Gene transfer of the Ly-3 chain gene of the mouse CD8 molecular complex: co-transfer with the Ly-2 polypeptide gene results in detectable cell surface expression of the Ly-3 antigenic determinants

Activation of stably silenced genes by recruitment of a synthetic de-methylating module

Stably silenced genes that display a high level of CpG dinucleotide methylation are refractory to the current generation of dCas9-based activation systems. To counter this, we create an improved activation system by coupling the catalytic domain of DNA demethylating enzyme TET1 with transcriptional activators (TETact). We show that TETact demethylation-coupled activation is able to induce transcription of suppressed genes, both individually and simultaneously in cells, and has utility across a number of cell types. Furthermore, we show that TETact can effectively reactivate embryonic haemoglobin genes in non-erythroid cells. We anticipate that TETact will expand the existing CRISPR toolbox and be valuable for functional studies, genetic screens and potential therapeutics.

Stably silenced genes with methylated CpG at the promoter are refractory to current CRISPR activation systems. Here the authors create a more robust activation system, TETact that recruits DNA-demethylating TET1 with transcriptional activators.

Control of the development of CD8αα+ intestinal intraepithelial lymphocytes by TGF-β

The molecular mechanisms directing the development of TCRαβ⁺CD8αα⁺ intestinal intraepithelial lymphocytes (IEL) are not thoroughly understood. Here we show that transforming growth factor-β (TGF-β) controls the development of TCRαβ⁺CD8αα⁺ IEL. Mice with either a TGF-β1 null mutation or a T cell-specific deletion of the TGF-β receptor I lacked TCRαβ⁺CD8αα⁺ IEL, whereas transgenic mice that over-expressed TGF-β1 had an increased population of TCRαβ⁺CD8αα⁺ IEL. Defective development of the TCRαβ⁺CD8αα⁺ IEL thymic precursors (CD4^-CD8^-TCRαβ⁺CD5⁺) was observed in the absence of TGF-β. In addition, we showed that TGF-β signaling induced CD8α expression in TCRαβ⁺CD8αα⁺ IEL thymic precursors and induced and maintained CD8α expression in peripheral populations of T cells. These data demonstrate a previously unrecognized role for TGF-β in the development of TCRαβ⁺CD8αα⁺ IEL and the expression of CD8 in T cells.

The Cytoplasmic Domain of Igα Is Necessary and Sufficient to Support Efficient Early B Cell Development

The B cell receptor complex (BcR) is essential for normal B lymphocyte function, and surface BcR expression is a crucial checkpoint in B cell development. However, functional requirements for chains of the BcR during development remain controversial. We have used retroviral gene transfer to introduce components of the BcR into chicken B cell precursors during embryonic development. A chimeric heterodimer, in which the cytoplasmic domains of chicken Igalpha and Igbeta are expressed by fusion with the extracellular and transmembrane domains of murine CD8alpha and CD8beta, respectively, targeted the cytoplasmic domains of the BcR to the cell surface in the absence of extracellular BcR domains. Expression of this chimeric heterodimer supported all early stages of embryo B cell development: bursal colonization, clonal expansion, and induction of repertoire diversification by gene conversion. Expression of the cytoplasmic domain of Igalpha, in the absence of the cytoplasmic domain of Igbeta, was not only necessary, but sufficient to support B cell development as efficiently as the endogenous BcR. In contrast, expression of the cytoplasmic domain of Igbeta in the absence of the cytoplasmic domain of Igalpha failed to support B cell development. The ability of the cytoplasmic domain of Igalpha to support early B cell development required a functional Igalpha immunoreceptor tyrosine-based activation motif. These results support a model in which expression of surface IgM following productive V(D)J recombination in developing B cell precursors serves to chaperone the cytoplasmic domain of Igalpha to the B cell surface, thereby initiating subsequent stages of development.

