Impaired survival of T helper cells in the absence of CD4

Human genetic and immunological dissection of papillomavirus-driven diseases: new insights into their pathogenesis

Human papillomaviruses (HPVs) are responsible for cutaneous and mucosal lesions. Persistent HPV infection remains a leading cause of uterine cancer in women, but also of cutaneous squamous cell carcinoma in patients with epidermodysplasia verruciformis (EV), and of rare and devastating benign tumors, such as 'tree-man' syndrome. HPV infections are usually asymptomatic or benign in the general population. Severe manifestations in otherwise healthy subjects can attest to inherited immunodeficiencies. The human genetic dissection of these cases has identified critical components of the immune response to HPVs, including the non-redundant roles of keratinocyte-intrinsic immunity in controlling β-HPVs, and of T cell-dependent adaptive immunity for controlling all HPV types. A key role of the CD28 T-cell costimulation pathway in controlling common warts due to HPVs was recently discovered. This review summarizes the state of the art in the human genetics of HPV infection, focusing on two key affected cell types: keratinocytes and T cells.

Human genetic dissection of papillomavirus-driven diseases: new insight into their pathogenesis

Human papillomaviruses (HPVs) infect mucosal or cutaneous stratified epithelia. There are 5 genera and more than 200 types of HPV, each with a specific tropism and virulence. HPV infections are typically asymptomatic or result in benign tumors, which may be disseminated or persistent in rare cases, but a few oncogenic HPVs can cause cancers. This review deals with the human genetic and immunological basis of interindividual clinical variability in the course of HPV infections of the skin and mucosae. Typical epidermodysplasia verruciformis (EV) is characterized by β-HPV-driven flat wart-like and pityriasis-like cutaneous lesions and non-melanoma skin cancers in patients with inborn errors of EVER1-EVER2-CIB1-dependent skin-intrinsic immunity. Atypical EV is associated with other infectious diseases in patients with inborn errors of T cells. Severe cutaneous or anogenital warts, including anogenital cancers, are also driven by certain α-, γ-, μ or ν-HPVs in patients with inborn errors of T lymphocytes and antigen-presenting cells. The genetic basis of HPV diseases at other mucosal sites, such as oral multifocal epithelial hyperplasia or juvenile recurrent respiratory papillomatosis (JRRP), remains poorly understood. The human genetic dissection of HPV-driven lesions will clarify the molecular and cellular basis of protective immunity to HPVs, and should lead to novel diagnostic, preventive, and curative approaches in patients.

Complete Multilineage CD4 Expression Defect Associated With Warts Due to an Inherited Homozygous CD4 Gene Mutation

Idiopathic T-CD4 lymphocytopenia (ICL) is a rare and heterogeneous syndrome characterized by opportunistic infections due to reduced CD4 T-lymphocytes (<300 cells/μl or <20% T-cells) in the absence of HIV infection and other primary causes of lymphopenia. Molecular testing of ICL has revealed defects in genes not specific to CD4 T-cells, with pleiotropic effects on other cell types. Here we report for the first time an absolute CD4 lymphocytopenia (<0.01 CD4⁺ T-cells/μl) due to an autosomal recessive CD4 gene mutation that completely abrogates CD4 protein expression on the surface membrane of T-cells, monocytes, and dendritic cells. A 45-year-old female born to consanguineous parents consulted because of exuberant, relapsing, and treatment-refractory warts on her hands and feet since the age of 10 years, in the absence of other recurrent infections or symptoms. Serological studies were negative for severe infections, including HIV 1/2, HTLV-1, and syphilis, but positive for CMV and EBV. Blood analysis showed the absence of CD4⁺ T-cells (<0.01%) with repeatedly increased counts of B-cells, naïve CD8⁺ T-lymphocytes, and particularly, CD4/CD8 double-negative (DN) TCRαβ⁺ TCRγδ^- T-cells (30% of T-cells; 400 cells/μl). Flow cytometric staining of CD4 using monoclonal antibodies directed against five different epitopes, located in two different domains of the protein, confirmed no cell surface membrane or intracytoplasmic expression of CD4 on T-cells, monocytes, and dendritic cells but normal soluble CD4 plasma levels. DN T-cells showed a phenotypic and functional profile similar to normal CD4⁺ T-cells as regards expression of maturation markers, T-helper and T-regulatory chemokine receptors, TCRvβ repertoire, and in vitro cytokine production against polyclonal and antigen-specific stimuli. Sequencing of the CD4 gene revealed a homozygous (splicing) mutation affecting the last bp on intron 7-8, leading to deletion of the juxtamembrane and intracellular domains of the protein and complete abrogation of CD4 expression on the cell membrane. These findings support previous studies in CD4 KO mice suggesting that surrogate DN helper and regulatory T-cells capable of supporting antigen-specific immune responses are produced in the absence of CD4 signaling and point out the need for better understanding the role of CD4 on thymic selection and the immune response.

