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D R Littman

Publications and source records attributed to D R Littman.

At least 91 records · Page 5Linked to original sources
PubMed1994

A subset of CD4+ thymocytes selected by MHC class I molecules.

To complete their maturation, most immature thymocytes depend on the simultaneous engagement of their antigen receptor [alpha beta T cell receptor (TCR)] and their CD4 or CD8 coreceptors with major histocompatibility complex class II or I ligands, respectively. However, a normal subset of mature alpha beta TCR+ thymocytes did not follow these rules. These thymocytes expressed NK1.1 and a restricted set of alpha beta TCRs that are intrinsically class I-reactive because their positive selection was class I-dependent but CD8-independent. These cells were CD4+ and CD4-8- but never CD8+, because the presence of CD8 caused negative selection. Thus, neither CD4 nor CD8 contributes signals that direct their maturation into the CD4+ and CD4-8- lineages.

Animals↗
PubMed1994

Signal transduction by lymphocyte antigen receptors.

Despite the differences in the antigens that they recognize and in the effector functions they carry out, B and T lymphocytes utilize remarkably similar signal transduction components to initiate responses. They both use oligomeric receptors that contain distinct recognition and signal transduction subunits. Antigen receptors on both cells interact with at least two distinct families of PTKs via common sequence motifs, ARAMs, in the cytoplasmic tails of their invariant chains, which have likely evolved from a common evolutionary precursor. Coreceptors appear to serve to increase the sensitivity of both of these receptor systems through events that influence ligand binding and signal transduction. The critical role of tyrosine phosphorylation of downstream signaling components, such as phospholipase C, is the net result of changes in the balance of the action of antigen receptor-regulated PTKs and PTPases. The identification of downstream effectors, including calcineurin and Ras, that regulate cellular responses, such as lymphokine gene expression, promises the future possibility of connecting the complex pathway from the plasma membrane to the nucleus in lymphocytes. Insight gained from studies of the signaling pathways downstream of TCR and BCR stimulation is likely to contribute significantly to future understanding of mechanisms responsible for lymphocyte differentiation and for the discrimination of self from nonself in developing and mature cells.

Amino Acid Sequence↗
PubMed1994

CD4+ T-lymphocytes are not required for murine resistance to the human filarial parasite, Brugia malayi.

Immunocompetent mice are nonpermissive for the development and maturation of the human filarial parasite, Brugia malayi. We and others have shown that the absence of T-lymphocytes, alone or in combination with B-lymphocytes, renders mice permissive to infection. In a previous study, we showed that mice lacking CD8+ T-lymphocytes are also completely nonpermissive for B. malayi, indicating that CD8+ T-lymphocytes are not an obligate requirement for resistance. In the present study, we have examined the role of CD4+ T-lymphocytes in resistance to filarial infection using two experimental systems. In the first, we used an anti-CD4 monoclonal antibody to deplete CD4+ T-cells in vivo in immunocompetent BALB/c mice. In the second system, we used mutant mice in which the gene encoding the CD4 antigen had been disrupted by homologous recombination, resulting in a lack of CD4+ T-cells. Challenge of either the anti-CD4 antibody depleted BALB/c mice or CD4 knockout mice with B. malayi infective-stage larvae demonstrated that mice lacking CD4+ T-lymphocytes were resistant to infection. These data indicate that CD4+ T-cells are not an obligate requirement for murine resistance to B. malayi.

Animals↗
PubMed1994

Thymocyte lineage commitment: is it instructed or stochastic?

Thymocytes co-expressing the CD4 and CD8 co-receptors differentiate into mature T cells that express either CD4 or CD8 and have helper or cytotoxic functions, respectively. Recent studies indicate that commitment to the CD4+ or CD8+ lineages occurs stochastically, but retention of the appropriate co-receptor is required to complete development.

Animals↗
PubMed1994

Disruption of T lymphocyte positive and negative selection in mice lacking the CD8 beta chain.

The CD4 and CD8 coreceptors have been shown to play significant roles in the differentiation and activation of helper and cytotoxic T lymphocytes (CTLs), respectively. Coordinate binding of coreceptor and T cell receptor (TCR) to the same major histocompatibility complex (MHC) molecule and coreceptor interaction with the tyrosine kinase p56lck are required for effective signaling. Whereas CD4 is a monomer, CD8 consists of either alpha alpha homodimers or alpha beta heterodimers. Signaling properties of CD8 have been ascribed to the alpha chain, which binds to both the MHC class I and to p56lck, respectively. To study CD8 beta specifically, we have generated mice defective in its expression. We observe a significant reduction in the numbers of CD8+ T cells, but these cells have normal CTL activity. By breeding CD8 beta null mice with animals expressing a class I-specific TCR transgene, we show that CD8 beta plays a significant role in both positive and negative selection of developing thymocytes.

