CD4 function in thymocyte differentiation and T cell activation

Modeling the effects of drugs of abuse on within-host dynamics of two HIV species

Injection drug use is one of the most significant risk factors associated with contracting human immunodeficiency virus (HIV), and drug users infected with HIV suffer from a higher viral load and rapid disease progression. While replication of HIV may result in many mutant viruses that can escape recognition of the host's immune response, the presence of morphine (a drug of abuse) can decrease the viral mutation rate and cellular immune responses. This study develops a mathematical model to explore the effects of morphine-altered mutation and cellular immune response on the within-host dynamics of two HIV species, a wild-type and a mutant. Our model predicts that the morphine-altered mutation rate and cellular immune response allow the wild-type virus to outcompete the mutant virus, resulting in a higher set point viral load and lower CD4 count. We also compute the basic reproduction numbers and show that the dominant species is determined by morphine concentration, with the mutant dominating below and the wild-type dominating above a threshold. Furthermore, we identified three biologically relevant equilibria, infection-free, mutant-only, and coexistence, which are completely characterized by the fitness cost of mutation, mutant escape rate, and morphine concentration.

Small Molecule Cyclotriazadisulfonamide Abrogates the Upregulation of the Human Receptors CD4 and 4-1BB and Suppresses In Vitro Activation and Proliferation of T Lymphocytes

The small molecule cyclotriazadisulfonamide (CADA) down-modulates the human CD4 receptor, an important factor in T cell activation. Here, we addressed the immunosuppressive potential of CADA using different activation models. CADA inhibited lymphocyte proliferation with low cellular toxicity in a mixed lymphocyte reaction, and when human PBMCs were stimulated with CD3/CD28 beads, phytohemagglutinin or anti-CD3 antibodies. The immunosuppressive effect of CADA involved both CD4⁺ and CD8⁺ T cells but was, surprisingly, most prominent in the CD8⁺ T cell subpopulation where it inhibited cell-mediated lympholysis. Immunosuppression by CADA was characterized by suppressed secretion of various cytokines, and reduced CD25, phosphoSTAT5 and CTPS-1 levels. We discovered a direct down-modulatory effect of CADA on 4-1BB (CD137) expression, a survival factor for activated CD8⁺ T cells. More specifically, CADA blocked 4‑1BB protein biosynthesis by inhibition of its co-translational translocation into the ER in a signal peptide-dependent way. Taken together, this study demonstrates that CADA, as potent down-modulator of human CD4 and 4‑1BB receptor, has promising immunomodulatory characteristics. This would open up new avenues toward chemotherapeutics that act as selective protein down-modulators to treat various human immunological disorders.

The Quest for the Best: How TCR Affinity, Avidity, and Functional Avidity Affect TCR-Engineered T-Cell Antitumor Responses

Over the past decades, adoptive transfer of T cells has revolutionized cancer immunotherapy. In particular, T-cell receptor (TCR) engineering of T cells has marked important milestones in developing more precise and personalized cancer immunotherapies. However, to get the most benefit out of this approach, understanding the role that TCR affinity, avidity, and functional avidity play on how TCRs and T cells function in the context of tumor-associated antigen (TAA) recognition is vital to keep generating improved adoptive T-cell therapies. Aside from TCR-related parameters, other critical factors that govern T-cell activation are the effect of TCR co-receptors on TCR–peptide-major histocompatibility complex (pMHC) stabilization and TCR signaling, tumor epitope density, and TCR expression levels in TCR-engineered T cells. In this review, we describe the key aspects governing TCR specificity, T-cell activation, and how these concepts can be applied to cancer-specific TCR redirection of T cells.

Correlation analyses of CpG island methylation of cluster of differentiation 4 protein with gene expression and T lymphocyte subpopulation traits

Objective

Cluster of differentiation 4 protein (CD4) gene is an important immune related gene which plays a significant role in T cell development and host resistance during viral infection.

Methods

In order to unravel the relationship of CpG island methylation level of CD4 gene with its gene expression and T lymphocyte subpopulation traits, we used one typical Chinese indigenous breed (Dapulian, DP) and one commercial breed (Landrace), then predicted the CpG island of CD4 gene, determined the methylation status of CpG sites by bisulfite sequencing polymerase chain reaction (BSP), and carried out the correlation analyses of methylation frequencies of CpG sites with mRNA expression and T lymphocyte subpopulation traits.

