Jonathan L. Linehan

Different routes of bacterial infection induce long-lived T H 1 memory cells and short-lived T H 17 cells

We used a sensitive method based on tetramers of peptide and major histocompatibility complex II (pMHCII) to determine whether CD4+ memory T cells resemble the T helper type 1 (TH1) and interleukin 17 (IL-17)-producing T helper (TH17) subsets described in vitro. Intravenous or intranasal infection with Listeria monocytogenes induced pMHCII-specific CD4+ naive T cells to proliferate and produce effector cells, about 10% of which resembled TH1 or TH17 cells, respectively. TH1 cells were also present among the memory cells that survived 3 months after infection, whereas TH17 cells disappeared. The short lifespan of TH17 cells was associated with small amounts of the antiapoptotic protein Bcl-2, the IL-15 receptor and the receptor CD27, and little homeostatic proliferation. These results suggest that TH1 cells induced by intravenous infection are more efficient at entering the memory pool than are TH17 cells induced by intranasal infection.

Single naive CD4+ T cells from a diverse repertoire produce different effector cell types during infection.

A naive CD4(+) T cell population specific for a microbial peptide:major histocompatibility complex II ligand (p:MHCII) typically consists of about 100 cells, each with a different T cell receptor (TCR). Following infection, this population produces a consistent ratio of effector cells that activate microbicidal functions of macrophages or help B cells make antibodies. We studied the mechanism that underlies this division of labor by tracking the progeny of single naive T cells. Different naive cells produced distinct ratios of macrophage and B cell helpers but yielded the characteristic ratio when averaged together. The effector cell pattern produced by a given naive cell correlated with the TCR-p:MHCII dwell time or the amount of p:MHCII. Thus, the consistent production of effector cell subsets by a polyclonal population of naive cells results from averaging the diverse behaviors of individual clones, which are instructed in part by the strength of TCR signaling.

Focused specificity of intestinal TH17 cells towards commensal bacterial antigens.

T-helper-17 (TH17) cells have critical roles in mucosal defence and in autoimmune disease pathogenesis. They are most abundant in the small intestine lamina propria, where their presence requires colonization of mice with microbiota. Segmented filamentous bacteria (SFB) are sufficient to induce TH17 cells and to promote TH17-dependent autoimmune disease in animal models. However, the specificity of TH17 cells, the mechanism of their induction by distinct bacteria, and the means by which they foster tissue-specific inflammation remain unknown. Here we show that the T-cell antigen receptor (TCR) repertoire of intestinal TH17 cells in SFB-colonized mice has minimal overlap with that of other intestinal CD4+ T cells and that most TH17 cells, but not other T cells, recognize antigens encoded by SFB. T cells with antigen receptors specific for SFB-encoded peptides differentiated into RORγt-expressing TH17 cells, even if SFB-colonized mice also harboured a strong TH1 cell inducer, Listeria monocytogenes, in their intestine. The match of T-cell effector function with antigen specificity is thus determined by the type of bacteria that produce the antigen. These findings have significant implications for understanding how commensal microbiota contribute to organ-specific autoimmunity and for developing novel mucosal vaccines.

Commensal–dendritic-cell interaction specifies a unique protective skin immune signature

The skin represents the primary interface between the host and the environment. This organ is also home to trillions of microorganisms that play an important role in tissue homeostasis and local immunity. Skin microbial communities are highly diverse and can be remodelled over time or in response to environmental challenges. How, in the context of this complexity, individual commensal microorganisms may differentially modulate skin immunity and the consequences of these responses for tissue physiology remains unclear. Here we show that defined commensals dominantly affect skin immunity and identify the cellular mediators involved in this specification. In particular, colonization with Staphylococcus epidermidis induces IL-17A+ CD8+ T cells that home to the epidermis, enhance innate barrier immunity and limit pathogen invasion. Commensal-specific T-cell responses result from the coordinated action of skin-resident dendritic cell subsets and are not associated with inflammation, revealing that tissue-resident cells are poised to sense and respond to alterations in microbial communities. This interaction may represent an evolutionary means by which the skin immune system uses fluctuating commensal signals to calibrate barrier immunity and provide heterologous protection against invasive pathogens. These findings reveal that the skin immune landscape is a highly dynamic environment that can be rapidly and specifically remodelled by encounters with defined commensals, findings that have profound implications for our understanding of tissue-specific immunity and pathologies.

