Thymic skewing of the CD4/CD8 ratio maps with the T-cell receptor alpha-chain locus

Evolutionary trade-offs constraining the MHC gene expansion: beyond simple TCR depletion model

The immune system is as much shaped by the pressure of pathogens as it is by evolutionary trade-offs that constrain its structure and function. A perfect example comes from the major histocompatibility complex (MHC), molecules that initiate adaptive immune response by presentation of foreign antigens to T cells. The remarkable, population-level polymorphism of MHC genes is assumed to result mainly from a co-evolutionary arms race between hosts and pathogens, while the limited, within-individual number of functional MHC loci is thought to be the consequence of an evolutionary trade-off between enhanced pathogen recognition and excessive T cell depletion during negative selection in the thymus. Certain mathematical models and infection studies suggest that an intermediate individual MHC diversity would thus be optimal. A recent, more direct test of this hypothesis has shown that the effects of MHC diversity on T-cell receptor (TCR) repertoires may differ between MHC classes, supporting the depletion model only for MHC class I. Here, we used the bank vole (Myodes=Cletronomys glareolus), a rodent species with variable numbers of expressed MHC genes, to test how an individual MHC diversity influences the proportions and TCR repertoires of responding T cell subsets. We found a non-linear relationship between MHC diversity and T cell proportions (with intermediate MHC numbers coinciding with the largest T cell proportions), perhaps reflecting an optimality effect of balanced positive and negative thymic selection. The association was strongest for the relationship between MHC class I and splenic CD8+ T cells. The CD8+ TCR richness alone was unaffected by MHC class I diversity, suggesting that MHC class I expansion may be limited by decreasing T cell counts, rather than by direct depletion of TCR richness. In contrast, CD4+ TCR richness was positively correlated with MHC class II diversity, arguing against a universal TCR depletion. It also suggests that different evolutionary forces or trade-offs may limit the within-individual expansion of the MHC class II loci.

The H2-A Class II molecule α/β-chain cis-mismatch severely affects cell surface expression, selection of conventional CD4+ T cells and protection against TB infection

Introduction

To dissect the role of the part of the H2 complex comprised of the MHC-II genes in the control of tuberculosis (TB) infection, we previously established a panel of recombinant congenic mouse strains bearing different segments of the H2 ^j haplotype on the B6 (H2 ^b) genetic background. Fine genetic mapping, gene sequencing and assessment of TB phenotypes resulted in identification of the H2-Ab gene as a major factor of TB control.

Methods

We further narrowed the MHC-II H2 ^j interval by spotting a new recombination event, sequencing newly established DNA configuration and establishing a mouse strain B6.I-103 in which j/b recombination occurred within the coding sequence of the H2-Ab gene.

Results

Unexpectedly, a novel H2-Aα ^b/Aβ^jE⁰ haplotype provided exclusively high susceptibility to TB challenge. Immunologic analysis revealed an altered CD4⁺ T-cell selection and maintenance in B6.I-103 mice, as well as seriously impaired expression of the H2-Aα^b/Aβ^j molecule on the surface of antigen presenting cells. Unlike previously reported cases of Class II malfunctioning, the defective phenotype arose not from strong structural mutations, but from regular recombination events within the MHC-II recombination hot spot region.

Discussion

Our findings provide evidence that Class II α/β-chain cis-allelic mismatches created by regular genetic recombination may severely affect immune system functioning. This issue is discussed in the context of the MHC evolution.

Clonal dynamics underlying the skewed CD4/CD8 ratio of mouse thymocytes revealed by TCR-independent barcoding

T cell differentiation in the thymus generates CD4⁺ helper and cytotoxic CD8⁺ cells as the two principal T cell lineages. Curiously, at the end of this complex selection process, CD4⁺ cells invariably outnumber CD8⁺ cells. Here, we examine the dynamics of repertoire formation and the emergence of the skewed CD4/CD8 ratio using high-resolution endogenous CRISPR/Cas9 barcoding that indelibly marks immature T cells at the DN2/DN3 pre-TCR stage. In wild-type mice, greater clone size of CD4⁺ cells and an intrinsically greater probability of Tcr β clonotypes for pMHCII interactions are major contributors to the skewed CD4/CD8 ratio. Clonal perturbations of thymocyte differentiation following the precocious expression of a rearranged iNKT invariant TCR α chain are due to loss of Tcr β clonotypes from the CD4 lineage-committed pre-selection repertoire. The present barcoding scheme offers a novel means to examine the clonal dynamics of lymphocyte differentiation orthogonal to that using TCR clonotypes.

CRSIPR-mediated barcoding of pre-TCR-expressing T cells is used to investigate the clonal dynamics of thymic T cells and reveals new fundamental understanding of how the skewed CD4/CD8 ratio arises during thymic selection.

Aging Aggravates Cachexia in Tumor-Bearing Mice

Simple Summary

Cachexia is a deadly disease that accompanies many different types of cancers. Animal studies on cachexia have so far mostly been conducted using young mice, while cancer in humans is a disease of high age. Mouse models used to date may therefore not be suitable to study cachexia with relevance to patients. By comparing young and old mice of three different strains and two different tumor types, we here show that the age of mice has a substantial effect on cachexia progression (specifically body weight, tissue weight, fiber size, molecular markers) that is dependent on the mouse strain studied. This is independent of glucose tolerance. The cachexia markers IL6 and GDF15 differ between ages in both mice and patients. Future studies on cachexia should consider the age and strain of mice.