Results

There was one CpG island predicted in the upstream −2 kb region and exon one of porcine CD4 gene, which located 333 bp upstream from the start site of gene and contained nine CpG sites. The correlation analysis results indicated that the methylation frequency of CpG_2 significantly correlated with CD4 mRNA expression in the DP and Landrace combined population, though it did not reach significance level in DP and Landrace separately. Additionally, 15 potential binding transcription factors (TFs) were predicted within the CpG island, and one of them (Jumonji) contained CpG_2 site, suggesting that it may influence the CD4 gene expression through the potential binding TFs. We also found methylation frequency of CpG_2 negatively correlated with T lymphocyte subpopulation traits CD4+CD8−CD3−, CD4−CD8+CD3− and CD4+/CD8+, and positively correlated with CD4−CD8+CD3+ and CD4+CD8+CD3+ (for all correlation, p<0.01) in DP and Landrace combined population. Thus, the CpG_2 was a critical methylation site for porcine CD4 gene expression and T lymphocyte subpopulation traits.

Conclusion

We speculated that increased methylation frequency of CpG_2 may lead to the decreased expression of CD4, which may have some kind of influence on T lymphocyte subpopulation traits and the immunity of DP population.

Generating Transgenic Mouse Models for Studying Celiac Disease

This chapter provides a brief overview of current animal models for studying celiac disease, with a focus on generating HLA transgenic mouse models. Human Leukocyte Antigen class II molecules have been a particular target for transgenic mice due to their tight association with celiac disease, and a number of murine models have been developed which had the endogenous MHC class II genes replaced with insertions of disease susceptible HLA class II alleles DQ2 or DQ8. Additionally, transgenic mice that overexpress interleukin-15 (IL-15), a key player in the inflammatory cascade that leads to celiac disease, have also been generated to model a state of chronic inflammation. To explore the contribution of specific bacteria in gluten-sensitive enteropathy, the nude mouse and rat models have been studied in germ-free facilities. These reductionist mouse models allow us to address single factors thought to have crucial roles in celiac disease. No single model has incorporated all of the multiple factors that make up celiac disease. Rather, these mouse models can allow the functional interrogation of specific components of the many stages of, and contributions to, the pathogenic mechanisms that will lead to gluten-dependent enteropathy. Overall, the tools for animal studies in celiac disease are many and varied, and provide ample space for further creativity as well as to characterize the complete and complex pathogenesis of celiac disease.

Multifunctional double-negative T cells in sooty mangabeys mediate T-helper functions irrespective of SIV infection

Studying SIV infection of natural host monkey species, such as sooty mangabeys, has provided insights into the immune changes associated with these nonprogressive infections. Mangabeys maintain immune health despite high viremia or the dramatic CD4 T cell depletion that can occur following multitropic SIV infection. Here we evaluate double-negative (DN)(CD3+CD4−CD8−) T cells that are resistant to SIV infection due to a lack of CD4 surface expression, for their potential to fulfill a role as helper T cells. We first determined that DN T cells are polyclonal and predominantly exhibit an effector memory phenotype (CD95+CD62L−). Microarray analysis of TCR (anti-CD3/CD28) stimulated DN T cells indicated that these cells are multifunctional and upregulate genes with marked similarity to CD4 T cells, such as immune genes associated with Th1 (IFNγ), Th2 (IL4, IL5, IL13, CD40L), Th17 (IL17, IL22) and T_FH (IL21, ICOS, IL6) function, chemokines such as CXCL9 and CXCL10 and transcription factors known to be actively regulated in CD4 T cells. Multifunctional T-helper cell responses were maintained in DN T cells from uninfected and SIV infected mangabeys and persisted in mangabeys exhibiting SIV mediated CD4 loss. Interestingly, TCR stimulation of DN T cells from SIV infected mangabeys results in a decreased upregulation of IFNγ and increased IL5 and IL13 expression compared to uninfected mangabeys. Evaluation of proliferative capacity of DN T cells in vivo (BrDU labeling) indicated that these cells maintain their ability to proliferate despite SIV infection, and express the homeostatic cytokine receptors CD25 (IL2 receptor) and CD127 (IL7 receptor). This study identifies the potential for a CD4-negative T cell subset that is refractory to SIV infection to perform T-helper functions in mangabeys and suggests that immune therapeutics designed to increase DN T cell function during HIV infection may have beneficial effects for the host immune system.