Efficient Transfection of Embryonic and Adult Stem Cells

The ability of embryonic stem cells and adult stem cells to differentiate into specific cell types holds immense potential for therapeutic use in cell and gene therapy. Realization of this potential depends on efficient and optimized protocols for genetic manipulation of stem cells. In the study reported here, we demonstrate the use of nucleofection as a method to introduce plasmid DNA into embryonic and adult stem cells with significantly greater efficiency than electroporation or lipid‐based transfection methods have. Using enhanced green fluorescent protein (eGFP) as a reporter gene, mouse embryonic stem cells were transfected both transiently and stably at a rate nearly 10‐fold higher than conventional methods. The transfected cells retained their stem cell properties, including continued expression of the stem cell markers SSEA1, Oct4, and Rex1; formation of embryoid bodies; differentiation into cardiomyocytes in the presence of appropriate inducers; and, when injected into developing blastocysts, contribution to chimeras. Higher levels of transfection were also obtained with human embryonic carcinoma and human embryonic stem cells. Particularly hard‐to‐transfect adult stem cells, including bone marrow and multipotent adult progenitor cells, were also transfected efficiently by the method of nucleofection. Based on our results, we conclude that nucleofection is superior to currently available methods for introducing plasmid DNA into a variety of embryonic and adult stem cells. The high levels of transfection achieved by nucleofection will enable its use as a rapid screening tool to evaluate the effect of ectopically expressed transcription factors on tissue‐specific differentiation of stem cells.

Hematopoietic Engraftment of Human Embryonic Stem Cell- Derived Cells Is Regulated by Recipient Innate Immunity

Human embryonic stem cells (hESCs) provide an important means to characterize early stages of hematopoietic development. However, the in vivo potential of hESC‐derived hematopoietic cells has not been well defined. We demonstrate that hESC‐derived cells are capable of long‐term hematopoietic engraftment when transplanted into nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Human CD45+ and CD34+ cells are identified in the mouse bone marrow (BM) more than 3 months after injection of hESCs that were allowed to differentiate on S17 stromal cells for 7–24 days. Secondary engraftment studies further confirm long‐term repopulating cells derived from hESCs. We also evaluated two mechanisms that may inhibit engraftment: host immunity and requirement for homing to BM. Treatment with anti‐ASGM1 antiserum that primarily acts by depletion of natural killer cells in transplanted mice leads to improved engraftment, likely due to low levels of HLA class I expressed on hESCs and CD34+ cells derived from hESCs. Intra‐BM injection also provided stable engraftment, with hematopoietic cells identified in both the injected and contra‐lateral femur. Importantly, no teratomas are evident in animals injected with differentiated hESCs. These results demonstrate that SCID‐repopulating cells, a close surrogate for hematopoietic stem cells, can be derived from hESCs. Moreover, both adaptive and innate immune effector cells may be barriers to engraftment of these cells.

Efficient and Stable Transgene Expression in Human Embryonic Stem Cells Using Transposon‐Mediated Gene Transfer

Efficient and stable genetic modification of human embryonic stem (ES) cells is required to realize the full scientific and potential therapeutic use of these cells. Currently, only limited success toward this goal has been achieved without using a viral vector. The Sleeping Beauty (SB) transposon system mediates nonviral gene insertion and stable expression in target cells and tissues. Here, we demonstrate use of the nonviral SB transposon system to effectively mediate stable gene transfer in human ES cells. Transposons encoding (a) green fluorescent protein coupled to the zeocin gene or (b) the firefly luciferase (luc) gene were effectively delivered to undifferentiated human ES cells with either a DNA or RNA source of transposase. Only human ES cells cotransfected with transposon‐ and transposase‐encoding sequences exhibited transgene expression after 1 week in culture. Molecular analysis of transposon integrants indicated that 98% of stable gene transfer resulted from transposition. Stable luc expression was observed up to 5 months in human ES cells cotransfected with a transposon along with either DNA or RNA encoding SB transposase. Genetically engineered human ES cells demonstrated the ability to differentiate into teratomas in vivo and mature hematopoietic cells in vitro while maintaining stable transgene expression. We conclude that the SB transposon system provides an effective approach with several advantages for genetic manipulation and durable gene expression in human ES cells.

