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Following the publication of the above article, the authors have requested a change in the authorship on the paper, and the revised list of authors is presented above; essentially, the ninth intended author, Giuseppe Salvo (G.S.), was inadverently omitted from the author list. G.S. contributed towards the T design and the preparation of the tagged ScFv. Therefore, the revised authors' names and affiliations, as they should have been presented in the original version of this paper, are as follows: Elisa Tremante1, Leonardo Sibilio2,7, Fabio Centola2,8, Nadine Knutti3,9, Gerd Holzapfel4, Isabella Manni5, Matteo Allegretti1, Paolo Lombardi6, Giuseppe Salvo2,10, Loredana Cecchetelli2, Karlheinz Friedrich3, Joachim BertraM4 and Patrizio Giacomini1. 1Oncogenomics and Epigenetics, IRCCS Regina Elena National Cancer Institute, Rome; 2Ibi Lorenzini, Aprilia, Italy; 3University Hospital Jena, Institute of Biochemistry II, Jena; 4IBA GmbH, Gottingen, Germany; 5SAFU, IRCCS Regina Elena National Cancer Institute, Rome; 6NaxosPharma, Novate Milanese, Milan, Italy. Correspondence to: Dr Patrizio Giacomini. Oncogenomics and Epigenetics, IRCCS Regina Elena National Cancer Institute, Via Elio Chianesi 53, 00144 Rome, Italy. E‑mail: patrizio.giacomini@ifo.gov.it. Present address: 7Menarini Biotech, Pomezia, Rome, Italy. Present address: 8Merck Serono Spa, Global Analytical Department, Guidonia Montecelio, Rome, Italy. Present address: 9University Hospital Jena Institute for Clinical Chemistry and Laboratory Diagnostics, Jena, Germany. Present address: 10External Quality Assurance (ExM), MSD Italia S.r.l., Via Vitorchiano 151, 00189 Rome.Italy. Furthermore, the Authors' contributions section should be amended to read as follows: Authors' contributions: ET and MA tested the TOOLBOX concept and performed the flow cytometry experiments. LS, FC, GS and LC designed and prepared the tagged ScFv. NK and KF designed and prepared the GFP promoter‑reporter construct. GH and JB designed and manufactured Strep‑Tactins. ET and IM performed animal studies. PL designed and manufactured NAX and NAXT compounds. PG conceptualized TOOLBOX and wrote the manuscript with the contribution of all authors. All authors have approved the final version of the manuscript. All the authors agree with the inclusion of Giuseppe Salvo as an author on this paper, and are grateful to the Editor for allowing them the opportunity to publish this Corrigendum. Furthermore, they apologize to the readership of the Journal for any inconvenience caused. [the original article was published in Oncology Reports 45: 77, 2021; DOI: 10.3892/or.2021.8028].

Herein, we describe TOOLBOX, a 3‑step modular nano‑assembly targeting system that permits the combinatorial exchange of antibody specificities and toxic payloads, introducing modularity in antibody‑drug conjugate (ADC) manufacturing. TOOLBOX integrates 3 building blocks: i) a recombinant antibody fragment (that in the selected setting targets the proto‑oncogene ERBB2) genetically fused to an 8 amino acid Strep‑Tag®; ii) a multivalent protein adapter, called Strep‑Tactin®; iii) two anticancer agents, e.g. DNA nanobinders and the maytansinoid DM1, both carrying a chemically attached Strep‑Tag that reversibly turns them into inactive prodrugs. Stoichiometrically optimized complexes of Strep‑Tagged antibody fragments and drugs, bridged by Strep‑Tactin, were specifically uptaken by breast cancer cells expressing ERBB2, and this unexpectedly resulted in conditional prodrug reactivation. A promoter‑reporter system showed that TOOLBOX inhibited downstream ERBB2 signaling not only in ERBB2‑overexpressing/‑amplified SK‑BR‑3 cells grown in vitro, but also in ERBB2‑low/non‑amplified BRC230 triple‑negative breast carcinoma cells xenotransplanted in nude mice. Thus, TOOLBOX is a modular ADC‑like nano‑assembly platform for precision oncology.