SIV infection of sooty mangabeys is generally characterized by maintained CD4 T cell levels and a lack of disease progression despite active virus replication. We have previously shown however, that dramatic loss of CD4 T cells can occur during SIV infection of mangabeys. This study investigates the potential for double negative (DN) T cells (which lack CD4 and CD8, and are refractory to SIV/HIV infection) to perform helper T cell functions. In our study, sooty mangabey DN T cells exhibited a memory phenotype and a diverse repertoire in their T cell receptors. Once stimulated, the DN T cells expressed multiple cytokines, indicating that they have the potential to function as helper T cells (a function normally undertaken by CD4+ T cells). In SIV infected mangabeys, DN T cells continue to function, proliferate in vivo, and maintain expression of homeostatic cytokine receptors on their surface. It is therefore likely that DN T cells have the potential to compensate for the loss of CD4 T cells during SIV infection. These studies indicate that increasing DN T cell levels and/or function during pathogenic HIV infection may provide one tangible component of a functional cure, and inhibit progression to clinical disease and AIDS

A genome-wide detection of copy number variations using SNP genotyping arrays in swine

Background

Copy Number Variations (CNVs) have been shown important in both normal phenotypic variability and disease susceptibility, and are increasingly accepted as another important source of genetic variation complementary to single nucleotide polymorphism (SNP). Comprehensive identification and cataloging of pig CNVs would be of benefit to the functional analyses of genome variation.

Results

In this study, we performed a genome-wide CNV detection based on the Porcine SNP60 genotyping data of 474 pigs from three pure breed populations (Yorkshire, Landrace and Songliao Black) and one Duroc × Erhualian crossbred population. A total of 382 CNV regions (CNVRs) across genome were identified, which cover 95.76Mb of the pig genome and correspond to 4.23% of the autosomal genome sequence. The length of these CNVRs ranged from 5.03 to 2,702.7kb with an average of 250.7kb, and the frequencies of them varied from 0.42 to 20.87%. These CNVRs contains 1468 annotated genes, which possess a great variety of molecular functions, making them a promising resource for exploring the genetic basis of phenotypic variation within and among breeds. To confirmation of these findings, 18 CNVRs representing different predicted status and frequencies were chosen for validation via quantitative real time PCR (qPCR). Accordingly, 12 (66.67%) of them was successfully confirmed.

Conclusions

Our results demonstrated that currently available Porcine SNP60 BeadChip can be used to capture CNVs efficiently. Our study firstly provides a comprehensive map of copy number variation in the pig genome, which would be of help for understanding the pig genome and provide preliminary foundation for investigating the association between various phenotypes and CNVs.

Double-negative T cells during HIV/SIV infections: potential pinch hitters in the T-cell lineup

PURPOSE OF REVIEW

This review summarizes the role of CD3+CD4-CD8- double-negative T cells, which have both regulatory and helper T-cell functions and may have the potential to compensate for the reduced levels of CD4 T cells during SIV/HIV infection.

RECENT FINDINGS

Double-negative T cells have been characterized in several human diseases and in murine models of autoimmunity and transplantation, where they exhibit both immunoregulatory and helper T-cell-like function. During the natural nonpathogenic SIV infection of African nonhuman primates, the lack of clinical disease progression is associated with the presence of double-negative T cells that maintain helper T-cell functions while remaining refractory to viral infection. Moreover, DN T cells may compensate for very low levels of CD4+ T cells observed in a cohort of SIV-infected sooty mangabeys that have remained free of clinical AIDS for over 10 years. These studies identify a potential for double-negative T cells to provide critical helper function during HIV infection.

SUMMARY

Double-negative T cells with some CD4+ T-cell functions are associated with a nonpathogenic outcome during SIV infection and represent a potential immune therapeutic target in HIV-infected patients.

Th2-polarised PrP-specific transgenic T-cells confer partial protection against murine scrapie

Several hurdles must be overcome in order to achieve efficient and safe immunotherapy against conformational neurodegenerative diseases. In prion diseases, the main difficulty is that the prion protein is tolerated as a self protein, which prevents powerful immune responses. Passive antibody therapy is effective only during early, asymptomatic disease, well before diagnosis is made. If efficient immunotherapy of prion diseases is to be achieved, it is crucial to understand precisely how immune tolerance against the prion protein can be overcome and which effector pathways may delay disease progression. To this end, we generated a transgenic mouse that expresses the ß-chain of a T cell receptor recognizing a PrP epitope presented by the class II major histocompatibility complex. The fact that the constraint is applied to only one TCR chain allows adaptation of the other chain according to the presence or absence of tolerogenic PrP. We first show that transgene-bearing T cells, pairing with rearranged α-chains conferring anti-PrP specificity, are systematically eliminated during ontogeny in PrP+ mice, suggesting that precursors with good functional avidity are rare in a normal individual. Second, we show that transgene-bearing T cells with anti-PrP specificity are not suppressed when transferred into PrP+ recipients and proliferate more extensively in a prion-infected host. Finally, such T cells provide protection through a cell-mediated pathway involving IL-4 production. These findings support the idea that cell-mediated immunity in neurodegenerative conditions may not be necessarily detrimental and may even contribute, when properly controlled, to the resolution of pathological processes.

