Subcellular distribution of Lck during CD4 T-cell maturation in the thymic medulla regulates the T-cell activation threshold.

Mature peripheral T cells respond to foreign but not to self-antigens. During development in the thymus, deletion of high-affinity self-reactive immature thymocytes contributes to tolerance of mature T cells. However, double-positive thymocytes are positively selected to survive if they respond to self-peptide-MHC complexes; thus, there must be mechanisms to prevent overt reactivity to those same complexes in the periphery. "Developmental tuning" is the active process through which T-cell receptor (TCR)-associated signaling pathways of single-positive (SP) thymocytes are attenuated to respond appropriately to self-peptide-MHC complexes in the periphery. We previously showed that MHC class II expression in the thymic medulla was necessary to tune CD4(+) SP (CD4 SP) thymocytes. CD4 SP thymocytes from mice lacking medullary MHC class II expression had inappropriately enhanced proximal TCR signaling to low-affinity self-ligands that was associated with altered cellular distribution of the tyrosine kinase Lck. Now, we report that activation of both tuned and untuned CD4 SP thymocytes is Lck-dependent. Untuned CD4 SP cells contain a pool of Lck with increased basal phosphorylation that is not associated with the CD4 coreceptor. Phosphorylation of this pool of Lck decreases with tuning. Immunogold transmission electron microscopy of membrane sheets permitted direct visualization of Lck. In the absence of tuning, a significant proportion of Lck and the TCR subunit CD3ζ are expressed on the same protein island; this close association of Lck and the TCR probably explains the enhanced activation of untuned CD4 SP cells. Thus, changes in membrane topography during thymic maturation determine the set point for TCR responsiveness.

The activation threshold of CD4+ T cells is defined by TCR/peptide-MHC class II interactions in the thymic medulla.

Immature thymocytes that are positively selected based upon their response to self-peptide-MHC complexes develop into mature T cells that are not overtly reactive to those same complexes. Developmental tuning is the active process through which TCR-associated signaling pathways of single-positive thymocytes are attenuated to respond appropriately to the peptide-MHC molecules that will be encountered in the periphery. In this study, we explore the mechanisms that regulate the tuning of CD4(+) single-positive T cells to MHC class II encountered in the thymic medulla. Experiments with murine BM chimeras demonstrate that tuning can be mediated by MHC class II expressed by either thymic medullary epithelial cells or thymic dendritic cells. Tuning does not require the engagement of CD4 by MHC class II on stromal cells. Rather, it is mediated by interactions between MHC class II and the TCR. To understand the molecular changes that distinguish immature hyperactive T cells from tuned mature CD4(+) T cells, we compared their responses to TCR stimulation. The altered response of mature CD4 single-positive thymocytes is characterized by the inhibition of ERK activation by low-affinity self-ligands and increased expression of the inhibitory tyrosine phosphatase SHP-1. Thus, persistent TCR engagement by peptide-MHC class II on thymic medullary stroma inhibits reactivity to self-Ags and prevents autoreactivity in the mature repertoire.

Oligoclonal CD4+ T cells promote host memory immune responses to Zwitterionic polysaccharide of Streptococcus pneumoniae.

Zwitterionic polysaccharides of the normal flora bacteria represent a novel class of antigens in that they correct systemic CD4(+) T-cell deficiencies and direct lymphoid organogenesis during colonization of the host. Presentation of these polysaccharides to CD4(+) T cells depends on major histocompatibility complex class II- and DM-dependent retrograde transport from lysosomes to the cell surface. Yet the phenotype and clonality of the immune response to the polysaccharide in the mature host immune system have not been studied. Using the zwitterionic capsular polysaccharide Sp1 of Streptococcus pneumoniae, a transient member of the bacterial flora, in an experimental mouse model of cellular immunity, we demonstrated the accumulation of TH1- and TH17-polarized CD4(+) CD44(high) CD62(low) CD25(-) memory T cells. Subcutaneous immunization with Sp1 resulted in an increase of serum immunoglobulin G (IgG), predominantly of the IgG1 subclass, and suggested the presence of a humoral memory response to the polysaccharide. CD4(+) T cells stimulated with polysaccharide in vitro and in vivo showed a nonrestricted pattern for the T-cell receptor (TCR) beta-chain variable region, as demonstrated by semiquantitative reverse transcription-PCR and flow cytometry. Clonotype mapping of in vivo and in vitro polysaccharide-activated CD4(+) T cells revealed clonotypic TCR transcripts. Taken together, the data show the induction of clonal expansion of CD4(+) T cells by polysaccharides of commensal bacteria. Cellular and humoral memory host responses imply the ability of these polysaccharides to mediate the expansion of T cells via recognition within the CDR3 region of the TCR.

