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By characterizing the poorly defined T cell composition in developing human fetal skin, Reitermaier et al. identify a unique population of TCR αβγδ+ T cells. These cells may contribute to early skin development and immune defense against invading pathogens in utero., T cells in human skin play an important role in the immune defense against pathogens and tumors. T cells are present already in fetal skin, where little is known about their cellular phenotype and biological function. Using single-cell analyses, we identified a naive T cell population expressing αβ and γδ T cell receptors (TCRs) that was enriched in fetal skin and intestine but not detected in other fetal organs and peripheral blood. TCR sequencing data revealed that double-positive (DP) αβγδ T cells displayed little overlap of CDR3 sequences with single-positive αβ T cells. Gene signatures, cytokine profiles and in silico receptor–ligand interaction studies indicate their contribution to early skin development. DP αβγδ T cells were phosphoantigen responsive, suggesting their participation in the protection of the fetus against pathogens in intrauterine infections. Together, our analyses unveil a unique cutaneous T cell type within the native skin microenvironment and point to fundamental differences in the immune surveillance between fetal and adult human skin., Graphical Abstract

CD4(+)FoxP3(+) regulatory T (T reg) cells comprise a separate lineage of T cells that are essential for maintaining immunological tolerance to self. The molecular mechanism(s) by which T reg cells mediate their suppressive effects remains poorly understood. One molecule that has been extensively studied in T reg cell suppression is transforming growth factor (TGF)-beta, but its importance remains controversial. We found that TGF-beta complexed to latency-associated peptide (LAP) is expressed on the cell surface of activated but not resting T reg cells. T reg cell LAP-TGF-beta plays an important role in the suppression of the proliferation of activated T cells, but it is not required for the suppression of naive T cell activation. More importantly, T reg cell-derived TGF-beta could generate de novo CD4(+)FoxP3(+) T cells in vitro from naive precursors in a cell contact-dependent, antigen-presenting cell-independent and alpha(V) integrin-independent manner. The newly induced CD4(+)FoxP3(+) T cells are suppressive both in vitro and in vivo. Transfer of activated antigen-specific T reg cells with naive antigen-specific responder T cells to normal recipients, followed by immunization, also results in induction of FoxP3 expression in the responder cells. T reg cell-mediated generation of functional CD4(+)FoxP3(+) cells via this TGF-beta-dependent pathway may represent a major mechanism as to how T reg cells maintain tolerance and expand their suppressive abilities.

Plasmacytoid dendritic cells (PDCs) play a pivotal role as cytokine-secreting accessory cells in the antimicrobial immune defense. In contrast, the capacity of PDCs to act as antigen-presenting cells in naive T cell priming remains unclear. By studying T cell responses in mice that lack conventional DCs (cDCs), and by the use of a PDC-specific antigen-targeting strategy, we show that PDCs can initiate productive naive CD4+ T cell responses in lymph nodes, but not in the spleen. PDC-triggered CD4+ T cell responses differed from cDC-driven responses in that they were not associated with concomitant CD8+ T cell priming. Our results establish PDCs as a bona fide DC subset that initiates unique CD4+ Th cell–dominated primary immune responses.

To determine how the α(1,3)fucosyltransferases Fuc-TIV and Fuc-TVII, and the selectin ligands they control may contribute to the adaptive immune response, contact hypersensitivity (CHS) was characterized in mice deficient in either or both enzymes. We find a substantial CHS deficiency in Fuc-TVII−/− mice, and a complete deficiency in Fuc-TIV−/−/Fuc-TVII−/− mice. These defects are not accounted for by alterations in the number or function of epidermal Langerhans cells required for cutaneous antigen processing and presentation. By contrast, defective CHS in Fuc-TVII−/− mice or Fuc-TIV−/−/Fuc-TVII−/− mice is attributed in part to prominent, or nearly complete deficiencies, respectively, in the complement of naive T lymphocytes available in lymph nodes for antigen-dependent activation, expansion, differentiation, and dissemination. Fuc-TVII deficiency also deletes expression of E- and P-selectin ligands by Th1 and T cytotoxic 1 (Tc1) lymphocytes, annuls T cell trafficking to inflamed cutaneous sites in vivo, and thereby controls an essential component of the efferent phase of the cutaneous immune response. These observations indicate that collaborative contributions of Fuc-TIV and Fuc-TVII to L-selectin ligand synthesis, and to lymphocyte recruitment, are requisite components of the primary cellular immune response, and assign an essential role to Fuc-TVII in control of E- and P-selectin ligand expression by Th1 and Tc1 lymphocytes.

