Tumor antigen-specific CD4+ T cells in cancer immunity: from antigen identification to tumor prognosis and development of therapeutic strategies

CD4(+) T cells comprise a large fraction of tumor infiltrating lymphocytes and it is now established that they may exert an important role in tumor immune-surveillance. Several CD4(+) T cell subsets [i.e. T helper (Th)1, Th2, T regulatory (Treg), Th17, Th22 and follicular T helper (Tfh)] have been described and differentiation of each subset depends on both the antigen presenting cells responsible for its activation and the cytokine environment present at the site of priming. Tumor antigen-specific CD4(+) T cells with different functional activity have been found in the blood of cancer patients and different CD4(+) T cell subsets have been identified at the tumor site by the expression of specific transcription factors and the profile of secreted cytokines. Importantly, depending on the subset, CD4(+) T cells may exert antitumor versus pro-tumor functions. Here we review the studies that first identified the presence of tumor-specific CD4(+) T cells in cancer patients, the techniques used to identify the tumor antigens recognized, the role of the different CD4(+) T cell subsets in tumor immunity and in cancer prognosis and the development of therapeutic strategies aimed at activating efficient antitumor CD4(+) T cell effectors.

CD36 folding revealed by conformational epitope expression is essential for cytoadherence of Plasmodium falciparum-infected red blood cells

CD36 is a membrane glycoprotein and a putative scavenger receptor expressed by several cell types. In capillary endothelial cells, it mediates the adherence of erythrocytes infected with Plasmodium falciparum. The CD36 sequence contains two hydrophobic domains located at the amino-and carboxyl-termini of the protein, but the topology of this protein and the functional significance of these domains are still not clearly defined. We generated soluble CD36-IgG chimeric molecules by fusion of the extracellular domains of CD36 with human immunoglobulin domains. The construct containing the N-terminal hydrophobic domain of CD36 was completely retained intracellularly as membrane-associated molecule, suggesting that the N-terminal hydrophobic domain of the CD36 is a real transmembrane domain and that CD36 has hairpin topology. A small amount of the CD36-IgG chimeric construct lacking both transmembrane domains escaped retention, was correctly processed, and accumulated in the extracellular medium as a soluble molecule. This CD36-IgG construct failed to bind Plasmodium falciparum-infected erythrocytes. Using monoclonal antibodies specific for either conformational or structural epitopes, we demonstrate that failure of this CD36-IgG construct to bind infected erythrocytes was due to incorrect folding of the soluble chimeric molecule.

To E or not to E? Can an IL-4-induced B cell choose between IgE and IgG4?

Parasite immunologists had known for some time that IgE-mediated hypersensitivity reactions are rare in patients with chronic helminth infections, even though basophils and mast cells in these patients are sensitized with antiparasite IgE and exposed, often continuously, to parasite antigens. The inhibition of allergic reactivity in chronic helminth infections is mainly due to IgG4 'blocking antibodies' in the serum of the infected individual. IgG4 do not fix complement and bind weakly to Fcgamma receptors. Thus, antigen binding by IgG4, unlike IgE, is likely to have no or minimally harmful consequences. The discovery that, similar to IgE, expression of IgG4 is IL-4-dependent and is an intermediate step in sequential switching from IgM to IgE makes it imperative to understand how the two isotypes are coregulated and whether the two responses can be uncoupled, selectively boosting IgG4 over IgE. The ultimate goal is to apply to allergy the lesson we learnt from helminth infections.

