Inducible functions in hybrids of a Lyt-2+ BW5147 transfectant and the 2C CTL line

MHC class Ib-restricted CD8 T cells differ in dependence on CD4 T cell help and CD28 costimulation over the course of mouse polyomavirus infection

We recently identified a protective MHC class Ib-restricted CD8 T cell response to infection with mouse polyomavirus. These CD8 T cells recognize a peptide from aa 139-147 of the VP2 viral capsid protein bound to the nonpolymorphic H-2Q9 molecule, a member of the Qa-2 family of β(2)m-associated MHC class Ib molecules. Q9:VP2.139-specific CD8 T cells exhibit an unusual inflationary response characterized by a gradual expansion over 3 mo followed by a stable maintenance phase. We previously demonstrated that Q9:VP2.139-specific CD8 T cells are dependent on Ag for expansion, but not for long-term maintenance. In this study, we tested the hypothesis that the expansion and maintenance components of the Q9:VP2.139-specific T cell response are differentially dependent on CD4 T cell help and CD28 costimulation. Depletion of CD4(+) cells and CD28/CD40L blockade impaired expansion of Q9:VP2.139-specific CD8 T cells, and intrinsic CD28 signaling was sufficient for expansion. In contrast, CD4 T cell insufficiency, but not CD28/CD40L blockade, resulted in a decline in frequency of Q9:VP2.139-specific CD8 T cells during the maintenance phase. These results indicate that the Q9:VP2.139-specific CD8 T cell response to mouse polyomavirus infection depends on CD4 T cell help and CD28 costimulation for inflationary expansion, but only on CD4 T cell help for maintenance.

Polyomavirus-infected dendritic cells induce antiviral CD8(+) T lymphocytes

CD8(+) T cells are critical for the clearance of acute polyomavirus infection and the prevention of polyomavirus-induced tumors, but the antigen-presenting cell(s) involved in generating polyomavirus-specific CD8(+) T cells have not been defined. We investigated whether dendritic cells and macrophages are permissive for polyomavirus infection and examined their potential for inducing antiviral CD8(+) T cells. Although dendritic cells and macrophages both supported productive polyomavirus infection, dendritic cells were markedly more efficient at presenting the immunodominant viral epitope to CD8(+) T cells. Additionally, infected dendritic cells, but not infected macrophages, primed anti-polyomavirus CD8(+) T cells in vivo. Treatment with Flt3 ligand, a hematopoietic growth factor that dramatically expands the number of dendritic cells, markedly enhanced the magnitude of virus-specific CD8(+) T-cell responses during acute infection and the pool of memory anti-polyomavirus CD8(+) T cells. These findings suggest that virus-infected dendritic cells induce polyomavirus-specific CD8(+) T cells in vivo and raise the potential for their use as cellular adjuvants to promote CD8(+) T cell surveillance against polyomavirus-induced tumors.

Replication timing of CD4 and CD8 in single-positive peripheral blood lymphocytes

The regulatory events leading to the mutually exclusive expression of CD4 and CD8 on peripheral lymphocytes are not fully understood. In particular, the association between DNA replication timing and transcriptional activity of these genes has not been previously investigated. Here, the replication kinetics of the CD4 and CD8 loci in mature single-positive T-cell populations have been examined using a novel approach to the separation of CD4(+) or CD8(+) lymphocytes into discrete cell cycle fractions and a competitive PCR replication timing assay. While the timing of replication of each of these loci is independent of their expression in mature CD4 or CD8 single positive T-cells, the replication of CD8, but not of CD4, shifts to a later time in S phase in transcriptionally silent HS68 fibroblast cells. These findings suggest that changes in DNA replication timing are associated with the developmentally regulated but not with the tissue-specific expression of CD4 and CD8.

