Mette D. Hazenberg

T cell receptor excision circles as markers for recent thymic emigrants: basic aspects, technical approach, and guidelines for interpretation

T cell differentiation in the thymus is characterized by a hierarchical order of rearrangement steps in the T cell receptor (TCR) genes, resulting in the joining of V, D, and J gene segments. During each of the rearrangement steps, DNA fragments between rearranging V, D, and J gene segments are deleted as circular excision products, the so-called TRECs (T cell receptor excision circles). TRECs are assumed to have a high over-time stability, but they can not multiply and consequently are diluted during T cell proliferation. It was recently suggested that quantitative detection of TRECs would allow for direct measurement of thymic output. The δRec-ψJα TREC appears to be the best marker, because the majority of thymocyte expansion occurs before this TREC is formed. However, apart from thymic output several other factors determine the TREC content of a T cell population, such as cell division and cell death. Likewise, the number of TRECs depends not only on thymic output, but also on the longevity of naive T cells. This warrants caution with regard to the interpretation of TREC data as measured in healthy and diseased individuals. δRec-ψJα TREC detection is a new and elegant tool for identification of recent thymic emigrants in the periphery, but further research is required for making quantitative estimations of thymic output with the use of TREC analysis.

Human innate lymphoid cells

Innate lymphoid cells (ILCs) are lymphoid cells that do not express rearranged receptors and have important effector and regulatory functions in innate immunity and tissue remodeling. ILCs are categorized into 3 groups based on their distinct patterns of cytokine production and the requirement of particular transcription factors for their development and function. Group 1 ILCs (ILC1s) produce interferon γ and depend on Tbet, group 2 ILCs (ILC2s) produce type 2 cytokines like interleukin-5 (IL-5) and IL-13 and require GATA3, and group 3 ILCs (ILC3s) include lymphoid tissue inducer cells, produce IL-17 and/or IL-22, and are dependent on RORγt. Whereas ILCs play essential roles in the innate immune system, uncontrolled activation and proliferation of ILCs can contribute to inflammatory autoimmune diseases. In this review, we provide an overview of the characteristics of ILCs in the context of health and disease. We will focus on human ILCs but refer to mouse studies if needed to clarify aspects of ILC biology.

Interleukin-12 and -23 Control Plasticity of CD127+ Group 1 and Group 3 Innate Lymphoid Cells in the Intestinal Lamina Propria

Human group 1 ILCs consist of at least three phenotypically distinct subsets, including NK cells, CD127(+) ILC1, and intraepithelial CD103(+) ILC1. In inflamed intestinal tissues from Crohns disease patients, numbers of CD127(+) ILC1 increased at the cost of ILC3. Here we found that differentiation of ILC3 to CD127(+) ILC1 is reversible in vitro and in vivo. CD127(+) ILC1 differentiated to ILC3 in the presence of interleukin-2 (IL-2), IL-23, and IL-1β dependent on the transcription factor RORγt, and this process was enhanced in the presence of retinoic acid. Furthermore, we observed in resection specimen from Crohns disease patients a higher proportion of CD14(+) dendritic cells (DC), which in vitro promoted polarization from ILC3 to CD127(+) ILC1. In contrast, CD14(-) DCs promoted differentiation from CD127(+) ILC1 toward ILC3. These observations suggest that environmental cues determine the composition, function, and phenotype of CD127(+) ILC1 and ILC3 in the gut.

Selective Decrease in Circulating Vα24 + Vβ11 + NKT Cells During HIV Type 1 Infection

CD1d-restricted NKT cells express an invariant TCR and have been demonstrated to play an important regulatory role in a variety of immune responses. Invariant NKT cells down-regulate autoimmune responses by production of type 2 cytokines and can initiate antitumor and antimicrobial immune responses by production of type 1 cytokines. Although defects in the (invariant) Vα24+Vβ11+ NKT cell population have been observed in patients with cancer and autoimmune diseases, little is known regarding the protective role of Vα24+Vβ11+ NKT cells in human infectious disease. In a cross-sectional study in HIV-1-infected individuals, we found circulating numbers of Vα24+Vβ11+ NKT cells to be reduced, independent of CD4+ T cell counts, CD4:CD8 ratios, and viral load. Because a small minority of Vα24+Vβ11+ NKT cells of healthy donors expressed HIV-1 (co)receptors and the vast majority of Vα24+Vβ11+ NKT cells in HIV-1-infected individuals expressed the Fas receptor, the depletion was more likely due to Fas-mediated apoptosis than to preferential infection of Vα24+Vβ11+ NKT cells by HIV-1. A longitudinal cohort study, in which patients were analyzed before seroconversion and 1 and 5 years after seroconversion, demonstrated that a large proportion of the depletion occurred within the first year postseroconversion. In this longitudinal study no evidence was found to support an important role of Vα24+Vβ11+ NKT cells in determining the rate of progression during HIV-1 infection.

Thymic output: a bad TREC record

TREC assays are used to detect recent thymic emigrants and quantitate thymic output. However, the longevity of naive T cells combined with T cell division suggest TREC data should be interpreted with caution.