Association of the porcine cluster of differentiation 4 gene with T lymphocyte subpopulations and its expression in immune tissues

Cluster of differentiation 4 (CD4) is mainly expressed on CD4⁺ T cells, which plays an important role in immune response. The aim of this study was to detect the association between polymorphisms of the CD4 gene and T lymphocyte subpopulations in pigs, and to investigate the effects of genetic variation on the CD4 gene expression level in immune tissues. Five missense mutations in the CD4 gene were identified using DNA pooling sequencing assays, and two main haplotypes (CCTCC and AGCTG) in strong linkage disequilibrium (with frequencies of 50.26% and 46.34%, respectively) were detected in the population of Large White pigs. Our results indicated that the five SNPs and the two haplotypes were significantly associated with the proportions of CD4⁻CD8⁻, CD4⁺CD8⁺, CD4⁺CD8⁻, CD4⁺ and CD4⁺/CD8⁺ in peripheral blood (p<0.05). Gene expression analysis showed the mRNA level of the CD4 gene in thymus was significantly higher than that in lymph node and spleen (p<0.05). However, no significant difference was observed between animals with CCTCC/CCTCC genotype and animals with AGCTG/AGCTG genotype in the three immune tissues (p>0.05). These results indicate that the CD4 gene may influence T lymphocyte subpopulations and can be considered as a candidate gene affecting immunity in pigs.

Increased numbers and suppressive activity of regulatory CD25(+)CD4(+) T lymphocytes in the absence of CD4 engagement by MHC class II molecules

Mechanisms of central and peripheral tolerance prevent autoimmunity. Regulatory T cells inhibit the activation of potentially auto-reactive T cells in peripheral lymphoid organs. In transgenic mice in which all MHC class II molecules are incapable of binding to CD4, class II MHC-restricted T cells preferentially differentiated into immunosuppressive, regulatory T cells. In these mutant MHC class II transgenic mice, a subset of CD4(+) T cells constitutively expressed moderately elevated levels of CD25 and potently inhibited interleukin-2 secretion by T cells from normal mice in a cell-to-cell, contact-dependent manner. Immunosuppressive activity depended on activation of the regulatory T cells. Thus, CD25(+)CD4(+) T cells from mutant MHC class II transgenic mice resembled phenotypically and functionally a major subset of natural regulatory T cells in normal mice, but were two to three-times more abundant. These results further clarify the mechanisms that govern the differentiation and maintenance of CD25(+)CD4(+) regulatory T cells, and present avenues for immunomodulation.

