High-level expression of recombinant immunoreactive thyroid peroxidase in the High Five insect cell line

Purification of recombinant human thyroid peroxidase (hTPO) from AD293 mammalian cells

Human thyroid peroxidase (hTPO) has been secretory expressed in AD293 mammalian cells. cDNA sequence of 'Gluc' (Gaussia luciferase) protein from Gaussia princeps was incorporated at the amino terminal of hTPO gene for secretion of targeted protein outside the mammalian cells. Augmentation of TPO clone in serum free mediums was investigated and a simplified purification procedure of hTPO has been reported here. Purified hTPO was further analyzed by SDS-PAGE and immunoblotting (western blotting). The relative molecular mass of hTPO was found to be 105kDa. This is the first report with respect to cost effective and simplified purification approach to get highest yield and purity of recombinant hTPO.

Breaking tolerance to thyroid antigens: changing concepts in thyroid autoimmunity

Thyroid autoimmunity involves loss of tolerance to thyroid proteins in genetically susceptible individuals in association with environmental factors. In central tolerance, intrathymic autoantigen presentation deletes immature T cells with high affinity for autoantigen-derived peptides. Regulatory T cells provide an alternative mechanism to silence autoimmune T cells in the periphery. The TSH receptor (TSHR), thyroid peroxidase (TPO), and thyroglobulin (Tg) have unusual properties ("immunogenicity") that contribute to breaking tolerance, including size, abundance, membrane association, glycosylation, and polymorphisms. Insight into loss of tolerance to thyroid proteins comes from spontaneous and induced animal models: 1) intrathymic expression controls self-tolerance to the TSHR, not TPO or Tg; 2) regulatory T cells are not involved in TSHR self-tolerance and instead control the balance between Graves' disease and thyroiditis; 3) breaking TSHR tolerance involves contributions from major histocompatibility complex molecules (humans and induced mouse models), TSHR polymorphism(s) (humans), and alternative splicing (mice); 4) loss of tolerance to Tg before TPO indicates that greater Tg immunogenicity vs TPO dominates central tolerance expectations; 5) tolerance is induced by thyroid autoantigen administration before autoimmunity is established; 6) interferon-α therapy for hepatitis C infection enhances thyroid autoimmunity in patients with intact immunity; Graves' disease developing after T-cell depletion reflects reconstitution autoimmunity; and 7) most environmental factors (including excess iodine) "reveal," but do not induce, thyroid autoimmunity. Micro-organisms likely exert their effects via bystander stimulation. Finally, no single mechanism explains the loss of tolerance to thyroid proteins. The goal of inducing self-tolerance to prevent autoimmune thyroid disease will require accurate prediction of at-risk individuals together with an antigen-specific, not blanket, therapeutic approach.

Production and characterization of a monoclonal antibody specific to Nef-associated factor 1 (Naf1)/A20-binding inhibitor of NF-kappaB activation (ABIN-1)

Cellular protein Naf1 (Nef-associated factor 1) or ABIN-1 (A20-binding inhibitor of NF-kappaB activation) is an important cellular protein, expressed in various human tissues and T-cell lines. Naf1 protein has two isoforms (Naf1alpha and Naf1beta) with different C-termini, produced by alternative splicing. Naf1alpha and Naf1beta have approximately 2800 and 2600 nucleotides, with an open reading frame of 1941 and 1781 nucleotides, encoding the 72-kDa Naf1alpha and 68-kDa Naf1beta proteins, respectively. In the present study, we generated a monoclonal antibody (MAb) against human Naf1, which recognizes full-length, endogenous Naf1 of both isotypes. For this purpose, recombinant 6xHis and myc-tagged N-terminal Naf1(38135), Naf1(N) protein was produced by using the baculovirus expression system. Recombinant Naf1(N) protein was used to immunize Balb/c mice, and a hybridoma cell line producing stable and highly specific MAb with strong affinity to Naf1 was established. We further characterized this antibody by immunofluorescent assay and Western blot analysis to confirm effectiveness in detecting recombinant and endogenous Naf1. By Western blot analysis of recombinant Naf1-N fusion proteins with overlapping N-terminal sequences, the epitope targeted by anti-Naf1 MAb was determined as the 81-88-amino acid region of human Naf1.

Structural and functional aspects of thyroid peroxidase

Thyroperoxidase (TPO) is the enzyme involved in thyroid hormone synthesis. Although many studies have been carried out on TPO since it was first identified as being the thyroid microsomal antigen involved in autoimmune thyroid disease, previous authors have focused more on the immunological than on the biochemical aspects of TPO during the last few years. Here, we review the latest contributions in the field of TPO research and provide a large reference list of original publications. Given this promising background, scientists and clinicians will certainly continue in the future to investigate the mechanisms whereby TPO contributes to hormone synthesis and constitutes an important autoantigen involved in autoimmune thyroid disease, and the circumstances under which the normal physiological function of this enzyme takes on a pathological role.