Role of CD8beta domains in CD8 coreceptor function: importance for MHC I binding, signaling, and positive selection of CD8+ T cells in the thymus

The contribution of the CD8beta subunit to CD8 coreceptor function is poorly understood. We now demonstrate that the CD8beta extracellular domain increases the avidity of CD8 binding to MHC I, and that the intracellular domain of CD8beta enhances association with two intracellular molecules required for TCR signal transduction, Lck and LAT. By assessing CD8+ T cell differentiation in CD8beta-deficient mice reconstituted with various transgenic CD8beta chimeric molecules, we also demonstrate that the intracellular and extracellular domains of CD8beta can contribute independently to CD8+ T cell development, but that both CD8beta domains together are most efficient. Thus, this study identifies the molecular functions of the CD8beta intracellular and extracellular domains and documents their contributions to CD8+ T cell development.

Human CD8 beta, but not mouse CD8 beta, can be expressed in the absence of CD8 alpha as a beta beta homodimer

The T cell coreceptor CD8 exists on mature T cells as disulfide-linked homodimers of CD8 alpha polypeptide chains and heterodimers of CD8 alpha- and CD8 beta-chains. The function of the CD8 alpha-chain for binding to MHC class I and associating with the tyrosine kinase p56lck was demonstrated with CD8 alpha alpha homodimers. CD8 alpha beta functions as a better coreceptor, but the actual function of CD8 beta is less clear. Addressing this issue has been hampered by the apparent inability of CD8 beta to be expressed without CD8 alpha. This study demonstrates that human, but not mouse, CD8 beta can be expressed on the cell surface without CD8 alpha in both transfected COS-7 cells and murine lymphocytes. By creating chimeric proteins, we show that the murine Ig domain of CD8 beta is responsible for the lack of expression of murine CD8 beta beta dimers. In contrast to CD8 alpha alpha, CD8 beta beta is unable to bind MHC class I in a cell-cell adhesion assay. Detection of this form of CD8 should facilitate studies on the function of the CD8 beta-chain and indicates that caution should be used when interpreting studies on CD8 function using chimeric protein with the murine CD8 beta beta Ig domain. In addition, we demonstrate that the Ig domains of CD8 alpha are also involved in controlling the ability of CD8 to be expressed. Mutation of B- and F-strand cysteine residues in CD8 alpha reduced the ability of the protein to fold properly and, therefore, to be expressed.

CD8 expression allows T cell signaling by monomeric peptide-MHC complexes

Physiologically, TCR signaling is unlikely to result from the cross-linking of TCR-CD3 complexes, given the low density of specific peptide-MHC complexes on antigen-presenting cells. We therefore have tested directly an alternative model for antigen recognition. We show that monomers of soluble peptide-MHC trigger Ca2+ responses in CD8alphabeta+ T cells. This response is not observed in CD8- T cells and when either the CD8:MHC or CD8:Lck interactions are prevented. This demonstrates that an intact CD8 coreceptor is necessary for effective TCR signaling in response to monomeric peptide-MHC molecules. We propose that this heterodimerization of TCR and CD8 by peptide-MHC corresponds to the physiological event normally involved during antigen-specific signal transduction.

The Cytoplasmic Domain of CD8β Regulates Lck Kinase Activation and CD8 T Cell Development

Previous studies have shown that CD8β plays a role in both enhancing CD8α-associated Lck kinase activity and promoting the development of CD8-lineage T cells. To examine the role of this enhancement in the maturation of CD8-lineage cells, we assessed CD8α-associated Lck kinase activity in both T cell hybridomas and thymocytes of mice expressing CD8β mutations known to impair CD8 T cell development. Lack of CD8β expression or expression of a cytoplasmic domain-deleted CD8β resulted in a severalfold reduction in CD8α-associated Lck kinase activity compared with that observed with cells expressing wild-type CD8β chain. This analysis indicated a critical role for the cytoplasmic domain of CD8β in the regulation of CD8α-associated Lck activity. Decreased CD8α-associated Lck activity observed with the various CD8β mutations also correlated with diminished in vivo cellular tyrosine phosphorylation. In addition, analysis of CD8β mutant mice (CD8β−/− or cytoplasmic domain-deleted CD8β transgenic) indicated that the degree of reduction in CD8α-associated Lck activity associated with each mutation correlated with the severity of developmental impairment. These results support the importance of CD8β-mediated enhancement of CD8α-associated Lck kinase activity in the differentiation of CD8 single-positive thymocytes.