Abstract

Background: Cancer is primarily a disease of high age in humans, yet most mouse studies on cancer cachexia are conducted using young adolescent mice. Given that metabolism and muscle function change with age, we hypothesized that aging may affect cachexia progression in mouse models. Methods: We compare tumor and cachexia development in young and old mice of three different strains (C57BL/6J, C57BL/6N, BALB/c) and with two different tumor cell lines (Lewis Lung Cancer, Colon26). Tumor size, body and organ weights, fiber cross-sectional area, circulating cachexia biomarkers, and molecular markers of muscle atrophy and adipose tissue wasting are shown. We correlate inflammatory markers and body weight dependent on age in patients with cancer. Results: We note fundamental differences between mouse strains. Aging aggravates weight loss in LLC-injected C57BL/6J mice, drives it in C57BL/6N mice, and does not influence weight loss in C26-injected BALB/c mice. Glucose tolerance is unchanged in cachectic young and old mice. The stress marker GDF15 is elevated in cachectic BALB/c mice independent of age and increased in old C57BL/6N and J mice. Inflammatory markers correlate significantly with weight loss only in young mice and patients. Conclusions: Aging affects cachexia development and progression in mice in a strain-dependent manner and influences the inflammatory profile in both mice and patients. Age is an important factor to consider for future cachexia studies.

Expansion of an Unusual Virtual Memory CD8+ Subpopulation Bearing Vα3.2 TCR in Themis-Deficient Mice

Deletion of the gene for Themis affects T cell selection in the thymus, which would be expected to affect the TCR repertoire. We found an increased proportion of cells expressing Vα3.2 (TRAV9N-3) in the peripheral CD8⁺ T cell population in mice with germline Themis deficiency. Analysis of the TCRα repertoire indicated it was generally reduced in diversity in the absence of Themis, whereas the diversity of sequences using the TRAV9N-3 V-region element was increased. In wild type mice, Vα3.2⁺ cells showed higher CD5, CD6 and CD44 expression than non-Vα3-expressing cells, and this was more marked in cells from Themis-deficient mice. This suggested a virtual memory phenotype, as well as a stronger response to self-pMHC. The Vα3.2⁺ cells responded more strongly to IL-15, as well as showing bystander effector capability in a Listeria infection. Thus, the unusually large population of Vα3.2⁺ CD8⁺ T cells found in the periphery of Themis-deficient mice reflects not only altered thymic selection, but also allowed identification of a subset of bystander-competent cells that are also present in wild-type mice.

Cell-intrinsic regulation of peripheral memory-phenotype T cell frequencies

Memory T and B lymphocyte numbers are thought to be regulated by recent and cumulative microbial exposures. We report here that memory-phenotype lymphocyte frequencies in B, CD4 and CD8 T-cells in 3-monthly serial bleeds from healthy young adult humans were relatively stable over a 1-year period, while Plasmablast frequencies were not, suggesting that recent environmental exposures affected steady state levels of recently activated but not of memory lymphocyte subsets. Frequencies of memory B and CD4 T cells were not correlated, suggesting that variation in them was unlikely to be determined by cumulative antigenic exposures. Immunophenotyping of adult siblings showed high concordance in memory, but not of recently activated lymphocyte subsets. To explore the possibility of cell-intrinsic regulation of T cell memory, we screened effector memory-phenotype T cell (TEM) frequencies in common independent inbred mice strains. Using two pairs from these strains that differed predominantly in either CD4 TEM and/or CD8 TEM frequencies, we constructed bi-parental bone marrow chimeras in F1 recipient mice, and found that memory T cell frequencies in recipient mice were determined by donor genotypes. Together, these data suggest cell-autonomous determination of memory T niche size, and suggest mechanisms maintaining immune variability.

Somatic Variation of T-Cell Receptor Genes Strongly Associate with HLA Class Restriction

Every person carries a vast repertoire of CD4⁺ T-helper cells and CD8⁺ cytotoxic T cells for a healthy immune system. Somatic VDJ recombination at genomic loci that encode the T-cell receptor (TCR) is a key step during T-cell development, but how a single T cell commits to become either CD4⁺ or CD8⁺ is poorly understood. To evaluate the influence of TCR sequence variation on CD4⁺/CD8⁺ lineage commitment, we sequenced rearranged TCRs for both α and β chains in naïve T cells isolated from healthy donors and investigated gene segment usage and recombination patterns in CD4⁺ and CD8⁺ T-cell subsets. Our data demonstrate that most V and J gene segments are strongly biased in the naïve CD4⁺ and CD8⁺ subsets with some segments increasing the odds of being CD4⁺ (or CD8⁺) up to five-fold. These V and J gene associations are highly reproducible across individuals and independent of classical HLA genotype, explaining ~11% of the observed variance in the CD4⁺ vs. CD8⁺ propensity. In addition, we identified a strong independent association of the electrostatic charge of the complementarity determining region 3 (CDR3) in both α and β chains, where a positively charged CDR3 is associated with CD4⁺ lineage and a negatively charged CDR3 with CD8⁺ lineage. Our findings suggest that somatic variation in different parts of the TCR influences T-cell lineage commitment in a predominantly additive fashion. This notion can help delineate how certain structural features of the TCR-peptide-HLA complex influence thymic selection.

HLA-DR*0401 expression in the NOD mice prevents the development of autoimmune diabetes by multiple alterations in the T-cell compartment

Several human HLA alleles have been found associated with type 1 diabetes (T1D), but their precise role is not clearly defined. Herein, we report that a human MHC class II (HLA-DR*0401) allele transgene that has been expressed into NOD (H-2(g7)I-E(null)) mice prone to T1D rendered the mice resistant to the disease. T1D resistance occurred in the context of multi-point T-cell alterations such as: (i) skewed CD4/CD8 T-cell ratio, (ii) decreased size of CD4(+)CD44(high) T memory pool, (iii) aberrant TCR Vβ repertoire, (iv) increased neonatal number of Foxp3(+) and TR-1(+) regulatory cells, and (v) reduced IFN-γ inflammatory response vs. enhanced IL-10 suppressogenic response of T-cells upon polyclonal and antigen-specific stimulation. The T-cells from NOD/DR4 Tg mice were unable to induce or suppress diabetes in NOD/RAG deficient mice. This study describes a multifaceted regulatory function of the HLA-DR*0401 allele strongly associated with the lack of T1D development in NOD mice.