Interactions between the mannose receptor and thyroid autoantigens

Thyroid autoantigens require internalization and processing by antigen-presenting cells to induce immune responses. Besides pinocytosis, antigen uptake can be receptor-mediated. The mannose receptor (ManR) has a cysteine rich domain (CR) and eight carbohydrate recognition domains (CRD) that bind glycosylated proteins. The TSH receptor (TSHR), thyroid peroxidase (TPO) and thyroglobulin (Tg) are glycoproteins. To investigate a role for the ManR in thyroid autoimmunity, we tested the interaction between these autoantigens and chimeric ManRs. Plasmids encoding the CR-domain linked to IgG-Fc (CR-Fc) and CDR domains 4-7 linked to IgG-Fc (CDR4-7-Fc) were expressed and purified with Protein A. Enzyme-linked immunosorbent assay (ELISA) plates were coated with human thyroid autoantigens and CR-Fc or CRD4-7-Fc binding detected with peroxidase-conjugated anti-IgG-Fc. CRD4-7-Fc binding was highest for the TSHR, followed by Tg and was minimal for TPO. CR-Fc bound to Tg but not to TSHR or TPO. The interaction between the TSHR and CRD-Fc was calcium-dependent; it was inhibited by mannose (not galactose), and required a glycosylated TSHR A-subunit. Moreover, precomplexing the TSHR A-subunit with CRD-Fc (but not CR-Fc), or adding mannose (but not galactose), decreased in vitro responses of splenocytes from TSHR-immunized mice. Our data indicate that the ManR may participate in autoimmune responses to Tg and the TSHR but not to TPO. Most important, ManR binding of heavily glycosylated TSHR A-subunits suggests a mechanism by which the minute amounts of A-subunit protein shed from the thyroid may be captured by antigen-presenting cells located in the gland or in draining lymph nodes.

Superiority of thyroid peroxidase DNA over protein immunization in replicating human thyroid autoimmunity in HLA-DRB1*0301 (DR3) transgenic mice

Murine experimental autoimmune thyroiditis (EAT), characterized by thyroid destruction after immunization with thyroglobulin (Tg), has long been a useful model of organ-specific autoimmune disease. More recently, porcine thyroid peroxidase (pTPO) has also been shown to induce thyroiditis, but these results have not been confirmed. When (C57BL/6 x CBA)F(1) mice, recently shown to be susceptible to mouse TPO-induced EAT, were immunized with plasmid DNA to human TPO (hTPO) and cytokines IL-12 or GM-CSF, significant antibody (Ab) titres were generated, but minimal thyroiditis was detected in one mouse only from the TPO + GM-CSF immunized group. However, after TPO DNA immunization of HLA-DR3 transgenic class II-deficient NOD mice, thyroiditis was present in 23% of mice injected with TPO + IL-12 or GM-CSF. We also used another marker for assessing the closeness of the model to human thyroid autoimmunity by examining the epitope profile of the anti-TPO Abs to immunodominant determinants on TPO. Remarkably, the majority of the anti-TPO Abs was directed to immunodominant regions A and B, demonstrating the close replication of the model to human autoimmunity. TPO protein immunizations of HLA-DR3 transgenic mice with recombinant hTPO did not result in thyroiditis, nor did immunization of other mice expressing HLA class II transgenes HLA-DR4 or HLA-DQ8, with differential susceptibility to Tg-induced EAT. Moreover, our efforts to duplicate exactly the experimental procedures used with pTPO also failed to induce thyroiditis. The success of hTPO plasmid DNA immunization of DR3(+) mice, similar to our reports on Tg-induced thyroiditis and thyrotropin receptor DNA-induced Graves' hyperthyroidism, underscores the importance of DR3 genes for all three major thyroid antigens, and provides another humanized model to study autoimmune thyroid disease.

Autoimmune response to the thyroid in humans: thyroid peroxidase--the common autoantigenic denominator

Autoimmunity to thyroid peroxidase (TPO), manifest as high affinity IgG class autoantibodies, is the common denominator of human thyroid autoimmunity, encompassing patients with overt hyper- or hypothyroidism as well as euthyroid individuals with subclinical disease. The identification and cloning of TPO (the "thyroid microsomal antigen") provided the critical tool for analyzing B and T cell reactivity to this major thyroid autoantigen. In particular, the availability of immunoreactive TPO permitted the isolation of essentially the entire repertoire of human monoclonal antibodies, a feat unparalled in an organ-specific autoimmune disease. These recombinant autoantibodies (expressed as Fab) provide insight into the genes encoding their H and L chains as well as the conformational epitopes on TPO with which serum autoantibodies interact. Analyses of TPO autoantibody epitopic "fingerprints" indicate a lack of epitope spreading as well as a genetic basis for their inheritance. Limited data are available for the responses and cytokine profiles of T cells to endogenously processed TPO. Moreover, the role of thyroid cells in initiating the autoimmune response to TPO, and of B cells in expanding and/or modulating the response of sensitized T cells, has yet to be established. Finally, because autoantibody (and likely T cell) responses to TPO parallel those to TSH receptor and thyroglobulin, manipulation of T and B cell responses to TPO may provide the basis for the development of immunospecific therapy for autoimmune thyroid disease in general.