Transport of Streptococcus pneumoniae capsular polysaccharide in MHC Class II tubules.

Bacterial capsular polysaccharides are virulence factors and are considered T cell-independent antigens. However, the capsular polysaccharide Sp1 from Streptococcus pneumoniae serotype 1 has been shown to activate CD4(+) T cells in a major histocompatibility complex (MHC) class II-dependent manner. The mechanism of carbohydrate presentation to CD4(+) T cells is unknown. We show in live murine dendritic cells (DCs) that Sp1 translocates from lysosomal compartments to the plasma membrane in MHCII-positive tubules. Sp1 cell surface presentation results in reduction of self-peptide presentation without alteration of the MHCII self peptide repertoire. In DM-deficient mice, retrograde transport of Sp1/MHCII complexes resulting in T cell-dependent immune responses to the polysaccharide in vitro and in vivo is significantly reduced. The results demonstrate the capacity of a bacterial capsular polysaccharide antigen to use DC tubules as a vehicle for its transport as an MHCII/saccharide complex to the cell surface for the induction of T cell activation. Furthermore, retrograde transport requires the functional role of DM in self peptide-carbohydrate exchange. These observations open new opportunities for the design of vaccines against microbial encapsulated pathogens.

In vitro generation of murine myeloid dendritic cells from CD34-positive precursors.

Dendritic cells (DCs) link the innate and adaptive immune system. Currently, murine DCs for cell biology investigations are developed from MHC class II-negative bone marrow (BM) precursor cells, non-depleted BM cells or BM monocytes in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Here we demonstrate an isolation procedure of functionally intact myeloid CD11c(+) CD11b(+) DCs derived from murine CD34-positive precursors. DCs derived from CD34(+) cells show functional internalization, maturation, cytokine secretion, MHC-restricted antigen presentation, and MHCII retrograde transport of antigens from the lysosomes to the cell surface. In comparison to the established method, the advantages of this isolation procedure are a shorter cultivation period, a superior transfection efficiency, the yield of a purer and more homogeneous population of immature DCs, and less consumption of cell culture medium and GM-CSF. The new isolation procedure and the functional quality of CD34(+) cell-derived murine myeloid DCs make them ideally suited for immunology and cell biology studies.

Origin and Role of a Subset of Tumor-Associated Neutrophils with Antigen-Presenting Cell Features in Early-Stage Human Lung Cancer.

Based on studies in mouse tumor models, granulocytes appear to play a tumor-promoting role. However, there are limited data about the phenotype and function of tumor-associated neutrophils (TANs) in humans. Here, we identify a subset of TANs that exhibited characteristics of both neutrophils and antigen-presenting cells (APCs) in early-stage human lung cancer. These APC-like "hybrid neutrophils," which originate from CD11b(+)CD15(hi)CD10(-)CD16(low) immature progenitors, are able to cross-present antigens, as well as trigger and augment anti-tumor T cell responses. Interferon-γ and granulocyte-macrophage colony-stimulating factor are requisite factors in the tumor that, working through the Ikaros transcription factor, synergistically exert their APC-promoting effects on the progenitors. Overall, these data demonstrate the existence of a specialized TAN subset with anti-tumor capabilities in human cancer.

Tumor-associated neutrophils stimulate T cell responses in early-stage human lung cancer.