Allelic exclusion of TCR α-chains upon severe restriction of Vα repertoire

Development of thymocytes through the positive selection checkpoint requires the rearrangement and expression of a suitable T cell receptor (TCR) α-chain that can pair with the already-expressed β-chain to make a TCR that is selectable. That is, it must have sufficient affinity for self MHC-peptide to induce the signals required for differentiation, but not too strong so as to induce cell death. Because both alleles of the α-chain continue to rearrange until a positively-selectable heterodimer is formed, thymocytes and T cells can in principle express dual α-chains. However, cell-surface expression of two TCRs is comparatively rare in mature T cells because of post-transcriptional regulatory mechanisms termed “phenotypic allelic exclusion”. We produced mice transgenic for a rearranged β-chain and for two unrearranged α-chains on a genetic background where endogenous α-chains could not be rearranged. Both Vα3.2 and Vα2 containing α-chains were efficiently positively selected, to the extent that a population of dual α-chain-bearing cells was not distinguishable from single α-chain-expressors. Surprisingly, Vα3.2-expressing cells were much more frequent than the Vα2 transgene-expressing cells, even though this Vα3.2-Vβ5 combination can reconstitute a known selectable TCR. In accord with previous work on the Vα3 repertoire, T cells bearing Vα3.2 expressed from the rearranged minilocus were predominantly selected into the CD8⁺ T cell subpopulation. Because of the dominance of Vα3.2 expression over Vα2 expressed from the miniloci, the peripheral T cell population was predominantly CD8⁺ cells.

The behavioral and immunological impact of maternal separation: a matter of timing

Maternal separation (MS), an early life stressful event, has been demonstrated to trigger neuropsychiatric disorders later in life, in particular depression. Experiments using rodents subjected to MS protocols have been very informative for the establishment of this association. However, the mechanism by which MS leads to neuropsychiatric disorders is far from being understood. This is probably associated with the multifactorial nature of depression but also with the fact that different research MS protocols have been used (that vary on temporal windows and time of exposure to MS). In the present study, MS was induced in rats in two developmental periods: for 6 h per day for 14 days between postnatal days 2-15 (MS2-15) and 7-20 (MS7-20). These two periods were defined to differ essentially on the almost complete (MS2-15) or partial (MS7-20) overlap with the stress hypo-responsive period. Behavioral, immunological, and endocrine parameters, frequently associated with depressive-like behavior, were analyzed in adulthood. Irrespectively from the temporal window, both MS exposure periods led to increased sera corticosterone levels. However, only MS2-15 animals displayed depressive and anxious-like behaviors. Moreover, MS2-15 was also the only group presenting alterations in the immune system, displaying decreased percentage of CD8(+) T cells, increased spleen T cell CD4/CD8 ratio, and thymocytes with increased resistance to dexamethasone-induced cell death. A linear regression model performed to predict depressive-like behavior showed that both corticosterone levels and T cell CD4/CD8 ratio explained 37% of the variance observed in depressive-like behavior. Overall, these findings highlight the existence of "critical periods" for early life stressful events to exert programing effects on both central and peripheral systems, which are of relevance for distinct patterns of susceptibility to emotional disorders later in life.

From pre-DP, post-DP, SP4, and SP8 Thymocyte Cell Counts to a Dynamical Model of Cortical and Medullary Selection

Cells of the mature αβ T cell repertoire arise from the development in the thymus of bone marrow precursors (thymocytes). αβ T cell maturation is characterized by the expression of thousands of copies of identical αβ T cell receptors and the CD4 and/or CD8 co-receptors on the surface of thymocytes. The maturation stages of a thymocyte are: (1) double negative (DN) (TCR⁻, CD4⁻ and CD8⁻), (2) double positive (DP) (TCR⁺, CD4⁺ and CD8⁺), and (3) single positive (SP) (TCR⁺, CD4⁺ or CD8⁺). Thymic antigen presenting cells provide the appropriate micro-architecture for the maturation of thymocytes, which “sense” the signaling environment via their randomly generated TCRs. Thymic development is characterized by (i) an extremely low success rate, and (ii) the selection of a functional and self-tolerant T cell repertoire. In this paper, we combine recent experimental data and mathematical modeling to study the selection events that take place in the thymus after the DN stage. The stable steady state of the model for the pre-DP, post-DP, and SP populations is identified with the experimentally measured cell counts from 5.5- to 17-week-old mice. We make use of residence times in the cortex and the medulla for the different populations, as well as recently reported asymmetric death rates for CD4 and CD8 SP thymocytes. We estimate that 65.8% of pre-DP thymocytes undergo death by neglect. In the post-DP compartment, 91.7% undergo death by negative selection, 4.7% become CD4 SP, and 3.6% become CD8 SP. Death by negative selection in the medulla removes 8.6% of CD4 SP and 32.1% of CD8 SP thymocytes. Approximately 46.3% of CD4 SP and 27% of CD8 SP thymocytes divide before dying or exiting the thymus.

The role of protein kinase cη in T cell biology

Protein kinase Cη (PKCη) is a member of the novel PKC subfamily, which also includes δ, ε, and θ isoforms. Compared to the other novel PKCs, the function of PKCη in the immune system is largely unknown. Several studies have started to reveal the role of PKCη, particularly in T cells. PKCη is highly expressed in T cells, and is upregulated during thymocyte positive selection. Interestingly, like the θ isoform, PKCη is also recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell. However, unlike PKCθ, which becomes concentrated to the central region of the synapse, PKCη remains in a diffuse pattern over the whole area of the synapse, suggesting distinctive roles of these two isoforms in signal transduction. Although PKCη is dispensable for thymocyte development, further analysis of PKCη- or PKCθ-deficient and double-knockout mice revealed the redundancy of these two isoforms in thymocyte development. In contrast, PKCη rather than PKCθ, plays an important role for T cell homeostatic proliferation, which requires recognition of self-antigen. Another piece of evidence demonstrating that PKCη and PKCθ have isoform-specific as well as redundant roles come from the analysis of CD4 to CD8 T cell ratios in the periphery of these knockout mice. Deficiency in PKCη or PKCθ had opposing effects as PKCη knockout mice had a higher ratio of CD4 to CD8 T cells compared to that of wild-type mice, whereas PKCθ-deficient mice had a lower ratio. Biochemical studies showed that calcium flux and NFκB translocation is impaired in PKCη-deficient T cells upon TCR crosslinking stimulation, a character shared with PKCθ-deficient T cells. However, unlike the case with PKCθ, the mechanistic study of PKCη is at early stage and the signaling pathways involving PKCη, at least in T cells, are essentially unknown. In this review, we will cover the topics mentioned above as well as provide some perspectives for further investigations regarding PKCη.