Cell type-dependent differences in thyroid peroxidase cell surface expression

Recently, it has been suggested that only approximately 2% of human thyroid peroxidase (hTPO(933)) reaches the surface of stably transfected (Chinese hamster ovary) cells, most being degraded intracellularly, and this might be representative of thyroid peroxidase (TPO) behavior in thyrocytes (Fayadat, L., Siffroi-Fernandez, S., Lanet, J., and Franc, J.-L. (2000) J. Biol. Chem. 275, 15948-15954). In agreement, in stably transfected Madin-Darby canine kidney clones, nonpermeabilized cells exhibit wild-type hTPO(933) immunofluorescence (apically) on <10% of that found in permeabilized cells, where an endoplasmic reticulum pattern is observed. Further, a C-terminally truncated, membrane-anchorless hTPO(848) is also retained in the endoplasmic reticulum of stably transfected Madin-Darby canine kidney cells. However, by contrast, in Chinese hamster ovary cells after transient transfection, hTPO(933) immunofluorescence is detected equally well in nonpermeabilized and permeabilized cells, indicating that a large portion of hTPO(933) is present at the cell surface; furthermore, hTPO(848) is efficiently secreted. Further, using an antiserum not cross-reacting with rat TPO, we find by immunofluorescence that in stable clones of PC Cl3 (rat) thyrocytes, considerably more ( approximately 50%) of the cells exhibit hTPO(933) at the cell surface. However, cell surface biotinylation and endoglycosidase H digestion assays appear to under-represent the extent of hTPO(933) transport, presumably because protein folding limits both Golgi carbohydrate modification and accessibility of lysines in the extracellular domain. We conclude that cell type-specific factors may facilitate stable expression of TPO at the cell surface of thyrocytes.

Recombinant human thyroid peroxidase produced in insect cells has similar properties to native human thyroid peroxidase

Purified native human thyroid peroxidase (nTPO) isolated from thyroid tissue and recombinant (r)TPO produced in High Five insect cells have been compared. nTPO and rTPO were purified to about 95% homogeneity and showed similar UV and visual spectra and similar 412 nm per 280 nm absorbance ratios (0.4 for nTPO and 0.4 for rTPO). The nTPO and rTPO guaiacol oxidation enzyme activities were about 1,000 guaiacol units per milligram of protein. TPO autoantibody binding characteristics of nTPO and rTPO were analyzed in an assay based on 125I-labeled nTPO and precipitation with protein A. In the assay, the effect of unlabeled nTPO or rTPO on TPO autoantibody binding from 25 patients sera was studied. Unlabeled nTPO or rTPO (from 0 to 160 ng/mL) inhibited the binding of TPO autoantibodies in a dose-dependent manner in the case of each serum studied (from 100% in the absence of unlabeled TPO to 5%-10% in the presence of 160 ng/mL of TPO). The inhibition profile for each serum was essentially identical in the case of both TPO preparations. The effect of TPO autoantibodies on enzyme activity of rTPO was analyzed after incubation of rTPO with TPO autoantibody-positive serum immunoglobulin G (IgG) (n = 12), TPO monoclonal antibodies reactive with two different epitopes on the TPO, IgG (n = 3) from glutamic acid decarboxylase autoantibody positive patient sera, and IgG (n = 3) from healthy blood donors. Effective complexing of TPO by TPO autoantibodies was tested by precipitating the complexes with solid phase protein A and measuring the TPO enzyme activity in the resulting supernatants. These studies showed that the TPO enzyme activity was not affected by incubation with TPO autoantibody-positive IgG or monoclonal antibodies despite effective complexing of the autoantibodies with TPO. Overall, our studies demonstrate that nTPO and rTPO produced in insect cells are very similar in terms of enzyme activity, UV and visible spectra, and reactivity with autoantibodies. Furthermore, in our study, TPO autoantibodies did not appear to inhibit TPO enzyme activity.