Composition of Innate Lymphoid Cell Subsets in the Human Skin: Enrichment of NCR+ ILC3 in Lesional Skin and Blood of Psoriasis Patients

Innate lymphoid cells (ILCs) are increasingly appreciated as important regulators of tissue homeostasis and inflammation. However, their role in human skin remains obscure. We found that healthy peripheral blood CD117(+) ILC3, lacking the natural cytotoxicity receptor (NCR) NKp44 (NCR(-) ILC3), CD117(-)NCR(-)CRTH2(-)CD161(+) ILC1, and CRTH2(+) ILC2, express the skin-homing receptor cutaneous lymphocyte antigen (CLA). NCR(+) ILC3 were scarce in peripheral blood. Consistently, we identified in normal skin ILC2 and NCR(-) ILC3, a small proportion of CD161(+) ILC1, and hardly any NCR(+) ILC3, whereas NCR(+) ILC3 were present in cultured dermal explants. The skin ILC2 and NCR(+) ILC3 subsets produced IL-13 and IL-22, respectively, upon cytokine stimulation. Remarkably, dermal NCR(-) ILC3 converted to NCR(+) ILC3 upon culture in IL-1β plus IL-23, cytokines known to be involved in psoriatic inflammation. In line with this observation, significantly increased proportions of NCR(+) ILC3 were present in lesional skin and peripheral blood of psoriasis patients as compared with skin and blood of healthy individuals, respectively, whereas the proportions of ILC2 and CD161(+) ILC1 remained unchanged. NCR(+) ILC3 from skin and blood of psoriasis patients produced IL-22, which is regarded as a key driver of epidermal thickening, suggesting that NCR(+) ILC3 may participate in psoriasis pathology.

Sparse production but preferential incorporation of recently produced naïve T cells in the human peripheral pool

In mice, recent thymic emigrants (RTEs) make up a large part of the naïve T cell pool and have been suggested to be a distinct short-lived pool. In humans, however, the life span and number of RTEs are unknown. Although 2H2O labeling in young mice showed high thymic-dependent daily naïve T cell production, long term up- and down-labeling with 2H2O in human adults revealed a low daily production of naïve T cells. Using mathematical modeling, we estimated human naïve CD4 and CD8 T cell half-lives of 4.2 and 6.5 years, respectively, whereas memory CD4 and CD8 T cells had half-lives of 0.4 and 0.7 year. The estimated half-life of recently produced naïve T cells was much longer than these average half-lives. Thus, our data are incompatible with a substantial short-lived RTE population in human adults and suggest that the few naïve T cells that are newly produced are preferentially incorporated in the peripheral pool.

Low-Level CD4+ T Cell Activation Is Associated with Low Susceptibility to HIV-1 Infection

Different features have been associated with low susceptibility to HIV type 1 (HIV-1) infection in exposed seronegative individuals. These include genetic make-up such as homozygosity for the CCR5-Δ32 allele and the presence of HIV-specific CTLs. We studied immune activation and immune responsiveness in relation to HIV-1 susceptibility in 42 high-risk seronegative (HRSN) participants of the Amsterdam Cohort Studies and 54 men from the same cohort who were seronegative at the moment of analysis but later became HIV seropositive. HRSN had higher naive (CD45RO CD27) CD4 and CD8 T cell numbers and lower percentages of activated (HLADR CD38, CD70) CD4 and proliferating (Ki67) CD4 and CD8 T cells, irrespective of previous episodes of sexually transmittable infections. Furthermore, whole blood cultures from HRSN showed lower lymphoproliferative responses than healthy laboratory controls. These data suggest that low levels of immune activation and low T cell responsiveness may contribute to low HIV susceptibility.

Idiopathic CD4+ T lymphopenia without autoimmunity or granulomatous disease in the slipstream of RAG mutations

A girl presented during childhood with a single course of extensive chickenpox and moderate albeit recurrent pneumonia in the presence of idiopathic CD4+ T lymphocytopenia (ICL). Her clinical condition remained stable over the past 10 years without infections, any granulomatous disease, or autoimmunity. Immunophenotyping demonstrated strongly reduced naive T and B cells with intact proliferative capacity. Antibody reactivity on in vivo immunizations was normal. T-cell receptor-Vβ repertoire was polyclonal with a very low content of T-cell receptor excision circles (TRECs). Kappa-deleting recombination excision circles (KRECs) were also abnormal in the B cells. Both reflect extensive in vivo proliferation. Patient-derived CD34+ hematopoietic stem cells could not repopulate RAG2(-/-)IL2Rγc(-/-) mice, indicating the lymphoid origin of the defect. We identified 2 novel missense mutations in RAG1 (p.Arg474Cys and p.Leu506Phe) resulting in reduced RAG activity. This report gives the first genetic clue for ICL and extends the clinical spectrum of RAG mutations from severe immune defects to an almost normal condition.

Naïve and Memory Cell Turnover as Drivers of CCR5-to-CXCR4 Tropism Switch in Human Immunodeficiency Virus Type 1: Implications for Therapy

ABSTRACT Early human immunodeficiency virus infection is characterized by the predominance of CCR5-tropic (R5) virus. However, in many individuals CXCR4-tropic (X4) virus appears in late infection. The reasons for this phenotypic switch are unclear. The patterns of chemokine receptor expression suggest that X4 and R5 viruses have a preferential tropism for naïve and memory T cells, respectively. Since memory cells divide approximately 10 times as often as naïve cells in uninfected individuals, a tropism for memory cells in early infection may provide an advantage. However, with disease progression both naïve and memory cell division frequencies increase, and at low CD4 counts, the naïve cell division frequency approaches that of memory cells. This may provide a basis for the phenotypic switch from R5 to X4 virus observed in late infection. We show that a model of infection using observed values for cell turnover supports this mechanism. The phenotypic switch from R5 to X4 virus occurs at low CD4 counts and is accompanied by a rapid rise in viral load and drop in CD4 count. Thus, low CD4 counts are both a cause and an effect of X4 virus dominance. We also investigate the effects of different antiviral strategies. Surprisingly, these results suggest that both conventional antiretroviral regimens and CCR5 receptor-blocking drugs will promote R5 virus over X4 virus.