Protein kinase C η is required for T cell activation and homeostatic proliferation

Protein kinase C η (PKCη) is abundant in T cells and is recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell; however, its function in T cells is unknown. We showed that PKCη was required for the activation of mature CD8+ T cells through the T cell receptor. Compared with wild-type T cells, PKCη-/- T cells showed poor proliferation in response to antigen stimulation, a trait shared with T cells deficient in PKCθ, which is the most abundant PKC isoform in T cells and was thought to be the only PKC isoform with a specific role in T cell activation. In contrast, only PKCη-deficient T cells showed defective homeostatic proliferation, which requires self-antigen recognition. PKCη was dispensable for thymocyte development; however, thymocytes from mice doubly deficient in PKCη and PKCθ exhibited poor development, indicating some redundancy between the PKC isoforms. Deficiency in PKCη or PKCθ had opposing effects on the relative numbers of CD4+ and CD8+ T cells. PKCη-/- mice had a higher ratio of CD4+ to CD8+ T cells compared to that of wild-type mice, whereas PKCθ-/- mice had a lower ratio. Mice deficient in both isoforms exhibited normal cell ratios. Together, these data suggest that PKCη shares some redundant roles with PKCθ in T cell biology and also performs nonredundant functions that are required for T cell homeostasis and activation.

A structural basis for varied αβ TCR usage against an immunodominant EBV antigen restricted to a HLA-B8 molecule

EBV is a ubiquitous and persistent human pathogen, kept in check by the cytotoxic T cell response. In this study, we investigated how three TCRs, which differ in their T cell immunodominance hierarchies and gene usage, interact with the same EBV determinant (FLRGRAYGL), bound to the same Ag-presenting molecule, HLA-B8. We found that the three TCRs exhibit differing fine specificities for the viral Ag. Further, via structural and biophysical approaches, we demonstrated that the viral Ag provides the greatest energetic contribution to the TCR-peptide-HLA interaction, while focusing on a few adjacent HLA-based interactions to further tune fine-specificity requirements. Thus, the TCR engages the peptide-HLA with the viral Ag as the main glue, such that neighboring TCR-MHC interactions are recruited as a supportive adhesive. Collectively, we provide a portrait of how the host's adaptive immune response differentially engages a common viral Ag.

Impact of MHC class II polymorphism on blood counts of CD4+ T lymphocytes in macaque

While the number of peripheral blood T lymphocytes and of their two main subsets (CD4+CD8- and CD4-CD8+) varies little in a given healthy individual, substantial variation is observed between individuals. It was proposed that these counts could be influenced by MHC polymorphisms because of the well-established role of MHC molecules in thymic T lymphocyte maturation and presentation of antigenic peptides to peripheral T lymphocytes. To test this hypothesis, we have chosen the crab-eating macaque (Macaca fascicularis), an animal model phylogenetically close to man. We selected the Philippine macaque population because of a restriction of the MHC polymorphism in this islander population. Peripheral blood lymphocytes were counted with an automated analyzer and T lymphocyte subsets were assessed by immunolabeling and flow cytometry. The MHC polymorphism was investigated in 200 unrelated subjects using 14 microsatellites markers distributed across the MHC and the DRB locus that was genotyped by denaturing gradient gel electrophoresis and sequencing. All markers were in Hardy-Weinberg equilibrium. Allelic associations were tested with the UNPHASED software. We revealed a significant influence of the MHC class II region on CD4+ T lymphocyte blood count with the largest effect associated with a two-locus haplotypes combining the DRACA allele 274 and the DRB haplotype #8a (p < 8 × 10(-7)). Our data should stimulate a similar association study of the CD4+ T cell counts in humans.

Evolutionarily conserved amino acids that control TCR-MHC interaction

The rules for the conserved reaction of alphabeta T cell receptors (TCRs) with major histocompatibility complex (MHC) proteins plus peptides are poorly understood, probably because thymocytes bearing TCRs with the strongest MHC reactivity are lost by negative selection. Thus, only TCRs with an attenuated ability to react with MHC appear on mature T cells. Also, the interaction sites between TCRs and MHC may be inherently flexible and hence difficult to spot. We reevaluated contacts between TCRs and MHC in the solved structures of their complexes with these points in mind. Relatively conserved amino acids in the TCR complementarity-determining regions (CDR) 1 and CDR2 are often used to bind exposed areas of the MHC alpha-helices. These areas are exposed because of small amino acids that allow somewhat flexible binding of the TCRs. The TCR amino acids involved are specific to families of variable (V) regions and to some extent different rules may govern the recognition of MHCI versus MHCII.

Quantitative analysis of T cell homeostatic proliferation

T cell homeostatic proliferation occurs on transfer of T cells into lymphopenic recipients; transferred cells undergo several rounds of division in the absence of specific antigen stimulation. For a quantitative analysis of this phenomenon, we applied a mathematical method to describe proliferating T cells to match peak distributions from actual CFSE dilution data. For in vitro stimulation of T cells with anti-CD3/anti-CD28, our simulation confirmed a high proportion of cells entering cell cycle with a low proportion undergoing apoptosis. When applied to homeostatic proliferation, it described striking differences in CD4 and CD8 T cell proliferation rates, and accurately predicted that successive divisions were accompanied by higher rates of apoptosis, limiting the accumulation of proliferating cells. Thus, the presence of multiple CFSE dilution peaks cannot be considered equivalent to lymphocyte expansion. Finally, genetic effects were identified that may help explain links between homeostatic proliferation and autoimmunity.