We report on the use of one-dimensional photonic crystals to detect clinically relevant concentrations of ERBB2/neu/Her2 in cell lysates. ERBB2 is a pivotal breast cancer biomarker and targetable oncogenic driver associated with aggressive breast cancer subtypes. To quantitate soluble ERBB2, we developed an optical platform that combines label-free and fluorescence detection modes. Such platform makes use of a sandwich assay in which the one-dimensional photonic crystals sustaining Bloch surface waves are tailored with a monoclonal antibody for highly specific biological recognition (BSW biochip). In a second step, a second antibody to ERBB2 quantitatively detects the bound analyte. The strategy of the present approach takes advantage of the combination of label-free and fluorescence techniques, making bio-recognition more robust and sensitive. In the fluorescence operation mode, the platform can attain the limit of detection 0.3 ng/mL (1.5 pM) for ERBB2 in cell lysates. Such resolution meets the international guidelines and recommendations (15 ng/mL) for diagnostic ERBB2 assays that in the future may help to more precisely assign therapies counteracting cancer cell proliferation and metastatic spread.

To present virus and tumor Ags, HLA class I molecules undergo a complex multistep assembly involving discrete but transient folding intermediates. The most extensive folding abnormalities occur in cells lacking the class I L chain subunit, called β2-microglobulin (β2m). Herein, this issue was investigated taking advantage of eight conformational murine mAbs (including the prototypic W6/32 mAb) to mapped H chain epitopes of class I molecules, four human mAbs to class I alloantigens, as well as radioimmunoprecipitation, in vitro assembly, pulse-chase, flow cytometry, and peptide-pulse/ELISPOT experiments. We show that endogenous (HLA-A1, -A66, and -B58) as well as transfected (HLA-A2) heavy chains in β2m-defective Burkitt lymphoma Daudi cells are capable of being expressed on the cell surface, although at low levels, and exclusively as immature glycoforms. In addition, HLA-A2 is: 1) partially folded at crucial interfaces with β2m, peptide Ag, and CD8; 2) receptive to exogenous peptide; and 3) capable of presenting exogenous peptide epitopes (from virus and tumor Ags) to cytotoxic T lymphocytes (bulk populations as well as clones) educated in a β2m-positive environment. These experiments demonstrate a precursor-product relationship between novel HLA class I folding intermediates, and define a stepwise mechanism whereby distinct interfaces of the class I H chain undergo successive, ligand-induced folding adjustments in vitro as well as in vivo. Due to this unprecedented class I plasticity, Daudi is the first human cell line in which folding and function of class I HLA molecules are observed in the absence of β2m. These findings bear potential implications for tumor immunotherapy.

Class II MHC genes (for example, the human HLA-DRA gene) are expressed at the cell surface in many professional and nonprofessional antigen-presenting cells in a variety of anatomical locations. Here, we report about 13 mouse transgenic lines (11 of which have not been previously described) generated with four distinct sets of DRA transgenes carrying progressive, informative 5' and 3' deletions. DRA expression was assessed in B lymphocytes, dendritic cells, macrophages, and extra-hematopoietic cells (particularly kidney epithelial cells). A compact transcriptional unit was identified that efficiently directs DRA expression [both constitutive and interferon (IFN)-gamma induced] in extra-hematopoietic tissues and dendritic cells. It extends from position -266 upstream of the transcription initiation site to position +119 downstream of the last DRA exon. The same fragment, however, did not efficiently direct IFN-gamma-induced DRA expression in macrophages, that required additional 5' sequences. Thus, IFN-gamma uses distinct promoter segments and mechanisms to up-regulate class II in different cell lineages. In contrast to previous results in transgenic mice expressing murine class II transgenes, we were unable to generate reproducible patterns of HLA-DRA expression in B cells.

The nonclassic class I human leukocyte antigen E (HLA-E) molecule engages the inhibitory NKG2A receptor on several cytotoxic effectors, including natural killer (NK) cells. Its tissue distribution was claimed to be wider in normal than in neoplastic tissues, and surface HLA-E was undetectable in most tumor cell lines. Herein, these issues were reinvestigated taking advantage of HLA-E-specific antibodies, immunohistochemistry, and biochemical methods detecting intracellular and surface HLA-E regardless of conformation. Contrary to published evidence, HLA-E was detected in a few normal epithelia and in a large fraction (approximately 1/3) of solid tumors, including those derived from HLA-E-negative/low-normal counterparts. Remarkably, HLA-E was detected in 30 of 30 tumor cell lines representative of major lymphoid and nonlymphoid lineages, and in 11 of 11, it was surface-expressed, although in a conformation poorly reactive with commonly used antibodies. Coexpression of HLA-E and HLA class I ligand donors was not required for surface expression but was associated with NKG2A-mediated protection from lysis by the cytotoxic cell line NKL and polyclonal NK cells from healthy donors, as demonstrated by antibody-mediated relief of protection in 10% to 20% of the tested target-effector combinations. NKG2A-mediated protection of additional targets became evident on NK effector blocking with antibodies to activating receptors (DNAM-1, natural cytotoxicity receptors, and NKG2D). Thus, initial evidence that the long-elusive HLA-E molecule is enhanced by malignant transformation and is functional in tumor cells is presented here, although its importance and precise functional role remain to be addressed in the context of a general understanding of the NK ligand-receptor network.

