Adam G. Schrum

Mechanism of T Cell Tolerance Induced by Myeloid-Derived Suppressor Cells

Ag-specific T cell tolerance plays a critical role in tumor escape. Recent studies implicated myeloid-derived suppressor cells (MDSCs) in the induction of CD8+ T cell tolerance in tumor-bearing hosts. However, the mechanism of this phenomenon remained unclear. We have found that incubation of Ag-specific CD8+ T cells, with peptide-loaded MDSCs, did not induce signaling downstream of TCR. However, it prevented subsequent signaling from peptide-loaded dendritic cells. Using double TCR transgenic CD8+ T cells, we have demonstrated that MDSC induced tolerance to only the peptide, which was presented by MDSCs. T cell response to the peptide specific to the other TCR was not affected. Incubation of MDSCs with Ag-specific CD8+ T cells caused nitration of the molecules on the surface of CD8+ T cells, localized to the site of physical interaction between MDSC and T cells, which involves preferentially only TCR specific for the peptide presented by MDSCs. Postincubation with MDSCs, only nitrotyrosine-positive CD8+ T cells demonstrated profound nonresponsiveness to the specific peptide, whereas nitrotyrosine-negative CD8+ T cells responded normally to that stimulation. MDSCs caused dissociation between TCR and CD3ζ molecules, disrupting TCR complexes on T cells. Thus, these data describe a novel mechanism of Ag-specific CD8+ T cell tolerance in cancer.

Different T Cell Receptor Signals Determine CD8+ Memory Versus Effector Development

Following infection, naïve CD8+ T cells bearing pathogen-specific T cell receptors (TCRs) differentiate into a mixed population of short-lived effector and long-lived memory T cells to mediate an adaptive immune response. How the TCR regulates memory T cell development has remained elusive. Using a mutant TCR transgenic model, we found that point mutations in the TCR β transmembrane domain (βTMD) impair the development and function of CD8+ memory T cells without affecting primary effector T cell responses. Mutant T cells are deficient in polarizing the TCR and in organizing the nuclear factor κB signal at the immunological synapse. Thus, effector and memory states of CD8+ T cells are separable fates, determined by differential TCR signaling.

T cell receptor engagement by peptide–MHC ligands induces a conformational change in the CD3 complex of thymocytes

The T cell receptor (TCR) can recognize a variety of cognate peptide/major histocompatibility complex (pMHC) ligands and translate their affinity into distinct cellular responses. To achieve this, the nonsignaling αβ heterodimer communicates ligand recognition to the CD3 signaling subunits by an unknown mechanism. In thymocytes, we found that both positive- and negative-selecting pMHC ligands expose a cryptic epitope in the CD3 complex upon TCR engagement. This conformational change is induced in vivo and requires the expression of cognate MHC. We conclude that TCR engagement with a cognate pMHC ligand induces a conformational change in the CD3 complex of thymocytes and propose that this marks an initial event during thymic selection that signals the recognition of self-antigen.

Junctate is a key element in calcium entry induced by activation of InsP3 receptors and/or calcium store depletion

In many cell types agonist-receptor activation leads to a rapid and transient release of Ca2+ from intracellular stores via activation of inositol 1,4,5 trisphosphate (InsP3) receptors (InsP3Rs). Stimulated cells activate store- or receptor-operated calcium channels localized in the plasma membrane, allowing entry of extracellular calcium into the cytoplasm, and thus replenishment of intracellular calcium stores. Calcium entry must be finely regulated in order to prevent an excessive intracellular calcium increase. Junctate, an integral calcium binding protein of endo(sarco)plasmic reticulum membrane, (a) induces and/or stabilizes peripheral couplings between the ER and the plasma membrane, and (b) forms a supramolecular complex with the InsP3R and the canonical transient receptor potential protein (TRPC) 3 calcium entry channel. The full-length protein modulates both agonist-induced and store depletion–induced calcium entry, whereas its NH2 terminus affects receptor-activated calcium entry. RNA interference to deplete cells of endogenous junctate, knocked down both agonist-activated calcium release from intracellular stores and calcium entry via TRPC3. These results demonstrate that junctate is a new protein involved in calcium homeostasis in eukaryotic cells.

The Proliferative Capacity of Individual Naive CD4+T Cells Is Amplified by Prolonged T Cell Antigen Receptor Triggering

Strong antigenic encounter by T cells rapidly induces immunological synapse formation and surface T cell receptor (TCR) downregulation. Although surface TCR expression can remain low for several days, T cells can still sustain antigenic signaling. It has been unclear whether prolonged antigenic signaling occurs in the absence of surface TCR replenishment, being maintained by a few “nondownregulatable” surface TCRs that might reside in a synaptosomal structure. Alternatively, the low surface TCR level induced by antigen might represent a dynamic state of expression involving continual surface TCR replenishment, reengagement by antigen, and ongoing downregulation. To resolve this issue, we studied in vivo–generated, dual-specificity primary naive CD4+ T cells. On these cells, antigenic stimulus exclusively downregulated antigen-specific, but not antigen-nonspecific, TCRs. In addition to providing a means to track TCR engagement, this also allowed us to use the antigen nonspecific TCR to track TCR expression in isolation from TCR engagement by antigen. Surface TCR replenishment began within the first day of stimulation, and occurred synchronously with continuous antigen-specific TCR engagement and downregulation. Furthermore, by enhancing CD25 expression, extended signaling through surface-replenishing TCRs significantly amplified the number of daughter cells generated by naive CD4+ T cells that had already committed to proliferate. This effect required TCR engagement and could not be substituted for by interleukin 2. These data demonstrate that TCR triggering and consumption can occur over an extended period of time, with a significant impact on the effector responses evoked from naive CD4+ T cells.