A peripheral CD4+ T cell precursor for naive, memory, and regulatory T cells

Mechanisms that control the size of the T cell pool, the ratio between naive cells and memory cells, the number and frequency of regulatory T cells, and T cell receptor (TCR) diversity are necessary to maintain immune integrity and avoid disease. We have previously shown that a subset of naive CD4(+) T cells, defined by the expression on their surface of a very low density of CD44 (CD44(v.low) cells), can inhibit wasting and wasting-associated lymphopenia in mice with cancer. In this study, we further investigate the properties of CD44(v.low) cells and show that they are significantly more efficient than the remaining naive (CD44(low) or CD44(int)) and memory CD4(+) cell subsets in reconstituting the overall size of the CD4(+) T cell pool, creating a T cell pool with a diverse TCR repertoire, generating regulatory T cells that express forkhead box P3 (FoxP3), and promoting homeostatic equilibrium between naive, memory, and Foxp3(+) regulatory T cell numbers. T cell population reconstitution by CD44(v.low) cells is thymus independent. Compared with CD44(int) cells, a higher percentage of CD44(v.low) cells express B cell leukemia/lymphoma 2, interleukin-7 receptor, and CD5. The data support a key role for CD4(+) CD44(v.low) cells as peripheral precursors that maintain the integrity of the CD4(+) T cell pool.

Lipoic acid downmodulates CD4 from human T lymphocytes by dissociation of p56(Lck)

Lipoic acid is an antioxidant that suppresses and treats a model of multiple sclerosis, experimental autoimmune encephalomyelitis. We now demonstrate that treatment of human PBMC and T cell lines with LA downmodulated CD4 expression in a concentration-dependent manner. LA treatment of Con A stimulated PBMC specifically removed CD4 from the T-cell surface, but not CD3. Epitope masking by LA was excluded by using monoclonal antibodies targeting different domains of CD4. Incubation on ice inhibited CD4 removal following LA treatment, suggesting that endocytosis was involved in its downmodulation. LA is in a unique category of compounds that induce CD4 downmodulation by various mechanisms (e.g., gangliosides). We hypothesized that LA might induce dissociation of p56(Lck) from CD4, thus leading to its downmodulation. Immunoblot analyses demonstrated reduced co-precipitation of p56(Lck) from Jurkat T-cells following LA treatment and precipitation of CD4. This unique immunomodulatory effect of LA warrants further investigation.

Involvement of avidity for major histocompatibility complex in homeostasis of naive and memory T cells

The requirements for survival and self-renewal of peripheral T cells and the nature of mechanisms controlling the size of the naive and memory pool are not completely understood. Here, we examine the involvement of the major histocompatibility complex (MHC) in survival and homeostatic expansion of naive and memory T cells. We show that the homeostatic behavior of naive T cell receptor (TCR)-transgenic T cells can be deduced by the expression levels of TCR and CD5, a negative regulator of TCR signaling. Both these factors determine the strength of TCR stimulation by MHC-derived signals. We further show that, similarly to naive T cells, MHC-derived signals influence the homeostatic expansion capacity of memory T cells under lymphopenic conditions. In contrast to naive T cells, however, memory T cells can reach a homeostatic equilibrium, in which survival/self-renewal of each clone is dissociated from their avidity for MHC-derived signals.

MHC-dependent survival of naïve T cells? A complicated answer to a simple question

The differentiation and survival of developing alpha beta thymocytes depends on effective T-cell receptor (TCR) signaling upon recognition of self peptide/major histocompatibility complex (MHC) molecule ligands. Although this concept is uniformly accepted with regard to immature thymocytes, there are conflicting reports as to whether or not MHC recognition is required for survival of mature peripheral naïve T cells. In this review, we assess these reports critically and conclude that in many cases, the differences observed in CD4(+) T-cell recovery between MHC-expressing and MHC-deficient animals can be attributed to proliferation occurring only in the MHC-expressing lymphopenic animals studied in these models systems, rather than to effects of MHC recognition on cell viability per se. Still other reports involve experimental manipulations that may have affected the intrathymic development of the T cells such that they receive a "poor" selecting signal, fail to fully mature, and thus behave more like thymocytes in their survival characteristics (i.e., show MHC dependence). With respect to CD8(+) T cells, we discuss data suggesting that some clones are more dependent upon the presence of MHC class I for survival than others. We propose that some CD8(+) T cells even in a wild-type host may behave like the manipulated CD4(+) T cells just described, and fail to mature completely with respect to their survival requirements. Although the proportion of CD8(+) cells in this MHC-dependent state is not known, the corresponding fraction among CD4(+) T cells seems to be rather small. Overall, our analysis of the available data suggests that most or all mature CD4(+) (and perhaps also many CD8(+)) T lymphocytes do not depend on self-recognition for their viability in the periphery.