Lymphoid organ development in rabbits: major lymphocyte subsets

Although rabbits represent an important animal model, little is known about the lymphoid organ development in this species. In the present study, lymphocyte subsets in peripheral blood, spleen, mesenteric and popliteal lymph nodes in newborn and 2-, 4-, 6- and 8-week old and adult were characterized. Lymphocyte subsets were detected using flow cytometry and monoclonal antibodies against rabbit CD4, CD8, T-cell-specific antigen and cross-reactive antibody against B-cell antigen CD79alpha. In neonates, lower numbers of T cells were detected in both peripheral blood and spleen than in mesenteric lymph nodes. In comparison with other compartments, CD79alpha(+) cells prevailed in the spleen. Post-natal development was characterized by a decreased CD4(+)/CD8(+) ratio due to increasing frequency of CD8(+) lymphocytes in all organs but mesenteric lymph nodes, where it was due to decreased numbers of CD4(+) lymphocytes. Another significant feature was the increase of B cells in peripheral blood and mesenteric lymph nodes.

A study of 82 extended HLA haplotypes in HFE-C282Y homozygous hemochromatosis subjects: relationship to the genetic control of CD8+ T-lymphocyte numbers and severity of iron overload

Background

It has been recently demonstrated that CD8+ T-lymphocyte numbers are genetically transmitted in association with the MHC class I region. The present study was designed with the objective of narrowing the region associated with the setting of CD8+ T-lymphocyte numbers in a population of C282Y homozygous hemochromatosis subjects, in whom a high prevalence of abnormally low CD8+ T-lymphocyte counts has been described.

Methods

The study includes 43 C282Y homozygous subjects fully characterized both phenotypically and genotypically. Clinical characterization includes measurements of iron parameters at diagnosis (transferrin saturation and serum ferritin), total body iron stores and T-cell immunophenotyping determined by flow cytometry. Genetic characterization includes HLA class I alleles (A, B and C) and four additional microsatellite markers (D6S265, D6S2222, D6S105 and D6S2239) spanning 5 Megabases in the 6p21.3 region.

Results

Eighty-two extended C282Y carrying haplotypes were defined. Single-locus analysis revealed that the HLA-A region was associated with CD8+ T-cell numbers. Multivariate analysis showed that the combinations of the most common HLA-A alleles (HLA-A*03, -A*02 and -A*01) were associated with significantly lower numbers of CD8+ T-lymphocytes (0.30 ± 0.14 × 10⁶/ml), in comparison with subjects carrying only one copy of those alleles (0.46 ± 0.19 × 10⁶/ml) and subjects without any copy of those alleles (0.79 ± 0.15 × 10⁶/ml;p = 0.0001). No differences were observed in CD8+ T-cell counts among control subjects carrying the same combinations of HLA-A alleles (0.47 ± 0.14; 0.45 ± 0.21 and 0.41 ± 0.17 × 10⁶/ml, respectively), therefore not supporting a direct effect of HLA specificity but rather an indirect association with a locus close to HLA-A. Multivariate analysis showed that the combination of the most common HLA-A alleles also have an impact on the clinical expression of HH in terms of iron stores, in males(p = 0.0009).

Conclusion

The present study provides evidence supporting an inextricable link between extended HLA haplotypes, CD8+ T-lymphocyte numbers and severity of iron overload in hereditary hemochromatosis(HH). It gives additional information to better define a candidate region involved in the regulation of CD8+ T-lymphocyte numbers. A new evolutionary hypothesis concerning the inheritance of the phenotype of low CD8+ T-lymphocyte numbers associated with particular ancestral HLA haplotypes carrying the C282Y mutation and its implication on the clinical heterogeneity of HH is discussed.

Low serum transferrin levels in HFE C282Y homozygous subjects are associated with low CD8+ T lymphocyte numbers

Hereditary hemochromatosis (HH) is a genetic iron overload disease, in the majority of cases associated with homozygosity for the C282Y mutation of the HFE gene. In spite of this genetic homogeneity, there is a great clinical heterogeneity among HH patients. Low CD8(+) lymphocyte numbers have been associated with a more severe expression of iron overload in HH patients, and in experimental models of iron overload. HH patients present low serum transferrin levels. Transferrin is an indispensable resource for lymphopoiesis. Lymphocyte homeostasis follows general ecology rules of population dynamics that involve competition for limiting resources. In the present study, we questioned whether transferrin levels could be associated with the anomalies seen previously in lymphocyte subset numbers in HH patients. Transferrin levels, total and subset T lymphocyte counts were done in 426 apparently healthy subjects genotyped for HFE. All HFE C282Y carriers presented significantly lower serum transferrin levels than the wild type group, a difference that could not be explained solely by the degree of iron overload. Significant differences were also seen in transferrin levels between males and females, with females presenting higher average serum Transferrin levels. In the population of subjects with Transferrin levels lower than 248 mg/dl, a positive correlation was seen between the peripheral CD8(+) lymphocyte numbers and serum transferrin levels (R(2) = 2.41; r = 0.16; P = 0.018). To test the possible limiting resource effect of transferrin, the correlation between transferrin levels and CD8(+) lymphocyte numbers was scrutinized in 34 HH patients, homozygous for the C282Y mutation. In the homozygous males, where the lowest average transferrin levels were seen, another highly significant correlation was observed between Transferrin levels and CD8(+) numbers. This correlation points to a possible role of transferrin as a limiting resource for MHC class I dependent lymphocyte proliferation, an effect that was not observed in C282Y homozygous female patients.