Regulatory role of CD8 in major histocompatibility complex-unrestricted tumoricidal activity of mouse T cells activated with anti-CD3 monoclonal antibody

Antigen-nonspecific CD8+ cytotoxic T cells induced with anti-CD3 monoclonal antibody (mAb) are able to kill tumor cells in a major histocompatibility complex (MHC)-unrestricted fashion. However, the role of CD8 in the MHC-independent tumoricidal activity of anti-CD3-activated killer T (AK-T) cells has not been investigated. Here we show that anti-CD8 alpha mAb inhibits, in a dose-dependent fashion, lysis of P815 and YAC-1 tumor cells by mouse AK-T cells. The inhibition of MHC-unrestricted cytotoxicity by anti-CD8 alpha mAb cannot be attributed to interference with an adhesion-like function of CD8 towards class I MHC molecules on the target cells because anti-CD8 alpha mAb (i) had equal inhibitory effects on the cytolysis of tumor target cells regardless of their relative level of class I MHC molecule expression and (ii) did not interfere with the formation of conjugates between AK-T cells and class I MHC-bearing P815 tumor cells. However, anti-CD8 alpha mAb abrogated AK-T cell granule exocytosis in the presence of P815 tumor cells, indicating a regulatory role for CD8 in the signal transduction events which result in lysis of the tumor target cells. Immunoblot analysis of the post-nuclear fraction of lysates from AK-T cells exposed to P815 tumor cells in the presence of anti-CD8 alpha mAb revealed reduced phosphorylation of tyrosine residues on a protein with an Mr of approximately 62 kDa. Taken together, these data suggest that CD8 is able to affect the tumoricidal activity of MHC-unrestricted AK-T cells independent of class I MHC molecules on the target cell.

The CD8 beta polypeptide is required for the recognition of an altered peptide ligand as an agonist

T cell activation is triggered by the specific recognition of cognate peptides presented by MHC molecules. Altered peptide ligands are analogs of cognate peptides which have a high affinity for MHC molecules. Some of them induce complete T cell responses, i.e. they act as agonists, whereas others behave as partial agonists or even as antagonists. Here, we analyzed both early (intracellular Ca2+ mobilization), and late (interleukin-2 production) signal transduction events induced by a cognate peptide or a corresponding altered peptide ligand using T cell hybridomas expressing or not the CD8 alpha and beta chains. With a video imaging system, we showed that the intracellular Ca2+ response to an altered peptide ligand induces the appearance of a characteristic sustained intracellular Ca2+ concentration gradient which can be detected shortly after T cell interaction with antigen-presenting cells. We also provide evidence that the same altered peptide ligand can be seen either as an agonist or a partial agonist, depending on the presence of CD8beta in the CD8 co-receptor dimers expressed at the T cell surface.

Molecular cloning of the feline CD8 beta-chain

Human mouse and rat CD8 have been described as being disulphide-linked heterodimers of alpha and beta chains. More recently the chicken alpha and beta chains were described. In the bovine and feline immune system only the z-chain was reported. In this study we have cloned and determined the nucleotide sequence of a cDNA encoding the beta-chain of the feline T-cell surface antigen CD8. Using a nested polymerase chain reaction- (PCR) and two primer pairs designed from the human CD8 beta cDNA nucleotide sequence, we amplified a 430 base pair fragment from a feline thymus cDNA library which was used as a probe for screening the feline library at high stringency. After three rounds of screening, five clones were isolated. A clone, named FTb-6, containing a 3.8 kilobase pair insert was mapped, sequenced and compared with the published sequences of the genes encoding the human, mouse, rat and avian CD8 beta. We have determined the primary structure of the feline CD8 beta. The feline CD8 beta has an open reading frame, 630 nucleotides in length encoding a protein with 210 amino acid residues and its composition showed that the feline molecule is a member of the immunoglobulin gene super family.

The peripheral T cell repertoire: independent homeostatic regulation of virgin and activated CD8+ T cell pools

Mature T cells may be produced in the thymus, or by expansion in the periphery. While thymus output of virgin cells ensures repertoire diversity, peripheral expansion increases the size of rare clones, and thus the efficiency of immune responses. We studied the role of both phenomena in the generation of the CD8+ T cell pool using RAG-/- female mice expressing a transgenic T cell receptor specific for the male antigen; nude mice injected with peripheral T cells; and euthymic irradiated chimeras injected with bone marrow and mature T cells. Our results show that the total number of virgin and activated T cells, each constituting about half of the peripheral T cell pool, was regulated independently, revealing an efficient mechanism to maintain repertoire diversity while optimizing the immune response.