Interactions between MHC molecules and co-receptors of the TCR

Genetic experiments indicate similarity between binding sites on MHC class I (MHCI) for CD8 and on MHCII for CD4, but the crystal structures of CD8/MHCI and CD4/MHCII complexes suggest critical differences between the interfaces in the two complexes. Biophysical analyses using ectodomains of co-receptors and MHC molecules demonstrate extremely fast kinetics and low-affinity interactions. Experiments with soluble multimeric MHC ligands suggest that CD4 and CD8 may differ in the mechanisms by which they promote the formation of ternary TCR/MHC/co-receptor complexes. Co-receptor-influenced duration of TCR signaling controls thymocyte selection. In naïve T cells, CD4/MHCII interactions may promote T-cell survival. Temporal and spatial analysis of TCR and CD4 co-clustering in the immunological synapse suggests that CD4 recruitment is regulated by the half-life of the initial TCR/MHCII complex. Diverse experimental systems have yielded conflicting data that have helped to formulate revised mechanistic models of co-receptor function.

Homeostatic competition among T cells revealed by conditional inactivation of the mouse Cd4 gene

Absence of CD4 impairs the efficiency of T cell receptor (TCR) signaling in response to major histocompatibility complex (MHC) class II-presented peptides. Here we use mice carrying a conditional Cd4 allele to study the consequences of impaired TCR signaling after the completion of thymocyte development. We show that loss of CD4 decreases the steady-state proliferation of T cells as monitored by in vivo labeling with bromo-deoxyuridine. Moreover, T cells lacking CD4 compete poorly with CD4-expressing T cells during proliferative expansion after transfer into lymphopenic recipients. The data suggest that T cells compete with one another during homeostatic proliferation, and indicate that the basis of this competition is TCR signaling.

Variations in serum IL-7 and 90K/Mac-2 binding protein (Mac-2 BP) levels analysed in cohorts of HIV-1 patients and correlated with clinical changes following antiretroviral therapy

Serum levels of interleukin-7 (IL-7), a non-redundant cytokine that plays a crucial role in lymphopoiesis, are known to be elevated in HIV-1-infected subjects. To examine further the association between levels of IL-7, CD4+ cell counts and viraemia, we analysed these parameters in a large cohort of HIV-1 patients along with serum levels of 90K, a marker of disease severity but with no established involvement in lymphopoiesis. While IL-7 levels were only found to correlate with CD4+ cell counts, 90K levels presented strong correlations with both CD4+ cell numbers and with plasma viral loads (VLs). These correlations were maintained in patients naive to treatment with antiretrovirals (n = 38) but were abolished when the analysis was restricted to the group receiving highly active antiretroviral therapy (HAART, n = 82). Moreover, although 90K levels were significantly reduced in patients on HAART, IL-7 levels continued to be elevated despite successful treatment. The influence of HAART on the variations in these serum parameters was further assessed in a longitudinal study on 32 subjects. The HAART-induced decrease in VLs and increase in CD4+ counts were found to correlate with a reduced serum level of 90K and IL-7, respectively. Nevertheless, following a median period of 33 months of immunological and virological successful HAART, serum levels of IL-7 continued to be significantly elevated compared with those detected in healthy controls. These findings suggest that immunotherapy with IL-7, aimed to replenish T-cell stock in HAART-treated subjects, may have a limited impact on the process of immune reconstitution.

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.