Human intestinal alkaline phosphatase-binding IgG in patients with severe bacterial infections

Impact of Unconjugated estriol (uE3) assay interference on prenatal screening tests

BACKGROUND

Unconjugated estriol (uE3) is an important biomarker in second trimester prenatal screening. Previous studies from our laboratory identified rare interference in the Beckman uE3 assay due to anti-ALP antibodies, which could be mitigated with a scavenger or heat-inactivated ALP (hALP). In the current study, 160 de-identified patient samples previously submitted for the Quad screen with low uE3 multiples of the median (MoM ≤0.50) were investigated for potential interference.

METHODS

A reagent pack spiking strategy with hALP was employed to understand if the interference could be identified and mitigated in a scalable manner. The 160 samples were measured using uE3 lot #920861 previously known to be subject to interference, lot #920861 spiked with hALP, and the vendor reformulated lot #922579. Samples were suspected to have interference if the percent difference in uE3 measurements was >50%. Pseudo-risks were calculated using a test patient environment to understand the screening impact due to the change in uE3 result.

RESULTS

Seventeen of the 160 samples had uE3 results that were >50% different between the hALP spiked and non-spiked reagent pack. Both original lot #920861 with hALP and reformulated lot #922579 identified the same 17 patients as having interference in lot #920861. Analysis of screening risks using a test patient environment showed that assay interference could result in false positives for one trisomy 21 and three trisomy 18 post-test risk calculations.

CONCLUSION

Our experiment of reagent pack spiking with hALP produced similar uE3 results to a reformulated reagent designed to address potential interference, demonstrating that this is a feasible strategy to screen for interference in a scalable manner. The vendor-provided reformulation addressed anti-ALP interference and improved the performance of the screen.

Aptameric Probe Specifically Binding Protein Heterodimer Rather Than Monomers

Dimerization of proteins occurs frequently and plays integral roles in biological processes. However, no single molecular probe is available for in situ detection of protein dimers on cells and tissues because of the difficulty of isolating complete protein dimers for probe preparation and screening, which has greatly hampered the biomedical study of protein dimers. Herein, a G-rich DNA aptamer (termed BG2) that only binds alkaline phosphatase (AP) heterodimers rather than monomers is reported. This aptamer is generated by the cell-SELEX (systematic evolution of ligands by exponential enrichment) technique and proves to fold into a duplex stabilized antiparallel G-quadruplex structure. Using BG2 as molecular probe, AP heterodimers are found to be expressed on several kinds of cancer cells. As an affinity ligand, BG2 could isolate AP heterodimers from cell lysate. BG2 is also demonstrated to be applicable for tumor imaging in mice xenografted with cells highly expressing AP heterodimers. AP isozymes are found in several tissues and blood throughout the body, but the function and tissue distribution of AP heterodimers are totally unknown; therefore, BG2 could serve as a molecular probe to uncover the mystery of AP heterodimers. The generation of aptameric probes by cell-SELEX will open up a new situation for the study of protein dimers.

An alkaline phosphatase transport mechanism in the pathogenesis of Alzheimer's disease and neurodegeneration

Systemic inflammation is associated with loss of blood-brain barrier integrity and neuroinflammation that lead to the exacerbation of neurodegenerative diseases. It is also associated specifically with the characteristic amyloid-β and tau pathologies of Alzheimer's disease. We have previously proposed an immunosurveillance mechanism for epithelial barriers involving negative feedback-regulated alkaline phosphatase transcytosis as an acute phase anti-inflammatory response that hangs in the balance between the resolution and the progression of inflammation. We now extend this model to endothelial barriers, particularly the blood-brain barrier, and present a literature-supported mechanistic explanation for Alzheimer's disease pathology with this system at its foundation. In this mechanism, a switch in the role of alkaline phosphatase from its baseline duties to a stopgap anti-inflammatory function results in the loss of alkaline phosphatase from cell membranes into circulation, thereby decreasing blood-brain barrier integrity and functionality. This occurs with impairment of both amyloid-β efflux and tau dephosphorylating activity in the brain as alkaline phosphatase is replenished at the barrier by receptor-mediated transport. We suggest systemic alkaline phosphatase administration as a potential therapy for the resolution of inflammation and the prevention of Alzheimer's disease pathology as well as that of other inflammation-related neurodegenerative diseases.