The transcription factor B cell-specific activator protein (BSAP) enhances both IL-4- and CD40-mediated activation of the human epsilon germline promoter

Induction of isotype switching to a particular C(H) gene correlates with the transcriptional activation of the same gene in germline configuration. Induction of correctly spliced germline transcripts is necessary to target a switch region for recombination and switching. Different cytokines activate transcription at different germline promoters. Because binding sites for the B cell-specific transcription factor BSAP are located upstream of several switch regions in the Ig locus, BSAP might play a role in isotype switching by regulating germline transcription. We investigated whether BSAP plays a role in the transcriptional regulation of the epsilon germline promoter in human B cells. We identified human EBV-negative B cell lines that express epsilon germline transcripts upon stimulation with IL-4. Electrophoretic mobility shift assay analysis showed that the human epsilon germline promoter binds BSAP. BSAP activity was expressed constitutively and was not affected by stimulation with IL-4 and/or anti-CD40 mAb. Reporter assays with constructs containing a luciferase gene driven by the epsilon germline promoter, with or without mutations in the BSAP binding site, showed that BSAP plays a role in both IL-4-dependent induction and CD40-mediated up-regulation of human epsilon germline transcription. Furthermore, epsilon germline promoter activity was abrogated in REH cells that express a BSAP polypeptide truncated in the trans-activation domain. Among the transcription factors that regulate epsilon germline expression, BSAP is unique, in that it is B cell-specific and is at the merging point of two signaling pathways that are distinct but both critical for the induction of IgE switching.

The transcription factor B cell-specific activator protein (BSAP) enhances both IL-4- and CD40-mediated activation of the human epsilon germline promoter

Induction of isotype switching to a particular C(H) gene correlates with the transcriptional activation of the same gene in germline configuration. Induction of correctly spliced germline transcripts is necessary to target a switch region for recombination and switching. Different cytokines activate transcription at different germline promoters. Because binding sites for the B cell-specific transcription factor BSAP are located upstream of several switch regions in the Ig locus, BSAP might play a role in isotype switching by regulating germline transcription. We investigated whether BSAP plays a role in the transcriptional regulation of the epsilon germline promoter in human B cells. We identified human EBV-negative B cell lines that express epsilon germline transcripts upon stimulation with IL-4. Electrophoretic mobility shift assay analysis showed that the human epsilon germline promoter binds BSAP. BSAP activity was expressed constitutively and was not affected by stimulation with IL-4 and/or anti-CD40 mAb. Reporter assays with constructs containing a luciferase gene driven by the epsilon germline promoter, with or without mutations in the BSAP binding site, showed that BSAP plays a role in both IL-4-dependent induction and CD40-mediated up-regulation of human epsilon germline transcription. Furthermore, epsilon germline promoter activity was abrogated in REH cells that express a BSAP polypeptide truncated in the trans-activation domain. Among the transcription factors that regulate epsilon germline expression, BSAP is unique, in that it is B cell-specific and is at the merging point of two signaling pathways that are distinct but both critical for the induction of IgE switching.

Regulation of human epsilon germline transcription: role of B-cell-specific activator protein

Germline transcripts initiate from promoters upstream of the immunoglobulin switch region, and are necessary to target the appropriate switch region for recombination and switching. Different cytokines activate transcription at the appropriate germline promoter. Because binding sites for B-cell-specific activator protein (BSAP) are located upstream of several switch regions in the immunoglobulin heavy chain gene cluster, BSAP might play a role in the regulation of germline transcription and isotype switching. We investigated whether BSAP plays a role in the transcriptional regulation of the epsilon germline promoter in human B cells. Our results showed that BSAP plays a role in both IL-4-dependent induction and CD40-mediated upregulation of human epsilon germline transcription. BSAP is unique among the transcription factors that regulate epsilon germline expression, because it is B cell specific, and is at the merging point of two signalling pathways that are critical for IgE switching.

Gene transfer-mediated generation of drug-resistant hemopoiesis

Autologous- or allogeneic-bone marrow transplantation are increasingly used to overcome the myelosuppressive effects of high dose chemotherapy administered to cancer patients. Transfer of the multidrug resistance (MDR) gene in hemopoietic progenitors has been proposed as a tool to administer higher and possibly more curative doses of chemotherapy. Murine models have demonstrated that retrovirus-mediated MDR transfer in bone marrow cells can render animals resistant to myeloablative doses of Taxol, and in vitro studies have shown that MDR-transduced human CD34+ cells can generate drug-resistant multipotential hemopoietic progenitors such as long term culture-initiating cells. Given these results, phase I clinical trials are currently under way to evaluate feasibility and treatment-related toxicity of MDR gene transfer in cancer patients by means of safe retroviral vectors. Finally, Taxol treatment of MDR transduced mice and human CD34+ cells have indicated that MDR is a dominant selectable marker in vitro and in vivo, and vectors carrying both MDR and non selectable genes such as beta-globin or glucocerebrosidase could be used in the next future for gene therapy of inherited disorders like thalassemia or Gaucher disease.