CD8 beta chain influences CD8 alpha chain-associated Lck kinase activity

The CD8 molecule plays an important role in the differentiation of CD8+ T cells in the thymus and in their normal function in the periphery. CD8 exists on the cell surface in two forms, the alpha alpha homodimer and the alpha beta heterodimer. Recent studies indicate an important role for the CD8 beta chain in thymic development of CD8+ T cells and suggest that signaling via CD8 alpha beta may be distinct from CD8 alpha alpha. To better understand these differences, we introduced the CD8 beta gene into a T cell hybridoma which only expressed the CD8 alpha alpha homodimer. In the parent hybridoma, cross-linking of the CD8 alpha chain led to minimal enhancement of CD8-associated Lck tyrosine kinase activity. In the CD8 beta+ transfectants, several observations suggested that CD8 beta modifies CD8 alpha-associated Lck tyrosine kinase activity: (a) in in vitro kinase assays, antibody-mediated crosslinking of CD8 alone, or CD8 cross-linking with the TCR, resulted in 10-fold greater activation of Lck kinase activity, compared to cells expressing CD8 alpha alpha alone; (b) in vivo, markedly enhanced tyrosine phosphorylation of several intracellular proteins was observed upon CD8 cross-linking with the TCR in CD8 alpha beta-expressing cells, compared to cells expressing CD8 alpha alpha alone; and (c) Lck association with CD8 alpha was stabilized by the coexpression of CD8 beta. Thus, the differential Lck kinase activation and tyrosine phosphorylation seen with CD8 alpha alpha vs. CD8 alpha beta may reflect the unique signaling capabilities of the CD8 beta molecule. These differences in signaling may, in part, account for the diminished ability to generate CD8 single positive thymocytes in mice bearing a homozygous disruption of the CD8 beta gene.

Reduced thymic maturation but normal effector function of CD8+ T cells in CD8 beta gene-targeted mice

CD8 is a cell surface glycoprotein on major histocompatibility complex class I-restricted T cells. Thymocytes and most peripheral T cells express CD8 as heterodimers of CD8 alpha and CD8 beta. The intestinal intraepithelial lymphocytes (IEL), which have been suggested to be generated extrathymically, express CD8 predominantly as homodimers of CD8 alpha. We have generated CD8 beta gene-targeted mice. CD8 alpha+ T cell population in the thymus and in most peripheral lymphoid organs was reduced to 20-30% of that in wild-type littermates. CD8 alpha expression on thymocytes and peripheral T cells also decreased to 44 and 53% of the normal levels, respectively. In contrast, neither the population size nor the CD8 alpha expression level of CD8 alpha+ IEL was reduced. This finding indicates that CD8 beta is important only for thymic-derived CD8+ T cells. The lack of CD8 beta reduces but does not completely abolish thymic maturation of CD8+ T cells. Our result also reveals the role of CD8 beta in regulating CD8 alpha expression on thymic derived T cells. Peripheral T cells in these mice were efficient in cytotoxic activity against lymphocytic choriomeningitis virus and vesicular stomatitis virus, suggesting that CD8 beta is not essential for the effector function of CD8+ T cells.

Requirement for CD8 beta chain in positive selection of CD8-lineage T cells

CD8 is either an alpha alpha homodimer or an alpha beta heterodimer, although most peripheral CD8-lineage T cells express only the CD8 alpha beta heterodimer. The physiological function of CD8 beta was elucidated with mice that were chimeric for the homozygous disruption of the CD8 beta gene. The CD8 beta-1- T cells developed normally to CD4+CD8+ stage, but did not efficiently differentiate further, which resulted in few peripheral CD8+ T cells. The number of peripheral CD8+ T cells was restored by transfer of an exogenous CD8 beta gene into CD8 beta-deficient T cells. Thus, CD8 beta is necessary for the maturation of CD8+ T cells.