It is generally accepted that prion-specific antibodies can protect against mouse scrapie infection. However, passive antibody therapy is limited to the lymphoinvasion stage of the disease. Active immunization has been attempted but the results have been disappointing. There is therefore a need for developing analytical models that will allow a fine dissection of the immune mechanisms at play in prion diseases and help distinguish between protective effects mediated by B cells and antibodies, and the effect of T cells. The aim of our study was to thoroughly examine T cell tolerance to the prion protein and to evaluate whether a pure specific population of T cells adoptively transferred to a normal host could proliferate and confer protection against scrapie. We designed a transgenic mouse in which the majority of T lymphocytes recognize the prion protein. Our key findings are that prion-specific T cells remain functional when transferred to normal recipients, even more so when the host is infected with scrapie, and confer partial protection against the disease by slowing down prion replication, in complete absence of anti-prion antibodies. Anti-prion T cells may therefore be considered as a therapeutic tool in the future.

Impact of the TCR signal on regulatory T cell homeostasis, function, and trafficking

Signaling through the T cell antigen receptor (TCR) is important for the homeostasis of naïve and memory CD4⁺ T cells. The significance of TCR signaling in regulatory T (Treg) cells has not been systematically addressed. Using an Ox40-cre allele that is prominently expressed in Treg cells, and a conditional null allele of the gene encoding p56^Lck, we have examined the importance of TCR signaling in Treg cells. Inactivation of p56^Lck resulted in abnormal Treg homeostasis characterized by impaired turnover, preferential redistribution to the lymph nodes, loss of suppressive function, and striking changes in gene expression. Abnormal Treg cell homeostasis and function did not reflect the involvement of p56^Lck in CD4 function because these effects were not observed when CD4 expression was inactivated by Ox40-cre.The results make clear multiple aspects of Treg cell homeostasis and phenotype that are dependent on a sustained capacity to signal through the TCR.

Profiling CD antigens on leukaemias with an antibody microarray

Cluster of differentiation (CD) antigens are defined when a surface molecule found on some members of a standard panel of human cells reacts with at least one novel antibody, and there is good accompanying molecular data. Monoclonal antibodies to surface CD antigens on leukocytes have been used for flow cytometry, and more recently to construct microarrays that capture live cells. These DotScan microarrays enable the rapid and highly parallel characterization of repertoires of CD antigens whose expression patterns may be correlated with discrete leukaemia subtypes, or used to define biomarker 'signatures' for non-hematological diseases. DotScan with fluorescence multiplexing enables profiling of CD antigens for minor subsets of cells, such as colorectal cancer cells and tumour-infiltrating lymphocytes from a surgical sample.

Regulated movement of CD4 in and out of the immunological synapse

The mechanism underlying the transient accumulation of CD4 at the immunological synapse (IS) and its significance for T cell activation are not understood. To investigate these issues, we mutated a serine phosphorylation site (S408) in the cytoplasmic tail of murine CD4. Preventing phosphorylation of S408 did not block CD4 recruitment to the IS; rather, it blocked the ability of CD4 to leave the IS. Surprisingly, enhanced and prolonged CD4 accumulation at the supramolecular activation cluster in the contact area had no functional consequence for T cell activation, cytokine production, or proliferation. Protein kinase C theta (PKCtheta)-deficient T cells also displayed enhanced and prolonged accumulation of wild-type CD4 at the IS, indicating that theta is the critical PKC isoform involved in CD4 movement. These findings suggest a model wherein recruitment of CD4 to the IS allows its phosphorylation by PKCtheta and subsequent removal from the IS. Thus, an important role for PKCtheta in T cell activation involves its recruitment to the IS, where it phosphorylates specific substrates that help to maintain the dynamism of protein turnover at the IS.