Animals↗
PubMed1993

CD4 function in thymocyte differentiation and T cell activation.

The ectodomains of the T cell surface glycoproteins CD4 and CD8 bind to membrane-proximal domains of MHC class II and class I molecules, respectively, while both cytoplasmic domains interact with the protein tyrosine kinase (PTK) p56lck (lck) through a shared cysteine-containing motif. Function of CD4 and CD8 requires their binding to the same MHC molecule as that recognized by the T cell antigen receptor (TCR). In vitro studies indicate that CD4-associated lck functions even in the absence of kinase activity. In vivo experiments show that, whereas helper T cell development is impaired in CD4-deficient mice, high level expression of a transgenic CD4 that cannot bind lck rescues development of this T cell subset. These studies suggest that CD4 is an adhesion molecule whose localization is regulated through protein-protein interactions of the associated PTK and whose function is to increase the stability of the TCR signalling complex by binding to the relevant MHC. The function of CD4 in development has been further studied in the context of how double positive (CD4+CD8+) thymocytes mature into either CD4+ T cells with helper function and TCR specificity for class II or into CD8+ T cells with cytotoxic function and specificity for class I. Studies using CD4-transgenic mice indicate that development of single positive T cells involves stochastic downregulation of either CD4 or CD8, coupled to activation of a cytotoxic or helper program, respectively, and subsequent selection based on the ability of the TCR and remaining co-receptor to engage the same MHC molecule.

Animals↗
PubMed1993

Helper T cells without CD4: control of leishmaniasis in CD4-deficient mice.

Expression of either the CD4 or CD8 glycoproteins discriminates two functionally distinct lineages of T lymphocytes. A null mutation in the gene encoding CD4 impairs the development of the helper cell lineage that is normally defined by CD4 expression. Infection of CD4-null mice with Leishmania has revealed a population of functional helper T cells that develops despite the absence of CD4. These CD8- alpha beta T cell receptor+ T cells are major histocompatibility complex class II-restricted and produce interferon-gamma when challenged with parasite antigens. These results indicate that T lymphocyte lineage commitment and peripheral function need not depend on the function of CD4.

Animals↗
PubMed1993

A kinase-independent function of Lck in potentiating antigen-specific T cell activation.

The lymphocyte-specific cytoplasmic protein-tyrosine kinase p56lck (Lck) is essential for T cell development and activation. Its association with the co-receptor molecules, CD4 and CD8, is required for potentiation of antigen-specific signals through the T cell antigen receptor. To study the mechanism of action of Lck, hybrid molecules consisting of the extracellular and transmembrane domains of CD4 fused to Lck or other Src family kinases were analyzed in an antigen-specific, CD4-dependent T cell hybridoma. Surprisingly, a chimera with a deletion of the Lck kinase domain was more active than the full-length protein. In contrast, point mutations in residues required for SH2 or kinase function resulted in moderately decreased activity, while a combination of these mutations rendered the chimera largely inactive. Different domains of CD4-associated Lck therefore have distinct functions that can independently contribute to T cell activation.

Amino Acid Sequence↗
PubMed1993

Helper T-cell development in the absence of CD4-p56lck association.

The CD4 and CD8 glycoproteins are expressed on helper and cytoxic T lymphocytes, respectively, and have important functions in the differentiation and activation of these cells. These molecules are thought to participate in signal transduction by binding to the same class II or class I major histocompatibility complex molecules that are engaged by the T-cell antigen receptor. The cytoplasmic domains of both CD4 and CD8 interact with the protein tyrosine kinase p56lck (refs 14-17), an essential participant in thymocyte maturation and T-cell activation. This interaction is required for effective in vitro responses to antigen, suggesting that signalling through p56lck is a major function of CD4 and CD8. Here we investigate the role of the CD4-p56lck interaction during T-lymphocyte development by expressing wild-type and truncated products of CD4 transgenes in mice that lack endogenous CD4 and hence have defective helper-cell development. We find that transgenic CD4, which cannot associate with p56lck, can nevertheless rescue the helper-cell lineage when overexpressed. This result indicates that the contribution of CD4 to lineage development need not involve signalling through p56lck, and provides insight into the general function of CD4 and CD8.

