Identification of cyclin B1 as a shared human epithelial tumor-associated antigen recognized by T cells.

We eluted peptides from class I molecules of HLA-A2.1(+) breast adenocarcinoma and loaded reverse phase high-performance liquid chromatography (HPLC) fractions onto dendritic cells to prime naive CD8(+) T cells. Fractions that supported growth of tumor-specific cytotoxic T lymphocytes were analyzed by nano-HPLC micro-ESI tandem mass spectrometry. Six HLA-A2.1-binding peptides, four 9-mers (P1-P4) differing in the COOH-terminal residue, and two 10-mers (P5 and P6) with an additional COOH-terminal alanine, were identified in one fraction. Peptide sequences were homologous to cyclin B1. We primed CD8(+) T cells from another HLA-A2.1(+) healthy donor with synthetic peptides and generated P4-specific responses. We also detected memory T cells specific for one or more of these peptides in patients with breast cancer and squamous cell carcinomas of the head and neck (SCCHN). T cells from one patient, restimulated once in vitro, could kill the tumor cell line from which the peptides were derived. Immunohistochemical analysis of tumor lines and tissue sections showed cyclin B1 overexpression and aberrant localization in the cytoplasm instead of the nucleus. Sequencing genomic DNA and cDNA corresponding to P1-P6 region showed that differences in COOH-terminal residues were not due to either DNA mutations or errors in transcription, suggesting a high error rate in translation of cyclin B1 protein in tumors.

Elafin drives poor outcome in high-grade serous ovarian cancers and basal-like breast tumors.

High-grade serous ovarian carcinoma (HGSOC) and basal-like breast cancer (BLBC) share many features including TP53 mutations, genomic instability and poor prognosis. We recently reported that Elafin is overexpressed by HGSOC and is associated with poor overall survival. Here, we confirm that Elafin overexpression is associated with shorter survival in 1000 HGSOC patients. Elafin confers a proliferative advantage to tumor cells through the activation of the MAP kinase pathway. This mitogenic effect can be neutralized by RNA interference, specific antibodies and a MEK inhibitor. Elafin expression in patient-derived samples was also associated with chemoresistance and strongly correlates with bcl-xL expression. We extended these findings into the examination of 1100 primary breast tumors and six breast cancer cell lines. We observed that Elafin is overexpressed and secreted specifically by BLBC tumors and cell lines, leading to a similar mitogenic effect through activation of the MAP kinase pathway. Here too, Elafin overexpression is associated with poor overall survival, suggesting that it may serve as a biomarker and therapeutic target in this setting.

Ion trajectories in an electrostatic ion guide for external ion source fourier transform ion cyclotron resonance mass spectrometry.

An electrostatic ion guide (EIG) that consists of concentric cylinder and central wire electrodes can transport ions efficiently from an external ion source to an ion cyclotron resonance (ICR) ion trap for mass analysis, with several advantages over current injection methods. Because the electrostatic force of the EIG captures ions in a stable orbit about the wire electrode, ions with initially divergent trajectories may be redirected toward the ICR ion trap for improved ion transmission efficiency. SIMION trajectory calculations (ion kinetic energy, 1-200 eV; elevation angle, 0.30 °; azimuthal angle, 0.360°) predict that ions of m/z 1000 may be transmitted through a strong (0.01 → 3.0-T) magnetic field gradient. Judicious choice of ion source position and EIG potential minimizes the spread in ion axial kinetic energy at the ICR ion trap. Advantages of the EIG include large acceptance angle, even for ions that have large initial kinetic energy and large radial displacement with respect to the central z-axis, low ion extraction voltage (5-20 V), and efficient trapping because ions need not be accelerated to high velocity to pass through the magnetic field gradient.

Primary ex vivo cultures of human fallopian tube epithelium as a model for serous ovarian carcinogenesis.