The regulation of CD4 and CD8 coreceptor gene expression during T cell development

The two major subsets of T lymphocytes in the peripheral immune system, the helper and cytotoxic T cells, are defined by their expression of either the CD4 or the CD8 glycoproteins, respectively. Expression of these molecules, which serve as coreceptors by interacting specifically with either MHC class II or class I molecules, also defines discrete stages of T cell development within the thymus. Thus, CD4+ and CD8+ single-positive (SP) thymocytes arise from common progenitor double positive (DP) cells that express both CD4 and CD8, during a process known as positive selection. The molecular mechanisms underlying the developmental choice toward the helper or cytotoxic lineage remain poorly understood. Because regulation of coreceptor gene expression appears to be coupled to the phenotypic choice of the differentiating T cell, it is likely that shared signaling pathways direct CD4 and CD8 transcription and the development of an uncommited DP thymocyte toward either the helper or cytotoxic lineage. Therefore, an understanding of how CD4 and CD8 expression is regulated will not only provide insights into transcriptional control mechanisms in T cells, but may also result in the identification of molecular factors that are involved in lineage choices during T cell development. In this review, we summarize recent progress that has been made toward an understanding of how CD4 and CD8 gene expression is regulated.

Hierarchical interactions of control elements determine CD8alpha gene expression in subsets of thymocytes and peripheral T cells

CD4 and CD8 are crucial for the development and function of T cells. An intergenic deoxyribonuclease I hypersensitive site region (cluster CIII) directs expression in mature CD8 T cells only. Here, we show that two further independent regions from the CD8 gene locus in conjunction with cluster CIII restore transgene expression in appropriate immature thymocytes. Deletion of two of the intergenic cluster CIII DNaseI-HSS in homozygous mutant mice affects expression of CD8alphaalpha homodimers on intraepithelial T cells (IEL), particularly on the gammadeltaTCR+ subset. Surprisingly, none of the thymocyte or peripheral alphabetaTCR T cell subsets are affected by this mutation, indicating hierarchical activation of these elements within the different T cell subsets.

Transporters associated with antigen processing (TAP)-independent presentation of soluble insulin to alpha/beta T cells by the class Ib gene product, Qa-1(b)

T cell hybridomas isolated from nonresponder H-2(b) mice immunized with pork insulin were stimulated by insulin in the presence of major histocompatibility complex (MHC)-unmatched antigen presenting cells. The restriction element used by these CD4(-) T cells was mapped to an oligomorphic MHC class Ib protein encoded in the T region and identified as Qa-1(b) using transfectants. The antigenic determinant was localized to the insulin B chain, and experiments with truncated peptides suggested that it is unexpectedly long, comprising most or all of the 30 amino acid B chain. The antigen processing pathway used to present insulin to the Qa-1(b)- restricted T cells does not require transporters associated with antigen processing (TAP), and it is inhibited by chloroquine. A wide variety of cell lines from different tissues efficiently present soluble insulin to Qa-1(b)-restricted T cells, and insulin presentation is not enhanced by phagocytic stimuli. Our results demonstrate that Qa-1(b) can function to present exogenous protein to T cells in a manner similar to MHC class II molecules. Therefore, this class Ib protein may have access to a novel antigen processing pathway that is not available to class Ia molecules.

Interaction of the nuclear matrix-associated region (MAR)-binding proteins, SATB1 and CDP/Cux, with a MAR element (L2a) in an upstream regulatory region of the mouse CD8a gene

Matrix-associated regions (MARs), AT-rich DNA segments that have an affinity for the nuclear matrix, have been shown to play a role in transcriptional regulation of eukaryotic genes. The present study demonstrates that a DNA element, called L2a, which has been implicated in the transcriptional regulation of the mouse CD8a gene encoding an important T cell coreceptor, is a MAR. Moreover, the identities of two nuclear proteins, L2a-P1 and L2a-P2, previously shown to bind to the L2a element, have been determined. The L2a-P1 protein found to be present in all CD8-positive T cell lines tested is SATB1, a known MAR-binding protein. The widely expressed L2a-P2 protein is CDP/Cux, a MAR-binding protein that has been associated with repression of gene transcription. Interaction of both proteins with the L2a element was studied using the missing nucleoside approach, DNase I footprinting, and electrophoretic mobility shift assays with wild type and mutant L2a elements. The data suggest that CDP/Cux bound to the L2a element is displaced by binding of SATB1 and the accompanying conformational change in the DNA lying between the primary binding sites of SATB1 and CDP/Cux. We suggest that displacement of CDP/Cux by SATB1 favors transcription of the CD8a gene, possibly by enhancing or altering its association with the nuclear matrix.