The lack of CD8 alpha cytoplasmic domain resulted in a dramatic decrease in efficiency in thymic maturation but only a moderate reduction in cytotoxic function of CD8+ T lymphocytes

The glycoprotein CD8 is believed to play an important role in the maturation and function of MHC class I-restricted T lymphocytes. CD8 has been proposed to function as a co-receptor of the TcR to participate in signal transduction, possibly through its cytoplasmic domain that binds to protein tyrosine kinase p56lck. A T cell-specific transgene encoding CD8 alpha truncated at the cytoplasmic domain ("tailless CD8 alpha"), was introduced into CD8 alpha-deficient mice. This animal model was used to study the role of the CD8 cytoplasmic domain in T cell ontogeny and function. "Tailless CD8 alpha" was expressed on the cell surface of thymocytes and peripheral T cells. A small population of peripheral CD4- T cells (6% of T lymphocytes) was found to have cell surface expression of "tailless CD8 alpha" and endogenous CD8 beta, indicating that these cells may belong to the CD8+ T cell lineage. A consistent result was obtained from CD8 alpha-deficient mice bearing the "tailless CD8 alpha" and the MHC class I-restricted 2C TcR transgenes. A small population of CD4- T cells expressing CD8 beta, the "tailless CD8 alpha" and the 2C TcR transgenes was present in the periphery of these mice in a selecting background, but was absent in a deleting background. When "tailless CD8 alpha" mice were infected with lymphocytic choriomeningitis virus (LCMV), the peripheral CD8+ CD4- T cell subset expanded dramatically and a significant LCMV-specific cytolytic activity was detected. The results suggest that the cytoplasmic portion of CD8 alpha is not absolutely required but dramatically enhances the efficiency of thymic maturation of CD8+ T cells. The lack of CD8 alpha cytoplasmic domain in peripheral CD8+ T cells does not abolish the generation of cytotoxicity in response to an in vivo LCMV infection, although the cytolytic activity is slightly reduced compared to that in control mice.

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.

Evolutionary relationships of the classes of major histocompatibility complex genes

A number of hypotheses have been proposed to account for the evolutionary origin of the classes of major histocompatibility complex (MHC) genes of vertebrates. According to one hypothesis the class II MHC evolved first, whereas another hypothesis holds that the class I MHC originated first as a result of a recombination between an immunoglobulin-like C-domain and the peptide-binding domain of an HSP70 heat-shock protein. A phylogenetic tree of C-domains from MHC and related molecules supports a relationship between the class II MHC alpha chain and beta 2-microglobulin and between the class II MHC beta-chain and the class I alpha chain. If this phylogeny is correct, the hypothesis that the class I MHC evolved by recombination with HSP70 is less parsimonious than the hypothesis that class II evolved first. Furthermore, when MHC peptide-binding domains are simultaneously aligned with HSP70 domains and with V-domains from members of the immunoglobulin superfamily, they are slightly more similar to the latter than to the former; and the class II alpha 1 and beta 1 domains show much greater similarity to each other than would be expected if they evolved from separate HSP70 domains. Thus, most evidence supports the hypothesis that the ancestral MHC molecule had a class II-like structure.

Presence of CD8 alpha-CD8 beta-positive TcR gamma/delta thymocytes in the fetal murine thymus and their in vitro expansion with interleukin-7

Several groups have described that a low percentage of in vitro cultured T cell receptor (TcR) gamma/delta cells express CD8. Contrary to TcR alpha/beta cells, however, CD8 on these TcR gamma/delta cells was shown to be a CD8 alpha homodimer. We describe here that addition of interleukin-7 (IL-7) to a short-term in vitro culture of fetal day 14 thymic lobes in an organ culture system or of fetal day 18 fetal thymocytes in cell suspension yields CD8 beta-positive TcR gamma/delta cells. This is not the result of IL-7-induced expression of CD8 beta on previously CD8 beta-negative cells. It is due to IL-7-induced expansion of CD8 alpha-CD8 beta-positive TcR gamma/delta cells which are shown to be present in the starting fetal thymocyte cell population.