Recent studies suggest that some serous ovarian carcinomas (SOCs) arise from the fallopian tube (FT) epithelium rather than the ovarian surface epithelium. This hypothesis places emphasis on the FT secretory epithelial cell as a cell-of-origin. Herein, we report the development of a novel ex vivo primary human FT epithelium culture system that faithfully recapitulates the in vivo epithelium, as shown by morphological, ultrastructural and immunophenotypic analyses. Mass spectrometry-based proteomics reveal that these cultures secrete proteins previously identified as biomarkers for ovarian cancer. We also use this culture system to study the response of the FT epithelium to genotoxic stress and find that the secretory cells exhibit a distinct response to DNA damage when compared with neighboring ciliated cells. The secretory cells show a limited ability to resolve the damage over time, potentially leaving them more susceptible to accumulation of additional mutagenic injury. This divergent response is confirmed with in situ studies using tissue samples, further supporting the use of this ex vivo culture system to investigate FT epithelial pathobiology. We anticipate that this novel culture system will facilitate the study of SOC pathogenesis, and propose that similar culture systems could be developed for other organ site-specific epithelia.

Wide-mass-range axialization for high-resolution Fourier-transform ion cyclotron resonance mass spectrometry of externally generated ions.

Ions generated in both continuous and pulsed-mode external ion sources are injected into an open-ended cylindrical Fourier-transform ion cyclotron resonance ion trap by means of an electrostatic ion guide consisting of concentric cylinder and wire electrodes to which a potential difference is applied. Broadband axialization during ion injection improves ion trapping efficiency by reducing the ion off-axis displacement. Significant improvements in both signal-to-noise ratio and mass resolving power, along with high mass selectivity are demonstrated for ions generated by external Cs+ secondary ion mass spectrometry or by matrix-assisted laser desorption/ionization.

An external source 7 T Fourier transform ion cyclotron resonance mass spectrometer with electrostatic ion guide.

An external source 7 T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer offers three main novel features. First, a 9-way ion-source cross allows for mounting of up to three ionization sources simultaneously, thereby minimizing 'downtime' for changing ion sources. Second, an electrostatic (wire-in-cylinder) ion guide transports the ions approximately 1.5 m from the ion source to the ion trap for mass analysis, through a large magnetic field gradient. Third, the system operates from a modular data system described elsewhere in this issue. Luteinizing hormone-releasing hormone (LH-RH) matrix-assisted laser desorption/ionization (MALDI) FTICR positive-ion mass spectra exhibit signal-to-noise ratio greater than 1000:1 and mass resolving power, m/delta m 50% > 100,000. Laser-induced fragmentation of bradykinin demonstrates the ability of the ion guide to transmit both molecular and fragment ions simultaneously. Ultra-high resolution (average resolving power approximately 400,000) was achieved for poly(ethylene glycol) of specified number-average molecular weight, Mn approximately 3400. Future installation of an electrospray source to the ion-source cross should allow for better characterization of the performance of the ion guide.

Structural characterization of phospholipids by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry.

Matrix-assisted laser desorption/ionization (MALDI) Fourier transform ion cyclotron resonance mass spectrometry provides for structural analysis of the principal biological phospholipids: glycerophosphatidylcholine, -ethanolamine, -serine, and -inositol. Both positive and negative molecular or quasimolecular ions are generated in high abundance. Isolated molecular ions may be collisionally activated in the source side of a dual trap mass analyzer, yielding fragments serving to identify the polar head group (positive ion mode) and fatty acid side chains (negative ion mode). Azimuthal quadrupolar excitation following collisionally activated dissociation refocuses productions close to the solenoid axis; subsequent transfer of product ions to the analyzer ion trap allows for high-resolution mass analysis. Cyro-cooling of the sample probe with liquid nitrogen greatly reduces matrix adduction encountered in the negative ion mode.

Attomole biomolecule mass analysis by matrix-assisted laser desorption/ionization Fourier transform ion cyclotron resonance.