A novel hypothesis for an alkaline phosphatase 'rescue' mechanism in the hepatic acute phase immune response

The liver isoform of the enzyme alkaline phosphatase (AP) has been used classically as a serum biomarker for hepatic disease states such as hepatitis, steatosis, cirrhosis, drug-induced liver injury, and hepatocellular carcinoma. Recent studies have demonstrated a more general anti-inflammatory role for AP, as it is capable of dephosphorylating potentially deleterious molecules such as nucleotide phosphates, the pathogenic endotoxin lipopolysaccharide (LPS), and the contact clotting pathway activator polyphosphate (polyP), thereby reducing inflammation and coagulopathy systemically. Yet the mechanism underlying the observed increase in liver AP levels in circulation during inflammatory insults is largely unknown. This paper hypothesizes an immunological role for AP in the liver and the potential of this system for damping generalized inflammation along with a wide range of ancillary pathologies. Based on the provided framework, a mechanism is proposed in which AP undergoes transcytosis in hepatocytes from the canalicular membrane to the sinusoidal membrane during inflammation and the enzyme's expression is upregulated as a result. Through a tightly controlled, nucleotide-stimulated negative feedback process, AP is transported in this model as an immune complex with immunoglobulin G by the asialoglycoprotein receptor through the cell and secreted into the serum, likely using the receptor's State 1 pathway. The subsequent dephosphorylation of inflammatory stimuli by AP and uptake of the circulating immune complex by endothelial cells and macrophages may lead to decreased inflammation and coagulopathy while providing an early upstream signal for the induction of a number of anti-inflammatory gene products, including AP itself.

A putative enzyme from various secretions specifically inhibits antibody-antigen interactions

Various human secretions (intestinal secretion, saliva, nasal mucus, lacrimal fluid) have been found to inhibit the binding of antibodies to their antigens. Various characteristics (e.g. time, pH, temperature dependence, affinity and size exclusion chromatography) suggested that the inhibitory activity was attributable to an enzyme. Further investigations revealed that this enzyme reacted with the Fab portion of immunoglobulin G, specifically with the heavy chain. It is assumed that it represents a novel immunoglobulin-specific protease since similar results were not obtained with proteolytic enzymes from human digestive organs e.g. pepsin, trypsin and chymotrypsin. Finally, investigating saliva it was demonstrated that the putative protease was not identical to enzymes from periodontal bacteria which are proteolytic for the Fc portion of immunoglobulins. The findings could be of general importance in the design of immunoassays which are to be applied to human (and possibly animal) secretions.

Origin and immunoregulation of autoantibodies against intestinal alkaline phosphatase

In the sera of patients with acute bacterial infections specific autoantibodies (sIAPa) of the immunoglobulin class G (IgG) were found which bind to intestinal alkaline phosphatase (IAP) through the Fab portion. This was demonstrated using immunoaffinity (IA) isolation of sIAPa from patients' sera (particularly bacterial meningitis and ventriculitis) digestion with pepsin, purification of F(ab')2 fragments on protein A and subsequently binding on IAP coupled to CNBr (cyanogen bromide)-activated Sepharose. Immunoblots using specific anti-Fc and anti-Fab antibodies showed that the bulk of F(ab')2 fragments had bound. Additionally, binding of native IAP to the F(ab')2 fragments was observed after separation of F(ab')2 fragments using isoelectric focusing (IEF), blotting onto nitrocellulose and incubation with IAP. Moreover, we have demonstrated the occurrence of natural anti-IAP autoantibodies (nIAPa) which were isolated from sera of healthy individuals using IA chromatography. Investigation of isotype distribution revealed that IgG but not IgM or IgA were predominant even among nIAPa. The nIAPa fraction exhibited lower binding efficiencies on IEF blots than the sIAPa fraction, however, in contrast to sIAPa, cross-reactions with other autoantigens were observed for nIAPa. NIAPa and sIAPa did not show subclass restriction. As revealed by IEF the spectrotypes of sIAPa were found to be patient-specific, poly- to oligoclonal and stable during longer periods.