Generation of novel killer hybridomas derived from proliferation-suppressed somatic cell hybrids between YACUT T cell lymphoma and normal lymphocytes activated in secondary mixed lymphocyte cultures

Somatic cell hybridization between the YACUT T cell lymphoma cell line with normal lymphocytes activated in secondary mixed lymphocyte cultures (MLCs) consistently yielded IL-2-dependent CD4- CD8 alpha+ beta- Fc gamma RIII+ hybrids with cytotoxic function. The hybrids expressed T cell receptors other than that of YACUT origin, and fusion of the YACUT with a CD8 alpha+ beta+ Fc gamma RIII- T cell line also yielded hybrids with an unexpected CD8 alpha+ beta- Fc gamma RIII+ phenotype, which two observations strongly suggested that CD8+ T cells became the parental cell of the hybrids. Prolonged growth of the hybrids with IL-2 resulted in the generation of autonomously growing hybrids (hybridomas) without abrogating the cytotoxic function. The hybridomas exhibited MHC-unrestricted cytotoxicity in a Ca(2+)-dependent manner without prior stimulation and also mediated antibody-dependent cellular cytotoxicity. These results indicate that novel killer hybridomas can be produced following cell transformation of proliferation-suppressed cytotoxic YACUT x MLC cell hybrids. The killer hybridomas may be of value for analyzing recognition mechanisms and molecules involved in MHC-unrestricted cell-mediated cytotoxicity.

Bop: a new T-cell-restricted gene located upstream of and opposite to mouse CD8b

In the course of transient expression studies undertaken to determine the location of the mouse CD8b gene promoter, two additional promoter activities were detected within 600 nucleotides upstream of the gene. One activity directs transcription in the same direction as CD8b but fails to transcribe the CAT reporter gene due to an apparent transcription-blocking element lying between it and the gene. The second activity directs transcription opposite to that of the CD8b gene. Northern hybridization with a probe consisting of nucleotides -875 to -550 relative to the site of CD8b transcription initiation revealed hybridizing species of 4 kilobases (kb) and 1.8 kb in poly-A-selected RNA from mouse thymus but not from any other tissues. Similar RNA species were detected in poly-A+ RNA from concanavalin A-stimulated spleen cells and several long-term CTL lines but not from the EL4 or BW5147 T-cell lines or the J558L myeloma. The mRNA species were most abundant in cells of a secondary mixed leukocyte culture which were greater than 95% CD8(+). Northern hybridizations using single-stranded unidirectional probes indicated that these mRNAs represent transcription opposite to the CD8b gene. The tissue and cell type distribution of this newly-discovered gene (designated Bop for CD8b opposite) are consistent with T-cell-specific and possibly CD8-positive T-cell-specific expression. The head-to-head arrangement of the Bop and CD8b genes is reminiscent of the arrangement of the Tap1 and Lmp2 genes, and the expression of the Bop gene in CD8-positive cells raises the possibility that these genes are involved in the same functional pathway.

The cysteine residues in the cytoplasmic tail of CD8 alpha are required for its coreceptor function

The cytoplasmic segment of the CD8 alpha polypeptide includes both a cysteine-containing motif that is required for its association with the tyrosine kinase p56lck, and two serine residues which are likely to be phosphorylated and involved in inside-out signaling phenomena. To determine the relative importance of these residues for CD8 function, a mouse T cell hybridoma expressing a T cell receptor specific for the class I major histocompatibility product H-2Kb was transfected with a set of CD8 alpha chain genes encoding polypeptides in which the cytoplasmic cysteine or serine residues were substituted with alanine. When challenged with Kb-transfected L cells, T cell transfectants expressing CD8 alpha beta or CD8 alpha alpha dimers with substituted cytoplasmic serine residues responded nearly as well as wild-type CD8 transfectants. In marked contrast, the CD8 alpha polypeptides bearing substitutions of both cytoplasmic cysteine residues were totally impaired in their ability to complement the co-expressed T cell receptor.