Significantly improved sensitivity for analysis of biomolecules by MALDI FT-ICR mass spectrometry is achieved by (i) microscope-monitored sample deposition onto a small indentation on the probe tip and (ii) multiple remeasurement of ions from a single laser shot. A simple modification to the solids probe tip allows for microdeposition of a few amols of analyte onto small indentation spots previously aligned with the laser beam. Ion multiple remeasurement of the same ion packet enhances the signal-to-noise ratio and thus extends the achievable FT-ICR MS detection limit. We demonstrate that FT-ICR can be used to detect parent and structurally significant fragment ions of peptides and phospholipids at low amol amounts. Positive ion mass spectra for approximately 90 amol of a mixture of angiotensin II and bradykinin, approximately 40 amol of dipalmitoylglycerophosphatidylcholine, and approximately 8 amol of substance P constitute the lowest reported detection limits to date for FT-ICR mass analysis of MALDI-generated ions.

Subfemtomole MS and MS/MS peptide sequence analysis using nano-HPLC micro-ESI fourier transform ion cyclotron resonance mass spectrometry.

Subfemtomole peptide sequence analysis has been achieved using microcapillary HPLC columns, with integrated nanoelectrospray emitters, coupled directly to a Fourier transform ion cyclotron resonance mass spectrometer. Accurate mass (+/-0.010 Da) peptide maps are generated from a standard six-protein digest mixture, whose principle components span a concentration dynamic range of 1000:1. Iterative searches against approximately 189000 entries in the OWL database readily identify each protein, with high sequence coverage (20-60%), from as little as 10 amol loaded on-column. In addition, a simple variable-flow HPLC apparatus provides for on-line tandem mass spectrometric analysis of tryptic peptides at the 400-amol level. MS/MS data are searched against approximately 280000 entries in a nonredundant protein database using SEQUEST. Accurate precursor and product ion mass information readily identifies primary amino acid sequences differing by asparagine vs aspartic acid (deltam = 0.98 Da) and glutamine vs lysine (deltam = 0.036 Da).

Serum protein immunogenicity: implications for liver xenografting.

The objectives of this study were threefold: (i) assess immunogenicity of donor plasma proteins following hepatic xenotransplantation, (ii) identify potential immunogens, and (iii) consider the implications of antibody formation against these plasma proteins in xenograft survival. We studied liver and heart xenografts in a concordant combination, hamster to rat. All grafts were examined at necropsy for evidence of rat immunoglobulin G (IgG) deposition. Cardiac xenografts were placed in recipients who had, or had not, been sensitized with hamster serum. Hepatic xenografts were placed in naive recipients to see if antibodies to hamster serum proteins could be eluted from the rejecting organ. Sera of immunized rats were examined for the presence of anti-hamster antibodies by immunoelectrophoresis and by Western blotting following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) separation of hamster serum. Antibodies in sera of immunized rats were compared with those eluted from rejecting livers. Candidate antigens were identified by tandem mass spectrometry, sequence analysis, and reference to protein databases. Results showed that sera of immunized rats recognized a minimum of four different antigens in hamster serum by immunoelectrophoresis, and a minimum of seven by the more sensitive SDS-PAGE Western blot. IgG eluted from rejecting livers bound three of seven candidate antigens recognized by sera of the immunized animals. Sequence analysis searches revealed proteinase inhibitors in each of the three SDS-PAGE bands common to the above samples. All of these candidate proteinase inhibitor immunogens share a common catabolic fate, uptake via the lipoprotein-related protein (LRP/alpha 2-macroglobulin receptor (CD91). Sensitization to hamster serum proteins hastened cardiac xenograft rejection in 30-50% of recipients (depending on sensitization protocol). Vascular deposition of rat IgG occurred in all rejecting xenografts. Antibody binding to proteinase inhibitors could disturb their functional activity and contribute to the pathogenesis of delayed xenograft rejection.