The nonclassical class I HLA-E molecule folds in the presence of peptide ligands donated by the signal sequences of permissive class I HLA alleles, with the aid of TAP and tapasin. To identify HLA-E-specific Abs, four monoclonals of the previously described MEM series were screened by isoelectric focusing (IEF) blot and immunoprecipitation/IEF on >30 single-allele class I transfectants and HLA-homozygous B lymphoid cells coexpressing HLA-E and HLA-A, -B, -C, -F, or -G. Despite their HLA-E-restricted reactivity patterns (MEM-E/02 in IEF blot; MEM-E/07 and MEM-E/08 in immunoprecipitation), all of the MEM Abs unexpectedly reacted with β2-microglobulin (β2m)-free and denatured (but not β2m-associated and folded) HLA-E H chains. Remarkably, other HLA-E-restricted Abs were also reactive with free H chains. Immunodepletion, in vitro assembly, flow cytometry, and three distinct surface-labeling methods, including a modified (conformation-independent) biotin-labeling assay, revealed the coexistence of HLA-E conformers with unusual and drastically antithetic features. MEM-reactive conformers were thermally unstable and poorly surface expressed, as expected, whereas β2m-associated conformers were either unstable and weakly reactive with the prototypic conformational Ab W6/32, or exceptionally stable and strongly reactive with Abs to β2m even in cells lacking permissive alleles (721.221), TAP (T2), or tapasin (721.220). Noncanonical, immature (endoglycosidase H-sensitive) HLA-E glycoforms were surface expressed in these cells, whereas mature glycoforms were exclusively expressed (and at much lower levels) in cells carrying permissive alleles. Thus, HLA-E is a good, and not a poor, β2m assembler, and TAP/tapasin-assisted ligand donation is only one, and possibly not even the major, pathway leading to its stabilization and surface expression.

Poor assembly of class I major histocompatibility HLA-C heavy chains results in their intracellular accumulation in two forms: free of and associated with their light chain subunit (beta(2)-microglobulin). Both intermediates are retained in the endoplasmic reticulum by promiscuous and HLA-dedicated chaperones and are poorly associated with peptide antigens. In this study, the eight serologically defined HLA-C alleles and the interlocus recombinant HLA-B46 allele (sharing the HLA-C-specific motif KYRV at residues 66-76 of the alpha1-domain alpha-helix) were compared with a large series of HLA-B and HLA-A alleles. Pulse-labeling experiments with HLA-C transfectants and HLA homozygous cell lines demonstrated that KYRV alleles accumulate as free heavy chains because of both poor assembly and post-assembly instability. Reactivity with antibodies to mapped linear epitopes, co-immunoprecipitation experiments, and molecular dynamics simulation studies additionally showed that the KYRV motif confers association to the HLA-dedicated chaperones TAP and tapasin as well as reduced plasticity and unfolding in the peptide-binding groove. Finally, in vitro assembly experiments in cell extracts of the T2 and 721.220 mutant cell lines demonstrated that HLA-Cw1 retains the ability to form a peptide-receptive interface despite a lack of TAP and functional tapasin, respectively. In the context of the available literature, these results indicate that a single locus-specific biosynthetic bottleneck renders HLA-C peptide-selective (rather than peptide-unreceptive) and a preferential natural killer cell ligand.