Animals↗
PubMed1993

Evidence for a stochastic mechanism in the differentiation of mature subsets of T lymphocytes.

Thymocytes that coexpress the CD4 and CD8 glycoproteins differentiate into mature CD4+ helper or CD8+ cytotoxic cells depending on whether their antigen receptors are specific for MHC class II or class I molecules, respectively. The mechanism of this decision process was investigated in mice whose T cell development was biased toward the class II-specific lineage. We found that constitutive expression of CD4 allows a developmentally arrested population of thymocytes that have mismatched class II-specific TCRs and the CD8 coreceptor to be rescued and to acquire a cytotoxic phenotype. This result is consistent with a two-step process of thymocyte maturation, in which there is stochastic down-regulation of either CD4 or CD8 and subsequent selection based on the ability of the TCR and remaining coreceptor to engage the same MHC molecule.

Animals↗
PubMed1993

Regulated expression of human CD4 rescues helper T cell development in mice lacking expression of endogenous CD4.

During T cell development, precursor thymocytes that co-express the CD4 and CD8 glycoproteins give rise to mature progeny expressing one of these molecules to the exclusion of the other. Continued expression of only CD4 is the hallmark of mature helper T cells, whereas cytotoxic T cells express CD8 and extinguish CD4. The differentiation program that generates the two T cell subsets is likely to be intimately tied to regulation of expression of these cell surface molecules. We now describe the use of a murine CD4 enhancer in the generation of transgenic mice expressing physiologic levels of human CD4. The transgene is appropriately regulated during T cell development and includes the necessary cis-acting sequences for extinguishing expression in the CD8 lineage. Furthermore, in mice whose endogenous CD4 gene is inactivated, the transgenic human CD4 mediates rescue of the CD4 lineage and restoration of normal helper cell functions. The generation of these mice exemplifies a general approach for developing reliable animal models for the human immune system.

Animals↗
PubMed1993

Cell fusion mediated by interaction of a hybrid CD4.CD8 molecule with the human immunodeficiency virus type 1 envelope glycoprotein does occur after a long lag time.

Several domains of CD4 have been suggested to play a critical role in events that follow its binding to the human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (gp120-gp41). It has been reported previously that cells expressing a chimeric molecule consisting of the first 177 residues of human CD4 attached to residues from the hinge, transmembrane, and cytoplasmic domains of human CD8 did not form syncytia with HIV-1-infected cells (L. Poulin, L.A. Evans, S. Tang, A. Barboza, H. Legg, D.R. Littman, and J.A. Levy, J. Virol. 65: 4893-4901, 1991). In contrast, we found that the hybrid CD4.CD8 molecule expressed in human cells did render them susceptible to fusion with cells expressing HIV-1IIIB or HIV-1RF envelope glycoproteins encoded by vaccinia virus recombinants, but only after long lag times. The lag time of membrane fusion mediated by the hybrid CD4.CD8 molecule was fivefold longer than that for the wild-type CD4 molecule. However, the rate of binding to and the affinity of soluble gp120 for membrane-associated CD4.CD8 were the same as for CD4. Both molecules were laterally mobile, as determined by patching experiments. Coexpression of the CD4.CD8 chimera with wild-type CD4 did not lead to interference in fusion but had an additive effect. Therefore, the proximal membrane domains of CD4 play an important role in determining the kinetics of postbinding events leading to membrane fusion. We hypothesize that the long lag time is due to the inability of the CD4.CD8-gp120-gp41 complex to undergo the rapid conformational changes which occur during the fusion mediated by wild-type CD4.

CD4 Antigens↗
PubMed1993

Truncation of the cytoplasmic domain of the simian immunodeficiency virus envelope glycoprotein increases env incorporation into particles and fusogenicity and infectivity.