Lifetime Incidence Risk of Alopecia Areata Estimated at 2.1% by Rochester Epidemiology Project, 1990–2009

Alopecia Areata (AA) is characterized by patchy, nonscarring, autoimmune-mediated hair loss, although many aspects of AA pathogenesis are unknown (Gilhar et al., 2012). The scalp is most commonly involved in clinically treated AA, but any hair-bearing surface of the body may be affected (Wasserman et al., 2007). Males and females of any age and hair color can have AA (Finner, 2011; Kyriakis et al., 2009). According to the First National Health and Nutrition Examination Survey conducted in the early 1970s, AA is fairly common; it was estimated to affect about 2 of every 1,000 people in the United States (Safavi, 1992). Dermatologists encounter AA in 0.7% to 4.0% of their patient populations (Price, 1991; Sharma et al., 1996; Tan et al., 2002). A previous study reported by Mayo Clinic and National Institutes of Health (Safavi et al., 1995) showed that the overall incidence of AA in Olmsted County, Minnesota, was 20.2 per 100,000 person-years from 1975 through 1989. A similar incidence rate for both sexes with a lifetime risk of 1.7% was observed.

Surface T-cell antigen receptor expression and availability for long- term antigenic signaling

Summary:  It is important to understand how T‐cell antigen receptor (TCR) engagement and signaling are regulated throughout an immune response. This review examines the dynamics of surface TCR expression and signaling capacity during thymic and effector T‐cell development. Although the TCR can undergo vast changes in surface expression, T cells remain capable of sustaining TCR engagement for long periods of time. This may be achieved by a combination of mechanisms that involve (a) controlling the quantity of surface TCR available for ligand interaction and (b) controlling the quality of surface TCR expression during T‐cell activation. TCR signaling itself appears to be one of the main quantitative modulators of surface TCR expression, and it can cause both downregulation and upregulation at different times of T‐cell activation. Recent studies indicate that the degree of upregulation is tunable by the strength of antigenic stimulation. There is evidence that qualitatively distinct forms of the TCR exist, and their potential role in sustained antigenic signaling is also discussed. A goal of future studies will be to better characterize these modulations in surface TCR expression and to clarify their impact on the regulation of immune responses.

High-sensitivity detection and quantitative analysis of native protein-protein interactions and multiprotein complexes by flow cytometry.

Most mechanisms of cell development, physiology, and signal transduction are controlled by protein-protein interactions. Immunoprecipitation of multiprotein complexes detected by flow cytometry (IP-FCM) is a means to quantitatively measure these interactions. The high sensitivity of this method makes it useful even when very little biomaterial is available for analysis, as in the case of rare primary cell subsets or patient samples. Detection of the T cell antigen receptor associated with the CD3 multiprotein complex from as few as 300 primary murine T cells is presented as an example. The method is compatible with quantitative flow cytometry techniques, making it possible to estimate the number of coimmunoprecipitated molecules. Both constitutive and inducible protein-protein interactions can be analyzed, as illustrated in related methodology using glutathione S-transferase–fusion protein pull-down experiments. IP-FCM represents a robust, quantitative, biochemical technique to assess native protein-protein interactions, without requiring genetic engineering or large sample sizes.

Ultrasensitive measurement of huntingtin protein in cerebrospinal fluid demonstrates increase with Huntington disease stage and decrease following brain huntingtin suppression

Quantitation of huntingtin protein in the brain is needed, both as a marker of Huntington disease (HD) progression and for use in clinical gene silencing trials. Measurement of huntingtin in cerebrospinal fluid could be a biomarker of brain huntingtin, but traditional protein quantitation methods have failed to detect huntingtin in cerebrospinal fluid. Using micro-bead based immunoprecipitation and flow cytometry (IP-FCM), we have developed a highly sensitive mutant huntingtin detection assay. The sensitivity of huntingtin IP-FCM enables accurate detection of mutant huntingtin protein in the cerebrospinal fluid of HD patients and model mice, demonstrating that mutant huntingtin levels in cerebrospinal fluid reflect brain levels, increasing with disease stage and decreasing following brain huntingtin suppression. This technique has potential applications as a research tool and as a clinical biomarker.

B7-H1 Expression on Old CD8+ T Cells Negatively Regulates the Activation of Immune Responses in Aged Animals

T cell responses are compromised in the elderly. The B7-CD28 family receptors are critical in the regulation of immune responses. We evaluated whether the B7-family and CD28-family receptors were differentially expressed in dendritic cells, macrophages, and CD4+ and CD8+ T cells from young and old mice, which could contribute to the immune dysfunction in the old. Although most of the receptors were equally expressed in all cells, >85% of the old naive CD8+ T cells expressed B7-H1 compared with 25% in the young. Considering that B7-H1 negatively regulates immune responses, we hypothesized that expression of B7-H1 would downregulate the function of old CD8+ T cells. Old CD8+ T cells showed reduced ability to proliferate, but blockade of B7-H1 restored the proliferative capacity of old CD8+ T cells to a level similar to young CD8+ T cells. In vivo blockade of B7-H1 restored antitumor responses against the B7-H1− BM-185–enhanced GFP tumor, such that old animals responded with the same efficiency as young mice. Our data also indicate that old CD8+ T cells express lower levels of TCR compared with young CD8+ T cells. However, following antigenic stimulation in the presence of B7-H1 blockade, the levels of TCR expression were restored in old CD8+ T cells, which correlated with stronger T cell activation. These studies demonstrated that expression of B7-H1 in old CD8+ T cells impairs the proper activation of these cells and that blockade of B7-H1 could be critical to optimally stimulate a CD8 T cell response in the old.