Receptor-ligand based molecular interaction to discover adjuvant for immune cell TLRs to develop next-generation vaccine
Introduction
Vaccination has appeared as most successful medical discovery, which always contributed in the benefit of public health. Those successful vaccines have saved the hundred thousands of lives worldwide. With the help of vaccination several diseases, specifically polio, measles and small pox have nearly been eradicated [1]. Development of the vaccine against several diseases has made a remarkable impact but there are still somedeadly human disease for which there are no vaccine available mentioning a few are malaria, leishmania, ulcer and tuberculosis. As per the World Health Organization (WHO) report of 2017, vaccination saved approx. 20 million lives, 500 million cases of infection from 2001 to 2020. The idea behind the vaccination against a specific pathogen/disease is to trigger strong adaptive response in human. TLRs available on immune cells permit for the specific signaling to activate immune defense system with lesser side effects as compare to the drugs having unusual mode of action. In so many ways, TLR family behaves as a “Pocket Knife” of the immune defense with its capacity of response against infections or diseases.
TLRs are leucine rich repeats and one of the largest studied members of pattern recognition factor family that binds with variety of Pattern Associated Molecular Patterns (PAMPs) from parasites, viruses, bacteria or fungi and binds with their DNA, RNA, proteins/peptides, lipids or peptidoglycan molecules. Upon extensive search of human genome, 10 functional TLRs has been identified and also found conserved between human and mice immune cells. This binding of ligands results in the dimerization of TLRs followed by recruitment of proteins at Toll IL-1 receptor (TIR) domain thus activates the intracellular signaling cascade. It is studied with the help of the crystal structure of TLR that the binding of the TLR with their corresponding ligand helps in the dimerization by creating a bridge. Thus far, there is no report for the mechanism showing signaling of TLR by protein ligands (protein adjuvants). Furthermore, if we look at the proteins of Leucine Rich Repeat (LRR) family, it is apparent that protein ligands are quite common for LRR receptor family [2]. This is because of their architecture, which is suitable for the development of protein-protein interaction. It is anticipated that the concave surfaces of LRR structures offer wide hydrophilic interaction that makes the protein ligand interaction suitable.
Over the past few years, advancement in molecular biology and bioinformatics as well as our extended knowledge about immune defense has led the development of “next generation” vaccine successful. Lower immunogenicity is the major obstruction of next generation of vaccine i.e. peptide subunit vaccine in attaining their potential. The best way to improve their efficacy is the development and screening of novel molecular adjuvants as human toll like receptors agonist. An adjuvant is a molecule, which helps the vaccine, in the long-term protection against the disease, via identification of vaccine (antigen) molecule by natural immune system. The natural immunity eventually stimulates host adaptive immune system [3]. It has been shown in the literature that in last two decades the majorly used chemical substance i.e. aluminium hydroxide as an adjuvant was not suitable (not generating strong immune response) when using for vaccines against some dreadful diseases like tuberculosis and malaria [4]. To induce the inflammation, a new adjuvant was proposed by Jules Freund named as Freund's Complete Adjuvant (FCA) where killed mycobacteria was co-injected with vaccine protein [5,6]. Though it turned out a strong adjuvant but found to be toxic in human [7].
A number of protein-based adjuvants have been described which includes heat shock proteins, bacterial flagellins, porins and bacterial lipopeptide signal peptides etc. [8]. These adjuvants are known for the fusion with subunit vaccine, which enables the production of recombinant protein as adjuvant-vaccine fusion proteins. Besides improving the efficiency, adjuvants also minimize the amount of antigen to be injected and total number of immunization needed to acquire satisfactory level of immune response [9]. Because of the role of microbial proteins in self and non-self differentiation and their capacity to enhance maturation of Antigen Presenting Cells, TLR agonists are considered to be the reliable adjuvant molecule [10]. There are some TLR agonist which has been tested and currently being used as adjuvants including Pam2CSK4, Lipopolysaccharide derivative (Monophosphoryl A) and flagellin etc. [11,12]. In recent decades, research has been conducted and shown that these bacterial proteins i.e. porins and flagellins have the immunomodulatory functions which is similar to the function of adjuvant. However, bacterial flagellin protein is widely known as TLR5 agonist [13,14]. In this study, we have identified the best TLR agonist for each human immune cell TLRs from the various known synthetic and microbial proteins/peptides reported.
Section snippets
Exploration of peptide adjuvants and human TLRs
In the first step of selection of adjuvant/agonist for human TLRs the known peptide adjuvants were excavated from the literature. To study binding of different already reported adjuvants with human TLRs, amino acid sequence of natural peptide adjuvants was retrieved in FASTA format from UniProt database (http://www.uniprot.org/uniprot) and the synthetic peptide adjuvant sequences were studied from the available literature. In case of human TLRs, PDB IDs were available on RCSB protein databank (//www.rcsb.org/
Exploration of peptide adjuvants and human TLRs
Using the PubMed search all the known adjuvants that were protein in nature, mined and tabulated. The sequences of natural adjuvants were retrieved in FASTA format from UniProt database. While sequence of synthetic adjuvants like TR-433, α-defensin, β-defensin, RS01, RS02, RS03, RS04, RS05, RS06, RS07, RS08, RS09, RS10, RS11 and RS12 were identified from the literature survey. All the details about the existing adjuvants, their respective TLRs have shown in Table 1 and the amino acid sequences
Discussion
Herein, we state for the first time a wide-ranging neck-to-neck comparison of binding affinity of adjuvant for human TLRs from the pool of different known protein adjuvants identified by literature survey. The adjuvant group is extremely diverse, but the study of a unique group of adjuvant i.e. protein adjuvant will be highly useful in terms of the conjugation/addition of adjuvant in the multi-epitope subunit protein vaccine to elicit the strong immunological response. The best strategy to
Conclusion
Adjuvants are important set of elements, which aid in the development of the robust immune stimulation to vaccines. The choice of an adjuvant for development of vaccine against particular type of infection/disease vaccine must be made based on their TLR activation. The right selection may always support the best choice to increase the functionality of upcoming vaccines against various pathogenic diseases. Therefore, our study is listing the best adjuvants for all human TLRs based upon their
CRediT authorship contribution statement
Nidhi Gupta: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Hansa Regar: Conceptualization, Methodology, Formal analysis, Writing - original draft, Writing - review & editing. Vijay Kumar Verma: Formal analysis, Writing - review & editing. Dhaneswar Prusty: Writing - review & editing. Amit Mishra: Writing - review & editing. Vijay Kumar Prajapati: Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing.
Declaration of competing interest
The Authors have declared no competing interest.
Acknowledgment
VKP is thankful to the Central University of Rajasthan for providing research facility.
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