To resolve primary (glycosylation-assisted) from secondary (glycosylation-independent) quality control steps in the biosynthesis of HLA (human leukocyte antigen) class I glycoproteins, the unique N-linked glycosylation site of the HLA-Cw1 heavy chain was deleted by site-directed mutagenesis. The non-glycosylated Cw1S88G mutant was characterized by flow cytometry, pulse-chase, co-immunoprecipitation, and in vitro assembly assays with synthetic peptide ligands upon transfection in 721.221 and 721.220 cells. The former provide a full set of primary as well as secondary chaperoning interactions, whereas the latter are unable to perform secondary quality control (e.g. proper class I assembly with peptide antigens) as a result of a functional defect of the HLA-dedicated chaperone tapasin. In both transfectants, Cw1S88G displayed a loss/weakening in its generic chaperoning interaction with calreticulin and/or ERp57 and became redistributed toward calnexin, known to bind the most unfolded class I conformers. Despite this, and quite unexpectedly, a weak interaction with the HLA-dedicated chaperone TAP was selectively retained in 721.221. In addition, the ordered, stepwise acquisition of thermal stability/peptide binding was disrupted, resulting in a heterogeneous ensemble of Cw1S88G conformers with unorthodox and unprecedented peptide assembly features. Because a lack of glycosylation and a lack of tapasin-assisted peptide loading have distinct, complementary, and additive effects, the former is separable from (and upstream of) the latter, e.g. primary quality control is suggested to supervise a crucial, generic folding step preliminary to the acquisition of peptide receptivity.

Whether human leukocyte antigen (HLA)-A, -B, -C expression has any predictive value on the prognosis of human malignancies remains controversial. Herein, monoclonal antibodies with preferential reactivity for HLA-A, HLA-B, and HLA-C (HCA2, HC10, and L31) were used to stain an archival collection of 291 formalin-fixed/paraffin-embedded tissues, comprising neoplastic lesions from stages II and III colon carcinoma patients (n=165), and the uninvolved, morphologically normal mucosae from a subset (n=126) of these patients. Marked staining variability was detected not only in the tumors as in previous studies, but also in the normal paired mucosae. HLA-A, -B, -C expression was similar in approximately two thirds of the available 126 normal/neoplastic pairs, confirming in vivo our previous observation that most tumor cells mimic the HLA phenotypes of their normal counterparts. Both up and down-regulation occurred in the remaining third of the pairs, but did not coincide with high and low expression, respectively, conventionally evaluated on the tumor lesion only. Remarkably, a "paired" evaluation, but not high or low expression in the tumor, was predictive of the clinical outcome. Deviations from the expression in the normal paired mucosa (both increases and decreases) of HCA2-reactive class I molecules (possibly HLA-A), and down-regulation of L31-reactive class I molecules (possibly HLA-C), particularly in tumors from stage II patients, correlated with poor 5-year overall and disease-free survival, hazard risk ranging from 2 to 6, approximately. Thus, a paired immunohistochemical comparison reveals a novel immune evasion strategy that may impact on the prognosis of colon carcinoma.

Three paired (from the same donor) sets of melanoma cells and normal melanocytes, established as early-passage cultures from metastatic lesions and the uninvolved skin of three patients, were comparatively cDNA profiled by macroarrays (approximately 1,200 genes) and reverse transcription (RT)-PCR. While 145 gene products were significantly (at least twofold) upregulated or downregulated in at least 1 pair, and 23 were in at least 2 pairs, only 3 (the signal transducer and activator of transcription STAT2, collagen type VI, and CD9) were concordantly modulated (downregulation) in all 3 pairs. Array results were validated by RT-PCR on a small panel of surgically removed nevocellular nevi and metastatic melanoma lesions, and by immunohistochemistry on a large panel of benign and malignant lesions of the nevomelanocytic lineage. The three gene products were downregulated at different stages of melanoma progression. STAT2 was detectable in nevi (5/5) and most primary melanomas (11/12), but was lost in 10/15 metastatic lesions. Collagen type VI was expressed in nevi (5/5) and primary melanomas below a Breslow thickness of 1 mm (3/3), but was lost in 24/24 primary melanomas above this threshold, and in metastatic melanomas (10/10). The tetraspanin CD9 molecule was expressed in 18/18 nevi, but was lost in 20/28 primary melanomas (including thin lesions), and in 24/52 metastatic lesions. These data provide the proof of principle that cDNA profiling of paired melanocyte/melanoma cultures sorts out novel, early signatures of melanocyte transformation that could contribute to the clinical management of patients at high risk of metastatic disease.