Electrospray ionization Fourier transform ion cyclotron resonance at 9.4 T.

We present the first results from a new electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer operated at a magnetic field of 9.4 T (i.e. > or = 2.4 T higher than for any prior FTICR instrument). The 9.4 T instrument provides substantially improved performance for large molecules (> or = 50% increase in mass resolving power) and complex mixtures (> or = 100% increase in dynamic range) compared to lower-field (< or = 6 T) instruments. The higher magnetic field makes possible larger trapped-ion population without introduction of significant space--charge effects such as spectral peak shift and/or distortion, and coalescence of closely-spaced resonances. For bovine ubiquitin (8.6 kDa) we observe accurate relative isotopic abundances at a signal-to-noise ratio greater than 1000:1, whereas a complete nozzle-skimmer dissociation electrospray ionization (ESI) FTICR mass spectrum of bovine carbonic anhydrase (29 kDa) is achieved from a single scan with a signal-to-noise ratio of more than 250:1. Finally, we are able to obtain mass resolving power, m/delta m > 200,000, routinely for porcine serum albumin (67 kDa). The present performance guides further modifications of the instrument, which should lead to significant further improvements.

Differences in the expression of human class I MHC alleles and their associated peptides in the presence of proteasome inhibitors.

We have studied the contributions of proteasome inhibitor-sensitive and -insensitive proteases to the generation of class I MHC-associated peptides. The cell surface expression of 13 different human class I MHC alleles was inhibited by as much as 90% or as little as 40% when cells were incubated with saturating concentrations of three different proteasome inhibitors. Inhibitor-resistant class I MHC expression was not due to TAP-independent expression or preexisting internal stores of peptides. Furthermore, it did not correlate with the amount or specificity of residual proteasome activity as determined in in vitro proteolysis assays and was not augmented by simultaneous incubation with multiple inhibitors. Mass spectrometry was used to directly characterize the peptides expressed in the presence and absence of proteasome inhibitors. The number of peptide species detected correlated with the levels of class I detected by flow cytometry. Thus, for many alleles, a significant proportion of associated peptide species continue to be generated in the presence of saturating levels of proteasome inhibitors. Comparison of the peptide-binding motifs of inhibitor-sensitive and -resistant class I alleles further suggested that inhibitor-resistant proteolytic activities display a wide diversity of cleavage specificities, including a trypsin-like activity. Sequence analysis demonstrated that inhibitor-resistant peptides contain diverse carboxyl termini and are derived from protein substrates dispersed throughout the cell. The possible contributions of inhibitor-resistant proteasome activities and nonproteasomal proteases residing in the cytosol to the peptide profiles associated with many class I MHC alleles are discussed.

Immunodominance among EBV-derived epitopes restricted by HLA-B27 does not correlate with epitope abundance in EBV-transformed B-lymphoblastoid cell lines.

Using synthetic peptides, the HLA-B27-restricted CTL response to EBV in asymptomatic virus carriers has been mapped to four epitope regions in EBV latent cycle Ags. One of these peptide-defined epitopes (RRIYDLIEL) tends to be immunodominant and is recognized in the context of all three B27 subtypes studied, B*2702, B*2704, and B*2705. The other peptide-defined epitopes induce responses only in the context of one subtype, the immunogenic combinations being RRARSLSAERY/B*2702, RRRWRRLTV/B*2704, and FRKAQIQGL/B*2705. We used immunoaffinity chromatography to isolate the naturally presented viral peptides associated with these MHC class I molecules on the surface of EBV-transformed B-LCL. Using CTL reconstitution assays in conjunction with mass spectrometry, we established that the naturally processed and presented peptides are identical with the previously identified synthetic sequences. Despite the subtype-specific immunogenicity of three of the four epitopes, all four epitope peptides were found in association with each of the three different HLA-B27 subtypes. Indeed, those peptides that failed to induce a response in the context of a particular HLA-B27 subtype were frequently presented at greater abundance by that subtype than were the immunogenic peptides. Furthermore, among the peptides that did induce a response, immunodominance did not correlate with epitope abundance; in fact the immunodominant RRIYDLIEL epitope was least abundant, being present at less than one copy per cell. The relationship of this unexpected finding to the persistence of EBV is discussed.

