Faculty Bibliography

T cells with helper activity can be found in mice that lack expression of the CD4 glycoprotein. The CD4 promoter is active in these cells; they respond to antigens presented by MHC class II molecules; they do not express CD8 and they do not depend on MHC class I for their development. By such criteria, these CD8- T cells resemble normal CD4+ helper T cells. The development of the helper lineage in CD4-null mice can be potentiated by expression of transgenes that encode either wild type CD4, or a deletion mutant of CD4 that lacks the cytoplasmic tail and therefore cannot interact with the tyrosine kinase p56lck. These observations suggest that CD4 is not absolutely required for the specification of the helper cell lineage. The role of the CD4 molecule in the development of T cells and possible mechanisms by which it achieves its functions are discussed

The lymphoid-specific Src family protein tyrosine kinase p56lck (Lck) is non-covalently associated with the cytoplasmic tail of CD4 and has an essential role in T cell activation. Engagement of ligand by the T cell antigen receptor (TCR) is followed by rapid tyrosine phosphorylation of several cellular proteins, including phospholipase C gamma 1 (PLC) and the TCR-associated CD3 zeta polypeptides. Tyrosine phosphorylation of PLC gamma 1 results in activation of PLC and subsequent phosphatidylinositol turnover. We have studied the effects of the CD4-associated Lck molecule on TCR-mediated activation of the protein tyrosine kinase (PTK) pathway in a murine T cell hybridoma. Antibodies against CD3 elicited the expected PTK activation, which was enhanced upon co-cross-linking of CD4. In contrast, anti-TCR-alpha beta antibodies had no effect on the PTK pathway unless CD4 was co-cross-linked. Antibody cross-linking of CD4 alone failed to induce the same pattern of tyrosine phosphorylation. Similar results were obtained when a chimeric protein consisting of the extracellular and transmembrane domains of CD4 linked to the intracellular Lck molecule was used in place of CD4. The tyrosine kinase activity of Lck was essential for the activity of the chimeric protein. Cross-linking of the CD4/Lck chimera to a CD8/zeta chimeric molecule also facilitated induction of the PTK pathway with anti-CD8 antibodies. Moreover, the interaction of the two chimeric proteins, either in vitro or in vivo, resulted in tyrosine phosphorylation of CD8/zeta. The effects of CD4/Lck on tyrosine phosphorylation and activation of PLC correlated well with the effects on PTK activation. Our results suggest that the Lck molecule positively regulates the TCR-coupled PTK pathway by phosphorylating tyrosines on the TCR-associated CD3 zeta polypeptides

The nef genes of HIV and SIV are dispensable in vitro, but are essential for viral spread and disease progression in vivo. Nef-induced down-regulation of CD4, the viral receptor, may be the key to this requirement

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

During development of T lymphocytes, differential regulation of expression of the CD4 and CD8 glycoproteins is coupled to the choice of one of two pathways of differentiation. Thymocytes that express both of these coreceptors commit to either the helper lineage, shutting off CD8, or the cytotoxic lineage, shutting off CD4. We have used transgenic mice to identify an intronic regulatory region that controls CD4 gene expression during development. This region selectively extinguishes transgene expression in CD4-CD8+, but not CD4+CD8- nor CD4+CD8+ T cells. It also represses gene expression in CD4-CD8- immature thymocytes, indicating that the CD4 gene is derepressed during differentiation from the CD4-CD8- to the CD4+CD8+ stage. The negative element(s) is both orientation and position independent and acts also on heterologous regulatory sequences. Its properties are functionally similar to those of silencers described in yeast and in Drosophila, suggesting that we have identified a developmentally regulated vertebrate transcriptional silencer

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

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

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

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