Activation of Lyt-2 associated with distant upstream insertion of an SL3-3 provirus

Two Lyt-2+ mutants of the T-cell lymphoma SL12.4.10 were selected by fluorescence activated cell sorting. Both mutants expressed Lyt-2 (CD8 alpha-chain) but not Lyt-3 (CD8 beta-chain). Derivatives of one Lyt-2+ mutant that expressed Lyt-3 could be isolated by sorting for Lyt-3+ cells. Southern blotting analysis indicated that both mutants had structural rearrangements within or immediately 3' of the Lyt-3 gene, accompanied by demethylation of at least one Hpa II site within the Lyt-2 gene. Gene cloning analysis of one mutant demonstrated that the structural rearrangement was due to insertion of an SL3-3 provirus 35 kb 5' to the Lyt-2 gene. It is likely that Lyt-2 gene activation is a direct or indirect consequence of proviral insertion at this site.

CD8 surface levels alter the fate of alpha/beta T cell receptor-expressing thymocytes in transgenic mice

The mature T cell receptor (TCR) repertoire is established on the basis of discriminative events involving binding of the TCR alpha and beta chains and CD4 or CD8 on immature thymocytes to major histocompatibility complex (MHC)/self-peptide complexes expressed in the thymus. To ask whether the strength of the interaction between a CD8/TCR complex and a MHC/self-peptide ligand plays a pivotal role in deciding the fate of a maturing thymocyte, we generated lines of transgenic mice that express distinct and elevated levels of CD8 alpha, approximately 2, 3, and 6-10 times. These lines were then crossed to a transgenic line expressing the class I-restricted TCR, 2C. We found that thymocytes expressing the 2C TCR in combination with the highest levels of CD8 were deleted on the H-2 Kb background that is normally positively selecting for the 2C TCR. In contrast, thymocytes coexpressing the 2C TCR and moderately elevated levels of CD8 were selected for maturation. These results demonstrate directly that CD8 levels can affect the developmental fate of a maturing thymocyte and argue in support of an affinity model for thymocyte selection.

Generation of anti-human CD8 beta-specific antibodies using transfectants expressing mixed-species CD8 heterodimers

The CD8 glycoprotein is a lymphocyte differentiation antigen comprised of two distinct polypeptide chains, alpha and beta, which have the capacity to form homodimeric (CD8 alpha/alpha) or heterodimeric (CD8 alpha/beta) cell surface complexes. The majority of monoclonal antibodies which recognize the human CD8 antigen react with the CD8 alpha chain, while a single mAb, referred to as T8/2T8-5H7 (or 2ST8-5H7), has been identified which binds to the CD8 alpha/beta heterodimer. In order to generate antibodies specific for CD8 beta, murine fibroblast transfectants were constructed which express the human CD8 beta chain in combination with either the human CD8 alpha chain or the murine CD8 alpha homologue, the Lyt-2 molecule. These transfectants were used to raise polyclonal heteroantisera in mice. Transfectants expressing human CD8 alpha/beta heterodimers induced moderate anti-CD8 alpha titers, but were weakly effective in generating anti-CD8 beta titers, despite high level cell surface expression of this protein. In contrast, transfectants expressing mixed-species CD8 heterodimers (murine CD8 alpha and human CD8 beta) induced high anti-CD8 beta titers in immunized mice. Following fusion of splenocytes from mice immunized with mixed-species CD8 transfectants, the mAb 5F2 was isolated which specifically recognizes the human CD8 beta chain. Unlike T8/2T8-5H7, the mAb 5F2 can bind the CD8 beta chain irrespective of its pairing partner, and can immunoprecipitate the CD8 beta protein from cells transfected with the CD8 beta gene in the absence of the human or mouse CD8 alpha gene product. Anti-CD8 beta antibodies should help elucidate the extent of biochemical heterogeneity of the CD8 beta protein, and will also be useful in defining the role of the CD8 beta protein in thymocyte and lymphocyte development, recognition and activation.

CD8 is needed for development of cytotoxic T cells but not helper T cells

A mutant mouse strain without CD8 (Lyt-2 and Lyt-3) expression on the cell surface has been generated by disrupting the Lyt-2 gene using embryonic stem cell technology. In these mice, CD8+ T lymphocytes are not present in peripheral lymphoid organs, but the CD4+ T lymphocyte population seems to be unaltered. Cytotoxic response of T lymphocytes from these mice against alloantigens and viral antigens is dramatically decreased. Proliferative response against alloantigens and in vivo help to B lymphocytes, however, are not affected. These data suggest that CD8 is necessary for the maturation and positive selection of class I MHC restricted cytotoxic T lymphocytes but is not required on any of the intermediate thymocyte populations (CD8+CD4-TcR- or CD4+CD8+TcRlow) during the development of functional class II MHC restricted helper T cells.