Synthesis, processing, and intracellular transport of CD36 during monocytic differentiation

CD36 is an integral membrane glycoprotein expressed by several cell types, including endothelial cells of the microvasculature, erythrocytes, platelets, and monocytes. In the monocytic lineage, CD36 is expressed during the late stages of differentiation in the bone marrow, in circulating monocytes, and in some tissue resident macrophages, and it is thought to mediate the phagocytosis of apoptotic cells and the endocytic uptake of modified lipoproteins. Here we analyze the synthesis, processing, and intracellular transport of CD36 in U937 and THP-1, two human cell lines representing different stages of monocytic maturation. In both cell lines, phorbol 12-myristate 13-acetate induces the expression of CD36. A 74-kDa intracellular precursor is first synthesized that has the hallmarks of a resident protein of the endoplasmic reticulum. The precursor protein is later processed into a mature form of 90-105 kDa which is transported to the cell surface. The kinetics of processing differ significantly in U937 and THP-1. These differences are specific for the CD36, as two unrelated proteins (CD11b and CD45R) are processed and transported to the surface at similar rates in the two cell lines. A 33-kDa endoglycosidase H-sensitive glycoprotein specifically associates with the 74-kDa precursor. Coprecipitation of gp33 correlates with slow processing of CD36 precursor, suggesting that gp33 may play a role in regulating the intracellular transport of CD36, during monocyte maturation.

Isolation and characterization of a mucin-glycoprotein from rat submandibular glands

A blood group A+ mucin-glycoprotein was purified from aqueous extracts of rat submandibular glands by sequential chromatography on columns of Sepharose CL-6B and Sephacryl S-300 in urea-containing buffers. Final purification was facilitated by reductive methylation which appeared to release contaminating (hydrophobic) peptides. Homogeneity of the purified mucin was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis at varying concentrations of acrylamide, lectin affinity chromatography, and Western blot analysis. In contrast to previously described preparations, the purified mucin contained only trace amounts of N-acetylglucosamine and aromatic amino acids. In addition, only low levels of basic amino acids were present.

Adsorption of human salivary mucins to hydroxyapatite

The interactions between low (MG2) and high (MG1) molecular-weight human submandibular-sublingual mucin and hydroxyapatite (OHAp) were compared using a quantitative assay. Data obtained appeared to empirically fit the Langmuir adsorption isotherm. Apparent affinity constants derived from this isotherm indicated that MG1 had a greater affinity for OHAp than did MG2. Inhibition studies revealed that salivary glycolipids inhibited the interaction of MG1 and OHAp without influencing MG2 adsorption. In contrast, MG2 adsorption to OHAp was markedly inhibited by cysteine-containing salivary phosphoprotein fractions. Collectively, the data indicate MG1 and MG2 differ in their interaction with OHAp.

Isolation and characterization of tracheobronchial mucin from a laryngectomee

A tracheobronchial mucin was isolated from the tracheobronchial secretion of a laryngectomee. It was purified by gel filtration on Sepharose CL-6B in Tris-urea buffer and rechromatography of excluded materials through the same gel matrix. It was homogeneous in 0.7% agarose-2% polyacrylamide electrophoresis under nonreducing conditions. Comparable analysis with 2-mercaptoethanol revealed at least 3 subunits. Based upon recoverable weight, the mucin was composed of 75% carbohydrate, 21% protein, and 3% sulfate. Oligosaccharides obtained by alkaline beta-elimination indicated O-glycosyl linkage to the peptide component. Marked heterogeneity of the carbohydrate side-chains was reflected in the preparation of 20 distinct oligosaccharides ranging in size from 4 to 17 residues.