mechanism of tolerance induced by nondepleting anti-CD4 mAb used in animal models for allograft studies, autoimmune pathologies, and for immunosuppressive therapies in humans.

Deletional and mutational analyses of the human CD4 gene promoter: characterization of a minimal tissue-specific promoter

In recent years, considerable interest has arisen in understanding the mechanisms of the CD4 gene transcriptional control which resulted in the identification of a promoter, enhancers and a silencer. While the murine CD4 gene promoter has been well studied, little is known about its human counterpart that we previously identified as a 1.1 kb region. Here, we show that the -170/+20 region represents a minimal tissue-specific promoter with a size compatible with its inclusion in viral vectors for gene therapy. In addition, mutational analyses provided evidence that this fragment contains three regions critical for transcriptional activity: an initiator-like sequence, an Ets consensus site and an ATF consensus site. The latter site is absent in the murine promoter, suggesting that some differences exist between the transcriptional control of the human and murine CD4 genes.

Costimulation of naive CD8(+) lymphocytes induces CD4 expression and allows human immunodeficiency virus type 1 infection

Human immunodeficiency virus type 1 (HIV-1) infection requires cell surface expression of CD4. Costimulation of CD8(+)/CD4(-) T lymphocytes by anti-CD3 and anti-CD28 antibodies or by allogeneic dendritic cells induced expression of CD4 and rendered these CD8 cells susceptible to HIV-1 infection. Naive CD45RA+ cells responded with greater expression of CD4 than did CD45RO+ cells. CD8(+) lymphocytes derived from fetal or newborn sources exhibited a greater tendency to express CD4, consistent with their naive states. This mechanism of infection suggests HIV-induced perturbation of the CD8 arm of the immune response and could explain the generally rapid disease progression seen in HIV-infected children.

Thymocyte selection: not by TCR alone

During development of T cells in the thymus, T-cell receptor (TCR)-mediated recognition of self-MHC/self-peptide complexes on thymic stroma dictates the developmental fate of immature CD4+CD8+ (double positive) thymocytes. Intriguingly, TCR-generated intracellular signals can elicit two entirely different cellular responses in such thymocytes: apoptosis or further differentiation. The critical issue in understanding end-stage T-cell development is how TCR occupancy can be perceived in such markedly different ways by the TCR. Here, we review the cytoplasmic and nuclear events that result from TCR signaling during thymocyte selection. Studies aimed at distinguishing molecular components involved in positive selection (resulting in signals for further differentiation) and negative selection (resulting in apoptosis) will help solve this fascinating feature of T-lymphocyte biology. We also discuss how non-TCR-derived signaling might serve to fine tune the TCR-driven selection events in thymocytes. Central to this aspect of the conceptual framework needed to explain thymocyte selection is the observation that thymic antigen-presenting cells appear to be specialized in the induction of either positive or negative selection. Finally, we suggest a hypothesis that integrates the facts currently available on developing thymocytes, and which may serve to refine our exploration of unresolved issues in thymocyte selection. This hypothesis expands our focus to include signals from receptors other than TCRs as modulating and amplifying factors in thymocyte signaling.

A potential role for Elf-1 in terminal transferase gene regulation

The terminal deoxynucleotidyltransferase (TdT) gene represents an attractive model for the analysis of gene regulation during an early phase of lymphocyte development. In previous studies, we identified a DNA element, termed D', which is essential for TdT promoter activity in immature lymphocytes, and two classes of D'-binding factors, Ikaros proteins and Ets proteins. Here, we report a detailed mutant analysis of the D' element which suggests that an Ets protein, rather than an Ikaros protein, activates TdT transcription. Since multiple Ets proteins are expressed in developing lymphocytes and are capable of binding to the D' element, DNA affinity chromatography was used to determine if one of the Ets proteins might bind to the D' element with a uniquely high affinity, thereby implicating that protein as a potential TdT activator. Indeed, one binding activity was greatly enriched in the high-salt eluates from a D' affinity column. Peptide microsequencing revealed that the enriched protein was Elf-1. Immunoblot analyses confirmed that in nuclear extracts, Elf-1 has a significantly higher affinity for the D' sequence than does another Ets protein, Ets-1. Transactivation and expression studies support the hypothesis that Elf-1 activates TdT transcription in immature T and B cells. Finally, a D' mutation which selectively reduces Elf-1 binding, but not the binding of other Ets proteins, was found to greatly reduce TdT promoter activity. Although Elf-1 previously had been implicated in the inducible activation of genes in mature T and B cells, our results suggest that it also plays an important role in regulating genes during an early phase of lymphocyte development.