The subset of dendritic cells (DCs) and the nature of the signal inducing DC maturation determine the capacity of DCs to generate polarized immune responses. In this study, we show that the ability of human monocyte-derived DCs (myeloid DC1) to promote T helper type 1 (Th1) or Th2 differentiation was also found to be critically dependent on stimulator/responder ratio. At a low ratio (1:300), mature DCs that have been differentiated after inflammatory (Staphylococcus aureus Cowan 1 or lipopolysaccharide) or T cell–dependent (CD40 ligand) stimulation induced naive T cells to become Th2 (interleukin [IL]-4+, IL-5+, interferon γ) effectors. Th2 differentiation was dependent on B7–CD28 costimulation and enhanced by OX40–OX40 ligand interactions. However, high DC/T cell ratio (1:4) favored a mixed Th1/Th2 cell development. Thus, the fact that the same DC lineage stimulates polarized Th1 or Th2 responses may be relevant since it allows the antigen-presenting cells to initiate an appropriate response for the signal received at the peripheral sites. Controlling the number and the rate of DC migration to the T cell areas in lymphoid tissues may be important for the therapeutic use of DCs.

T cell tolerance to parenchymal self-antigens is thought to be induced by encounter of the T cell with its cognate peptide–major histocompatibility complex (MHC) ligand expressed on the parenchymal cell, which lacks appropriate costimulatory function. We have used a model system in which naive T cell receptor (TCR) transgenic hemagglutinin (HA)-specific CD4+ T cells are adoptively transferred into mice expressing HA as a self-antigen on parenchymal cells. After transfer, HA-specific T cells develop a phenotype indicative of TCR engagement and are rendered functionally tolerant. However, T cell tolerance is not induced by peptide–MHC complexes expressed on parenchymal cells. Rather, tolerance induction requires that HA is presented by bone marrow (BM)–derived cells. These results indicate that tolerance induction to parenchymal self-antigens requires transfer to a BM-derived antigen-presenting cell that presents it to T cells in a tolerogenic fashion.

A diverse T cell repertoire is essential for a vigorous immune response to new infections, and decreasing repertoire diversity has been implicated in the age-associated decline in CD8 T cell immunity. In this study, using the well-characterized mouse influenza virus model, we show that although comparable numbers of CD8 T cells are elicited in the lung and lung airways of young and aged mice after de novo infection, a majority of aged mice exhibit profound shifts in epitope immunodominance and restricted diversity in the TCR repertoire of responding cells. A preferential decline in reactivity to viral epitopes with a low naive precursor frequency was observed, in some cases leading to “holes” in the T cell repertoire. These effects were also seen in young thymectomized mice, consistent with the role of the thymus in maintaining naive repertoire diversity. Furthermore, a decline in repertoire diversity generally correlated with impaired responses to heterosubtypic challenge. This study formally demonstrates in a mouse infection model that naturally occurring contraction of the naive T cell repertoire can result in impaired CD8 T cell responses to known immunodominant epitopes and decline in heterosubtypic immunity. These observations have important implications for the design of vaccine strategies for the elderly.

The differentiation of antigen-stimulated naive CD4 T cells into T helper (Th)1 or Th2 effector cells can be prevented in vitro by transforming growth factor (TGF)-beta and anti-interferon (IFN)-gamma. These cells proliferate and synthesize interleukin (IL)-2 but not IFN-gamma or IL-4, and can differentiate into either Th1 or Th2 cells. We have now used two-color Elispots to reveal substantial numbers of primed cells producing IL-2 but not IL-4 or IFN-gamma during the Th1- or Th2-biased immune responses induced by soluble proteins or with adjuvants. These cells were CD4(+)CD44(high) and were present during immediate and long-term immune responses of normal mice. Naive T cell receptor for antigen (TCR) transgenic (DO11.10) T cells were primed in vivo after adoptive transfer into normal hosts and FACS((R)) cloned under conditions that did not allow further differentiation. After clonal proliferation, aliquots of each clone were cultured in Th1- or Th2-inducing conditions. Many in vivo-primed cells were uncommitted, secreting IL-2 but not IL-4 or IFN-gamma at the first cloning step, but secreting either IL-4 or IFN-gamma after differentiation in the appropriate conditions. These in vivo-primed, uncommitted, IL-2-producing cells may constitute an expanded pool of antigen-specific cells that provide extra flexibility for immune responses by differentiating into Th1 or Th2 phenotypes later during the same or subsequent immune responses.