The V beta repertoire of mouse gut homodimeric alpha CD8+ intraepithelial T cell receptor alpha/beta + lymphocytes reveals a major extrathymic pathway of T cell differentiation

Gut intraepithelial lymphocytes (IEL) contain two independent T cell receptor alpha/beta + T cell populations, with different V beta repertoires. In DBA/2 mice (Mlsa, IE+), the CD4+ and heterodimeric alpha/beta CD8+ thymodependent T cell pool shows the same deletion of V beta 6, 8.1, and 11+ cells as found in peripheral lymphoid organs. In contrast, such deletions are not observed in the pool of IEL bearing homodimeric alpha CD8+ chains, in which these V beta families are frequently observed in high amounts. The size of this gut homodimeric alpha CD8+ IEL pool and its different V beta repertoire selection demonstrate the existence of a major extrathymic pathway of T cell differentiation with a gut-restricted localization. The large amount of the thymo-independent, homodimeric alpha CD8+ IEL found in the small bowel may contribute to a first line of defense against exogenous superantigens.

Two gut intraepithelial CD8+ lymphocyte populations with different T cell receptors: a role for the gut epithelium in T cell differentiation

Mouse gut intraepithelial lymphocytes (IEL) consist mainly (90%) of two populations of CD8+ T cells. One bears heterodimeric alpha/beta CD8 chains (Lyt-2+, Lyt-3+), a T cell receptor (TCR) made of alpha/beta chains, and is Thy-1+; it represents the progeny of T blasts elicited in Peyer's patches by antigenic stimulation. The other bears homodimeric alpha/alpha CD8+ chains, contains no beta chain mRNA, and is mostly Thy-1- and TCR-gamma/delta + or -alpha/beta +; it is thymo-independent and does not require antigenic stimulation, as shown by its presence: (a) in nude and scid mice; (b) in irradiated and thymectomized mice repopulated by T-depleted bone marrow cells bearing an identifiable marker; (c) in thymectomized mice treated by injections of monoclonal anti-CD8 antibody, which lead to total depletion of peripheral CD8+ T lymphocytes; and (d) in germ-free mice and in suckling mice. In young nude mice, alpha/alpha CD8 chains, CD3-TCR complexes, and TCR mRNAs (first gamma/delta) are found on IEL, while they are not detectable on or in peripheral or circulating lymphocytes or bone marrow cells. IEL, in contrast to mature T cells, contain mRNA for the RAG protein, which is required for the rearrangement of TCR and Ig genes. We propose that the gut epithelium (an endoderm derivative, as the thymic epithelium) has an inductive property, attracting progenitors of bone marrow origin, and triggering their TCR rearrangement and alpha/alpha CD8 chains expression, thus giving rise to a T cell population that appears to belong to the same lineage as gamma/delta thymocytes and to recognize an antigenic repertoire different from that of alpha/beta CD8+ IEL.

Selective expression of CD8 alpha (Ly-2) subunit on activated thymic gamma/delta cells

Although most cells of the T cell receptor (TcR) gamma/delta lineage are CD4-CD8-, some gamma/delta cells express CD8. We show here that mouse thymic gamma/delta cells express CD8 following in vitro activation by concanavalin A. Staining with monoclonal and polyclonal antibodies to the CD8 alpha (Ly-2) and CD8 beta (Ly-3) subunits indicates that only CD8 alpha is expressed by these activated TcR gamma/delta cells. Furthermore, immunoprecipitation and reduced/nonreduced gel analysis reveals that thymic gamma/delta cells express CD8 alpha as a homodimer. In contrast, similarly activated TcR alpha/beta cells express a conventional CD8 alpha/CD8 beta heterodimer.