Infiltrating inflammatory cells are highly prevalent within the tumor microenvironment and mediate many processes associated with tumor progression; however, the contribution of specific populations remains unclear. For example, the nature and function of tumor-associated neutrophils (TANs) in the cancer microenvironment is largely unknown. The goal of this study was to provide a phenotypic and functional characterization of TANs in surgically resected lung cancer patients. We found that TANs constituted 5%-25% of cells isolated from the digested human lung tumors. Compared with blood neutrophils, TANs displayed an activated phenotype (CD62L(lo)CD54(hi)) with a distinct repertoire of chemokine receptors that included CCR5, CCR7, CXCR3, and CXCR4. TANs produced substantial quantities of the proinflammatory factors MCP-1, IL-8, MIP-1α, and IL-6, as well as the antiinflammatory IL-1R antagonist. Functionally, both TANs and neutrophils isolated from distant nonmalignant lung tissue were able to stimulate T cell proliferation and IFN-γ release. Cross-talk between TANs and activated T cells led to substantial upregulation of CD54, CD86, OX40L, and 4-1BBL costimulatory molecules on the neutrophil surface, which bolstered T cell proliferation in a positive-feedback loop. Together our results demonstrate that in the earliest stages of lung cancer, TANs are not immunosuppressive, but rather stimulate T cell responses.

The modulation of adaptive immune responses by bacterial zwitterionic polysaccharides.

The detection of pathogen-derived molecules as foreign particles by adaptive immune cells triggers T and B lymphocytes to mount protective cellular and humoral responses, respectively. Recent immunological advances elucidated that proteins and some lipids are the principle biological molecules that induce protective T cell responses during microbial infections. Polysaccharides are important components of microbial pathogens and many vaccines. However, research concerning the activation of the adaptive immune system by polysaccharides gained interest only recently. Traditionally, polysaccharides were considered to be T cell-independent antigens that did not directly activate T cells or induce protective immune responses. Here, we review several recent advances in "carbohydrate immunobiology". A group of bacterial polysaccharides that are known as "zwitterionic polysaccharides (ZPSs)" were recently identified as potent immune modulators. The immunomodulatory effect of ZPSs required antigen processing and presentation by antigen presenting cells, the activation of CD4 T cells and subpopulations of CD8 T cells and the modulation of host cytokine responses. In this review, we also discuss the potential use of these unique immunomodulatory ZPSs in new vaccination strategies against chronic inflammatory conditions, autoimmunity, infectious diseases, allergies and asthmatic conditions.

Effect of B7-2 and CD40 signals from activated antigen-presenting cells on the ability of zwitterionic polysaccharides to induce T-Cell stimulation.

Carbohydrates have been thought to stimulate immune responses independently of T cells; however, zwitterionic polysaccharides (ZPSs) from the capsules of some bacteria elicit potent CD4+-T-cell responses in vivo and in vitro. We demonstrated that HLA-DR on professional antigen-presenting cells (APCs) is required for ZPS-induced T-cell proliferation in vitro (15). Recently, it was shown that ZPSs are processed to low-molecular-weight carbohydrates by a nitric oxide-mediated mechanism in endosomes and locate in the major histocompatibility complex class II pathway (5, 15). The effect of the ZPS-mediated expression of HLA-DR and costimulatory molecules on the APC and T-cell engagement and subsequent T-cell activation has not been elucidated. Herein, we report that ZPS-mediated induction of HLA-DR-surface expression and T-cell proliferation are maximally enhanced after incubation of APCs for 8 h with ZPS. Treatment of APCs with bafilomycin A inhibits the up-regulation of ZPS-mediated HLA-DR surface expression and leads to inhibition of T-cell proliferation. Monoclonal antibodies (MAbs) to the costimulatory molecules B7-2 and CD40L specifically block ZPS-mediated T-cell activation, while a MAb to B7-1 does not. Surface expression of B7-2 and B7-1 but not of CD40 is maximally enhanced at 8 to 16 h of treatment of APCs with ZPS. The results demonstrate that the cellular immune response to ZPS depends on the translocation of HLA-DR to the cell surface and requires costimulation via B7-2 and CD40 on activated APCs. The implication is that activation of ZPS-specific T cells requires an orchestrated arrangement of both presenting and costimulatory molecules to form an immunological synapse.