Identification and modulation of a naturally processed T cell epitope from the diabetes-associated autoantigen human glutamic acid decarboxylase 65 (hGAD65).

T cell recognition of autoantigens is critical to progressive immune-mediated destruction of islet cells, which leads to autoimmune diabetes. We identified a naturally presented autoantigen from the human islet antigen glutamic acid decarboxylase, 65-kDa isoform (GAD65), by using a combination of chromatography and mass spectrometry of peptides bound by the type I diabetes (insulin-dependent diabetes mellitus, IDDM)-associated HLA-DR4 molecule. Peptides encompassing this epitope-stimulated GAD65-specific T cells from diabetic patients and a DR4-positive individual at high risk for developing IDDM. T cell responses were antagonized by altered peptide ligands containing single amino acid modifications. This direct identification and manipulation of GAD65 epitope recognition provides an approach toward dissection of the complex CD4(+) T cell response in IDDM.

Proteasomes can either generate or destroy MHC class I epitopes: evidence for nonproteasomal epitope generation in the cytosol.

Proteasomes have been implicated in the production of the majority of peptides that associate with MHC class I molecules. We used two different proteasome inhibitors, the peptide aldehyde N-acetyl-L-leucyl-L-leucyl-L-norleucinal (LLnL) and the highly specific inhibitor lactacystin, to examine the role of proteasomes in generating peptide epitopes associated with HLA-A*0201. Neither LLnL nor lactacystin was able to completely block the expression of the HLA-A*0201. Furthermore, the effects of LLnL and lactacystin on the expression of different categories of specific epitopes, TAP independent vs TAP dependent and derived from either cytosolic or membrane proteins, were assessed. As predicted, presentation of two TAP-dependent epitopes was blocked by LLnL and lactacystin, while a TAP-independent epitope that is processed in the endoplasmic reticulum was unaffected by either inhibitor. Surprisingly, both LLnL and lactacystin increased rather than inhibited the expression of a cytosolically transcribed and TAP-dependent peptide from the influenza A virus M1 protein. Mass spectrometric analyses of in vitro proteasome digests of a synthetic 24 mer containing this epitope revealed no digestion products of any length that included the intact epitope. Instead, the major species resulted from cleavage sites within the epitope. Although cleavage at these sites was inhibitable by LLnL and lactacystin, epitope-containing species were still not produced. We conclude that proteasomes may in some cases actually destroy epitopes that would otherwise be destined for presentation by class I molecules. These results suggest that some epitopes are generated by nonproteasomal proteases in the cytosol.

Cutting edge: the HLA-A*0101-restricted HY minor histocompatibility antigen originates from DFFRY and contains a cysteinylated cysteine residue as identified by a novel mass spectrometric technique.

In this report, we describe the use of novel mass spectrometry instrumentation to identify a male-specific minor histocompatibility Ag restricted by HLA-A*0101 (A1-HY). This Ag has the sequence IVDC*LTEMY, where C* represents a cysteine disulfide bonded to a second cysteine residue. The core peptide sequence is found in the protein product of DFFRY, a Y chromosome gene not previously identified as the source of an HY Ag. The male-specific form of the peptide differs from its X chromosomal counterpart by the substitution of serine for the C* residue. Both peptides are expressed on the cell surface at 30 or fewer copies per cell. However, A1-HY-specific CTL recognize the DFFRY-derived peptide at a 1500-fold lower dose than the female homologue. Thus, these studies have identified a new source of HY epitopes and provide additional information about the influence of posttranslational modifications of class I-associated peptides on T cell recognition.