PCR cloning and baculovirus expression of human lactoperoxidase and myeloperoxidase

Lactoperoxidase (LPO) and myeloperoxidase (MPO) have been identified previously in human milk. These peroxidases have antimicrobial activity and presumably contribute to the protective functions of milk. In this study, we amplified genes encoding LPO and MPO from human mammary gland cDNA by the polymerase chain reaction (PCR). These genes were expressed in a baculovirus-insect cell system. Peroxidase activity was observed in the culture supernatant of Tricoplusia ni cells infected with the recombinant viruses and the levels increased upon addition of delta-aminolevulinic acid. Purified recombinant human LPO and MPO, both with a molecular mass of about 80 kDa, showed properties similar to bovine LPO and human MPO, respectively, in terms of absorption spectrum, sensitivity to dapsone, specificity for chloride ions, and reactivity with anti-bovine LPO or anti-MPO antibodies. Our data suggest that this expression system is useful for studying the catalytic mechanism and biological significance of these human peroxidases.

Analysis of autoantibody epitopes on human thyroid peroxidase

A number of studies have indicated that the major autoantibody epitopes on human thyroid peroxidase (TPO) are conformational and are formed by two overlapping immunodominant regions on the TPO molecule. In order to investigate further autoantibody reactivity with TPO, we have studied the TPO binding characteristics of sera from patients with autoimmune thyroid disease (n = 20), autoimmune adrenal disease (Addison's disease; n = 8) and apparently healthy blood donors (n = 9) using recombinant TPO expressed with a series of truncations and internal deletions. This material was obtained using an in vitro transcription/translation system in the presence of 35S-methionine and the reactivity of TPO autoantibodies tested in an immunoprecipitation assay. In addition, we have studied the effects of denaturing purified recombinant TPO by reduction and/or sodium dodecyl sulphate on its reactivity with TPO autoantibodies by Western blotting analysis. These studies show that TPO autoantibodies can recognise TPO in Western blotting analysis when large amounts of purified TPO are run on the gels and the blotted proteins renatured prior to addition of antibody. Under these conditions TPO autoantibodies in all 20 Graves' or Hashimoto's sera tested reacted strongly with blots of non-reduced TPO but reduction of TPO had a marked effect on the ability of autoantibodies to recognise it in Western blotting analysis. Analysis of TPO autoantibody binding to 35S-labelled TPO proteins containing N-terminal, central or C-terminal deletions indicated that all modifications studied caused a statistically significant lowering of binding. In the case of some modifications, there were differences in the reactivity of TPO autoantibodies in sera from patients with Addison's disease compared to TPO autoantibodies in autoimmune thyroid disease and/or healthy blood donor sera. Overall, our results of analysis of T PO autoantibody binding in Western blotting and with modified TPO proteins in immunoprecipitation assays suggest that the main autoantibody binding sites on the TPO molecule involve extensive amino acid sequences. Our studies also suggest that TPO autoantibodies from patients with autoimmune thyroid disease, Addison's disease and apparently healthy blood donors show some differences in epitope recognition on TPO and this approach may allow differentiation between disease related and unrelated TPO autoantibodies.

Distinct immunological and biochemical properties of thyroid peroxidase purified from human thyroid glands and recombinant protein produced in insect cells

The biosynthesis of thyroid hormone from thyroglobulin is catalysed by thyroid peroxidase (TPO), an integral membrane protein. TPO is also a major autoantigen in autoimmune thyroid disease and autoantibodies to TPO are markers for disease activity. Large quantities of purified TPO are essential for elucidating its structure and understanding its role in disease activity. We describe the high yield purification of full-length recombinant human TPO from baculovirus infected insect cells and compare it to purified native TPO from human thyroid glands. In contrast to native human TPO, the human TPO produced in insect cells as a recombinant protein was insoluble and resistant to solubilisation in detergents. Reversible substitution of lysine residues with citraconic anhydride led to increased solubility of the recombinant TPO, allowing high-yield purification by monoclonal antibody chromatography. The purified enzyme preparation was shown to be TPO by its reactivity with monoclonal and polyclonal antibodies by enzyme linked immunosorbent assay and Western blotting. Both the human and recombinant purified TPO preparations also react with sera from patients with autoimmune thyroid disease, although the binding of conformational dependent autoantibodies was considerably lower to the recombinant TPO than to the native TPO. This suggests that the recombinant TPO may differ in some aspects of its tertiary structure. The purified recombinant TPO was devoid of enzyme activity, in contrast to the enzymatically active, purified human TPO preparations. Both preparations contained comparable amounts of haem (R(z)=0.269), but a shift in the Soret band of recombinant TPO (402 nm) from that of natural TPO (409 nm) indicates that the lack of enzymatic activity of the recombinant enzyme may be due to changes in the protein backbone surrounding the haem. Both the purified native and recombinant TPO, under non-denaturing conditions, show evidence of high molecular mass oligomers, although the latter preparation is prone to a greater degree of aggregation. In conclusion, our studies indicate that recombinant TPO generated in insect cells is conformationally distinct from the native TPO, is insoluble and enzymatically inactive, consistent with the difficulties associated with its purification and crystallisation.