A signaling role for the cytoplasmic segment of the CD8 alpha chain detected under limiting stimulatory conditions

To test for the functional importance of the cytoplasmic segment of the CD8 molecule, a mouse T-cell hybridoma expressing a T-cell receptor specific for the class I major histocompatibility complex product H-2Kb was transfected with a set of CD8 alpha-chain (Ly-2) and/or beta-chain (Ly-3) genes encoding polypeptides with carboxyl-terminal truncations or substitutions. When challenged with Kb-positive splenocytes, transfectants expressing Ly-2 homodimers that lacked cytoplasmic tails responded nearly as effectively as wild-type Ly-2 transfectants. However in marked contrast to the wild-type Ly-2 transfectants, tailless Ly-2 transfectants were greatly impaired in their ability to respond to Kb-transfected L cells. Coexpression of the Ly-3 gene did not restore this impaired response. The unique functional property of the Ly-2 alpha cytoplasmic segment was further supported by the analysis of a chimeric Ly-3 subunit in which the cytoplasmic segment was replaced by the one from the Ly-2 alpha subunit. When associated with a soluble Ly-2 subunit lacking a transmembrane segment, the chimeric Ly-3 was indeed sufficient to restore the response to Kb-transfected L cells. Since the lateral mobility of the tailless Ly-2 molecules on the cell surface was nearly identical to that of the wild-type Ly-2 molecules, their partially impaired function may indicate that they have lost their cis-acting signaling properties but retained their ability to bind class I products of the major histocompatibility complex.

Differential expression and regulation of the human CD8 alpha and CD8 beta chains

The CD8 glycoprotein is expressed by thymocytes, mature T cells and natural killer (NK) cells and has been implicated in the recognition of monomorphic determinants on major histocompatibility complex (MHC) Class I antigens, and in signal transduction during the course of T-cell activation. Both human and rodent CD8 antigens are comprised of two distinct polypeptide chains, alpha and beta. The majority of monoclonal antibodies (mAb) reactive with the human CD8 antigen bind the CD8 alpha chain, while a single mAb, T8/2T8-5H7, has been identified which binds to the CD8 alpha/beta heterodimer. While the two chains of CD8 have been presumed to be coordinately expressed in normal T cells, this is not always the case. Northern blot analysis of a panel of T-cell leukemias and normal cells demonstrate that CD8 alpha and CD8 beta are not invariably co-transcribed and phenotypic analysis of fresh and interleukin 2 (IL-2) expanded peripheral blood mononuclear cells (PBMC) confirm that the CD8 alpha and CD8 beta chains are differentially expressed at the cell surface. Four distinct subpopulations of CD8+ cells have been identified based on the expression of CD8 alpha/alpha or CD8 alpha/beta complexes: (1) T-cell receptor (TcR) alpha beta+ T cells which are CD8 alpha+/beta+; (2) TcR alpha beta+ T cells which are CD8 alpha+/beta-; (3) TcR gamma delta+ T cells which are CD8 alpha+/beta- and (4) natural killer (NK) cells which are CD8 alpha+/beta-. We also demonstrate the down-regulation of the CD8 alpha/beta heterodimers from the surface of a CD8+ T-cell clone following treatment with phorbol myristate acetate (PMA) while CD8 alpha/alpha homodimers remain on the cell surface. This observation demonstrates that a) a CD8+ T-cell clone can express both CD8 alpha/alpha homodimers and CD8 alpha/beta heterodimers and b) these two complexes do not have identical biological properties. Together, these data suggest that CD8 alpha/alpha and CD8 alpha/beta dimers may not subserve identical functions. The differential contribution of these two CD8 complexes should be considered in models of T-cell-mediated cytotoxicity and T-cell activation.

The human Lyt-3 molecule requires CD8 for cell surface expression

We have previously identified a monoclonal antibody, T8/2T8-5H7, which clustered serologically with CD8 monoclonal antibodies, but lacked reactivity with L cell transfectants expressing the human CD8 molecule (Lyt-2 homologue). Based on these observations, we postulated that T8/2T8-5H7 might recognize the human Lyt-3 gene product. To test this hypothesis, we have isolated a full-length cDNA encoding the human Lyt-3 molecule and have characterized its product in additional transfection experiments. The results of these studies indicate that the human Lyt-3 cDNA encodes a product recognized by the antibody T8/2T8-5H7. Interestingly, the human Lyt-3 molecule cannot be expressed alone, but requires the human Lyt-2 homologue for efficient cell surface expression. A heterodimer composed of the human Lyt-2 and Lyt-3 molecules may have importance in T cell-target cell interactions.