Transcriptional regulation of CD4 gene expression by T cell factor-1/beta-catenin pathway

By interacting with MHC class II molecules, CD4 facilitates lineage development as well as activation of Th cells. Expression of physiological levels of CD4 requires a proximal CD4 enhancer to stimulate basic CD4 promoter activity. T cell factor (TCF)-1/beta-catenin pathway has previously been shown to regulate thymocyte survival via up-regulating antiapoptotic molecule Bcl-xL. By both loss and gain of function studies, in this study we show additional function of TCF-1/beta-catenin pathway in the regulation of CD4 expression in vivo. Mice deficient in TCF-1 displayed significantly reduced protein and mRNA levels of CD4 in CD4+ CD8+ double-positive (DP) thymocytes. A transgene encoding Bcl-2 restored survival but not CD4 levels of TCF-1(-/-) DP cells. Thus, TCF-1-regulated survival and CD4 expression are two separate events. In contrast, CD4 levels were restored on DP TCF-1(-/-) cells by transgenic expression of a wild-type TCF-1, but not a truncated TCF-1 that lacks a domain required for interacting with beta-catenin. Furthermore, forced expression of a stabilized beta-catenin, a coactivator of TCF-1, resulted in up-regulation of CD4. TCF-1 or stabilized beta-catenin greatly stimulated activity of a CD4 reporter gene driven by a basic CD4 promoter and the CD4 enhancer. However, mutation of a potential TCF binding site located within the enhancer abrogated TCF-1 and beta-catenin-mediated activation of CD4 reporter. Finally, recruitment of TCF-1 to CD4 enhancer was detected in wild-type but not TCF-1 null mice by chromatin-immunoprecipitation analysis. Thus, our results demonstrated that TCF/beta-catenin pathway enhances CD4 expression in vivo by recruiting TCF-1 to stimulate CD4 enhancer activity.

Evolution of the thymus size in response to physiological and random events throughout life

During embryogenesis and in the early stages of life, the thymus is a crucial organ for the generation of the T cell repertoire. T cells are generated from hematopoietic stem cells already differentiated to precursor T cells in the bone marrow. These cells enter the thymus guided by chemotactic factors secreted by this organ. The complex maturation process takes place that ensures self-tolerance and homeostasis. Thymocytes that show autoreactivity do not leave the thymus, but rather die by apoptosis. The final percentage of mature T cells that survive to migrate from the thymus to the periphery is very low: at most 5%, under optimal conditions. The highest migration occurs in childhood and adulthood, at least in mice and humans; however, it declines throughout life and is minimal in the elderly. Under normal circumstances, the thymus commences involution soon after birth, and this involution correlates with the capacity to export mature T cells to the periphery. Hormones, cytokines, and neurotransmitters all play a role in this age-associated process, but the reasons for and mechanisms of this involution remain unknown. Apart from physiological conditions that change throughout life and govern age-related thymus evolution, random states and events provoked by intrinsic or extrinsic factors can induce either thymus involution, as in reversible transient thymic hypoplasias, or thymic hyperplasias. The age-associated involution, unlike transient involutions, follows a regular pattern for all individuals, though there are clear differences between the sexes. Nevertheless, even the age-associated involution seems to be reversible, raising the possibility of therapeutic strategies aimed at enhancing thymus function in the elderly.

Loss of Ly-6A.2 expression on immature developing T cells in the thymus is necessary for their normal growth and generation of the Vbeta T-cell repertoire

Stage-specific expression of a number of cell-surface and signaling proteins is critical for normal development of T cells in the thymus. Equally important may be the loss of expression/signaling of developmentally regulated proteins for proper transitioning of developing T cells into thymic subsets. Ly-6A.2 exhibits a regulated pattern of expression on T cells maturing in the thymus, and dysregulating its expression results in arrest of developing T cells within the CD3-CD4-CD8- triple negative (TN) stage where the normal expression of Ly-6A.2 is extinguished. To further characterize the mechanisms underlying this block, we examined whether cell signaling and/or cell adhesion properties of the Ly-6A.2 molecule influenced the block in T-cell development. Analysis of bone marrow chimeras generated by injecting CFSE-labeled Ly-6A.2 transgenic bone marrow cells into irradiated syngeneic non-transgenic mice revealed normal trafficking of developing T cells from the cortex into the medulla. Production of LAT but not p56lck was diminished in CD4-CD8- DN cells from Ly-6A.2 dysregulated mice when compared with control littermates. Dysregulated expression of Ly-6A.2 did not suppress endogenous TCR-Vbeta expression. Finally, dysregulated expression of Ly-6A.2 enhanced apoptosis of an immature CD4+CD8+ (DP) subset of developing cells and altered the selected TCR-Vbeta repertoire. Taken together, these observations indicate that the termination of Ly-6A.2 expression and signaling within the CD4-CD8-CD3- subset of developing T cells is an important checkpoint during normal thymic development.