Class I MHC H chains assemble with β2-microglobulin (β2m) and are loaded with peptide Ags through multiple folding steps. When free of β2m, human H chains react with Abs to linear epitopes, such as L31. Immunodepletion and coimmunoprecipitation experiments, performed in this study, detected a preferential association of L31-reactive, β2m-free H chains with calnexin in β2m-defective cells, and with calreticulin and TAP in β2m-expressing cells. In β2m-defective cells, the accumulation of calnexin-bound H chains stoichiometrically exceeded their overall accumulation, a finding that supports both chaperoning preferences and distinct sorting abilities for different class I folds. No peptide species, in a mass range compatible with that of the classical class I ligands, could be detected by mass spectrometry of acidic eluates from L31-reactive HLA-Cw1 H chains. In vitro assembly experiments in TAP-defective T2 cells, and in cells expressing an intact Ag-processing machinery, demonstrated that L31 H chains are not only free of, but also unreceptive to, peptides. L31 and HC10, which bind nearly adjacent linear epitopes of the α1 domain α helix, reciprocally immunodepleted free HLA-C H chains, indicating the existence of a local un-/mis-folding involving the N-terminal end of the α1 domain α helix and peptide-anchoring residues of the class I H chain. Thus, unlike certain murine free H chains, L31-reactive H chains are not the immediate precursors of conformed class I molecules. A model inferring their precursor-product relationships with other known class I intermediates is presented.

The ultimate outcome of an immune response (escape or surveillance) depends on a delicate balance of opposing signals delivered by activating and inhibitory immune receptors expressed by cytotoxic T lymphocytes and natural killer cells. In this light, loss and down-regulation of human leukocyte antigens (HLA) class I molecules, while important for keeping tumors below the T-cell detection levels, may incite recognition of missing self. Conversely, the maintenance of normal levels of expression (or even up-regulation) may be favorable to tumors, at least in certain cases. In this study, we took advantage of a previously characterized panel of 15 early passage tumor cell lines (mainly from melanoma and lung carcinoma lesions) enriched with class I-low phenotypes. These cells were systematically characterized by Northern and/or Western blotting (e.g., mini-transcriptome/mini-proteome analysis) for the expression of HLA-A, -B, -C, beta(2)-microglobulin, and the members of the "antigen processing machinery" of class I molecules (LMP2, LMP7, TAP1, TAP2, tapasin, calreticulin, calnexin, and ERp57). In addition, we established four pairs of cultures, each comprising melanoma cells and normal melanocytes from the same patient. We found that approximately 97% of the 185 tested gene products are expressed (although often weakly), and in many cases coordinately regulated in 18 of 19 tumor cell lines. Linked expression patterns could be hierarchically arranged by statistical methods and graphically described as a class I HLA "coordinome." Deviations (both down- and up-regulation) from the coordinome expression pattern inherited from the normal, paired melanocyte counterpart, were allowed but limited in magnitude, as if melanoma cells were trying to keep a "low profile" HLA phenotype. We conclude that irreversible HLA loss is a rare event, and class I expression in tumor cells almost invariably results from reversible gene regulatory (rather than gene disruption) events.

Peptide trimming in the endoplasmic reticulum (ER), the final step required for the generation of most HLA class I-binding peptides, implicates the concerted action of two aminopeptidases, ERAP1 and ERAP2. Because defects in the expression of these peptidases could lead to aberrant surface HLA class I expression in tumor cells, we quantitatively assayed 14 EBV-B cell lines and 35 human tumor cell lines of various lineages for: 1) expression and enzymatic activities of ERAP1 and ERAP2; 2) ER peptide-trimming activity in microsomes; 3) expression of HLA class I H chains and TAP1; and 4) surface HLA class I expression. ERAP1 and ERAP2 expression was detectable in all of the EBV-B and tumor cell lines, but in the latter it was extremely variable, sometimes barely detectable, and not coordinated. The expression of the two aminopeptidases corresponded well to the respective enzymatic activities in most cell lines. A peptide-trimming assay in microsomes revealed additional enzymatic activities, presumably contributed by other unidentified aminopeptidases sharing substrate specificity with ERAP2. Interestingly, surface HLA class I expression showed significant correlation with ERAP1 activity, but not with the activity of either ERAP2 or other unidentified aminopeptidases. Transfection with ERAP1 or ERAP2 of two tumor cell lines selected for simultaneous low expression of the two aminopeptidases resulted in the expected, moderate increases of class I surface expression. Thus, low and/or imbalanced expression of ERAP1 and probably ERAP2 may cause improper Ag processing and favor tumor escape from the immune surveillance.