Involvement of the major histocompatibility complex region in the genetic regulation of circulating CD8 T-cell numbers in humans

Variability in T-lymphocyte numbers is partially explained by a genetic regulation. From studies in animal models, it is known that the Major Histocompatibility Complex (MHC) is involved in this regulation. In humans, this has not been shown yet. The objective of the present study was to test the hypothesis that genes in the MHC region influence the regulation of T-lymphocyte numbers. Two approaches were used. Association studies between T-cell counts (CD4(+) and CD8(+)) or total lymphocyte counts and HLA class I alleles (A and B) or mutations in the HFE (C282Y and H63D), the hemochromatosis gene, in an unrelated population (n = 264). A second approach was a sibpair correlation analysis of the same T-cell counts in relation to HLA-HFE haplotypes in subjects belonging to 48 hemochromatosis families (n = 456 sibpairs). In the normal population, results showed a strong statistically significant association of the HLA-A*01 with high numbers of CD8(+) T cells and a less powerful association with the HLA-A*24 with low numbers of CD8(+) T cells. Sibpair correlations revealed the most significant correlation for CD8(+) T-cell numbers for sibpairs with HLA-HFE-identical haplotypes. This was not observed for CD4(+) T cells. These results show that the MHC region is involved in the genetic regulation of CD8(+) T-cell numbers in humans. Identification of genes responsible for this control may have important biological and clinical implications.

Nonobese diabetic CD4 lymphocytosis maps outside the MHC locus on chromosome 17

Genetic control of homeostasis of peripheral CD4+ lymphocyte levels is incompletely understood. Recent genome scans have linked mouse peripheral CD4 levels to chromosome 17, with strongest linkage to the Ea region. Nonobese diabetic (NOD) mice demonstrate peripheral T-cell lymphocytosis, and previous studies also suggested that the MHC region might control this phenotype. Here we confirm that loci on Chr 17 control NOD peripheral CD4 lymphocytosis. An elevated NOD CD4:CD8 ratio maps to the same region, and we show it is due to increased numbers of CD4+ cells. However, using NOD MHC congenic mice, we demonstrate that the MHC region is excluded, and that NOD peripheral lymphocytosis is controlled by genetic intervals adjacent to the MHC region on Chr 17.

Thymocyte-intrinsic genetic factors influence CD8 T cell lineage commitment and affect selection of a tumor-reactive TCR

Selection of immature CD4CD8 double-positive (DP) thymocytes for CD4 or CD8-lineage commitment is controlled by the interaction of the TCR with stromal cell-expressed peptide/MHC. We show that thymocyte-intrinsic genes influence the pattern of expression of a MHC class I-restricted transgenic (tg) TCR so that in DBA/2 mice, DP thymocytes with a characteristically high expression of tg TCR, infrequently transit to CD8 single-positive thymocytes. In contrast, in B10.D2 mice, the same tg TCR is expressed at lower levels on a subpopulation of DP thymocytes that more frequently transit to CD8 single-positive thymocytes. These characteristics were not influenced by thymic stromal components that control positive selection. Radiation chimeras reconstituted with a mixture of BM from tg TCR mice of the two genetic backgrounds revealed that the relative frequency of transit to the CD8 lineage remained thymocyte-intrinsic. Identifying the gene products whose polymorphism controls CD8 T cell development may shed new light on the mechanisms controlling T cell commitment/selection in mice other than the most studied "C57BL/6"-based strains.

Surprisingly minor influence of TRAV11 (Valpha14) polymorphism on NK T-receptor mCD1/alpha-galactosylceramide binding kinetics

Defects in natural killer T (NK T) cell function and of interleukin-4 -production in SJL and NOD mice have been linked to susceptibility to autoimmune disease. As SJL and NOD mice both carry the T-cell receptor (TCR) alpha-chain locus "c" (Tcra(c)) haplotype, found in few other strains, we have attempted to determine the influence of Tcra polymorphism on NK T-cell recognition of ligand, selection, and immune responses. The majority of NK T cells use an "invariant" TRAV11J15 (previously called AV14J18 or Valpha14 Jalpha281) alpha- chain paired with either TRBV13-2, BV29, or BV1 to recognize ligands presented by mCD1 molecules, including the glycolipid alpha-galactosylceramide (alpha-GalCer). Sequencing of TRAV11 from the mouse strains B10.A (encoding the Tcra(b) haplotype), B10.A- Tcra(c), and NOD (Tcra(c)) shows that Tcra(c) has a single TRAV11 gene (TRAV11*01) and that Tcra(b) has a single expressed gene (TRAV11*02), plus a closely related pseudogene. There is no apparent difference in alpha-chain J-region usage or in the CDR3alpha sequence at the TRAV11-J15 junction between the haplotypes in TRAV11-bearing NK T cells. Using Biacore and tetramer-binding and decay assays, we have determined that the interaction between Tcra(c) TRAV11*01 NK T TCR and the mCD1/alpha-GalCer complex is slightly weaker than that of Tcra(b) (i.e., TRAV11*02) NK T TCR. These differences are minor compared with differences between agonist and antagonist ligands in other TCR systems, suggesting that it is unlikely that TCR polymorphism explains the defect in NK T cells in the autoimmune mouse strains.

Genetic polymorphisms in mouse genes regulating age-sensitive and age-stable T cell subsets

To see whether genetic polymorphisms regulate inter-individual differences in T cell subset levels, we have conducted a genome scan in two populations of mice, bred as the progeny of a cross between CB6F1 females and C3D2F1 males. The data document quantitative trait loci (QTL) with statistically significant effects on CD4, CD8, and CD8 memory T cells, and on subsets of CD4 and CD8 T cells that express P-glycoprotein. Some of the loci detected were robust, in the sense that they produced effects of similar size both in mated female mice, and in a population that included male and female virgin animals. Some of the effects were stable, in that they were apparent at both 8 and 18 months of age, but others were age-specific, showing effects either at 8 or at 18 months but not at both ages. Genes that had an effect on the same T cell subset were in almost all cases additive rather than epistatic, and their combined effects could produce large overall effects, leading in the most dramatic case to a two-fold difference in CD8 memory cells. The analysis also documented two QTL, on chromosomes 4 and 13, that regulate an age-sensitive composite index of T cell subset pattern which has been shown previously to be a predictor of life expectancy in these mice. The analysis thus reveals both subset-specific genes and others which modulate the overall pattern of age-sensitive changes in T cell subset distributions.