CD4 promotes breadth in the TCR repertoire

A diverse population of MHC class II-restricted CD4 lineage T cells develops in mice that lack expression of the CD4 molecule. In this study, we show that the TCR repertoire selected in the absence of CD4 is distinct, but still overlapping in its properties with that selected in the presence of CD4. Immunization of mice lacking CD4 caused the clonal expansion of T cells that showed less breadth in the range of Ag-binding properties exhibited by their TCRs. Specifically, the CD4-deficient Ag-specific TCR repertoire was depleted of TCRs that demonstrated low-affinity binding to their ligands. The data thus suggest a key role for CD4 in broadening the TCR repertoire by potentiating productive TCR signaling and clonal expansion in response to the engagement of low-affinity antigenic ligands.

Regulatory CD4(+) T cells expressing endogenous T cell receptor chains protect myelin basic protein-specific transgenic mice from spontaneous autoimmune encephalomyelitis

The development of T cell-mediated autoimmune diseases hinges on the balance between effector and regulatory mechanisms. Using two transgenic mouse lines expressing identical myelin basic protein (MBP)-specific T cell receptor (TCR) genes, we have previously shown that mice bearing exclusively MBP-specific T cells (designated T/R-) spontaneously develop experimental autoimmune encephalomyelitis (EAE), whereas mice bearing MBP-specific T cells as well as other lymphocytes (designated T/R+) did not. Here we demonstrate that T/R- mice can be protected from EAE by the early transfer of total splenocytes or purified CD4(+) T cells from normal donors. Moreover, whereas T/R+ mice crossed with B cell-deficient, gamma/delta T cell-deficient, or major histocompatibility complex class I-deficient mice did not develop EAE spontaneously, T/R+ mice crossed with TCR-alpha and -beta knockout mice developed EAE with the same incidence and severity as T/R- mice. In addition, MBP-specific transgenic mice that lack only endogenous TCR-alpha chains developed EAE with high incidence but reduced severity. Surprisingly, two-thirds of MBP-specific transgenic mice lacking only endogenous TCR-beta chains also developed EAE, suggesting that in T/R+ mice, cells with high protective activity escape TCR-beta chain allelic exclusion. Our study identifies CD4(+) T cells bearing endogenous alpha and beta TCR chains as the lymphocytes that prevent spontaneous EAE in T/R+ mice.

Sequestration of CD4-Associated Lck from the TCR Complex May Elicit T Cell Hyporesponsiveness in Nonobese Diabetic Mice

The Lck protein tyrosine kinase associates noncovalently with the cytoplasmic domain of CD4. Upon ligand engagement of the TCR, CD4-associated Lck is rapidly activated and recruited to the TCR complex. Coupling of this complex to an intracellular signaling pathway may result in T cell proliferation. Previously, we reported that thymocytes from nonobese diabetic (NOD) mice (≥6 wk of age) exhibit a proliferative hyporesponsiveness after TCR stimulation, which is associated with defective TCR-mediated signaling along the protein kinase C/Ras/mitogen-activated protein kinase pathway of T cell activation. Here, we investigated whether differential association of Lck with TCR or CD4 mediates the control of NOD thymocyte hyporesponsiveness. We demonstrate that less CD4-associated Lck is recruited to the TCR in activated NOD thymocytes than in control thymocytes. This CD4-mediated sequestration of Lck from the TCR correlates with the increased binding of CD4-associated Lck through its Src homology 2 domain to free TCRζ and CD3γε chains on the plasma membrane. Sequestration of Lck by CD4 does not occur in activated thymocytes from 3-wk-old NOD mice and is only apparent in thymocytes from NOD mice &gt;5 to 6 wk of age. This diminished recruitment of CD4-associated Lck to the TCR is not mediated by an increase in the amount of CD8-associated Lck. Thus, impaired recruitment of CD4-associated Lck to the TCR complex may represent an early event that results in deficient coupling of the TCR complex to downstream signaling events and gives rise to NOD thymocyte hyporesponsiveness.

The function of the CD4 coreceptor in the development of T cells

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.

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.