Immunocapture-Selected Reaction Monitoring Screening Facilitates the Development of ELISA for the Measurement of Native TEX101 in Biological Fluids

Monoclonal antibodies that bind the native conformation of proteins are indispensable reagents for the development of immunoassays, production of therapeutic antibodies and delineating protein interaction networks by affinity purification-mass spectrometry. Antibodies generated against short peptides, protein fragments, or even full length recombinant proteins may not bind the native protein form in biological fluids, thus limiting their utility. Here, we report the application of immunocapture coupled with selected reaction monitoring measurements (immunocapture-SRM), in the rapid screening of hybridoma culture supernatants for monoclonal antibodies that bind the native protein conformation. We produced mouse monoclonal antibodies, which detect in human serum or seminal plasma the native form of the human testis-expressed sequence 101 (TEX101) protein—a recently proposed biomarker of male infertility. Pairing of two monoclonal antibodies against unique TEX101 epitopes led to the development of an ELISA for the measurement of TEX101 in seminal plasma (limit of detection: 20 pg/ml) and serum (limit of detection: 40 pg/ml). Measurements of matched seminal plasma samples, obtained from men pre- and post-vasectomy, confirmed the absolute diagnostic specificity and sensitivity of TEX101 for noninvasive identification of physical obstructions in the male reproductive tract. Measurement of male and female serum samples revealed undetectable levels of TEX101 in the systemic circulation of healthy individuals. Immunocapture-SRM screening may facilitate development of monoclonal antibodies and immunoassays against native forms of challenging protein targets.Monoclonal antibodies that bind the native form of a protein are indispensable for the development of sensitive immunoassays, production of therapeutic antibodies and for studying protein interaction networks by affinity purification-mass spectrometry (1, 2). Large-scale purification of native proteins from biological samples may be challenging, so recombinant proteins or protein fragments are often used for antibody production. Antibodies produced against short peptides, protein fragments, or even full length recombinant proteins, however, may not bind the native protein conformation present in biological fluids, thus limiting the utility of antibodies. Rapid screening of antibody-producing hybridoma clones for native protein binders requires highly specific and sensitive assays, performed under nondenaturing conditions. Here, we report the capability of an immunocapture-SRM assay to facilitate fast screening of hybridoma cultures for monoclonal antibodies that recognize the native conformation of testis-expressed sequence 101 (TEX101)¹ protein in biological fluids.Recently, we discovered, verified, and validated two proteins, testis-specific protein TEX101 and epididymis-specific protein ECM1, as biomarkers for the differential diagnosis of azoospermia (3, 4). Combination of TEX101 and ECM1 proteins measured in seminal plasma could differentiate between normal spermatogenesis, obstructive azoospermia (OA), and nonobstructive azoospermia (NOA) with very high diagnostic sensitivity and specificity. TEX101 levels in seminal plasma also facilitated classification of NOA subtypes of hypospermatogenesis, maturation arrest and Sertoli cell-only syndrome (5). A clinical laboratory test for TEX101 in seminal plasma may confirm the success of vasectomy or vasovasostomy, eliminate diagnostic testicular biopsies, and predict the success of sperm cell retrieval for assisted reproduction.Human TEX101 is a membrane GPI-anchored protein encoded by the TEX101 gene, located in the 19q13.31 region of chromosome 19. According to the Human Protein Atlas, TEX101 expression is restricted to testicular tissue and male germ cells, with no evidence of expression in any other human tissue or cell type (6). Investigation of the function of mouse TEX101 demonstrated its direct role in fertilization (7–9).We initially measured TEX101 levels in seminal plasma by mass spectrometry-based selected reaction monitoring (SRM) and immuno-SRM assays, with limits of detection of 120 and 5 ng/ml, respectively (4, 5). However, because of the ultra-wide range of TEX101 concentrations in seminal plasma of infertile and healthy men (0.5 ng/ml to 50,000 ng/ml) and theoretically zero levels for some azoospermic patients, a sensitive TEX101 immunoassay is required to develop a clinical laboratory test. In addition to immunoassay, monoclonal antibodies against native TEX101 would allow investigating its interactome and revealing its functional role in spermatogenesis and male fertility. Because TEX101 may emerge as a novel biomarker of male infertility, in this work we focused on the development of an ELISA for sensitive measurement of TEX101 in seminal plasma and serum.Our initial efforts to develop a TEX101 immunoassay using commercially available polyclonal antibodies were not successful. We found that commercial antibodies recognized only the denatured form of TEX101 and were useful for immunohistochemistry and Western blots, but not for the analysis of native TEX101 in seminal plasma. Here, we describe the production of mouse monoclonal antibodies against native TEX101, screening of antibody-producing clones by the two-step immunocapture and SRM assay, development of a sensitive ELISA and measurement of TEX101 in seminal plasma and serum (Fig. 1).Open in a separate windowFig. 1.Pipeline for the production of mouse monoclonal anti-TEX101 antibodies and screening of colonies using two-step immunocapture-SRM assay. Screening included the coating of microtiter plates with sheep anti-mouse IgG antibodies, the addition of hybridoma cell supernatants, incubation with seminal plasma containing the native form of TEX101 followed by trypsin digestion and SRM analysis. Two-step immunocapture followed by SRM detection facilitated rapid screening of antibody-producing colonies and provided the following advantages: no requirement for previously developed TEX101 antibodies, small scale antibody production on 96-well plates, screening of low amounts of the newly-produced antibodies and direct selection of antibodies against the native form of TEX101. Eventually, all positive clones were expanded and a sensitive immunofluorescent assay for TEX101 was developed in seminal plasma and serum.