Growth of macaque simian immunodeficiency virus (SIVmac) in certain cloned human T-cell lines, such as HUT.78, selects for isolates containing a premature stop codon within the cytoplasmic domain of the transmembrane envelope glycoprotein. In contrast, propagation of virus in macaques or in their cultured T cells favors replication of virus containing the full-length envelope glycoprotein. To elucidate the causes of this phenomenon, we used a human immunodeficiency virus pseudotyping system to assess the effects on infectivity of the cytoplasmic domains of envelope glycoproteins obtained from SIVmac1A11 and SIVmac239. These envelopes contain truncated and full-length cytoplasmic domains, respectively. By analyzing human immunodeficiency virus particles containing selectable genes pseudotyped with each glycoprotein or with chimeric derivatives, we found that truncation of the cytoplasmic domain resulted in a significant advantage in viral entry into HUT.78 T cells and CD4+ U87.MG glial cells. Truncation of the cytoplasmic domain significantly enhanced both envelope density on particles and envelope-mediated cell-to-cell fusion. It is likely that one or both of these effects contribute to the observed differences in infectivity and to the selection of virions with short cytoplasmic tails in human T cells.

Amino Acid Sequence↗
PubMed1993

A heterodimer of HEB and an E12-related protein interacts with the CD4 enhancer and regulates its activity in T-cell lines.

A T-lymphocyte-specific enhancer located 13 kb upstream of the murine CD4 gene was recently shown to be required for the developmentally regulated expression of CD4. We have previously identified three nuclear protein binding sites in this enhancer; one of these sites, CD4-3, is essential for expression and contains two E-box core motifs (CANNTG) adjacent to each other in the sequence TAACAGGTGTCAGCTGGT. In electrophoretic mobility shift assays using the CD4-3 oligonucleotide as a probe, three nuclear protein complexes, termed CD4-3A, -B, and -C, were detected with nuclear extracts from T-cell lines. CD4-3A, which involves nuclear protein binding to the 5' E-box, was detected only with nuclear extracts from lymphoid cells. Specific antisera were used to show that the CD4-3A complex contains a heterodimer or heterooligomer of basic helix-loop-helix transcriptional factors, E12 or a related factor and HEB, which is expressed predominantly in thymus. Consistent with this finding, in vitro-translated E12 and HEB proteins, as homodimers or heterodimers, bound preferentially to the 5' E-box. Point mutations in the 5' E-box, but not in the 3' E-box, abolished CD4 enhancer activity. Furthermore, overexpression of Id, a protein that forms inactive heterodimers with E12/E47, blocked CD4 enhancer activity in T cells. These results suggest that a heterodimer composed of HEB and E12 or a closely related protein plays a critical role in CD4 enhancer function by interacting with the 5' E-box motif of the CD4-3 site in vivo.

Amino Acid Sequence↗
PubMed1992

Requirement for CD8-major histocompatibility complex class I interaction in positive and negative selection of developing T cells.

The interaction of the T cell surface glycoprotein CD8 with major histocompatibility complex (MHC) class I molecules on target cells is required for effective T cell activation. Mutations in the alpha 3 domain of the MHC class I molecule can disrupt binding to CD8, yet leave antigen presentation unaffected. Here we show that such a mutation can interfere with positive and negative selection of T cells bearing T cell receptors (TCRs) that interact specifically with the mutant class I molecule. Autoreactive T cells in male mice expressing a transgenic TCR specific for the male antigen H-Y and H-2Db were not deleted in the context of a transgenic Db molecule bearing a mutation at residue 227. Similarly, CD8+ cells were not positively selected in female mice expressing both the TCR and mutant class I transgenes. Endogenous MHC class I molecules were competent to bind CD8, but were unable to rescue the defect, indicating a requirement for coordinate recognition of antigen/MHC by a complex of the TCR and CD8 coreceptor for both positive and negative selection of thymocytes.

Animals↗
PubMed1992

Disruption of CD8-dependent negative and positive selection of thymocytes is correlated with a decreased association between CD8 and the protein tyrosine kinase, p56lck.

The CD4 and CD8 coreceptor molecules on immature thymocytes participate in T cell repertoire selection. To examine more definitively the role of CD4 and CD8 in the negative and positive selection of immature thymocytes, we generated transgenic mice with elevated surface CD4 expression and mated them with mice expressing a transgenic T cell receptor. Augmented CD4 expression was found to markedly alter CD8-dependent negative and positive selection of T cells specific for the male (H-Y) antigen presented by H-2Db major histocompatibility complex class I molecules. Moreover, the cytoplasmic tail of CD4 was essential for effecting these alterations, since the overexpression of tailless CD4 molecules failed to influence the outcome of CD8-dependent selection. The inhibition of positive and negative selection in double-transgenic mice expressing the full-length CD4 molecule was associated with a decreased interaction between the protein tyrosine kinase p56lck and CD8. These results strongly implicate p56lck in T cell repertoire selection.

Animals↗