A system that allows the study, in a gentle fashion, of the role of MHC molecules in naive T cell survival is described. Major histocompatibility complex class II–deficient mice were engineered to express Eα chains only in thymic epithelial cells in a tetracycline (tet)-controllable manner. This resulted in tet-responsive display of cell surface E complexes, positive selection of CD4+8– thymocytes, and generation of a CD4+ T cell compartment in a class II–barren periphery. Using this system, we have addressed two unresolved issues: the half-life of naive CD4+ T cells in the absence of class II molecules (3–4 wk) and the early signaling events associated with class II molecule engagement by naive CD4+ T cells (partial CD3 ζ chain phosphorylation and ZAP-70 association).

In a screen designed to identify genes that regulate T cell receptor (TCR)/CD3-mediated apoptosis, we found that high level expression of CD43 protected T cell hybridomas from activation-induced cell death. The protection appears to result from its capacity to block Fas-mediated death signals rather than from inhibition of the upregulation of Fas and/or Fas ligand after T cell stimulation. We found that peripheral CD4(+) T cells can be divided into two subsets based on the level of CD43 surface expression. The CD4(+)CD43(low) subset exhibits a naive T cell phenotype, being CD62L(high)CD45RB(high)CD44(low), whereas CD4(+)CD43(high) cells exhibit a memory phenotype, being CD62L(low)CD45RB(low)CD44(high). Recent studies have demonstrated that engagement of TCR and Fas induces naive CD4(+) T cells to undergo apoptosis, and the same treatment enhances the proliferation of memory CD4(+) T cells. We confirm here that peripheral CD4(+)CD43(high) T cells are resistant to TCR/CD3-mediated cell death. These results suggest that the expression levels of CD43 on naive and memory CD4(+) T cells determine their susceptibility to Fas-dependent cell death and that high level expression of CD43 may be used as a marker to define CD4(+) memory T cells. Expression of CD43 provides a novel mechanism by which tumor cells expressing abnormally high levels of CD43 may escape Fas-mediated killing.

Because of the low frequency of T cells for any particular soluble protein antigen in unprimed animals, the requirements for naive T cell responses in specific antigens have not been clearly delineated and they have been difficult to study in vitro. We have taken advantage of mice transgenic for the V beta 3/V alpha 11 T cell receptor (TCR), which can recognize a peptide of cytochrome c presented by IEk. 85-90% of CD4+ T cells in these mice express the transgenic TCR, and we show that almost all such V beta 3/V alpha 11 receptor-positive cells have a phenotype characteristic of naive T cells, including expression of high levels of CD45RB, high levels of L-selectin (Mel-14), low levels of CD44 (Pgp-1), and secretion of interleukin 2 (IL-2) as the major cytokine. Naive T cells, separated on the basis of CD45RB high expression, gave vigorous responses (proliferation and IL-2 secretion) to peptide antigen presented in vitro by a mixed antigen-presenting cell population. At least 50% of the T cell population appeared to respond, as assessed by blast transformation, entry into G1, and expression of increased levels of CD44 by 24 h. Significant contributions to the response by contaminating memory CD4+ cells were ruled out by demonstrating that the majority of the CD45RB low, L-selectin low, CD44 high cells did not express the V beta 3/V alpha 11 TCR and responded poorly to antigen. We find that proliferation and IL-2 secretion of the naive CD4 cells is minimal when resting B cells present peptide antigen, and that both splenic and bone marrow-derived macrophages are weak stimulators. Naive T cells did respond well to high numbers of activated B cells. However, dendritic cells were the most potent stimulators of proliferation and IL-2 secretion at low cell numbers, and were far superior inducers of IL-2 at higher numbers. These studies establish that naive CD4 T cells can respond vigorously to soluble antigen and indicate that maximal stimulation can be achieved by presentation of antigen on dendritic cells. This model should prove very useful in further investigations of activation requirements and functional characteristics of naive helper T cells.