Recognition of a specific self-peptide: self-MHC class II complex is critical for positive selection of thymocytes expressing the D10 TCR

We examined the specificity of positive and negative selection by using transgenic mice carrying a variant of the D10 TCR. We demonstrate that a point mutation at position 51 within the CDR2alpha segment significantly reduces the avidity of this TCR for its cognate ligand, but does not impact recognition of nonself MHC class II molecules. Although structural studies have suggested that this TCR site interacts with the MHC class II beta-chain, the avidity of this TCR for its ligand and the function of the T cell can be reconstituted by a point mutation in the bound antigenic peptide. These data demonstrate that the bound peptide can indirectly alter TCR interactions by influencing MHC structure. Remarkably, reducing the avidity of this TCR for a specific antigenic peptide-MHC ligand has a dramatic impact on thymic selection. Positive selection of thymocytes expressing this TCR is nearly completely blocked, whereas negative selection on allogenic MHC class II molecules remains intact. Therefore, the recognition of self that promotes positive selection of the D10 TCR is highly peptide-specific.

Linkage analysis of variations in CD4:CD8 T cell subsets between C57BL/6 and DBA/2

The ratio of CD4 T cells to CD8 T cells (CD4:CD8 ratio) varies over two-fold between C57BL/6 and DBA/2 mice for both T cell precursors in the thymus and mature T cells in the periphery. Correlation analysis of the CD4:CD8 ratio in thymic precursors vs peripheral T cells in F2 and backcross mice found that thymic precursor ratios are inherited independently from those in the periphery, indicating that the CD4:CD8 ratios in these populations are affected by distinct genetic mechanisms. A genome scan of progeny in the phenotypic extremes identified three quantitative trait loci (QTLs). Trmq1 (for T cell ratio modifier QTL 1) was detected in the telomeric end of c6 (peak marker D6Mit15 at 74 cM) and had a maximum LOD score of 4.6. Trmq2, in the telomeric half of c2, peaked at D2MIT483 and had a maximum LOD score of 3.41. Both of these QTLs impacted the CD4:CD8 ratios in peripheral T cells and had no impact on variation in this ratio among thymic precursors. However, heterozygosity for the H2 complex was suggestively associated (LOD score of 2.43) with increases in CD4 T cells among T cell precursors in the thymus. All of these QTLs were affected by epistatic interactions, indicating that additional modifiers in the B6 and DBA/2 genomes modulate this phenotype.

A ligand for the chemokine receptor CCR7 can influence the homeostatic proliferation of CD4 T cells and progression of autoimmunity

Homeostasis of T cell numbers in the periphery implies an ability of lymphocytes to sense cell numbers. Although the mechanisms are unknown, we find that the chemokine CCL21 (also known as TCA4, SLC, 6Ckine), a ligand for the chemokine receptor CCR7, can regulate homeostasis of CD4 (but not CD8) T cells. In the absence of CCR7 ligands, transferred CD4 T cells failed to expand in lymphopenic hosts, whereas in the presence of CCL21 overexpression, homeostatic CD4 T cell proliferation occurred even in nonlymphopenic recipients. Ag-specific CD4 T cells transferred into Ag-expressing mice proliferated and induced autoimmunity only in lymphopenic recipients. Pancreatic expression of CCL21 was sufficient to replace the requirement for lymphopenia in the progression of autoimmune disease. These results suggest that CD4 T cells use local concentrations of CCR7 ligands as an index of T cell steady state numbers and that homeostatic expansion of the T cell population may be a contributing factor in the development of autoimmune disease.

Pulmonary T cells and eosinophils: coconspirators or independent triggers of allergic respiratory pathology?

Etiologic discussions of allergic respiratory pathology frequently engender rabid constituencies of pro-T cell or proeosinophil disciples, each claiming, often with religious fervor, the importance of their leukocyte. However, increasing evidence suggests that the exclusionary rhetoric from either camp is inadequate to explain many of the pathologic changes occurring in the lung. Data from both asthmatic patient and mouse models of allergic respiratory inflammation suggest that, in addition to cell-autonomous activities, T-cell and eosinophil interactions may be critical to the onset and progression of pulmonary pathology. These studies also suggest that T-lymphocyte subpopulations and eosinophils communicate by means of both direct cell-cell interactions and through the secretion of inflammatory signals. Collectively, the data support an expanded view of T-cell and eosinophil activities in the lung, including both immunoregulative activities and downstream effector functions impinging directly on lung function.

Distinct mechanisms contribute to generate and change the CD4:CD8 cell ratio during thymus development: a role for the Notch ligand, Jagged1

In adult life, the high CD4:CD8 cell ratio observed in peripheral lymphoid organs originates in the thymus. Our results show that the low peripheral CD4:CD8 cell ratio seen during fetal life also has an intrathymic origin. This distinct production of CD4(+)CD8(-) and CD4(-)CD8(+) thymocytes is regulated by the developmental age of the thymic stroma. The differential expression of Notch receptors and their ligands, especially Jagged1, throughout thymus development plays a key role in the generation of the different CD4:CD8 cell ratios. We also show that the intrathymic CD4:CD8 cell ratio sharply changes from fetal to adult values around birth. Differences in the proliferation and emigration rates of the mature thymocyte subsets contribute to this change.

