Marion Pepper

Acute Gastrointestinal Infection Induces Long-Lived Microbiota-Specific T Cell Responses

Recognizing Escaped Commensals In order to coexist peacefully, the billions of bacteria in our gut and our immune system have reached a détente. An intestinal mucosal firewall exists, so bacteria remain localized to the gut, where the immune system is tightly regulated so that these bacteria are tolerated. Enteric infections, however, lead to a breach in this mucosal firewall, resulting in exposure of the peripheral immune system to the intestinal bacterial contents. What is the result? Using oral Toxoplasma gondii infection in mice, Hand et al. (p. 1553, published online 23 August) show that, besides the T. gondii–specific T cell response, a commensal bacteria–specific T cell response is elicited. The CD4+ T cell–specific response was tracked to a commensal-derived flagellin, and these T cells expanded after T. gondii infection and formed long-lived memory cells able to respond to subsequent challenges. Thus, enteric infections can lead to the formation of commensal bacteria–specific, long-lived memory T cells that reside throughout the body—which may play a role in intestinal pathologies such as inflammatory bowel disease. Enteric infections induce lasting adaptive immunity against commensal bacteria that may play a role in intestinal problems. The mammalian gastrointestinal tract contains a large and diverse population of commensal bacteria and is also one of the primary sites of exposure to pathogens. How the immune system perceives commensals in the context of mucosal infection is unclear. Here, we show that during a gastrointestinal infection, tolerance to commensals is lost, and microbiota-specific T cells are activated and differentiate to inflammatory effector cells. Furthermore, these T cells go on to form memory cells that are phenotypically and functionally consistent with pathogen-specific T cells. Our results suggest that during a gastrointestinal infection, the immune response to commensals parallels the immune response against pathogenic microbes and that adaptive responses against commensals are an integral component of mucosal immunity.

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.

Origins of CD4+ effector and central memory T cells

Lineage-committed effector CD4+ T cells are generated at the peak of the primary response and are followed by heterogeneous populations of central and effector memory cells. Here we review the evidence that T helper type 1 (TH1) effector cells survive the contraction phase of the primary response and become effector memory cells. We discuss the applicability of this idea to the TH2 cell, TH17 helper T cell, follicular helper T cell (TFH cell) and induced regulatory T cell lineages. We also discuss how central memory cells are formed, with an emphasis on the role of B cells in this process.

Tracking epitope-specific T cells.

The tracking of antigen-specific T cells in vivo is a useful approach for the study of the adaptive immune response. This protocol describes how populations of T cells specific for a given peptide–major histocompatibility complex (pMHC) epitope can be tracked based solely on T-cell receptor (TCR) specificity as opposed to other indirect methods based on function. The methodology involves the adoptive transfer of TCR transgenic T cells with defined epitope specificity into histocompatible mice and the subsequent detection of these cells through the use of congenic or clonotypic markers. Alternatively, endogenous epitope-specific T cells can be tracked directly through the use of pMHC tetramers. Using magnetic bead-based enrichment and advanced multiparameter flow cytometry, populations as small as five epitope-specific T cells can be detected from the peripheral lymphoid organs of a mouse. The adoptive transfer procedure can be completed within 3 h, whereas analysis of epitope-specific cells from mice can be completed within 6 h.

Type I interferons directly inhibit regulatory T cells to allow optimal antiviral T cell responses during acute LCMV infection

Inhibition of T reg cells by type I IFNs is necessary for the generation of optimal antiviral T cell responses during acute LCMV infection.

Plasmacytoid Dendritic Cells Are Activated by Toxoplasma gondii to Present Antigen and Produce Cytokines

Infection with the parasite Toxoplasma gondii leads to the induction of a Th1-type response dominated by IFN-γ production and control of this pathogen. Cells of the innate immune system are essential in initiating this response both through the production of IL-12 as well as the presentation of parasite-derived Ags to MHC-restricted T cells. Although dendritic cells (DCs) have been implicated in these events, the contribution of individual DC populations remains unclear. Therefore, multiparameter flow cytometry was used to identify and characterize subsets of murine DCs during acute toxoplasmosis. This approach confirmed that infection leads to the expansion and activation of conventional DC (cDC) subsets. Unexpectedly, however, this analysis further revealed that plasmacytoid DCs are also expanded and that these cells up-regulate MHC class II and costimulatory molecules associated with their acquired ability to prime naive CD4+ T cells. Furthermore, T. gondii-activated plasmacytoid DCs produce high levels of IL-12 and both plasmacytoid DC maturation and cytokine production are dependent on TLR11. Together these studies suggest that pDCs are a prominent DC subset involved in the initial stages of T. gondii infection, presenting parasite Ags and producing cytokines that are important for controlling infection.

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.

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.

Somatically Hypermutated Plasmodium-Specific IgM+ Memory B Cells Are Rapid, Plastic, Early Responders upon Malaria Rechallenge

Summary Humoral immunity consists of pre-existing antibodies expressed by long-lived plasma cells and rapidly reactive memory B cells (MBC). Recent studies of MBC development and function after protein immunization have uncovered significant MBC heterogeneity. To clarify functional roles for distinct MBC subsets during malaria infection, we generated tetramers that identify Plasmodium-specific MBCs in both humans and mice. Long-lived murine Plasmodium-specific MBCs consisted of three populations: somatically hypermutated immunoglobulin M+ (IgM+) and IgG+ MBC subsets and an unmutated IgD+ MBC population. Rechallenge experiments revealed that high affinity, somatically hypermutated Plasmodium-specific IgM+ MBCs proliferated and gave rise to antibody-secreting cells that dominated the early secondary response to parasite rechallenge. IgM+ MBCs also gave rise to T cell-dependent IgM+ and IgG+B220+CD138+ plasmablasts or T cell-independent B220−CD138+ IgM+ plasma cells. Thus, even in competition with IgG+ MBCs, IgM+ MBCs are rapid, plastic, early responders to a secondary Plasmodium rechallenge and should be targeted by vaccine strategies.

Virulence of Toxoplasma gondii Is Associated with Distinct Dendritic Cell Responses and Reduced Numbers of Activated CD8+ T Cells

The Toxoplasma gondii population consists of multiple strains, defined by genotype and virulence. Previous studies have established that protective immunity to this organism is mediated by IL-12, which drives T cells to produce IFN-γ. Paradoxically, although type I and type II strains of T. gondii both induce IL-12 and IFN-γ in the mouse, type I parasites are lethal, whereas type II strains establish chronic infection. The cellular basis for these strain-dependent differences remains unclear. To better understand these events, the CD8+ T cell and dendritic cell (DC) responses to transgenic, OVA-expressing type I RH (RH OVA) and type II Prugniuad (Pru OVA) parasites were examined. Pru OVA-infected mice developed a robust DC response at the site of infection and the draining lymph node and generated a population of endogenous OVA-specific CD8+ T cells. In contrast, RH OVA-infected mice had fewer DCs and OVA-specific CD8+ T cells. RH OVA-infected mice given preactivated OVA-specific CD8+ T cells were protected, suggesting that reduced DC-derived signals contributed to the low OVA-specific CD8+ T cell numbers observed during type I infection. Indeed, DC depletion prior to Pru OVA infection resulted in a failure to generate activated OVA-specific CD8+ T cells, and IL-12p70 treatment during RH OVA infection modestly increased the number of Ag-specific cells. Together, these data are consistent with a model of immunity to T. gondii in which strain-dependent DC responses shape the generation of Ag-specific CD8+ T cells and determine the outcome of infection.