Removing T-cell epitopes with computational protein design.

Significance Proteins represent the fastest-growing class of pharmaceuticals for a diverse range of clinical applications. Computational protein design has the potential to create a novel class of therapeutics with tunable biophysical properties. However, the immune system reacts to T-cell epitope sequences in non-human proteins, leading to neutralization and elimination by the immune system. Here, we combine machine learning with structure-based protein design to identify and redesign T-cell epitopes without disrupting function of the target protein. We test the method experimentally, removing T-cell epitopes from GFP and Pseudomonas exotoxin A while maintaining function. Immune responses can make protein therapeutics ineffective or even dangerous. We describe a general computational protein design method for reducing immunogenicity by eliminating known and predicted T-cell epitopes and maximizing the content of human peptide sequences without disrupting protein structure and function. We show that the method recapitulates previous experimental results on immunogenicity reduction, and we use it to disrupt T-cell epitopes in GFP and Pseudomonas exotoxin A without disrupting function.

Tolerance is established in polyclonal CD4+ T cells by distinct mechanisms, according to self-peptide expression patterns

Studies of repertoires of mouse monoclonal CD4+ T cells have revealed several mechanisms of self-tolerance; however, which mechanisms operate in normal repertoires is unclear. Here we studied polyclonal CD4+ T cells specific for green fluorescent protein expressed in various organs, which allowed us to determine the effects of specific expression patterns on the same epitope-specific T cells. Peptides presented uniformly by thymic antigen-presenting cells were tolerated by clonal deletion, whereas peptides excluded from the thymus were ignored. Peptides with limited thymic expression induced partial clonal deletion and impaired effector T cell potential but enhanced regulatory T cell potential. These mechanisms were also active for T cell populations specific for endogenously expressed self antigens. Thus, the immunotolerance of polyclonal CD4+ T cells was maintained by distinct mechanisms, according to self-peptide expression patterns.

Robust Antigen Specific Th17 T Cell Response to Group A Streptococcus Is Dependent on IL-6 and Intranasal Route of Infection

Group A streptococcus (GAS, Streptococcus pyogenes) is the cause of a variety of clinical conditions, ranging from pharyngitis to autoimmune disease. Peptide-major histocompatibility complex class II (pMHCII) tetramers have recently emerged as a highly sensitive means to quantify pMHCII-specific CD4+ helper T cells and evaluate their contribution to both protective immunity and autoimmune complications induced by specific bacterial pathogens. In lieu of identifying an immunodominant peptide expressed by GAS, a surrogate peptide (2W) was fused to the highly expressed M1 protein on the surface of GAS to allow in-depth analysis of the CD4+ helper T cell response in C57BL/6 mice that express the I-Ab MHCII molecule. Following intranasal inoculation with GAS-2W, antigen-experienced 2W:I-Ab-specific CD4+ T cells were identified in the nasal-associated lymphoid tissue (NALT) that produced IL-17A or IL-17A and IFN-γ if infection was recurrent. The dominant Th17 response was also dependent on the intranasal route of inoculation; intravenous or subcutaneous inoculations produced primarily IFN-γ+ 2W:I-Ab+ CD4+ T cells. The acquisition of IL-17A production by 2W:I-Ab-specific T cells and the capacity of mice to survive infection depended on the innate cytokine IL-6. IL-6-deficient mice that survived infection became long-term carriers despite the presence of abundant IFN-γ-producing 2W:I-Ab-specific CD4+ T cells. Our results suggest that an imbalance between IL-17- and IFN-γ-producing CD4+ T cells could contribute to GAS carriage in humans.