Monophosphoryl lipid A (MPLA) (Synthetic, TLR4/Th1) vaccine adjuvant

Rabies, one of the world’s most threatening zoonoses, causes a deadly disease of the central nervous system (CNS). Until now, vaccination with rabies vaccines has been the most effective measure for the prevention and control of this disease. Currently, inactivated rabies vaccines are widely used in humans and domestic animals. However, the humoral immune responses induced by inactivated rabies vaccines are relatively low and multiple vaccinations are required to achieve protective immunity. Supplementation with an adjuvant is a convenient way to improve the immunogenicity of inactivated rabies vaccines.

 In this study, we found that monophosphoryl lipid A (MPLA), a known TLR4 agonist, could significantly promote the maturation of bone marrow-derived dendritic cells (BMDC) via a TLR4-dependent pathway in vitro and the maturation of conventional ones DCs (cDCs) in vivo. 

We also found that MPLA, acting as an adjuvant to inactivated rabies vaccines, could significantly facilitate the formation of follicular helper T cells (Tfh), germinal center B cells (GC) and plasma cells (PCs) and consequently increase the production of RABV -Specific total IgG, IgG2a, IgG2b and the virus neutralizing antibodies (VNAs). In addition, MPLA could increase the survival ratio of mice challenged with virulent RABV. Taken together, our results show that MPLA as an adjuvant enhances the intensity of humoral immune responses by activating the cDC-Tfh-GC-B axis. Our findings will help improve the effectiveness of conventional rabies vaccines. Germinal center (GC) B cellsand plasma cells (PCs), thereby increasing the production of RABV-specific total IgG, IgG2a, IgG2b and the virus neutralizing antibodies (VNAs). In addition, MPLA could increase the survival ratio of mice challenged with virulent RABV. 

Show summaryour results that MPLA as an adjuvant enhances the intensity of humoral immune responses by activating the cDC-Tfh-GC-B axis. Our results will help improve the effectiveness of conventional rabies vaccines. Germinal center B cells (GC) and plasma cells (PCs), thereby increasing the production of RABV-specific total IgG, IgG2a, IgG2b and the virus neutralizing antibodies (VNAs). In addition, MPLA could increase the survival ratio of mice challenged with virulent RABV. Taken together, our results show that MPLA as an adjuvant enhances the intensity of humoral immune responses by activating the cDC-Tfh-GC-B axis.

Our results will help improve the effectiveness of conventional rabies vaccines. Our results show reported that MPLA as an adjuvant enhances the intensity of humoral immune responses by activating the cDC-Tfh-GC-B axis. Our results will help improve the effectiveness of conventional rabies vaccines. Our results show that MPLA as an adjuvant enhances the intensity of humoral immune responses by activating the cDC-Tfh-GC-B axis. Our findings will help improve the effectiveness of conventional rabies vaccines.

introduction

Rabies virus (RABV), a negative-strand single RNA virus belonging to the  Lyssavirus genus  within the  Rhabdoviridae  family, is still responsible for 59,000–61,000 human deaths annually, mostly in developing countries . The RABV genome encodes five structural proteins including the nucleocapsid protein (N), phosphoprotein (P), matrix protein (M), glycoprotein (G), and large polymerase (L) [  4]. After autocleavage of the first 19 amino acids (aa), defined as the signal peptide (sp) of the G protein precursor, the mature G protein (1–505 aa) is formed, which consists of the ectodomain at the 5′ end ( et, 1 -439 aa), the transmembrane domain (tm, 440-461 aa) and the cytoplasmic tail (ct, 462-505 aa), access the virion surface. Importantly , G protein is the only protein on the virion surface and is primarily responsible for interacting with receptors expressed on the cell surface. Furthermore, G protein is the only protein that induces virus neutralizing antibodies (VNA) [  4  ].

Pre-exposure prophylaxis (PrEP) and post-exposure prophylaxis (PEP) are the most important methods of rabies prevention and control. 

In recent years, recombinant virus vectors such as poxvirus, paramyxovirus, and adenovirus have become promising for research and development of novel rabies vaccines. Nevertheless, both attenuated RABV and the recombinant virus can exhibit potential virulence, which can become a major barrier to obtaining licenses in many countries. Inactivated vaccines are still widely used in humans and pets due to their high level of safety. However, the efficacy of inactivated RABV vaccines is relatively low compared to live attenuated vaccines and more than one vaccination is required to achieve protective immunity. Adjuvant supplementation is a practical strategy to enhance the immunogenicity of inactivated RABV vaccines. So far, the aluminum adjuvant for the inactivated rabies vaccine is in preclinical study  and the PIKA adjuvant (a synthetic double-stranded RNA analogue) has progressed to a phase II study in healthy adults. PIKA-containing rabies vaccine is more effective in preventing rabies compared to adjuvant-free vaccines due to its ability to activate Toll-like receptor 3 (TLR3) signaling.

TLRs are promising immune sensors and play a crucial role in defense against pathogenic microbial infections. Several previous studies reported that TLR4, as an innate immune sensor, recognizes both microbial and endogenous ligands and initiates an immediate immune response to them . Our previous study showed that the High Mobility Group Box 1 protein (HMGB1), known as TLR4 ligand, could enhance humoral immunity by activating dendritic cells (DC). Furthermore, the well-recognized TLR4 agonist monophosphoryl lipid A (MPLA) could induce a potent immune response of type 1 CD4T helper cells (Th1), which plays a crucial role in affinity maturation of antibodies and was recently approved as an adjuvant of human papillomavirus (HPV) vaccine in Europe and the US. 

However, RABV-specific VNA and the protective effect of immunization with rabies vaccines supplemented with MPLA have not been studied. In this study, the effect of MPLA as an adjuvant to an inactivated rabies vaccine was evaluated in a mouse model. Our results indicate that MPLA could enhance RABV-specific VNA and protect against a virulent RABV challenge.