Genetic control of peripheral TCRAV usage by representation in the preselection repertoire and MHC allele-specific overselection

TCRAV segments contribute significantly to MHC restriction as illustrated by their general preference for either the CD4 or CD8 T cell subset and additional, MHC allele-specific overselection during T cell differentiation. The 10-fold over-representation of the TCRAV8S2 (VA8S2) segment in CD8 over CD4 T cells by the RT1(f) haplotype of LEW.1F rats provides the most striking example of MHC allele-specific overselection of a VA segment reported so far. Also in alloreactivity, VA8S2(+) CD8 cells from RT1(f-) rats are preferentially expanded by RT1(f+) stimulators. We have identified the class I molecule, A(f), mediating VA8S2 overselection and report that it differs only in four amino acids at the MHC-TCR interface from the class I molecule A(a), which is neutral with regard to selection of VA8S2. We also provide an extensive survey of the TCRAV8 family and show that among 14 functional VA8 segments in LEW rats, the dramatic A(f)-dependent overselection is unique for VA8S2. Surprisingly, VA8S2 expression in CD8 T cells of RT1(f+) rats derived from a Sprague-Dawley stock was only 3% as compared to the 12% observed in LEW.1F. The VA8S2 segment of Sprague-Dawley (VA8S2(SD)) differs from VA8S2 of the LEW background (VA8S2(l)) in only two amino acids, one of which is located in CDR2 and could thus participate in allele-specific recognition of A(f). However, analysis of the pre- and postselection thymic repertoires of Sprague-Dawley and LEW.1F rats and of the repertoire of CD8 cells from both strains expanded in the alloreactive response to RT1(f) revealed that the difference in VA8S2 representation between the two backgrounds is explained by differential availability in the preselection repertoires and not by a difference in overselection. Sequence comparisons of A(f) and A(a) and of both VA8S2 segments suggest a predominant role of CDR1 in hyper-reactivity to A(f). Thus, the VA composition of the mature TCR repertoire is influenced by TCRA: locus polymorphisms at two levels: the regulation of VA usage in the preselection repertoire and the composition of structural elements which contribute to specific VA-MHC interactions during thymic selection.

Homeostasis of alpha beta TCR+ T cells

Cytokines contribute to T cell homeostasis at all stages of T cell existence. However, the particular cytokine involved varies as T cells progress from a naïve through an activated to a memory state. In many cases the important cytokines are members of the interleukin 2 subfamily of the short-chain type I cytokines. A case is made for the idea that the evolutionary divergence of the short-chain family allowed for concurrent divergence in leukocytes.

Selecting and maintaining a diverse T-cell repertoire

To provide a T-cell population that will respond promptly to foreign antigen, the immune system looks inward, using the variety of self-antigens to select and maintain a diverse repertoire of receptors. A protective immune system must include a T-lymphocyte population that is poised to respond to foreign antigenic peptides presented by self-major histocompatibility complex molecules. As the organism cannot predict the precise pathogen-derived antigens that will be encountered, the system uses the diverse array of self-peptides bound to self-major histocompatibility complex molecules, not only to select a receptor repertoire in the thymus, but also to keep naïve T cells alive and 'ready for action' in the periphery.

Amino Acids Specifying MHC Class Preference in TCR Vα2 Regions

Some TCR variable regions are preferentially expressed in CD4+ or CD8+ T cells, reflecting a predilection for interacting with MHC class II or class I molecules. The molecular basis for MHC class bias has been studied previously, in particular for Vα3 family members, pointing to a dominant role for two amino acid positions in complementary-determining regions (CDRs) 1 and 2. We have evaluated the generality of these findings by examining the MHC class bias of Vα2 family members, an attractive system because it shows more variability within the CDR1 and -2, exhibits variation in the framework regions, and includes a member for which the crystal structure has been determined. We find that preferential recognition of MHC class I or II molecules does not always depend on residues at the same positions of CDR1 and -2; rules for one family may be reversed in another. Instead, there are multiple influences exerted by various CDR1/2 positions as well as the CDR3s of both the TCR α- and TCR β-chains.

Allelic exclusion of the T cell receptor alpha-chain: developmental regulation of a post-translational event

Allelic exclusion of the alpha and beta chains of the T cell receptor is maintained by different mechanisms. Exclusion of the beta-chain is primarily by allowing the successful rearrangement of only one of the two beta-chain loci. In the case of the alpha-chain, rearrangement on both chromosomes is very common, as is expression of alpha-chain mRNA and protein encoded by both loci. For the most part, however, functional alpha-chain allelic exclusion is maintained at the cell surface after positive selection in the thymus. The mechanism by which this is accomplished is not yet known, but recent evidence indicates that it is an active process coupled to signalling through the T cell receptor.

Reciprocal Expression in CD4 or CD8 Subsets of Different Members of the Vα11 Gene Family Correlates with Sequence Polymorphism

Previous staining studies with TCR Vα11-specific mAbs showed that Vα11.1/11.2 (AV11S1 and S2) expression was selectively favored in the CD4+ peripheral T cell population. As this phenomenon was essentially independent of the MHC haplotype, it was suggested that AV11S1 and S2 TCRs exert a preference for recognition of class II MHC molecules. The Vα segment of the TCR α-chain is suggested to have a primary role in shaping the T cell repertoire due to selection for class I or II molecules acting through the complementarity determining regions (CDR) 1α and CDR2α residues. We have analyzed the repertoire of Vα11 family members expressed in C57BL/6 mice and have identified a new member of this family; AV11S8. We show that, whereas AV11S1 and S2 are more frequent in CD4+ cells, AV11S3 and S8 are more frequent in CD8+ cells. The sequences in the CDR1α and CDR2α correlate with differential expression in CD4+ or CD8+ cells, a phenomenon that is also observed in BALB/c mice. With no apparent restriction in TCR Jα usage or CDR3α length in C57BL/6, these findings support the idea of Vα-dependent T cell repertoire selection through preferential recognition of MHC class I or class II molecules.