Three monoclonal antibodies against the serpin protease nexin-1 prevent protease translocation

In vitro identification of neutralizing epitopes of Rhipicephalus microplus serpin 17 (RmS-17)

BACKGROUND

Rhipicephalus microplus poses a significant problem for livestock worldwide and is primarily controlled with synthetic acaricides. The continuous use of acaricides results in the selection of resistance and causes environmental harm. Vaccination presents an alternative solution to this problem, although searching for the suitable antigen is still a work in progress. Salivary proteins hold promise for inclusion in vaccine formulation due to their roles in modulating host responses, assisting blood feeding and pathogen transmission. Serpins are a class of proteinase inhibitors and are among the molecules found in tick saliva that modulate host blood coagulation, inflammation, and adaptive immune responses. Previous studies have demonstrated the potential of R. microplus serpin 17 (RmS-17) to interfere with the host's defenses, and antibodies have been shown to neutralize its effects. This makes RmS-17 an putative target for vaccine development.

METHODS

Epitope mapping of RmS-17 was achieved using in silico approach combining linear B-cell epitope and antigenicity predictor. In addition, epitope mapping using overlapping peptides in an ELISA screening was used. The serpin tridimensional structure and the epitopes spatial location within the molecule were determined. Peptides were synthetized based on the predictions and used for the production of rabbit anti-sera. Purified IgG's were used to assess the antibodies capacity to neutralize RmS-17.

RESULTS

Through in silico mapping, nine potential B cell epitope regions were screened, with p1RmS-17 and p2RmS-17 selected for the experiment based on antigen prediction. In the ELISA screening using overlapping peptides, eight antibody-binding regions were identified, and p3RmS-17 and p4RmS-17 were chosen. Antibodies raised against p3RmS-17 and p4RmS-17 partially neutralized RmS-17 activity.

CONCLUSION

It was found that antibodies against a single epitope are sufficient to partially neutralize RmS-17 activity. These findings support the possibility of using an epitope-based vaccine for immunization against R. microplus.

Priming of SARS-CoV-2 S protein by several membrane-bound serine proteinases could explain enhanced viral infectivity and systemic COVID-19 infection

The ongoing COVID-19 pandemic has already caused over a million deaths worldwide, and this death toll will be much higher before effective treatments and vaccines are available. The causative agent of the disease, the coronavirus SARS-CoV-2, shows important similarities with the previously emerged SARS-CoV-1, but also striking differences. First, SARS-CoV-2 possesses a significantly higher transmission rate and infectivity than SARS-CoV-1 and has infected in a few months over 60 million people. Moreover, COVID-19 has a systemic character, as in addition to the lungs, it also affects the heart, liver, and kidneys among other organs of the patients and causes frequent thrombotic and neurological complications. In fact, the term "viral sepsis" has been recently coined to describe the clinical observations. Here I review current structure-function information on the viral spike proteins and the membrane fusion process to provide plausible explanations for these observations. I hypothesize that several membrane-associated serine proteinases (MASPs), in synergy with or in place of TMPRSS2, contribute to activate the SARS-CoV-2 spike protein. Relative concentrations of the attachment receptor, ACE2, MASPs, their endogenous inhibitors (the Kunitz-type transmembrane inhibitors, HAI-1/SPINT1 and HAI-2/SPINT2, as well as major circulating serpins) would determine the infection rate of host cells. The exclusive or predominant expression of major MASPs in specific human organs suggests a direct role of these proteinases in e.g., heart infection and myocardial injury, liver dysfunction, kidney damage, as well as neurological complications. Thorough consideration of these factors could have a positive impact on the control of the current COVID-19 pandemic.

Inhibition of an active zymogen protease: the zymogen form of matriptase is regulated by HAI-1 and HAI-2

The membrane-bound serine protease matriptase belongs to a rare subset of serine proteases that display significant activity in the zymogen form. Matriptase is critically involved in epithelial differentiation and homeostasis, and insufficient regulation of its proteolytic activity directly causes onset and development of malignant cancer. There is strong evidence that the zymogen activity of matriptase is sufficient for its biological function(s). Activated matriptase is inhibited by the two Kunitz-type inhibitor domain-containing hepatocyte growth factor activator inhibitors 1 (HAI-1) and HAI-2, however, it remains unknown whether the activity of the matriptase zymogen is regulated. Using both purified proteins and a cell-based assay, we show that the catalytic activity of the matriptase zymogen towards a peptide-based substrate as well as the natural protein substrates, pro-HGF and pro-prostasin, can be inhibited by HAI-1 and HAI-2. Inhibition of zymogen matriptase by HAI-1 and HAI-2 appears similar to inhibition of activated matriptase and occurs at comparable inhibitor concentrations. This indicates that HAI-1 and HAI-2 interact with the active sites of zymogen and activated matriptase in a similar manner. Our results suggest that HAI-1 and HAI-2 regulate matriptase zymogen activity and thus may act as regulators of matriptase trans(auto)-activation. Due to the main localisation of HAI-2 in the ER and HAI-1 in the secretory pathway and on the cell surface, this regulation likely occurs both in the secretory pathway and on the plasma membrane. Regulation of an active zymogen form of a protease is a novel finding.

A slow-cycling subpopulation of melanoma cells with highly invasive properties

Melanoma is a heterogeneous tumor with different subpopulations showing different proliferation rates. Slow-cycling cells were previously identified in melanoma, but not fully biologically characterized. Using the label-retention method, we identified a subpopulation of slow-cycling cells, defined as label-retaining cells (LRC), with strong invasive properties. We demonstrate through live imaging that LRC are leaving the primary tumor mass at a very early stage and disseminate to peripheral organs. Through global proteome analyses, we identified the secreted protein SerpinE2/protease nexin-1 as causative for the highly invasive potential of LRC in melanomas.

An antibody that prevents serpin polymerisation acts by inducing a novel allosteric behaviour

Serpins are important regulators of proteolytic pathways with an antiprotease activity that involves a conformational transition from a metastable to a hyperstable state. Certain mutations permit the transition to occur in the absence of a protease; when associated with an intermolecular interaction, this yields linear polymers of hyperstable serpin molecules, which accumulate at the site of synthesis. This is the basis of many pathologies termed the serpinopathies. We have previously identified a monoclonal antibody (mAb4B12) that, in single-chain form, blocks α1-antitrypsin (α1-AT) polymerisation in cells. Here, we describe the structural basis for this activity. The mAb4B12 epitope was found to encompass residues Glu32, Glu39 and His43 on helix A and Leu306 on helix I. This is not a region typically associated with the serpin mechanism of conformational change, and correspondingly the epitope was present in all tested structural forms of the protein. Antibody binding rendered β-sheet A - on the opposite face of the molecule - more liable to adopt an 'open' state, mediated by changes distal to the breach region and proximal to helix F. The allosteric propagation of induced changes through the molecule was evidenced by an increased rate of peptide incorporation and destabilisation of a preformed serpin-enzyme complex following mAb4B12 binding. These data suggest that prematurely shifting the β-sheet A equilibrium towards the 'open' state out of sequence with other changes suppresses polymer formation. This work identifies a region potentially exploitable for a rational design of ligands that is able to dynamically influence α1-AT polymerisation.